The effects of postsynaptic levels of cyclic AMP on excitatory and inhibitory responses of an identified central neuron.

The effects of cAMP on Mauthner (M-) cell excitatory and inhibitory responses were studied in vivo. Cyclic AMP was iontophoresed into the M-cell lateral dendrite, after which we monitored the changes in cellular responsiveness to stimulation of 2 classes of identified presynaptic cells: (1) excitatory fibers from the posterior branch of the ipsilateral eighth cranial nerve and (2) inhibitory interneurons activated by the M-cell collateral and commissural networks. We found that postsynaptic injections of cAMP increased the magnitudes of the electrically and chemically mediated EPSPs from the eighth nerve and enhanced M-cell inhibitory responses as well. Furthermore, cAMP augmented paired-pulse facilitation of both types of excitatory potentials. No effects on input conductance, resting membrane potential, or presynaptic spike width were observed. All effects were mimicked by aminophylline, a cAMP-dependent phosphodiesterase inhibitor but not by 5'-AMP, the cAMP breakdown product. These results provide evidence for second-messenger modulation in vivo of electrotonic and chemical synaptic potentials at mixed synapses. In addition, they provide the first evidence for cAMP modification of a glycinergic receptor-channel complex. Preliminary results from some of these experiments have been reported previously (Wolszon and Faber, 1986).