Desensitization Of Nicotinic Receptors Research Articles

Antinociceptive effects of the alkaloid epibatidine: Further studies on involvement of nicotinic receptors

AbstractEpibatidine is a potent analgetic agent with high affinity for nicotinic receptors. The antinociceptive effects of epibatidine are blocked by both competitive and noncompetitive nicotinic antagonists. The L‐type calcium channel activator Bay K 8644 potentiates the antinociceptive effects of epibatidine, while nifedipine antagonizes the antinociceptive effects. Acute treatment with chlorisondamine leads to long‐term blockade of the antinociceptive effects of epibatidine. The antinociceptive effects of epibatidine are reduced in mice rendered tolerant to the behavioral effects of nicotine by chronic nicotine or caffeine treatment. Epibatidine has very high affinity for the major central nicotinic receptor to which [3H]nicotine binds. However, unlike nicotine and cytisine, epibatidine is very potent at ganglionic‐type nicotinic receptors. Epibatidine in cultured cells causes desensitization of both ganglionic and neuromuscular nicotinic receptors. Thus, like nicotine, chronic treatment with epibatidine might be expected to lead to tolerance. These studies establish epibatidine as a tool for the study of nicotinic pathways involved in pain perception. © 1995 Wiley‐Liss, Inc.

Enhancement by 5-hydroxytryptamine and analogues of desensitization of neuronal and muscle nicotinic receptors expressed in Xenopus oocytes.

1. The action of 5-hydroxytryptamine (5-HT) on neuronal and muscle nicotinic acetylcholine receptors (nicotinic AChR) expressed in Xenopus oocytes was studied. 2. 5-HT enhanced the rate of desensitization of the acetylcholine (ACh) current response in all receptor subtypes investigated (muscle, alpha beta 2 gamma delta and alpha 4 beta 2), acting in a dose-dependent manner. 3. 5-HT also reduced the peak current elicited by ACh in a dose-dependent manner. The IC50 value for the muscle type receptor was 227 +/- 0.44 microM, and 166 +/- 0.47 microM and 283 +/- 0.28 microM for the combinations alpha beta 2 gamma delta and alpha 4 beta 2 respectively. 4. The effect of 5-HT on the responses to ACh (10 microM) was found to be independent of membrane voltage over the range tested (-80 to -10 mV), and to be readily reversed by washout. 5. The action of 5-HT could be mimicked by structurally similar molecules. The homologue tryptamine was less potent than 5-HT in blocking the ACh current, with an IC50 of 1.0 +/- 0.02 mM. Ketanserin, a 5-HT2 receptor antagonist, was more potent than 5-HT, the IC50 being 49.0 +/- 1.4 microM. 6. We postulate that a highly conserved portion of the tertiary structure of nicotinic AChRs, which includes some part of the ACh binding site, has affinity for 5-HT and structural analogues.

Postsynaptic nicotinic receptor desensitized by non-contractile Ca2+ mobilization via protein kinase-C activation at the mouse neuromuscular junction.

1. Non-contractile Ca2+ mobilization (unaccompanied by muscle contraction) was initiated by nerve stimulation in the presence of neostigmine (more than 0.03 microM) at the endplate region of mouse diaphragm muscles. In the process of nicotinic receptor desensitization, the depressant effect of non-contractile Ca2+ on contractile Ca2+ mobilization was investigated by measurement of Ca(2+)-aequorin luminescence. 2. When the phrenic nerve was stimulated with paired pulses having intervals of 150, 300, 600, 1000 and 2000 ms, contractile Ca2+ transients were elicited during the generation of non-contractile Ca2+ mobilization. The amplitude of the contractile Ca2+ transients elicited by the second pulse (S2) was depressed at the shorter pulse intervals, but recovered to the initial contractile response (S1) at longer pulse intervals. 3. The extent of depression of S2 was enhanced by increasing the concentration of neostigmine (0.03 to 0.3 microM). When a low concentration (0.05 microM) of pancuronium, a competitive nicotinic antagonist, completely blocked non-contractile Ca2+ mobilization, the depression of S2 was diminished. 4. The depression of S2 was enhanced when the peak amplitude of non-contractile Ca2+ mobilization was raised by increasing the external Ca2+ concentration from 1.3 to 5 mM. 5. Staurosporine (10 nM), a protein kinase-C inhibitor, diminished the depression of S2 despite large amounts of non-contractile Ca2+ mobilization. The diminishing effect of staurosporine was counteracted by TPA (0.1 microM), a protein kinase-C activator. 6. These findings suggest that non-contractile Ca2+ mobilization may enhance the desensitization of the postsynaptic nicotinic receptor via activation of protein kinase-C at the neuromuscular junction.

Loss of neonatal hypoxia tolerance after prenatal nicotine exposure: Implications for sudden infant death syndrome

Maternal cigarette smoking has a high correlation with Sudden Infant Death Syndrome, a condition in which cardiorespiratory failure occurs during an hypoxic episode, as in sleep apnea. Pregnant rats were given nicotine infusions of 2 or 6 mg/kg/day throughout gestation, regimens that produce plasma nicotine levels spanning the range in smokers. The day after birth, animals in the high dose group displayed excessive mortality during hypoxic challenge. These animals were found to be deficient in an essential response component, namely adrenomedullary catecholamine release that is required to maintain neonatal cardiac rhythm during hypoxia; the defect was in adrenal cell function rather than in altered innervation or nicotinic receptor desensitization. We also examined brainstem and forebrain noradrenergic mechanisms that are involved in neonatal respiratory control. The nicotine group showed suppressed spontaneous neuronal activity, but were hyperresponsive to hypoxia. As these projections are inhibitory for respiration, the nicotine-induced sensitization would be expected to contribute to respiratory arrest during hypoxia. Prenatal nicotine exposure may thus provide a useful animal model with which to study the physiological mechanisms that underlie Sudden Infant Death Syndrome, while at the same time providing a biological explanation for the association of the syndrome with smoking.

Ganglionic nicotinic acetylcholine receptor activation by the novel agonist ABT‐418

AbstractABT‐418 was functionally characterized as a neuronal nicotinic acetylcholine receptor (nAChR) channel agonist using preparations that contain nAChRs characteristic of the ganglionic subtypes. In PC12 cells, ABT‐418, like (−)nicotine, activated an inward current that decayed within seconds in the continued presence of agonist. ABT‐418 was 4‐fold less potent than (−)nicotine (EC50 = 214 ± 30 μM and 52 ± 4 μM, respectively) while the efficacy of ABT‐418 was not significantly different from (−)nicotine when the peak response amplitude was measured. Responses to 300 μM ABT‐418 were reversibly inhibited 81 ± 3% by 10 μM mecamylamine, 38 ± 1% by 10 μM dihydro‐β‐erythroidine, and 82 ± 2% by 100 μM dihydro‐β‐erythroidine. These nAChR antagonists affected the response to (−)nicotine similarly. Furthermore, responses to maximal concentrations of ABT‐418 (3 mM) and (−)nicotine (1 mM) were not additive, consistent with ABT‐418 and (−)nicotine acting through the same receptor(s). However, the Hill coefficient for ABT‐418 (1.18 ± 0.20) was smaller than that for (−)nicotine (1.77 ± 0.18), and high concentrations of ABT‐418 appeared to elicit a more rapidly decaying response than did (−)nicotine. In the rat superior cervical sympathetic ganglion also, ABT‐418 was 2.5‐fold less potent than (−)nicotine in blocking nicotinic transmission, presumably through nicotinic receptor desensitization. These studies provide the most direct evidence that ABT‐418 activates nicotinic cholinergic channels, and suggest that ABT‐418 would have reduced potency compared to (−)nicotine in peripheral ganglia, consistent with the reduced side effect liability of this novel nAChR agonist. © Wiley‐Liss, Inc.

Nicotine-induced acute tolerance: Studies involving schedule-controlled behavior

The major goal of the present study was to examine acute tolerance to nicotine-induced disruption of operant behavior following a single, noncontingent injection. Rats were trained to lever press for food reinforcement under a fixed ratio-30 schedule. Once trained, rats were injected with either saline or nicotine (0.8 mg/kg) in their home cages. After either a 90- or 180-min delay, each rat was injected with nicotine (0.4 mg/kg) and placed in the operant chamber for a 30-min behavioral evaluation session. This experiment was replicated with slight modifications 1 week later. The results of the present study suggest that 0.8 mg/kg of nicotine produces acute tolerance to the response rate decreasing effects of 0.4 mg/kg of nicotine. Because the tolerance-producing dose of nicotine was injected while rats were not in the test environment, they did not have an opportunity to practice the target behavior while under the influence of the drug. Hence, the acute tolerance observed in this study appears to be, at least partly, pharmacological (vs. behavioral) in nature, and may be related to a desensitization of central nicotinic acetylcholinergic receptors (nAChRs).

General anesthetics modify the kinetics of nicotinic acetylcholine receptor desensitization at clinically relevant concentrations.

General anesthetics are thought to induce anesthesia through their actions on ligand-gated ion channels. One such channel, the nicotinic acetylcholine receptor (nAcChoR), can be found in different subtypes in the central nervous system and at the periphery in the neuromuscular junction. The latter subtype of the nAcChoR is a useful model for examining interactions between general anesthetics and ligand-gated ion channels, because it can be isolated and purified in sufficient quantities to allow for biophysical and biochemical studies. This study examines the actions of general anesthetics on agonist-induced conversion of the nAcChoR to inactive desensitized conformational states. Nicotinic acetylcholine receptor membranes were purified from the electric organ of Torpedo nobiliana. Agonist-induced desensitization was characterized from the time-dependent increase in fluorescence intensity that results from the binding of the fluorescent acetylcholine analog, Dns-C6-Cho, to the nAcChoR. Mixing Dns-C6-Cho with nAcChoR-rich membranes results in an increase in fluorescence that is characterized by four rate processes. Concentrations of isoflurane and butanol, which range from subclinical to toxic increase the rates of the third and fourth components of fluorescence, corresponding to fast and slow desensitization, respectively. At concentrations that are twice their EC50s for anesthesia, isoflurane, butanol, chloroform, methanol, and cyclopentanemethanol increase the apparent rates of fast and slow desensitization by an average of 92 +/- 22% and 108 +/- 22%, respectively. The concentration range over which general anesthetics modify the kinetics of nAcChoR desensitization is similar to those reported for anesthetic actions on the GABAA receptor. Thus, the nAcChoR, like other members of this superfamily, is a sensitive target of general anesthetics.

Facilitation of nicotinic receptor desensitization at mouse motor endplate by a receptor-operated Ca 2+ channel blocker, SK&F 96365

When acetylcholinesterase was inhibited by neostigmine, SK&F 96365 (1-{ β-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl}-1 H-imidazole hydrochloride) at 10 μM caused no effect on the amplitude of single endplate potentials (e.p.p.s) but shortened the decay time in mouse phrenic nerve-diaphragm preparations. However, SK&F 96365 inhibited high-frequency stimulation-evoked long-lasting depolarization of the endplate region and accelerated the run-down of trains of e.p.p.s which were eliminated within 1 s. After a train of stimulation, SK&F 96365 produced a post-tetanic depression of single e.p.p.s. The post-tetanic effect gradually dissipated with full restoration in 10–15 s. During a train of stimulation, SK&F 96365 also depressed miniature endplate potentials (m.e.p.p.s), which were restored after termination of stimuli in parallel with the recovery of e.p.p. The decay times of miniature endplate currents during recovery phases changed slightly. In control preparations not treated with neostigmine, however, SK&F 96365 did not alter the amplitude and decay time of m.e.p.p.s or e.p.p.s but accelerated the decay of succinylcholine-induced endplate depolarizations. The results suggest that SK&F 96365 facilitates nicotinic receptor desensitization in addition to blocking receptor-operated Ca 2+ channels.

Synaptic transmission from splanchnic nerves to the adrenal medulla of guinea‐pigs.

1. Membrane potentials were recorded with conventional intracellular microelectrodes from chromaffin cells in isolated, bisected adrenal glands from guinea-pigs. 2. All cells were electrically excitable and responded to depolarizing current with all-or-nothing action potentials that were blocked by tetrodotoxin. 3. Input resistance was 180 +/- 14 M omega and this was lower than that reported for isolated chromaffin cells using patch electrodes. 4. All cells responded to transmural stimulation with action potentials that arose from excitatory synaptic potentials in response to the excitation of one or more preganglionic fibres, many having strong synaptic action. Other fibres had weaker synaptic action but in all cases, maximal transmural stimulation caused depolarization well above threshold for action potential initiation. 5. Spontaneous excitatory synaptic potentials were observed whose frequency was greatly increased by repetitive stimulation at 10 or 30 Hz. 6. No evidence was found for the desensitization of nicotinic receptors in response to acetylcholine released from presynaptic nerve terminals. 7. These experiments show that there are many similarities between the responses to splanchnic nerve stimulation of guinea-pig chromaffin cells in situ and sympathetic ganglion cells from the same species.

Desensitization of Nicotine‐Stimulated [3H]Dopamine Release from Mouse Striatal Synaptosomes

Potential desensitization of brain nicotinic receptors was studied using a [3H]dopamine release assay. Nicotine-stimulated [3H]dopamine release from mouse striatal synaptosomes was concentration-dependent with an EC50 of 0.33 +/- 0.13 microM and a Hill coefficient of 1.44 +/- 0.18. Desensitization by activating concentrations of nicotine had a similar EC50 and a half-time of 35 s. Concentrations of nicotine that evoked little release also induced a concentration-dependent desensitization (EC50 = 6.9 +/- 3.6 nM, t1/2 = 1.6-2.0 min, nH = 1.02 +/- 0.01). Both types of desensitization produced a maximum 75% decrease in [3H]dopamine release. Recovery from desensitization after exposure to low or activating concentrations of nicotine was time-dependent with half-times of 6.1 min and 12.4 min, respectively. Constants determined for binding of [3H]nicotine to striatal membrane at 22 degrees C included a KD of 3.7 +/- 0.5 nM, Bmax of 67.5 +/- 2.2 fmol/mg, and Hill coefficient of 1.07 +/- 0.06. Association of nicotine with membrane binding sites was biphasic with half-times of 9 s and 1.8 min. The fast rate process contributed 37% of the total reaction. Dissociation was a uniphasic process with a half-time of 1.6 min. Comparison of constants determined by the release and binding assays indicated that the [3H]-nicotine binding site could be the presynaptic receptor involved in [3H]dopamine release in mouse striatal synaptosomes.

Desensitization of nicotinic receptors on inhibitory interneurons as a possible pathophysiological mechanism in schizophrenia

Desensitization of nicotinic receptors on inhibitory interneurons as a possible pathophysiological mechanism in schizophrenia

Desensitization of embryonic nicotinic acetylcholine receptors expressed in Xenopus oocytes

Xenopus oocytes where injected with mRNA coding for the embryonic type of the bovine nicotinic acetylcholine receptor (nAChR). Pulses of actylcholine (ACh) were applied to outside-out patches of the injected oocytes with the liquid filament switch. With high concentrations of ACh (10 −3 M), the ACh response desensitized rapidly; the decay could be fitted with a single exponential with a time constant of about 50 ms. In double pulse experiments, recovery from desensitization was tested. Complete recovery was observed after 5 s. The affinity of ACh to the desensitized receptor was measured by preincubating the outside-out patch with different concentrations of ACh. Preincubation with 10 −6 M ACh led to complete desensitization of the nAChRs on the patch. The results can be fitted with the molecular scheme describing activation and desensitization of the nAChR which was developed recently for embryonic nAChRs of mouse muscle.

Two components in the adrenal nicotinic secretory response revealed by cobalt ramps

Prolonged stimulation with nicotine (50 μM) enhanced the secretion of catecholamines from perfused cat adrenal glands. The profile of secretion consisted of a quick activation phase to peak of 7.68 μg/min followed by a second inactivation phase which exhibited a t 1 2 of 3.75 min. Sustained stimulation with a solution enriched in K + (59 mM) also evoked a transient secretory response, with a peak release of 8.62 μg/2 min and a t 1 2 for inactivation of 4.8 min. Co 2+ (10 mM) blocked the nicotinic response by 58% and the K +-evoked secretory response by over 96%. In the presence of Co 2+; (5 mM), continuous perfusion with nicotine produced a transient but large initial secretory response; the gradual decrease of the extracellular Co 2+ concentration, [Co 2+] 0, as a continuous ramp allowed the development of a second component of secretion which inactivated later on. When the glands were continuously stimulated with 59 mM K + in the presence of Co 2+, the first component of secretion was missing: the second component appeared as [Co 2+] 0 decreases as a ramp. In similar experiments performed in low-Na + solution (10 mM Na +), only the first secretion component evoked by nicotine was observed. This finding suggests that the second component of secretion depends on Na + entry through the nicotinic receptor, on the ensuing cell depolarization and on Ca 2+ entry through voltage-dependent Ca 2+ channels. These data are compatible with the idea that the nicotinic receptor-mediated catecholamine secretory response in the cat adrenal chromaffin cell has two components: the first seem to be triggered by Ca 2+ entry through the nicotinic receptor channels, and the second by Ca 2+ entry through voltage-dependent Ca 2+ channels. The data also suggest that Co 2+ ions protect against the Ca 2+-dependent desensitization of nicotinic receptors.

Desensitization of junctional and extrajunctional nicotinic ACh receptors expressed in Xenopus oocytes

Desensitization of nicotinic acetylcholine receptors (AChRs) was studied using the Xenopus oocyte expression system. Mouse and cat extrajunctional AChRs expressed in oocytes desensitized more slowly than Torpedo AChRs. Substitution of the mouse γ subunit into the Torpedo AChR reduced the rate of desensitization, making it similar to the mouse AChR. Likewise, substitution of the Torpedo γ subunit into the mouse extrajunctional AChR increased the rate of desensitization, making it similar to the Torpedo AChR. Furthermore, mouse junctional AChRs in which the γ subunit was replaced by the ϵ subunit desensitized more rapidly than either mouse or cat extrajunctional AChRs, and resembled Torpedo AChRs. Thus, in addition to other properties, junctional and extrajunctional AChRs differ with respect to desensitization, suggesting a possible role of desensitization in normal development of neuromuscular junction.

Nicotinic acetylcholine receptor desensitization is regulated by activation-induced extracellular adenosine accumulation

Adenosine modulation of nicotinic ACh receptor (nAChR) function was studied in primary cultures of rat skeletal muscle. Activation of the nAChR by carbachol increased extracellular adenosine concentration in a dose-dependent manner. Furthermore, carbachol activation of the nicotinic receptor resulted in a twofold increase in cAMP levels in the muscle cells. The carbachol-dependent increase in cAMP levels was inhibited by adenosine receptor antagonists as well as by nicotinic receptor antagonists. These results suggest that the increased cAMP levels were due to adenosine receptor activation by the extracellular adenosine accumulated on nAChR activation. Others have shown that desensitization of the nAChR by agonist is mediated, in part, by phosphorylation. Since we found that nicotinic cholinergic agonists also cause adenosine accumulation with concomitant cAMP increases, we determined whether the accumulated adenosine has a role in desensitization. We found that the adenosine receptor antagonist, BW1434U, significantly inhibited carbachol-induced nAChR desensitization, indicating that extracellular adenosine is involved in nAChR desensitization. Our data suggest that nAChR function is regulated via a feedback mechanism mediated by adenosine released from muscle on activation of the nAChR.

Differential potentiation by calcium antagonists of neuromuscular blockade induced by pancuronium and succinylcholine in cats in vivo.

The effects of several calcium antagonists (verapamil, nicardipine and two diltiazem isomers, d-cis and l-cis diltiazem) alone and associated to non-depolarizing (pancuronium) and depolarizing (succinylcholine) neuromuscular blockers, were evaluated on sciatic nerve-tibialis anterior muscle preparations from cats in vivo. The calcium antagonists used (at 0.1 and 0.5 mg/kg iv) did not modify the height of muscular twitches elicited indirectly. However, these agents potentiated in a dose-dependent way the neuromuscular blockade induced by iv pancuronium (2-40 micrograms/kg) and succinylcholine (6-200 micrograms/kg). The order of potency in increasing the effects of pancuronium was nicardipine much greater than d-cis diltiazem greater than or equal to verapamil, whereas the order of potency in enhancing succinylcholine effects was d-cis diltiazem greater than or equal to verapamil much greater than nicardipine. The effects of diltiazem were stereoselective, thus the potentiation induced by d-cis diltiazem was significantly greater in all cases than that induced by l-cis diltiazem, which suggests that calcium channel blockade plays a role in these interactions. However, other mechanisms such as calcium antagonists-induced nicotinic receptor desensitization may also be involved.

Regulation of acetylcholine receptor desensitization in mouse myotubes by cytosolic cyclic AMP

Whole-Cell currents activated by bath applications of acetylcholine (ACh) (0–30 μM) were recorded from patch-clamped myotubes of the mouse C2 cell line. Increasing concentrations of forskolin caused a dose-dependent fast decay of ACh-activated currents as compared to the long-lasting ACh-currents in control cells. The forskolin-induced modulation of nicotinic ACh receptor (nAChR) desensitization was proportional to the drug-induced elevation in the cyclic AMP (cAMP) cellular. Furthermore, an increase in the rate of decay of the ACh-current response, which paralleled an elevation in cAMP cellular content, was caused by treatment with a calcitonin gene-related peptide (1 μM), 8-Br-cAMP) (0.5 mM), or by loading the myotubes with cAMP. These results therefore indicate that the desensitization of nAChR is a cAMP-related process in C2-myotubes.

Desensitization of the nicotinic acetylcholine receptor: molecular mechanisms and effect of modulators.

1. Loss of response after prolonged or repeated application of stimulus is generally termed desensitization. A wide variety of phenomena occurring in living organisms falls under this general definition of desensitization. There are two main types of desensitization processes: specific and non-specific. 2. Desensitization of the nicotinic acetylcholine receptor is triggered by prolonged or repeated exposure to agonists and results in inactivation of its ion channel. It is a case of specific desensitization and is an intrinsic molecular property of the receptor. 3. Desensitization of the nicotinic acetylcholine receptor at the neuromuscular junction was first reported by Katz and Thesleff in 1957. Desensitization of the receptor has been demonstrated by rapid kinetic techniques and also by the characteristic "burst kinetics" obtained from single-channel recordings of receptor activity in native as well as in reconstituted membranes. In spite of a number of studies, the detailed molecular mechanism of the nicotinic acetylcholine receptor desensitization is not known with certainty. The progress of desensitization is accompanied by an increase in affinity of the receptor for its agonist. This change in affinity is attributed to a conformational change of the receptor, as detected by spectroscopic and kinetic studies. A four-state general model is consistent with the major experimental observations. 4. Desensitization of the nicotinic acetylcholine receptor can be potentially modulated by exogenous and endogenous substances and by covalent modifications of the receptor structure. Modulators include the noncompetitive blockers, calcium, the thymic hormone peptides (thymopoietin and thymopentin), substance P, the calcitonin gene-related peptide, and receptor phosphorylation. Phosphorylation is an important posttranslational covalent modification that is correlated with the regulation and desensitization of the receptor through various protein kinases. 5. Although the physiological significance of desensitization of the nicotinic receptor is not yet fully understood, desensitization of receptors probably plays a significant role in the operation of the neuronal networks associated in memory and learning processes. Desensitization of the nicotinic receptor could also possibly be related to the neuromuscular disease, myasthenia gravis.

Arg-Lys-Asp-Val-Tyr (thymopentin) accelerates the cholinergic-induced inactivation (desensitization) of reconstituted nicotinic receptor.

1. The thymic hormone thymopoietin blocks neuromuscular transmission and was proposed (Goldstein, 1974) as a modulator of synaptic conductivity. 2. The cholinergic-induced inactivation of nicotinic receptor reconstituted into asolectin lipid vesicles was studied in the presence and in the absence of thymopentin, a synthetic pentapeptide corresponding to positions 32-36 of thymopoietin. 3. The present data show that thymopentin accelerates desensitization of the nicotinic acetylcholine receptor, supporting the aforementioned physiological role proposed for thymopoietin. 4. They also suggest that the hormone itself and/or a yet unidentified hormine-derived peptide fragment may act as an endogenous ligand for nicotinic acetylcholine receptor desensitization.

Antagonism of the nicotine-induced changes of the striatal dopamine metabolism in mice by mecamylamine and pempidine.

The ability of nicotinic receptor blockers, mecamylamine and pempidine, to antagonize the changes in striatal dopamine (DA) metabolism induced by repeated nicotine administration was studied. The contents of DA and its metabolites 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured. Mice kept at 20-22 degrees C were given nicotine, 3 mg/kg, s.c., four times, at 30 min intervals, and sacrificed 20 min after the last dose. Hexamethonium, 10 mg/kg, i.p., was administered at 30 min before the first nicotine dose in order to prevent the peripheral effects of nicotine. Mecamylamine, 0.6 or 10 mg/kg, i.p., and pempidine, 0.6 or 20 mg/kg, i.p., were given at 60 min before sacrifice. Mecamylamine and pempidine decreased clearly the striatal 3-MT content, which suggests that the nigrostriatal dopaminergic neurons are physiologically controlled by a stimulatory nicotinic mechanism. The repeatedly administered nicotine caused deep hypothermia, and increased the striatal DOPAC content but decreased the 3-MT and HVA contents. The small dose of mecamylamine, which was the only dose found to effectively antagonize the nicotine-induced hypothermia, antagonized the decrease of HVA content. The large but not the small doses of mecamylamine and pempidine antagonized the nicotine-induced increase of DOPAC content but none of the doses studied antagonized the decrease of 3-MT content. Thus it seems that nicotine decreases the 3-MT content by a mechanism distinct from the mechanism mediating the increase of the DOPAC content. The decreased 3-MT content most probably results from desensitization of nicotinic cholinergic receptors (nAChR) and following decrease of cholinergic regulation of nigrostriatal dopaminergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)