Regional selectivity of neuroleptic drugs: An argument for site specificity

Brain Research Bulletin, Vol. 11, pp. 215-218, 1983. 0 Ankho International

Inc. Printed in the U.S.A.

Regional Selectivity of Neuroleptic Drugs: An Argument For Site Specificity RICHARD

L. BORISON

AND

BRUCE

I. DIAMOND

Department of Psychiatry, Downtown V.A. Medical Center Medical College of Georgia, Augusta, GA 30910

BORISON, R. L. AND B. I. DIAMOND. Regional selectivity of neuroleptic drugs: An argumenf fur site specijkity. BRAIN RES BULL 11(2)215-218, 1983.-Differences among neuroleptic drugs in their abilities to produce extrapyramidal side-effects have alternatively been ascribed to their inherent anticholinergic effect or their preferential action with particular brain areas. Animal behavioral studies in rats, with the intralimbic or intrastriatal injection of dopamine demonstrate that atypical neuroleptics, i.e., clozapine, thioridazine, show a greater relative preference for blocking dopamine’s limbic actions. Studies using the rat model of tardive dyskinesia (neuroleptic-induced behavioral hypersensitivity), suggest that the atypical neuroleptics are again unique. Receptor ligand studies in rat brain and human brain, using JH-spiroperidol, show that haloperidol produces a preferential blockade of striatal receptor sites, whereas clozapine and thioridazine are most active at limbic sites. Biochemical data in animals and clinical data in man are reviewed to further support the concept of site specificity. Atypical neuroleptics

Dopamine receptors

Extrapyramidal system

Limbic system

by a stainless injection needle attached to polyethylene tubing which was connected to a microliter syringe. Ail soiutions used for intracerebral injection were dissolved in artificial cerebrospinal fluid. Animals were allowed to move freely while the drug was infused at a rate of 0.5 pi/minute. The injector was allowed to stay in place one additional minute at the termination of drug injection to allow for diffusion of fluid from the injector tip; total volume of injection was 1 ~1. Prior to receiving intracerebral injections, animals had received in some cases daily, five week, intraperitoneal injections of either phenyiethylamine (PEA) (50 mg/kg) or d-amphetamine (3.75 mg/kg), which produced intense stereotyped behavior. Animals were placed into wire mesh cages and observed by two independent blind observers using a rating scale for locomotor activity and stereotypy [5]. If atypical neuroieptic agents truly have a specific action on iimbic rather than striatal dopamine receptors, they should then be unable to produce the striatal dopamine receptor denervation hypersensitivity that occurs secondary to receptor blockade. To test this hypothesis, while male Sprague-Dawiey rats were used in an animal paradigm for tardive dyskinesia. In this model, animals received daily administration of an antipsychotic drug during a three week period, after which the animals then received daily saline injections over five days. To assess dopamine receptor sensitization, the animals were challenged with the direct-acting dopamine agonist, apomorphine, in a dose which fails to produce a behavioral response in control animals. In biochemical studies, receptor i&and studies were performed from discrete brain areas of rat and human brain. In the case of rat brain, areas were dissected immediately after sacrificing animals. Human brain material was collected within six hours of death and, with the aid of a

CONVENTIONAL antipsychotic drugs possess as their most common side-effect, the ability to produce extrapyramidai side-effects (EPSE). Certain of these agents produce a low incidence of EPSE and have been termed atypical neuroieptics. Although controversial, it has previously been explained that the atypical neuroleptics, i.e., ciozapine and thioridazine, produce few EPSE due to their inherent anticholine@ activities. As we shall review, this widely held explanation only partially explains the clinical experience with ciozapine and thioridazine, and fails to explain why tardive dyskinesia is associated less, rather than more, with their usage. An alternative hypothesis in explaining the action of atyp icai neuroieptics is that these drugs more selectively interact with iimbic rather than striatal dopamine receptors. This hypothesis more consistently explains the clinical actions of these drugs. In this paper we will review animal and clinical data to test the hypothesis of whether atypical neuroleptics exert their actions via selective blockade of brain dopamine receptors. METHOD

Subjects were albino male Sprague-Dawiey rats. Animals received stereotactically placed bilateral stainless steel cannuiae into the nucleus accumbens and caudate putamen nucleus [ 151. The cannuiae were 0.8 mm in diameter, and fitted with an indwelling obturator. To check cannuia placement after experimentation, animals were sacrificed by cervical dislocation, the brains fixed in 10% formalin, and then cut into 50 Frn slices on a cryostat and imbedded in paraffin. Behavioral scores of animals were used only when it was verified that the cannuia was in the nucleus accumbens or the caudate-putamen nucleus. Upon testing animals, the cannuia obturator was replaced

215

neuropathologist. areas from the limbic system (olfactory tubercle. nucleus accumbens) and extrapyramidal system (caudate nucleus, putamen) were dissected. Tissues were homogenized and prepared in a sodium-potassiumphosphate buffer. Receptor ligand binding was carried out using ‘IH-spiroperidol [4] and specific binding was ascertained using (+)-butaclamol. RESUL’i’S

We found that the intralimbic injection of cloazpine (30 pg) significantly decreased PEA stereotypy to 60% of controf values. while non-significantly antagonizing d-amphetamine behavior. At a dose of 10 pg. clozapine significantly blocked PEA behavior while failing to affect d-amphetamine stereotypy. Thioridazine at 30 gg was without actions. yet at 60 /*g it produced a 71% blockade of PEA stereotypy, while failing to affect d-amphetamine-induced behavior. By comparison. chlorpromazine (30 ~8) produced a modest, but statistically significant. 42% antagonism to phenylethy~am~ne stereotypy, while antagonizing by only 15% the stereotypy produced by d-amphetamine. In contrast. at lower doses ( 10 ,ug) it failed to have significant effects on stereotypies. The intralimbic injection of haloperidol (2 pg) significantly antagonized PEA stereotypy by 43%’ while not affecting d-amphetamine activity, and at 5 pg produced a greater antagonism to PEA behavior while not having a statistically significant effect upon d-amphetamine-induced stereotypy. Furthermore, fluphenazine hydrochloride (2 fig and 5 pg) failed to alter the stereotyped behavior elicited by d-amphetamine administration. but reduced PEA stercotyped behavior sign~~cantly. In other experiments we injected dopamine ( IOOgg) bilaterally into either the nucleus accumbens or caudate-putamen nucleus and quantified locomotor and stereotypic activity. We found that when animals were pretreated with clozapine (10. 20, or 30 mgikg, IP) there was a significantly greater inhibition of dopamine-induced behavior via the accumbens as compared to the caudate-putamen. This same trend was observed when animals were pretreated with thioridazine (IS. 30. or 60 mg/kg) or haloperidol (0.25 or 0.5 mgikg). In the animal model for tardive dyskinesia, we found that thioridazine produced a short-lived sensitization of striatal dopamine receptors whereas halop~~dol and ~uphenazin~ produced more marked and longer-lived receptor sensittzation. Other animal studies, in particular primate studies [IO. 131 have also demonstrated dyskinetogenic (dopamine receptor hypersensitivity induction) potential. and have also found agents such as haloperidol to be much more potent in inducing this phenomenon. In receptor binding studies in rat brain (Table 1) we found that the IC,,, for clozapine and thioridazine was much lower in hmbic rather than striatal regions. When comparing limbic versus striatal IC;,,. the two atypical neuroleptics show a clear specificity of action for interacting with Iimbic receptor sites. In human brain, it was found that the high afftnity dopamine receptor site showed a I&=0.8 nM. Using striatal tissue, haloperidol and butaclamol were the most potent in displacing the tritiated ligand. whereas clozapine and thioridazine were several orders of magnitude less potent. In contrast. all four neuroleptics were within the same order of magnitude in their abilities to block limbic dopamine receptors. Interestingly. prochlorperazine (Compazine), an antiemetic drug that produces a high incidence of EPSE, has a

TABLE ,‘H-SPIROPERIDOI.

BINDING

I

“IO KAI BRAIh

H~ll~)perido~

IC,,, Limbic IC,,, Striatal

16 ?0 IO00

?‘!I) 17 X0

Ic’,,, inkl,

lc‘,(/ Limhlc-_ -.. IC-,, Strialul

Olfactory Tubercle Nucleus Accumbens Caudate-Putamen

Clo2apnIe ‘l‘hiortdazine

KEC;IOKS

517 so 59

/I 01. o.‘G O.JO.0 ?J iY ll. 1 :h

values less thnn I indicate pret’et-etttial hindmg to llmhic wuctures

high affinity for extrapyramtdal dopamine receptors but a low affinity for limbic dopamme receptors, which may account for the very weak antipsychotic properties of thib drug. In com~ring receptor blockade m the fimbic rather than extrapyramidal system. clozapine and thio~dazine clearly showed a site specificity for blocking limbic dopamine receptors. This site specificity may therefore account for the reiative paucity of acute and chronic EPSE observed in patients treated with thioridazine or clozapine. and may explain why these two drugs produce less tardive dyskinesia than is seen with the other neuroleptic drugs, which are more lihely to block striatal rather than limbic dopamine receptors. DISC’USSION

It has been generally accepted that certain neuroleptic agents produce a low incidence of EPSE due to their concomitant anticholinergic acttons. This concept has been abetted by muscarinic receptor binding studies in rat brain [20], which demonstrate that agents such as thioridazine are several hundred to several thousand-fold more potent as an anticholinergic agent than other drugs such as fluphenazine or haloperidol. It must now be critically questioned as to whether these data bear any relevance to i/r ~VVOanimal studies or clinical trials with atypical neuroleptic agents. It is clear from animal studies that the addition of an antichohnergic antiparkinsonian to a conventional neuroleptic does not mimic the actions of the atypical drugs f 141. thus contradicting 01 t’itvo work. Similarly in well controlled clinical studies it is seen that an atypical phenothiazine such as thioridazine produces markedly less EPSE. but has a clinical profile of anticholinergic side-effects not substantially different from fluphenazine (Table 2). It may then be concluded that irr \-irrc> studies of anticholinergic actions may be of heuristic value, but clinical and behavioral tests in man and animals do not differentiate the various neuroleptics according to their anticholinergic activities. There is a clear consensus that drugs such as clozapine ot thio~dazine produce a more pronounced effect on the biochemistry of limbic rather than striatal dopaminergic neurons, as measured by increased dopamine turnover Cl, 3. 6, 7. 8. 16, 191. Our behavioral studies, in which neuroleptics are tested in antagonizing the direct actions of dopamine in the nucleus accumbens or caudate-putamen nucleus, clearly show that agents such as clozapine or thioridazine have much weaker relative actions in the blockade of striatal as opposed to limbic receptors. Since it is beheved that the blockage of striatal dopamine receptors produces tardive dyskinesia in man and behavioral hypersensitivity in

217

NEUROLEPTICS TABLE 2 ANTICHLORINERGIC

ANTIPARKINSONIAN

MEDICATIONS, NEUROLEPTICS AND SIDE-EFFECTS

Percent with Extrapyramidal Side-Effects (EPSE)

Percent with Anticholinergic Side-Effects

Lasky et (11. 1121

Galbrecht and Klett ]19]

Lasky et ul. [12]

Galbrecht and Klett [9]

23.5 15.7 39.8

16.2 8.0 35.7

35.0 31.0 24.0

21.0 19.0 15.0

Chlorpromazine Thioridazine Fluphenazine

Numbers between parenthesis indicate reference cited.

animals, this paradigm has been used as a model for tardive dyskinesia [18]. In these tests we demonstrated that thioridazine, as opposed to haloperidol or fluphenazine, was less active in this model, providing yet another line of reasoning as to the low activity of the atypical neuroleptics in the striatum. A final argument as to the direct actions of neuroleptics in discrete brain areas can be examined in receptor binding studies. Concordant with both the behavioral and biochemical data cited above, we found that atypical neuroleptics preferentially bind to limbic rather than striatal binding sites in rat brain. Our finding corroborates earlier findings [I l] of a site specificity of receptor binding in rodent brain. These results are further extended in our studies of human brain. Again in concordance with clinical observations, we found that clozapine and thioridazine are relatively weak in blocking striatal dopamine recognition sites, whereas haloperidol and butaclamol were several orders of magnitude more active. This may explain why haloperidol effectively masks the movements

dive dyskinesia,

associated

with striatal disease

Huntington’s

states (tar-

chorea, Gilles de la Tourette

syndrome) at low oral milligram doses which per se are subthreshold for producing antipsychotic actions; thioridazine is ineffective in masking striatal movement disorders. Conversely, all four neuroleptics showed equal potencies in blocking limbic dopamine receptors. The limbic system is the neuroanatomical area best describing psychotic behavior [17] and our results are again consistent with the clinical reality that all available neuroleptic agents produce equivalent antipsychotic actions. In our brief review we have demonstrated that widely held beliefs concerning EPSE and anticholinergic actions are not supported by clinical data. Furthermore, we have presented behavioral and biochemical data demonstrating a site specific action of certain atypical neuroleptics, which correlates with and may explain the clinical actions of these drugs. The clinical relevance of this work may best be understood in its relation to a major side-effect of neuroleptic drugs, namely tardive dyskinesia. Drugs with a limbic site specificity of action, i.e., thioridazine and clozapine, should clinically produce less tardive dyskinesia, and this prediction is supported by clinical data [2].

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by

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