Effects of "non-quantal" acetylcholine on postsynaptic membrane senstitivity: Effects of ouabain,and mimicry by exogenous acetylcholine

The development of postsynaptic potentiation (PSP) and desensitization due to "non-quantal" acetylcholine that occurs when acetylcholinesterase (AChE) is inhibited was studied using the Na,K-ATPase inhibitor, ouabain, to alter (initially increasing, then decreasing) the level of non-quantal acetylcholine secretion, and exogenous acetylcholine. When ouabain increased non-quantal secretion the time constant () of the miniature end-plate current (MEPC) decay increased, i.e., PSP developed. The later the application of ouabain relative to inhibition of AChE, the greater the degree of PSP. During the next phase when non-quantal secretion was inhibited the MEPC time course shortened more rapidly than in the controls, i.e., desensitization occurred. If ouabain abolished non-quantal secretion before AChE had been inhibited did not change, and neither PSP nor desensitization developed. When AChE was not inhibited ouabain had no effect on . When ACh was continuously applied at 20 nmol·liter^–1, similar to the nonquantal concentration, the shortening of slowed down, and the signal amplitude declined more rapidly than in controls. Addition of exogenous ACh (50 nmol·liter^–1) after acceleration of MEPC decay had developed caused to increase to its initial value. The combined appearance of PSP and desensitization during the action of non-quantal ACh, and the sustained desensitization after removal of released ACh from the synaptic cleft are discussed.S. V. Kurashova Institute of Medicine, Russian Federation Ministry of Public Health, Kzan. Translated from Neirofiziologiya, Vol. 24, No. 4, pp. 396–404, July–August, 1992.     

Effects of vinblastine and colchicine on the postsynaptic membrane at the frog neuromuscular junction

The possible effects of the alkaloids vinblastine and colchicine on the postsynaptic membrane of the frog neuromuscular junction were investigated using voltage-clamp techniques. Concentrations of vinblastine and colchicine which had been shown to exert no effect on the amplitude and duration of miniature endplate currents (MEPC) and the current-voltage relationship of low-quantal endplate currents (EPC) together with the coefficient of voltage-dependent EPC decay did produce a considerable rise in the amplitude of response to iontophoretically applied acetylcholine (ACh). In addition, vinblastine and colchicine accelerate MEPC and EPC during acetylcholine esterase inhibition while further depressing the amplitude of multi-quantal EPC succeeding at the rate of 10 Hz as well as response to regular (5–10 Hz) application of ACh from a micropipet. The dosage-frequency effects of vinblastine and colchicine on the postsynaptic membrane (as described) are presumed to be unconnected with the action of these agents on muscle fiber cytoskeleton but the results of accelerated desensitization of cholinoreceptors.S. V. Kurashov Medical Institute, Kazan. Translated from Neirofiziologiya, Vol. 20, No. 1, pp. 75–81, January–February, 1988.     

End plate currents at the physiological level of quantal secretion and after potentiation of transmitter release by 4-aminopyridine

The role of postysynaptic potentiation (PSP) and asynchronous secretion of acetylcholine (ACh) in the generation of multiquantal currents and end plate currents (EPC) was investigated under voltage clamp conditions in transected sartorius muscle of the frog before and after 4-aminopyridine (4-AP) treatment. Compared with miniature EPC (MEPC), showing an average quantum content of 249, multiquantal EPC has a larger amplitude, longer rise-time, and longer decay-time (_epc). Magnesium ions (6–10 mM) reduce the amplitude and _spec of EPC without affecting its rise-time. Rhythmical stimulation (10 Hz for 60 sec) results in reduced amplitude and but increased rise-time of EPC. D-turbocurarine (5×10^–7 M) and -bungarotoxin (1×10^–5 gm/ml) diminishes the difference between _epc and _mepc. In the presence of 4-AP, all these effects are much more pronounced. It is proposed that asynchronous secretion of ACh from motor nerve teminals causes prolongation of the rise-time and reduction of the amplitude of EPC but has little or no effect upon the decay rate of EPC. The slow decay of multiquantal EPC, both in the absence and in the presence of 4-AP, is almost entirely due to postynaptic interaction of ACh quanta, i.e., PSP.Kurashov Meidcal Institute, Kazhan. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 48–56, January–February, 1991.     

Aftercurrent in submucous neurons of guinea pig

Membrane currents elicited by iontophoretic applications of acetylcholine (ACh currents) were recorded from neurons of the guinea pig submucous plexus using a whole-cell patch-clamp recording technique. The ACh currents declined to 5–20% of their peak amplitude during about 2 sec-long application of ACh. After the end of ACh application, a transient increase of the ACh current (the aftercurrent, AC) was observed. The most probable mechanisms responsible for the ACh current decline and for the appearance of the AC are transitions of nicotinic ACh receptors first to a desensitized state, and then to the state with an open ionic channel, respectively.Neirofiziologiya/Neurophysiology, Vol. 25, No. 4, pp. 291–296, July–August, 1993     

Effects of "non-quantal" acetylcholine on miniature endplate currents in mice

The possibility of postsynaptic potentiation (PSP) and desensitization developing due to nonquantal acetylcholine (ACh) secretion was investigated in mouse diaphragm with reference to time-amplitude relationships of miniature endplate currents (MEPC). The H effect (which characterizes nonquantal secretion (NS) of ACh) fell to zero over 3 h under the action of armine-induced inhibition of acetylcholinesterase (AChE) at a temperature of 20°C. A decline in the decay time constant () of MEPC unaccompanied by observable alteration in MEPC amplitude occurred at the same time. This accelerated decay of MEPC was not observed in the absence of NS (the early stages of denervation). Start of NS did not show any effect on maximum retardation of MEPC decay due to AChE inhibition, indicating that no PSP sets in under the effects of non-quantal secretion. The effect of decline in accelerated with a rise in temperature; it could be reproduced with neostigmine replacing armine, while remained unchanged in the time spells investigated with AChE in its active state. Non-quantal ACh is not thought to produce substantial retardation of MEPC decay, although it does bring about desensitization, signs of which may be partially masked owing to concurrent onset of PSP.S. V. Kurashov Medical Institute, Kazan'. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 507–513, July–August, 1990.     

Postsynaptic potentiation and desensitization at the frog neuromuscular junction produced by repeated stimulation of the motor nerve

Investigations were performed on a split neuromuscular preparation of frog sartorial muscle during acetylcholinesterase inhibition. A study was made of the part played in postsynaptic potentiation (PSP) and desensitization (DS) in changes in the amplitude and time course of miniature endplate currents (MEPC) recorded directly after regular stimulation of the motor nerve at a frequency of 10 Hz for 5 or 60 sec, producing short and long series of multiquantal endplate currents (EPC) respectively. After the short train the amplitude of MEPC could hardly be distinguished from initial level, while the decay time constant (MEPC) increased by 32%, indicating PSP. Comparable but more pronounced biphasic changes occurred in the time course of endplate currents. These effects were not observed when acetylcholinesterase was uninhibited. Both PSP and DS were restored when 1×10^–6 M exogenous acetylcholine was added to the bath. The ratio between them could be changed by aprodifen — a substance which accelerates desensitization.S. V. Kurashov Medical Institute, Ministry of Public Health of the RSFSR, Kazan'. I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 645–654, September–October, 1986.     

Comparison of Pre- and postsynaptic process kinetics in neuro-muscular synapse using the end-plate current model

The effect of correlation between the kinetics of acetylcholine (ACh) quanta release and the kinetics of the postsynaptic membrane channel activity on end-plate currents (EPCs) was studied in model experiments. The presynaptic process was described by a transmitter secretion probability distribution (SPD), and the postsynaptic process by characteristicsof miniature EPC (MEPC) with standard amplitude and variable decay duration (_MEPC). The SPD was represented by a curve with an S-like rise and exponential decay. The main portion of the data was analyzed after being adjusted to 20°C. The effect of the EPC amplitude loss (the difference between the EPC amplitudes in cases of synchronous and non-synchronous release of the ACh quanta) due to temporal dispersion of the transmitter secretion process was 43% at _MEPC=1.0 msec and 20% at _MEPC=4–5 msec under conditions when acetylcholinesterase (AChE) was inhibited.When _MEPC varied over a wide range, the effect of the EPC amplitude loss could be described by a curve with steeply and gently sloping portions; the boundary between these two portions corresponded to approximately 1.0 msec _MEPC, which is nearly the channel lifetime at resting membrane potential. The rise time of the EPC only slightly related to changes in _MEPC. The rate of EPC decay exceeded that in MEPC only when the lifetimes of the ACh-activated ionic channels were lower than their physiological values. In this case, the kinetics of transmitter release became a factor determining the EPC decay time course.Three variants of an increase in temporal dispersion of the ACh quanta secretion were modeled for constant _MEPC: one resulting from an increase in the SPD rising phase; a second resulting from equal increases in the SPD rising and declining phases, and a third resulting from an increase in the SPD declining phase. The results of the first variant corresponded most closely to experimental data on changes in EPC during a short-term period of rhythmical activity. This was also characterized by minimal effect of EPC amplitude loss.It has been suggested that the interrelation between the parameters of SPD and _MEPC is an important factor regulating the amplitude and duration of the postsynaptic signal. The maintenance of this interrelation at a constant level ensures reliability of synaptic transmission.Translated from Neirofiziologiya, Vol. 25, No. 2, pp. 126–132, March–April, 1993.     

Mechanisms of tubocurarine action on nicotinic cholinoreceptors of rat sympathetic ganglia neurons

The effects of tubocurarine (TC) on current induced by acetylcholine (ACh) in neurons of rat upper cervical ganglia were investigated using techniques for voltage-clamping at the membrane. Reinforcement of TC-induced blockade was achieved by paired application of ACh following prior activation of nicotinic cholinoreceptors, indicating that TC blocked the channels opened by ACh. On average, the TC-open channel complex persisted for 9.8±0.5 sec (n=7) at –50 mV and 20–24°C. It was found that increases exponentially with hyperpolarization at the membrane (a shift in membrane potential of 61 mV corresponds to an e-fold change). Suppression of ACh-induced current (ACh current) was eliminated completely under the effects of 3–30 M with depolarization of up to 80–100 mV at the membrane. Suppression of ACh current produced by membrane potential at negative levels is intensified with increasing doses of ACh. Findings would indicate that blockade of ionic channels opened by ACh is the only mechanism of TC action on nicotinic cholinoreceptors in rat sympathetic ganglia.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 5, pp. 672–680, September–October, 1988.     

Structural Evaluation of Distant Homology. A 3-D Model of the Ligand Binding Domain of the Nicotinic Acetylcholine Receptor Based on Acetylcholinesterase: Consistency with Experimental Data

Acetylcholine is a ligand for both acetylcholinesterases and nicotinic acetylcholine receptors. Hence, at least some local sequence and structural similarities between the acetylcholinesterases and the receptors which recognize acetylcholine (ACh) might be expected. Peterson [2] produced an alignment of the ACh binding region between these two types of ACh–binding molecules, featuring a number of well conserved residues. The extent of this region of sequence similarity suggests the possible existence of a common ancenstral ACh binding module. To attempt to further validate Petersons sequence alignment we have built a homology model of the ACh binding domain of the human neuromuscular nicotinic acetylcholine receptor based on the structure of acetylcholinesterase from Torpedo californica. Using this 3–D model we have examined the residues which were previously shown to interact with the endogenous ligand by various methods (mapping, site–directed mutagenesis). The consistency of such data with the model provides further support for a structural similarity and possibly a divergent evolutionary relationship between the ACh–binding domains of these two classes of proteins. Results suggest that this model may be able to contribute to an understanding of the structure and function of the ACh receptor. Using this case as an example, we propose that 3–dimensional computer modeling can be used as a tool to evaluate distant homologies when adequate experimental data (e.g., site–directed mutagenesis) is available.Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1007/s0089460020046     

Physiological properties of nicotinic acetylcholine receptors of sympathetic ganglionic neurons

The basic properties of nicotinic acetylcholine receptors of the neurons of a sympathetic ganglion responsible for the performance by these receptors of their main function — initiation of an electric current through the postsynaptic membrane — and determining the particular features of the acetylcholine receptors of these neurons by contrast with receptors of other objects, are described. Stoichiometric relations of the recognition center of the acetylcholine receptors with the transmitter, the relative strength of various agonists, and the method of action of -bungarotoxin on this center are indicated; the "life-time" and conductance of the ion channel are described. On the basis of "life-time" two groups of acetylcholine receptors are distinguished: synaptic (long-living) and extrasynaptic (short-living). Selective blockers of acetylcholine receptors of ganglionic neurons, namely bis-ammonium compounds, have two types of effect (competitive and channel-blocking), caused by the action of the blocker on two different regions of the receptor molecule, respectively. Since the channel-blocking action develops at lower concentrations than the competitive, and since it correlates closely with the ganglion-blocking effect, it is concluded that it is the first of these which determines the properties of selective blockers of acetylcholine receptors.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 319–326, May–June, 1984.     

Postsynaptic potentiation and desensitization in frog neuromuscular junction

The fractional increase in ACh responses that occurs at the beginning of each train of iontophoretically applied ACh pulses has been examined at the frog neuromuscular junction at room temperature, in the presence of active cholinesterase, during desensitization produced by a rapid sequence (every 20 s) of short (5 Hz, 5 s) iontophoretic trains of ACh. The fractional increase in ACh responses, which is used as an indicator of postsynaptic potentiation, becomes progressively greater with ACh application, often markedly (greater than 100%), although ACh responses are greatly reduced (as much as 90%) owing to desensitization. Clearly postsynaptic potentiation can exist concomitantly with desensitization. In addition, the dose-response curve is shifted to the right and its maximal response is diminished. The shift in the dose-response curve to the right, which can explain greater postsynaptic potentiation, is unlikely to be caused by accumulation of "monoligand-bound ACh receptor complexes," since experiments were done with active cholinesterase. The shift probably results from a greater number of desensitized receptors which, because of their large affinity for ACh molecules, serve as "high affinity traps." A small decrease of the maximal dose-response suggests only a small fractional decrease in the number of activable receptors, whereas a large shift to the right indicates a large fractional increase in the number of desensitized receptors. It appears that prior to ACh application only a small fraction of all receptors are desensitized. Alternatively, the shift to the right occurs because the cooperative action of ACh on receptors increases during desensitization.     

ATP-induced currents in submucous plexus neurons of the guinea pig small intestine

ATP-induced membrane durrents in the submucous neurons of the guinea pig small intestine were studied using the whole-cell patch-clamp recording technique. Being applied at –50 mV. ATP activated an inward non-selective cationic current in 68.3% of the investigated neurons. An increase in ATP concentration within the 1–1,000 µM range resulted in the s-like increase in the amplitude of ATP-induced current. The EC₅₀ was 150.0±18.5 µM, while the Hill number was 1.6. The current was selectively activated by ATP and was not blocked by P2 purinoreceptor antagonist suramin (50–300 µM).,-Methylene-ATP (100–200 µM) and,-methylene-ATP (100–200µM), which are P2-purinoreceptor agonists, as well as adenosine (100–300 µM), exerted no effects. Reactive blue 2, if applied up to 4 min, enhanced ATP-induced current, while its longer application partially suppressed this current. In most submucous neurons, acetylcholine (ACh) likewise activated an inward cationic current. The amplitude of ACh-induced current was lower if ACh was applied during a long-lasting application of ATP than if ACh only was applied. Hexamethonium (50 µM), d-tubocurarine (20–40 µM), and trimethaphan (30 µM) completely and reversibly blocked ACh-induced currents, regardless of the presence of ATP, and did not affect ATP-induced currents. The results suggest that ATP-induced currents in submucous neurons are due to activation of a unique type of P2 purinoreceptors, which function in connection with nicotinic ACh receptors.Neirofiziologiya/Neurophysiology, Vol. 28, No. 2/3, pp. 100–110, March–June, 1996.     

Voltage dependence of acetylcholine-activated membrane conductance in sympathetic ganglion neurons

Acetylcholine-induced membrane conductance was investigated in superior cervical ganglion neurons using a patch-clamp technique. It was found that hyperpolarization and depolarization produce an increase and a reduction in acetylcholine (ACh) conductance. This reduction was unconnected with either reversal of the current induced by iontophoretic ACh application or the presence of Ca ions in the external solution. The time constant of relaxation (_r) of this current, produced by a jump in membrane potential, was found to increase e-fold when the membrane was hyperpolarized by 70 mV, matching the voltage dependence of ACh conductance. This led to the hypothesis that voltage-dependent ACh-induced conductance is entirely determined by the voltage dependence of nicotinic receptor channel gating kinetics.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 2, pp. 167–171, March–April, 1988.     

Voltage-dependent effect of tetraethylammonium on the nicotinic acetylcholine receptors of rat superior cervical ganglion neurons

The effect of tetraethylammonium (TEA) on the currents evoked in neurons of the rat superior cervical ganglion by iontophoretic application of acetylcholine (ACh) was studied using a whole-cell patch-clamp recording technique. Tetraethylammonium was used at a concentration of about 20 µM, providing no blocking effect on the ACh-induced membrane currents at a range of positive membrane potentials and reducing these currents recorded at a range of negative membrane potentials by about half. The blocking effect of TEA increased with hyperpolarization within the –50 to –90 mV membrane potential range, and did not depend on the membrane potential level within a range of 0 to –50 mV. The analysis of dose dependence showed that both the voltage-dependent and the voltage-independent blocking effects are due to TEA competitive action on the ganglionic nicotinic acetylcholine receptors (nAChR). The results suggest that the TEA-induced competitive blockade is voltage-dependent.Neirofiziologiya/Neurophysiology, Vol. 27, No. 1, pp. 63–66, January–February, 1995.     

Participation of the adenylate cyclase system in the postsynaptic mechanism of heterosynaptic facilitation

In an analysis of the postsynaptic mechanism of heterosynaptic facilitation, changes in the amplitude of the excitatory postsynaptic current (EPSC) and the current evoked by application of acetylcholine (ACh current), acting on the adenylate cyclase system of the LC-1 and RC-1 neurons of the molluskPlanorbis corneus, were compared. Both responses are n-cholinergic and depend on the membrane conductivity for Na⁺ and K⁺. Application of serotonin led to a 100–300% increase in the amplitude of the EPSC and (in most cases) the ACh current. However, in 30% of the cases, the increase in the EPSC was accompanied by a decrease in the ACh current. This is probably due to the different contributions of Na⁺ and K⁺ to the mechanism of activation of the conductivity of th channel-receptor complex of the nonsynaptic cell membrane. The influence of serotonin on the EPSC and ACh current was simulated by the action of phosphodiesterase blockers and adenylate cyclase activators. Phosphodiesterase activators and protein kinase blockers reversibly inhibited the EPSC and ACh current. Thus, activation of the adenylate cyclase system, mediated by the action of serotonin, promotes the development of a postsynaptic mechanism of formation of heterosynaptic facilitation of the EPSC in the command neurons of the mollusk.A. A. Bogomolets Institute of Physiology, Ukrainian Academy of Sciences, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 6, pp. 676–683, November–December, 1991.     

Postsynaptic effects of substance P in frog neuromuscular junction

We have studied the effect of substance P on the end-plate currents (EPC) and the miniature EPC (MEPC) after acetylcholine esterase (ACE) inhibition in the cut neuromuscular preparation of the frog sartorius muscle using the voltage-clamp technique. At concentrations of 5·10^–7–1·10^–6 moles/liter substance P had no effect on the amplitude and the time characteristics of single EPC and MEPC but promoted prolongation of EPC decay on repetitive stimulation of the nerve with a frequency of 10/sec, indicating intensification of postsynaptic potentiation. Elevation of the concentration of the given peptide to 5·10^–6 moles/liter led to the shortening of the decay of single EPC and a more marked depression of the EPC amplitude in the trains as compared to the control, reflecting a decrease in the sensitivity of the postsynaptic membrane to the mediator, i.e., development of desensitization.S. V. Kurashov State Medical Institute, Kazan. I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Leningrad. Translated from Neirofiziologiya, Vol. 23, No. 4, pp. 436–441, July–August, 1991.     

Contribution of a humoral factor to postsynaptic sensitization in heterosynaptic facilitation

It was shown that heterosynaptic facilitation develops in the cerebral ganglia giant neurons of the freshwater gastropod molluskPlanorbis corn eus due to diffuse neurohumoral influences on pre- and postsynaptic structures and not local synaptic action on presynaptic mechanisms. It was also found that n-cholinergic synaptic mechanisms come under this facilitatory influence. Serotonin is the source of facilitation in neurons of bothPlanorbis corneus cerebral ganglion and those of the aplysia abdominal ganglion. Seeing that: a) conditioning stimuli facilitate the effects produced by iontophoretic acetylcholine application, as well as n-cholinergic synaptic transmission and b) the amplitude of EPSP and acetylcholine potential increase 4–6 times during facilitation when the input impedance of the post-synaptic membrane is increased by just 20%, it was deduced that the postsynaptic membrane of the giant neuron makes a significant contribution to heterosynaptic facilitation of the sensitization of n-cholinergic receptors. The part played by n-cholinergic receptors of the postsynaptic membrane in heterosynaptic facilitation and conditioned reflex habituation is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 2, pp. 250–259, March–April, 1986.     

Effects of voltage and temperature changes on postsynaptic potentiation at the frog neuromuscular junction

The difference in the decay time constants of multiquantal endplate currents (EPC) produced by presenting paired stimuli 100 msec apart was measured during experiments on transversely cut neuromuscular preparations of the frog sartorius muscle. When acetylcholinesterase was inhibited by 3×10^–6 M prostigmine, decay time of the 2nd EPC (₂) was 39±8% longer than that of the first (₁) due to postsynaptic potentiation. It was found that degree of potentiation was not affected by membrane potential level within the –30 to –120 mV range. Several effects were produced by a drop in temperature: an increase in EPC decay time constant and in that of miniature endplate currents (MEPC) in particular, a slight drop in MEPC amplitude, and a reduction in EPC quantal content. By comparing paired EPC of equal quantal content at different temperatures it was found that potentiation was more pronounced at 12°C than at 22°C and the temperature coefficient Q₁₀ at which ₂ exceeds ₁ was 2.0±0.2 (n=7). The processes determining postsynaptic potential are clearly not voltage-dependent but have a complex dependence on temperature. Quantal content of EPC falls with reduced temperature, thereby restraining potentiation, while helping to retain residual transmitter activity.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Kurashov Medical Institute, Kazan'. Translated from Neirofiziologiya, Vol. 18, No. 4, pp. 512–518, July–August, 1986.     

Nicotinic acetylcholine receptors on a cholinergic nerve terminal in the cockroach,Periplaneta americana

Summary Intracellular microelectrode recording and ionophoretic application of carbamylcholine (CCh) were used to compare the cholinergic sensitivity of postsynaptic dendrites of an identified neurone with that of an identified presynaptic cholinergic axon.The axon of the lateral filiform hair sensory neurone (LFHSN) in the first-instar cockroachPeriplaneta americana was found to be as sensitive to CCh as the dendritic regions of giant interneurone 3 (GI 3). The CCh response of both neurones was unaffected by replacing Ca²⁺ with Mg²⁺, confirming that the ACh receptors are present on the neurones under test. The CCh response of both neurones was mimicked by ionophoretic application of nicotine. The responses were blocked by 10^–5 M mecamylamine and 10^–6 M d-tubocurarine and were not affected by muscarinic antagonists, suggesting that the ACh receptors present on GI 3 and LFHSN are predominantly nicotinic.The muscarinic agonist oxotremorine and the antagonists atropine and quinuclidinyl benzilate had no modulatory effect on LFHSN-GI 3 synaptic transmission.The latency of the LFHSN response to CCh was consistent with the hypothesis that ACh receptors are situated on the main axon/terminal within the neuropil of the ganglion. It has previously been shown that this region of the axon does not form output synapses (Blagburn et al. 1985a). This indirect evidence indicates that presynaptic or extrasynaptic ACh receptors are present in the membrane of a cholinergic axon.LFHSN was depolarized by synaptically-released ACh after normal or evoked spike bursts, suggesting that the nicotinic ACh receptors act as autoreceptors. However, it was not possible to obtain direct evidence to support the hypothesis that these receptors modulate ACh release.Abbreviations CCh carbamylcholine - GI giant interneurone - FHSN filiform hair sensory neurone - LFHSN lateral filiform hair sensory neurone - R _in input resistance - V depolarization - V _m resting potential     

Intracellular regulation of neuronal nicotinic cholinergic receptors

Effects of substances affecting intracellular secondary messengers on the membrane currents evoked by ionophoretic application of acetylcholine (ACh currents) and on the excitatory postsynaptic currents (EPSC) evoked by single stimuli applied to preganglionic nerve fibres, were studied in neurones of the rat isolated superior cervical ganglion. Forskolin, the protein kinase A activator, and isobutyl-methyxanthine, the phosphodiesterase inhibitor, decreased the ACh currents. Neither forskolin nor isobutyl-methylxanthine affected the EPSC amplitude or the EPSC decay time constant. Phorbol ester, the protein kinase C activator, decreased the ACh current but did not affect either EPSC amplitude or the EPSC decay time constant. Thapsigargin, the intracellular calcium releaser, decreased the ACh current and the EPSC amplitude but did not affect the EPSC decay time constant. The data obtained suggest that nicotinic acetylcholine receptors (nAChRs) of ganglion neurones are not modulated through the pathways involving protein kinase A or protein kinase C. The nAChRs sensitivity to both exogenous and nerve-released acetylcholine is reduced by intracellular calcium without affecting kinetics of their ionic channels.