Postsynaptic modulation of synaptic efficacy at mixed synapses on the Mauthner cell.

Extracellular application of dopamine in the synaptic bed of the lateral dendrite of the goldfish Mauthner (M-) cell enhances both the electrical and chemical components of the mixed excitatory postsynaptic potential (EPSP) evoked by ipsilateral eighth nerve stimulation (Pereda et. al., 1992). We describe here results of experiments designed to determine the locus of action of dopamine and the underlying cellular mechanisms. This amine acts independently on the two modes of transmission, since (1) the percentage increases in the two were not correlated, (2) the time courses of their modifications were independent, and (3) the observed increases in synaptic responses cannot be attributed to a generalized effect on M-cell input conductance, which was increased by dopamine, a change that would rather be expected to shunt the synaptic potentials. Also, dopamine does not produce presynaptic spike broadening and does not modify paired-pulse facilitation, two indications that it acts postsynaptically. The alterations in the mixed EPSP are presumably due to activation of a postsynaptic cAMP-dependent phosphorylation pathway. Specifically, they did not occur if the cAMP-dependent protein kinase inhibitor PKI5-24 was injected intradendritically prior to dopamine application, and they could, on the other hand, be mimicked by injections of the catalytic subunit of the cAMP-dependent protein kinase, PKACAT. In contrast, neither manipulation altered the M-cell input conductance directly or affected the dopamine-induced increase in conductance, suggesting this effect of dopamine is cAMP independent.(ABSTRACT TRUNCATED AT 250 WORDS)