Mechanisms underlying intracellular signal transduction of the slow IPSP in submucous neurones of the guinea-pig caecum.

1. Intracellular recordings were obtained from submucous plexus neurones of the guinea-pig caecum. 2. The resting membrane conductance displayed two types of inward rectification: one which developed at potentials more negative than -70 mV, and another that occurred at potentials more negative than the potassium equilibrium potential. The former inward rectification was blocked by extracellular caesium (Cs+; 1-2 mM) and the latter was blocked by Cs+ (1-2 mM) or barium (Ba2+; 30-100 microM). 3. The noradrenaline-induced current measured by subtraction of the current-voltage (I-V) relation before and after adding the agonist also showed an inward rectification around the resting potential. Ba2+ (30-100 microM) blocked both the outward and inward current induced by noradrenaline. The noradrenaline current was not affected by Cs+ (1-2 mM). Both the slow IPSP and the slow IPSC (inhibitory postsynaptic current) were reduced by Ba2+, but not by Cs+. 4. During the intracellular injection of guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S), multiple repetitive stimulation or repeated applications of noradrenaline produced irreversible membrane hyperpolarizations with a decreased membrane input resistance, until the membrane had approached the potassium equilibrium potential. 5. Pertussis toxin (1-40 micrograms/ml) abolished both the slow IPSP and the noradrenaline hyperpolarization without affecting the nicotinic fast EPSP or the slow EPSP. 6. Superfusion with a Ca(2+)-free, high-Mg2+ (12 mM) solution caused a membrane depolarization associated with an increased input resistance. It eliminated the Ca2+ spikes, the slow after-hyperpolarizations following the spikes, and the synaptic potentials within 3 min. Prolonged exposure (longer than 20 min) to this solution resulted in a progressive decline of the noradrenaline hyperpolarization. 7. Intracellular injection of ethylene glycol-bis(beta-aminoethylether)N,N,N',N'-tetraacetic acid (EGTA) reduced the slow IPSP and the noradrenaline hyperpolarization. Superfusion with a membrane-permeable Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM; 10-200 microM) reduced the noradrenaline hyperpolarization. 8. Procaine reversibly reduced the slow IPSP and noradrenaline hyperpolarization without affecting the fast EPSP or slow EPSP at concentrations up to 300 microM.(ABSTRACT TRUNCATED AT 400 WORDS)