Characteristics of synaptic input to three classes of sympathetic neurone in the coeliac ganglion of the guinea-pig.

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1. Intracellular recordings from sympathetic neurones in the isolated coeliac ganglion of guinea-pigs have been used to define the synaptic input to three subtypes of neurone, classified on the basis of their discharge during maintained depolarizing current as phasic neurones, neurones with prolonged after-hyperpolarizations (LAH), and tonic neurones. 2. The three classes of neurone were distributed characteristically in different parts of the ganglion. 3. Passive membrane properties differed between the three neurone types. Mean input resistance was highest in phasic neurones and was inversely related to the size of the prolonged calcium-activated potassium conductance in LAH neurones. Mean input time constant was highest in tonic neurones, because of significantly higher cell capacitance. 4. Phasic and LAH neurones usually received one suprathreshold ('strong') as well as several subthreshold excitatory synaptic potentials (ESPs) from the ipsilateral splanchnic nerve. In general, the amplitude and number of splanchnic inputs were greater, and the occurrence of two strong inputs more common, in phasic than in LAH neurones. The input to tonic neurones was small and usually subthreshold, even with supramaximal splanchnic stimulation. In a few (mostly tonic) neurones lying close to the midline, small ESPs were evoked by contralateral splanchnic stimulation. 5. Antidromic action potentials were evoked in more than half of all neurones by high voltage coeliac nerve stimulation. In addition, multiple small subthreshold ESPs were recorded in virtually all tonic neurones (99%) on coeliac nerve stimulation. In contrast, coeliac stimulation rarely evoked a few very small ESPs in LAH neurones (9%), but no synaptic response in phasic neurones. 6. In about half of the tonic neurones tested (but no phasic or LAH neurones), small ESPs were evoked by stimulation of the intermesenteric nerve. 7. Slow depolarization elicited by repetitive activation of splanchnic and coeliac nerve trunks, at voltages supramaximal for the fast cholinergic responses, were recorded from about half of both phasic and tonic neurones, but only one of twenty-four LAH neurones. These responses commonly faded during subsequent trials, so that it was difficult to characterize them. 8. The data indicate that the three broad groups of coeliac neurone, classified on the basis of their voltage- and calcium-dependent potassium conductances, receive different patterns of synaptic input. The differences may be related to the three major functions of vasoconstriction, motility and mucosal secretion in the small intestine.

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