- Email: [email protected]
Integrative peptides
Brcrin Rrscwrch
Bulk~ir~.
Vol. 14. pp. 525-528,
1985. i- Ankho
International
Inc. Printed
0361-9230185 $3.00 + .OO
in the U.S.A.
Integrative Peptides BARTLEY Dcpnrtmcnt
of Psychology,
G. HOEBEL
Princeton
Unilsersity,
Princeton,
NJ 08544
HOEBEL, B. G. IIIIP~YN~~Wpcptkks. BRAIN RES BULL 14(6) 525-528, 1985.-Angiotensin and CCK are presented as examples of neuropeptides that might function in the body and the brain as hormones and neurotransmitters to integrate physiological functions with psychological functions. It is suggested that they be called, integrative peptides, to distinguish them from the more limited functions, or less well documented functions, of the other regulatory peptides. Integrative
peptides
Hormones
Neurotransmitters
Angiotensin
Cholecystokinin
well studied. If it is verified that angiotensin causes thirst, thoughts, and plans to get water, and if LHRH causes libido, thoughts and plans for reproduction, and CCK causes satiety, thoughts and plans to avoid eating, then these peptides will indeed be involved in cognition as well as physiology and behavior. Their overall function might then be to integrate all three. Many peptides have been discovered in places that have little to do with homeostasis, such as the retina [ 1I]. Therefore the concept of integrative peptides, and the heuristic value of the idea, may be seriously flawed if it turns out that these peptides also serve functions irrelevant to the physiology-behavior-cognition axis. There may be no logical way of distinguishing the relevant from the irrelevant. For example, suppose angiotensin can increase blood pressure, water reabsorption, drinking, subjective thirst, and maybe even water-related associations and memory of where to find water. Then angiotensin could be classified as an integrating peptide, and this would provide a useful organizing principle defining a system for fluid homeostasis in all its aspects. When angiotensin is then discovered in the retina, the olfactory bulb or the spinal cord, it would be incumbent on the theorist either to find a logical link to fluid balance, or to question the heuristic value of the theory. Peptides can act as either messengers or effecters. Genetic DNA produces RNA which assembles amino acids into linear arrays which are then cleaved to liberate peptides which spring into three-dimensional conformations. Some serve as mobile transmitters; others have a tail section which is built into a membrane surface so they can act as receptors. Messengers and receptors act in a cascade leading to cellular responses [21,22]. If the medium is the message for integration then we must consider both the mobile integrating peptides and their receptors. Perhaps receptor characteristics as determined by labelled antibody research will clarify a distinction between angiotensin-the-integrator and angiotensin-the-irrelevant. Peptides may act as long range hormones, short range neurohormones or synaptic neurotransmitters. Some peptidergic neurons are interneurons; others are long and diffusely ramifying as if to modulate many brain systems. Some of the diffuse type contain both monoamines and one or more peptides. Neurosecretory cells which synthesize amines and peptides have a common embryonic origin [38].
REGULATORY peptides are those which regulate cell function (./ R~gul Pept). Current evidence suggests that a subset of these act in the body and the brain to integrate physiological and psychological functions. Let us call these “integrating peptides” or “integrative peptides” [16]. Olds [36] suggested that peptides serve a drive function, but there is a possibility that the peptides in question integrate physiology with learning and memory as well as motivation; therefore the Sherringtonian concept of CNS integration is preferable to the Hullian concept of drive. A prime example of an integrative peptide is angiotensin which integrates water balance, blood pressure, drinking behavior and thirst. Another is LHRH which integrates reproductive readiness and sexuality [3, 22, 351. They may also integrate brain with body. The concept of peptides as regulators grew out of research on hypothalamic releasing factors which regulate pituitary function, but now we know they can do much more [ 18,291. Some of these same peptides were found to function as neurotransmitters released by axonal branches reaching into the surrounding hypothalamus and beyond. Other peptides such as angiotensin and CCK were first discovered in the periphery where they control organ functions. Then they were found to have parallel functions when injected into the ventricles [10,33]. Next their action was observed pharmacologically in localized brain sites. Most recently they have turned up in brain neurons projecting to these pharmacologically active sites where the peptides may act as neurotransmitters. In this brief review, we will not make a distinction between neurotransmitters and neuromodulators; peptides may act in either fashion. It could be argued that there is no need to rename any of the regulatory peptides. There is the danger of creating a new name for old intervening variables which introduce explanatory fictions. However, the term, regulatory, is already an expression of a teleological view (to regulate), and a gestaltist view (regulation as something greater than a control system for parts). It is a small, philosophical step to advocate “integrative peptides” as those which integrate physiological, behavioral and mental gestalts which are greater than the sum of regulated control systems. Homeostasis is a state of balance or dynamic equilibrium between safe boundary conditions. Three types of regulatory processes maintain this balance. They are physiological, behavioral and cognitive. Cognitive processes have not been
525
526
HOEBHL.
Most of their cell bodies are found in the hypothalamus, pituitary, pineal gland, also in cell clusters in the midbrain and hindbrain and scattered diffusely in glands and organs of the periphery. Peptides that co-exist with the monoamines are candidates to serve as co-transmitters [17]. To claim that a peptide is a neurotransmitter or co-transmitter requires the traditional tenets: evidence of synthesis in the cell, release by appropriate stimuli, substitution by exogenous agonists, termination of synaptic actions, and ph~ma~olo~cal antagonism [223. The evidence suggests that norepineph~ne is a hormone in the blood, and a neurohormone which satisfies the requirements for a neurotransmitter in the brain [1.5,23]. Angiotensin is also approaching this status. In sum, embryonic neurosecretory cells can remain as neurosecretory cells, or develop into endocrine cells or neurons. All three types that derive from the common precursor use the same chemical messages whether they are located in the brain, pituitary, or peripheral glands. Thus brain neurons may contain “pituitary hormones” or “adrenal hormones” or “hypothalmic releasing factors.” Their mechanism of action on cell membranes may be similar in some cases [50]. This is a great opportunity to understand the brain by comparing the well known peripheral ho~onal functions of these chemicals with their possible brain function. This strategy can be applied to catecholamines (e.g., dopamine, [47]), steroids (e.g., aldosterone, [12,48] or corticosterone, [23]) and peptides, for example angiotensin and cholecystokinin. ANGIOTENSI~
IS AN INTEGRATIVE
PEPTIDE
Angiotensin is a fluid control peptide which is present in both the blood and the brain without much crossover under normal blood pressure conditions [28,37]. It may also be secreted into the ventricles in some animals, but in rats is rapidly degraded. This raises a question as to which source of the peptide affects behavior [39,42]. Is it blood angiotensin, neuronal angiotensin, CSF ~giotensin or all three. If it acts by any two of these three routes, then this is one way that integration could occur. Blood-borne angiotensin would gain access to the brain through the fenestrated capillaries of the circumventricular organs such as the subfornical organ. If angiotensin gained access to the CSF, it could enter through the ventricular wall to reach sensitive brain sites in the anterior ventral region of the third ventricle [2]. Neural angiotensin has been discovered in nerves connecting these two sites and other brain regions [24]! Neurotransmitter status for angiotensin is suggested by reports of precursors and enzymes for its synthesis [ 13,221, by immunolo~~al revelation of angiotensin cell bodies in the subfo~c~ organ [24], and by bbckade of an~otensin drinking by the receptor antagonist, saralasin [lo]. Tract tracing studies by Miselis [30,31] provide additional evidence that the subfornical organ is involved in mediating many of the brain’s water balance responses. The projections from the subfornical organ are to appropriate parts of the preoptic area and hy~th~~us for drinking behavior, vasopressin release and a pressor response. Further work [25,26] substantiates these findings and indicates that some of the neural components in the projection from the SF0 contain angiotensin. It is conceivable, therefore that the subfomical organ (SFO) leaks blood-borne angiotensin onto angiotensinreceptive cells which informs them indirectly that hypovolemia has been detected. SF0 neurons presumably carry the information (be it negative or positive feedback) to the tissue near the anterior third ventricular region and su-
009 I
I
I
::
0 18
2
Proglumide
I
1
0 75
1 50
Dose lug, unilateral)
FIG. 1. Dose-response curve showing food intake per 3-hour test following unibterat injection of pro~umide, or pro~ium~de plus sulfated-CCK-8, in rats (n=8); open circle on the ordinate is food intake after saline injection. *Significantly greater than intake with saline. fsignificantly less than proglumide alone.
praoptic f41] or medial preoptic (MPO) region where action potentials would release an~otensin from nerve terminals. Thus angiotensin, the neurotr~smitter, may carry information about angiotensin, the hormone. One effect of angiotensin in the MPO is possibly to affect changes in osmotic circuits for vasopressin release to reabsorb water [39] and perhaps improve memory [20]. Thus angiotensin may have a coordinating function for these two fluid control peptides which together raise blood pressure, reabsorb water and cause drinking. Epstein [lo] reviews the role of angiotensin as the hormone which defends blood volume in a number of ways. If we consider together all of angiotensin’s functions in the regulation of physiological control systems for blood pressure, plus coordination of multiple systems for fluid balance in~ludin8 thirst and sodium appetite, we have a clear case of an “integrative peptide.” It integrates in the sense that it affects different, but homeostatically-related, functions; it integrates in the sense that it affects related functions both peripherally and centrally, and it may also integrate in the strongest sense that it could act at some of the same brain receptor sites when circulating systemically as a hormone or released locally as a neurotr~smitter. CCK MAY BE AN INTEGRATIVE
PEPTIDE
CCK is a physiologically active peptide for digestion and a psychologically active peptide for satiety. Thus it may be an integrating peptide. After extensive debate it appears that CCK can be either quieting and satiating, or distressive and aversive depending on the tests one uses [8,44]. Learned aversion and place preference tests point to aversive proper-
INTEGRATIVE
521
PEPTIDES
ties 1491. Sham feeding
tests, behavioral observation and sequence leading to sleepiness [43]. Satiety can be sensory specific [32,40]. This suggests that there are different kinds, stages and degrees of satiety which may, in the extreme, shade into sensory specific aversion. CCK given peripherally depends on vagal sensory nerves to conduct the satiety information to the brain (see Smith’s paper, this conference). The sensory vagus projects to the nucleus tractis solitarius (NTS) where its input could represent the peripheral CCK-induced signal, or the input could increase brain sensitivity to CCK circulating as a hormone. Blood-borne CCK could presumably pass the blood-brain barrier in a circumventricular organ such as the area postrema which overlies part of the NTS. Circulating CCK could act by itself, but more likely in combination with other synergistic neural or chemical inputs. Taste neurons project to the NTS, so it is interesting that peripheral CCK inhibited intake of sweet water but not plain water [51]. Thus CCK inputs and gustatory inputs may interact. Instead of cutting the vagus to prevent the effects of CCK, another approach is to block the CCK receptors. When sheep were given intragastric proglumide, a CCK antagonist, they ate more food than the control group [l]. This confirmed that somewhere there are CCK receptors that normally curb food intake. In addition to the gut, they could also be in the brain. CCK has some of the characteristics of a neurotransmitter. It is localized within neurons where it is synthesized; it is released by depolarization, and it has high affinity receptors ]331. The search for central CCK satiety mechanisms is proceeding much like the angiotensin story. Lesions, immunohistochemistry and pharmacology are all contributing to this fast developing field. NTS lesions prevented the CCK-induced decrease in exploration and locomotion which normally follows peripheral injections [5]. The NTS projects via the parabrachial region to many parts of the brain [45], including some that contain immunoreactive CCK receptors. One of these is the paraventricular nucleus (PVN) alongside the third ventricle in the hypothalamus. Lesions in the PVN also blocked peripheral CCK-induced hypoactivity [ 191. CCK itself has been discovered within the vagus, the NTS, the parabrachial nucleus, in a path to the ventromedial hypothalamus and in the PVN [34, 53, 541. Thus CCK might conceivably be a satiety transmitter in the brain as well as a peripheral satiety stimulus. EEG recordings
suggest
a satiety
Central injection of CCK does not always suppress feeding even when injected into the NTS [4); however CCK suppressed norepineph~ne-induced feeding in the PVN [27]. Conversely to find out if brain CCK receptors are necessary for satiety, Baile’s group injected CCK-antibody intraventricularly in sheep. This induced feeding [7]. We found that the CCK antagonist, proglumide (0.18-l ..5 pglunitalerally), injected into the PVN causes rats to eat. The effect was dose related and counteracted by simultaneously applied CCK [9]. When the injectors were lowered an extra 1.5 mm the effect of proglumide was significantly reduced, although the animals still ate more than with saline. This reduction in effect makes it unlikely that reflux or diffusion into the ventricles explains the phenomenon. Proglumide is a weak blocker [ 141and has never been shown to bind to CNS CCK receptors; therefore we are testing a new CCK antagonist, benzotrypt, which has greater potency and a different structure. Benzotript injected in the PVN unilaterally induced feeding comparable to that of proglumide, but the threshold dose was ten times lower. In summary, it appears that CCK contributes to satiety both by an action in the gut where it is a peptide hormone, and in the PVN where it could be a peptide neurotransmitter. Comprehensive evidence for neurotransmitter status is a long way off; however it is suggestive that neurons containing CCK have been traced from vagal sensory nuclei as far forward as the hypothalamus. When these results can be confirmed and extended, CCK may qualify as an “integrative peptide.” In any case the heuristic value of the theory is evident. This theory also led us to test the intestinal peptide, neurotensin, in the PVN. Neurotensin suppressed norepinephrine-induced feeding and deprivation-induced feeding [46]. The problem with this theoretical approach as stated at the outset, is the difficulty in incorporating other peptidergic effects with no apparent relation to the integration theme 134,521. In the case of CCK the anomalous behavioral results include CCK-induced potentiation of dopamine-mediated locomotion [6] and CCK-induced reinforcement (Hoebel et crl. unpublished). These effects occurred when CCK was injected in the nucleus accumbens where it is a cotransmitter with dopamine. These effects of CCK may be a different story completely, or conceivably CCK from the accumbens flows through the ventricular system to influence the hypothalamus. We do not know. ACKNOWLEDGEMENTS
This work was supported by PHS grant MH-35740 and a grant from The Campbell Soup Company. We also thank E. R. Squibb and Sons Inc. for CCK-8 and A. H. Robins Co. for proglumide.
REFERENCES 1. Baile. C. A. and M. A. Della-Fera. Peptidergic control of food intake in food-producing animals. Fed Pror 43: 2898-2902, 1984. 2. Brody. M. J. and A. K. Johnson. Role of the anteroventrai third ventricle region in fluid and electrolyte balance, arterial pressure regulation, and hypertension. In: Frontiers in Naurtwndoc’rinnlogy. vol 6, edited by L. Martini and W. F. Ganong. New York: Raven Press, 1980, pp. 24%292. 3. Chappel, S. C. Neuroendocrine regulation of luteinizing hormone and follicle stimulating hormone: A review. Lifb Sci 36: 97-103, 1985. 4. Crawley, J. N. Neurochemical investigation of the afferent pathway from the vagus nerve to the nucleus tractus solitarius in syndrome” mediating the “satiety induced by systemic cholecystokinin. Peptidrs 6: SuppI I, 133-137, 1985.
5 Crawley, J. N. and J. S. Schwaber. Abolition of the behavioral effects of cholecystokinin following bilateral radiofrequency lesions of the parvocellular subdivision of the nucleus tractus solitarius. f&&r Rr.7 295: 289-299, 1984. 6 Crawley, J. N., J. A. Stivers, L. K. Blumstein and S. M. Paul. Cholecystokinin potentiates dopamine-mediated behaviors: Evidence for modulation specific to a site of co-existence. J Ncurosci, in press. 7. Della-Fera, M. A., C. A. Baile, B. S. Schneider and J. A. Grinker. Cholecystokinin antibody injected in cerebral ventricles stimulates feeding in sheep. Scirnce 212: 687-89, 1981.
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8. Deutsch, J. A. Controversies
HGEBEI., in food intake regulation. In: The
Neuruf Basis of Feeding and ReBvard, edited by B. G. Hoebel and
D. Novin. Brunswick, ME: Haer Institute. 1982. uo. 137-148. 9. Dorfman, D., P. Scott and B. G. Hoebel: Feeding inducedby the cholecystokinin antagonist, proglumide, injected in the paraventricular region of the hypothalamus. Sot Neurosci A bstr 10: 652, 1984. 10. Epstein, A. N. The physiology of thirst. In: The Physiological ~e~hanjs~~s of ~ot~~lufion, edited bv D. W. Pfaff. New York: Springer-Verlag, 1982, pp. 164-214. . 11 Eriksen, E. F. and L. I. Larsson. Neuropeptides in the retina: Evidence for differential topographical localization. Peptides 2: 153-157, 1981. 12, Fluharty, S. J. and A. N. Epstein. Sodium appetite elicited by intracerebroventricular infusion of angiotensin II in the rat: II. Synergistic interaction with systemic mineralocorticoids. Behni Neurosci 97: 746-758, 1983. 13. Ganten, D., G. Speck, P. Schelling and T. Unger. The brain renin-angiotensin system. In: Advances in Biochemical Psyehopharmacology, edited by J. B. Martin, S. Reichlin and K. L. Bick. New York: Raven Press, 1981, pp. 359-372. 14. Gardener, J. D. and R. T. Jensen. Cholecystokinin receptor antagonists. Am J Physiof 246: G471-G476, 1984. 15, Hoebel, B. G. Neurotransmitters in the control of feeding and its rewards: Mono~ines, opiates, and brain-gut peptides. In: Eating and Its Disorders, edited by A. J. Stunkard and E. Stellar. New York: Raven Press, 1984, pp. 15-38. 16. Hoebel, B. G. The neuropsychology of motivation. In: Stevens Handbook of Experimental Psychology, edited by R. C. Atkinson, R. J. Hermstein, G. Lindzey and R. D. Lute. New York: Wiley and Sons, in preparation. 17. Hokfelt, T.. J. M. Lundberg, L. Skirboi, 0. Johansson, M. Schultzberg and S. R. Vincent. Co-existence of classical transmitters and peptides in neurons. In: Cu-Trtrnsmission, edited bv A. C. Cuello. London: Macmillan, 1982. 18. Kastin, A. J., J. E. Zadina, W. A. Banks and M. V. Graf. Misleading concepts in the field of brain peptides. Peptides 5: Suppl 1, 249-253, 1984. 19. Kiss, J. Z. and J. Crawley. Cholecystokinin-induced “satiety” is blocked by paravent~cular nucIeus lesions. In press. 20. Koob, G. and F. E. Bloom. Memory, learning, and adaptive behaviors. In: Brain Peptides, edited by D. T. Krieger, M. J. Brownstein and J. B. Martin. New York: Wiley and Sons, 1983, pp. 369-388. 21. Krieger, D. T. Brain peptides: What, where, why? Science 222: 975-985, 1983. 22. Krieger, D. T., M. J. Brownstein and J. B. Martin teds.). Brain Peptides. New York: Wiley and Sons, 1983. 23. Leibowitz, S. F. Neurochemical systems of the hypothalamus: Control of feeding and drinking behavior and water-electrolyte excretion. In: Handbook of the Hypothalamus. ~013, Part A, Behavioral Studies of the Hypothalamus, edited by P. J. MorPane and J. Pankseon. . _. New York: Marcel Dekker. 1980, .PP. 299-437.
24. Lind, R. W.. L. W. Swanson and D. Ganten. Angiotensin II immunoreactivity in neural afferents and efferents of the subfornical organ of the rat. Brain Res 321: 209-215, 1984. 25. Lind, R. W., G. W. Van Hoesen and A. K. Johnson. An HRP study of the connections of the subfornical organ of the rat. J Camp Neural 210: 265-277, 1982. 26. Lind, R. W., R. L. Thunhorst and A. K. Johnson. The subfomical organ and the integration of multiple factors in thirst. Physirti Behav 32: 69-74, 1984. 27. McCaleb, M. L. and R. D. Myers. Cholecystokinin acts on the system” involved in feeding. hypothalamic “noradrenergic Peptides 1: 47-49, 1980. 28. Meisenberg, G. and W. H. Simmons. Peptides and the bloodbrain barrier. Life Sci 32: 2611-2623, 1983. 29. Me&es, J. Interactions between hypothalamic neurotransmitters and pituitary function. Fed Proc-39: 2888, 1980. 30. Miselis. R. R.. R. E. Shaoiro, P. J. Hand. Subfomic~ organ efferents to neural systems for control of body water. S&&e 205: 1022-1025, 1979.
3 I. Miselis, R. R. The efferent projections of the subfornical organ of the rat: A circumventricular organ within a neural circuitry subserving water balance. Brain Ra& 230: l-23. 1981. 32. Mook, D. G., J. A. Brane. L. R. Kushner and J. A. Whitt. Glucose solution intake in the rat: The specificity of postingestive satiety. Appetife 4: l-9, 1983. 33. Morley, J. E. The ascent of cholecystokinin (CCK)-from gut to brain. Life Sci 30: 479-493, 1982. 34. Morley, J. E., A. S. Levine, B. A. Gosnell and C. J. Billington. Neuropeptides and appetite: Cont~bution of neuropharmacological modeling. Fed Proc 43: 2903-2907, 1984. 35. Nemeroff, C. B. and A. J. Prange, Jr. Peptides and psychoneuroendocrinology. Arch Gen Psychiatc 35: 999-1010, 1978. 36. Olds, J. Drives amI Reinjinx.ementA: Behuvioral Studies oj Hypathafamic Functions. New York: Raven, 1977.
37. Pardridge, W. M. Neuropeptides Anna Rev Physjo~ 45: 73-82,
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38. Pearse, A. G. E. and T. T. Takor. Embryology
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41. 42.
43 44
45. 46.
47.
48.
of the diffuse neuroendocrine system and its relationship to the common peptides. Fed Proc 38: 2288-2293, 1979. Reid, I. A. Actions of angiotensin II on the brain: Mechanisms and physiologic role. Am J Physiol 246: F533-543, 1984. Rolls, B. J. How variety and palatability can stimulate appetite. Mutr JMl 5: 7186, 1979. Sgro, S., A. V. Ferguson and L. P. Renaud. Subfornical organsupraoptic nucleus connections: An electrophysiologic study in the rat. Brain Res 303: 7-13, 1984. Simpson, J. B. The circumventricular organs and the central actions of angiotensin. Neuroendocrino/ogy 32: 241256, 1981, Smith, G. P. and 3. Gibbs. Gut peptides and postprandial satiety. FeJ Proc 43: 2889-2892. 1984. Smith, G. P., J. Gibbs and P. J. Kulkosky. Relationships between brain-gut peptides and neurons in the control of food intake. In: The Neural Basis oj’Feeding und Rrward, edited by B. G. Hoebel and D. Novin. Brunswick, ME: Haer Institute, 1982, pp. 149165. Smith, 0. A. and J. L. DeVito. Central neural integration for the control of autonomic responses associated with emotion. Anrru Ret, Ncurosc~i 7: 43-65, 1984. Stanley, B. G., B. G. Hoebel and S. F. Leibowitz. Neurotensin: Effects of hypothalamic and intravenous injections on eating and drinking in rats. Peptides 4: 493-500, 1983. Stricker, E. M. Brain neurochemistry and the control of food intake. In: Hundbook oj’Behuviora/ Neurobiology, edited by E. Satinoff and P. Teitelbaum, New York: Plenum, 1983, pp. 329366. Stricker, E. M. Thirst and sodium appetite after colloid treatment in rats: Role of the renin-angiotensin-aidosterone system.
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49. Swerdlow. N. R.. D. van der Koov. G. F. Koob and J. R. Wenger. Cholecystokinin produces conditioned placeaversions, not place-preferences, in food-deprived rats: Evidence against involvement in satiety. Lift .%i 32: 2087-2093, 1983. 50. Szego, C. M. Mechanisms of hormone action: Parallels in receptor-mediated signal propagation for steroid and peptide effectors Life Sci 35: 2383-23%. 1984. 51. Waldbillin, R. J. and M. G’Callaghan. Hormones and hedonics cholecystokinin and taste: A possible behavioral mechanism of action. Phrsiol Behuv 25: 25-30, 1980. 52. Yalow, R.-S., J. Eng and E. Straus. The role of CCK-like peptides in appetite regulation. Adv ~etab~~~ Disord 10: 43-S-456, 1983.
53. Zaborszky, L. M., C. Beinfeld, M. Palkovits and L. Heimer. Brainstem projection to the hypothalamic ventromedial nucleus in the rat: A CCK-containing long ascending pathway. Bmin Res 303: 225-231, 1984. 54. Zarbin, M. S., J. K. Wamsley. R. B. Innis and M. J. Kuhar. Chofecystokinin receptors: Presence and axonal flow in the rat vagus nerve. Life Sci 29: 697-705. 1981.
Bulk~ir~.
Vol. 14. pp. 525-528,
1985. i- Ankho
International
Inc. Printed
0361-9230185 $3.00 + .OO
in the U.S.A.
Integrative Peptides BARTLEY Dcpnrtmcnt
of Psychology,
G. HOEBEL
Princeton
Unilsersity,
Princeton,
NJ 08544
HOEBEL, B. G. IIIIP~YN~~Wpcptkks. BRAIN RES BULL 14(6) 525-528, 1985.-Angiotensin and CCK are presented as examples of neuropeptides that might function in the body and the brain as hormones and neurotransmitters to integrate physiological functions with psychological functions. It is suggested that they be called, integrative peptides, to distinguish them from the more limited functions, or less well documented functions, of the other regulatory peptides. Integrative
peptides
Hormones
Neurotransmitters
Angiotensin
Cholecystokinin
well studied. If it is verified that angiotensin causes thirst, thoughts, and plans to get water, and if LHRH causes libido, thoughts and plans for reproduction, and CCK causes satiety, thoughts and plans to avoid eating, then these peptides will indeed be involved in cognition as well as physiology and behavior. Their overall function might then be to integrate all three. Many peptides have been discovered in places that have little to do with homeostasis, such as the retina [ 1I]. Therefore the concept of integrative peptides, and the heuristic value of the idea, may be seriously flawed if it turns out that these peptides also serve functions irrelevant to the physiology-behavior-cognition axis. There may be no logical way of distinguishing the relevant from the irrelevant. For example, suppose angiotensin can increase blood pressure, water reabsorption, drinking, subjective thirst, and maybe even water-related associations and memory of where to find water. Then angiotensin could be classified as an integrating peptide, and this would provide a useful organizing principle defining a system for fluid homeostasis in all its aspects. When angiotensin is then discovered in the retina, the olfactory bulb or the spinal cord, it would be incumbent on the theorist either to find a logical link to fluid balance, or to question the heuristic value of the theory. Peptides can act as either messengers or effecters. Genetic DNA produces RNA which assembles amino acids into linear arrays which are then cleaved to liberate peptides which spring into three-dimensional conformations. Some serve as mobile transmitters; others have a tail section which is built into a membrane surface so they can act as receptors. Messengers and receptors act in a cascade leading to cellular responses [21,22]. If the medium is the message for integration then we must consider both the mobile integrating peptides and their receptors. Perhaps receptor characteristics as determined by labelled antibody research will clarify a distinction between angiotensin-the-integrator and angiotensin-the-irrelevant. Peptides may act as long range hormones, short range neurohormones or synaptic neurotransmitters. Some peptidergic neurons are interneurons; others are long and diffusely ramifying as if to modulate many brain systems. Some of the diffuse type contain both monoamines and one or more peptides. Neurosecretory cells which synthesize amines and peptides have a common embryonic origin [38].
REGULATORY peptides are those which regulate cell function (./ R~gul Pept). Current evidence suggests that a subset of these act in the body and the brain to integrate physiological and psychological functions. Let us call these “integrating peptides” or “integrative peptides” [16]. Olds [36] suggested that peptides serve a drive function, but there is a possibility that the peptides in question integrate physiology with learning and memory as well as motivation; therefore the Sherringtonian concept of CNS integration is preferable to the Hullian concept of drive. A prime example of an integrative peptide is angiotensin which integrates water balance, blood pressure, drinking behavior and thirst. Another is LHRH which integrates reproductive readiness and sexuality [3, 22, 351. They may also integrate brain with body. The concept of peptides as regulators grew out of research on hypothalamic releasing factors which regulate pituitary function, but now we know they can do much more [ 18,291. Some of these same peptides were found to function as neurotransmitters released by axonal branches reaching into the surrounding hypothalamus and beyond. Other peptides such as angiotensin and CCK were first discovered in the periphery where they control organ functions. Then they were found to have parallel functions when injected into the ventricles [10,33]. Next their action was observed pharmacologically in localized brain sites. Most recently they have turned up in brain neurons projecting to these pharmacologically active sites where the peptides may act as neurotransmitters. In this brief review, we will not make a distinction between neurotransmitters and neuromodulators; peptides may act in either fashion. It could be argued that there is no need to rename any of the regulatory peptides. There is the danger of creating a new name for old intervening variables which introduce explanatory fictions. However, the term, regulatory, is already an expression of a teleological view (to regulate), and a gestaltist view (regulation as something greater than a control system for parts). It is a small, philosophical step to advocate “integrative peptides” as those which integrate physiological, behavioral and mental gestalts which are greater than the sum of regulated control systems. Homeostasis is a state of balance or dynamic equilibrium between safe boundary conditions. Three types of regulatory processes maintain this balance. They are physiological, behavioral and cognitive. Cognitive processes have not been
525
526
HOEBHL.
Most of their cell bodies are found in the hypothalamus, pituitary, pineal gland, also in cell clusters in the midbrain and hindbrain and scattered diffusely in glands and organs of the periphery. Peptides that co-exist with the monoamines are candidates to serve as co-transmitters [17]. To claim that a peptide is a neurotransmitter or co-transmitter requires the traditional tenets: evidence of synthesis in the cell, release by appropriate stimuli, substitution by exogenous agonists, termination of synaptic actions, and ph~ma~olo~cal antagonism [223. The evidence suggests that norepineph~ne is a hormone in the blood, and a neurohormone which satisfies the requirements for a neurotransmitter in the brain [1.5,23]. Angiotensin is also approaching this status. In sum, embryonic neurosecretory cells can remain as neurosecretory cells, or develop into endocrine cells or neurons. All three types that derive from the common precursor use the same chemical messages whether they are located in the brain, pituitary, or peripheral glands. Thus brain neurons may contain “pituitary hormones” or “adrenal hormones” or “hypothalmic releasing factors.” Their mechanism of action on cell membranes may be similar in some cases [50]. This is a great opportunity to understand the brain by comparing the well known peripheral ho~onal functions of these chemicals with their possible brain function. This strategy can be applied to catecholamines (e.g., dopamine, [47]), steroids (e.g., aldosterone, [12,48] or corticosterone, [23]) and peptides, for example angiotensin and cholecystokinin. ANGIOTENSI~
IS AN INTEGRATIVE
PEPTIDE
Angiotensin is a fluid control peptide which is present in both the blood and the brain without much crossover under normal blood pressure conditions [28,37]. It may also be secreted into the ventricles in some animals, but in rats is rapidly degraded. This raises a question as to which source of the peptide affects behavior [39,42]. Is it blood angiotensin, neuronal angiotensin, CSF ~giotensin or all three. If it acts by any two of these three routes, then this is one way that integration could occur. Blood-borne angiotensin would gain access to the brain through the fenestrated capillaries of the circumventricular organs such as the subfornical organ. If angiotensin gained access to the CSF, it could enter through the ventricular wall to reach sensitive brain sites in the anterior ventral region of the third ventricle [2]. Neural angiotensin has been discovered in nerves connecting these two sites and other brain regions [24]! Neurotransmitter status for angiotensin is suggested by reports of precursors and enzymes for its synthesis [ 13,221, by immunolo~~al revelation of angiotensin cell bodies in the subfo~c~ organ [24], and by bbckade of an~otensin drinking by the receptor antagonist, saralasin [lo]. Tract tracing studies by Miselis [30,31] provide additional evidence that the subfornical organ is involved in mediating many of the brain’s water balance responses. The projections from the subfornical organ are to appropriate parts of the preoptic area and hy~th~~us for drinking behavior, vasopressin release and a pressor response. Further work [25,26] substantiates these findings and indicates that some of the neural components in the projection from the SF0 contain angiotensin. It is conceivable, therefore that the subfomical organ (SFO) leaks blood-borne angiotensin onto angiotensinreceptive cells which informs them indirectly that hypovolemia has been detected. SF0 neurons presumably carry the information (be it negative or positive feedback) to the tissue near the anterior third ventricular region and su-
009 I
I
I
::
0 18
2
Proglumide
I
1
0 75
1 50
Dose lug, unilateral)
FIG. 1. Dose-response curve showing food intake per 3-hour test following unibterat injection of pro~umide, or pro~ium~de plus sulfated-CCK-8, in rats (n=8); open circle on the ordinate is food intake after saline injection. *Significantly greater than intake with saline. fsignificantly less than proglumide alone.
praoptic f41] or medial preoptic (MPO) region where action potentials would release an~otensin from nerve terminals. Thus angiotensin, the neurotr~smitter, may carry information about angiotensin, the hormone. One effect of angiotensin in the MPO is possibly to affect changes in osmotic circuits for vasopressin release to reabsorb water [39] and perhaps improve memory [20]. Thus angiotensin may have a coordinating function for these two fluid control peptides which together raise blood pressure, reabsorb water and cause drinking. Epstein [lo] reviews the role of angiotensin as the hormone which defends blood volume in a number of ways. If we consider together all of angiotensin’s functions in the regulation of physiological control systems for blood pressure, plus coordination of multiple systems for fluid balance in~ludin8 thirst and sodium appetite, we have a clear case of an “integrative peptide.” It integrates in the sense that it affects different, but homeostatically-related, functions; it integrates in the sense that it affects related functions both peripherally and centrally, and it may also integrate in the strongest sense that it could act at some of the same brain receptor sites when circulating systemically as a hormone or released locally as a neurotr~smitter. CCK MAY BE AN INTEGRATIVE
PEPTIDE
CCK is a physiologically active peptide for digestion and a psychologically active peptide for satiety. Thus it may be an integrating peptide. After extensive debate it appears that CCK can be either quieting and satiating, or distressive and aversive depending on the tests one uses [8,44]. Learned aversion and place preference tests point to aversive proper-
INTEGRATIVE
521
PEPTIDES
ties 1491. Sham feeding
tests, behavioral observation and sequence leading to sleepiness [43]. Satiety can be sensory specific [32,40]. This suggests that there are different kinds, stages and degrees of satiety which may, in the extreme, shade into sensory specific aversion. CCK given peripherally depends on vagal sensory nerves to conduct the satiety information to the brain (see Smith’s paper, this conference). The sensory vagus projects to the nucleus tractis solitarius (NTS) where its input could represent the peripheral CCK-induced signal, or the input could increase brain sensitivity to CCK circulating as a hormone. Blood-borne CCK could presumably pass the blood-brain barrier in a circumventricular organ such as the area postrema which overlies part of the NTS. Circulating CCK could act by itself, but more likely in combination with other synergistic neural or chemical inputs. Taste neurons project to the NTS, so it is interesting that peripheral CCK inhibited intake of sweet water but not plain water [51]. Thus CCK inputs and gustatory inputs may interact. Instead of cutting the vagus to prevent the effects of CCK, another approach is to block the CCK receptors. When sheep were given intragastric proglumide, a CCK antagonist, they ate more food than the control group [l]. This confirmed that somewhere there are CCK receptors that normally curb food intake. In addition to the gut, they could also be in the brain. CCK has some of the characteristics of a neurotransmitter. It is localized within neurons where it is synthesized; it is released by depolarization, and it has high affinity receptors ]331. The search for central CCK satiety mechanisms is proceeding much like the angiotensin story. Lesions, immunohistochemistry and pharmacology are all contributing to this fast developing field. NTS lesions prevented the CCK-induced decrease in exploration and locomotion which normally follows peripheral injections [5]. The NTS projects via the parabrachial region to many parts of the brain [45], including some that contain immunoreactive CCK receptors. One of these is the paraventricular nucleus (PVN) alongside the third ventricle in the hypothalamus. Lesions in the PVN also blocked peripheral CCK-induced hypoactivity [ 191. CCK itself has been discovered within the vagus, the NTS, the parabrachial nucleus, in a path to the ventromedial hypothalamus and in the PVN [34, 53, 541. Thus CCK might conceivably be a satiety transmitter in the brain as well as a peripheral satiety stimulus. EEG recordings
suggest
a satiety
Central injection of CCK does not always suppress feeding even when injected into the NTS [4); however CCK suppressed norepineph~ne-induced feeding in the PVN [27]. Conversely to find out if brain CCK receptors are necessary for satiety, Baile’s group injected CCK-antibody intraventricularly in sheep. This induced feeding [7]. We found that the CCK antagonist, proglumide (0.18-l ..5 pglunitalerally), injected into the PVN causes rats to eat. The effect was dose related and counteracted by simultaneously applied CCK [9]. When the injectors were lowered an extra 1.5 mm the effect of proglumide was significantly reduced, although the animals still ate more than with saline. This reduction in effect makes it unlikely that reflux or diffusion into the ventricles explains the phenomenon. Proglumide is a weak blocker [ 141and has never been shown to bind to CNS CCK receptors; therefore we are testing a new CCK antagonist, benzotrypt, which has greater potency and a different structure. Benzotript injected in the PVN unilaterally induced feeding comparable to that of proglumide, but the threshold dose was ten times lower. In summary, it appears that CCK contributes to satiety both by an action in the gut where it is a peptide hormone, and in the PVN where it could be a peptide neurotransmitter. Comprehensive evidence for neurotransmitter status is a long way off; however it is suggestive that neurons containing CCK have been traced from vagal sensory nuclei as far forward as the hypothalamus. When these results can be confirmed and extended, CCK may qualify as an “integrative peptide.” In any case the heuristic value of the theory is evident. This theory also led us to test the intestinal peptide, neurotensin, in the PVN. Neurotensin suppressed norepinephrine-induced feeding and deprivation-induced feeding [46]. The problem with this theoretical approach as stated at the outset, is the difficulty in incorporating other peptidergic effects with no apparent relation to the integration theme 134,521. In the case of CCK the anomalous behavioral results include CCK-induced potentiation of dopamine-mediated locomotion [6] and CCK-induced reinforcement (Hoebel et crl. unpublished). These effects occurred when CCK was injected in the nucleus accumbens where it is a cotransmitter with dopamine. These effects of CCK may be a different story completely, or conceivably CCK from the accumbens flows through the ventricular system to influence the hypothalamus. We do not know. ACKNOWLEDGEMENTS
This work was supported by PHS grant MH-35740 and a grant from The Campbell Soup Company. We also thank E. R. Squibb and Sons Inc. for CCK-8 and A. H. Robins Co. for proglumide.
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