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A novel view of opiate tolerance
Pergamon
in Neumirnmunology Vol. 6, pp. 265-277, 1996 0 1996 Elsevier Science Ltd. All rights reserved Printed in Great Britain. 0960-5428/96 $32.00
Advances
Copyright
PII: SO960-5428(96)00022-8
A novel view of opiate tolerance George B. Stefano” #I, Berta Scharrer$, Michel
Thomas and Gregory L. Fricchione”
Saket§ll
V Biljinger*$,
*Multidisciplinary Center for the Study of Aging, Old Westbury Neuroscience Research Institute, State University of New York at Old Westbury, Old Westbury, New York, NY 11568, USA Psychiatry Consultation Service, Division of Psychiatry, Brigham and Women’s Hospital, Harvard Medical School, 75 Francis Street, Boston, MA 02 115,USA $Cardiac Research Program, Cardiovascular Center, University Medical Center at SUNY, Stony Brook, NY 11794, USA $Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York, NY 10461, USA IlLaboratoire de Phylogenie moleculaire des annelides, EA DRED 1027, SN3, Universite des sciences et technologies de Lille, F-59655 Villeneuve d’Ascq cedex, France Keywords-Morphine,
dopamine,
tolerance,
/.Q opiate receptor, psychiatry,
depression,
schizophrenia.
Summary Introduction
Opiate substances occur as natural compounds in various invertebrate and vertebrate neural tissues. Recently we have discovered a novel opiate alkaloid-selective and opioid peptideinsensitive receptor, designated p3, that provides further evidence of the existence of separate morphine processes. Interestingly morphine biosynthesis appears to be linked to the dopamine pathway. Based on studies documenting the presence of morphine after stress, e.g., trauma, it is noted that this signal substance emerges after a timely delay. From this we speculate that this molecule can serve a specific effect to downregulate physiological processes after stress. We conclude that tolerance represents a natural process that terminates its action. In this regard a morphine hypothesis may be essential to a complete picture of motive circuitry. A speculative view of the psychiatric implications in schizophrenia, depression, and autism are presented with this in mind. Copyright 0 1996 Elsevier Science Ltd
¶Corresponding
author.
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Studies on the pharmacological properties of morphine and morphine-like substances have long been exclusively concerned with the effects of exogenous opiates, a family of important analgesic and antinociceptive drugs. This picture began to change after the discovery (Lord e~al., 1977) that morphine can bind to the same receptors known to be used by endogenous opioid peptides. An important step forward was the demonstration of endogenous opiates in various vertebrate tissues including the nervous system (e.g., Donnerer et al., 1987). At this turning point, the focus of interest shifted to include a study of the possible roles played by a new group of endogenous messenger molecules under physiological as well as pathological conditions (see Fricchione et al., 1994; Stefano and Scharrer, 1994). A primary effort began, directed toward the search for specific sites of production and action of endogenous morphine in the nervous system. Corresponding immune system studies also started. The extension of this work to include higher invertebrates added another dimension to
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the insights gained (Stefano et al., 1993). Before turning our attention to the discussion of the presence and possible functional significance of opiates in the two organ systems mentioned, the presumed synthetic pathway of morphine and the identification of opiate binding sites will be briefly reviewed. Biosynthesis
of opiate
alkaloids
The first report on the occurrence of morphine in a vertebrate animal, a toad, was made by Spector and colleagues (Oka et&., 1985) which followed an seminal study by Ginztler and colleagues (1976). Soon thereafter, several morphine-related substances, including thebaine, and codeine, were found in a variety of tissues, e.g., liver, spleen, adrenal, skin and hypothalamus of several mammalian species (see Stefano and Scharrer, 1994). These results obtained by use of HPLC, RIA and physical chemistry provided data for the construction of the biosynthetic pathway (or pathways) of endogenous morphine originating from dopamine (DA) (see Stefano and Scharrer, 1994).
‘Dihydromorphine
receptors
The first demonstration by Kosterlitz and coworkers (Lord et al., 1977) that exogenous morphine can bind to receptors in the mammalian brain indicated that it shares these sites with those used by endogenous opioid substances (e.g., enkephalins). Since then, growing information on the multiplicity of available receptor types has led to the understanding that, depending on their site of action, opioid peptides as well as opiate alkaloids may bind to more than one opiate receptor subtype Recently, our laboratory has proposed the existence of a third p receptor, ps, located on the immunocytes and neural tissues of the invertebrate Mytilus as well as on human monocytes and other human and mammalian tissues (Dobrenis et al., 1995; Makman, 1994; Stefano etal., 1993, 1995a; Stefano and Scharrer, 1996). The novelty and selectivity of this site was made apparent when a variety of opioid peptides, tested by two methods, were ineffective in displacing specifically bound
et al., 1993). By
The lack of interaction of S- and p-opioid peptide ligands with this novel receptor site justified its classification as type pa (Stefano et al., 1993). In addition, we have just demonstrated that this receptor is present on human endothelial cells where it mediates the release of nitric oxide (NO) which causes vasodilation (Stefano et al., 1995a). Furthermore, opioid peptides do not exert this action. Recent studies from our laboratory now indicate this same action is occurring in other tissues expressing this receptor (human macrophages and granulocytes and invertebrate immunocytes and microglia, suggesting morphine is acting, in part, by being coupled to NO release (Liu et al., 1996; Magazine et al., 1996). The significance of these findings is increased with the demonstration of ~a binding to neural tissues (Cruciani et al., 1994; see Stefano et al., 1996; Stefano and Scharrer, 1996). Opiate
Morphine
(Stefano
contrast, the opiate alkaloid p ligands were quite potent while kappa ligands dynorphin 1-I 7 and ethyl-keto-cyclasocine (EKC) were very weak.
alkaloids
in the nervous
system
The discovery of the presence of endogenous morphine in the nervous system of diverse animals (Donnerer et al., 1987; Fricchione et al., 1994; Stefano et al., 1993) has generated interest in determining their locations, sites of action. and functional significance. In attempts to identify the specific sites of production and release of these opiates, neuronal rather than glial elements come to mind. Our attention is directed to the class of catecholaminergic neurons, since DA is a presumed precursor in the synthetic pathway of morphine (see Stefano and Scharrer, 1994). Recently, Bianchi et al. (1993) reported the presence of reaction products to morphine in perikarya, fibers, and terminals of neurons in discrete areas of the brain and spinal cord of the rat. These neurons also accumulated and stored [‘HI morphine that was slowly infused intracerebroventricularly. They concluded that the endogenous opiates act within the central nervous system by binding to the F receptors present in these areas,
Opiate tolerance a view supported by the observation of immunoreactive material in structures resembling synaptic terminals (see Bianchi et al., 1994). However, these results do not suffice to arrive at a decision as to the possible existence of a separate subgroup of ‘morphinergic neurons’.
Opiate
alkaloids
and the immune
system
The concept of a functional relationship between endogenous opiates and the immune system is based on the demonstration of such material in thecirculationandofspecialopiatereceptors(ps) on immune cells of vertebrates and invertebrates (Stefano et al., 1993). Effects of exogenous morphine on cells of this system have been known for almost 100 years (Atchard et& 1909; Cantacuzene, 1898). The effects of opiate alkaloids on these cells differ from those of various opioid peptides tested. Moreover, morphine has recently been demonstrated to upregulate neutral endopeptidase 24.11 in human granulocytes (Stefano et al., 1994), an observation made many years earlier in neural tissues (Malfroy etal., 1978). In all animals tested thus far, the administration of morphine tended to inhibit or reduce immunocyte activity, i.e., chemotaxis, cellular velocity, phagocytosis (Stefano and Scharrer, 1994; Stefano et al., 1993), and cellular responsiveness to peptidergic signals that also happen to be substrates of neutral endopeptidase (Stefano et al., 1994). Recently, we were able to show that the effect of Met-enkephalin and deltorphin on immune cells is mediated viaa highly specific 6, receptor (Stefano et al., 1992a), and the opposite effect of morphine via a novel ~~ receptor sharing no cross reactivity with opioid peptides (Stefano er al., 1993). It is of interest that opiate alkaloids tend to act at higher concentrations (lo-* M) than, for example, Met-enkephalin which stimulates immunocyte activity at lo-” M (Scharrer and Stefano, 1994; Stefano et al., 1992b; Stefano, 1994). This last point will be the subject of speculation later on in this review.
Activation
267 of the pu, receptor
One feature of the morphine hypothesis that is difficult to explain at present is that extremely high concentrations of morphine are required to activate the 11Xreceptor (Kd in all tissues examined is in the 15-50 nm range; Stefano etal., 1993, 1995a). In this regard, as noted below in various stress studies, the levels of naturally occurring morphine do not reach the presumed level to ensure their operation. However, the measurement of endogenous opiate levels is taken from examining large body compartment fluids (blood) not, for example, in a ‘morphinergic’ synapse. Indeed the levels in highly localized areas, e.g. synapse or ‘blood’ cell origin points, may be higher. This is equally true for invertebrates (Stefano et& 1993, 1995b). However, there are reports that upon prolonged incubation of various cellular systems with morphine its actions, i.e., immunocyte downregulation, emerge (Chao et al., 1994; Stefano et al., 1995b). With this in mind, we suspect that pX activation can occur with prolonged exposure to morphine at somewhat lower concentrations (Fig. 1). In other words, given morphine’s relative stability, hours in plasma compared to enkephalin’s minutes (Reisine and Pasternak, 1996; Shipp et al., 1990) by increasing the duration of its presence in the environment, immunocyte inhibition may occur at lower concentrations of morphine. Furthermore, since it can be metabolized to morphine 6-glucoronide, a compound also exhibiting opiate actions (Dobrenis et al., 1995), prolonged exposure to target tissues is also increased and thus, this may be the mechanism to achievep.,activationatrelativelylowerconcentrations. Indeed the existing data support this mechanism, since morphine levels once they increase remain high for a prolonged period of vs 95 pmol/ml in time, i.e., 2-3 pmol/ml invertebrate hemolymph (Stefano et al., 1993, 1995b, unpublished). In unpublished studies on morphine levels in human plasma we found (G. Stefano; Y. Liu; T. V. Bilfinger and E. Tonnesen, unpublished) levels increasing from 1-3 to 400-800 pmol/ml 2 days post coronary artery
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Minutes -
B. Morphine
Receptor
?Tgg
\v
Hours Fig. I. Morphine has a long half-life compared to that found for the various enkephalins in human plasma. (A) Opioid peptides, i.e., enkephalins must reach a optimal concentration to be effective in a short period of time. (B) Opiate alkaloids, i.e., morphine, can initiate activity at lower concentrations since their half-life in plasma is in hours. Thus, if an opiate molecule is present in a particular bodily environment at a lower concentration than its specific receptor K,,, it may still initiate an action due to its prolonged presence.
bypass surgery, remaining high (500 pmol/ml) 3-5 days post surgery. These patients (16) did not receive morphine during the course of their hospital experience. Interestingly, this same phenomenon occurred in invertebrates subjected to trauma (Stefano et al., 1995b). In examining the immunocyte responsive state we found that during trauma or surgery they were hyperactivated (Stefano and Bihinger, 1993; Stefano et al., 1995b). However, during the morphine increase on days 2 and 3, the cells became refractory to stimulation (Stefano el al., 1995b, unpublished). From day 4 to 5, following a period of hypersensitivity (possible a rebound effect), their responsiveness to stimulatory molecules returned to ‘normal’ (Stefano et&., 1995b, unpublished). In in vitro incubations of immunocytes with morphine the cells first become ‘deactivated’, then supersensitive followed by normal activity in the continued presence of morphine. Thus, an important factor in ~a activation may be the presence of morphine and its metabolite over an extended period of time, since
morphine levels, as far as we can ascertain today, do not reach the high levels required to activate this receptor. In this discussion of the possible activities of endogenous opiates we are guided by information collected in numerous studies on the pharmacological responses to the administration of exogenous morphine, discussed in the preceding section. One feature that appears to be characteristic of exogenous opiate compounds, exemplified by their known antinociceptive effects as well as substance abuse potential, is that they lower cellular thresholds for activation under a variety of physiological and pathological conditions. It is, therefore, reasonable to speculate that endogenous opiates may act in a similar capacity, wherever a situation calls for it, especially since the levels rise after a delay. The presence of opiate alkaloids in the circulation and of special opiate receptors on immunocytes, demonstrated in vertebrates as well as
Opiate tolerance invertebrates, enables these compounds to participate directly in autoimmunoregulatory activities. These direct activities may be judged to be largely of an inhibitory nature. In addition, circulating opiates may contribute to the sum total of directives mediated by signal molecules reaching the central nervous system from various sources, including the immune system. There seems to be general agreement on the fact that serious or life-threatening challenges create a state of alertness, brought about by the instant release of stimulatory messenger molecules (catecholamines, opioid peptides and others, e.g., cytokines), during which all available energies are directed toward meeting the emergency (see Fricchione and Stefano, 1994). What should be considered to be equally important is that these stimulatory signals need to be stopped as soon as they are no longer required, so as to prepare the organism for a subsequent challenge. Endogenous morphine would seem to be an appropriate candidate to meet this demand. For example, during major surgical interventions, the immunosuppressive immunocyte effect of adrenocorticotropin (Smith et al., 1992) and interleukin-10 (see Stefano et al., 1996) produced by immunocytes may not suffice to lower the hyperstimulation of granulocytes and macrophages, which is attributable to their trauma induced acute phase response release of excitatory biological response modifiers (i.e., cytokines) (Stefano etal., 1993, 1995a). It seems reasonable to suggest that, under these circumstances, morphine may be called upon to downregulate the process so as to restore the normal level of activity. The validity of this proposal is supported by tests carried out with blood samples taken from patients during cardiopulmonary bypass operations. In preparations exposed to morphine, signs of cellular activity were significantly less pronounced than in untreated samples (Stefano and Bilfinger, 1993; Stefano et al., 1995a, unpublished data). The results of another experiment, indicative of the downregulating capacity of endogenous morphine under conditions of stress, deserves attention, since it was first reported in an invertebrate. The design was to follow the
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sequence of activities generated by subjecting the Mytilus to stressful interventions (electrical shock, prevention of valve movements; see Stefano etal., 1991). The immediate response was activation of the animal’s defense system, as judged by conformational changes (becoming ameboid and mobile) in its immunocytes, and interpreted as being brought about by the release of endogenous opioid peptides and additional molecules. Twenty-four hours later, when the state of alertness had subsided, the return of the immunoactive hemocytes to a more ‘inactive’ conformation was found to concur with a temporary but significant rise in the morphinelike content of nervous tissue and hemolymph (Stefano et al., 1993, 1995a). The conformation of the immunocytes observed at this point in time resembled that of unstressed animals exposed to exogenous morphine. Comparable studies in vertebrates showed a marked increase in morphine concentration in the spinal cord of rats suffering from chronic pain elicited by experimental arthritis (Donnerer et al., 1987). The same was true for animals subjected to prolonged food deprivation in which the morphine content of brains was higher than in controls (Lee and Spector, 1991). The insights gained from the study of the various traumatic situations cited suggest that in the hierarchy of available downregulating mechanisms, morphine operates as a strong back-up system. The observation that this secondary system comes into effect after a latency period during which endogenous opiate levels rise, is in line with the fact that the p3 opiate receptor has an affinity constant in the range of lo-* M. Evidently, the availability of a network of effective immunostimulatory agents has great survival value for vertebrates and invertebrates alike. It is, therefore, understandable that the development of its elements, including those operating in immunoregulation, can be traced far back on the evolutionary scale. The need for the operation of more than one immunosuppressive mechanism is as obvious as that for the availability of effective immunostimulatory agents. It mollusc
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is our belief that it is one of morphine’s important tasks to meet this vital demand. In addition, endogenous morphine and related opiates may be presumed to engage in a variety of other activities, for example some operating within the confines of the nervous system. However, most of these questions are still unanswered, and efforts for their solution will undoubtedly gain momentum in years to come (see Fricchione et al., 1994). Tolerance
With the above discussion in mind we will now consider the phenomenon of tolerance with regard to endogenous opiate compounds. With continued exposure to the same dose of an opiate various physiological systems exhibit a decrease in their response. This phenomenon is referred to as tolerance. As with the study of opiates our historic interest in this phenomenon is focused around antinociception. However, given that morphine is a naturally occurring signal substance we must ask another question. Since tolerance occurs, what would its ‘normal’ function be? We have examined the need for ‘turn-on’ molecules as well as the need for ‘turn-off’ molecules in various systems. Now we must consider what turns off or downregulates this ‘off system, i.e., morphine. We believe the answer, in part, to this question is in the phenomenon of tolerance. Once the downregulatory process has been initiated and the level of these molecules (i.e., morphine) rise to competitively overcome the influences of the initial stimulatory molecules (Stefano et al., 1993), the inhibitory molecule’s level, i.e. morphine (as also noted by the Kd for morphine on the pa receptor, approx. 15-50 nM), would be hard to overcome, as would be its continued presence (Fig. 2). Thus, stimulatory signal molecules could not activate the system during this downregulation unless their concentrations rose well above those levels in the initial event. Indeed, at this moment, the activities generated by an additional phase of excitatory molecule release would upset the now instituted downregulation, given its competitive and reversible
signal molecule nature. This is especially true of et al., 1993, 1995b). morphine (Stefano Therefore, the only mechanism that can effectively diminish the inhibitory actions over a relatively short period of time would be one in which the very same effector cell system, progressively becomes desensitized to the presence of morphine (Fig. 2). In this way, the downregulating influence would be terminated regardless of the concentration of morphine present during a single event. Thus, the effector cells become tolerant. Interestingly, tolerance would only set in once downregulation had been achieved. Thus, the ‘brake’ would be administered on a ‘need’ basis along with the dynamic capability of progressive adjustments in this process if required. Moreover, tolerance, once achieved, also would allow for further stimulation of the system if it was required, since morphine’s presence would not be ‘sensed’ (Fig. 2). In summary, tolerance represents a dynamic mechanism that can be used to augment various regulatory processes whether they be involved in excitation or inhibition. Simply stated, tolerance is a process that allows for the termination of morphine’s action while it is still present in the environment. It is still present in the environment because it is a general, yet specific, mechanism operating only at concentrations above basal levels, concentrations that would terminate excitatory processes. However, tolerance ensures that immuno-inhibition, for example, does not last to the point whereby the organism would be compromised due to a lack of a functioning system over an extended period of time. Thus, desensitization sets in and allows the various processes to be stimulated and operational once more. Clearly, the timely ‘rebound’ of the immune and nervous processes involved with opiate actions provides for a successful mechanism to ensure survival as has been demonstrated (see Stefano et al., 1995b). Since tolerance has been demonstrated in invertebrates and vertebrates the use of this strategy becomes even more evident (see Stefano et al., 1980).
Opiate
tolerance
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Fig. 2. Physiological significance of tolerance. (A) Stimulatory immunocyte signal molecule, i.e., TNF, activates a cell to become mobile and ameboid. (B) On first and prolonged exposure to morphine (see Fig. 1) it downregulates the cell (round and immobile). (C) However, with time, even though morphine is still present it becomes tolerant to morphine’s presence and ready for activation once more (D). This activation can occur in the presence of morphine. Indeed, to further downregulate this morphine tolerant cell more opiate is required. (E-F) In the immediate downregulated state the cell can still be activated by higher levels of a stimulatory agent (G). Thus, the morphine induced downregulation is terminated by tolerance or it can be overcome by higher doses of the stimulatory agent. This process becomes quite efficient in regulating the alert state of immunocytes and possibly neurons as well. Dependence/addiction
In drug dependence one can see tolerance develop with a decreased response to the actions of a drug. Thus, in order to achieve the same effect one has to take a larger dose. The phenomenon of dependence occurs upon the withdrawal of the drug, which produces behavioral signs opposite of those desired. Furthermore, associated psychological dependence involves compulsive drug-seeking behavior. In all animals there are normal behaviors which can be said to be based on a compulsive ‘foundation’. It would be interesting to speculate that addiction emerges from tolerance if the presence and level of endogenous opiates do not or cannot return to their previous or prestimulation low levels. In this scenario, once tolerance occurs, the opiate molecules, i.e., morphine, remain relatively elevated. In this event, in order
to further lower the threshold of activation, e.g., immunosuppression, one would require even greater increases in morphine levels due to tolerance. It would be to an organism’s benefit to have such an immune mechanism, since this would allow for a more dynamic response to antigenic challenge, for example. Thus, an animal would have several levels of immune responsiveness to call upon. Indeed, we surmise, the lack or dysfunction of such a system may lead to various pathologies, i.e., hyperactive cell disorders autoimmune disorders). However, given the above, the dynamics of passing through many levels of tolerance may, in time, adversely affect the organism. Clearly, operating at different morphine levels may force a ‘system/process’ to continually re-establish its threshold for excitability due to tolerance setting
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in. This endogenous trauma may manifest itself, as noted above, in the display of opposite ‘behaviors’ i.e., excitability of nerve or immune cells. Indeed, one may associate immune stimulation with morphine actions due to the fact the process has become tolerant and thus supersensitive when morphine availability ceases (Stefano et al., 1995b). In a concurrent neuronal morphine receptor supersensitive state, if the stimuli/state became cognitive, one would actively seek out opiates. In this regard, addictive or compulsive behavior may be viewed as a process indicative of morphine insufficiency. A subpopulation of individuals seeking continuously higher levels of opiates (i.e., those addicted) may be operating from a morphine insufficiency status. Therefore, addictive behavior may be viewed as a phenomenon that reflects opiate insufficiency in an organism with attendant alterations in neuro- and immunoregulation.
Normal Morphine
Indeed, what may initiate the cascade of tolerance in a potential addict, an individual who may have an endogenous morphine biosynthesis and/or excessive degradation problem, is the very first experience with such a substance (Fig. 3). For in this experience a subpopulation of individuals may for the first time ‘feel’ normal. This might reflect the ‘neurological susceptibility’ to drug addiction referred to by Dole and Nyswander in their 1967 metabolic theory of addiction (Dole and Nyswander, 1967). Normal is defined as being in the sense a diabetic feels being given insulin (Goldstein, 1991). However, in the morphine insufficiency scenario, as morphine is administered, tolerance develops because it is the normal way endogenous morphine’s presence is downregulated. Thus, a person enters into this cycle of dependence as they continually seek to regain and maintain the ‘normal’ feeling; often overshooting into a state of intoxication. This
Excitation is momentary or prolonged as a function
-
Morphine _ Activity goes back to F “normal”
I
of strength
Morphine I 3.
Excitation Morphine
insufficiency
-
abnormally prolonged
+ Activity goes back to “normal”
l-l Tolerance then addiction Fig. 3.
1 -
lnorder to feel “normal” one to take
must continue morphine
The pathological significance of the morphine insufficiency hypothesis. Under normal circumstances cellular excitatory states can be downregulated by morphine, representing one of many downregulating signal molecules. In the morphine insufficiency scenario an excitatory state may be prolonged. In an individual having such a deficit in this downregulating mechanism, being exposed to an opiate alkaloid may temporarily ‘fix’ the problem. In this regard, the individual may seek this agent once the ‘fix’ is gone due to tolerance or dissipation ofthe active concentration. This individual seeking to be ‘normal’ again must continually take the agent, initiating a cycle of tolerance and dependence.
Opiate tolerance hypothesis offers the explanation for the phenomenon that some individuals treated for drug addiction are not ‘cured’. In this specific case the reason is simple, the endogenous opiate insufficiency has not been addressed, and by providing the substance one compromises the existing regulatory processes, i.e., tolerance. Speculation psychiatry
on brain
motive
circuitry
and
In 199 1, Goldstein updated the Dole and Nyswander metabolic theory of addiction and focused on opioid neuropeptide stimulation of preceptors on inhibitory GABA neurons which modulate ventral tegmental DA neurons (Goldstein, 1991). The resultofsuchFstimulationisdisinhibitionofthese DA circuits, leading to DA stimulation at the nucleus accumbens and the subjective reward response. Recently we speculated that endogenous morphine may also be essential to a complete picture of brain motive circuitry (Fricchione etal., 1994). A hypothetical DA-morphine pathway was reviewed and a role in addictive disorders was offered (Fricchione et al., 1994). This hypothesis gathers support from recent genetic findings regarding the Al allele of the D2-DA receptor (DRD2) gene in alcoholics and cocaine abusers, which suggest that certain substance abusers seek intoxication as compensation for an inherently underactive DA-mediated reward system (Hyman and Nestler, 1996). Given this speculative hypothesis of morphine insufficiency it is of interest to consider it in regard to a potential influence in the motive circuits. This does not detract from the presence and actions of other opioid signalling processes in these areas but may serve to add another dimension. Hyman and Nestler have recently reviewed how opiates are hypothesized to act on the brain motive circuitry (Compton et al., 1996). While there are opioid receptors directly on limbic neurons and inhibitory kappa dynorphin receptors on DA neurons in the ventral tegmental area (VTA), opiate p receptors responsive to morphine lie on VTA GABA interneurons and perform an inhibitory function on GABA inhibition, thereby indirectly
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disinhibiting VTA DA neurons as Goldstein (199 1) suggests. In this scenario we surmise endogenous morphine deficiency syndrome would tend to enhance GABAergic inhibition of VTA DA activity. Cell bodies in the mesolimbicmesocortical DA brain reward system are located in the VTA. Terminal fields in this key neural circuitry include the nucleus accumbens (NAS), the anterior cingulate cortex (ACC) and the dorsolateral pre-frontal cortex (DLPFC). The interaction among DA, GABA, glutamate, acetylcholine and serotonin among other neurotransmitter systems, is known to be central to our understanding of brain reward system functioning in this network (see Fricchione et al., 1994). In a morphine insufficiency scenario the p receptor on VTA GABA intemeurons may become hypersensitive, setting the stage for an amplified effect when morphine is reintroduced. The result would be morphine-induced inhibition of GABA which, in turn, would lead to DA disinhibition. Of course, along with chronic morphine availability tolerance would develop. Thus, one way to counteract a putative morphine deficiency syndrome would be to administer exogenous morphine acutely. This may lead to long-term difficulties, however, repeated acute use may eventually cause receptor level alterations and withdrawal symptoms which are only counteracted by chronic use of morphine, cocaine and alcohol, leading to addiction. What may be occurring when these substances are used chronically is an attempt to compensate through tyrosine hydroxylase and aldehyde effects in the DA-endogenous alkaloid biosynthetic pathway toward endogenous codeine and morphine production. This could be thought of as a ‘priming the pump’ strategy doomed to failure if the cause for the endogenous morphine deficiency is a defect in the DA-endogenous alkaloid biosynthetic pathway which has, in effect, left the well dry. While endogenous opiates will be expected to normally operate under conditions of amplification and tolerance, an endogenous morphine deficiency would create a naturally occurring addiction state, i.e., a constellation of
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physiological and psychological phenomena reflective of starved supersensitive amplified opiate receptors. After making the association between the mesocortical-mesohmbic DA system and opiates (both exogenous and endogenous) active at local k receptors, it follows that there may be other psychiatric implications in addition to those regarding addictive illness. The mesolimbicmesocortical DA system in addition to the mesostriatal DA system appears to play an important role in schizophrenia (Fig. 4; Lipska and Weinberger, 1993). Some schizophrenics are now thought to exhibit hypofrontality with ventricular enlargement and atrophy on neuroimaging and frontal deficits on neuropsychological testing (Weinberger etal., 1992). Data from animal studies show that PFC damage reduces the threshold for activation of the mesolimbic DA system under stress conditions. This leads to a state of hyperresponsiveness to stress. Hypofrontality may be associated with reduction of DA turnover in the PFC, which can be brought about by hippocampal lesions (Fricchione et al., 1994). Hippocampal disarray has been reported in schizophrenia (Lipska and Weinberger, 1993). Morphine immunoreactivity has been found in the hippocampus (Bianchi et al., 1993) as well as in mesocorticol-
imbic and mesostriatal regions and thus may contribute to the modulation of DA function in schizophrenic disorder. Changes in amplification and tolerance effects of sufficient magnitude involving endogenous opiates could thereby contribute to positive and negative symptom presentations in schizophrenia. The clinical observation of higher pain tolerance in some schizophrenics There
may also be explained
is also
evidence
on this basis.
of mesolimbic
DA
in depression (Kapur and Mann, 1992). Lower cerebrospinal fluid (CSF) homovanillic acid (HVA) levels in depressed patients, increased incidence of depression in Parkinson’s disease and with DA-depleting agents, and the enhancement of DA transmission with electroconvulsive therapy all point to a role for DA dysfunction in depressive disorders. As we mentioned, drugs such as cocaine, morphine, as well as psychostimulants and nicotine that result in self-administration due to reinforcement properties are associated with increased VTA DA neuron firing, activation of mesohmbic pathways, and an increase in NAS extracellular DA concentration (DiChiara etal., 1990). The implications of these relationships for our understanding of depression are still unclear, but we can again speculate that a naturally occurring opiate may be playing system
involvement
Low Morphine -
Increase GABA Inhibition of VTA DA
Add Morphine -
Tolerance *
VTA-GABA hypersensitived
DA Disinhibition
///I Schizophrenia Fig. 4. Morphine deficiency would be expected to permit GABA intemeuron psychiatric sequela, in addition to a predisposition to addiction, would be plementation or endogenous excess would be expected to decrease GABA would be an increase in VTA DA output. Psychiatric consequences may schizophrenia, autism and delirium.
Autism
Delirium
inhibition of VTA DA. A potential dysthymic mood. Morphine supinhibition of VTA DA. The result be reflected in such disorders as
Opiate tolerance a role through mechanisms of amplification and tolerance. We can also speculate that endogenous morphine is involved in the pathogenesis of autism, especially given the fact that narcotic antagonists have shown some efficacy in reducing autistic symptomatology, as have the DA antagonist neuroleptics (Herman et al., 1986; Campbell 1987). Opiate-DA interaction may also be important in the pathophysiology of delirium (Fig. 4).
Conclusion
The brain reward system interaction between F-mediated opiate processes and the mesolimbic-mesocortical DAsystem is central to all mammalian behavior, including that of humans. The balancing of human behavior in terms of pleasure and pain occurs in this motive circuitry, thus imparting an assessment of survival value. In part this assessment will be based on the relative state-amplified vs tolerant-of opiate receptor system processes. Those individuals with addictions, schizophrenia, depression, autism and other psychiatric disorders, find themselves dealing with core survival issues against the background of their biological endowments, particularly in the form of their individual mesolimbic-mesocortical systems. In this context, naturally occurring morphine may play a crucial role in modulating mesolimbic-mesocortical DA sensitization in response to stress. In this regard opiate tolerance must be examined, especially given its critical actions in regulating immune function. This is especially true, given the interaction of microglia and neurons (see Fricchione et al., 1996).
Acknowledgments
We are grateful to Dr. Mary Weitzman for valuable bibliographic assistance. We acknowledge the following grant support: NIMH-NIDA COR 17 138, NIDA 09010, the Research Foundation of SUNY (CBS) and NIH NS 22344-08 (BS).
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References Atchard, C., Bernard, H. and Gagneux, C. (1909) Action de la morphine sur les proprietes leucocytaires. Leuco-diagnostic du morphinisme. Bull. Mem. Sot. Med. Hosp. Paris 28:958. Bianchi, E., Alessandrini, C., Guama, M. and Tagliamonte, A. (1993). Endogenous codeine and morphine are stored in specific brain neurons. Brain Res. 627:210-215. Bianchi, E., Guama, M. and Tagliamonte, A. (1994). Immunocytochemical localization of endogenous codeine and morphine. Adv. Neuroimmunol. 4:83-92. Campbell, M. (1987). Drug treatment of infantile autism: the past decade. In: Psychopharmacology: The third generation of progress, ed. H. Y. Meltzer, pp. 1225. New York, Raven Press. Cantacuzene, J. (1898). Nouvelles recherches sur led mode de destruction des vibrions dans l’organisme. Ann. Inst. Pasteur 12:274-300. Chao, C. C., Gekker, G., Sheng, W. S., Hu, S., Tsang, M. and Peterson, P. K. (1994). Priming effect of morphine on the production of tumor necrosis factorcwby microglia: implications in respiratory burst activity and human immunodeficiency virus-l expression. J. Exp. Pharmacol. Ther: 269: 198-203. Compton, P. A., Anglin, M. D., Khalsa-Denison, E. and Paredes, A. (1996). The D2 dopamine receptor gene, addiction and personality: clinical correlates in cocaine abusers. Biol. Psychiatry 39:302-304. Cruciani, R. A., Dvorkin, B., Klinger, H. P. and Makman, M. H. (1994). Presence in neuroblastoma cells of a ,u~ receptor with selectivity for opiate alkaloids but without affinity for opioid peptides. Brain Res. 667:229-237. DiChiara, G., Acquas, E. and Carboni, E. (1990). Dopamine and drug-induced motivation. In: Doparnine andMental Depression, eds G. L. Gessa and G. Serra, pp. 27-38. Oxford, Pergamon Press. Dobrenis, K., Makman, M. H. and Stefano, G. B. (1995). Occurrence of the opiate alkaloid-selective pll receptor in mammalian microglia, astrocytes and kupffer cells. Brain Res. 686:239-248. Dole, V. P. and Nyswander, M. E. (1967). Heroine addiction - a metabolic disease. Arch. Intern. Med. 120: 19-24. Donnerer, J., Cardinale, G., Coffey, J., Lisek, C. A., Jardine, I. and Spector, S. (1987). Chemical characterization and regulation of endogenous
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morphine and codeine in the rat. J. Pharmacol. Exp. Ther 242583-587. Fricchione, G. L., Bilfinger, T. V. and Stefano, G. B. (1996). The macrophage and neuropsychiatric disorders. Neuropsychiarry, Neuropsychol. Behavior Neurol. 9: 16-29. Fricchione, G. L., Mendoza, A. and Stefano, G. B. (1994). Morphine and its psychiatric implications. Adv. Neuroimtnunol. 4: 117-132. Fricchione, G. L. and Stefano, G. B. (1994). The stress response and autoimmunoregulation. Adv. Neuroimmunol. 4: 13-28. Gintzler, A. R., Levy, A. and Spector, S. (1976). Antibodies as a means of isolating and characterizing biologically active substances: presence of a non-peptide morphine-like compound in the central nervous system. Proc. NatlAcad. Sci. USA 73:2312-2136. Goldstein, A. (1991). Heroine addiction: neurobiology, pharmacology, and policy. J. Psychoactive Drugs 23:123-133. Herman, B. H., Hammock, M. K., Arthur-Smith, A., Egan, J. Chatoor, I., Zeinick, N., Corradine, M., Appelgate, K., Boeckx, R. L. and Sharp, S. D. (1986). Role of opioid peptides in autism: effects of acute administration of naltrexone. Sot. Neurosci. 12:172-173. Hyman, S. E. and Nestler, E. J. (1996). Initiation and adaptation: a paradigm for understanding psychotropic drug action. Am. J. Psychiatn, 153: 15 l-l 62. Kapur, S. and Mann, J. J. (1992). Role of the dopaminegic system in depression. BioI. fsychiatp32: l-17. Lee, C. S. and Spector, S. (1991). Changes of endogenous morphine and codeine contents in fasting rat. J. Pharmacol. Exp. Ther 257:647-650. Lipska, B. K. and Weinberger, D. R. (1993). Cortical regulation of the mesolimbic dopamine system: implications for schizophrenia. In: Limbic motor circuits and neuropsychiatry, eds I? W. Kalivas and C. D. Barnes, pp. 329-349. CRC Press, Boca Raton. Liu, Y., Shenouda, D., Bilfinger, T. V., Stefano, M. L., Magazine, H. I. and Stefano, G. B. (1996). Morphine stimulates nitric oxide release from invertebrate microglia. Bruin Res. 722: 125-I 3 1. Lord, J. A. H., Waterfield, A. A., Hughes, J. and Kosterlitz, H. W. (1977). Endogenous opioid peptides: multiple agonists and receptors. Nature 267:495-499. Magazine, H. I., Liu, Y., Bilfinger, T. V., Fricchione, G. L. and Stefano, G. B. (I 996). Morphine-induced conformational changes in immunocytes, endothelial
cells and microglia are mediated by nitric oxide. J. Immunol. 156: 4845-4850. Makman, M. H. (1994). Morphine receptors in immunocytes and neurons. Adv. Neuroimmunol. 4:69-82. Malfroy, B., Swerts, J. P., Guyon, A., Roques, B. P. and Schwartz, J. C. (1978). High affinity enkephalin degrading peptidase in brain is increased after morphine. Nature 276:523-526. Oka, K., Kantrowitz, J. D. and Spector, S. (1985). Isolation of morphine from toad skin. Proc. Nat1 Acad. Sci. USA 82: 1852-l 854. Reisine, T. and Pasternak, G. W. (1996). Opioid analgesics and antagonists. In The Pharmacological Basis of Therapeutics, eds J. G. Hardman and L. E. Limbird, pp. 521-556. New York, McGraw Hill. Scharrer, B. and Stefano, G. B. (1994). Neuropeptides and autoregulatory immune processes. In: Neuropeptides and Immunoregulation, eds B. Scharrer, E. M. Smith and G. B. Stefano, pp. l-18. Berlin, Springer. Shipp, M. A., Stefano, G. B., D’Adamio, L., Switzer, S. N., Howard, F. D., Sinisterra, J. I., Scharrer, B. and Reinherz, E. (1990). Downregulation of enkephalin-mediated inflammatory responses by CD 1O/neutral endopeptidase 24.11. Nature 347:394-396. Smith, E. M., Hughes, T. K., Hashemi, F. and Stefano, G. B. (1992). Immunosuppressive effects of ACTH and MSH and their possible significance in human immunodeficiency virus infection. Proc. NatIAcad. Sci. USA 89:782-786. Stefano, G. B. (1994). Pharmacological and binding evidence for opioid receptors on vertebrate and invertebrate blood cells. In: Neuropeptides and Immunoregulafion, eds B. Scharrer, E. M. Smith and G. B. Stefano, pp. 139-15 1. Berlin, Springer. Stefano, G. B. and Bilfinger, T. V. (1993). Human neutrophil and macrophage chemokinesis induced by cardiopulmonary bypass: loss of DAME and IL-l chemotaxis. J. Neuroimmunol. 47: 189-l 98. Stefano, G. B., Cadet, P., Dokun, A. and Scharrer, B. (1991). A neuroimmunoregulatory-like mechanism responding to electrical shock in the marine bivalve Myrilus edulis. Brain, Behav. Immunity 41323-329. Stefano, G. B., Digenis, A., Spector, S., Leung, M. K., Bilfinger, T. V., Makman, M. H., Scharrer, B. and Abumrad, N. N. (1993). Opiate like substances in an invertebrate, a novel opiate receptor on invertebrate and human immunocytes, and a role in
Opiate immunosuppression. Proc. Nat1 Acad. Sci. USA 90:11099-11103. Stefano, G. B., Hartman, A., Bilfinger, T. V, Magazine, H. I., Liu, Y., Casares, F. and Goligorsky, M. S. (1995a). Presence of the pcLsopiate receptor in endothelial cells: Coupling to nitric oxide production and vasodilation. J. Biol. Chem. 270:30290-30293. Stefano, G. B., Hiripi, L., Rozsa, K. S. and Salanki, J. (1980). Behavioral effects of morphine on the land snail Helixpomatia: demonstration of tolerance. In: Neurobiology of Invertebrates, ed. J. Salanki, pp. 285-295. New York, Pergamon Press. Stefano, G. B., Kushnerik, V., Rodriquez, M. and Bilfinger, T. V. (1994). Inhibitory effect of morphine on granulocyte stimulation by tumor necrosis factor and substance P. ht. J. Immunopharmacol. 16:329-334. Stefano, G. B., Leung, M. K., Bilfinger, T. V. and Scharrer, B. (1995b). Effect of prolonged exposure to morphine on responsiveness of human and invertebrate immunocytes to stimulatoty molecules. J. Neuroimmunol. 63: 175-l 8 1. Stefano, G. B., Melchiorri, P., Negri, L., Hughes, T. K. J. and Scharrer, B. (1992a). (D-Ala2)-Deltorphin I binding and pharmacological evidence for a special
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subtype of delta opioid receptor on human and invertebrate immune cells. Proc. NatlAcad. Sci. USA 89:93 16-9320. Stefano, G. B., Paemen, L. R. and Hughes, T. K. J. (1992b). Autoimmunoregulation: differential modulation of CDlO/neutral endopeptidase 24.11 by tumor necrosis factor and neuropeptides. J. Neuroimmunol. 41:9-14. Stefano, G. B. and Scharrer, B. (1994). Endogenous morphine and related opiates, a new class of chemical messengers. Adv. Neuroimmunol. 4:57-68. Stefano, G. B. and Scharrer, B. (1996). The presence of the p-3 opiate receptor in invertebrate neural tissues. Comp. Biochem. Physiol. 113C:369-373. Stefano, G. B., Scharrer, B., Smith, E. M., Hughes, T. K., Magazine, H. I., Bilfinger,T. V., Hartman,A. R., Fricchione, G. L., Liu, Y. and Makman, M. H. (1996). Opioid and opiate immunoregulatory processes. Crit. Rev. Immunol. 16: 109-144. Weinberger, D. R., Berman, K. F., Suddath, R. and Torrey, E. F. (1992). Evidence for dysfunction of a pre-frontal-limbic network in schiophrenia: an MRI and rCBF study of discordant monozygotic twins. Am. J. Psychiatry 149:890-897.
in Neumirnmunology Vol. 6, pp. 265-277, 1996 0 1996 Elsevier Science Ltd. All rights reserved Printed in Great Britain. 0960-5428/96 $32.00
Advances
Copyright
PII: SO960-5428(96)00022-8
A novel view of opiate tolerance George B. Stefano” #I, Berta Scharrer$, Michel
Thomas and Gregory L. Fricchione”
Saket§ll
V Biljinger*$,
*Multidisciplinary Center for the Study of Aging, Old Westbury Neuroscience Research Institute, State University of New York at Old Westbury, Old Westbury, New York, NY 11568, USA Psychiatry Consultation Service, Division of Psychiatry, Brigham and Women’s Hospital, Harvard Medical School, 75 Francis Street, Boston, MA 02 115,USA $Cardiac Research Program, Cardiovascular Center, University Medical Center at SUNY, Stony Brook, NY 11794, USA $Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York, NY 10461, USA IlLaboratoire de Phylogenie moleculaire des annelides, EA DRED 1027, SN3, Universite des sciences et technologies de Lille, F-59655 Villeneuve d’Ascq cedex, France Keywords-Morphine,
dopamine,
tolerance,
/.Q opiate receptor, psychiatry,
depression,
schizophrenia.
Summary Introduction
Opiate substances occur as natural compounds in various invertebrate and vertebrate neural tissues. Recently we have discovered a novel opiate alkaloid-selective and opioid peptideinsensitive receptor, designated p3, that provides further evidence of the existence of separate morphine processes. Interestingly morphine biosynthesis appears to be linked to the dopamine pathway. Based on studies documenting the presence of morphine after stress, e.g., trauma, it is noted that this signal substance emerges after a timely delay. From this we speculate that this molecule can serve a specific effect to downregulate physiological processes after stress. We conclude that tolerance represents a natural process that terminates its action. In this regard a morphine hypothesis may be essential to a complete picture of motive circuitry. A speculative view of the psychiatric implications in schizophrenia, depression, and autism are presented with this in mind. Copyright 0 1996 Elsevier Science Ltd
¶Corresponding
author.
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Studies on the pharmacological properties of morphine and morphine-like substances have long been exclusively concerned with the effects of exogenous opiates, a family of important analgesic and antinociceptive drugs. This picture began to change after the discovery (Lord e~al., 1977) that morphine can bind to the same receptors known to be used by endogenous opioid peptides. An important step forward was the demonstration of endogenous opiates in various vertebrate tissues including the nervous system (e.g., Donnerer et al., 1987). At this turning point, the focus of interest shifted to include a study of the possible roles played by a new group of endogenous messenger molecules under physiological as well as pathological conditions (see Fricchione et al., 1994; Stefano and Scharrer, 1994). A primary effort began, directed toward the search for specific sites of production and action of endogenous morphine in the nervous system. Corresponding immune system studies also started. The extension of this work to include higher invertebrates added another dimension to
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in Neuroimmunology
the insights gained (Stefano et al., 1993). Before turning our attention to the discussion of the presence and possible functional significance of opiates in the two organ systems mentioned, the presumed synthetic pathway of morphine and the identification of opiate binding sites will be briefly reviewed. Biosynthesis
of opiate
alkaloids
The first report on the occurrence of morphine in a vertebrate animal, a toad, was made by Spector and colleagues (Oka et&., 1985) which followed an seminal study by Ginztler and colleagues (1976). Soon thereafter, several morphine-related substances, including thebaine, and codeine, were found in a variety of tissues, e.g., liver, spleen, adrenal, skin and hypothalamus of several mammalian species (see Stefano and Scharrer, 1994). These results obtained by use of HPLC, RIA and physical chemistry provided data for the construction of the biosynthetic pathway (or pathways) of endogenous morphine originating from dopamine (DA) (see Stefano and Scharrer, 1994).
‘Dihydromorphine
receptors
The first demonstration by Kosterlitz and coworkers (Lord et al., 1977) that exogenous morphine can bind to receptors in the mammalian brain indicated that it shares these sites with those used by endogenous opioid substances (e.g., enkephalins). Since then, growing information on the multiplicity of available receptor types has led to the understanding that, depending on their site of action, opioid peptides as well as opiate alkaloids may bind to more than one opiate receptor subtype Recently, our laboratory has proposed the existence of a third p receptor, ps, located on the immunocytes and neural tissues of the invertebrate Mytilus as well as on human monocytes and other human and mammalian tissues (Dobrenis et al., 1995; Makman, 1994; Stefano etal., 1993, 1995a; Stefano and Scharrer, 1996). The novelty and selectivity of this site was made apparent when a variety of opioid peptides, tested by two methods, were ineffective in displacing specifically bound
et al., 1993). By
The lack of interaction of S- and p-opioid peptide ligands with this novel receptor site justified its classification as type pa (Stefano et al., 1993). In addition, we have just demonstrated that this receptor is present on human endothelial cells where it mediates the release of nitric oxide (NO) which causes vasodilation (Stefano et al., 1995a). Furthermore, opioid peptides do not exert this action. Recent studies from our laboratory now indicate this same action is occurring in other tissues expressing this receptor (human macrophages and granulocytes and invertebrate immunocytes and microglia, suggesting morphine is acting, in part, by being coupled to NO release (Liu et al., 1996; Magazine et al., 1996). The significance of these findings is increased with the demonstration of ~a binding to neural tissues (Cruciani et al., 1994; see Stefano et al., 1996; Stefano and Scharrer, 1996). Opiate
Morphine
(Stefano
contrast, the opiate alkaloid p ligands were quite potent while kappa ligands dynorphin 1-I 7 and ethyl-keto-cyclasocine (EKC) were very weak.
alkaloids
in the nervous
system
The discovery of the presence of endogenous morphine in the nervous system of diverse animals (Donnerer et al., 1987; Fricchione et al., 1994; Stefano et al., 1993) has generated interest in determining their locations, sites of action. and functional significance. In attempts to identify the specific sites of production and release of these opiates, neuronal rather than glial elements come to mind. Our attention is directed to the class of catecholaminergic neurons, since DA is a presumed precursor in the synthetic pathway of morphine (see Stefano and Scharrer, 1994). Recently, Bianchi et al. (1993) reported the presence of reaction products to morphine in perikarya, fibers, and terminals of neurons in discrete areas of the brain and spinal cord of the rat. These neurons also accumulated and stored [‘HI morphine that was slowly infused intracerebroventricularly. They concluded that the endogenous opiates act within the central nervous system by binding to the F receptors present in these areas,
Opiate tolerance a view supported by the observation of immunoreactive material in structures resembling synaptic terminals (see Bianchi et al., 1994). However, these results do not suffice to arrive at a decision as to the possible existence of a separate subgroup of ‘morphinergic neurons’.
Opiate
alkaloids
and the immune
system
The concept of a functional relationship between endogenous opiates and the immune system is based on the demonstration of such material in thecirculationandofspecialopiatereceptors(ps) on immune cells of vertebrates and invertebrates (Stefano et al., 1993). Effects of exogenous morphine on cells of this system have been known for almost 100 years (Atchard et& 1909; Cantacuzene, 1898). The effects of opiate alkaloids on these cells differ from those of various opioid peptides tested. Moreover, morphine has recently been demonstrated to upregulate neutral endopeptidase 24.11 in human granulocytes (Stefano et al., 1994), an observation made many years earlier in neural tissues (Malfroy etal., 1978). In all animals tested thus far, the administration of morphine tended to inhibit or reduce immunocyte activity, i.e., chemotaxis, cellular velocity, phagocytosis (Stefano and Scharrer, 1994; Stefano et al., 1993), and cellular responsiveness to peptidergic signals that also happen to be substrates of neutral endopeptidase (Stefano et al., 1994). Recently, we were able to show that the effect of Met-enkephalin and deltorphin on immune cells is mediated viaa highly specific 6, receptor (Stefano et al., 1992a), and the opposite effect of morphine via a novel ~~ receptor sharing no cross reactivity with opioid peptides (Stefano er al., 1993). It is of interest that opiate alkaloids tend to act at higher concentrations (lo-* M) than, for example, Met-enkephalin which stimulates immunocyte activity at lo-” M (Scharrer and Stefano, 1994; Stefano et al., 1992b; Stefano, 1994). This last point will be the subject of speculation later on in this review.
Activation
267 of the pu, receptor
One feature of the morphine hypothesis that is difficult to explain at present is that extremely high concentrations of morphine are required to activate the 11Xreceptor (Kd in all tissues examined is in the 15-50 nm range; Stefano etal., 1993, 1995a). In this regard, as noted below in various stress studies, the levels of naturally occurring morphine do not reach the presumed level to ensure their operation. However, the measurement of endogenous opiate levels is taken from examining large body compartment fluids (blood) not, for example, in a ‘morphinergic’ synapse. Indeed the levels in highly localized areas, e.g. synapse or ‘blood’ cell origin points, may be higher. This is equally true for invertebrates (Stefano et& 1993, 1995b). However, there are reports that upon prolonged incubation of various cellular systems with morphine its actions, i.e., immunocyte downregulation, emerge (Chao et al., 1994; Stefano et al., 1995b). With this in mind, we suspect that pX activation can occur with prolonged exposure to morphine at somewhat lower concentrations (Fig. 1). In other words, given morphine’s relative stability, hours in plasma compared to enkephalin’s minutes (Reisine and Pasternak, 1996; Shipp et al., 1990) by increasing the duration of its presence in the environment, immunocyte inhibition may occur at lower concentrations of morphine. Furthermore, since it can be metabolized to morphine 6-glucoronide, a compound also exhibiting opiate actions (Dobrenis et al., 1995), prolonged exposure to target tissues is also increased and thus, this may be the mechanism to achievep.,activationatrelativelylowerconcentrations. Indeed the existing data support this mechanism, since morphine levels once they increase remain high for a prolonged period of vs 95 pmol/ml in time, i.e., 2-3 pmol/ml invertebrate hemolymph (Stefano et al., 1993, 1995b, unpublished). In unpublished studies on morphine levels in human plasma we found (G. Stefano; Y. Liu; T. V. Bilfinger and E. Tonnesen, unpublished) levels increasing from 1-3 to 400-800 pmol/ml 2 days post coronary artery
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Minutes -
B. Morphine
Receptor
?Tgg
\v
Hours Fig. I. Morphine has a long half-life compared to that found for the various enkephalins in human plasma. (A) Opioid peptides, i.e., enkephalins must reach a optimal concentration to be effective in a short period of time. (B) Opiate alkaloids, i.e., morphine, can initiate activity at lower concentrations since their half-life in plasma is in hours. Thus, if an opiate molecule is present in a particular bodily environment at a lower concentration than its specific receptor K,,, it may still initiate an action due to its prolonged presence.
bypass surgery, remaining high (500 pmol/ml) 3-5 days post surgery. These patients (16) did not receive morphine during the course of their hospital experience. Interestingly, this same phenomenon occurred in invertebrates subjected to trauma (Stefano et al., 1995b). In examining the immunocyte responsive state we found that during trauma or surgery they were hyperactivated (Stefano and Bihinger, 1993; Stefano et al., 1995b). However, during the morphine increase on days 2 and 3, the cells became refractory to stimulation (Stefano el al., 1995b, unpublished). From day 4 to 5, following a period of hypersensitivity (possible a rebound effect), their responsiveness to stimulatory molecules returned to ‘normal’ (Stefano et&., 1995b, unpublished). In in vitro incubations of immunocytes with morphine the cells first become ‘deactivated’, then supersensitive followed by normal activity in the continued presence of morphine. Thus, an important factor in ~a activation may be the presence of morphine and its metabolite over an extended period of time, since
morphine levels, as far as we can ascertain today, do not reach the high levels required to activate this receptor. In this discussion of the possible activities of endogenous opiates we are guided by information collected in numerous studies on the pharmacological responses to the administration of exogenous morphine, discussed in the preceding section. One feature that appears to be characteristic of exogenous opiate compounds, exemplified by their known antinociceptive effects as well as substance abuse potential, is that they lower cellular thresholds for activation under a variety of physiological and pathological conditions. It is, therefore, reasonable to speculate that endogenous opiates may act in a similar capacity, wherever a situation calls for it, especially since the levels rise after a delay. The presence of opiate alkaloids in the circulation and of special opiate receptors on immunocytes, demonstrated in vertebrates as well as
Opiate tolerance invertebrates, enables these compounds to participate directly in autoimmunoregulatory activities. These direct activities may be judged to be largely of an inhibitory nature. In addition, circulating opiates may contribute to the sum total of directives mediated by signal molecules reaching the central nervous system from various sources, including the immune system. There seems to be general agreement on the fact that serious or life-threatening challenges create a state of alertness, brought about by the instant release of stimulatory messenger molecules (catecholamines, opioid peptides and others, e.g., cytokines), during which all available energies are directed toward meeting the emergency (see Fricchione and Stefano, 1994). What should be considered to be equally important is that these stimulatory signals need to be stopped as soon as they are no longer required, so as to prepare the organism for a subsequent challenge. Endogenous morphine would seem to be an appropriate candidate to meet this demand. For example, during major surgical interventions, the immunosuppressive immunocyte effect of adrenocorticotropin (Smith et al., 1992) and interleukin-10 (see Stefano et al., 1996) produced by immunocytes may not suffice to lower the hyperstimulation of granulocytes and macrophages, which is attributable to their trauma induced acute phase response release of excitatory biological response modifiers (i.e., cytokines) (Stefano etal., 1993, 1995a). It seems reasonable to suggest that, under these circumstances, morphine may be called upon to downregulate the process so as to restore the normal level of activity. The validity of this proposal is supported by tests carried out with blood samples taken from patients during cardiopulmonary bypass operations. In preparations exposed to morphine, signs of cellular activity were significantly less pronounced than in untreated samples (Stefano and Bilfinger, 1993; Stefano et al., 1995a, unpublished data). The results of another experiment, indicative of the downregulating capacity of endogenous morphine under conditions of stress, deserves attention, since it was first reported in an invertebrate. The design was to follow the
269
sequence of activities generated by subjecting the Mytilus to stressful interventions (electrical shock, prevention of valve movements; see Stefano etal., 1991). The immediate response was activation of the animal’s defense system, as judged by conformational changes (becoming ameboid and mobile) in its immunocytes, and interpreted as being brought about by the release of endogenous opioid peptides and additional molecules. Twenty-four hours later, when the state of alertness had subsided, the return of the immunoactive hemocytes to a more ‘inactive’ conformation was found to concur with a temporary but significant rise in the morphinelike content of nervous tissue and hemolymph (Stefano et al., 1993, 1995a). The conformation of the immunocytes observed at this point in time resembled that of unstressed animals exposed to exogenous morphine. Comparable studies in vertebrates showed a marked increase in morphine concentration in the spinal cord of rats suffering from chronic pain elicited by experimental arthritis (Donnerer et al., 1987). The same was true for animals subjected to prolonged food deprivation in which the morphine content of brains was higher than in controls (Lee and Spector, 1991). The insights gained from the study of the various traumatic situations cited suggest that in the hierarchy of available downregulating mechanisms, morphine operates as a strong back-up system. The observation that this secondary system comes into effect after a latency period during which endogenous opiate levels rise, is in line with the fact that the p3 opiate receptor has an affinity constant in the range of lo-* M. Evidently, the availability of a network of effective immunostimulatory agents has great survival value for vertebrates and invertebrates alike. It is, therefore, understandable that the development of its elements, including those operating in immunoregulation, can be traced far back on the evolutionary scale. The need for the operation of more than one immunosuppressive mechanism is as obvious as that for the availability of effective immunostimulatory agents. It mollusc
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is our belief that it is one of morphine’s important tasks to meet this vital demand. In addition, endogenous morphine and related opiates may be presumed to engage in a variety of other activities, for example some operating within the confines of the nervous system. However, most of these questions are still unanswered, and efforts for their solution will undoubtedly gain momentum in years to come (see Fricchione et al., 1994). Tolerance
With the above discussion in mind we will now consider the phenomenon of tolerance with regard to endogenous opiate compounds. With continued exposure to the same dose of an opiate various physiological systems exhibit a decrease in their response. This phenomenon is referred to as tolerance. As with the study of opiates our historic interest in this phenomenon is focused around antinociception. However, given that morphine is a naturally occurring signal substance we must ask another question. Since tolerance occurs, what would its ‘normal’ function be? We have examined the need for ‘turn-on’ molecules as well as the need for ‘turn-off’ molecules in various systems. Now we must consider what turns off or downregulates this ‘off system, i.e., morphine. We believe the answer, in part, to this question is in the phenomenon of tolerance. Once the downregulatory process has been initiated and the level of these molecules (i.e., morphine) rise to competitively overcome the influences of the initial stimulatory molecules (Stefano et al., 1993), the inhibitory molecule’s level, i.e. morphine (as also noted by the Kd for morphine on the pa receptor, approx. 15-50 nM), would be hard to overcome, as would be its continued presence (Fig. 2). Thus, stimulatory signal molecules could not activate the system during this downregulation unless their concentrations rose well above those levels in the initial event. Indeed, at this moment, the activities generated by an additional phase of excitatory molecule release would upset the now instituted downregulation, given its competitive and reversible
signal molecule nature. This is especially true of et al., 1993, 1995b). morphine (Stefano Therefore, the only mechanism that can effectively diminish the inhibitory actions over a relatively short period of time would be one in which the very same effector cell system, progressively becomes desensitized to the presence of morphine (Fig. 2). In this way, the downregulating influence would be terminated regardless of the concentration of morphine present during a single event. Thus, the effector cells become tolerant. Interestingly, tolerance would only set in once downregulation had been achieved. Thus, the ‘brake’ would be administered on a ‘need’ basis along with the dynamic capability of progressive adjustments in this process if required. Moreover, tolerance, once achieved, also would allow for further stimulation of the system if it was required, since morphine’s presence would not be ‘sensed’ (Fig. 2). In summary, tolerance represents a dynamic mechanism that can be used to augment various regulatory processes whether they be involved in excitation or inhibition. Simply stated, tolerance is a process that allows for the termination of morphine’s action while it is still present in the environment. It is still present in the environment because it is a general, yet specific, mechanism operating only at concentrations above basal levels, concentrations that would terminate excitatory processes. However, tolerance ensures that immuno-inhibition, for example, does not last to the point whereby the organism would be compromised due to a lack of a functioning system over an extended period of time. Thus, desensitization sets in and allows the various processes to be stimulated and operational once more. Clearly, the timely ‘rebound’ of the immune and nervous processes involved with opiate actions provides for a successful mechanism to ensure survival as has been demonstrated (see Stefano et al., 1995b). Since tolerance has been demonstrated in invertebrates and vertebrates the use of this strategy becomes even more evident (see Stefano et al., 1980).
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tolerance
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Fig. 2. Physiological significance of tolerance. (A) Stimulatory immunocyte signal molecule, i.e., TNF, activates a cell to become mobile and ameboid. (B) On first and prolonged exposure to morphine (see Fig. 1) it downregulates the cell (round and immobile). (C) However, with time, even though morphine is still present it becomes tolerant to morphine’s presence and ready for activation once more (D). This activation can occur in the presence of morphine. Indeed, to further downregulate this morphine tolerant cell more opiate is required. (E-F) In the immediate downregulated state the cell can still be activated by higher levels of a stimulatory agent (G). Thus, the morphine induced downregulation is terminated by tolerance or it can be overcome by higher doses of the stimulatory agent. This process becomes quite efficient in regulating the alert state of immunocytes and possibly neurons as well. Dependence/addiction
In drug dependence one can see tolerance develop with a decreased response to the actions of a drug. Thus, in order to achieve the same effect one has to take a larger dose. The phenomenon of dependence occurs upon the withdrawal of the drug, which produces behavioral signs opposite of those desired. Furthermore, associated psychological dependence involves compulsive drug-seeking behavior. In all animals there are normal behaviors which can be said to be based on a compulsive ‘foundation’. It would be interesting to speculate that addiction emerges from tolerance if the presence and level of endogenous opiates do not or cannot return to their previous or prestimulation low levels. In this scenario, once tolerance occurs, the opiate molecules, i.e., morphine, remain relatively elevated. In this event, in order
to further lower the threshold of activation, e.g., immunosuppression, one would require even greater increases in morphine levels due to tolerance. It would be to an organism’s benefit to have such an immune mechanism, since this would allow for a more dynamic response to antigenic challenge, for example. Thus, an animal would have several levels of immune responsiveness to call upon. Indeed, we surmise, the lack or dysfunction of such a system may lead to various pathologies, i.e., hyperactive cell disorders autoimmune disorders). However, given the above, the dynamics of passing through many levels of tolerance may, in time, adversely affect the organism. Clearly, operating at different morphine levels may force a ‘system/process’ to continually re-establish its threshold for excitability due to tolerance setting
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in. This endogenous trauma may manifest itself, as noted above, in the display of opposite ‘behaviors’ i.e., excitability of nerve or immune cells. Indeed, one may associate immune stimulation with morphine actions due to the fact the process has become tolerant and thus supersensitive when morphine availability ceases (Stefano et al., 1995b). In a concurrent neuronal morphine receptor supersensitive state, if the stimuli/state became cognitive, one would actively seek out opiates. In this regard, addictive or compulsive behavior may be viewed as a process indicative of morphine insufficiency. A subpopulation of individuals seeking continuously higher levels of opiates (i.e., those addicted) may be operating from a morphine insufficiency status. Therefore, addictive behavior may be viewed as a phenomenon that reflects opiate insufficiency in an organism with attendant alterations in neuro- and immunoregulation.
Normal Morphine
Indeed, what may initiate the cascade of tolerance in a potential addict, an individual who may have an endogenous morphine biosynthesis and/or excessive degradation problem, is the very first experience with such a substance (Fig. 3). For in this experience a subpopulation of individuals may for the first time ‘feel’ normal. This might reflect the ‘neurological susceptibility’ to drug addiction referred to by Dole and Nyswander in their 1967 metabolic theory of addiction (Dole and Nyswander, 1967). Normal is defined as being in the sense a diabetic feels being given insulin (Goldstein, 1991). However, in the morphine insufficiency scenario, as morphine is administered, tolerance develops because it is the normal way endogenous morphine’s presence is downregulated. Thus, a person enters into this cycle of dependence as they continually seek to regain and maintain the ‘normal’ feeling; often overshooting into a state of intoxication. This
Excitation is momentary or prolonged as a function
-
Morphine _ Activity goes back to F “normal”
I
of strength
Morphine I 3.
Excitation Morphine
insufficiency
-
abnormally prolonged
+ Activity goes back to “normal”
l-l Tolerance then addiction Fig. 3.
1 -
lnorder to feel “normal” one to take
must continue morphine
The pathological significance of the morphine insufficiency hypothesis. Under normal circumstances cellular excitatory states can be downregulated by morphine, representing one of many downregulating signal molecules. In the morphine insufficiency scenario an excitatory state may be prolonged. In an individual having such a deficit in this downregulating mechanism, being exposed to an opiate alkaloid may temporarily ‘fix’ the problem. In this regard, the individual may seek this agent once the ‘fix’ is gone due to tolerance or dissipation ofthe active concentration. This individual seeking to be ‘normal’ again must continually take the agent, initiating a cycle of tolerance and dependence.
Opiate tolerance hypothesis offers the explanation for the phenomenon that some individuals treated for drug addiction are not ‘cured’. In this specific case the reason is simple, the endogenous opiate insufficiency has not been addressed, and by providing the substance one compromises the existing regulatory processes, i.e., tolerance. Speculation psychiatry
on brain
motive
circuitry
and
In 199 1, Goldstein updated the Dole and Nyswander metabolic theory of addiction and focused on opioid neuropeptide stimulation of preceptors on inhibitory GABA neurons which modulate ventral tegmental DA neurons (Goldstein, 1991). The resultofsuchFstimulationisdisinhibitionofthese DA circuits, leading to DA stimulation at the nucleus accumbens and the subjective reward response. Recently we speculated that endogenous morphine may also be essential to a complete picture of brain motive circuitry (Fricchione etal., 1994). A hypothetical DA-morphine pathway was reviewed and a role in addictive disorders was offered (Fricchione et al., 1994). This hypothesis gathers support from recent genetic findings regarding the Al allele of the D2-DA receptor (DRD2) gene in alcoholics and cocaine abusers, which suggest that certain substance abusers seek intoxication as compensation for an inherently underactive DA-mediated reward system (Hyman and Nestler, 1996). Given this speculative hypothesis of morphine insufficiency it is of interest to consider it in regard to a potential influence in the motive circuits. This does not detract from the presence and actions of other opioid signalling processes in these areas but may serve to add another dimension. Hyman and Nestler have recently reviewed how opiates are hypothesized to act on the brain motive circuitry (Compton et al., 1996). While there are opioid receptors directly on limbic neurons and inhibitory kappa dynorphin receptors on DA neurons in the ventral tegmental area (VTA), opiate p receptors responsive to morphine lie on VTA GABA interneurons and perform an inhibitory function on GABA inhibition, thereby indirectly
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disinhibiting VTA DA neurons as Goldstein (199 1) suggests. In this scenario we surmise endogenous morphine deficiency syndrome would tend to enhance GABAergic inhibition of VTA DA activity. Cell bodies in the mesolimbicmesocortical DA brain reward system are located in the VTA. Terminal fields in this key neural circuitry include the nucleus accumbens (NAS), the anterior cingulate cortex (ACC) and the dorsolateral pre-frontal cortex (DLPFC). The interaction among DA, GABA, glutamate, acetylcholine and serotonin among other neurotransmitter systems, is known to be central to our understanding of brain reward system functioning in this network (see Fricchione et al., 1994). In a morphine insufficiency scenario the p receptor on VTA GABA intemeurons may become hypersensitive, setting the stage for an amplified effect when morphine is reintroduced. The result would be morphine-induced inhibition of GABA which, in turn, would lead to DA disinhibition. Of course, along with chronic morphine availability tolerance would develop. Thus, one way to counteract a putative morphine deficiency syndrome would be to administer exogenous morphine acutely. This may lead to long-term difficulties, however, repeated acute use may eventually cause receptor level alterations and withdrawal symptoms which are only counteracted by chronic use of morphine, cocaine and alcohol, leading to addiction. What may be occurring when these substances are used chronically is an attempt to compensate through tyrosine hydroxylase and aldehyde effects in the DA-endogenous alkaloid biosynthetic pathway toward endogenous codeine and morphine production. This could be thought of as a ‘priming the pump’ strategy doomed to failure if the cause for the endogenous morphine deficiency is a defect in the DA-endogenous alkaloid biosynthetic pathway which has, in effect, left the well dry. While endogenous opiates will be expected to normally operate under conditions of amplification and tolerance, an endogenous morphine deficiency would create a naturally occurring addiction state, i.e., a constellation of
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physiological and psychological phenomena reflective of starved supersensitive amplified opiate receptors. After making the association between the mesocortical-mesohmbic DA system and opiates (both exogenous and endogenous) active at local k receptors, it follows that there may be other psychiatric implications in addition to those regarding addictive illness. The mesolimbicmesocortical DA system in addition to the mesostriatal DA system appears to play an important role in schizophrenia (Fig. 4; Lipska and Weinberger, 1993). Some schizophrenics are now thought to exhibit hypofrontality with ventricular enlargement and atrophy on neuroimaging and frontal deficits on neuropsychological testing (Weinberger etal., 1992). Data from animal studies show that PFC damage reduces the threshold for activation of the mesolimbic DA system under stress conditions. This leads to a state of hyperresponsiveness to stress. Hypofrontality may be associated with reduction of DA turnover in the PFC, which can be brought about by hippocampal lesions (Fricchione et al., 1994). Hippocampal disarray has been reported in schizophrenia (Lipska and Weinberger, 1993). Morphine immunoreactivity has been found in the hippocampus (Bianchi et al., 1993) as well as in mesocorticol-
imbic and mesostriatal regions and thus may contribute to the modulation of DA function in schizophrenic disorder. Changes in amplification and tolerance effects of sufficient magnitude involving endogenous opiates could thereby contribute to positive and negative symptom presentations in schizophrenia. The clinical observation of higher pain tolerance in some schizophrenics There
may also be explained
is also
evidence
on this basis.
of mesolimbic
DA
in depression (Kapur and Mann, 1992). Lower cerebrospinal fluid (CSF) homovanillic acid (HVA) levels in depressed patients, increased incidence of depression in Parkinson’s disease and with DA-depleting agents, and the enhancement of DA transmission with electroconvulsive therapy all point to a role for DA dysfunction in depressive disorders. As we mentioned, drugs such as cocaine, morphine, as well as psychostimulants and nicotine that result in self-administration due to reinforcement properties are associated with increased VTA DA neuron firing, activation of mesohmbic pathways, and an increase in NAS extracellular DA concentration (DiChiara etal., 1990). The implications of these relationships for our understanding of depression are still unclear, but we can again speculate that a naturally occurring opiate may be playing system
involvement
Low Morphine -
Increase GABA Inhibition of VTA DA
Add Morphine -
Tolerance *
VTA-GABA hypersensitived
DA Disinhibition
///I Schizophrenia Fig. 4. Morphine deficiency would be expected to permit GABA intemeuron psychiatric sequela, in addition to a predisposition to addiction, would be plementation or endogenous excess would be expected to decrease GABA would be an increase in VTA DA output. Psychiatric consequences may schizophrenia, autism and delirium.
Autism
Delirium
inhibition of VTA DA. A potential dysthymic mood. Morphine supinhibition of VTA DA. The result be reflected in such disorders as
Opiate tolerance a role through mechanisms of amplification and tolerance. We can also speculate that endogenous morphine is involved in the pathogenesis of autism, especially given the fact that narcotic antagonists have shown some efficacy in reducing autistic symptomatology, as have the DA antagonist neuroleptics (Herman et al., 1986; Campbell 1987). Opiate-DA interaction may also be important in the pathophysiology of delirium (Fig. 4).
Conclusion
The brain reward system interaction between F-mediated opiate processes and the mesolimbic-mesocortical DAsystem is central to all mammalian behavior, including that of humans. The balancing of human behavior in terms of pleasure and pain occurs in this motive circuitry, thus imparting an assessment of survival value. In part this assessment will be based on the relative state-amplified vs tolerant-of opiate receptor system processes. Those individuals with addictions, schizophrenia, depression, autism and other psychiatric disorders, find themselves dealing with core survival issues against the background of their biological endowments, particularly in the form of their individual mesolimbic-mesocortical systems. In this context, naturally occurring morphine may play a crucial role in modulating mesolimbic-mesocortical DA sensitization in response to stress. In this regard opiate tolerance must be examined, especially given its critical actions in regulating immune function. This is especially true, given the interaction of microglia and neurons (see Fricchione et al., 1996).
Acknowledgments
We are grateful to Dr. Mary Weitzman for valuable bibliographic assistance. We acknowledge the following grant support: NIMH-NIDA COR 17 138, NIDA 09010, the Research Foundation of SUNY (CBS) and NIH NS 22344-08 (BS).
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