In vitro pharmacologic characterization of a cholinergic receptor on outer hair cells

Acetylcholine (ACh) is the major neurotransmitter released from the efferent fibers in the cochlea onto the outer hair cells (OHCs). The type of ACh receptor on OHCs and the events subsequent to receptor activation are unclear. Therefore we studied the effect of agonists and antagonists of the ACh receptor on isolated OHCs from the guinea pig. OHCs were recorded from in whole cell voltage and current clamp configuration. ACh induced an increase in outward K+ current (IACh) which hyperpolarized the OHCs. No desensitization to ACh application was observed. Cs+ replaced K+ in carrying the IACh. The IACh is Ca(2+)-dependent, time and voltage sensitive, and different from the IKCa induced by depolarization of the membrane potential. When tested at 100 microM, several agonists also induced outward current responses (acetylcholine > suberyldicholine > or = carbachol > DMPP) whereas nicotine, cytisine and muscarine did not. The IACh response to 10 microM ACh was blocked by low concentrations of traditional and non-traditional-nicotinic antagonists (strychnine > curare > bicuculline > alpha-bungarotoxin > thimethaphan) and by higher concentrations of muscarinic antagonists (atropine > 4-DAMP > AF-DX 116 > pirenzepine). Pharmacologically, the ACh receptor on OHCs is nicotinic.

Nicotinic receptor activation facilitates GABAergic neurotransmission in the avian lateral spiriform nucleus

Whole-cell patch clamp recordings were performed in embryonic chick brain slices to characterize responses to nicotinic receptor activation in the mesencephalic lateral spiriform nucleus. Using intracellular recording, we previously reported the presence of functional high-affinity nicotinic sites in this nucleus that are insensitive to blockade with kappa- and alpha-bungarotoxin. We now report that nicotinic agonists not only produce an inward current in these cells, but also elicit a massive increase in the frequency of spontaneous postsynaptic currents without changing the amplitude distribution or risetime and decay kinetics of these events. The nicotinic receptor antagonist, dihydro-beta-erythroidine, blocks both the postsynaptic inward current and the enhancement of spontaneous postsynaptic currents. The spontaneous currents reverse at or near the chloride ion equilibrium potential and are completely blocked by 10 microM bicuculline, indicating that these events are likely to be GABAergic inhibitory postsynaptic currents. The nicotinic agonist-induced enhancement in inhibitory postsynaptic current frequency is blocked by 1.0 microM tetrodotoxin, demonstrating that the effect is mediated through the activation of voltage-dependent sodium channels. Nicotinic receptors are widely distributed in the central nervous system and in some cases are thought to modulate the release of various neurotransmitters. Our results show that activation of nicotinic receptors facilitates inhibitory neurotransmission in the avian lateral spiriform nucleus by increasing the frequency of spontaneous GABAergic postsynaptic currents. These data support a role for nicotinic receptors in the regulation of GABA release from nerve terminals in this nucleus.

Isoflurane inhibits nicotinic acetylcholine receptor-mediated 22Na+ influx and muscarinic receptor-evoked cyclic GMP production in cultured bovine adrenal medullary cells

The effects of isoflurane on 22Na+ influx, 45Ca2+ influx, catecholamine secretion and cyclic GMP production induced by three kinds of secretagogue (nicotinic agonists, veratridine and a high concentration of K+) have been investigated using cultured bovine adrenal medullary cells. (1) Isoflurane (1-6%) inhibited catecholamine secretion stimulated by carbachol, nicotine and dimethyl-4-phenylpiperazinium in a concentration-dependent manner. Isoflurane suppressed carbachol-evoked 22Na+ influx and 45Ca2+ influx at concentrations similar to those which suppressed catecholamine secretion. The inhibition of catecholamine secretion by isoflurane was not overcome by increasing the concentration of carbachol. (2) The inhibitory effects of isoflurane on veratridine-induced 22Na+ influx, 45Ca2+ influx and catecholamine secretion became evident when the concentration of isoflurane was raised to 4-6%, i.e. 2-3 fold higher than the concentrations (1-2%) employed clinically. (3) High K(+)-evoked 45Ca2+ influx and catecholamine secretion were not affected by isoflurane (1-6%). (4) Isoflurane (1-6%) attenuated the production of cyclic GMP caused by muscarine, but not that caused by atrial natriuretic peptide or by sodium nitroprusside. These results suggest that isoflurane, at clinical anesthetic concentrations, inhibits nicotinic acetylcholine receptor-mediated cell responses as well as muscarinic receptor-mediated cyclic GMP production in adrenal medullary cells.

Cardiovascular responses to cholinergic agonists in sinoaortic denervated rats

1. The cardiovascular effect of systemic nicotinic and muscarinic cholinergic stimulation were studied in conscious sham operated and sinoaortic denervated (SAD) rats, 7 days after the corresponding operation. 2. The administration of the nicotinic ganglionic agent, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 50-100 micrograms.kg-1, i.v.) induced a fall of heart rate that was significantly higher in SAD rats than in sham rats. DMPP induced in sham rats an increase of arterial pressure but in SAD animals a biphasic response: an initial hypotension followed by an increase of arterial pressure. 3. Under muscarinic blockade, DMPP only showed a pressor and tachycardic action in both groups of rats without differences between them. 4. The muscarinic agonist, carbachol (0.1-10 micrograms.kg-1, i.v.) showed the same hypotensive and bradycardic action in both groups of rats. 5. Our results suggest that after 7 days of SAD, differences in the response to DMPP between sham and denervated animals could be due to the loss of baroreflex mechanisms. The increased bradycardic effect of DMPP in SAD rats could be mediated by a supersensitivity of parasympathetic ganglionic nicotinic receptors, whilst the sympathetic ganglionic nicotinic receptors remained unaltered. On the other side, the cardiovascular muscarinic responses to carbachol remain unaffected in SAD rats.

Taurine and myocardial noradrenaline

Taurine (CAS 107-35-17) is an anticonvulsant used also as an adjunct in the treatment of cardiovascular disorders. Therefore, we studied its effects noradrenergic transmission in the isolated rabbit heart prelabelled with 3H-noradrenaline. At the concentrations of 1 and 10 mmol/l taurine treatment was without effect on the neuronal and extraneuronal uptake of noradrenaline by the myocardial tissue. At the highest concentration, it decreased the spontaneous release of the transmitter and enhanced its catabolism. Without any significant effect on tyramine-induced noradrenaline release, taurine decreased the release of the amine induced by dimethylphenylpiperazinium and nerve stimulation. These results suggested that taurine may reduce the peripheral sympathetic activity by accelerating noradrenaline catabolism and decreasing its release probably via its ability to prevent a rise of intracellular calcium ion.

Blockade of nicotinic receptor-mediated release of dopamine from striatal synaptosomes by chlorisondamine and other nicotinic antagonists administered in vitro

1. Central nicotinic receptor function examined in vitro, by measuring nicotine-induced [3H]-dopamine release from rat striatal synaptosomes. 2. The agonists (-)-nicotine, acetylcholine, 1,1-dimethyl-4-phenylpiperazinium (DMPP) and cytisine (10(-7)-10(-4) M) all increased [3H]-dopamine release in a concentration-dependent manner. Cytisine did not produce a full agonist response, compared to the other agonists. 3. The actions of nicotine, acetylcholine and cytisine were largely dependent on external Ca2+. In contrast, DMPP (10(-5) and 10(-4) M) evoked a marked release of [3H]-dopamine even in the absence of Ca2+. Nevertheless, in the presence of external Ca2+, responses to DMPP were completely blocked by the nicotinic antagonists chlorisondamine and mecamylamine (5 x 10(-5) M); in the absence of external Ca2+, blockade was only partial. 4. Chlorisondamine, mecamylamine and dihydro-beta-erythroidine (10(-8)-10(-4) M) produced a concentration-dependent block of responses to nicotine (10(-6) M). Approximate IC50 values were 1.6, 0.3 and 0.2 x 10(-6), respectively. Chlorisondamine and mecamylamine blocked responses to nicotine (10(-7)-10(-4) M) insurmountably, whereas dihydro-beta-erythroidine behaved in a surmountable fashion. 5. The occurrence of use-dependent block was tested by briefly pre-exposing the synaptosomes to nicotine during superfusion with antagonist, and determining the response to a subsequent nicotine application. Consistent with a possible channel blocking action, brief pre-exposure to agonist increased the antagonist potency of chlorisondamine (approximately 25 fold). No significant use-dependent block was detected with dihydro-beta-erythroidine.

Developmental changes in the response of rat isolated duodenum to nicotine

Developmental changes in the response to ganglionic stimulants, nicotine and dimethylphenylpiperazinium, were investigated in rat isolated duodenum by recording isotonic mechanical activity. The duodenal response to nicotine/dimethylphenylpiperazinium (3 x 10(-7) to 10(-3) M) in neonatal rats was contraction, which was blocked by hexamethonium, tetrodotoxin and hyoscine. The response to nicotine/dimethylphenylpiperazinium (10(-6) to 10(-4) M) in the adult duodenum was relaxation, which was blocked by tetrodotoxin and hexamethonium, but by neither guanethidine nor hyoscine. The transition of the response to nicotine/dimethylphenylpiperazinium from contraction to relaxation occurred at around the 3rd postnatal week. Nicotine-induced relaxation of adult duodenum was significantly inhibited by preincubation with alpha-chymotrypsin, a proteolytic enzyme, and a combination of nucleotide pyrophosphatase and 8-phenyltheophylline, a P1 purinoceptor antagonist. Nicotine-induced relaxation was desensitized by alpha, beta-methylene ATP, a stable P2x purinoceptor agonist. These results suggest that the contractile response of isolated duodenum to nicotine is mediated through cholinergic transmission in neonatal rats and the relaxant response is mediated through non-adrenergic, non-cholinergic transmission, which involves both peptidergic and purinergic transmission, in adult rats.

5-HT3 receptors do not mediate vagally-induced relaxation or contraction of the isolated stomach of the guinea-pig

1. We have tested whether 5-HT3 receptors mediate vagally-induced relaxation or contraction of the isolated stomach of the guinea-pig. 2. The antagonists of 5-HT3 receptors, ondansetron (1-10 microM) or metoclopramide (1-30 microM) did not inhibit vagally-induced relaxations at low concentrations but partially inhibited them at 30 microM or 60 microM, respectively. These higher concentrations of ondansetron and metoclopramide also inhibited relaxations induced by 1,1-dimethyl-4-phenylpiperazinium iodide (30 microM) but did not affect those induced by glyceryl trinitrate (0.7-1.1 microM). 3. Desensitization to 5-HT (100 microM) or 2-Me-5-HT (100 microM) did not affect relaxations or contractions induced by vagal stimulation. 4. Ondansetron (30 microM) or metoclopramide (60 microM) did not inhibit vagally-induced gastric contraction. 5. Thus, 5-HT3 receptors do not mediate vagally-induced relaxation or contraction in the guinea-pig stomach.

Inhibition of neuronal nicotinic acetylcholine receptors by imipramine and desipramine

The actions of two structurally related tricyclic antidepressants on neuronal nicotinic acetylcholine receptors were investigated in human neuroblastoma (SY-SY5Y) cells, using whole-cell patch-clamp recordings. Both desipramine and imipramine reversibly inhibited inward currents evoked by application of the nicotinic receptor agonist dimethylphenylpiperazinium iodide (30-300 microM) with IC50 values of 0.17 microM and 1.0 microM respectively (holding potential -70 mV). The degree of current inhibition caused by either tricyclic compound was unaffected by agonist concentration (30-300 microM). The effects of desipramine were voltage-independent over the range -40 mV to -100 mV, and inhibition caused by imipramine only increased very slightly with membrane hyperpolarization over the same range. These results indicate that tricyclic antidepressants can inhibit neuronal nicotinic acetylcholine receptors by mechanisms which are distinct from their actions at non-neuronal nicotinic acetylcholine receptors.

Opposite modulation by muscarinic M1 and M3 receptors of acetylcholine release from guinea pig ileum as measured directly

Muscarinic receptor agonist and antagonist effects on acetylcholine release evoked by electrical or dimethylphenylpiperazinium stimulation from guinea pig ileum were evaluated by measuring acetylcholine with a high performance liquid chromatography-electrochemical detector system. AF102B (cis-2-methylspiro-(1,3-oxathiolane-5,3')-quinuclidine), a muscarinic M1 receptor agonist, increased markedly the evoked release of acetylcholine. In contrast, pirenzepine decreased the evoked acetylcholine release. 4-DAMP (4-diphenyl-acetoxy-N-methylpiperidine methiodide) and p-F-HHSiD (p-fluoro-hexahydrosiladifenidol), muscarinic M3 antagonists, increased the release of acetylcholine. Atropine enhanced acetylcholine release to a similar extent while bethanechol reduced the electrically evoked acetylcholine release. This reduction was virtually unaffected by methoctramine, but was antagonized by 4-DAMP or p-F-HHSiD. The results from direct determination of acetylcholine suggest that, in contrast to autoinhibition by stimulation of muscarinic M3 receptors, stimulation of presynaptic muscarinic M1 receptors is predominantly involved in enhancement of the acetylcholine release from guinea pig ileal nerves, and that AF102B functions as a muscarinic M1 agonist in this peripheral neuron.

Nicotinic depolarizations of rat medial pontine reticular formation neurons studied in vitro

Either muscarinic or nicotinic cholinergic activation of the medial pontine reticular formation evokes a behavioral state, indistinguishable in most respects from that of natural rapid eye movement sleep. However, the presence of physiologically relevant nicotonic receptors has not been described. Intracellular current and single electrode voltage clamp recordings were used to analyse the electrophysiological responses of rat medial pontine reticular formation neurons to nicotinic activation in vitro. In response to the nicotonic agonist, 1,1-dimethyl-4-phenylpiperazinium iodine, depolarization in association with an inward current was observed in 70% of the medial pontine reticular formation neurons. This effect was insensitive to the muscarinic antagonist atropine and the nicotinic ganglionic antagonists mecamylamine and hexamethonium. However, the neuromuscular nicotinic antagonist D-tubocurare and dihydro-beta-erythroidine were effective. This is consistent with a cholinergic activation of medial pontine reticular formation neurons evoking a rapid eye movement sleep-like behavioral state, at least in part, by nicotinic receptors on these neurons.

The arginine/nitric oxide pathway modulates sphincter of Oddi motor activity in guinea pigs and rabbits

BACKGROUND

Nitric oxide acts as a neurotransmitter of nonadrenergic, noncholinergic pathways. The present study examined the role of the L-arginine/NO pathway on sphincter of Oddi motility.

METHODS

In anesthetized guinea pigs and rabbits, intracholedochal pressure was recorded by a perfused catheter whose open tip was maintained close to the sphincter of Oddi lumen. The contractile responses to cholecystokinin or bethanechol were recorded before and after treatment with either a specific NO synthetase inhibitor (NG-nitro-L-arginine methyl ester) or a donor of NO (sodium nitroprusside). The effect of NO synthase inhibition on isometric tension generated by muscle from rabbit sphincter of Oddi was tested in vitro.

RESULTS

Tonic pressure and phasic contractions significantly increased after NO synthase inhibition; responses to cholecystokinin and bethanechol were enhanced. In contrast, sodium nitroprusside reduced the response to cholecystokinin. Isolated muscle from rabbit sphincter of Oddi relaxed in response to the nicotinic agonist dimethyl-4-phenylpiperazinium. Relaxation was eliminated by either the neurotoxin tetrodotoxin or by NG-nitro-L-arginine methyl ester. NO synthase inhibition also suppressed the relaxatory response induced by electrical field stimulation. Calcium-dependent activity of NO synthase was detected in fresh homogenates from guinea pig and rabbit sphincter of Oddi tissue.

CONCLUSIONS

NO that is locally generated by a constitutive NO synthase regulates sphincter of Oddi motor function.

Expression of endogenous NMDAR1 transcripts without receptor protein suggests post-transcriptional control in PC12 cells

Expression of RNA for the NMDAR1 subunit of the N-methyl-D-aspartate receptor was detected by Northern hybridization in both nerve growth factor-differentiated and undifferentiated rat pheochromocytoma (PC12) cells. The NMDA receptor type 1 (NMDAR1) message in PC12 cells was similar in size to that expressed in hippocampal neurons. PC12 cell cDNAs that were amplified by polymerase chain reaction with primers flanking the coding region of NMDAR1 corresponded to the NMDAR1 splice variant NMDA receptor type 1 isoform C (NMDAR1C). Using calcium imaging or patch-clamp recording, no functional NMDA-gated ion channels were found in PC12 cells. A monoclonal antibody against NMDAR1 was developed in order to investigate whether or not NMDAR1 protein was present in PC12 cells. Only trace amounts of NMDAR1 protein were found in native PC12 cells. However, expression of NMDAR1 protein was detected in PC12 cells that were transfected with an expression vector containing an NMDAR1C clone under control of a cytomegalovirus promoter. These findings suggest that the expression of NMDAR1 protein in PC12 cells may be controlled by post-transcriptional mechanisms. The PC12 cell line may serve as a model system for the study of the transcriptional, post-transcriptional, and translational regulation of NMDAR1. Furthermore, the presence of NMDAR1 RNA in a particular cell type may not necessarily indicate expression of NMDAR1 protein.

The nicotinic acetylcholine receptor of the bovine chromaffin cell, a new target for dihydropyridines

The effects of 1,4-dihydropyridine derivatives on divalent cation transients and catecholamine release stimulated by either high K+ or the nicotinic receptor agonist dimethyl-phenyl-piperazinium (DMPP) have been compared in bovine adrenal chromaffin cells. The activation of Ca2+ entry pathways was followed by measuring 45Ca2+ or Mn2+ uptake, or by the changes of [Ca2+]i in fura-2-loaded chromaffin cells. Various dihydropyridine Ca2+ channel blockers (nimodipine, PCA50938, nifedipine, nitrendipine, furnidipine) abolished the DMPP-mediated effects, but prevented only partially the activation by high [K+]0 of 45Ca2+ uptake. The IC50 for DMPP-induced activation was around 1 microM. The L-type Ca2+ channel activator Bay K 8644 potentiated the uptake of 45Ca2+ induced by K+ depolarization at concentrations between 10 nM and 1 microM, but completely inhibited the uptake of 45Ca2+ by DMPP (IC50, 0.9 microM). Both high [K+]0 and DMPP produced membrane depolarization as measured using bis-oxonol. The DMPP-evoked, but not the K(+)-evoked membrane depolarization was prevented by Na+ removal, suggesting that the depolarization was due to Na+ entry through the acetylcholine receptor ionophore. Nimodipine at 10 microM abolished the depolarization induced by DMPP, leaving the K(+)-evoked depolarization unaffected. Tetrodotoxin (2 microM) did not affect the DMPP- or high K(+)-mediated cell depolarization. Whole-cell inward current evoked by 100 microM DMPP (IDMPP) was measured in cells voltage-clamped at -80 mV. Nimodipine (10 microM) reduced IDMPP by 36%; Bay K 8644 (10 microM) inhibited IDMPP by 67%. DMPP-evoked catecholamine release from superfused chromaffin cells was reduced by over 90% with 10 microM nimodipine; in contrast, K(+)-evoked release was decreased by 20%.(ABSTRACT TRUNCATED AT 250 WORDS)

Residues 1 to 80 of the N-terminal domain of the beta subunit confer neuronal bungarotoxin sensitivity and agonist selectivity on neuronal nicotinic receptors

Standard two electrode voltage clamp techniques were used to investigate the response of neuronal nicotinic acetylcholine receptors, expressed in Xenopus oocytes, to various agonists and neuronal bungarotoxin (NBT). The beta subunit is an important determinant of the receptor's pharmacological profile. Co-expression of alpha 4 and beta 2 subunits produced a receptor that was relatively insensitive to cytisine and nicotine and inhibited by NBT, whilst the alpha 4 beta 4 combination produced a receptor that was highly sensitive to cytisine and nicotine but resistant to toxin. The first 80 amino acids of the N-terminal domain of the beta subunit are implicated in these characteristics, since the combination of alpha 4 with a hybrid beta subunit comprising amino acids 1-->80 of beta 2 and 81-->416 of beta 4 became relatively insensitive to nicotine and cytisine and resistant to inhibition by neuronal bungarotoxin.

Electrogenic ion secretion in proximal, mid and distal colon from fed and starved mice

1. Electrogenic ion transport was monitored in vitro as the short-circuit current (Isc in microA/cm2) across proximal, mid and distal colon removed from fed and 48 hr-starved Swiss albino mice (Mus muscaris). 2. Electrogenic secretion was induced either with serosal bethanechol (muscarinic agonist), DMPP (nicotinic agonist) or dibutyryl-cyclic AMP (DbcAMP). Proximal and distal colon from starved mice showed greater electrogenic secretion in response to bethanechol than those the fed controls while DMPP and DbcAMP did not activate the hypersecretion. 3. In the distal colon, starvation induced a large increase in the basal Isc that was unaffected by mucosal amiloride but was inhibited by tetrodotoxin (TTX) and by diphenylamine-2-carboxylic acid (DPC) unlike the fed basal Isc. Bethanechol activated a biphasic response consisting of a transient decrease in the Isc followed by a sustained increase both of which were significantly greater in the starved than the fed tissue and were inhibited by TTX, DPC and atropine but not hexamethonium. 4. Starvation enhances the secretory response to muscarinic activation in proximal and distal colon and induces an increased basal electrogenic (Cl-) secretion in the distal colon stimulated by an augmented neural tone.

In vivo evidence for adrenal catecholamine release mediated by nonnicotinic mechanism: local medullary effect of VIP

The aim of the present study was 1) to develop a model in anesthetized dogs in which local infusion of a given substance could be made to the adrenal gland without any systemic effects and 2) to show in this model the potential existence of a nonnicotinic mechanism involved in adrenal catecholamine secretion. Plasma catecholamine concentrations were determined by an high-performance liquid chromatography method. The local infusion into the left adrenolumbar artery (0.5 ml/min for 1 min) of either dimethylphenylpiperazinium (0.3-3.0 micrograms/ml) or vasoactive intestinal peptide (0.1-10.0 micrograms/ml) resulted in dose-dependent increases in both epinephrine and norepinephrine secretions. Neither aortic pressure nor plasma catecholamine levels altered during the drug infusion. The net increases in adrenal catecholamine secretion obtained with dimethylphenylpiperazinium were abolished by the nicotinic blockade with pentolinium (2 mg/ml, 0.5 ml/min for 2 min). However, the net catecholamine responses to vasoactive intestinal peptide remained unaffected in the presence of the same dose of pentolinium. The results suggest that there exists a nonnicotinic mechanism that may be implicated in the local regulation of medullary catecholamine secretion in the dog adrenal gland. This model may be a useful tool for studying local physiological role(s) of various neurotransmitters and neuromodulators in the adrenal secretory function in vivo.

Alterations of immune status induced by the sympathetic nervous system: immunomodulatory effects of DMPP alone and in combination with morphine

The aim of the present study was to explore the involvement of the sympathetic nervous system (SNS) in the immunomodulatory effects of morphine in Lewis rats and to assess the effects of alterations in SNS activity on immune status. In the first experiment, sympathetic tone was elevated by administering the ganglionic stimulant 1,1-dimethyl-4-phenylpiperazinium (DMPP) in doses of 0, 0.01, 0.1, and 1.0 mg/kg, sc, 5 min before the sc administration of 15 mg/kg morphine or saline. Animals were sacrificed 1 h after the morphine injection and multiple in vitro immune assays were then conducted. Although DMPP did not significantly enhance morphine's suppressive effects in the spleen and blood mitogen stimulation assays or the splenic natural killer (NK) cell assay, DMPP alone produced effects on immune status in saline-treated animals. Therefore, a second experiment was conducted to examine the immunomodulatory effects of increasing peripheral sympathetic outflow in greater detail. Animals were administered a wider dose range of DMPP (0, 0.005, 0.05, 0.5, and 5.0 mg/kg, sc) 30 min prior to sacrifice and an expanded repertoire of immune assays was conducted. DMPP dose-dependently suppressed the mitogenic responsiveness of splenic T lymphocytes, splenic NK cell activity, and interleukin-2 (IL-2) and gamma-interferon production by stimulated splenocytes. DMPP did not alter the total number of splenic leukocytes or the proliferative response of splenic B lymphocytes. In the mesenteric lymph nodes, DMPP had no effect on mitogenic responsiveness, the production of IL-2 or the total number of leukocytes. In the blood, however, DMPP increased mitogenic responsiveness at intermediate doses and decreased proliferation at higher doses. DMPP also dose-dependently decreased the number of blood leukocytes/ml. Taken together, these results indicate that increasing peripheral sympathetic outflow results in profound effects on immune status that depend upon the degree to which SNS activity is altered, the compartment of the immune system, and the lymphocyte subtype.

Nicotinic and muscarinic modulations of excitatory synaptic transmission in the rat prefrontal cortex in vitro

The importance of the cholinergic innervation of the neocortex in cognitive functions has been shown in a number of clinical and animal studies. Until recently, attempts to study the mode of action of acetylcholine in the neocortex have concentrated on muscarinic effects, whereas cholinergic actions mediated by nicotinic receptors have been difficult to demonstrate. The present work was undertaken to study the mechanism of action of nicotinic agents on cortical neurons and compare it to muscarinic effects by means of intracellular recordings in a slice preparation. The study was performed in the prelimbic area of the rat prefrontal cortex, a cortical region particularly involved in cognitive processes. Recordings were made from pyramidal cells located in layers II/III and synaptic potentials were evoked by stimulation of superficial cortical layers. Iontophoretic applications of nicotinic agonists (nicotine, dimethylphenylpiperazinium, cytisine) increased the amplitude of the monosynaptic excitatory postsynaptic potential mediated by non-N-methyl-D-aspartate glutamate receptors in 14% (22/159) of cells. This effect was abolished by the selective nicotinic blocker, neuronal bungarotoxin (IC50 = 0.6-0.7 microM) and by dihydro-beta-erythroidine (IC50 = 20-30 microM), whereas hexamethonium, mecamylamine, curare and alpha-bungarotoxin were ineffective. The nicotinic agonists did not change resting membrane potential, input resistance or current-voltage relationship. They also did not affect the depolarizations produced by glutamate applied by iontophoresis in the somatic or dendritic area. In contrast, the muscarinic agonists (muscarine, acetyl-beta-methylcholine) decreased the amplitude of the excitatory postsynaptic potential in 100% of the neurons tested. Atropine was more effective (IC50 = 0.08 microM) than pirenzepine (IC50 = 2 microM) to antagonize the muscarinic action. These effects were observed in the absence of any direct postsynaptic change in membrane potential or input resistance, provided that the site of the iontophoretic application was more than 100 microM distant from the soma. The muscarinic agonists did not influence the actions of iontophoretically applied glutamate. These results suggest that nicotinic and muscarinic agonists modulate excitatory synaptic transmission mediated at dendritic sites by non-N-methyl-D-aspartate glutamate receptors, possibly through a presynaptic action. Thus ascending cholinergic systems may take part in information processing in the prefrontal cortex through the control of ongoing excitation to pyramidal cells.

Agonist pharmacology of the neuronal alpha 7 nicotinic receptor expressed in Xenopus oocytes

The potencies and efficacies of seven agonists at chick alpha 7 nicotinic receptors expressed in Xenopus oocytes were determined by whole cell recording. (+)-Anatoxin-a was the most potent agonist (EC50 = 0.58 microM) and acetylcholine was the least potent (EC50 = 320 microM). The rank order of agonist potencies was: (+)-anatoxin-a >> cytisine > (-)-nicotine > (+)-nicotine > DMPP > 1-acetyl-4-methylpiperazine methiodide > acetylcholine. DMPP evoked only very small currents: comparison of maximally effective agonist concentrations showed that DMPP was only one-fifth as efficacious as other agonists. Previously published IC50 values for rat brain [125I]alpha-bungarotoxin sites show a similar agonist profile, and the identity of homo-oligomeric alpha 7 receptors with native alpha-bungarotoxin-sensitive neuronal nicotinic receptors is discussed.

Nicotinic agonists antagonize quantal size increases and evoked release at frog neuromuscular junction

1. Previous studies at the frog neuromuscular junction showed that quantal size can be increased two- to fourfold by a variety of treatments, including prior exposure to hypertonic solution (which activates protein kinase A) and insulin (which acts via an unknown pathway). Size increases largely because quanta contain more acetylcholine (ACh). 2. Now the effects of cholinergic agonists on the increases in quantal size have been studied. One muscle from a frog was kept for 2 h in hypertonic sodium gluconate solution. The miniature endplate potential (MEPP) sizes were measured in saline: they had increased about fourfold. The paired muscle went through the same experimental sequence, except that an agonist was added to the hypertonic gluconate solution. Again MEPP sizes were measured in saline. The increase in quantal size was significantly depressed by 0.2 microM 1,1-dimethyl-4-phenyl- piperazinium (DMPP). The sequence of effectiveness of agonists was: DMPP > carbachol > or = ACh = cytisine > oxytremorine. This sequence suggests that the receptor belongs in the nicotinic class. 3. Quantal size is doubled after 1 h in insulin. One micromolar carbachol largely blocked the size increase. 4. The effects of cholinergic antagonists were tested by keeping the experimental muscle in hypertonic gluconate solution containing 1 microM carbachol plus an antagonist. The controls were paired muscles kept in hypertonic gluconate solution (without carbachol or antagonist). MEPP sizes were measured in saline. The depressing action of carbachol on the increase in MEPP size was blocked by 0.2 microM neuronal-bungarotoxin (nBTX). The sequence of effectiveness of antagonists was: nBTX > trimethaphan > d-tubocurarine (dTC). Ten micromolar atropine (without carbachol) depressed the increase in quantal size. Therefore, the antagonist potency of atropine could not be adequately tested. Carbachol action was not blocked by 10 microM hexamethonium or 10 microM mecamylamine. 5. Once quanta are made large they can be converted back to normal size by cholinergic agonists. Muscles in which quantal size had been enlarged were exposed to hypertonic solutions containing the agonist. Quantal size was reduced to a fraction of its former value when the hypertonic solution contained 1 microM carbachol- or 1 microM DMPP. One micromolar oxytremorine had no effect. Carbachol still reduced quantal size when applied in low-Ca2+ solutions, so it does not appear to act by elevating intracellular [Ca2+]. 6. Previous work suggested that the treatments produce a subpopulation of large quanta that are positioned for release.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium entry through nicotinic receptor channels and calcium channels in cultured rat superior cervical ganglion cells

1. Patch-clamp techniques in conjunction with indo-1 fluorescent measurements were used to measure increases in intracellular free calcium concentration and membrane conductance induced by the activation of nicotinic and calcium channels in cultured rat sympathetic neurons. 2. Under voltage-clamp conditions, pressure application of the nicotinic agonist DMPP (1,1-dimethyl-4-phenylpiperazinium iodide, 100 microM, 100 ms) increased [Ca2+]i by 193 +/- 26 nM at a clamp potential of -60 mV. This was accompanied by an inward current of -4.53 +/- 0.89 nA, giving a mean ratio of the delta (Ca2+]i to the total inward charge transfer of 42.7 nmoles per litre of free calcium per nanocoulomb of charge (M/q ratio). 3. The DMPP-induced current and associated delta [Ca2+]i were reduced by mecamylamine (100 nM-10 microM) but were unaffected by alpha-bungarotoxin (100 nM) or cadmium (100 microM). 4. The M/q ratio was not affected by the holding potential (from -80 to -40 mV) but was a function of the external calcium concentration. 5. The M/q ratio was reduced by increasing the intracellular calcium buffering capacity and increased by heparin but not affected by ryanodine or by depletion of the caffeine-sensitive calcium store. 6. Under the same recording conditions, we quantified the increase in [Ca2+]i associated with activation of the voltage-dependent calcium current. On average at -60 mV, the M/q ratio of this highly calcium-selective permeability was 1961 mM nC-1, which is 46 times that obtained for the nicotinic channel. 7. Assuming constant-field theory, ion-substitution experiments suggest that in 2.5 mM external calcium, the permeability sequence for the nicotinic conductance was Cs+ < Li+ < Na+ < K+ < Ca2+. 8. We conclude that the nicotinic channels in rat sympathetic neurones are significantly permeant to Ca2+ and that the influx of Ca2+ through these channels is the principal cause of the rise in [Ca2+]i seen under voltage clamp.

Identification of acetylcholine receptors in adult rat trigeminal ganglion neurons

Nicotinic acetylcholine receptors (nAChRs) were identified in a subpopulation of cultured adult rat trigeminal ganglia (TG) neurons by whole-cell patch-clamp recordings. Dimethylphenylpiperazinium (DMPP), a nAChR agonist, induced inward currents in 21/68 of TG neurons having soma diameters greater than 28 microns. These currents were inhibited by hexamethonium, mecamylamine and atropine, indicating the presence of neuronal ganglionic-type nAChRs. This interpretation is consistent with the finding that the nicotine- or DMPP-induced currents were not inhibited by alpha-bungarotoxin (alpha-Bng) in 5 of the 9 cells tested with this compound. However, in 2 of the 9 cells tested, the DMPP-induced currents were completely inhibited by alpha-Bng, and in the remaining two cells tested, the currents were partially inhibited by alpha-Bng. About 22% of the cells having diameters > or = 28 microns were specifically labeled with FITC-labeled alpha-Bng, whereas only 2% of the cells with soma diameters < 28 microns were labeled. These data taken together suggest that more than one subtype of nAChR is present in TG.