Regulation of Histamine Turnover via Muscarinic and Nicotinic Receptors in the Brain

To clarify the regulation of central histaminergic (HAergic) activity by cholinergic receptors, the effects of drugs that stimulate the cholinergic system on brain histamine (HA) turnover were examined, in vivo, in mice and rats. The HA turnover was estimated from the accumulation of tele-methylhistamine (t-MH) during the 90-min period after administration of pargyline (65 mg/kg, i.p.). In the whole brain of mice, oxotremorine, at doses higher than 0.05 mg/kg, s.c., significantly inhibited the HA turnover, this effect being completely antagonized by atropine but not by methylatropine. A large dose of nicotine (10 mg/kg, s.c.) also significantly inhibited the HA turnover. This inhibitory effect was antagonized by mecamylamine but not by atropine or hexamethonium. A cholinesterase inhibitor, physostigmine, at doses higher than 0.1 mg/kg, s.c., significantly inhibited the HA turnover. This effect was antagonized by atropine but not at all by mecamylamine. None of these cholinergic antagonists used affected the steady-state t-MH level or HA turnover by themselves. In the rat brain, physostigmine (0.1 and 0.3 mg/kg, s.c.) also decreased the HA turnover. This inhibitory effect of physostigmine was especially marked in the striatum and cerebral cortex where muscarinic receptors are present in high density. Oxotremorine (0.2 mg/kg, s.c.) and nicotine (1 mg/kg, s.c.) also decreased the HA turnover in the rat brain. However, these effects showed no marked regional differences. These results suggest that the stimulation of central muscarinic receptors potently inhibits the HAergic activity in the brain and that strong stimulation of central nicotinic receptors can also induce a similar effect.     

Effect of gamma-aminobutyric acid on corticosterone secretion: involvement of the noradrenergic system.

The administration of gamma-aminobutyric acid (GABA) in the brain right lateral ventricle reduces serum corticosterone levels, and induces significant variations of hypothalamus biogenic amines in conscious male rats. After pretreatment with either alpha 1-adrenergic (prazosin) or alpha 2-adrenergic (yohimbine) blocking agents, the inhibitory effect of GABA on ACTH secretion was prevented. However, we observed that pretreatment with a beta-adrenergic blocking agent (propranolol), did not preventing the inhibitory effect of GABA on serum corticosterone levels. These results indicate that GABA has an inhibitory effect on ACTH secretion mediated by the activation of alpha 1 and alpha 2-adrenergic receptors.     

Characterization of N-[3H]Methylcarbamylcholine Binding Sites and Effect of N-Methylcarbamylcholine on Acetylcholine Release in Rat Brain

The present experiments show that N-[3H]-methylcarbamylcholine ([3H]MCC) binds specifically and with high affinity to rat hippocampus, frontal cortex, and striatum. The highest maximal density of binding sites was apparent in frontal cortex and the lowest in hippocampus. [3H]MCC binding was potently inhibited by nicotinic, but not muscarinic, agonists and by the nicotinic antagonist dihydro-beta-erythroidine in all three brain regions studied. The effect of unlabeled MCC on acetylcholine (ACh) release from slices of rat brain was tested. The drug significantly enhanced spontaneous ACh release from slices of hippocampus and frontal cortex, but not from striatal slices. This effect of MCC to increase ACh release from rat hippocampus and frontal cortex was antagonized by the nicotinic antagonists dihydro-beta-erythroidine and d-tubocurarine, but not by alpha-bungarotoxin or by the muscarinic antagonist atropine. The MCC-induced increase in spontaneous ACh release from hippocampal and frontal cortical slices was not affected by tetrodotoxin. The results suggest that MCC might alter cholinergic transmission in rat brain by a direct activation of presynaptic nicotinic receptors on the cholinergic terminals. That this alteration of ACh release is apparent in hippocampus and frontal cortex, but not in striatum, suggests that there may be a regional specificity in the regulation of ACh by nicotinic receptors in rat brain.     

Inhibition of associative long-term depression by activation of β-adrenergic receptors in rat hippocampal CA1 synapses

The aim of this study was to investigate the role of β-adrenergic receptors in modulating associative long-term depression (LTD) at CA1 synapses in rat hippocampal slices. Standard extracellular electrophysiological techniques were employed to record field excitatory post-synaptic potential (fEPSP) activity and to induce associative LTD. Two independent Schaffer collateral pathways were elicited in hippocampal CA1 areas. In one (weak) pathway, the stimulating intensity was adjusted to elicit small fEPSP activity (20–30% of the maximum response). In contrast, 80–90% of the maximum response was evoked in the other (strong) pathway. Associative LTD of weak pathway could be induced by paired stimulation of weak and the strong pathways, repeated 100 times at 0.167 Hz. The associative LTD of weak pathway was NMDA receptor- and phophatase 2B dependent, because bath application of 50 μM D, L-AP5 or 10 μM cypermethrin blocked its induction. Bath application of 1 μM isoproterenol inhibited associative LTD, and this effect was blocked by timolol, suggesting the involvement of β-adrenergic receptors. The inhibitory effect of β-adrenergic receptors on LTD induction was blocked in slices pretreated with inhibitors of protein kinase A and mitogen-activated protein kinase, suggesting that these signal cascades are downstream effectors following activation of β-adrenergic receptors. Nevertheless, bath application of timolol or cypermethrin alone did not have significant effect on associative LTD induction, suggesting neither endogenous function of β-adrenergic receptor nor endogenous PKA activity does have a role in associative LTD induction.     

beta-Adrenergic receptor subtypes in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus.

The subtype of beta-adrenergic receptors in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus was studied. Pigment of denervated melanophores in isolated, split caudal fins was preliminarily aggregated by incubating the specimens in a physiological saline containing 10 microM phentolamine and 30-100 microM verapamil or 2-10 nM melatonin, and the responses of the melanophores to a beta-adrenergic agonist added to the incubating medium were recorded photoelectrically. The beta-adrenergic agonists noradrenaline, adrenaline, isoproterenol, salbutamol and, dobutamine were all effective in evoking a dispersion of melanophore pigment in the presence of phentolamine and verapamil or melatonin. The pigment-dispersing effect of noradrenaline (beta 1-selective agonist) was inhibited by metoprolol (beta 1-selective antagonist), propranolol,- and butoxamine. Whereas, the effect of salbutamol (beta 2-selective agonist) was hardly inhibited by metoprolol, though it was considerably inhibited by propranolol and ICI-118551. It was estimated that beta 1- and beta 2-adrenergic receptors coexist at ratios of 8.6:91.4, in the melanophore of Tridentiger trigonocephalus, and 25:75, in the melanophore of Chasmichthys gulosus, through the analyses of Hofstee plots of the effects of the beta-adrenergic drugs. It was suggested that the relation between the pigment-dispersing effect of a beta-adrenergic agonist on the melanophores and the concentration of the drug follows mass action kinetics, when the effect is mainly caused by the activation of beta 2-adrenergic receptors of the melanophores. However, when it is mainly caused by the activation of beta 1-adrenergic receptors of the melanophores, the relation does not follow mass action kinetics.     

Cholinergic modulation of microglial activation by alpha 7 nicotinic receptors

Almost all degenerative diseases of the CNS are associated with chronic inflammation. A central step in this process is the activation of brain mononuclear phagocyte cells, called microglia. While it is recognized that healthy neurons and astrocytes regulate the magnitude of microglia-mediated innate immune responses and limit excessive CNS inflammation, the endogenous signals governing this process are not fully understood. In the peripheral nervous system, recent studies suggest that an endogenous 'cholinergic anti-inflammatory pathway' regulates systemic inflammatory responses via alpha 7 nicotinic acetylcholinergic receptors (nAChR) found on blood-borne macrophages. These data led us to investigate whether a similar cholinergic pathway exists in the brain that could regulate microglial activation. Here we report for the first time that cultured microglial cells express alpha 7 nAChR subunit as determined by RT-PCR, western blot, immunofluorescent, and immunohistochemistry analyses. Acetylcholine and nicotine pre-treatment inhibit lipopolysaccharide (LPS)-induced TNF-alpha release in murine-derived microglial cells, an effect attenuated by alpha 7 selective nicotinic antagonist, alpha-bungarotoxin. Furthermore, this inhibition appears to be mediated by a reduction in phosphorylation of p44/42 and p38 mitogen-activated protein kinase (MAPK). Though preliminary, our findings suggest the existence of a brain cholinergic pathway that regulates microglial activation through alpha 7 nicotinic receptors. Negative regulation of microglia activation may also represent additional mechanism underlying nicotine's reported neuroprotective properties.     

Non-adrenergic mechanisms of inhibition of the contractile activity of the cat small intestine by histamine, bradykinin, and met-enkephalin

I. a. histamine and bradykinin caused both an activation and an inhibition of jejunal contractile activity. The inhibitory effect was preserved after blockade of muscarinic cholinoreceptors, alpha- and beta-adrenoceptors, and abolished with the nicotinic cholinoceptor blockade. Metenkephalin inhibited the jejunal contractile activity after first activating it. The inhibitory effect of the peptide was preserved after blockade of alpha- and beta-adrenoceptors as well as the nicotinic cholinoceptors. The data obtained suggest that the non-adrenergic inhibitory effect of metenkephalin on intestinal contractions was the result of its depressing action on motor cholinergic neurons, whereas the inhibition of intestinal contractile activity with histamine and bradykinin resulted from their activating action on cholinergic interneurons which activate non-adrenergic inhibitory neurons through nicotinic cholinoceptors.     

Selective inhibition of β2-adrenergic receptor-mediated cAMP generation by activation of the P2Y2 receptor in mouse pineal gland tumor cells

Rhythmic noradrenergic signaling from the hypothalamic clock in the suprachiasmatic nucleus to the pineal gland causes an increase in intracellular cAMP which regulates the circadian fluctuation of melatonin synthesis. The activation of phospholipase C (PLC)-coupled P2Y(2) receptors upon treatment with ATP and UTP exclusively inhibited the isoproterenol-stimulated cAMP production in mouse pineal gland tumor cells. However, the activation of other PLC-coupled receptors including P2Y(1) and bombesin receptors had little or no effect on the isoproterenol-stimulated cAMP production. Also, ATP did not inhibit cAMP production caused by forskolin, prostaglandin E(2), or the adenosine analog NECA. These results suggest a selective coupling between signalings of P2Y(2) and beta(2)-adrenergic receptors. The binding of [(3)H]CGP12177 to beta(2)-adrenergic receptors was not effected by the presence of ATP or UTP. Ionomycin decreased the isoproterenol-stimulated cAMP production, whereas phorbol 12-myristate 13-acetate slightly potentiated the isoproterenol response. Chelation of intracellular Ca(2+), however, had little effect on the ATP-induced inhibition of cAMP production, while it completely reversed the ionomycin-induced inhibition. Treatment of cells with pertussis toxin almost completely blocked the inhibitory effect of nucleotides. Pertussis toxin also inhibited the nucleotide-induced increase in intracellular Ca(2+) and inositol 1,4,5-trisphosphate production by 30-40%, suggesting that the ATP-mediated inhibition of the cAMP generation and the partial activation of PLC are mediated by pertussis toxin-sensitive G(i)-protein. We conclude that one of the functions of P2Y(2) receptors on the pineal gland is the selective inhibition of beta-adrenergic receptor-mediated signaling pathways via the inhibitory G-proteins.     

A simple test for enhanced guanyl nucleotide exchange in brain adenylate cyclase systems activated by neurotransmitters

The mechanism of receptor-induced activation of adenylate cyclase has been proposed to involve an enhanced exchange of GDP for GTP. The kinetics of this process have not been investigated so far in the brain due to a spontaneous activation of the enzyme by guanyl nucleotides, which precludes the ability to follow receptor-dependent events. We show that it is possible to investigate the mechanism of receptor action in such systems by using a combination of guanosine 5'-(beta-gamma-imino)triphosphate (Gpp(NH)p) and guanosine 5'-(2-O-thio)diphosphate (GDP beta S). In pineal membranes, beta-adrenergic agonists increase the rate of adenylate cyclase activation by 10 or 100 microM Gpp(NH)p about 40-fold (0.023-0.9 min-1 kact) and decrease the inhibitory potency of GDP beta S nearly 1000-fold. As a result, 100 microM GDP beta S which blocks 90% of the activation by 10 microM Gpp(NH)p has no inhibitory effect in the presence of 10 microM Gpp(NH)p and 10 microM noradrenaline or isoproterenol. In caudate nucleus, dopamine does not appear to increase the rate of activation of adenylate cyclase by 10 microM Gpp(NH)p. Nevertheless, 100 microM GDP beta S blocks 90% of the activation by 10 microM Gpp(NH)p but has no inhibitory effects in the presence of dopamine. Thus, one can demonstrate that even weakly activating receptors have the capacity to facilitate a functional exchange of GDP beta S for Gpp(NH)p and measure the efficacy of the interaction between the receptor and the functionally linked guanyl nucleotide subunit.     

Chronic dietary administration of tryptophan prevents the development of deoxycorticosterone acetate salt induced hypertension in rats

Hypertension developed within 3 to 5 weeks in uninephrectomized rats administered deoxycorticosterone acetate (DOCA) at a dose of 850 micrograms X kg-1 X day-1 via Silastic tubes and given isotonic saline to drink. Chronic dietary administration of tryptophan (25 and 50 g/kg of food) to DOCA-treated rats reduced their exaggerated intake of NaCl solution and attenuated the elevation of blood pressure induced by treatment with DOCA alone. Treatment with tryptophan also protected against the reduction in urinary concentrating ability during a 24-h dehydration that is characteristic of DOCA-treated rats. Other tests assessed the responsiveness to the beta-adrenergic agonist, isoproterenol. These included measurement of drinking and heart rate following acute administration of isoproterenol. The characteristically depressed drinking and chronotropic responses of DOCA-treated rats to acute administration of isoproterenol were unaffected by tryptophan. Responsiveness to angiotensin II (AII) was also tested by assessment of dipsogenic and metabolic responses to acute administration of AII. The increased drinking and tail skin temperature responses to administration of AII, characteristic of DOCA-treated rats, were reduced in a graded fashion by treatment with graded doses of tryptophan. The specific binding of AII to its receptors in membranes form the diencephalon of the brain was increased by treatment with DOCA but was returned to control level by concomitant treatment with tryptophan. The content of serotonin in the mesencephalon of the brain was not changed significantly by treatment with tryptophan, but the content of 5-hydroxyindole acetic acid in the same region increased significantly, suggesting that turnover of serotonin was increased by chronic treatment with tryptophan. The cardiac hypertrophy characteristic of treatment with DOCA was attenuated significantly by chronic treatment with tryptophan, while the low, resting plasma renin activity of the DOCA-treated group was unchanged. These results suggest that tryptophan provides significant protection against the development of DOCA-induced hypertension, polydipsia, polyuria, and cardiac hypertrophy in rats. It also reduces the hyperresponsiveness to treatment with AII, possibly by decreasing the specific binding of AII to its receptors. It also appears to increase the turnover of serotonin in the brain. Whether either one or all of these is responsible for the antihypertensive effect of tryptophan remains for further study.     

Nicotinic Receptor-Mediated Release of Noradrenaline in the Human Neuroblastoma SH-SY5Y

Abstract: Dimethylphenylpiperazinium iodide (a nicotinic agonist) evokes noradrenaline release from human neuroblastoma SH-SY5Y cells that have been pretreated with 12- O -tetradecanoylphorbol 13-acetate for 8 min. This effect of dimethylphenylpiperazinium iodide was inhibited by 1 μ M mecamylamine but not by 1 μ M atropine, which suggests that SH-SY5Y cells express nicotinic receptors coupled to the release of noradrenaline. Dimethylphenylpiperazinium iodide-evoked release was enhanced by 5 μ M Bay K 8644 (an L-type calcium agonist) and inhibited by 1 μ M nifedipine. Dimethylphenylpiperazinium iodide depolarised SH-SY5Y cells and enhanced the level of intracellular calcium in cells loaded with fura 2. The effects of dimethylphenylpiperazinium iodide on noradrenaline release, depolarisation, and intracellular calcium levels were all inhibited by 1 μ M desmethylimipramine. The results of this study show that nicotinic receptors in SH-SY5Y cells stimulate noradrenaline release by activation of L-type calcium channels.     

Neosurugatoxin blocks nicotinic acetylcholine receptors in the brain

Neosurugatoxin, a neurotoxin isolated from the Japanese ivory mollusc ($\text{Babylonia japonica}$) is a nicotinic antagonist with a specificity towards ganglionic nicotinic receptors. At low concentration (5 × 10^?8 M) neosurugatoxin inhibited the release of [³H]dopamine evoked by 1,1-dimethyl-4-phenylpiperazinium (DMPP) from rat striatal nerve terminals, without affecting the response to K⁺-depolarisation. In contrast, αbungarotoxin did not antagonise the action of DMPP. Neosurugatoxin also inhibited [³H] nicotine binding to rat brain membranes but had no effect on [¹²⁵I]αbungarotoxin binding to the same tissue preparation. These results support the view that functional nicotinic receptors in the CNS resemble ganglionic nicotinic receptors. Neosurugatoxin has considerable potential as a useful probe for such receptors in the brain.     

Evidence that muscimol acts in the forced swimming test by activating the rat dopaminergic system

Muscimol as well as catecholaminergic drugs reduce immobility time in the forced swimming test. In view of the fact that GABAergic drugs may facilitate some brain catecholaminergic functions, we investigated as to whether or not muscimol would reduce immobility time through activation of catecholaminergic mechanisms. The effect of muscimol (2 mg/Kg i.p.) on reduction of immobility time was prevented by intraperitoneal alpha-methyl-para-tyrosine (250 mg/Kg i.p.), which reduces brain catecholamine content, haloperidol (0.5 mg/Kg) and sulpiride (100 and 50 mg/Kg), antidopaminergic drugs, and meta-chlorphenyl-piperazine (0.6 and 1.25 mg/Kg), a serotonergic agonist, but not by clonidine (0.1 mg/Kg), an alpha2-adrenoceptor agonist, d, 1-propranolol (5 mg/Kg), an antagonist of beta-adrenergic receptors, or subcutaneous prazosin (3 mg/Kg), an alpha1-adrenolytic drug. Our findings indicate that a) muscimol reduces immobility time by stimulating dopaminergic neurons and b) activation of the serotonergic system antagonizes muscimol effect.     

An antibody directed against residues 100-119 within the alpha-helical domain of Galpha(s) defines a novel contact site for beta-adrenergic receptors.

A polyclonal antiserum that recognizes residues 100-119 within the alpha-helical domain of Galpha(s) (K-20) caused a dissociation of G(s) into its component subunits and activated a cholera toxin-sensitive high affinity GTPase. Consistently, the antibody mimicked the stimulatory effects of the beta-adrenergic agonist, isoproterenol, on adenylyl cyclase, which is mediated by Galpha(s), and its inhibitory action on NADPH-dependent H(2)O(2) generation, a Gbetagamma-mediated response. A peptide corresponding to the target sequence of K-20 not only neutralized the receptor-mimetic effects of the antibody but inhibited the whole spectrum of isoproterenol action as well, including its antagonistic effects on adenylyl cyclase and NADPH-dependent H(2)O(2) generation. By contrast, COOH-terminal anti-Galpha(s) selectively inhibited the stimulatory effect of isoproterenol on cAMP formation without affecting its inhibitory effect on NADPH-dependent H(2)O(2) generation. The data are consistent with the concept that beta-adrenergic receptors interact with multiple sites on Galpha(s) each playing a distinct role, and strongly suggest that antibody K-20 defines a novel contact site for beta-adrenergic receptors that localizes to the alpha-helical domain and is essential for eliciting the complete spectrum of beta-adrenergic responses.     

Analysis of the role of adrenergic receptors in the regulation of small intestine motor activity in sheep

In sheep with chronic fistulae of the small intestine and rumen the participation of alpha- and beta-adrenergic receptors in the regulation of the motor activity of the small intestine was studied by the method of pharmacological analysis. The movements of the fistulated parts of the alimentary tract were recorded by the balloon method. Slow intravenous infusion of isoprenaline inhibited the contractions of the small intestine. This inhibitory effect of isoprenaline was abolished by propranolol. Intravenous phenylephrine inhibited the motor activity of this intestinal part as well. The effect of phenylephrine was abolished by pretreatment with dihydroergotamine. In the small intestine of sheep stimulation of the alpha and beta adrenergic receptors decrease the motor activity of intestine.     

Kyotorphin has a novel action on rat cardiac muscle

Several endogenous peptides for G-protein-coupled receptors have been found to play physiological roles in muscle contraction in addition to their well-demonstrated actions in other tissues. To further identify such peptides, we screened over 400 peptides using an isometric tension assay of rat papillary muscle. Here, we report that kyotorphin, which is known as an analgesic dipeptide, has a cardiac effect. Although kyotorphin had no effect on the twitch tension itself, it inhibited beta-adrenergic agonist isoprenaline-induced increases in twitch tension in a dose-dependent manner. Leu-Arg, a selective antagonist of kyotorphin, reversed this inhibitory effect. The inhibitory effect was also reversed by naloxone, an opioid receptor antagonist. These results suggest that kyotorphin may release opioid peptides from rat cardiac muscle and have an indirect regulatory role in beta-adrenergic action through cross-talk with opioid receptors.     

Pre-synaptic nicotinic and D2 receptors functionally interact on dopaminergic nerve endings of rat and mouse nucleus accumbens

The existence of pre-synaptic auto- and hetero receptors which modulate neurotransmitter release is well documented. Emerging evidence show that in some cases these pre-synaptic receptors may also cross-talk with each other. The aim of the present work was to investigate whether acetylcholine receptors (nAChRs) and dopamine (DA) autoreceptors, which are both able to modulate DA release, functionally interact on the same nerve endings. We used rat and mouse nucleus accumbens synaptosomes pre-labeled with [³H]DA and exposed to nicotinic and dopaminergic receptor ligands. Both nicotinic agonists and 4-aminopyridine (4-AP) provoked [³H]DA release which was inhibited by quinpirole and blocked by sulpiride and raclopride. Both the inhibitory effect of quinpirole and the stimulatory effect of (−)nicotine did not change when the nAChRs or the DA receptors were desensitized. (−)Nicotine and 4-AP were able to stimulate [³H]DA overflow also in mouse synaptosomes and this overflow was partially inhibited by quinpirole. In the β₂ knockout mice quinpirole was still able to inhibit the [³H]DA overflow elicited by 4-AP. To conclude: in rat and mouse the (−)nicotine evoked-release can be modulated by D₂/D₃ autoreceptors present on the DA terminals and nAChRs function is independent from D₂/D₃ autoreceptors which themselves may function independently from the activation of nAChRs.     

Evidence for a complex influence of nicotinic acetylcholine receptors on hippocampal serotonin release

The effects of nicotine on 5-hydroxytryptamine (5-HT) release from serotonergic nerve endings in rat dorsal hippocampal slices were studied. Nicotine (50-500 microM:) caused a concentration-dependent increase in 5-HT release. This effect was antagonised by mecamylamine (0.5 microM:), indicating an action at nicotinic receptors. Nicotine-evoked 5-HT release was not affected by tetrodotoxin (3 microM:), cadmium chloride (0.1 mM:), or the absence of Ca(2+) or Na(+) in the superfusion medium. Unexpectedly, higher concentrations of mecamylamine alone (1-50 microM:) increased 5-HT release. This suggested the presence of inhibitory input to 5-HT neurones and that these inhibitory neurones possess tonically active nicotinic receptors. The effect of mecamylamine (50 microM:) on 5-HT release was reduced by the muscarinic M(1) receptor agonist, McN-A-343 (100 microM:), but pirenzepine (0.005-1 microM:), which blocks M(1) receptors, alone increased 5-HT release. Hippocampal serotonergic neurones are known to possess both excitatory nicotinic receptors and inhibitory M(1) receptors. Although there may be several explanations for our results, one possible explanation is that nicotine stimulates 5-HT release by activating nicotinic heteroreceptors on 5-HT terminals. Mecamylamine (0.5 microM:) antagonises this effect, but higher concentrations increase 5-HT release indirectly by blocking the action of endogenous acetylcholine on nicotinic receptors situated on cholinergic neurones that provide muscarinic inhibitory input to 5-HT neurones.     

Activation of the inhibitory GTP-binding protein of adenylate cyclase, Gi, by beta-adrenergic receptors in reconstituted phospholipid vesicles

beta-Adrenergic receptors and the inhibitory GTP-binding protein, Gi of the adenylate cyclase system were reconstituted into phospholipid vesicles by the method described previously for reconstituting receptors and the stimulatory GTP-binding protein, Gs (Brandt, D. R., Asano, T., Pedersen, S. E., and Ross, E. M. (1983) Biochemistry 22, 4357-4362). In the receptor-Gi vesicles, beta-adrenergic agonists stimulated both the high-affinity binding of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) to Gi and GTPase activity to an extent similar to that observed in vesicles containing beta-adrenergic receptors and Gs. Stimulation required receptors and displayed appropriate beta-adrenergic specificity. The prior treatment of receptor-Gi vesicles with islet-activating protein (pertussis toxin) plus NAD markedly inhibited both the isoproterenol-stimulated binding of GTP gamma S and the isoproterenol-stimulated GTPase activity. No contamination of Gi by Gs was apparent. These data suggest that receptors that typically stimulate adenylate cyclase activity may also activate the inhibitory system, perhaps as one mechanism of desensitization.     

Serotonin blocks the facilitatory action of muscarinic and nicotinic agents in the hippocampus in vivo

The inhibitory effect of serotonin, released iontophoretically, on acetylcholine-induced facilitation of population spikes evoked by fimbria-commissural stimulation was studied in the CA1 region of rat hippocampus in vivo. After serotonin was applied for 2.6 +/- 0.8 min, acetylcholine's action was inhibited in 39 cases out of 57 (68.4%), by 68.9 +/- 23.1%, irrespective of whether serotonin alone increased or reduced the population spike. Spiperone, used as a 5-hydroxytryptamine1A (5-HT1A) antagonist, suppressed the inhibitory action of serotonin in 14 of 21 tests. Serotonin had similar effects on population spike facilitations induced by acetyl-beta-methylcholine and dimethylphenylpiperazinium. Thus serotonin, probably acting on 5-HT1A receptors, blocks effectively but indiscriminately all cholinergic facilitations, whether mediated by nicotinic or muscarinic receptors.