Membrane currents underlying the cholinergic slow excitatory post-synaptic potential in the rat sympathetic ganglion

Neuropharmacology of the muscarinic antagonist telenzepine in myenteric ganglia of the guinea-pig small intestine

Intracellular recording methods were used to investigate the actions of the putative M1 muscarinic receptor antagonist telenzepine on the electrical and synaptic behavior of myenteric neurons. Telenzepine had no effect on resting membrane potential, input resistance, excitability and antidromic potentials in both AH/type 2 and S/type 1 neurons, when applied in concentrations of 0.1-2000 nM, although higher concentrations (10-100 microM) did have a significant non-specific effect on the postsynaptic membrane. Micromolar concentrations of telenpzepine (1-2 microM) had no effect on excitatory responses to substance P, vasoactive intestinal peptide, the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium or the nicotinic action of acetylcholine. Nicotinic fast excitatory postsynaptic potentials were also unaffected by 2 microM telenzepine. In contrast, at submicromolar concentrations (100 nM), telenzepine abolished responses to either muscarine or the muscarinic component of the acetylcholine response. The excitatory effect of muscarine at postsynaptic M1 receptors was dose dependently inhibited by telenzepine (0.1-1000 nM) at concentrations which had no effect on the electrical properties of the cells. This effect was slowly reversible, usually requiring more than 60 min for significant recovery. The threshold dose of telenzepine as an antagonist of the muscarinic depolarization in AH/type 2 neurons was in the range of 0.1-1 nM. The IC50 concentration of telenzepine needed to abolish the response was 8.5 nM. A small proportion of stimulus-evoked slow excitatory postsynaptic potentials in both AH/type 2 and S/type 1 cells were abolished by 1 microM telenzepine, while the majority of them remained unaffected, indicating that some slow excitatory postsynaptic potentials are mediated by the muscarinic action of released acetylcholine.(ABSTRACT TRUNCATED AT 250 WORDS)

The light-suppressible K+ conductance and evaluation of internal messenger candidates in the molluscan extraocular photoreceptor

A photoreceptor potential produced by a decrease in membrane conductance was not thought to occur in any invertebrate photoreceptors. However, we have found that the molluscan extraocular photoreceptor, A-P-1 responds to light with a depolarizing receptor potential due to a decrease in K+ conductance, so that the photoresponse associated with a decrease in membrane conductance is not unique to the vertebrate photoreceptor. The properties that the light-suppressible K+ conductance is time- and voltage-dependent are explained by comparison with those of the single channel conductance obtained in patch-clamp of both vertebrate and invertebrate photoreceptors. The noise analysis of the light-induced current suggest that this macroscopic light-suppressible conductance consists of channels. It is concluded that the light-suppressible K+ conductance is mediated by hydrolysis of cGMP which reduces internal cGMP, in agreement with the cGMP hypothesis of vertebrate phototransduction and that the hydrolysis may be modified by IP3.

Autoradiographic localization of muscarinic receptors on cultured, peptide-containing neurones from newborn rat superior cervical ganglion

In order to identify subpopulations of cultured rat superior cervical ganglion (SCG) neurones which express muscarinic receptors, a combination of immunocytochemistry and autoradiography was performed on these cultures. Antibodies to vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY) were used to immunostain cultures that had previously been labelled with the irreversible muscarinic antagonist, [3H]propylbenzylylcholine mustard (PrBCM). Binding sites for [3H]PrBCM were observed on a large subpopulation of 65-85% of the ganglionic neuronal cell bodies. Specific labelling was not associated with non-neuronal cells found in these cultures. Approximately 60% of the SCG neurones were NPY-like immunoreactive (NPY-LI), a high proportion of which expressed muscarinic receptors. Five to 10% of the SCG neurones were VIP-LI, a small subpopulation of which displayed [3H]PrBCM binding sites. Receptor distribution on cell bodies was usually uniform, but occasionally, regions of high receptor density were seen. Dense networks of both varicose and non-varicose NPY-LI fibres were seen throughout the culture, a subpopulation of which expressed muscarinic receptors. Occasional VIP-LI fibres were also labelled with silver grains for [3H]PrBCM, but in less abundance than those for NPY-LI fibres. Conversely, neurones expressing muscarinic receptors were often immunonegative for either VIP or NPY: therefore, the identity of some of the neurones which express muscarinic receptors remains to be determined.

Potassium currents of acutely isolated adult rat superior cervical ganglion neurons

K+ currents of adult rat superior cervical ganglion neurons were studied using the voltage-clamp technique. Neuronal somata were dissociated from the ganglion using an enzymatic dispersion technique and voltage-clamped using the whole-cell patch-clamp technique. In solutions designed to isolate K+ currents, depolarization from a prepulse potential of -100 mV induced both transient and sustained outward current components. The transient current was completely eliminated by depolarization to -50 mV. The remaining sustained current component could be separated further into Ca2+-sensitive and Ca2+-insensitive components by superfusion with a Ca2+-free external solution. The transient current, which could be isolated by digital subtraction, rose rapidly and decayed over the subsequent 80 ms. Reversal potential determinations in different K+-containing solutions demonstrated that the current was carried primarily by K+. The transient current showed voltage-dependent inactivation, showing 50% inactivation near -87 mV and was completely inactivated at potentials more positive than -60 mV. The transient current recovered from inactivation with a voltage-dependent time course, the time course of inactivation decreasing with hyperpolarization. This transient outward current had characteristics of IA. The sustained Ca2+-insensitive outward current showed little decay over 800 ms and was also carried primarily by K+. This current component had characteristics similar to the delayed rectifier. A third sustained outward current eliminated by superfusion with Ca2+-free external solution had characteristics similar to the Ca2+-dependent K+ current.

Depolarization of human neuroblastoma cells as a result of muscarinic receptor-induced rise in cytosolic Ca2+

The role of intracellular free Ca2+ in muscarinic-receptor linked depolarization of SH-SY5Y neuroblastoma cells has been determined by using the bisoxonol membrane potential probe DiBaC4-(3) and intracellular Ca2+ indicator fura-2 respectively. Carbachol and the Ca2+ ionophore, ionomycin, at concentrations which caused similar rises in intracellular Ca2+ increased the bisoxonol fluorescence (depolarization) to the same extent. The membrane potential responses, but not the changes in intracellular Ca2+, were dependent on extracellular Na+. Ionomycin depletion of intracellular Ca2+ with EGTA and ionomycin or loading the cells with a Ca2+ buffer, BAPTA, reduced the carbachol-induced depolarization. The results suggest that a rise in intracellular Ca2+ may cause depolarization through an increase in the Na+ permeability.

Secondary late components of the muscarinic postsynaptic potentials, in rabbit superior cervical ganglion

The well known muscarinic slow excitatory polysynaptic potential (s-EPSP) of rabbit superior cervical ganglion (SCG) peaking at about 1-2 s and lasting 5-10 s, is immediately followed by an abrupt change in slope to a longer, lower depolarizing phase. A brief dip in the level of depolarization (DP) often separates the two depolarizing phases. The secondary phase of s-EPSP rises to its own peak at about 25 s; total duration 60-120 s. With repetition of orthodromic volleys secondary s-EPSP builds up more gradually than initial s-EPSP, but more rapidly than slow-slow (ss-) EPSP. The later 'secondary' depolarizing phase along with the antecedent 'dip in DP' are, like the 'initial' s-EPSP, eliminated by a muscarinic antagonist, quinuclinidyl benzilate hydrochloride (QNB). This distinguishes secondary s-EPSP from the even slower rising non-cholinergic ss-EPSP. The ss-EPSP, although relatively small in the responses to the usual 3-pulse test stimuli, rises to an extraordinary amplitude (equal to the compound action potential) during a 10 s-120 s train of pulses. Gallamine blocked most of the slow IPSP component in test responses but not initial or secondary s-EPSP. A preganglionic conditioning train (10/s for 2 min) induced a long-term-enhancement (LTE) of secondary s-EPSP lasting greater than 3 h, with maximum postconditioning percentage increases greater than for initial s-EPSP. Also enhanced was the dip in DP, now forming a deeper notch between initial and secondary s-EPSPs; this attains a maximum at about 30 min postconditioning but thereafter progressively loses the enhancement by about 90 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Burst-patterned stimulation promotes nicotinic transmission in isolated perfused rat sympathetic ganglia

1. Intracellular recordings of small nicotinic excitatory postsynaptic potentials (EPSPs) were made from rostral cells in superior cervical ganglia (SCG) of rats during and after test stimulation of small preganglionic fibre bundles, while perfusing the isolated ganglia via their arterial vasculature. Perfusion, in contrast to superfusion of desheathed ganglia, (a) produced much more rapid and complete equilibration of drugs and ions at synaptic sites, (b) greatly reduced depression of EPSPs during high-frequency stimulation, and (c) largely prevented slowing of conduction, presumably by minimizing accumulation of K+ in the intercellular spaces surrounding these sites. 2. Preganglionic inputs were found to fall into two major groups: those in which the EPSP amplitude during 200 pulse trains was facilitated and others in which it was depressed as stimulation frequency in the train was increased from 2 to 20 Hz or from 0.2 to 1.25 Hz. Both the facilitation and the depression were presynaptic, since they occurred without changes in miniature EPSP amplitude. 3. The maximum maintained facilitation was reached at 5-10 Hz with a value 1.26 times the 1.0 Hz control. This was associated with an increase in the binomial parameter n. While long 20 Hz trains produced a similar facilitation to an early plateau, and an increase in n, EPSP amplitude declined as the train progressed. This was associated with a decrease in the binomial parameter p. 4. Unlike the 20 Hz trains, stimulation with 0.5 s long, 20 Hz bursts given every 8 s produced a marked potentiation in facilitating units and this was maintained for as long as the stimulation was continued (3-11 min). Burst-patterned potentiation was 1.66 times larger than the facilitation evoked by tonic stimulation at the same average frequency (1.25 Hz), and more than twice that achieved with long, 200 pulse trains. The potentiation was associated with increases in both n and p in the first EPSP of the burst and mainly with an increase in n at the end of the burst. Potentiation persisted unchanged for about 30 s following the return to control 0.2 Hz stimulation, before declining to control levels over the next 2-3 min. Depressing units on average showed neither burst-patterned potentiation nor post-burst-patterned potentiation. 5. All inputs tested in Locke solution in which Ca2+ was reduced to 0.5 mM with addition of 1.2 mM-Mn2+ or 3.8 mM-MgCl2 exhibited a pronounced facilitation within each burst but no extension of potentiation into ensuing bursts. Both burst-patterned potentiation and the post-burst-patterned potentiation were abolished.(ABSTRACT TRUNCATED AT 400 WORDS)

Substance P-mediated membrane currents in voltage-clamped guinea pig inferior mesenteric ganglion cells

Responses to substance P (SP) and to hypogastric nerve stimulation were recorded from voltage-clamped guinea pig inferior mesenteric ganglion (IMG) neurons, and compared with those to muscarine. Muscarine produced a voltage-dependent inward current accompanied by a reduced input conductance and inhibition of IM a time- and voltage-dependent K+-current (Brown and Adams: Nature 283:673-676, 1980). SP also produced an inward current, accompanied by a fall in input conductance (20 out of 31 cells) or a rise in input conductance (7 out of 31 cells). The fall in input conductance was not accompanied by an inhibition of M-current (unlike frog ganglia: Adams et al.: British Journal of Pharmacology 79:330-333, 1983) or an inhibition of the inward rectifier current (unlike globus pallidus neurons: Stanfield et al.: Nature 315:498-501, 1985). Repetitive hypogastric nerve stimulation (10-20 Hz, 2-10 s) produced a slow inward postsynaptic current lasting 1-3 min, with decreases or increases of input conductance matching those produced by SP. The postsynaptic current did not show a consistent or reproducible change in amplitude on varying the holding potential between -90 and -25 mV. It is concluded that SP and hypogastric stimulation produce complex and variable changes in ionic conductance in IMG neurons.

Regulation of two ion channels by a common muscarinic receptor-transduction system in a vertebrate neuron

In bullfrog sympathetic ganglion cells, muscarine produced an inward current (Imus) through the activation of a subtype (M1) of muscarinic acetylcholine receptor (mAChR) by suppressing an outward M-current (IM), and/or activating cation-selective current (ID; see below). The former was induced with a potency (Kd = 0.5 microM) higher than the latter (Kd = 5 microM) before and after blocking a fraction of the receptor with an irreversible blocker. Activators of protein kinase C mimicked muscarine's actions. Blocking IM by Ba2+ increased ID. These results suggest that activation of M1-mAChR both closes M-channel and opens cation-selective D-channel through phosphoinositide breakdown and the subsequent activation of protein kinase C and that a difference in potency at the last step of the cascade determines the order in which channels are regulated.

Three types of muscarinic conductance changes in sympathetic neurons discriminately evoked by the different concentrations of acetylcholine

Three different types of changes in membrane conductance were induced in isolated mammalian sympathetic neurons by the muscarinic actions of acetylcholine at different levels of concentration. These include a voltage-independent increase and two decreases, voltage-dependent and -independent, respectively. Muscarinic receptors in sympathetic neurons are thus shown to be coupled to multiple ionic channels, even within a single cell, and controlled by the different concentrations of a neurotransmitter.