5-Fluoronicotine,noranhydroecgonine, and pyridyl-methylpyrrolidine release acetylcholine and biogenic amines in rat cortex in vivo

The effects of nicotinic receptor agonists 5-fluoronicotine, noranhydroecgonine and pyridyl-methylpyrrolidine on the cortical release of acetylcholine (ACh), norepinephrine (NE), dopamine (DA) and serotonin (5-HT) were investigated with microdialysis in rat. 5-Fluoronicotine significantly elevated ACh to 76% above basal values and DA to 69% above baseline. Pyridyl-methylpyrrolidine significantly increased the release of ACh to 39% above basal values and NE to 63% above baseline. Noranhydroecgonine significantly elevated NE to 64% above basal values and DA to 147% above baseline. 5-Fluoronicotine did not affect NE release; pyridylmethylpyrrolidine did not alter DA release; and noranhydroecgonine did not significantly elevate ACh release. None of these agonists increased the release of 5-HT. All responses were blocked by prior administration of mecamylamine, a nicotinic receptor antagonist. The distinctive neurotransmitter-related profiles for the three agonists are suggestive of activity at subtypes of nicotinic receptors, an effect that may be related to the structural diversity of these compounds.     

Effect of L-prolyl-L-leucyl-glycinamide (MIF-I) on some neutrotransmitter-receptor interactions

Some neurotransmitter-receptor interactions have been studied in an attempt to determine how L-prolyl-L-leucyl-glycinamide (MIF-I) exerts its antiparkinson effect. MIF-I affected neither the contractile responses of isolated mouse vas deferens and guinea pig ileum to noradrenaline, acetylcholine, substance P and histamine, nor the inhibitory effects of dopamine and GABA on the rat vas deferens and guinea pig ileum. MIF-I, as well as L-leucine and Pro-Leu, antagonized the contractile response of the ileum to 5-hydroxytryptamine (5-HT). Behavioural tests were used to examine the action of MIF-I on CNS transmitter-receptor interactions. MIF-I did not modify the circling produced by either dopamine agonists in nigro-striatal lesioned rats of 5-HT agonists in rats with a lesion of the medial raphe nucleus. MIF-I affected neither 5-hydroxytryptophan-induced head twitches in mice, which is a measure of 5-HT receptor stimulation, nor striatally-evoked head turning in the rat, which is a model for brain GABA function. It is concluded that MIF-I, at the doses used, does not directly modify the function of any of the CNS transmitter examined. Other possibilities to explain its antiparkinson action are discussed.     

Release and effect ofγ-Aminobutyric acid (GABA) on rat pineal melatonin productionin vitro

1. 3H-gamma-Aminobutyric acid (GABA) release elicited by a depolarizing K+ stimulus or by noradrenergic transmitter was examined in rat pineals in vitro. 2. The release of 3H-GABA was detectable at a 20 mM K+ concentration in medium and increased steadily up to 80 mM K+. 3. In a Ca2+-free medium 3H-GABA release elicited by 30 mM K+, but not that elicited by 50 mM K+, became blunted. 4. Norepinephrine (NE; 10(-6)-10(-4) M) stimulated 3H-GABA release from rat pineal explants in a dose-dependent manner. 5. The activity of 10(-5) M NE on pineal GABA release was suppressed by equimolecular amounts of prazosin or phentolamine (alpha 1- and alpha 1/alpha 2-adrenoceptor blockers, respectively) and was unaffected by propranolol (beta-adrenoceptor blocker). 6. The alpha 1-adrenoceptor agonist phenylephrine (10(-7)-10(-5) M) and the beta-adrenoceptor agonist isoproterenol (10(-5) M) mimicked the GABA releasing activity of NE, while 10(-7) M isoproterenol failed to affect it; the alpha 2-adrenoceptor agonist clonidine (10(-7)-10(-5) M) did not modify 3H-GABA release. 7. The addition of 10(-4) M GABA or of the GABA transaminase inhibitor gamma-acetylenic GABA or aminooxyacetic acid inhibited the melatonin content and/or release to the medium in rat pineal organotypic cultures. 8. GABA at concentrations of 10(-5) M or greater partially inhibited the NE-induced increase in melatonin production by pineal explants. 9. The depressant effect of GABA on melatonin production was inhibited by the GABA type A receptor antagonist bicuculline; bicuculline alone increased the pineal melatonin content. Baclofen, a GABA type B receptor agonist, did not affect the pineal melatonin content or release. 10. The decrease in serotonin (5-HT) content of rat pineal explants brought about by NE was not modified by GABA; GABA by itself increased 5-HT levels. 11. These results indicate that (a) GABA is released from rat pineals by a depolarizing stimulus of K+ through a mechanism which is partially Ca2+ dependent; (b) NE releases rat pineal GABA via interaction with alpha 1-adrenoceptors; (c) GABA inhibits melatonin production in vitro via interaction with GABA type A receptor sites; and (d) GABA's effect on NE-induced melatonin release does not correlate with the lack of effect on the NE-induced decrease in pineal 5-HT content.     

Modulation of In Vivo Striatal Transmitter Release by Nitric Oxide and Cyclic GMP

Abstract: The effects of nitric oxide (NO) and cyclic GMP on in vivo transmitter release in the rat striatum were investigated using microdialysis sampling in urethane-anaesthetised animals. The NO release-inducing substances S -nitrosoacetylpenicillamine (SNAP), S -nitrosoglutathione (SNOG), and sodium nitroprusside (SNP) increased extracellular concentrations of aspartate (Asp), glutamate (Glu), γ-aminobutyric acid (GABA), taurine (Tau), acetylcholine (ACh), and serotonin (5-HT). Dopamine (DA) concentrations were decreased by SNAP but were increased by SNOG and SNP. An NO scavenger, haemoglobin, blocked or reduced the effects of SNAP on transmitter release. However, the control carrier compounds for SNAP, SNOG, and SNP (penicillamine, glutathione, and potassium ferricyanide, respectively, which do not induce release of NO) also increased GABA, Tau, DA, and 5-HT concentrations. When NO gas was given directly by dissolving it in degassed Ringer's solution, DA concentrations decreased significantly, and those of Asp, Glu, GABA, Tau, ACh, and 5-HT increased. These effects of NO gas were all inhibited by coadministration of haemoglobin and for GABA, Tau, ACh, and DA showed some calcium dependency. The cyclic GMP agonists 8-bromo-cyclic GMP and dibutryl-cyclic GMP stimulated dose-dependent increases in Asp, Glu, GABA, Tau, ACh, DA, and 5-HT concentrations. Increased striatal transmitter release in response to NO may therefore be mediated by its stimulatory action on cyclic GMP formation. NO inhibition of DA release may be mediated indirectly through its stimulation of local cholinergic and GABAergic neurones.     

GABA evoked ACh release from isolated guinea pig ileum

To identify the target cells of GABAergic neurons located in the myenteric plexus, the action of γ-aminobutyric acid (GABA) on the release of acetylcholine (ACh) and on the contractions was studied using the isolated guinea pig ileum. GABA evoked a release of ³H-ACh from the contracting ileum, under conditions of loading with ³H-choline. As both the GABA-evoked release of ³H-ACh and the contractions were inhibited by bicuculline, tetrodotoxin and furosemide, but not by hexamethonium, this release seems to be evoked through GABA receptors which are bicuculline sensitive and associated with the Cl- ion channel.     

Intracellular recording of the frog dorsal root single fibres' responses mediated by the GABAA and 5-HT-receptors

The effects of GABA, bicuculline and 5-HT on primary afferents in the isolated spinal cord of the frog Rana ridibunda were studied. Bath application of GABA (1 mM) reduced the primary afferent depolarisation (PAD) in IX segment of the spinal cord evoked by X dorsal root stimulation (57 +/- 8% of initial level, n = 5, p < 0.05). The action potentials (AP) recorded in dorsal root afferents was also suppressed under the GABA action (74 +/- 9%, p < 0.05). Bath application of bicuculline (50 microM) reduced the PAD (21 +/- 7%), n = 6, p < 0.05), meanwhile the AP in dorsal root afferents was resistant against the bicuculline action. Bath application of 5-HT (25 microM) depressed the PAD (34 +/- 7%, n = 7, p < 0.05) and the amplitude of the AP recorded from the single afferent fibre in dorsal column (76 +/- 6%, n = 7, p < 0.05). In contrast to GABA, 5-HT more effectively suppressed the late phase of the PAD evoked by X dorsal root stimulation and caused (76 +/- 6%, n = 7, p < 0.05) an alteration of the AP shape. All effects induced by these drugs were reversible. The mechanisms of GABA and 5-HT modulation of spinal cord afferent income are discussed.     

Gamma-aminobutyric acid modulation of acetylcholine-induced contractions of a smooth muscle from an echinoderm (Sclerodactyla briareus)

This study provides pharmacological evidence for the presence of GABAergic neurons innervating the longitudinal muscle of the body wall (LMBW) of holothurians. Gamma-aminobutyric acid (GABA) A and B receptor subtypes were both present in this system and regulated spontaneous contractions as well as responses to acetylcholine (ACh) that stimulated contraction of the LMBW. GABA dose-dependently relaxed the resting tone of the LMBW. GABA (10(-5) M) inhibited ACh-induced (10(-4) M) contractions by 20%. The GABA B agonist, baclofen, relaxed the LMBW, an effect potentiated by GABA. Pretreatment with baclofen (10(-4) M) inhibited ACh (10(-4) M) contractions of the LMBW by 50%. Phaclofen, a GABA receptor B antagonist, caused a dose-dependent increase in resting tension. Phaclofen-induced (10(-5) M) contractions were reversed by the addition of GABA or baclofen (10(-4) M) and potentiated by the addition of another GABA B receptor antagonist, 2-hydroxy-saclofen (10(-5) M). Pretreatment with phaclofen (10(-5) M) caused a marked potentiation of ACh-induced (10(-4) M) contractions by 101%. 2-Hydroxy-saclofen (10(-5) M) had a toxic effect on the LMBW, rendering it completely unresponsive either to ACh or to a second exposure to GABA, and so exhibiting cross-desensitization. Muscimol, a GABA A receptor agonist, had no effect on the resting tension of the LMBW. Curiously, pretreatment of the muscle with muscimol (10(-5) M) potentiated ACh-evoked (10(-4) M) contractions by nearly 20%. Bicuculline (10(-5) M), a GABA A receptor antagonist, generated large, sustained contractions and partially blocked GABA-induced (10(-4) M) relaxation. Like 2-hydroxy-saclofen, bicuculline (10(-5) M) had a profound cross-desensitizing effect on the LMBW to subsequent exposures to GABA and ACh. ACh was unable to potentiate the sustained contractions induced by bicuculline.     

Modulation of the Electrically Evoked Release of 5-[3H]Hydroxytryptamine from Rat Cerebral Cortex: Effects of Alpidem, CL 218872, and Diazepam

The effect of omega (benzodiazepine)-receptor agonists, antagonists, and inverse agonists on the electrically evoked release of 5-[3H]hydroxytryptamine ([3H]5-HT) was studied in superfused slices of the rat frontal cerebral cortex. The electrically evoked release of [3H]5-HT was enhanced by nanomolar concentrations of diazepam and the selective omega 1-receptor agonists alpidem and CL 218872. The omega 1/omega 2- and omega 1-receptor antagonists flumazenil and CGS 8216, respectively, did not modify the electrically evoked release of [3H]5-HT. The omega 3-receptor agonist Ro 5-4864 and the omega 1-receptor inverse agonist ethyl-beta-carboline-3-carboxylate on their own did not affect the electrically evoked release of [3H]5-HT. On the other hand, the inverse agonist 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester (DMCM), at micromolar concentrations, inhibited both the spontaneous and the evoked release of [3H]5-HT. The facilitation of the electrically evoked release of [3H]5-HT by diazepam, alpidem, or CL 218872 was potentiated by gamma-aminobutyric acid (GABA). Exposure to flumazenil and CGS 8216 antagonized the facilitation by diazepam, alpidem, or CL 218872 of [3H]5-HT release. The inhibition of the release of [3H]5-HT by DMCM was not modified by exposure to either flumazenil, CGS 8216, or GABA. The inhibitory effect of DMCM was not observed when monoamine oxidase activity was inhibited by pargyline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute vasorelaxant effects of metformin and attenuation by stimulation of sympathetic agonist release

We recently discovered 1) that intravenous injection of the antidiabetic drug metformin in the rat rapidly reduces arterial pressure elevations maintained by the alpha-adrenoceptor agonist phenylephrine (PE) and 2) that direct administration of metformin to isolated rat tail arterial tissue rings rapidly relaxes PE-induced contractions. To further characterize this potential direct vasodilator action, we examined effects of metformin on contractions induced not only by PE but also by norepinephrine (NE) and by nonadrenergic agonists (5-hydroxytryptamine, 5HT; arginine vasopressin, AVP). Also, because the rat tail artery contains abundant adrenergic nerve endings we conducted these tests not only in arterial rings with nerve endings intact but in rings in which they had been removed by pretreatment with 6-hydroxydopamine. In intact rings, metformin at levels from approximately 0.2 to 20 mmol/L rapidly relaxed half-maximal contractions induced by PE and NE similarly and to a markedly greater degree than contractions induced by 5-HT (p<0.05). Metformin did not relax AVP-induced contractions. In addition, removal of adrenergic nerve endings facilitated metformin's relaxant effects (p<0.05). Thus, the acute vasodilator action of metformin appears 1) to be selectively more powerful on arterial smooth muscle contractions induced adrenergically versus nonadrenergically and 2) to be buffered by a possible metformin-induced release of endogenous NE from adrenergic nerve endings. Such results were not seen during relaxation produced by either the calcium channel inhibitor nifedipine or the nitrovasodilator nitroprusside suggesting that metformin's effects are mediated by other mechanisms.     

Effects of transmitters on pyramidal and nonpyramidal neurons in hippocampal slices

Investigations were performed on the effects of acetylcholine (ACh), norepinephrine (NE), 5-hydroxytryptamine (5-HT), and -aminobutyric acid (GABA) on the background firing of the three following groups of field CA₃ neurons in guinea pig hippocampal slices: nonpyramidal neurons of the stratum radiatum moleculare (NSR), stratum pyramidale cells with single spike discharges (SD units), and those with complex discharge patterns (CD units) within the same layer. The action of ACh and NE on presumed interneurons of the pyramidal layer (IPL) was also investigated; CD units were found to differ from the remaining groups, which reacted similarly to the transmitters tested. It was shown that NE, 5-HT, and GABA inhibited the activity of CD cells, while ACh produced inhibitory-activating response in 50% of these units. Both NE and ACh exerted a monophasic activating effect on NSR, ISP, and SD, however, while 5-HT and GABA induced activation in a proportion of NSR and SD cells, as well as inhibitory response. The excitatory effects produced by ACh, NE, and 5-HT on NSR persisted during blockade of synaptic transmission, indicating that associated afferent fibers may be acting directly on these cells.Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino-on-Oka. Translated from Neirofiziologiya, Vol. 20, No. 1, pp. 64–74, January–February, 1988.     

The release of gamma-aminobutyric acid, glutamate, and acetylcholine from striatal slices: a mass fragmentographic study

The release processes of endogenous Acetylcholine (ACh), gamma-aminobutyric acid (GABA), glutamate (Glu) and glutamine (GLN) were studied in superfused guinea-pig caudatal slices. Basal ACh release remained constant for up to 2 h, while the basal release of GABA, Glu and GLN declined to half or less of its initial values after 1 h of superfusion. Electrical stimulation increased the ACh release by 700-800% and that of GABA by 80% whereas it decreased the output of Glu by 50% and failed to modify the GLN efflux. KCl (25 nM) increased the output of ACh by 400%, that of GABA by approximately 500% and decreased that of Glu by 40%. Substituting of CaCl(2) by MgCl(2) in the superfusion medium reduced the basal efflux of GABA, Glu and GLN. Under these conditions, no evoked release of ACh or of GABA was detected, following electrical or KCl stimulation. Tetrodotoxin 5 x 10(-7) decreased the basal ACh release by 60% and increased the GABA efflux by 40%. The toxin abolished the stimulus-evoked ACh efflux but scarcely affected that of GABA. These results are consistent with a possible neurotransmitter role of ACh and GABA in the striatum and show some differences in the ionic mechanisms underlying GABA and ACh release.     

Motility of isolated mammalian gastric fundus

Gastric fundus strips (GFS) of rats, guinea-pigs, rabbits, cats and humans, but not of dogs, show spontaneous phasic contractions in vitro. Rabbit, cat, dog and human GFS exhibit dose-dependent responses to acetylcholine (ACh), histamine (His), serotonin (5-HT) and KCl. Guinea-pig drug-induced responses are not dose-dependent. Rat GFS does not respond to His. ED50S for KCl are always higher than for other agonists. ED50S for 5-HT are lower than for other drugs in rats and cats, but not different in other species. Maximal GFS contractions induced by ACh are usually higher than those induced by other drugs.     

5-HT-mediated inhibition of cardiovagal baroreceptor reflex response during defense reaction in the rat

Previous studies showed that the cardiac response of the baroreceptor reflex (bradycardia) is inhibited during the defense reaction evoked by direct electrical or chemical stimulation of the periaqueductal gray (dPAG) in the rat. Whether central serotonin and nucleus tractus solitarius (NTS) serotonin(3) (5-HT(3)) receptors might participate in this inhibition was investigated in urethane-anesthetized and atenolol-pretreated rats. Our results showed that both electrical and chemical stimulation of the dPAG produced a drastic reduction of the cardiovagal component of the baroreceptor reflex triggered by either intravenous administration of phenylephrine or aortic nerve stimulation. This inhibitory effect of dPAG stimulation on the baroreflex bradycardia was not observed in rats that had been pretreated with p-chlorophenylalanine (300 mg/kg ip daily for 3 days) to inhibit serotonin synthesis. Subsequent 5-hydroxytryptophan administration (60 mg/kg ip), which was used to restore serotonin synthesis, allowed the inhibitory effect of dPAG stimulation on both aortic and phenylephrine-induced cardiac reflex responses to be recovered in p-chlorophenylalanine-pretreated rats. On the other hand, in nonpretreated rats, the inhibitory effect of dPAG stimulation on the cardiac baroreflex response could be markedly reduced by prior intra-NTS microinjection of granisetron, a 5-HT(3) receptor antagonist, or bicuculline, a GABA(A) receptor antagonist. These results show that serotonin plays a key role in the dPAG stimulation-induced inhibition of the cardiovagal baroreceptor reflex response. Moreover, they support the idea that 5-HT(3) and GABA(A) receptors in the NTS contribute downstream to the inhibition of the baroreflex response caused by dPAG stimulation.     

The Release of γ-Aminobutyric Acid, Glutamate, and Acetylcholine from Striatal Slices: A Mass Fragmentographic Study

Abstract: The release processes of endogenous Acetylcholine (ACh), γ-aminobutyric acid (GABA), glutamate (Glu) and glutamine (GLN) were studied in superfused guinea-pig caudatal slices. Basal ACh release remained constant for up to 2 h, while the basal release of GABA, Glu and GLN declined to half or less of its initial values after 1 h of superfusion. Electrical stimulation increased the ACh release by 700–800% and that of GABA by 80% whereas it decreased the output of Glu by 50% and failed to modify the GLN efflux. KCl (25 mM) increased the output of ACh by 400%, that of GABA by approximately 500% and decreased that of Glu by 40%. Substituting of CaCl₂ by MgCl₂ in the superfusion medium reduced the basal ACh release by 70% whereas no differences were observed in the basal efflux of GABA, Glu and GLN. Under these conditions, no evoked release of ACh or of GABA was detected, following electrical or KCl stimulation. Tetrodotoxin 5 × 10^-7 M decreased the basal ACh release by 60% and increased the GABA efflux by 40%. The toxin abolished the stimulus-evoked ACh efflux but scarcely affected that of GABA. These results are consistent with a possible neurotransmitter role of ACh and GABA in the striatum and show some differences in the ionic mechanisms underlying GABA and ACh release.     

Serotonin and forskolin enhance both magnitude of contraction and relaxation rate of Aplysia dorsal extrinsic muscle independently of acetylcholine receptor

Hexamethonium bromide (Hex. Br.) blocks acetylcholine (ACh) elicited contractions but not electrically elicited contractions of isolated preparations of Aplysia californica dorsal extrinsic muscle. Serotonin (5-hydroxytryptamine, 5-HT) enhances both magnitude and relaxation rate of ACh and electrically elicited contractions. In the presence of Hex. Br., 5-HT still exhibits its modulatory effects on electrically elicited contractions. Forskolin enhances both magnitude and relaxation rate of ACh and electrically elicited contractions. Forskolin (10(-5) M) increases the cyclic AMP content of the accessory radula closer and dorsal extrinsic muscles.     

Diurnal Variation in the Function of Serotonin Terminals in the Rat Hypothalamus

The high-affinity binding of [3H]imipramine is associated with the serotonin (5-hydroxytryptamine; 5-HT) transporter in the brain and in platelets. In the rat hypothalamus it has been reported that the density of these sites is increased in the dark period of the day, and this could result in an alteration in the release of 5-HT. The electrically evoked release of [3H]5-HT was thus studied in preloaded hypothalamic slices prepared from rats kept under 12:12 h light/dark or dark/light schedules. The fractional release of [3H]5-HT evoked by electrical stimulation, but not by the 5-HT releasing agent fenfluramine, was significantly decreased during the dark period when compared with the light period. The effects of the 5-HT reuptake blocker citalopram, of the two 5-HT autoreceptor agonists 5-methoxytryptamine and RU 24969, and of the 5-HT autoreceptor antagonist methiothepin on the release of [3H]5-HT were the same in both groups of rats. In conclusion, the release of [3H]5-HT from prelabelled rat hypothalamic slices is decreased during the dark period of the day. This modification is not reflected by changes in the effects of citalopram, an inhibitor of 5-HT reuptake, to modify the overflow of [3H]5-HT. The sensitivity and efficacy of agonists of the 5-HT autoreceptor are the same during the light and dark periods of the day.     

Antagonism of relaxation to isoproterenol caused by agonist interactions

The interaction of contractile agonists on the relaxation elicited with isoproterenol (ISO) was studied in 112 tracheal smooth muscle (TSM) strips from 20 dogs in vitro. Strips were contracted to the same active target tension (TT) with acetylcholine (ACh), histamine (HIS), serotonin (5-hydroxytryptamine, 5-HT), potassium chloride (KCl), or the combinations of ACh + HIS, ACh + 5-HT, HIS + KCl, HIS + 5-HT (50% TT from each agonist). Although a less potent agonist, adding HIS to cause 50% of the TT reduced the concentration of ACh to elicit the remaining 50% TT and substantially altered relaxation by ISO compared with HIS alone [concentration required to achieve 50% relaxation (RC50) = 9.2 +/- 2.4 X 10(-8) vs. 9.0 +/- 4.4 X 10(-9) M to HIS alone; P less than 0.003]. Relaxation for TSM strips contracted with ACh + HIS was comparable to that elicited from the same TT with ACh alone, although concentrations required in combination were lower than for either agonist alone. Trachealis strips contracted equivalently with KCl + HIS also had augmented contraction and attenuated relaxation (RC50 = 3.7 +/- 0.8 X 10(-8) M; P less than 0.015 vs. HIS alone). However, combinations of 5-HT + ACh and 5-HT + HIS did not alter relaxation to ISO from that elicited by the weaker agonist alone. We demonstrate that TSM relaxation depends on the combination of agonists eliciting contraction and may be inhibited substantially by interactions among contractile agonists.     

Calcium dependent release of acetylcholine and gamma-aminobutyric acid from the rabbit retina.

The concurrent release of endogenous ACh and GABA from the retina (in the presence of physostigmine) was measured using either an eye-cup preparation in rabbits anaesthetized with urethane or isolated rabbit retinas. There was a spontaneous resting release of ACh and GABA from the dark adapted retina of ca 5 and 160 pmol min-1 respectively. Stimulation of the initially dark adapted retina in vivo with flickering light (0.1-20 Hz) increased the release of ACh by up to 5 times the spontaneous resting release but did not cause a detectable increase in GABA release. The maximum light-evoked release of ACh was about 24 pmol min-1/retina and occurred at a frequency of 10 Hz. However, the maximum release of ACh per flash occurred at 0.1 Hz at which frequency the average ACh release per flash from one amacrine cell was ca 2.35 x 10(-18) mol. Exposure of the retina to the potent inhibitors of GABA uptake, SKF89976A and SKF100330A markedly reduced the resting release of ACh and abolished the light-evoked release of ACh but did not enable a light-evoked release of GABA to be detected. Bicuculline blocked the inhibitory actions of both SKF89976A and SKF100330A on ACh release but the combination of bicuculline and uptake inhibitor did not result in a light-evoked release of GABA. In contrast, KCl (20 mM) applied locally to the retina in vivo resulted in the release of both ACh and GABA (61 and 2.6-fold respectively). KCl (20 mM) also evoked large increases in ACh and GABA release from isolated rabbit retinas in room light (13.5 and 3.4-fold respectively). The K-evoked release of ACh and GABA from the rabbit retina both in vivo and in vitro was calcium dependent. These experiments are the first in which endogenous ACh and GABA release from the retina have been simultaneously measured and suggest that the release mechanisms for these transmitters are fundamentally similar.     

Role of serotonin receptors in panic-like behavior induced by nitric oxide in the rat dorsolateral periaqueductal gray: effects of chronic clomipramine treatment

Local administration of serotonin (5-HT) receptor agonists inhibits panic-like reactions induced by electrical stimulation of the rat dorsolateral periaqueductal grey (dlPAG). This anti-aversive effect is enhanced by chronic treatment with anti-panic drugs such as clomipramine. Since nitric oxide (NO) may mediate panic-like behavior in the dlPAG, we tested the hypothesis that chronic clomipramine treatment would also potentiate the effects of locally injected 5-HT-receptor agonists on panic-like reactions induced by intra-dlPAG injection of an NO-donor (SIN). After 21 days of daily i.p. injections of saline or clomipramine (10 mg/kg) male Wistar rats received local injections of saline, the 5-HT(1A)-receptor agonist 8-OH-DPAT (8 nmol) or the 5-HT2A/2C-receptor agonist DOI (16 nmol) followed by saline or SIN (150 nmol). NO-induced panic-like reactions were inhibited by DOI, but not by 8-OH-DPAT. Chronic clomipramine did not modify these effects but tended to produce anti-aversive effect by itself. In chronically clomipramine treated animals 8-OH-DPAT potentiated NO-induced panic-like reactions. The results indicate that the panic-like effects of NO in the dlPAG may be attenuated by 5-HT2A/2C-, but not by 5-HT1A-receptors. The anti-aversive effect of DOI is not modified by chronic clomipramine treatment.     

Type A and B gaba receptors mediate inhibition of acetylcholine release from cholinergic nerve terminals in the superior cervical ganglion of rat

The effects of γ-amino-n-butyric acid (GABA), (+)bicuculline, isoguvacine and 3-(4-chlorophenyl)-4-aminobutyrate [(±)baclofen] on the K-induced release of [³H]acetylcholine (ACh) were studied in the superior cervical ganglia of the rat in vitro. GABA and isoguvacine inhibited [³H]ACh release and these inhibitions were reversible by (+)bicuculline. Furthermore, the release of [³H]ACh was also inhibited by (±)baclofen. In receptor-binding studies, binding of [³H]GABA to membrane preparations from the superior cervical ganglia was inhibited by both (±)baclofen and (+)bicuculline. It is concluded that the inhibitory effect of GABA on the release of ACh can be mediated by GABA_A(bicuculline-sensitive) and by GABA_B (baclofen-activated) receptors. Our findings are compatible with the existence of a non-synaptic GABAergic inhibitory system involving GABA_A and GABA_B receptors on cholinergic nerve terminals in the superior cervical ganglion of rat.