Muscarinic excitation and inhibition of neurons in the submucous plexus of the guinea-pig caecum

Abstract

Intracellular recordings were made from neurons in the submucous plexus of the guinea-pig caecum. Muscarinic agonists (acetylcholine, bethanechol and muscarine) depolarized about 70%, and hyperpolarized about 30% of the submucous plexus neurons. Low concentrations of pirenzepine reversibly antagonized both responses. The measured dissociation constants (KD) of 10-30 nM for the depolarizations and 1-3 nM for the hyperpolarizations suggest that each response was mediated by muscarinic M1 cholinoceptors. The muscarinic depolarization and hyperpolarization were associated with a decreased and an increased conductance, respectively, and the reversal potential for the muscarinic responses varied as the potassium concentration varied, always being around the potassium equilibrium potential. In cells depolarized by muscarinic agonists these agents appeared to decrease a potassium conductance that could also be inactivated by substance P. In approximately 30% of the submucous neurons, the slow inhibitory postsynaptic potential, elicited in response to single or repetitive focal stimuli (1-10 pulses at 20-40 Hz), appeared to consist of a large component which was sensitive to the blocking action of idazoxan (100-300 nM) and a small component which was idazoxan-insensitive. The latter (muscarinic slow inhibitory postsynaptic potential) was completely abolished by pirenzepine. The concentrations of pirenzepine which caused a 50% depression ranged from 5 to 20 nM. The muscarinic slow inhibitory postsynaptic potential was increased in amplitude and duration by physostigmine (100-300 nM). The muscarinic slow inhibitory postsynaptic potential was accompanied by a decrease in membrane input resistance, and was reversed in polarity near the potassium equilibrium potential. When muscarine induced a hyperpolarization and/or focal stimulation elicited a muscarinic slow inhibitory postsynaptic potential in the presence of idazoxan (100-300 nM), the intracellular injection of guanosine 5'-O-(3-thiotriphosphate) produced a progressive membrane hyperpolarization during which the muscarinic hyperpolarizing responses were attenuated. It is concluded that the muscarine-induced reduction in potassium conductance is mediated through a muscarinic M1 receptor which has a relatively low affinity for pirenzepine. The muscarine-induced increase in potassium conductance is probably produced by the association of a guanine nucleotide-binding regulatory protein with another muscarinic M1 receptor that has a relatively high affinity for pirenzepine.