The correlation between hypothalamic and midbrain biogenic amine content and the intensity of the emotional-defensive arousal in cats

Changes in levels of noradrenaline (NA), dopamine (DA), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the medial hypothalamus and the midbrain central grey matter were determined and correlated with various levels of an emotional-defensive arousal in cats, evoked by an intrahypothalamic injection of carbachol. At the initial stage of excitation, 3 min after the injection of carbachol, the levels of NA, DA, and 5-HIAA in the medial hypothalamus increased, whereas in the central grey matter only the 5-HIAA level was elevated. During the maximum excitation (when the vocalization attained the peak level) the level of amines and 5-HIAA decreased in the medial hypothalamus; in the central grey matter only the levels of 5-HT and 5-HIAA declined. In the final phase of response, 5 min after the disappearance of vocalization, the 5-HT level in both structures increased. These results confirm the notion that the noradrenergic, dopaminergic and serotonergic systems participate in the central regulation of the emotional-defensive behavior, and show that the hypothalamus and midbrain play different roles in this regulation. The results also indicate that neurochemical mechanisms are similar in different types of emotional-defensive behavior. It seems that catecholaminergic systems play a stimulating role, while the serotonergic system inhibits the carbachol-induced defensive response.     

Pharmacological activity of N-methyl-carbamylcholine, a novel acetylcholine receptor agonist with selectivity for nicotinic receptors

N-Methyl-carbamylcholine (also called N-methyl-carbachol) is an analogue of the mixed muscarinic-nicotinic acetylcholine receptor agonist, carbachol. Previous studies have provided evidence that radiolabelled N-methyl-carbachol can bind selectively to nicotinic acetylcholine receptors in rat brain. To determine whether N-methyl-carbachol acts as an agonist or an antagonist at nicotine and/or muscarinic receptor sites, the present study examined the pharmacological activity of this compound on some cholinergically innervated tissues. N-Methyl-carbachol, like carbachol, depolarized rat isolated sympathetic ganglia and these effects were inhibited by a nicotinic antagonist, d-tubocurarine, but not by a muscarinic antagonist, atropine. Exposure of rat sympathetic ganglia to N-methyl-carbachol blocked the compound action potential generated in ganglia by stimulation of the pre-ganglionic trunk; this effect of N-methyl-carbachol was likely due to desensitization of the nicotinic response. N-Methyl-carbochol, like carbachol, stimulated the release of [3H]noradrenaline from cultured adrenal medullary cells that had been pre-loaded with [3H]noradrenaline; these effects were largely inhibited by a nicotinic antagonist, mecamylamine, while atropine produced less blockade. N-Methyl-carbachol contracted the frog isolated rectus abdominis muscle and the effect was completely blocked by d-tubocurarine. By contrast, contracture of the rectus abdominis produced by carbachol was partially inhibited by either atropine or d-tubocurarine. N-Methyl-carbachol, like carbachol, contracted the rat isolated ileum and these effects were completely blocked by atropine; however, N-methyl-carbachol was about 42 times less potent than carbachol for this effect. Intravenous injection of N-methyl-carbachol, like nicotine, to the rat produced a transient decrease followed by a more sustained rise in blood pressure while carbachol produced only a sustained decrease in blood pressure. The effects of N-methyl-carbachol and nicotine on blood pressure were blocked by pretreatment of the animal with a nicotinic antagonist, hexamethonium. N-methyl-carbachol, like nicotine, stimulated the release of [3H]dopamine from rat striatal synaptosomes, pre-loaded with [3H]dopamine; release induced by either N-methyl-carbachol or nicotine was inhibited by mecamylamine but not by atropine. In rat cerebral cortical slices pre-loaded with [3H]inositol, carbachol, but not N-methyl-carbachol, stimulated the accumulation of [3H]inositol-1-phosphate, an effect blocked by atropine but not by mecamylamine.(ABSTRACT TRUNCATED AT 400 WORDS)     

中枢N受体对M受体介导体温降低作用的调节

在清醒大鼠上，可进中枢的Ｎ受体拮抗剂美加明可对抗Ｎ受体激动剂烟碱降低体温的作用，增强Ｍ受体激动剂氧化震颤素降低体温的作用；不进中枢的外周Ｎ受体拮抗剂六甲溴铵不影响烟碱和氧化震颤素的上述作用．每天３次ｓｃ烟碱２．５ｍｇ·ｋｇ－１，连续７ｄ，大鼠对烟碱降低体温的作用产生慢性耐受后，氧化震颤素降低体温的作用无显著变化．提示以体温变化为指标，美加明封闭中枢Ｎ受体功能后，中枢Ｍ受体对其激动剂的敏感性增强；反复给予烟碱诱导中枢Ｎ受体功能失敏后，中枢Ｍ受体对其激动剂的敏感性无显著变化．     

Studies of carbachol-induced wet-dog shake behavior in rats

Intraventricular administration of carbachol chloride evoked wet-dog shakes (WDS) in rats in a dose-related manner. WDS induced by carbachol at the dose of 20 g were antagonized by scopolamine, atropine, cyproheptadine, morphine, clonidine, phentolamine, haloperidol, and l-5-hydroxytryptophan (5-HTP).Methergoline, propranolol, bicuculline, and aminooxyacetic acid had no effect on carbachol-induced shaking behavior. The present experiments show the existence of different types of shaking behavior, not exclusively related to the stimulation of central 5-HT structures.     

Chronic administration of clomipramine prevents the increase in serotonin and noradrenaline induced by chronic stress

The effects of chronic clomipramine administration (15 mg/kg daily for 23 days) on changes in serotonin (5-hydroxytryptamine, 5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and noradrenaline (NA) induced by chronic stress have been studied in the rat brain. Chronic stress increased 5-HT in midbrain, pons and hippocampus, 5-HIAA in frontal cortex, midbrain, pons and hippocampus, and NA in midbrain and striatum. Chronic clomipramine significantly decreased the levels of 5-HT in most regions. In hypothalamus, hippocampus and perhaps in frontal cortex this effect possibly reflects decreased synthesis caused by an action on presynaptic 5-HT receptors. However, in midbrain, pons and striatum decreased 5-HT could not be attributed to a decrease in its synthesis since 5-HIAA also increased. This drug treatment also reduced NA in all regions except the striatum. Nevertheless, conclusions on NA synthesis or turnover cannot be drawn since only NA levels were measured. When administered concurrently, chronic clomipramine prevented the increases in 5-HT, 5-HIAA and NA produced by chronic stress. These results are in good accordance with previous findings showing that chronic antidepressant treatment also prevented behavioural disturbances induced by chronic stress.     

Absence of 5-HT3 and cholinergic mechanisms in improgan antinociception

Improgan, an analgesic derived from histamine antagonists, acts in the brain stem to activate descending non-opioid, pain-relieving circuits, but the mechanism of action of this drug remains elusive. Because improgan has a moderate affinity for 5-HT(3) receptors, and, since cholinergic and serotonergic drugs can modulate descending analgesic circuits, roles for 5-HT(3), nicotinic and muscarinic receptors in improgan antinociception were presently investigated in rats. Improgan (80 microg, icv) induced nearly maximal inhibition of hot plate and tail flick nociceptive responses, and these actions we unaffected by antagonists of muscarinic (atropine, 5.9 mg/kg, i.p.) and nicotinic (mecamylamine, 2 mg/kg, i.p.) receptors. Control experiments verified that these antagonist treatments were maximally effective against muscarinic and nicotinic antinociception in both tests. In addition, improgan antinociception was unaffected by icv pretreatment with a 5-HT(3) antagonist (ondansetron, 20 microg). When given alone, icv treatment with neither this antagonist nor a 5-HT(3) agonist (m-chlorophenylbiguanide, 1000 nmol, icv) modified thermal nociceptive latencies. These results show no role for supraspinal cholinergic and 5-HT(3) receptors in improgan antinociception. The findings help to narrow the search for the relevant mediators of the action of this novel analgesic agent.     

Pressor response to posterior hypothalamic administration of carbachol is mediated by muscarinic M3 receptor.

Unilateral microinjection of the acetylcholine receptor agonist carbachol into the posterior hypothalamic nucleus evokes a pressor response in the conscious, freely moving rat. To further localize this response 3.3 or 5.5 nmol of carbachol was microinjected in a volume of 50 nl directly into and outside the region of the posterior hypothalamic nucleus. Administration of carbachol outside the posterior hypothalamic nucleus failed to evoke a change in blood pressure indicating that the carbachol-induced pressor response is mediated from the posterior hypothalamic nucleus. Since posterior hypothalamic administration of atropine completely blocks the carbachol-induced increase in blood pressure and atropine blocks the three pharmacologically identified muscarinic receptor subtypes, methylatropine and progressively more selective muscarinic antagonists were administered into the posterior hypothalamic nucleus prior to 5.5 nmol of carbachol. Microinjection of the M1/M2/M3 muscarinic antagonist methylatropine (0.19-12.5 nmol), the M1/M3 antagonist 4-DAMP (4-diphenylacetoxy-N-methylpiperidine; 0.9-3.6 nmol), the M1 antagonist pirenzepine (9.5-38 nmol), the M2 antagonist methoctramine (5.5-44 nmol), or the M3 antagonist p-F-HHSiD (para-fluoro-hexahydro-sila-difenidol; 2.1-8.3 nmol) inhibited the peak increase in mean arterial pressure and the area under the curve of the change in mean arterial pressure versus time plot in a dose-dependent manner. Log ID50s calculated for the antagonists from the dose-response curves were found to correlate significantly with the log Kis of the antagonists for the muscarinic M3 receptor subtype. These results demonstrate that the increase in mean arterial pressure evoked by microinjection of carbachol into the posterior hypothalamic nucleus is mediated by the muscarinic M3 receptor.     

Differential effects of neuroleptic and serotonergic drugs on amphetamine-induced hypothermia in mice

Intraventricular administration of amphetamine in mice produced hypothermia. Pretreatment with the dopaminergic (DA) receptor antagonist haloperidol reduced this response, whereas pretreatment with pimozide, sulpiride or cis-flupentixol did not. The direct DA agonist apomorphine strongly potentiated the hypothermia. Pretreatment with the serotonergic (5-HT) receptor blocker cyproheptadine also potentiated the hypothermia. Depletion of 5-HT in brain by p-chlorophenylalanine and accumulation of 5-HT induced by fluoxetine had no effect. In contrast, stimulation of 5-HT receptors by quipazine reduced the hypothermic effect of amphetamine. The inhibitor of catecholamine synthesis alpha-methyl-p-tyrosine, the alpha-adrenergic blocker phentolamine and the muscarinic antagonist atropine failed to alter the hypothermia. It was concluded that DA and 5-HT mechanisms are involved in amphetamine-induced hypothermia in mice and that these two systems display a functional antagonism.     

The effect of niaprazine on the turnover of 5-hydroxytryptamine in the rat brain

Niaprazine (60 mg/kg i.p.) increased rat brain 5-hydroxyindole acetic acid (5-HIAA) concentrations 30 min after treatment, and reduced them at 3-8 hr after treatment. Rat brain 5-hydroxytryptamine (5-HT) levels were unchanged. Niaprazine also produced a short-lasting depletion of rat brain noradrenaline (NA) and dopamine (DA). Pretreatment with alpha-phenyl-alpha-propyl-benzeneacetic acid, 2-(diethylamino) ethyl ester hydrochloride (SKF 525A) (75 mg/kg i.p.) potentiated the increase in 5-HIAA and depletion of catecholamines produced 1 hr after niaprazine, but abolished the reduction in 5-HIAA produced 8 hr after the drug. This suggested that a metabolite might be responsible for the delayed reduction in 5-HIAA levels. A potential metabolite, p-fluoro-phenylpiperazine (FPP) (5-40 mg/kg i.p.) reduced rat brain 5-HIAA and 3,4-dihydroxyphenyl acetic acid (DOPAC), and inhibited 5-HT and NA uptake in vitro. Unlike niaprazine, FPP produced no behavioural sedation, but in large doses produced a behavioural syndrome indicative of serotonergic stimulation. Studies of the metabolism of 14C-niaprazine in rats indicated the presence of a urinary metabolite with the same chromatographic characteristics as FPP. These results suggest that niaprazine itself depletes brain catecholamines and increases 5-HT turnover, while a metabolite, FPP, subsequently reduces the turnover of 5-HT and DA.     

Impaired performance of rats in the Morris swim-maize test late in abstinence following long-term sodium barbital treatment

Rats were tested for place learning in the Morris swim maze on days 110-114 of abstinence following 48 weeks of treatment with sodium barbital. A retarded acquisition of the swim-maze task, that could not be ascribed to motor impairments, was found in the barbital-treated rats. There was a significant difference in brain weight, but there were no significant differences between the control and barbital-treated rats in the frontal cortical concentrations of noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), nor in the intra- and extrasynaptosomal activities of cerebral cortical monoamine oxidase towards NA and 5-HT. Postsynaptically, neither the cerebral cortical inositol phospholipid breakdown responses to carbachol and NA (mediated by muscarinic and alpha 1-adrenergic receptors, respectively), nor the striatal and cortical densities of muscarinic receptors labelled by [3H]quinuclidinyl benzilate [( 3H]QNB) were found significantly to be altered in the barbital-treated rats. A strong correlation between the density of striatal and cortical [3H]QNB binding sites was seen for the barbital-treated (r = 0.91) but not for the control (r = -0.05) rats. It is suggested that the deficit in performance of the barbital-treated rats in the Morris maze may be related to a cholinergic dysfunction.     

Natriuresis, kaliuresis and antidiuresis induced by central carbachol injection are mediated by muscarinic M1 receptors

The present study investigated the effect of selective muscarinic antagonists on natriuresis, kaliuresis and antidiuresis induced by intracerebroventricular (i.c.v.) injection of carbachol in the rat. The muscarinic antagonists were given by i.c.v. injection 1 min before carbachol (1 microgram/rat). 4-Diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP), a rather selective M1 and M3 receptor antagonist, was the most potent inhibitor of carbachol-induced natriuresis, kaliuresis and antidiuresis, its ID50 being respectively 0.12, 0.04 and 0.56 nmol/rat. Pirenzepine, a selective M1 antagonist, potently inhibited the above mentioned carbachol effects, its ID50 being 1.85, 3.25 and 1.49 nmol/rat, respectively. On the other hand, the M2-selective antagonist methoctramine and the M3-selective antagonist p-fluoro-hexahydro-sila-difenidol were very weak inhibitors. Methoctramine at doses up to 60 nmol/rat produced non statistically significant inhibition of carbachol-induced natriuresis, kaliuresis and antidiuresis. Para-fluoro-hexahydro-sila-diphenidol showed an ID50 of 64.4 nmol/rat on carbachol-induced natriuresis, while at the maximum dose employed, 100 nmol/rat, the inhibition of carbachol-induced kaliuresis and antidiuresis was lower than 50%. The rank order of potency of the antagonists tested proved to be related to their pA2 values for muscarinic M1 receptors, suggesting that this receptor subtype mediates the central effects of cholinergic mechanisms on water and electrolyte excretion.     

Functional and molecular identification of 5-hydroxytryptamine receptors in rabbit pulmonary artery: involvement in complex regulation of noradrenaline release

The rabbit pulmonary artery was used to examine whether presynaptic serotonin (5-HT) receptors modulate noradrenaline (NA) release also in this blood vessel and to confirm their presence with molecular biology techniques. Arteries preincubated with [3H]NA were superfused in the presence of the alpha2-adrenoceptor blocker rauwolscine and the effects of 5-HT receptor ligands on the electrically evoked 3H overflow were determined. The 5-HT4 receptor agonist cisapride inhibited 3H overflow in a manner sensitive to blockade by atropine. The 5-HT1B/1D receptor agonist 5-carboxamidotryptamine inhibited 3H overflow only in the presence of atropine. The 5-HT4 and 5-HT1B/1D receptor agonists 5-HT and 5-methoxytryptamine reduced 3H overflow in the absence and presence of atropine, and this effect was blocked by methiothepin, a non-selective 5-HT receptor antagonist, in the presence of atropine. PCR with cDNAs derived from reverse transcribed blood vessel mRNA suggested the expression of the 5-HT1B, 5-HT1D and 5-HT4 receptors, the latter being highly homologous to the human one. In conclusion, the cholinergic nerves are endowed with excitatory 5-HT4 receptors mediating release of acetylcholine which, in turn, activates muscarine receptors on the sympathetic nerves leading to inhibition of NA release. Blockade of the presynaptic muscarinic receptors involved is necessary to disclose an inhibition of NA release via 5-HT1B/1D receptors. Taking results reported in the literature into account, the 5-HT1D and 5-HT4 receptors identified by molecular biology techniques probably are located predominantly on the noradrenergic and cholinergic neurons, respectively.     

Suppression of pentylenetetrazol-induced seizures by hydralazine associated with 5-hydroxytryptaminergic system in rat brain

The effects of hydralazine on the central nervous system were studied in rats. Administration of hydralazine (10 mg/kg, i.p.) transiently, but significantly suppressed the seizures elicited by pentylenetetrazol (PTZ). The suppressive actions were potentiated in the animals pretreated with either reserpine or p-chlorophenylalanine, but not alpha-methyltyrosine. Methysergide, an antagonist of 5-hydroxytryptamine (5-HT) receptors, could abolish the effect of hydralazine on the tonic component of the seizures, but unlikely that on the clonic one. Although 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels in the brain were both significantly increased after the administration of hydralazine, the increased levels of 5-HIAA reached the peak level earlier than those of 5-HT did. In 5-HT turnover, hydralazine did not change the 5-HT synthesis rate, but the drug inhibited the elimination of 5-HIAA from the brain. The accumulation of 5-HIAA after the inhibition of the acid transport system by probenecid was transiently, but significantly increased in the animals treated with hydralazine. The potency of the suppressive effects of hydralazine on PTZ-induced seizures was in parallel with the rate of 5-HIAA formation in the brain. These results suggest that hydralazine might antagonize the PTZ-induced seizures at least partly by modulating the activation in the central 5-HT-ergic system.     

The effects of cinepazide on the content, biosynthesis and turnover of noradrenaline, dopamine and 5-hydroxytryptamine in the rat brain under room air and hypoxia

The effects of cinepazide, a vasodilator, on the content, biosynthesis and turnover of noradrenaline (NA), dopamine (DA) and 5-hydroxytryptamine (5-HT) in the rat brain were examined under room air and hypoxia (10% O2, 90% N2). Under room air, cinepazide had no significant effects on the content of NA, DA, 5-HT and 5-hydroxyindoleacetic acid (5-HIAA), the accumulation of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) after central decarboxylase inhibition, and the depletion of NA, DA and 5-HT after synthesis inhibition. After 2 hr-exposure to hypoxia, the content of NA, 5-HT and 5-HIAA was decreased, whereas the content of DA was unchanged. The accumulation of DOPA and 5-HTP was decreased. The depletion of DA and 5-HT was inhibited by hypoxia, whereas the depletion of NA was unaffected. Under hypoxic conditions, cinepazide had no effects on the content of NA, DA and 5-HT, the accumulation of DOPA and 5-HTP, and the depletion of NA and DA, whereas cinepazide increased both the rate of 5-HT depletion and the content of 5-HIAA. The present data suggest that cinepazide selectively stimulates the functional activities of 5-HT neurons in the brain, which are depressed by hypoxia.     

Is pindolol a mixed agonist-antagonist at central serotonin (5-HT) receptors?

The effects of the non-selective beta-adrenergic blocking agent pindolol upon central monoaminergic neurotransmission in rats were studied by means of in vivo biochemical methods. It was found that (-)-pindolol elicited a clearcut, selective, dose-dependent and stereospecific reduction of brain 5-HT synthesis rate. The synthesis reduction was not accompanied by any change in the tissue tryptophan levels and could not be prevented by depleting the monoamine stores by means of reserpine. Furthermore, in non-pretreated animals, (-)- but not (+)-pindolol (nor the beta 1- and beta 2-selective adrenoceptor antagonists metoprolol, betaxolol and ICI 118,551) decreased the 5-HIAA level and the 5-HIAA/5-HT ratio while neither enantiomer altered the concentrations of 5-HT or NA, or DA and its metabolites. It is suggested that these effects of (-)-pindolol may be due to direct stimulation of 5-HT receptors in the CNS. The action of the compound is discussed within the context of literature data indicating its ability to act as an antagonist of certain other aspects of 5-HT receptor activation. The possibility is considered that, in addition to its beta-adrenergic properties, (-)-pindolol is a mixed agonist-antagonist at central 5-HT receptors.     

Nicotinic acetylcholine receptor (nACh-R) agonist-induced changes in brain monoamine turnover in mice

The aim of the present study was to evaluate the effects of nicotinic acetylcholine receptor (nACh-R) agonists such as (−)-nicotine and related compounds on brain monoamine turnover. A single administration of (−)-nicotine (0.04, 0.2, 1.0, and 5.0 mg/kg SC) increased both noradrenaline (NA) and dopamine (DA) turnover in a dose-dependent manner, and the maximum effects were achieved 30 min after treatment with (−)-nicotine (1.0 mg/kg). The effect of (−)-nicotine on serotonin (5-HT) turnover was complicated; 5-HT turnover was increased at a low dose of (−)-nicotine (0.04 mg/kg) but decreased at a high dose (1.0 mg/kg). The (−)-nicotine (1.0 mg/kg)-induced changes in monoamine turnover were blocked by pretreatment with the centrally acting nACh-R channel blocker mecamylamine (2.0 mg/kg IP) but not by hexamethonium (2.0 mg/kg IP). These findings indicate that systemically administered (−)-nicotine can enhance brain NA and DA turnover and affect 5-HT turnover, both of which are mediated by central nACh-R. The changes in the monoamine turnover induced by (±)-anabasine were similar to those induced by (−)-nicotine, while (−)-lobeline and (−)-cytisine had little effect, and 1,1-dimethyl-4-phenyl-piperazinium (DMPP) increased NA and 5-HT turnover but not DA turnover at all doses tested. (S)-3-Methyl-5-(1-methyl-2-pyrrolidinyl)isoxazole (ABT-418), a selective neuronal nACh-R agonist, increased NA, DA and 5-HT turnover, but had a weaker effect on DA turnover than NA and 5-HT turnover. In addition, 9-amino-1,2,3,4-tetrahydroacridine (THA), an acetylcholine esterase inhibitor, also increased monoamine turnover in the brain. Pretreatment with mecamylamine completely blocked the THA-induced increase in NA and 5-HT turnover, but not in DA turnover, suggesting that the nACh-R system is involved in the THA-induced increase in brain NA and 5-HT turnover. On the other hand, (−)-cytisine, a partial agonist for the β2 subunit containing nACh-R, completely inhibited the nACh-R agonist-and THA-induced increases in NA turnover, but not in DA turnover, and normalized the changes in 5-HT turnover. In conclusion, the subtypes of nACh-Rs mediating DA turnover may be different from those mediating NA and 5-HT turnover in the CNS. Received: 3 April 1996 /Final version: 18 June 1996     

Carbachol activates a novel sodium current in isolated guinea pig ventricular myocytes via M2 muscarinic receptors.

Carbachol induces a novel tetrodotoxin-resistant Na+ current in guinea pig ventricular myocytes bathed in Tyrode's solution with 20 mM Cs+. This action of carbachol, which initiates a series of reactions that culminates in a catecholamine-independent positive inotropic effect, occurs through muscarinic rather than nicotinic cholinoceptive sites. The concentrations of muscarinic antagonists required to suppress the carbachol-induced current by 50% were 2.1 nM, 270 nM, and 1700 nM for atropine, AF-DX 116, and pirenzepine, respectively. These results indicate that an M2-selective antagonist, AF-DX 116, is more potent than an M1-selective antagonist, pirenzepine, as an inhibitor. The M1-selective agonist McN-A-343 did not induce an inward current and blocked that caused by carbachol, in a rapid and reversible manner. This finding is also consistent with the conclusion that the muscarinic receptor involved in the regulation of myocardial Na+ channels by carbachol cannot be distinguished from the M2 subtype of such receptors. Treatment with pertussis toxin did not affect the ability of carbachol to induce an inward current in ventricular myocytes and reversed the current activated by carbachol in atrial cells from outward to inward. The electrophysiological and pharmacological nature of the carbachol-induced current in ventricular myocytes is very similar to that of the acetylcholine-induced current in Xenopus oocytes transfected with porcine M2, but not M1, muscarinic receptors. In both preparations, Na+ is the dominant charge carrier, intracellular Ca2+ is not involved in opening the Na+ channel, and an M2 receptor is involved.     

cGMP formation and phosphoinositide turnover in rat brain slices are mediated by pharmacologically distinct muscarinic acetylcholine receptors.

The cGMP response and the accumulation of inositol monophosphate (IP) induced by carbachol were compared in slices of different rat brain structures. Basal cGMP and the responses of cGMP to carbachol appeared dependent on the concentration of added Ca2+, suggesting that distinct Ca(2+)-mediated and Ca(2+)-sensitive muscarinic receptor-mediated mechanisms stimulate guanylate cyclase. Regional responses of cGMP to carbachol or to direct stimulation of guanylate cyclase with sodium nitroprusside were markedly distinct, indicating that a major proportion of guanylate cyclase in the cortex, an intermediate proportion in other forebrain regions, and only a minor proportion in the brainstem is sensitive to muscarinic receptor stimulation. The regional patterns of IP and cGMP responses to carbachol were different in the forebrain. Maximal IP accumulation was found in the cortex, whereas cGMP responses were highest in the hippocampus. Moreover, IP and cGMP formation in the hippocampus were differently antagonized by atropine, 4-diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP), the M2-receptor subtype-preferring antagonist AF-DX 116 and the M1-selective antagonist pirenzepine. These data support the notion that the IP formation induced by carbachol in the forebrain predominantly is mediated by muscarinic receptors of the M1 subtype, and indicate the involvement of muscarinic receptors of the M3 subtype in the carbachol-induced cGMP formation.     

The carbachol-induced enhancement of desynchronized sleep signs is dose dependent and antagonized by centrally administered atropine

Considerable data show that microinjection of carbachol into the pontine reticular formation produces a desynchronized (D) sleep-like state. The present study examined the hypothesis that this carbachol-induced enhancement of D sleep signs is mediated by muscarinic, cholinergic receptors. This hypothesis was tested by quantifying the dose-dependent effects of centrally administered carbachol on the D sleep-like state and by pretreating the animals with centrally administered atropine. Six dosages of carbachol were microinjected into the pontine reticular formation of conscious cats and polygraphic measures of behavioral state were recorded. The percentage, latency, duration, frequency, and time course of the carbachol-induced D sleep-like state were dose dependent. Centrally administered atropine competitively antagonized the ability of carbachol to induce the D sleep-like state, whereas pontine administration of L-glutamate did not significantly alter D sleep. These data demonstrate that muscarinic, cholinergic receptors within the pontine reticular formation mediate the phenomenon of cholinoceptive D sleep sign enhancement.