Pituitary adenylate cyclase-activating polypeptide (PACAP) increases excitability of guinea pig cardiac neurons, an effect mediated by PACAP-selective PAC1 receptors. In dissociated guinea pig cardiac neurons, PACAP causes a positive shift of the voltage dependence of activation of the hyperpolarization-activated nonselective cation current ( Ih). This observation suggested that an enhancement of Ih contributed to the increase in excitability in neurons within whole-mount cardiac ganglia preparations. To evaluate the role of Ih in the PACAP-induced increase in excitability, we compared the increase in action potentials generated by 10 nM PACAP in control neurons and in neurons treated with ZD7288 (10 or 100 μM) or CsCl (2 or 2.5 mM), drugs known to inhibit Ih. In control cells exposed to PACAP, 1-s depolarizing current pulses elicited multiple action potential firing in 79% of the neurons. In ZD7288- or CsCl-containing solutions, the 10 nM PACAP-induced increase in excitability was markedly suppressed, with 7% and 21% of the neurons generating multiple action potentials, respectively. Prior results indicated that PACAP initiates depolarization by activating an inward current, which is separate from its enhancement of Ih. Here, we show that a PACAP-induced depolarization was comparable in control neurons and neurons bathed in a CsCl-containing solution, an observation indicating that CsCl did not interfere with activation of the PAC1 receptor by PACAP. Additional experiments indicated that pretreatment with the putative M current ( IM) inhibitor 1 mM BaCl2, but not 10 μM XE991, initiated multiple firing in a majority of neurons, with resting potentials maintained at approximately −60 mV. Furthermore, in Ba2+-treated cells, 10 nM PACAP increased the number of action potentials generated. Our results indicate that PACAP enhancement of Ih, rather than inhibition of IM and other 1 mM Ba2+-sensitive K+ currents, is a key ionic mechanism contributing to the peptide-induced increase in excitability for neurons within whole-mount cardiac ganglia preparations.
Electrophysiology of guinea-pig supraoptic neurones: role of a hyperpolarization-activated cation current in phasic firing.
