Peptidergic and muscarinic excitation at amphibian sympathetic synapses.

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Abstract
A single-electrode voltage clamp was used to study the slow muscarinic and late slow peptidergic excitatory post-synaptic currents (EPSC) in B cells of the paravertebral sympathetic ganglia of the bull-frog [Rana catesbeiana]. Conductance decreases were measured during peptidergic EPSC in nearly all cells at clamped potentials near the resting level. In .apprx. 1/2 of the cells the size of the peptidergic EPSC increased with hyperpolarization and in some of these cells conductance increases were found at hyperpolarized levels. In the remaining cells conductance decreases occurred at all levels of membrane potential tested, and in a few of these the polarity of the EPSC reversed at hyperpolarized potentials. A similar diversity was observed among muscarinic EPSC. At least 2 simple ionic mechanisms are required to explain the heterogeneous voltage dependencies observed: a conductance decrease primarily to K+ that dominates at depolarized potentials and a conductance increase to other ions that is more prominent at hyperpolarized potentials. The proportion of these 2 mechanisms appears to differ among B cells. The 2 slow EPSC recorded in the same neuron had the same voltage dependence and were accompanied by the same conductance changes in each of 8 cells despite differences between cells. The muscarinic EPSC was reduced during the peptidergic EPSC in each of 25 neurons tested over a range of membrane potentials. Externally-applied luteinizing hormone releasing hormone (LHRH) produced currents with the same voltage dependence and conductance changes as the nerve-evoked peptidergic EPSC in each of 15 cells tested. Bethanechol, a muscarinic agonist, and LHRH produced currents with the same voltage dependence and conductance changes in each of the 12 cells studied. In several cells a saturating response to a prolonged application of LHRH completely occluded the response to bethanechol, and vice versa. Slow currents were recorded from dissociated cell bodies in response to bethanechol and LHRH; these responses exhibited the same diversity of voltage dependence and conductance changes as was observed in intact ganglia. Activation of muscarinic and peptidergic receptors may control shared ionic mechanisms in single ganglion cells.