M1 and M2 muscarinic receptors mediate excitation and inhibition of guinea‐pig intracardiac neurones in culture.

Abstract

SUMMARY1. The effects of muscarine upon intracardiac neurones cultured from ganglia within the atria and interatrial septum of the newborn guinea-pig heart were studied using intracellular recording techniques.2. Muscarine applied to the neuronal soma typically produced a biphasic change in membrane potential which consisted of a small hyperpolarization followed by a (lepolarization. In addition, muscarine (0 01-10 /tM) inhibited the calcium-dependent, after-hyperpolarization (AHP) and greatly increased the number of action potentials that could be evoked by a given depolarizing current.3. The hyperpolarization was associated with a decrease in input resistance and it reversed to become a depolarization at a potential of -865 mV. This response was antagonized by 4-diphenylacetoxy-N-methyl-piperidine (4-DAMP; 100 nM) and AF-DX 116 (500 nM), but was unaffected by pirenzepine (0 1-5 ftM).4. Two types of slow depolarization were observed in the presence of muscarine. The most common was associated with an increase in input resistance in the potential range -70 to -40 mV. Pirenzepine (100 nM) selectively antagonized this response, 4-DAMP (100 nM) similarly antagonized the response, but was non-selective. AF-DX 116 (0 5-5 /IM) showed no antagonist effect. The less common depolarization (5 % of cells) had a long latency and was associated with a decrease in input resistance.5. Muscarine reduced the duration of the action potential and inhibited the AHP.Cadmium chloride (100,UM) mimicked these actions of muscarine. Application of muscarine immediately following a train of action potentials did not inhibit the AHP, suggesting that muscarine did not directly inhibit the calcium-activated potassium current (VK(Ca)). Muscarine-induced depression of the slow AHP was antagonized by 4-DAMP (100 nM) but was not antagonized by either pirenzepine (01-05,um) orAF-DX 116 (05-5/tM).6. It is concluded that the muscarine-induced depolarization of guinea-pig intracardiac neurones results from reduction of a potassium conductance similar to the M-conductance, through activation of M1 muscarinic receptors. The hyper-

Potassium currents in adult rat intracardiac neurones.

“…8A). (Adams et al 1982a;Adams et al 1982b;Allen & Burnstock, 1990a), application of muscarine (10 /SM) reversibly blocked IM in intracardiac neurones (n =4, Fig. 8C).…”

“…A similar activation threshold of inward rectifier is reported in myenteric spike after-hyperpolarization neurones (Galligan et al 1989). In addition, to function as a gain control, IM may be involved in muscarinic synaptic transmission (Brown, 1988;Allen & Burnstock, 1990a;Xi-Moy, Randall & Wurster, 1993). Cholinergic slow excitatory postsynaptic potentials have been recorded in canine intracardiac ganglion neurones (Xi-Moy et at.…”

“…The background K+ current may also contribute to the resting potential in the bullfrog sympathetic and cultured parasympathetic intracardiac neurones (Jones, 1989;Xu & Adams, 1992 1987). In both the guinea-pig and canine intracardiac ganglia, inhibition of M-current initiated spontaneous spikings in quiescent cells (Allen & Burnstock, 1990a;Xi et al 1992). Are there intrinsic modulatory inputs which may inactivate the inhibitory or shunting conductances, thereby leading to spontaneous firing in intracardiac neurones?…”

Local anaesthetic blockade of neuronal nicotinic ACh receptor‐channels in rat parasympathetic ganglion cells

“…This current appears to be identical with the MI-activated current described in cultured intracardiac neurones of the guinea-pig (Allen & Burnstock, 1990a). In contrast, QX-222 reversibly inhibited the ACh-evoked transient inward current and steady-state current.…”

“…Mammalian intracardiac neurones possess both nicotinic and muscarinic ACh receptors (Hassall et al, 1987;Seabrook et al, 1990). In mammalian cultured cardiac neurones, ACh elicits a rapidly activating current mediated by nicotinic ACh-receptors (Fieber & Adams, 1991a;Adams & Nutter, 1992;Selyanko & Skok, 1992) and a slowly activating current mediated by muscarinic AChreceptors (Allen. & Burnstock, 1990a;Selyanko & Skok, 1992).…”