Peripheral GABAergic mechanisms

TIPS - May 1985 Soc. Neurosci Abstr. 9, 290 13 Luttinger, D., Nemeroff, C.B. and Prange, A. J. (1982) Brain Res. 237, 183192 14 Ervin, G. N., Birkemo, L. S., Nemeroff, C.B. and Prange, A.J. (1981) Nature (London) 91, 73-76 15 Kalivas, P. W., Nemeroff, C.B. and Prange, A.H. (1984) Neuroscience 11, 919-930

205 16 Kalivas, P. W., Burgess, S. K., Nemeroff, C.B. and Prange, A.J. (1983) Neuroscience 8, 495-505 17 Glimcher, P. W., Margolin, D.H., Giovino, A. A. and Hoebel, B. G. (1984) Brain Res. 291, 119-124 18 Agnati, T. F., Fuxe, K., Benfenati, F. and Battistini, N. (1983) Acta Physiol. Scand. 119, 459-461 19 Govoni, S., Hong, J. S., Wang, H.-Y. T.

Peripheral GABAergic mechanisms Sandor L. Erd6 The early assumption that ~,-aminobutyric acid (GABA) is present in vertebrates and plays a neurotransmitter role solely in the CNS has been disproved. Increasing evidence has shown that GABA is present and may even be a neurotransmitter in certain peripheral tissues of mammals. In spite of the fact that specific GABAergic responses have also been demonstrated in numerous peripheral tissues, little attention has been paid to the functional importance of GABAergic mechanisms outside the brain. In the present review Sfindor L. Erd6 summarizes the current knowledge of GABAergic systems in peripheral tissues and emphasizes the functional relevance of GABA in a series of peripheral organs. G A B A has been identified in nearly 30 different peripheral tissues of various m a m m a l s (see Ref. 1). In most tissues examined, GABA concentrations were rather low, i.e. b e l o w 1% of the cerebral level. O t h e r tissues, however, such as the oviduct, the pancreatic islets and the ovary of the rat or the myenteric plexus of the gut, contain h i g h GABA concentrations, comp a r a b l e w i t h the cerebral level. Surprisingly, the rat oviduct was f o u n d to b e more than twice as rich in GABA as the b r a i n 1. It should be n o t e d that in certain peripheral organs (kidney, oviduct) GABA levels s h o w marked species differences. Up to now, little information has b e e n available concerning the cellular location of GABA in peripheral tissues. In the myenteric plexus, GABA is likely to be present in n e u r o n s 2, w h e r e a s in pancreatic islets it is p r e s u m a b l y located in B-cells3. In adrenals, G A B A is p r e s e n t m a i n l y in chromaffin cells. In other peripheral organs, the GABA-containing cells have not yet b e e n identified.

Sandor Erdb is a research worker at the Pharmacological Research Centre, Chemical Works of Gedeon Richter Ltd., H-1475 Budapest, PO Box 27, Hungary

Biosynthesis and metabolism The major biosynthetic pathw a y for GABA in the brain is catalysed b y L-glutamate decarboxylase (GAD) although absence of GAD in a particular tissue does not disprove the local formation of GABA from e n d o g e n o u s substances other than glutamate. GAD activity has been detected in at least 20 different peripheral tissues. Significant activity of the enzyme, however, has been demonstrated only in a few tissues, such as in the kidney, the heart, the pancreatic islets, the m y e n teric plexus and the oviduct. Immunochemical and biochemical studies have revealed that different types of GAD are present in cerebral neurons and in certain non-neural, peripheral tissues such as in the k i d n e y and the heart 4. The properties of GAD from oviduct and from brain tissue, however, seem identical, indicating that the neural type of the enzyme m a y also be present in peripheral organs 5. Other lines of evidence indicate that glutamate decarboxylation is not the sole p a t h w a y for GABA formation in peripheral tissues. Andersson, Fogel and others reported significant GABA formation from putrescine, in vivo, in a series of peripheral organs such as

and Costa, E. (1980) J. Pharmacol. Exp. Ther. 215, 413-417 20 Widerlov, E., Lindstrom, L. H., Besev, G., Manberg, P.J., Nemeroff, C.B., Breese, G.R., Kizer, I.S. and Prange, A.J. (1982) Am. J. Psychiatry 139, 11221126 21 Nemeroff, C. B., Youngblood, W.W., Manberg, P.J., Prange, A.J., Jr. and Kizer, J. S. (1983) Science 221,972-975

the small intestine, liver, kidney, spleen, lung, heart, ovary, uterus, thyroid, etc. Particularly high formation rates were observed in h u m a n neoplastic thyroid tissue. The first step of this biosynthetic p a t h w a y is catalysed b y d i a m i n e oxidase which converts putrescine (1,4-diaminobutane) to GABAaldehyde (4-aminobutyric aldehyde). An aldehyde dehydrogenase forms GABA from the precursor. The scheme of the alternative biosynthetic routes and the major catabolic p a t h w a y are shown in Fig. 1. Recently, Fernandez et al. demonstrated that, in the rat oviduct in vivo, nearly the same amount of GABA can be synthesized from putrescine and from glutamate. (The putrescine pathw a y has recently also been identified in brain tissue). Detectable activity of GABA transaminase the enzyme primarily responsible for GABA degradation i n t h e brain, has been demonstrated in a series of peripheral tissues, (e.g. gut, kidney, liver, oviduct, blood vessels etc.4). Alternative catabolic p a t h w a y s for peripheral GABA have not yet been studied in detail.

GABA uptake and release The high-affinity re-uptake systems for GABA in the brain are generally thought to be responsible for inactivation and reutilization of the e n d o g e n o u s amino acid. Active GABA uptake mechanisms which might serve similar functions, have also been identified in certain peripheral tissues (thyroid, ovary, oviduct, gut, urinary bladder, adrenals, liver, etc.). The GABA-accumulating cells in the myenteric plexus of the gut and in the urinary b l a d d e r proved to be mainly neurons. Hepatocytes in the liver, chromaffin cells in the adrenals and blood platelets also possess active GABA uptake systems. In thyroid, the follicle cells were found to accumulate GABA predominantly. A GABA release process that is

~) 1985,ElsevierSciencePublishersB.V.,Amsterdam 0165- 6147/85/$O200

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rather similar to the calciumdependent neuronal release mechanisms has been demonstrated in the myenteric plexus, the urinary bladder and the adrenals. Also, preliminary data indicate that GABA can be released from preloaded slices of the rabbit oviduct by electric stimuli (Erd6 et al., unpublished observations). No attempt has been made to examine the possible GABA release systems in other peripheral tissues. It can be proposed that active GABA transport systems are present in a series of peripheral tissues, but the neuronal-like nature of these mechanisms seems to be convincingly documented for only a few tissues. GABA receptors

In mammalian tissues two pharmacologically and functionally distinct types of GABA receptors have been identified. One of them, the classical GABA receptor (GABAA) is sensitive to the antagonist bicuculline. GABAB is bicuculline-insensitive and can be selectively stimulated by baclofen. Both subpopulations have been found not only in the brain, but also in various peripheral organs 6,7.

glutamate

Different approaches have been used to identify specific GABA receptors in peripheral tissues. Receptor ligand binding experiments revealed the presence of specific GABA binding sites (GABAA) in the ovary, the oviduct, the uterus, in the adrenals, on certain blood vessels and in blood platelets. The baclofenbinding procedure developed recently7, however, has not yet been used for characterization of GABAB binding sites in peripheral tissues. Using physiological approaches, specific, GABA receptor-mediated responses have been demonstrated in smooth muscle tissues (gut, oviduct, urinary bladder, gall-bladder, vas deferens, blood vessels, anococcygeus muscle), and in endocrine organs (pancreas, adrenals, antral mucosa of the stomach). These are discussed below. Electrophysiological and pharmacological experiments have rev e a l e d that GABA receptors in peripheral tissues are located on neuronal elements of the autonomic ganglia (both sympathetic and parasympathetic!). They are present on neuronal cell bodies, axons and dendrites (see Ref. 8). GABAB receptors in the oviduct

putrescine

~ diamine I oxJdase

~l:tamatcarbo~ylas,

G A B A - aldehyde

aldehydedehydrogenase GABA

I GABA- transaminase succinic semi aldehyde

I snccinicsemialdehydedehydrogenase succinate Fig. I Metabolic pathways of GABA in peripheral tissues.

are located most probably on smooth muscle cells 9. The presence of GABA receptors on secretory cells, i.e. pancreatic islets, and adrenal chromaffin cells, has also been documented. GABAergic innervation

Taking into account the identified elements of GABAergic systems in a series of peripheral tissues, the ubiquitary occurrence of GABAergic mechanisms in mammalian tissues seems reasonable to assume. Convincing evidence, however, for the neurotransmitter function of GABA outside the CNS, is as yet available only for the myenteric plexus of the 811t2. This is not surprising considering that systematic studies attempting to clarify the neurotransmitter nature - or at least the neuronal localization - of GABA in peripheral tissues other than the gut, have not yet been completed. The incomplete information regarding the elements of GABAergic systems in certain peripheral tissues is shown in Table I. P h y s i o l o g i c a l r e s p o n s e s to G A B A - smooth muscle tissues

The contractility of various segments of the gut 1° from different mammals can be modulated by local GABA receptors. GABA elicits contractions via GABAA receptors located on postganglionic cholinergic neurons in the myenteric plexus. On the other hand, GABAB receptors in the gut mediate relaxation by inhibition of postganglionic cholinergic nerves. Interactions between GABA and 5-HT in the gut have also been demonstrated. GABA stimulates the spontaneous contractility of isolated rabbit oviduct9 via GABAB receptors most likely located on smooth muscle cells. Similar responses can be observed in uterine strips (Erd6 and Riesz, unpublished observations). The motility of the isolated urinary bladder 11 can be inhibited by GABA. This bicuculline-sensitive effect is presumably mediated via GABAA receptors on cholinergic nerves. The same phenomenon can be observed also in situ. Acetylcholine release can be evoked by GABA from isolated strips of gall-bladder 12. This effect is likely mediated by GABAA

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ated on lactotrophes inhibit prolactine secretion, GABAA and GABAB receptors on melanotrophes stimulate and inhibit melanocyte-stimulating hormone secretion, respectively. The inhibition of growth hormone secretion via bicuculline- sensitive GABA receptors on somato-

receptors located on postganglionic cholinergic neurons. GABA inhibits the release of noradrenaline and the twitch response to electrical stimulation in isolated vas deferens 6. These actions are thought to be mediated via GABAB receptors located on adrenergic nerve terminals.

TABLE I. GABAergic parameters in certain peripheral tissues.

Tissue Adrenals Anococcygeus muscle Blood vessels Gut Kidney Ovary Oviduct Pancreatic islets Stomach Urinary bladder Vas deferens

GABA

GAD

GABA trans aminase

+

+

+

+

+

+ + +

+ + +

+ + +

+

+

uptake

release

+

+

+

+

+ +

+ +

+

+

+

+ +

+ +

+

+

functional receptors response + +

+ +

+ +

+ +

+

+

+ + + + +

+ + + + +

surface of secretory cells. It seems reasonable to assume that intrinsic GABAergic mechanisms in the stomach 16 may modulate the endocrine function of the organ. In perfused dog pancreas 17, GABA increases the release of somatostatin, inhibits insulin secretion, but does not influence glucagon secretion. In healthy h u m a n subjects, a high oral dose of the GABAB receptor agonist baclofen increased the plasma levels of insulin and glucagon TM. Similar effects of GABA but not muscimol (GABAA receptor agonist) has been reported in another clinical study. These findings indicate that local GABAergic mechanisms do influence the endocrine pancreatic functions 3. Some of the effects of GABA may be mediated via GABAB receptors. P h y s i o l o g i c a l r e s p o n s e s to G A B A - other t i s s u e s

In isolated pulmonary artery 13, GABA inhibits the contractile response and the release of noradrenaline via bicuculline-insensitive (GABAB?) receptors. Also, GABA relaxes the basilar artery, but the latter effect is likely m e d i a t e d b y GABAA sites on vascular elements (see also Ref. 4). It depresses the contractile response of the isolated anococcygeus muscle 2° to field stimulation. This effect is considered to be due to an action via GABAB receptors on the excitatory adrenergic nerves. These receptor-mediated actions of GABA m a y be involved, in situ, in the local modulation of important living functions such as the regulation of enteric motility, ovum transport and fertility, urination and tissue blood flow.

P h y s i o l o g i c a l r e s p o n s e s to G A B A - endocrine tissues

Hormone secretion from anterior, posterior and intermediate lobes of pituitary can be significantly influenced b y GABA and related drugs. Although the results concerning certain hormones are often contrasting, most of the GABAergic effects seem to be mediated via central actions through hypothalamic releasing factors. Nevertheless, there is a growing body of evidence that GABA may also act directly on secretory cells of pituitary. For instance, GABAA receptors loc-

trophes, has also been suggested. O n the other hand secretion of hormones such as luteinizing hormone, adrenocorticotropin, thyrotropin and vasopressin, seem to be influenced b y GABA at the hypothalamic but not at the pituitary level. Although the indirect actions of GABA are often more pronounced, the involvement of local GABA receptors in the modulation of pituitary hormone secretion appears convincingly documented. For a recent review, see Ref. 14. GABA releases catecholamines from the isolated, perfused adrenal medulla TM via GABAA receptors. This suggests the involvement of local GABAergic mechanisms in the regulation of adrenal medullary function. Preliminary results indicate that GABA increases ovarian blood flow and estradiol secretion, but decreases the release of progesterone via a local action (Erd6, Varga, Horv~ith, unpublished observations). It is uncertain, however, whether these effects would be mediated via specific receptors or not. Nevertheless, the above observations may indicate the functional relevance of ovarian GABAergic system. GABA stimulates the release of gastrin, but inhibits somatostatin secretion from antral mucosa, in vitro. Both actions are exerted via bicuculline- sensitive receptors, proposed to be located on the

GABA depresses or facilitates respiratory and sympathetic vasomotor 6,19 activities through both GABAA and GABAB receptors, at different sites of the respiratory and cardiovascular reflex pathways. []

[]

[]

The specific GABAergic responsiveness of a series of autonomically innervated organs and the occurrence of GABA and related enzymes in most of these tissues might suggest that GABA has a role as a neurotransmitter in certain peripheral tissues other than the gut although hitherto, no attempt has been made to visualize GABAergic neurons in such peripheral organs. Morphological evidence for and against the GABAergic innervation of these tissues has not yet been provided. In addition, GABA has been proved to be present in peripheral tissues in cell types other than neurons. Thus, functions for GABA, other than neurotransmission, should also be considered. Goodyer and others suggested a possible role of GABA in renal ammoniagenesis and in the function of proximal tubular cells. The liver m a y be involved in the regulation of serum GABA levels via active GABA transport processes. Accumulation of GABA in hepatocytes may lead to an eleva-

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tion of serum GABA concentrations which may contribute to a liver failure. Baxter, Sandraval and others raised the possibility of a role of GABA i n regulation of protein synthesis, growth process, maturation, differentiation and cell proliferation. A function of GABA i n energy metabolism of certain peripheral tissues has also been hypothesized. The presence of GABA in pancreatic B-cells and chromaffin cells of adrenals might suggest that GABA is a humoral factor in these tissues. Regardless of the cellular location and function of GABA, peripheral GABAergic mechanisms may be involved in the regulation of a series of important living functions. Thus, the GABAergic systems in tissues showing specific physiological responsiveness to GABA should be regarded as potential targets for drug action. Future efforts may lead to the development of GABAergic drugs with a

wide therapeutic efficiency which act at peripheral level and do not cross the blood - brain barrier. References

1 Erd6, S. L., Rosdy, B. and Szporny, L. (1982) J. Neurochem. 38, 1174-1176 2 Jessen, K. R., Hills, J.M., Dennison, M. E. and Mirski,R. (1983)Neuroscience 10, 1427-1442 3 Taniguchi, H., Okada, Y., Kobayashi, T., Murakami, K. and Baba, S. (1979)in Proinsulin, Insulin, C-Peptide (Baba, S., Kaneko, T. and Yanaihara, N., eds), pp. 335-347, Excerpta Medica, Amsterdam, Oxford 4 Wu, J. Y. (1982) in Problems in GABA Research from Brain to Bacteria (Okada, Y. and Roberts, E. eds), pp. 40-54, Excerpta Medica, Amsterdam, Oxford Princeton 5 Erd6, S. L., Kiss, B. and Szporny, L. (1984) J. Neurochern. 43, 1532-1537 6 Bowery, N. G., Doble, A., Hill, D. R., Hudson, A. L, Shaw, J.S., Turnbull, M.J. and Warrington, R. (1981) Eur. J. Pharmacol. 71, 53-70 7 Hill, D. R. and Bowery, N.G. (1981) Nature (London) 290, 149-152 8 Morris, M. E., DiConstanzo, G. A. and Werman, R. (1983) Brain Res. 278, 117126

Modulation mechanisms in cell-cell recognition Gerald M. Edelman Cell surface glycoproteins mediating cell-cell adhesion have been characterized only in the last ten years. Still more recently, evidence has accrued which has allowed theories of the regulation of cell-cell interactions to be developed. Gerald Edelman discusses the regulatory role of cell adhesion molecules (CAMs) in cell-cell recognition and opens up new areas for pharmacological investigation: what is the relationship of CAMs to channels, receptors and neurotransmitters, and what regulates these regulators? The discovery a n d characterization i n the last decade of cell surface glycoproteins that mediate cell-cell adhesion s has made it possible to ask some p o i n t e d questions a b o u t the principles governing the formation of specific tissue structures such as neuromuscular junctions, epithelial organs, and neural maps. Perhaps the most significant of these questions concerns the regulation of cell-cell interactions d u r i n g early development. Do these occur as the result of interactions of large n u m b e r s of genetically p r e d e t e r m i n e d cell Gerald Edelman is Vincent Astor Distinguished Professor at The Rockefeller University, 1230 York Avenue, New York, N.Y, 10021, USA 1985, Elsevier Science Publishers B.V., A m s t e r d a m

surface markers that are placespecific? Or do they occur by d y n a m i c and epigenetically determ i n e d alterations of the a m o u n t s or b i n d i n g m e c h a n i s m s of a relatively small n u m b e r of molecules, with c o n s e q u e n t changes in higher level d r i v i n g forces such as cell m o v e m e n t ? The accumulating evidence favors this second possibility. Several specific mechanisms of m o d u l a t i o n of cell adhesion molecules (CAMs) have n o w been correlated with key histogenetic events; i n addition, a series of p e r t u r b a t i o n experiments have confirmed the importance of these molecules i n morphogenetic sequences. In this short survey, I propose to discuss some examples of such sequences in order to

0165 - 6147/85/$02.00

9 ErdS, S. L., Riesz, M., K~rpiiti, E. and Szporny, L. (1984) Eur. J. Pharmacol. 99, 333-336 10 Ong, J. and Kerr, D. I. B. (1983) Eur. J. Pharmacol. 86, 9-17 11 Taniyama, K., Kusunoki, M. and Tanaka, C. (1983) Eur. J. Pharmacol. 89, 16,3-166 12 Saito, N., Taniyama,K. and Tanaka,C. (1984) Naunyn-Schmied. Arch. Pharmacol. 326, 45-48 13 Starke, K. and Weitzell, R. (1980) J. Auton. Pharmacol. 1, 45-51 14 DeFeudis,F. V. (1984)TrendsPharmacol. Sci. 5, 152-156 15 Yamanaka, K., Yamada, S., Okada, T. and Hayashi, E. (1983)Jap. J. Pharmacol. 33, 1049-1055 16 Harty, R. F. and Franklin, P. A. (1983) Nature (London) 303, 623--624 17 Kawai,K. and Unger,R. H. (1983)Endocrinology, 113, 111-113 18 Passariello, N., Giugliano, D., Torella, R., Sgambato, S., Coppola, L. and Frascolla, N. (1982) J. Clin. Endocrinol. Metab., 54, 1145-1149 19 Laney,P. M. (1982)in Problemsin GABA Research from Brain to Bacteria (Okada, Y. and Roberts, E. eds), pp. 137-146, Excerpta Medica, Amsterdam, Oxford, Princeton 20 Hughes, P. R., Morgan, P. F. and Stone, T. W. (1982)Br. J. Pharmacol.77, 691-695 illustrate the role of CAMs in cellcell recognition. Before doing so, however, it may be valuable to describe the basic findings in s u m m a r y fashion. Extensive reviews 1,2 have b e e n published and m a y be consulted for more details o n earlier work as well as for bibliographic background. Cell adhesion molecules occur in all vertebrates, are present at cell surfaces and are the major means by which cells sort out 3 in development. Different CAMs have different b i n d i n g specificities but, within any specificity, the chief means by which cell interaction is altered during development is b y modulation of their amount, of their distribution on cell parts or of their b i n d i n g mechanisms. Such local cell surface modulations occur in orderly sequences d u r i n g tissue development. The regulation of these sequences is a major factor in d e t e r m i n i n g cell migration and epithelial interactions. The examples I have chosen to illustrate these principles and conclusions are: (1) the formation of the neural plate d u r i n g induction; (2) the migration of neural crest cells to form peripheral nervous structures; (3) some sites of secondary embryonic induction; (4) the development of the cerebellum; and (5) retinotectal maps. In