- Email: [email protected]
The mucosa as a target tissue for gut neuropeptides
TINS - January 1984
2 family rather than the individual that is transmitted the gene received from their affected parent. The safety of chorionic the unit to be tested? There is still a little time for these .biopsy as a prenatal procedure for obthorny problems to be discussed and taining fetal DNA should be clear (hopefully) ’ resolved. though there within the next year. and this would would seem little doubt that Gx or a allow a prenatal prediction to be made successor to it will be applied in predic- at 8-10 weeks gestationA. Finally, there is the possibility that tion. Perhaps, however, one should also stress the positive aspects. which are therapy which modifies the course of the disease could be forthcoming. If this likely to outweigh the controversial ones. Since the genetic risk for most were the case. then it will be of direct relatives. when age-adjusted. is well benefit for an individual to be detected under 50%. the majority of results will early. quite apart from any genetic conpredict normality and will relieve many sideration. The discovery of Gusella and relatives of a serious burden of risk. colleagues should be a spur to all those and Indeed. many people will not even working on the neurochemical require testing if their at-risk parent is pharmacological aspects of HC and predicted to be free from the gene. related diseases to produce new theraPrenatal prediction will still be an peutic approaches: in this way one can option even for those predicted as affec- offer hope to those who will in the near future have to live with the strong likelited. and may help others where a definihood, rather than the possibility. that tive prediction is not feasible for themselves by showing that they have not they will develop the disorder.
The mucosa as a target tissue for gut neuropeptides Several of the new peptidergic neurotransmitters found in enteric neurons can modulate ion and electrolyte transport across the intestinal mucosa. In some cases. receptors for these substances are localized directly on mucosal cells while in other cases their effects are mediated by the release of further substances from enteric neurons. Electrical field stimulation can be used to demonstrate the modulation of mucosal function by the enteric nervous system.
The enteric nervous system is a rich source of many of the newly discovered peptide neurotransmitters as well as more traditional substances such as acetylcholine. Using immunohistochemical techniques for the localization of both peptides and acetylcholine, the transmitters in both the myenteric plexus and submucous plexus are currently being mapped out le3. In some cases such as the guinea-pig ileum, the transmitters existing in a large percentage of enteric neurons have been identified’. In the case of some transmitters, the morphology of the neurons - the lengths of their projections and the portions of gastrointestinal tract they innervate have also been determined”‘. What are the targets for these various gut neurotransmitters and what physiological functions do they regulate? We have known for many years that both gut smooth muscle and enteric nerves are sensitive to many substances including those that act as gut neurotransmitters. It is only recently, however, that the sensitivity of the gut mucosa to the same agents has been established. What are the functions of the gut mucosa and how are
they modulated by enteric neurotransmitters? The mucosa is a transporting epithelium. Certain cells in it are responsible for the transport of ions and electrolytes between the lumen of the gut and the bloodstream. In addition. the mucosa also contains many endocrine cells that can secrete amines and peptides into the bloodstream and also possibly into the lumen. In an interesting series of studies, Hubel has demonstrated the influence of the enteric nervous system on mucosal function%13. If a piece of small intestine is subjected to electrical field stimulation, a large increase in the short circuit current (I,,) across the mucosa can be recorded. It can be shown that this increase in I,, is due to a stimulation of active anion secretion by the tissue. The stimulation of I,, can be blocked by tetrodotoxin, indicating the involvement of enteric nerves in the response. The addition of atropine has a small inhibitory effect in some species and no effect in others. Thus, the release of neurotransmitter(s) in addition to acetylcholine must be involved. There are several candidates for these neurotransmitters. The action
Reading list Gusell~. J. F.. Wexler. N. S.. Conneally. P. M.. Naylor. S. L.. Anderson. M. A., Tanzi. R. E.. Watkins. P. C., Ottina, K.. Wallace, M. R.. Sakaguchi. A. Y., Young, A. B.. Shoulson. I.. Bonilla. E. and Martin, J. B. (1983) Nr~ntre (London) 306. 234-238 A., Conncally, P. M.. Pcricak-Vance. M. Merritt. A. D.. Roos, R. P.. Vance, J. M.. Yu. P. L.. Norton. J. A.. Jr and Antel. J. P. (1979) in Huflriflgrof?‘s D&eme (Advances in
Nectrologv. Vol. 231 (Chase, T. N.. Wexler. N. S. and Barbeau. A.. cds), Raven Press. New York Kingston. H. M.. Thomas. N. S. T.. Pearson. P. L.. Sarfarazi. M. and Harper. P. S. (1983) 1. Med.
Gene/.
20. 2S-258
Elks. R. G.. Williamson. Coleman. D. V. and Horwell.
Eugl. 1. Med.
R.. D.
Niazi. (1983)
M.. N.
368. 143%1435 P. S. HARPER
Pro/cssor
ond Consultonr in Medical Genefics. Medical Genetics. Department of Medicine. Welsh Norional School of Medicine. Heoh Park. Cardiff CF4 4XN. UK. Secliou
of
of different substances on ion transport across the mucosa can be studied in vitro. Several gut neuropeptides can be shown to increase I,, in the ileum. These include substance P, neurotensin, bombesin, vasoactive intestinal polypeptide (VIP) and bradykinin”. Somatostatin and the enkephalins produce the opposite effects, that is a decrease in I,,14. Some of these substances, for example VIP and substance P, appear to exert direct effects on receptors localized on mucosal cells. Others such as neurotensin and the enkephalins appear to exert indirect effects mediated by nerves in the submucous plexus14. Both substance P and VIP are good candidates for mediating the effects of electrical field stimulation. Thus, cell bodies of the submucous plexus are frequently found to contain both these peptides. Moreover, both sets of neurons send processes into the mucosa. Another possibility is that electrical field stimulation reduces the release of a substance that normally decreases I,,; however, this possibility seems rather unlikely. Experiments to distinguish between the possible involvement of substance P and VIP have not yet been performed. However, the two peptides have different mechanisms of action. Thus, VIP stimulates adenylate cyclase’5 in mucosal cells whereas the action of substance P appears to involve calcium 16. The idea that gut neurotransmitters can modulate the functions of the mucosa has several important consequences. The actions of certain substances previously attributed to their effects on
TINS - Jonuq~
3
1954
gastrointestinal motility are currently being re-evaluated. For example. opiates are the most widely used anti-diarrhea1 agents. These effects were thought to be a result of the ability of opiates to slow gastrointestinal transit”. However recently opiates. particularly enkephalins. have been shown to have powerful antisecretory effects’J.‘“.‘“. The current view therefore is that such actions probably play an important role in mediating the anti-diarrhea1 actions of opiates. Indeed. the realization that the transporting properties of the mucosa are susceptible to modulation by the whole spectrum of gut neurotransmitters has ushered in a new era in our approach to the pharmacology of the gastrointestinal system. RI’3IARD J. CIILLER Prqbor. Deparme~r Physiological Scietice.~, Chiccrgo. IL M)(s27. USA.
o/ Plrarmacolo~ical Uni\~errify 01
atrd Chicago.
Reading list I Schultzbcrg.
M..
Dreyfus.
C.
F..
Gershon,
M. D.. Hokfelt. T.. Elde. R. P.. Nilsson. G.. Said. S. I. and Goldstein. M. (1980) Nnrroscience 5. 6,X!h74-l 2 Jcsscn. K. R.. SaTfrey. M. J.. van Noorden, S.. Bloom. S. R.. Polak. J. and Burnstock. G. ( I 980) Neuroscience 5. I 7 I 7-l 735 3 Furness. J. B. and Costa. M. (19X0) Akroscience 5. l-20 -I Furncss. J. B.. Costa. M. and Miller. R. J. ( 19X3) 5 Costa.
Newoscie~~ce M.. Fumcss.
I. J. and 4 I 1424
Cuello.
A.
8. 65~3-664 J. B..
Llewllyn-Smith.
L. (1981)
Neuroscience
6.
6 Costa. M.. Fumess. J. B.. Llewilvn-Smith. I. J.. Davies. B. and Oliver. J. (1980) Nellroscience 5. X41-852 7 Furness. J. B. and Costa. M. (1979) Neurusci.
x
Lnr. IS. 199-204 Furness. J. B.. Costa. ( 1981) Gmrroemerolo~~
M. and Walsh. X0. 1557-1561
9 Hubel. K. A. and Schirazi. emerologv X3. 6-08 IO Hubcl. K. A. (1978) /. IO39- I 047 K. A. and Callahan. II Hubel. Phyiol. 239. G K. A. I2 Hubcl. G2l I-G216 K. A. I3 Hubel. GjOI-G506 IJ Tapper. E. J. GJ57-GJhX
S. (1982) Ch.
J. H. Grarro-
fnvesr.
D. (1980)
62.
Am.
J.
I X-G22 (1981)
Am.
J.
fhysiol.
240.
( 1983)
Am.
J.
fhysiol.
244.
(19X3)
Am.
J.
Physiol.
244.
C. S.. Kimbcrg. D. V.. Shecrin. IS Schwartz. H. E.. Field. M. and Said. S. 1. (1974) /. Ch. lme~r. 54. 536-54 I6 Kachur. J. F.. Miller. R. J.. Field. M. and Rivicr. J. (1982) J. Pl~crrmrrcoL Exp Ther. 220. 456463 D. W. (1981) Gawuenrerology 80. I7 Powell. m64ux I8 Dobbins. J.. Racusen. L. and Binder. H. J. (19X0) J. C/h. lwesr. 66. I%28 I9 Kachur. J. F.. Miller. R. J. and Field. M. ( 1980) Proc. Narl Acad. Sci. USA 77. 2753-2756
Neurotensin
- a status report
Nelrrorensin, n hnsic tridecapeptide, wasfirst isolatedfrom bovine hypothalamus a little over ten years ago. Since then its presencehas been demonstruted in u wide variety of CNS locations us well ns in N variety of peripheral h.ssries.Neurotensin has been shown to be concentrated in vesicles,and to be releasedfrom regions rich in neurotensin-like immltnorenctivity by depolarizing stimuli. In this short urticle Michel Goedert clssesses the occlrm&uing evidencefor nerrrotemin’srole asa neurotransmitter, Neurotensin was discovered by Can-away and Leeman in extracts from bovine hypothalamus’.‘. and radioimmunoassay (RIA) studies rapidly showed neurotensin-like immunoreactivity to be present in synaptosomes and to be released by depolarizing stimuli: neurotensin receptors have been demonstrated by conventional ligand-binding techniques and by autoradiography’. Structure-activity studies have indicated that the carboxy-terminal end of the neurotensin molecule is required for receptor binding and biological activity. Xenopsin. a peptide isolated from the skin of Xenopu Inevi$. and two peptides isolated from chicken intestineJ.” share marked sequence homologies with the carboxy-terminal end of mammalian neurotensin (Table I). Whereas xenopsinlike immunoreactivity is undetectable in central and peripheral tissuesof several mammalian species”. the peptide (Ly?Asn”] neurotensin ((8-13)‘. is present in rat brain and intestine’. The fact that it competes only very poorly with tritiated neurotensin for the specific neurotensin-binding site in rat brains probably implies the existenceof multiple receptors for the neurotensin family. As is the case for most peptides whose amino acid sequence is known and for which antibodies are available. we have far more information on the distribution of neurotensin than on its possible functions.
Neurotensin-dopamine interactions There is considerable circumstantial evidence for an interaction between neurotensin and dopamine. and neurotensin has some actions in common with neuroleptic drugs”. It is found in nerve fibres in the olfactory tubercle, the nucleus accumbens and the striatum’, and the presence of neurotensin receptors hasbeen demonstrated on dopaminergic cells in the substantia nigra”‘. Electrophysiologically, neurotensin selectively excites dopaminergic neurons in the pars compacta of the substantia nigra’ ‘. The injection of neurotensin into the ventral tegmental area leads to
a haloperidol-blockable increase in locomotor activity and this effect is accompanied by an increase in dopamine tumover’. Recent in-vivo and in-vitro studies have also demonstrated that neurotensin can release dopamine from a variety of brain areas”.‘“.“. Much less is known about the influence of dopamine on the neurotensin system. The administration of neuroleptics leads to an apparent increase in neurotensin turnover and to a substantial increase in neurotensin content in dopamine-rich areas. However, this effect could be due to a direct influence on neurotensin synthesis and may be independent of dopamine-receptor blockade. The low levels of neurotensin and the incomplete knowledge of its exact anatomical rela- ’ tionship to the dopamine system make it difficult to study the effects of dopamine on neurotensin. The situation is complicated further by the finding that dopamine and neurotensin coexist in various parts of the rat CNS’s.‘“. The recent finding that in the cat the basal ganglia contain considerably higher amounts of neurotensin than in the rat” may mean that such problems are best approached by studies on the former species. Anti-nociception and hypothermia Following direct injection into the brain, neurotensin producesmarked antinociception and hypothermia. In general. such studies require very high concentrations of peptide for a maximal effect (up to 100 kg), as compared with the total brain content of neurotensin (10 ng for the rat). It therefore seems legitimate to question the physiological significance of these results. However, in the absence of selective antagonists. studies such as these represent the only way of finding out what neurotensin might be doing in the CNS. Neurotensin is one of the most potent anti-nociceptive substancesknown, being more potent than morphine on a molar basislx. Its action is insensitive to naloxone and probably takes place at the level of the brain stem. where neurotensin-like immunoreactivity and neurotensin receptors are present. The hypothermic effect of
2 family rather than the individual that is transmitted the gene received from their affected parent. The safety of chorionic the unit to be tested? There is still a little time for these .biopsy as a prenatal procedure for obthorny problems to be discussed and taining fetal DNA should be clear (hopefully) ’ resolved. though there within the next year. and this would would seem little doubt that Gx or a allow a prenatal prediction to be made successor to it will be applied in predic- at 8-10 weeks gestationA. Finally, there is the possibility that tion. Perhaps, however, one should also stress the positive aspects. which are therapy which modifies the course of the disease could be forthcoming. If this likely to outweigh the controversial ones. Since the genetic risk for most were the case. then it will be of direct relatives. when age-adjusted. is well benefit for an individual to be detected under 50%. the majority of results will early. quite apart from any genetic conpredict normality and will relieve many sideration. The discovery of Gusella and relatives of a serious burden of risk. colleagues should be a spur to all those and Indeed. many people will not even working on the neurochemical require testing if their at-risk parent is pharmacological aspects of HC and predicted to be free from the gene. related diseases to produce new theraPrenatal prediction will still be an peutic approaches: in this way one can option even for those predicted as affec- offer hope to those who will in the near future have to live with the strong likelited. and may help others where a definihood, rather than the possibility. that tive prediction is not feasible for themselves by showing that they have not they will develop the disorder.
The mucosa as a target tissue for gut neuropeptides Several of the new peptidergic neurotransmitters found in enteric neurons can modulate ion and electrolyte transport across the intestinal mucosa. In some cases. receptors for these substances are localized directly on mucosal cells while in other cases their effects are mediated by the release of further substances from enteric neurons. Electrical field stimulation can be used to demonstrate the modulation of mucosal function by the enteric nervous system.
The enteric nervous system is a rich source of many of the newly discovered peptide neurotransmitters as well as more traditional substances such as acetylcholine. Using immunohistochemical techniques for the localization of both peptides and acetylcholine, the transmitters in both the myenteric plexus and submucous plexus are currently being mapped out le3. In some cases such as the guinea-pig ileum, the transmitters existing in a large percentage of enteric neurons have been identified’. In the case of some transmitters, the morphology of the neurons - the lengths of their projections and the portions of gastrointestinal tract they innervate have also been determined”‘. What are the targets for these various gut neurotransmitters and what physiological functions do they regulate? We have known for many years that both gut smooth muscle and enteric nerves are sensitive to many substances including those that act as gut neurotransmitters. It is only recently, however, that the sensitivity of the gut mucosa to the same agents has been established. What are the functions of the gut mucosa and how are
they modulated by enteric neurotransmitters? The mucosa is a transporting epithelium. Certain cells in it are responsible for the transport of ions and electrolytes between the lumen of the gut and the bloodstream. In addition. the mucosa also contains many endocrine cells that can secrete amines and peptides into the bloodstream and also possibly into the lumen. In an interesting series of studies, Hubel has demonstrated the influence of the enteric nervous system on mucosal function%13. If a piece of small intestine is subjected to electrical field stimulation, a large increase in the short circuit current (I,,) across the mucosa can be recorded. It can be shown that this increase in I,, is due to a stimulation of active anion secretion by the tissue. The stimulation of I,, can be blocked by tetrodotoxin, indicating the involvement of enteric nerves in the response. The addition of atropine has a small inhibitory effect in some species and no effect in others. Thus, the release of neurotransmitter(s) in addition to acetylcholine must be involved. There are several candidates for these neurotransmitters. The action
Reading list Gusell~. J. F.. Wexler. N. S.. Conneally. P. M.. Naylor. S. L.. Anderson. M. A., Tanzi. R. E.. Watkins. P. C., Ottina, K.. Wallace, M. R.. Sakaguchi. A. Y., Young, A. B.. Shoulson. I.. Bonilla. E. and Martin, J. B. (1983) Nr~ntre (London) 306. 234-238 A., Conncally, P. M.. Pcricak-Vance. M. Merritt. A. D.. Roos, R. P.. Vance, J. M.. Yu. P. L.. Norton. J. A.. Jr and Antel. J. P. (1979) in Huflriflgrof?‘s D&eme (Advances in
Nectrologv. Vol. 231 (Chase, T. N.. Wexler. N. S. and Barbeau. A.. cds), Raven Press. New York Kingston. H. M.. Thomas. N. S. T.. Pearson. P. L.. Sarfarazi. M. and Harper. P. S. (1983) 1. Med.
Gene/.
20. 2S-258
Elks. R. G.. Williamson. Coleman. D. V. and Horwell.
Eugl. 1. Med.
R.. D.
Niazi. (1983)
M.. N.
368. 143%1435 P. S. HARPER
Pro/cssor
ond Consultonr in Medical Genefics. Medical Genetics. Department of Medicine. Welsh Norional School of Medicine. Heoh Park. Cardiff CF4 4XN. UK. Secliou
of
of different substances on ion transport across the mucosa can be studied in vitro. Several gut neuropeptides can be shown to increase I,, in the ileum. These include substance P, neurotensin, bombesin, vasoactive intestinal polypeptide (VIP) and bradykinin”. Somatostatin and the enkephalins produce the opposite effects, that is a decrease in I,,14. Some of these substances, for example VIP and substance P, appear to exert direct effects on receptors localized on mucosal cells. Others such as neurotensin and the enkephalins appear to exert indirect effects mediated by nerves in the submucous plexus14. Both substance P and VIP are good candidates for mediating the effects of electrical field stimulation. Thus, cell bodies of the submucous plexus are frequently found to contain both these peptides. Moreover, both sets of neurons send processes into the mucosa. Another possibility is that electrical field stimulation reduces the release of a substance that normally decreases I,,; however, this possibility seems rather unlikely. Experiments to distinguish between the possible involvement of substance P and VIP have not yet been performed. However, the two peptides have different mechanisms of action. Thus, VIP stimulates adenylate cyclase’5 in mucosal cells whereas the action of substance P appears to involve calcium 16. The idea that gut neurotransmitters can modulate the functions of the mucosa has several important consequences. The actions of certain substances previously attributed to their effects on
TINS - Jonuq~
3
1954
gastrointestinal motility are currently being re-evaluated. For example. opiates are the most widely used anti-diarrhea1 agents. These effects were thought to be a result of the ability of opiates to slow gastrointestinal transit”. However recently opiates. particularly enkephalins. have been shown to have powerful antisecretory effects’J.‘“.‘“. The current view therefore is that such actions probably play an important role in mediating the anti-diarrhea1 actions of opiates. Indeed. the realization that the transporting properties of the mucosa are susceptible to modulation by the whole spectrum of gut neurotransmitters has ushered in a new era in our approach to the pharmacology of the gastrointestinal system. RI’3IARD J. CIILLER Prqbor. Deparme~r Physiological Scietice.~, Chiccrgo. IL M)(s27. USA.
o/ Plrarmacolo~ical Uni\~errify 01
atrd Chicago.
Reading list I Schultzbcrg.
M..
Dreyfus.
C.
F..
Gershon,
M. D.. Hokfelt. T.. Elde. R. P.. Nilsson. G.. Said. S. I. and Goldstein. M. (1980) Nnrroscience 5. 6,X!h74-l 2 Jcsscn. K. R.. SaTfrey. M. J.. van Noorden, S.. Bloom. S. R.. Polak. J. and Burnstock. G. ( I 980) Neuroscience 5. I 7 I 7-l 735 3 Furness. J. B. and Costa. M. (19X0) Akroscience 5. l-20 -I Furncss. J. B.. Costa. M. and Miller. R. J. ( 19X3) 5 Costa.
Newoscie~~ce M.. Fumcss.
I. J. and 4 I 1424
Cuello.
A.
8. 65~3-664 J. B..
Llewllyn-Smith.
L. (1981)
Neuroscience
6.
6 Costa. M.. Fumess. J. B.. Llewilvn-Smith. I. J.. Davies. B. and Oliver. J. (1980) Nellroscience 5. X41-852 7 Furness. J. B. and Costa. M. (1979) Neurusci.
x
Lnr. IS. 199-204 Furness. J. B.. Costa. ( 1981) Gmrroemerolo~~
M. and Walsh. X0. 1557-1561
9 Hubel. K. A. and Schirazi. emerologv X3. 6-08 IO Hubcl. K. A. (1978) /. IO39- I 047 K. A. and Callahan. II Hubel. Phyiol. 239. G K. A. I2 Hubcl. G2l I-G216 K. A. I3 Hubel. GjOI-G506 IJ Tapper. E. J. GJ57-GJhX
S. (1982) Ch.
J. H. Grarro-
fnvesr.
D. (1980)
62.
Am.
J.
I X-G22 (1981)
Am.
J.
fhysiol.
240.
( 1983)
Am.
J.
fhysiol.
244.
(19X3)
Am.
J.
Physiol.
244.
C. S.. Kimbcrg. D. V.. Shecrin. IS Schwartz. H. E.. Field. M. and Said. S. 1. (1974) /. Ch. lme~r. 54. 536-54 I6 Kachur. J. F.. Miller. R. J.. Field. M. and Rivicr. J. (1982) J. Pl~crrmrrcoL Exp Ther. 220. 456463 D. W. (1981) Gawuenrerology 80. I7 Powell. m64ux I8 Dobbins. J.. Racusen. L. and Binder. H. J. (19X0) J. C/h. lwesr. 66. I%28 I9 Kachur. J. F.. Miller. R. J. and Field. M. ( 1980) Proc. Narl Acad. Sci. USA 77. 2753-2756
Neurotensin
- a status report
Nelrrorensin, n hnsic tridecapeptide, wasfirst isolatedfrom bovine hypothalamus a little over ten years ago. Since then its presencehas been demonstruted in u wide variety of CNS locations us well ns in N variety of peripheral h.ssries.Neurotensin has been shown to be concentrated in vesicles,and to be releasedfrom regions rich in neurotensin-like immltnorenctivity by depolarizing stimuli. In this short urticle Michel Goedert clssesses the occlrm&uing evidencefor nerrrotemin’srole asa neurotransmitter, Neurotensin was discovered by Can-away and Leeman in extracts from bovine hypothalamus’.‘. and radioimmunoassay (RIA) studies rapidly showed neurotensin-like immunoreactivity to be present in synaptosomes and to be released by depolarizing stimuli: neurotensin receptors have been demonstrated by conventional ligand-binding techniques and by autoradiography’. Structure-activity studies have indicated that the carboxy-terminal end of the neurotensin molecule is required for receptor binding and biological activity. Xenopsin. a peptide isolated from the skin of Xenopu Inevi$. and two peptides isolated from chicken intestineJ.” share marked sequence homologies with the carboxy-terminal end of mammalian neurotensin (Table I). Whereas xenopsinlike immunoreactivity is undetectable in central and peripheral tissuesof several mammalian species”. the peptide (Ly?Asn”] neurotensin ((8-13)‘. is present in rat brain and intestine’. The fact that it competes only very poorly with tritiated neurotensin for the specific neurotensin-binding site in rat brains probably implies the existenceof multiple receptors for the neurotensin family. As is the case for most peptides whose amino acid sequence is known and for which antibodies are available. we have far more information on the distribution of neurotensin than on its possible functions.
Neurotensin-dopamine interactions There is considerable circumstantial evidence for an interaction between neurotensin and dopamine. and neurotensin has some actions in common with neuroleptic drugs”. It is found in nerve fibres in the olfactory tubercle, the nucleus accumbens and the striatum’, and the presence of neurotensin receptors hasbeen demonstrated on dopaminergic cells in the substantia nigra”‘. Electrophysiologically, neurotensin selectively excites dopaminergic neurons in the pars compacta of the substantia nigra’ ‘. The injection of neurotensin into the ventral tegmental area leads to
a haloperidol-blockable increase in locomotor activity and this effect is accompanied by an increase in dopamine tumover’. Recent in-vivo and in-vitro studies have also demonstrated that neurotensin can release dopamine from a variety of brain areas”.‘“.“. Much less is known about the influence of dopamine on the neurotensin system. The administration of neuroleptics leads to an apparent increase in neurotensin turnover and to a substantial increase in neurotensin content in dopamine-rich areas. However, this effect could be due to a direct influence on neurotensin synthesis and may be independent of dopamine-receptor blockade. The low levels of neurotensin and the incomplete knowledge of its exact anatomical rela- ’ tionship to the dopamine system make it difficult to study the effects of dopamine on neurotensin. The situation is complicated further by the finding that dopamine and neurotensin coexist in various parts of the rat CNS’s.‘“. The recent finding that in the cat the basal ganglia contain considerably higher amounts of neurotensin than in the rat” may mean that such problems are best approached by studies on the former species. Anti-nociception and hypothermia Following direct injection into the brain, neurotensin producesmarked antinociception and hypothermia. In general. such studies require very high concentrations of peptide for a maximal effect (up to 100 kg), as compared with the total brain content of neurotensin (10 ng for the rat). It therefore seems legitimate to question the physiological significance of these results. However, in the absence of selective antagonists. studies such as these represent the only way of finding out what neurotensin might be doing in the CNS. Neurotensin is one of the most potent anti-nociceptive substancesknown, being more potent than morphine on a molar basislx. Its action is insensitive to naloxone and probably takes place at the level of the brain stem. where neurotensin-like immunoreactivity and neurotensin receptors are present. The hypothermic effect of