m1-m5 muscarinic receptor distribution in rat CNS by RT-PCR and HPLC

Intrathecal administration of liposomal neostigmine prolongs analgesia in mice

BACKGROUND

There is substantial evidence that cholinomimetic drugs increase pain threshold. However, the profound side effects of these agents have limited their clinical use either as analgesics or as analgesic adjuncts. A delivery system that would assure a slow and sustained drug release may be of value in ameliorating the problem of untoward effects.

METHODS

The acetylcholinesterase inhibitor neostigmine was encapsulated into multilamellar lipid vesicles composed of phosphocholine and cholesterol. Three doses of plain or liposomal neostigmine were administered by the intrathecal route to mice (n=8-10/group), and analgesic duration was quantified by tail flick test. The doses were chosen based on preliminary experiments, which showed the maximum tolerated intrathecal doses of plain and liposomal neostigmine formulation were 0.625 microg and 80 microg, respectively. Two other doses for each formulation were then derived by 1:1 serial dilutions. Results were compared using survival analysis.

RESULTS

The median antinociceptive duration for plain neostigmine was 0.33, 0.99 and 1.0 h for the 0.115, 0.312 and 0.625 microg doses, respectively. For the liposomal formulation, the median antinociceptive duration was 1.0, 1.5 and 6.0 h for the 20, 40 and 80 microg doses, respectively.

CONCLUSIONS

Liposomal neostigmine provides prolonged spinal antinociception, and permits the safe administration of a relatively large dose, because drug is gradually released from the liposomal depot. This technology holds promise for the development of a clinically useful neostigmine formulation to provide spinal analgesia.

RT-PCR reveals muscarinic acetylcholine receptor mRNA in the pre-Bötzinger complex

Muscarinic receptors mediate the postsynaptic excitatory effects of acetylcholine (ACh) on inspiratory neurons in the pre-Bötzinger complex (pre-BötC), the hypothesized site for respiratory rhythm generation. Because pharmacological tools for identifying the subtypes of the muscarinic receptors that underlie these effects are limited, we probed for mRNA for these receptors in the pre-BötC. We used RT-PCR to determine the expression of muscarinic receptor subtypes in tissue punches of the pre-BötC taken from rat medullary slices. Cholinergic receptor subtype M(2) and M(3) mRNAs were observed in the first round of PCR amplification. All five subtypes, M(1)-M(5), were observed in the second round of amplification. Our results suggest that the majority of muscarinic receptor subtypes in the pre-BötC are M(2) and M(3), with minor expression of M(1), M(4), and M(5).

Cholinergic dilation of cerebral blood vessels is abolished in M(5) muscarinic acetylcholine receptor knockout mice

The M(5) muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M(1)-M(5)) to be cloned. At present, the physiological relevance of this receptor subtype remains unknown, primarily because of its low expression levels and the lack of M(5) receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M(5) receptor-deficient mice (M5R(-/-) mice). M5R(-/-) mice did not differ from their wild-type littermates in various behavioral and pharmacologic tests. However, in vitro neurotransmitter release experiments showed that M(5) receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M(5) receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M(5) receptors led to changes in vascular tone by using several in vivo and in vitro vascular preparations. Strikingly, acetylcholine, a powerful dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5R(-/-) mice. This effect was specific for cerebral blood vessels, because acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5R(-/-) mice. Our findings provide direct evidence that M(5) muscarinic receptors are physiologically relevant. Because it has been suggested that impaired cholinergic dilation of cerebral blood vessels may play a role in the pathophysiology of Alzheimer's disease and focal cerebral ischemia, cerebrovascular M(5) receptors may represent an attractive therapeutic target.

Neurotransmitters in the thalamus relaying visceral input to the insular cortex in the rat

Neurotransmitters relaying ascending visceral information were examined by comparing the response of neurons in the insular cortex to vagal stimulation (0.8 Hz, 2 mA) before and after neurotransmitter antagonist injections (200 nl) in the ventroposterior parvocellular nucleus of the thalamus (VPpc). Cobalt (10 mM; presynaptic blocker) and kynurenate (100 microM; nonspecific excitatory amino acid antagonist) injections in the VPpc resulted in an attenuation (73-100 and 38-98%, respectively) of the evoked cortical response. Injections of the specific N-methyl-D-aspartate (NMDA) antagonist DL-2-amino-5-phosphonopentanoic acid (200 microM and 2 mM) did not affect the vagally evoked response, whereas the nonspecific non-NMDA antagonist L-glutamic acid diethylester (200 microM) attenuated the vagally evoked response by 66-100%. Three concentrations of the DL-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA)-specific antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 and 200 microM and 2 mM) attenuated the vagally evoked cortical response by 29 +/- 9, 31 +/- 10, and 59 +/- 8%, respectively. The more selective AMPA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (200 microM and 2 mM) inhibited the vagally evoked cortical response by 53 +/- 8 and 52 +/- 3%, respectively. Phentolamine (0.1 and 1.0 microM), a general alpha-adrenergic antagonist, and picrotoxin (0.1 and 1.0 microM), a GABA(A) antagonist, did not affect the vagally evoked response. Atropine, a muscarinic cholinergic antagonist, decreased the vagally evoked response by 40 +/- 2% at a concentration of 0.1 microM, but a higher concentration of 1.0 microM had no effect. These results indicate that the non-NMDA excitatory amino acid receptor is necessary for the relay of visceral information in the VPpc. Muscarinic receptors may modulate visceral neuronal excitability in the VPpc, although the exact interaction between the inhibitory (m2) and excitatory (m3 or m5) muscarinic receptor types found in the thalamus is not known.

Cholinergic systems and schizophrenia: primary pathology or epiphenomena?

Post mortem schizophrenia research has been driven first by the dopamine and then the glutamate hypotheses. These hypotheses posit primary pathology in pathways dependent upon dopamine or glutamate neurotransmission. Although the dopamine and glutamate hypotheses retain considerable theoretical strength, neurobiological findings of altered dopamine or glutamate activity in schizophrenia do not explain all features of this disorder. A more synthetic approach would suggest that focal pathological change in either the prefrontal cortex or mesial temporal lobe leads to neurochemical changes in multiple neurotransmitter systems. Despite the limited experimental evidence for abnormal cholinergic neurotransmission in psychiatric disorders, increased understanding of the role of acetylcholine in the human brain and its relationship to other neurotransmitter systems has led to a rapidly growing interest in the cholinergic system in schizophrenia. This review focuses on the basic anatomy of the mammalian cholinergic system, and its possible involvement in the neurobiology of schizophrenia. Summaries of cholinergic cell groups, projection pathways, and receptor systems, in the primate and human brain, are followed by a brief discussion of the functional correlations between aberrant cholinergic neurotransmission and the signs and symptoms of schizophrenia.

Acetylcholine becomes the major excitatory neurotransmitter in the hypothalamus in vitro in the absence of glutamate excitation

Glutamate and GABA are two major fast neurotransmitters (excitatory and inhibitory, respectively) in the CNS, including the hypothalamus. They play a key role in the control of excitation/inhibition balance and determine the activity and excitability of neurons in many neuronal circuits. Using neuronal cultures, whole-cell recording, Ca(2+) imaging, and Northern blots, we studied the compensatory regulation of neuronal activity during a prolonged decrease in glutamate excitation. We report here that after a chronic (6-17 d) blockade of ionotropic glutamate receptors, neurons in hypothalamic cultures revealed excitatory electrical and Ca(2+) synaptic activity, which was not elicited in the control cultures that were not subjected to glutamate blockade. This activity was suppressed with acetylcholine (ACh) receptor antagonists and was potentiated by eserine, an inhibitor of acetylcholinesterase, suggesting its cholinergic nature. The upregulation of ACh receptors and the contribution of ACh to the control of the excitation/inhibition balance in cultures after a prolonged decrease in glutamate activity were also demonstrated. Enhanced ACh transmission was also found in chronically blocked cerebellar but not cortical cultures, suggesting the region-specific character of glutamate-ACh interactions in the brain. We believe that in the absence of glutamate excitation in the hypothalamus in vitro, ACh, a neurotransmitter normally exhibiting only weak activity in the hypothalamus, becomes the major excitatory neurotransmitter and supports the excitation/inhibition balance. The increase in excitatory ACh transmission during a decrease in glutamate excitation may represent a novel form of neuronal plasticity that regulates activity and excitability of neurons during the glutamate/GABA imbalance.

Nippostrongylus brasiliensis: infection induces upregulation of acetylcholinesterase activity on rat intestinal epithelial cells

Expression of cholines terases and muscarinic acetylcholine receptors in the jejunal mucosa has been investigated during infection of rats with the nematode parasite Nippostrongylus brasiliensis. Selective expression of m3 receptors was observed on epithelial cells from uninfected rats and animals 7 days postinfection, and saturation binding with [(3)H]quinuclidinyl benzilate indicated that receptor expression on cell membranes was unaltered by infection. Butyrylcholinesterase was highly expressed in mucosal epithelia, but acetylcholinesterase was present at low levels in uninfected animals. In contrast, discrete foci of intense acetylcholinesterase activity were observed on the basement membrane of intestinal epithelial cells in animals infected with N. brasiliensis. This was demonstrated to be due to upregulation of expression of endogenous enzyme, which peaked at Day 10 postinfection and subsequently declined to preinfection levels. It is suggested that this occurs in response to hyper-activation of the enteric nervous system as a result of infection, and may benefit the host by limiting excessive fluid secretion due to cholinergic stimulation.

Protractive effects of chronic treatment with an acutely sub-toxic regimen of diisopropylflurophosphate on the expression of cholinergic receptor densities in rats

Individuals chronically exposed to low levels of organophosphate insecticides may present with subtle impairments in cognition. In addition, low level diisopropylflurophosphate (DFP) exposure (0.25 mg/kg per day for 2 weeks) in rats resulted in protracted working memory impairment [29]. The current studies attempt to show a temporal relationship between the DFP-induced impairment in performance of a spatial memory task and the protracted decrease in the expression of cholinergic receptors and acetylcholinesterase in specific brain regions. Cholinergic receptors labeled with the ligands [(3)H]epibatidine and [(3)H]AFDX-384 were affected to a much greater extent and for a longer period of time than were both acetylcholinesterase activities and cholinergic receptors labeled with [(3)H]QNB. Pre-testing administration of nicotine was shown to completely reverse this DFP-induced impairment in memory-related task performance. Additionally, prophylaxis with pyridostigmine bromide (PB) caused DFP-treated animals to exhibit near normal levels of memory-related task performance. These results are consistent with the development of a protracted phase of learning impairment to sub-acute DFP exposure, which may involve the loss of hippocampal nicotinic receptors, and may be prevented or reversed by PB or nicotine, respectively.

Effects of the selective M1 muscarinic receptor antagonist dicyclomine on emotional memory

The nonselective muscarinic antagonist scopolamine is known to impair the acquisition of some learning tasks such as inhibitory avoidance. There has been recent research into the effects of this drug in contextual fear conditioning and tone fear conditioning paradigms. The purpose of the present study was to assess the role of the selective M1 muscarinic antagonist dicyclomine in these paradigms and in the inhibitory avoidance test. Rats were administered different doses of dicyclomine or saline 30 min before acquisition training. The animals were tested 24 hr later, and it was observed that 16 mg/kg of dicyclomine impaired both contextual fear conditioning and inhibitory avoidance. However, dicyclomine (up to 64 mg/kg) did not affect tone fear conditioning. These results suggest that the selective M1 muscarinic antagonist dicyclomine differentially affects aversively motivated tasks known to be dependent on hippocampal integrity (such as contextual fear conditioning and inhibitory avoidance) but does not affect similar hippocampus-independent tasks.

Selective blockade of muscarinic receptor subtypes in the brain stem reticular formation in rats: effects on anesthetic requirements

Muscarinic involvement in the modulation of general anesthesia was examined in the rat with a cannula implanted in the pontine reticular formation. Atropine microinjected into the reticular formation reversed the minimum alveolar concentration (MAC) reducing effect of carbachol on halothane anesthesia, but M(1) or M(3) antagonist had no effect. An M(2) antagonist reduced the MAC of halothane following saline and carbachol. The results suggest that any of the muscarinic receptor subtypes in this region do not independently mediate the cholinomimetic effect on halothane anesthesia.

Adaptive changes in M1 muscarinic receptors localized to specific rostral brain regions during and after morphine withdrawal

Morphine-dependent rats were allowed to undergo withdrawal by abrupt discontinuation of the drug. The regional expression of brain M1 muscarinic receptors was measured directly by autoradiographic determination with [(3)H] pirenzepine, and indirectly by quantifying the relative levels of M1 mRNA encoding the receptor protein. Patterns of receptor changes after morphine treatment were in general agreement using the two methods. Frontal cortical samples derived from morphine-dependent rats exhibited a 28% increase in M1 receptor mRNA measured at the end of the infusion. At the peak of the withdrawal, M1 mRNA levels for dependent rats were much lower (33.4%) than those for control rats. Hippocampal samples derived from morphine-dependent rats exhibited no changes in M1 mRNA levels after the morphine infusion. During the peak of withdrawal, however, hippocampal M1 mRNA levels were reduced (57%) compared with levels for controls. The M1 mRNA levels remained at this reduced degree of expression even after withdrawal symptoms had subsided. Addition of diisopropylflurophophate (DFP) to the morphine infusion schedule inhibited the adaptive changes in M1 mRNA levels induced by morphine. During the peak period of withdrawal, M1 mRNA levels in the hippocampus declined by only 18% as compared with 57% for the morphine control group. The adaptive decrease in hippocampal M1 receptors after withdrawal subsided may reflect prolonged heightened cholinergic activity in an area where such cholinergic innervation plays an important role in memory.

Involvement of M3 muscarinic receptors of the spinal cord in formalin-induced nociception in mice

Subcutaneous injection of formalin into a paw of mice caused two distinct phases of licking and biting, first phase (1-5 min) and the second phase (7-30 min) after the injection. The muscarinic antagonist atropine (0.1-10 ng, i.t.) and the M(3) receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) (0.1-20 ng, i.t.) inhibited the second phase of this response, whereas higher doses of atropine (20-100 ng, i.t.) did not cause inhibition. The M(1) muscarinic receptor antagonist pirenzepine (10-100 ng, i.t.) did not inhibit either the first or the second phase response, but a high dose of pirenzepine (1000 ng, i.t.) tended to inhibit the second phase response. On the other hand, the M(2) muscarinic receptor antagonist 11-¿(2-[(diethylamino)methyl]-1-piperidinyl¿acetyl)-5, 11-dihydro-6H-pyrido(2,3-b)(1,4)benzodiazepine-6-one (AF-DX116; 10-1000 ng, i.t.) had no effect on either the first or the second phase of response. The opioid receptor antagonist naloxone did not affect the 4-DAMP-induced anti-nociceptive response. The i.t. injection of the acetylcholinesterase inhibitor neostigmine (25 ng) significantly inhibited only the second phase. The acetylcholine (ACh) depletor hemicholinium-3 (HC-3) (1 microg, i.t.) completely abolished the 4-DAMP-induced anti-nociceptive response. The ACh content of the spinal cord was significantly increased 14 min after formalin injection. This significant increase in the ACh content was inhibited by pretreatment with 4-DAMP (10 ng, i.t.). These results suggest that endogenous ACh in the spinal cord acts as a transmitter anti-nociception, and that ACh release regulated by presynaptic M(3) muscarinic receptors in the spinal cord is involved in the second phase of nociception induced by formalin.

Spontaneously hypertensive rats cholinergic hyper-responsiveness: central and peripheral pharmacological mechanisms

1. The mechanisms and the subtypes of muscarinic receptors implicated in the cardiovascular effects of physostigmine were investigated in conscious normotensive and spontaneously hypertensive rats. 2. Intravenous (i.v.) physostigmine (50 microg kg-1) induced in both strains a long pressor response, accompanied by a bradycardia. This pressor response was larger in spontaneously hypertensive (+41+/-6 mmHg) than in Wistar-Kyoto (+25+/-2 mmHg) rats (P<0.05). 3. Pretreatment with atropine sulphate (0.4 mg kg-1 i.v.), completely abolished the physostigmine-induced pressor response in both normotensive and hypertensive rats. In both strains, the physostigmine pressor response was significantly reduced by the systemic administration of either an alpha1-adrenoceptor antagonist (prazosin, 1 mg kg-1) or a V1A-vasopressin receptor antagonist (AVPX, 20 microg kg-1). This physostigmine pressor effect was completely abolished in both strains when both antagonists were administered concomitantly. 4. In WKY rats, the pressor response to physostigmine (50 microg kg-1 i.v.) was inhibited in a dose-dependent manner by i. c.v. administration of atropine (ID50=3.70 nmoles), the M1 receptor antagonist pirenzepine (ID50=10.71 nmoles), the M2 receptor antagonist methoctramine (ID50=4.31 nmoles), the M3 receptor antagonist p-F-HHSiD (ID50=60.52 nmoles) and the M4 receptor antagonist tropicamide (ID50=214.20 nmoles). In the hypertensive strain, the ID50 were found to be significantly higher for atropine (7.34 nmoles), pirenzepine (21.60 nmoles) and p-F-HHSiD (139.50 nmoles) (P<0.05). 5. The present results indicate that physostigmine acts in normotensive and spontaneously hypertensive rats, through stimulation of both central M2 and M1 cholinoceptors to induce a rise in blood pressure mediated by an increase in plasma vasopressin and sympathetic outflow. Moreover, our results suggest that some modifications of the M1 receptor subtypes in terms of expression or affinity could be responsible for the hyper-responsiveness of the hypertensive strain to cholinomimetic agents.

Amnesic effects of the anticholinergic drugs, trihexyphenidyl and biperiden: differences in binding properties to the brain muscarinic receptor

An amnesic effect of anticholinergic drugs was previously described from several behavioral studies. We examined this effect induced by trihexyphenidyl and biperiden, clinically used in the parkinsonism and schizophrenic patients, by using passive avoidance tasks. Both of these drugs (0.1-10 mg/kg, s.c.) showed dose-dependent amnesic effects in the acquisition and retrieval phases. However, the effect induced by trihexyphenidyl was transient, whereas that of biperiden was long-lasting. To clarify the reason for the different duration of the amnesic activity, binding to the muscarinic receptor was examined. In the Scatchard analysis, trihexyphenidyl competed with [(3)H]quinuclidinyl benzilate ([(3)H]QNB) on the muscarinic receptor (showed increased K(d) and unchanged B(max) value), while biperiden decreased [(3)H]QNB binding (B(max) value) significantly. Furthermore, in an exchange assay for receptor inactivation, trihexyphenidyl binding to muscarinic receptors was exchanged by [(3)H]QNB completely, but biperiden decreased the exchangeable binding of [(3)H]QNB in a dose dependent manner (0.1-100 nM). These results suggested that the binding of trihexyphenidyl and biperiden to muscarinic receptor might be completely reversible and partially irreversible, respectively, whereas the K(i) values of these two drugs were similar. In conclusion, this difference in binding property may explain the difference in the time-course of the amnesic effect induced by trihexyphenidyl and biperiden.

Rapid eye movement sleep induction by vasoactive intestinal peptide infused into the oral pontine tegmentum of the rat may involve muscarinic receptors

In rats, rapid eye movement sleep can be induced by microinjection of either the cholinergic agonist carbachol or the neuropeptide vasoactive intestinal peptide into the oral pontine reticular nucleus. Possible involvement of cholinergic mechanisms in the effect of vasoactive intestinal peptide was investigated using muscarinic receptor ligands. Sleep-waking cycles were analysed after infusion into the oral pontine reticular nucleus of vasoactive intestinal peptide (10 ng in 0.1 microl), carbachol (20 ng), atropine (200 ng) and pirenzepine (50, 100 ng), performed separately or in combination at 15-min intervals. The increase in rapid eye movement sleep due to the combined infusion of vasoactive intestinal peptide and carbachol (+58.7+/-4.6% for 8 h, P<0.05) was not significantly different from that induced by each compound separately. The enhancement of rapid eye movement sleep by vasoactive intestinal peptide was totally prevented by infusion of atropine, but not pirenzepine, a relatively selective M1 antagonist. On their own, none of the latter two compounds affected the sleep-waking cycle. Quantitative autoradiographic studies using [3H]quinuclidinyl benzylate (1 nM) and pirenzepine (0.5 microM) indicated that muscarinic receptors correspond to pirenzepine-insensitive binding sites in the oral pontine reticular nucleus. In vitro, vasoactive intestinal peptide (1-100 nM) significantly increased (+30-40%) the specific binding of [3H]quinuclidinyl benzylate to the oral pontine reticular nucleus in rat brain sections. This effect appeared to be due to an increased density, with no change in affinity, of pirenzepine-insensitive binding sites in this area. These data suggest that pirenzepine-insensitive muscarinic binding sites are involved in the induction of rapid eye movement sleep by vasoactive intestinal peptide at the pontine level in the rat.

Antinociception after both peripheral and intrathecal injection of oxotremorine is modulated by spinal nitric oxide

The present study investigated the role of spinal nitric oxide (NO) in the antinociception induced by intraperitoneal (i.p.) and intrathecal (i.th.) injection of oxotremorine. The experiments were carried out on male Wistar rats, which had cannulas chronically implanted in the lumbar enlargement of the spinal cord. Antinociceptive effects were evaluated using a tail-flick and a paw pressure test. To raise the spinal NO level, the rats received the NO donor, 3-morpholino-sydnonimine (SIN-1, 10 and 100 microg/5 microl); to lower the NO level, the inhibitor of NO synthase, N-nitro-L-arginine methyl ester (L-NAME, 50 and 400 microg/5 microl), was administered. Both those substances were injected i.th. Systemic injections of oxotremorine (0.02 and 0.1 mg/kg) produced a significant increase in the thermal nociceptive threshold, while the mechanical threshold was affected only by the higher dose (0.1 mg/kg) of the muscarinic agonist. I.th. injections of oxotremorine (0.1 ng, 1 ng, 1 microg/5 microl) produced significant antinociception in both those tests. I.th. administration of SIN-1 in doses which themselves did not affect the nociceptive threshold antagonized both the peripheral and central oxotremorine antinociception. I.th. administration of L-NAME (50 and 400 microg/5 microl) did not change the nociceptive threshold, but dose-dependently potentiated the effects of oxotremorine injected i.p. in both tests; however, the effect of i.th. administration of oxotremorine was potentiated only in the tail-flick test. Our results demonstrate that irrespective of the way of its injection, the antinociceptive effect of oxotremorine is modulated by activity of the spinal NO. Moreover, our results further support the hypothesis that NO present in the spinal cord exerts pronociceptive effects.

The effect of pregnancy and contractile activity on bladder muscarinic receptor subtypes

In the rabbit bladder, pregnancy and prolonged bladder contractions decrease both muscarinic receptor density and contractile response, whereas newborns show enhanced muscarinic contractile response. Although the M(2) receptor predominates in rabbit bladder, we and others have shown that the affinity of a series of subtype selective muscarinic antagonists for inhibition of muscarinic agonist-induced contractions is most consistent with the pharmacologically defined M(3) receptor directly mediating smooth muscle contraction. Bladders from fetal rabbits, gravid rabbits, and male rabbits exposed to 4 hr of induced spontaneous contractions were used to determine whether changes in receptor density and contractility are due to a selective decrease in either the M(2) or M(3) muscarinic receptor subtype. To determine organ specificity, the heart and uterus were also studied. Gravid rabbits of 3 weeks' gestation and their fetal rabbits were studied. In male rabbits, bladder contractions were induced for 4 hr by ligating the catheterized penis at its base. Muscarinic receptor density and subtype distribution were determined by radioligand binding and immunoprecipitation. Receptor density was 24% lower in gravid bladder body, unchanged in gravid bladder base, 54% lower in gravid uterus, 115% higher in fetal bladders, and 34% lower after induced bladder contractions. Immunoprecipitation showed greater M(2) receptors than M(3) in all tissues studied, whereas M(l) and M(4) receptors were undetectable. There was no difference from control in the ratio of M(2) to M(3) receptor in any tissues except that a greater proportion of M(3) receptors was found in male vs. female bladders. Changes in contractile response to cholinergic stimulation in the gravid, fetal, and experimental detrusor instability model are associated with changes in total receptor density and not solely with changes in the M(3) receptor subtype that mediates bladder smooth muscle contraction. Neurourol. Urodynam. 18:511-520, 1999.

Prevention of morphine-induced muscarinic (M2) receptor adaptation suppresses the expression of withdrawal symptoms

Treatment of opiate addiction is generally directed at the suppression of withdrawal symptoms through maintenance of the 'addicted' state with methadone. Yet relatively little is known regarding the neural substrates that contribute to, and maintain the prolonged state of withdrawal experienced by addicts. Opiates can profoundly alter the dynamics of brain and peripheral cholinergic systems, and central administration of anticholinergic drugs in dependent rats has been shown to decrease the expression of precipitated withdrawal symptoms. The purpose of this study was to determine whether the adaptive changes to M2 muscarinic receptors in autonomic centers are linked to the expression of withdrawal phenomena. During the peak period of withdrawal, there was a significant increase in both the expression of M2 muscarinic receptors and its corresponding mRNA within the rostral ventrolateral medulla, a primary vasomotor region. That most of these changes in receptor expression were adaptive in nature was suggested by the fact that when the acetylcholinesterase inhibitor DFP was co-administered with morphine, both the increased mRNA expression and the appearance of withdrawal symptoms were inhibited. Thus, interference with morphine-induced M2 muscarinic receptor adaptation in critical brain regions was correlated with a reduction in the development of physical dependence.

The effects of brucine and alcuronium on the inhibition of [3H]acetylcholine release from rat striatum by muscarinic receptor agonists

1. Radioligand binding experiments indicate that the affinity of muscarinic receptors for their agonists may be enhanced by allosteric modulators. We have now investigated if brucine can enhance the inhibitory effects of muscarinic receptor agonists on the electrically evoked release of [3H]acetylcholine ([3H]ACh) from superfused slices of rat striatum. 2. The evoked release of [3H]ACh was inhibited by all agonists tested (i.e., furmethide, oxotremorine-M, bethanechol and oxotremorine). 3. Brucine enhanced the inhibitory effects of furmethide, oxotremorine-M and bethanechol on the evoked [3H]ACh release without altering the inhibitory effect of oxotremorine. 4. Alcuronium was applied for comparison and found to diminish the inhibitory effect of furmethide on the evoked [3H]ACh release. 5. The results demonstrate that it is possible both to enhance and diminish the functional effects of muscarinic receptor agonists by allosteric modulators. 6. The direction of the observed effects of brucine and alcuronium on [3H]ACh release fully agrees with the effects of these modulators on the affinities of human M4 receptors for furmethide, oxotremorine-M, bethanechol and oxotremorine, as described by Jakubik et al. (1997). This supports the view that the presynaptic muscarinic receptors responsible for the autoinhibition of ACh release in rat striatum belong to the M4 muscarinic receptor subtype.

Virus- and interferon-induced loss of inhibitory M2 muscarinic receptor function and gene expression in cultured airway parasympathetic neurons

Viral infections increase vagally mediated reflex bronchoconstriction. Decreased function of inhibitory M2 muscarinic receptors on the parasympathetic nerve endings is likely to contribute to increased acetylcholine release. In this study, we used cultured airway parasympathetic neurons to determine the effects of parainfluenza virus and of interferon (IFN)-gamma on acetylcholine release, inhibitory M2 receptor function, and M2 receptor gene expression. In control cultures, electrically stimulated acetylcholine release increased when the inhibitory M2 receptors were blocked using atropine (10(-)5 M) and decreased when these receptors were stimulated using methacholine (10(-)5 M). Acetylcholine release was increased by viral infection and by treatment with IFN-gamma (300 U/ml). In these cells, atropine did not further potentiate, nor did methacholine inhibit, acetylcholine release, suggesting decreased inhibitory M2 receptor function and/or expression. Using a competitive reverse transcription-polymerase chain reaction method, we demonstrated that M2 receptor gene expression was decreased by more that an order of magnitude both by virus infection and by treatment with IFN. Thus, viral infections may increase vagally mediated bronchoconstriction both by directly inhibiting M2 receptor gene expression and by causing release of IFN-gamma which inhibits M2 receptor gene expression.

Pharmacological profile of CEB-1957 and atropine toward brain muscarinic receptors and comparative study of their efficacy against sarin poisoning

This study consists of two parts, first to compare the pharmacological profile of atropine and CEB-1957 substance toward muscarinic receptor subtypes. In various rat brain structures, binding properties were determined by competition experiments of [3H]pirenzepine, [3H]AF-DX 384, and [3H]4-DAMP in quantitative autoradiography of M1, M2, and M3 muscarinic receptor subtypes, respectively. Competition curves have shown that atropine presents similar nanomolar inhibition constants toward each subtype, while CEB-1957 has distinct affinities (Ki from 0.26 to 73 nM) with the following range order: M3 > or = M2 > M1. The second part is to compare atropine and CEB-1957 (in combination with pralidoxime) for their ability to protect against the lethality induced by 2 x LD50 of the acetylcholinesterase inhibitor sarin. CEB-1957 reduced the mortality at doses 10 times lower than atropine. Finally, from these results, it is proposed that a selective blockade of M2 and M3 receptor subtypes could play a pivotal role in the protective effect against sarin poisoning.

Telenzepine-sensitive muscarinic receptors on rat pancreatic acinar cells

To identify the muscarinic subtype present on the rat pancreatic acinar cell, we examined the effects of different muscarinic receptor antagonists on amylase secretion and proteolytic zymogen processing in isolated rat pancreatic acini. Maximal zymogen processing required a concentration of carbachol 10- to 100-fold greater (10(-3) M) than that required for maximal amylase secretion (10(-5) M). Although both secretion and conversion were inhibited by the M3 antagonist 4-diphenylacetoxy-N-methyl-piperidine (4-DAMP) (50% inhibition approximately 6 x 10(-7) M and 1 x 10(-8) M, respectively), the most potent inhibitor was the M1 antagonist telenzepine (50% inhibition approximately 5 x 10(-10) M and 1 x 10(-11) M, respectively). Pirenzepine, another M1 antagonist, and the M2 antagonist methoctramine did not reduce amylase secretion or zymogen processing in concentrations up to 1 x 10(-5) M. Analysis of acinar cell muscarinic receptor by PCR revealed expression of both m1 and m3 subtypes. The pancreatic acinar cell has a distinct pattern of muscarinic antagonist sensitivity (telenzepine >> 4-DAMP > pirenzepine) with respect to both amylase secretion and zymogen conversion.

Expression of cholinergic markers in the pons of Flinders rats

The Flinders sensitive line of rats (FSL rats) have an altered REM sleep pattern which includes a shorter REM sleep latency and an increased percentage of REM sleep [R.M. Benca, D.H. Overstreet, M.A. Gilliland, D. Russell, B.M. Bergmann, W.H. Obermeyer, Increased basal REM sleep but no difference in dark induction or light suppression of REM sleep in Flinders rats with cholinergic supersensitivity, Neuropsychopharmacology 15 (1996) 45-51; P.J. Shiromani, D.H. Overstreet, D. Levy, C.A. Goodrich, S.A. Campbell, J. C. Gillin, Increased REM sleep in rats selectively bred for cholinergic hyperactivity, Neuropsychopharmacology 1 (1988) 127-133]. Cholinergic mechanisms have been implicated in REM sleep generation [reviewed in P.J. Shiromani, J.C. Gillin, S.J. Henriksen, Acetylcholine and the regulation of REM sleep: basic mechanisms and clinical implication for affective illness and narcolepsy, Annu. Rev. Pharmacol. Toxicol. 27 (1987) 137-156]. In the present study, specific aspects of the cholinergic system were examined in the pontine region of the FSL rats. The number of cholinergic neurons in the LDT and PPT were not different in FSL and control rats. Analysis of steady state levels of mRNAs encoding the acetylcholine synthesizing protein, choline acetyltransferase (ChAT) or the m2, m3 and m5 muscarinic receptor subtypes were also comparable in FSL and control rats. These data raise the possibility that the cellular events underlying the altered REM sleep pattern in FSL rats may include mechanisms that effect the muscarinic or nicotinic receptor in the pons.

Nicotine administration decreases the number of binding sites and mRNA of M1 and M2 muscarinic receptors in specific brain regions of rat neonates

Nicotine has been shown to delay the developmental increase of muscarinic receptors in brain of rat neonates. In this study, we have examined the muscarinic receptor binding sites and corresponding messenger RNA in rat neonates, whose mothers received nicotine in the drinking water, using competition binding study and northern blot analysis. On postnatal 14th day, nicotine treatment led to a 61% reduction in the proportion of high-affinity sites for pirenzepine (M1-subtype) to total [3H]quinuclidinyl benzilate binding sites in cerebral cortex in rat neonate. Nicotine treatment also decreased that for 11-2[[2-[(diethylamino)methyl]-1-piperidinyl]-5,11-dihydro-6H-pyrido [2,3-b] [1,4] benzodiazepin-6-one (AF-DX 116) (M2-subtype) by 77% in cerebellum on postnatal 14th day. The levels of m1 and m2 muscarinic receptor messenger RNAs in the same brain regions were also decreased in the same day after nicotine treatment. On postnatal 35th day, no difference was observed in binding sites or in messenger RNA levels of the brain regions tested (cerebral cortex, midbrain, hippocampus, cerebellum and brainstem). Our results show that nicotine reduces muscarinic receptor subtypes in developing rat brain, in part, through suppression of the messenger RNA expression, but that the effects are different among brain regions and subtypes.

Prejunctional M1 facilitory and M2 inhibitory muscarinic receptors mediate rat bladder contractility

Subtype-selective muscarinic antagonists effects on carbachol-induced and electric field-stimulated contractility of rat bladder were compared in vitro. Schild plot analysis of cumulative carbachol dose-response curves in the presence of antagonists was consistent with M3-mediated bladder contractions. However, nerve-evoked contractions were inhibited 15% at 30 Hz (P < 0.01) by 10 nM pirenzepine (M1-selective antagonist), whereas 10 nM methoctramine (M2-selective antagonist) increased these contractions by 17% at 30 Hz (P < 0.01). Identical doses had no effect on carbachol-induced contractions, indicating prejunctional M1 facilitory and M2 inhibitory receptors. m1 Receptors could not be identified by subtype-selective antibodies, nor could the m1 transcript be identified by Northern hybridization. However, m1, m2, m3, and m4 transcripts were identified in rat bladder using the reverse transcriptase-polymerase chain reaction, providing support for the existence of the m1 subtype. In conclusion, strong evidence is provided for the existence of prejunctional M1 facilitory and M2 inhibitory and postjunctional M3 receptors modulating contractility in the rat urinary bladder.

Characterization of muscarinic cholinergic receptor subtypes in rat prostate

The purpose of this study was to characterize the muscarinic receptor subtypes in the individual lobes of the rat prostate. Immunoprecipitation was performed on homogenates of these 3 lobes using antibodies to the m1-m4 muscarinic receptor subtypes. Reverse transcriptase polymerase chain reaction assays (RT-PCR) were also performed using primers specific for each of the five muscarinic receptor subtypes (m1-m5). The susceptibility of the receptors to degradation by endogenous prostate proteases was assessed by mixing rat ventral prostate with rat heart (m2) and rat parotid (m3) prior to immunoprecipitation. In the ventral lobe, transcripts for the m1-m4 subtypes were amplified whereas in the dorsal and lateral lobes only the m2 and m3 sets of primers amplified PCR products of the predicted size. Immunoprecipitation of the ventral lobe resulted in predominantly m3 receptors, while the majority of receptors immunoprecipitated from lateral and dorsal lobes were the m2 subtype. The m3 muscarinic subtype was apparently susceptible to degradation by prostate proteases whereas the m2 subtype was not. These results demonstrate a regional distribution in the subtypes of muscarinic receptors in the rat prostate, and a greater susceptibility of the m3 receptor to degradation during immunoprecipitation than the m2 subtype.

Quantitative reverse transcription-PCR-HPLC for nerve growth factor mRNA using a deletion RNA as an internal standard

We have developed a convenient method for the routine measurement of the absolute amount of nerve growth factor (NGF) mRNA in tissue samples. The method consists of RNA extraction, amplification by reverse transcription-PCR and detection by high-performance liquid chromatography. The addition of a deletion mutant RNA to tissue samples as an internal standard enabled correction for RNA recovery during extraction, and the target mRNA and the internal standard were both amplified with the same PCR primers. The conditions were optimized so that the procedure was conducted in the region where the calibration curve was linear, thereby allowing high reproducibility and reliability. The method was applied to the measurement of NGF mRNA in tissues such as skin and skeletal muscle, where the levels are too low to be easily detected by Northern blotting analysis: skin, 14.1 +/- 4.6 fg/mg tissue and skeletal muscle, 11.0 +/- 2.2 fg/mg tissue (mean +/- SD, n = 10). The coefficient of variation of this method was less than 2.8%. This approach should also be applicable to the routine assay of the absolute amount of other mRNAs present at low levels in tissues.

Carbachol-induced inositol phosphate formation in rat striatum is mediated by both M1 and M3 muscarinic receptors

In vibratome-cut slices from rat striatum and in the presence of 10 mM LiCl, the cholinergic agonist carbachol stimulated the accumulation of total [3H]inositol phosphates (EC50 11+/-1 microM and maximum effect 546+/-36% of basal). The response to 100 microM carbachol (497+/-24% of basal) was inhibited by muscarinic antagonists (1 microM), the rank order of efficacy being 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; 100% inhibition) approximately pirenzepine (98+/-3%) > p-fluoro analog of hexahydro-sila-difenidol (pFHHSiD; 90+/-3%) >> methoctramine (32+/-7%) approximately tropicamide (30+/-10%). Antagonist inhibition curves best fit to a single-site model for 4-DAMP (pKi 8.9+/-0.2) whereas, for both pirenzepine and pFHHSiD, the best fit was to the two-site model. The pKi values for the high-affinity (8.3+/-0.2) and low-affinity (6.9+/-0.2) components for pirenzepine-mediated inhibition corresponded to those reported for M1 and M3 receptors, respectively. The pKi values for the high-affinity (8.2+/-0.3) and low-affinity (7.0+/-0.2) components for pFHHSiD inhibition were in good agreement with those reported for M3 and M1 receptors, respectively. Altogether these results indicate that carbachol-induced [3H]inositol phosphate formation in rat striatal slices is mediated by both M1 and M3 muscarinic receptors.

Selective M1 muscarinic receptor antagonists disrupt memory consolidation of inhibitory avoidance in rats

The effect of three different M1 muscarinic antagonists, pirenzepine, biperiden, and trihexyphenidyl on memory consolidation was investigated. Rats were trained in a one-trial step-through inhibitory avoidance task and injected intraperitoneally immediately afterwards, either with pirenzepine, biperiden, or trihexyphenidyl (dose range from 0 to 16 mg/kg). The non-selective antimuscarinic compound scopolamine, was also administered for comparison. One day later, rats were tested for retention. Results show that biperiden, trihexyphenidyl and scopolamine produced a dose-dependent impairment of inhibitory avoidance consolidation, while pirenzepine had no effect. The amnestic state produced by biperiden and trihexyphenidyl was comparable to that observed after the administration of scopolamine. These results indicate that the selective blockade of the central M1 muscarinic receptors interfere with memory consolidation of inhibitory avoidance and suggest that this receptor subtype is critically involved in mnemonic functions.

Immunocytochemical localization of muscarinic m2 receptor in the rat spinal cord

Muscarinic receptors have been implied in the regulation of spinal cord functions. The m2 subtype has been found to be one of the major muscarinic receptors expressed in the spinal cord. In order to determine the precise cellular localization of m2 receptor in the rat spinal cord, immunocytochemistry was performed using a commercially available specific antibody. In the dorsal horn, strong m2 immunoreactivity was detected in the substantia gelatinosa. In the ventral horn, m2 immunoreactivity was detected in patches that were associated with cell groups of motor neurons. At higher magnification, m2 immunoreactivity was detected in neuronal processes and many of them were seen in close apposition to m2-negative perikarya and proximal dendrites of motor neurons. These results indicate that m2 receptor immunoreactivity is localized in different neuronal elements of the spinal cord.

Increased expression of M2 muscarinic receptor mRNA and binding sites in the rostral ventrolateral medulla of spontaneously hypertensive rats

A significant body of evidence suggests that the development and maintenance of elevated blood pressure in the spontaneously hypertensive rat (SHR), a genetic model for essential hypertension, is due at least partly to a central hyper-cholinergic state. For example, this strain responds with an exaggerated pressor response to pharmacological stimulation of central muscarinic receptors in certain brain regions compared to normotensive Wistar Kyoto rats (WKY). At least part of the enhanced response to central muscarinic receptor stimulation in SHR is due to the altered expression of post-synaptic receptors. In the present study, the reverse transcriptase-polymerase chain reaction and autoradiographic techniques were used to estimate the relative levels of mRNA and density of receptor binding sites for the five subtypes of muscarinic receptors within the rostral ventrolateral medulla (RVL) of SHR and WKY. Adult (12-week-old) SHR exhibited an increase in the levels of both M2 muscarinic mRNA, and M2 receptor binding sites in RVL compared to age-matched normotensive WKY. Similarly, 4-week-old pre-hypertensive SHR exhibited increased levels of M2 mRNA in whole medulla oblongata, and an increase in the number of binding sites for M2 receptors in the RVL. Since the RVL is known to integrate tonic cholinergic sympathoexcitatory input, these results suggest that the increased expression of M2 muscarinic receptors in this region represents one neurochemical correlate for the maintenance of excessive central efferent sympathetic nervous activity in the SHR. Since the neurochemical change precedes the development of hypertension, the altered medullary M2 receptor expression may play a role as an initiating or predisposing factor for the development of hypertension in SHR.

Role of central neural mechanisms in the regulation of hepatic glucose metabolism

Central monoamine neurotransmitters affect blood glucose homeostasis. Activation of central cholinergic, noradrenergic histaminergic, and serotonergic neurons rapidly increase hepatic glucose output via the sympathetic nervous system. Acute hyperglycemia is mediated by three distinct pathways: the action of epinephrine on the liver, the action of glucagon on the liver, and the direct innervation of the liver. The relative contribution of these factors to hyperglycemia can be altered by diet and the kinds of neurotransmitters evoked in the central nervous system, but the magnitude of epinephrine secretion is closely related to the magnitude of hyperglycemia. On the other hand, neuropharmacological stimulation of central cholinergic muscarinic receptors, histaminergic H1 receptors, and serotonergic 5-HT2 receptors increases hypothalamic noradrenergic neuronal activity, which is associated with hyperglycemia. In contrast, central GABAA receptors play an inhibitory role in the regulation of hepatic glucose metabolism. Thus, central monoaminergic neurons could be linked together, and play a homeostatic role in the regulation of hepatic glucose metabolism.

The M5 (m5) receptor subtype: fact or fiction?

By comparison to the other subtypes of muscarinic receptors, very little is known about the binding properties, locations, mechanisms and physiological functions of the M5 (m5)* receptor subtype. Studies of the m5 receptor have been hampered by the lack of m5-selective ligands or antibodies and a source that endogenously expresses predominantly the m5 receptor subtype. We have developed a pharmacological labeling strategy using the non-selective muscarinic antagonist [3H]NMS, in the presence of muscarinic antagonists and toxins in green mamba venom to occlude the m1-m4 receptor subtypes, to selectively label the m5 receptor subtype. This m5-selective labeling approach, along with those developed for the other four receptor subtypes, has permitted for the first time a comparison of the relative expression levels and anatomical localizations of the five muscarinic receptor subtypes in the brain. The distribution profile of the m5 receptor is distinct from the other four subtypes and is enriched in the outer layers of the cortex, specific subfields of the hippocampus, caudate putamen, olfactory tubercle and nucleus accumbens. These studies have also demonstrated that the levels of m5 receptor protein expression are apparently higher and more widespread than anticipated from previous in situ hybridization and immunoprecipitation studies. Taken together, the results suggest a unique and potentially physiologically important role for the m5 receptor subtype in modulating the actions of acetylcholine in the brain.

The effects of cholinergic drugs on rat neocortical high-voltage spindles in ketanserin-treated rats

To investigate the roles of the cholinergic system and 5-HT2 receptors in the modulation of thalamocortical oscillations, we studied the effects of systemic (s.c.) administration of anticholinesterases (physostigmine, tetrahydroaminoacridine) and muscarinic acetylcholine receptor agonists (pilocarpine, oxotremorine) on spontaneous thalamically generated rhythmic neocortical high-voltage spindles in adult rats pretreated with either saline or ketanserin, a 5-HT2 receptor antagonist. Ketanserin at 20.0 mg/kg increased the number of high-voltage spindles. In saline-treated rats, tetrahydroaminoacridine 3.0 and 9.0 mg/kg was able to decrease high-voltage spindles, whereas in ketanserin 20.0 mg/kg-treated rats only the highest dose of tetrahydroaminoacridine (9.0 mg/kg) decreased high-voltage spindles. Both doses of physostigmine, 0.12 and 0.36 mg/kg, decreased high-voltage spindles in both saline and ketanserin 20.0 mg/kg-treated rats. Lower doses of tetrahydroaminoacridine (1.0 mg/kg) and physostigmine (0.06 mg/kg) were ineffective in both saline- and ketanserin 20.0 mg/kg-treated rats. Pilocarpine 3.0 mg/kg and oxotremorine 0.1 and 0.9 mg/kg decreased high-voltage spindles in saline-treated rats. However, in rats treated with ketanserin 20.0 mg/kg, only the lower doses of pilocarpine (0.3 and 1.0 mg/kg) and oxotremorine (0.03 mg/kg) were able to decrease the high-voltage spindles. The results suggest that activation of the cholinergic system and activation of 5-HT2 receptors have additive effects in the suppression of thalamocortical oscillations and related neocortical high-voltage spindles in rats, thus maintaining effective information processing in thalamocortical networks.

Spinal muscarinic cholinergic and nitric oxide systems in cardiovascular regulation

Pharmacological activation of muscarinic receptors located in the thoracic spinal cord evokes a marked increase in blood pressure and heart rate. We have previously demonstrated that the cardiovascular response to stimulation of spinal cord muscarinic cholinergic receptors is dependent upon a pharmacologically described ascending spino-bulbar pathway. The purpose of the study was to determine whether the blood pressure and heart rate responses to intrathecal (i.t.) injection of the muscarinic cholinergic receptor agonist carbachol are mediated by a local nitric oxide (NO)-generating system. Freely moving rats were previously prepared with chronic indwelling i.t. and intra-arterial catheters. Both the pressor and tachycardic responses produced by i.t. injection of carbachol were inhibited in a dose-dependent manner by i.t. pre-treatment with the NO synthase inhibitor N-omega-Nitro-L-arginine methylester (L-NAME). To confirm the site of action of the drugs employed in conscious rats, a separate group of rats was anesthetized, and using surgical procedures previously developed in this laboratory, drug distribution was limited specifically to the lower thoracic spinal cord. When carbachol was administered by i.t. injection and localized to the lower thoracic area, muscarinic cholinergic receptor stimulation again produced a marked pressor response, but without the accompanying tachycardia. The ability of N-omega-Nitro-L-arginine methylester (L-NAME) to inhibit the pressor response to carbachol in conscious rats was confirmed in anesthetized rats, although higher doses of L-NAME than those employed in conscious rats were required. L-NAME-induced inhibition of the carbachol-evoked pressor response was reversed by the L-, but not the D-isomer, of arginine. Moreover, i.t. pre-treatment with Methylene blue, that interferes with NO production and function, effectively inhibited the expression of the pressor response to i.t. injection of carbachol. The 'anti-muscarinic' action of L-NAME was not due to a direct interaction with spinal muscarinic receptors, as L-NAME did not significantly displace [3H]methyl-scopolamine from spinal cord membranes in vitro. The results of this study support the hypothesis that spinal muscarinic cholinergic receptors participate in a sympathoexcitatory pathway that interacts either directly or indirectly with a local NO-generating system involved in the regulation of blood pressure.

Chronic nicotine treatment delays the developmental increase in brain muscarinic receptors in rat neonate

Developmental increase in the muscarinic receptors ([3H]quinuclidinyl benzylate binding sites) of rat neonate brain (cerebral cortex and cerebellum) was significantly inhibited by chronic nicotine treatment of the dams during pregnancy and lactation. However, development of the nicotinic receptors ([3H]cytisine binding sites) was not inhibited and rather was up-regulated in the cerebral cortex and brainstem by the nicotine treatment. Such inhibition and up-regulation were not seen in nicotine-withdrawn rats after birth. These results suggest that nicotine treatment during lactation may cause a remarkable delay in development of muscarinic neurotransmission in rat neonates.

N- and C-terminal substance P fragments modulate striatal dopamine outflow through a cholinergic link mediated by muscarinic receptors

The present study investigated whether the modulatory effects of substance P and substance P fragments on striatal dopamine release involve a cholinergic link. Rat striatal slices were incubated with substance P, substance P(1-4), substance P(1-7), substance P(5-11) and substance P(8-11) in the absence or presence of various agents which modify cholinergic transmissions, and endogenous dopamine outflow was measured using high-performance liquid chromatography. The incubation of striatal slices with substance P and its N- and C-terminal fragments (1 nM) induced a significant overflow of endogenous dopamine. Neostigmine (150 nM) potentiated the effects of substance P and its fragments, whereas the incubation with hemicholinium-3 (50 microM) abolished the effects of the peptides on dopamine outflow. The acetylcholinesterase inhibitor and the inhibitor of choline uptake did not have intrinsic effects on dopamine outflow. The muscarinic antagonist atropine (1 microM) reversed completely the effects of substance P and its fragments, whereas the nicotinic antagonists dihydro-beta-erythroidine (0.5 microM) and pempidine (10 microM) were devoid of effects. None of the cholinergic antagonists modified dopamine outflow. The results suggest that substance P and several N- and C-terminal substance P fragments activate cholinergic neurons in striatal slices. The released acetylcholine induces an increased dopamine outflow, mediated by muscarinic receptors. These observations represent additional evidence which supports the functional interactions between substance P, acetylcholine and dopamine in the striatum. Furthermore, they show that substance P fragments may exert neuromodulatory effects through mechanisms similar to those underlying the effects of the parent peptide.

Muscarinic receptor subtypes in subpopulations of human blood mononuclear cells as analyzed by RT-PCR technique

In this study we have analysed the expression of mRNA encoding the m1-m5 mAChR subtypes in human blood mononuclear cells and subpopulations of lymphocytes using reverse transcriptase-polymerase chain reaction (RT-PCR) technique. Total RNA was extracted from human blood mononuclear cells. T cells, monocytes. EB virus transformed B cells and from two leukemic cell lines and analysed by RT-PCR. Our results indicate that mRNAs for the m3, m4 and m5 muscarinic subtypes are expressed in mononuclear cells and purified T cells while m1 and m2 mRNAs were not detected in these cells. No m1-m5 subtype mRNA was detected in B cells and monocytes, indicating absence of muscarinic receptors in these cells. The expression of muscarinic subtypes in the leukemic T cell line. Peer, and the promyelocytic leukemic cell line HL-60 was different from peripheral mononuclear cells. Both m3 and m5 subtypes were expressed in Peer cells but not the m4 subtype, whereas the m4 and m5 subtypes detected in HL-60 cells.

Carbachol stimulates inositol phosphate formation in rat thalamus slices through muscarinic M3-receptor activation

In cross-chopped slices from rat thalamus and in the presence of 10 mM LiC1, the cholinergic agonist carbachol stimulated the accumulation of total [3H]inositol phosphates ([3H]IP2 = [3H]IP1 + [3H]IP2 + [3H]IP3). Best-fit values for the concentration-response curve for carbachol after 60 min incubation yielded an EC50 of 44 +/- 6 microM, maximum effect of 199 +/- 6% of basal accumulation and Hill coefficient (nH) of 1.1 +/- 0.1. Carbachol-induced [3H]IPs accumulation was inhibited by 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; pKi 9.1) and the p-fluoro analogue of hexahydro-sila-difenidol (pF-HHSiD; pKi 8.1). Concentration-response curves for carbachol were shifted to the right in a parallel fashion by pirenzepine (100, 300 and 100 nM). A Schild plot of the data was linear (slope 0.95 +/- 0.04) and yielded a log KD for pirenzepine of -6.8 +/- 0.1. Taken together, these results suggest that carbachol-induced inositol phosphate accumulation in rat thalamus is mediated by muscarinic M3-receptors.

Acetylcholine as a mitogen: muscarinic receptor-mediated proliferation of rat astrocytes and human astrocytoma cells

The mitogenic effect of muscarinic receptor agonists in glial cells has been characterized in rat cortical astrocytes and human 132 1N1 astrocytoma cells. The muscarinic receptor agonist carbachol caused a dose- and time-dependent increase in proliferation, as measured by [3H]thymidine incorporation. The mitogenic effect was mimicked by several muscarinic, but not nicotinic receptor agonists, and was blocked by muscarinic receptor antagonists. Reverse transcription-polymerase chain reaction (RT-PCR) experiments indicated the presence of m2, m3 and to a lesser degree, m5 muscarinic receptor mRNA in both astrocytes and astrocytoma cells. Proliferation experiments with subtype-specific muscarinic receptor antagonists suggest that carbachol-induced proliferation is due to activation of muscarinic M3 receptors. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) also stimulated glial cell proliferation. Down-regulation of protein kinase C, or the protein kinase C antagonist 1,5-(isoquinolynsulfanyl)-2-methylpiperazine dihydrochloride (H7) blocked proliferation induced by either TPA or carbachol. Of other neurotransmitters tested, histamine caused glial cell proliferation, norepinephrine and gamma-aminobutyric acid were ineffective, while serotonin and glutamate inhibited basal or serum-stimulated proliferation.

Changes in rat brain muscarinic receptors after inhibitory avoidance learning

It is widely accepted that cerebral acetylcholine is necessary for learning and memory, but little is known about the type of muscarinic receptors involved in these functions. To investigate this problem, [3H]-N-methyl-scopolamine which binds to different types of muscarinic receptors, [3H]-Pirenzepine an M1 receptor antagonist, and [3H]-Oxotremorine-M which binds mainly to M2 receptors, were used as ligands to look for possible changes in muscarinic receptor density in neostriatum (NEO), hippocampus (HIP), amygdala (AMY), and temporo-parietal neocortex (CTX), after testing for retention of inhibitory avoidance, trained with high or low footshock intensities. After low reinforcement there was an M1 postsynaptic receptor up-regulation in NEO, HIP, and CTX, and an M2 presynaptic receptor down-regulation in HIP, which suggests a concerted pre- and postsynaptic cholinergic activation in this area. An up-regulation of both M1 and M2 receptors was detected in CTX of low and high footshocked animals, which indicates the presence of a cortical postsynaptic M2 receptor.

A quantitative autoradiographic study of muscarinic cholinergic receptor subtypes in the brains of pyrithiamine-treated rats

Previous studies describe decreased acetylcholine synthesis in brain as well as neurobehavioral evidence for a central muscarinic cholinergic deficit in pyrithiamine-induced thiamine-deficient rats. In order to further evaluate this possibility, quantitative autoradiographic procedures using [3H]quinuclidinyl benzilate (for total muscarinic binding sites), [3H]pirenzepine (for muscarinic M1 sites) and [3H]AF-DX 384 (for muscarinic M2 sites) were performed at early (presymptomatic) and late (symptomatic) stages of thiamine deficiency induced in rats by administration of the central thiamine antagonist, pyrithiamine. No significant alterations in densities of M1, M2 or total muscarinic binding sites were observed in any brain structure evaluated at either early or late stages of thiamine deficiency. These findings do not support a major role for modifications of muscarinic cholinergic function in the pathogenesis of the neurological symptoms of thiamine deficiency.

Identification of m3, m4 and m5 subtypes of muscarinic receptor mRNA in human blood mononuclear cells

In this study we made use of the Reverse transcription-polymerase chain reaction to analyze the expression of mRNA for the five subtypes of muscarinic acetylcholine receptors in human blood mononuclear cells. mRNA for m3, m4 and m5 subtypes was detected, while mRNA for m1 and m2 muscarinic receptors was not found. Similar results were obtained for three different healthy human subjects studied. Interestingly, the m5 subtype was expressed at higher levels in blood mononuclear cells than in cerebral cortex. To our knowledge this is the first time that m5 muscarinic receptor mRNA has been found outside of the central nervous system.

Spinal NMDA receptor--nitric oxide mediation of the expression of morphine withdrawal symptoms in the rat

Previous studies in this laboratory have demonstrated that cholinergic receptors within the spinal cord play an important role in the expression of naloxone-precipitated withdrawal symptoms in the morphine-dependent rat. Related cardiovascular studies in non-dependent animals have demonstrated that this spinal cholinergic system is linked to a glutamatergic, NMDA pressor pathway which also involves the participation of a nitric oxide (NO) generating system. The purpose of this study was to determine whether spinal NMDA receptors and/or NO are involved in the expression of morphine withdrawal symptoms. Rats bearing previously implanted intrathecal (IT) catheters were dependent on morphine following chronic i.a. infusion of increasing doses over 5 days. Naloxone (0.5 mg/kg) was administered via the i.a. line to precipitate withdrawal; and both cardiovascular and behavioral symptoms were recorded over 60 min. Pretreatment 20 min before naloxone with IT injection of either of the NMDA receptor antagonists, MK-801 or AP-7 (100-200 nmol), produced a significant reduction in the expression of both the cardiovascular and behavioral symptoms of up to about 60%. IT pretreatment with the NO synthase inhibitor L-NAME--a methyl ester derivative of L-arginine, also produced a dose-dependent, L-arginine reversible inhibition of the cardiovascular (mainly the pressor) component of withdrawal, but had no significant effect on the expression of behavioral signs. In contrast, IT pretreatment with L-NOARG and L-NMMA, non-ester analogs of L-arginine, significantly inhibited the expression of the behavioral signs of withdrawal but did not alter the pressor component. A combined pretreatment with L-NAME and L-NOARG resulted in suppression of both pressor and behavioral components of withdrawal. The anti-withdrawal actions of either class of NO synthase inhibitor could not be attributed to blockade of local muscarinic receptors. These findings are consistent with a role for both spinal NMDA receptors and a NO generating system in the expression of both the behavioral and autonomic components of naloxone-precipitated withdrawal. They also suggest that different structural analogs of L-arginine have different profiles of activity in this regard--opening the possibility that different isozymes of NO synthase located within the same spinal region mediate different physiological or behavioral functions.

Ultrastructural immunolocalization of muscarinic acetylcholine receptor in the dorsal thalamus of rat

The ultrastructural distribution of muscarinic acetylcholine receptor (mAChR) in the dorsal thalamus of the adult rat was studied by means of pre-embedding immunocytochemistry using the monoclonal antibody M35. mAChR immunoreactivity (ir) was present with variable intensity in the different thalamic nuclei, but with a similar subcellular localization. Labeling was restricted to neuronal cell bodies and dendrites, where it was both in the cytoplasm and along the cytoplasmic side of the plasma membrane, in areas post-synaptic to small terminals with round clear vesicles but also in non-synaptic areas. Glial cells were unlabeled. By combining the pre-embedding immunostaining for mAChR with post-embedding immunogold labeling for GABA it was shown that GABAergic terminals made synaptic contacts with cholinoceptive structures, but no mAChR ir was present at their post-synaptic sites.