Modulation of AMPA receptor-mediated ion current by pituitary adenylate cyclase-activating polypeptide (PACAP) in CA1 pyramidal neurons from rat hippocampus

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neurotrophic and neuromodulatory peptide, was recently shown to enhance NMDA receptor-mediated currents in the hippocampus (Macdonald, et al. 2005. J Neurosci 25:11374-11384). To check if PACAP might also modulate AMPA receptor function, we tested its effects on AMPA receptor-mediated synaptic currents on CA1 pyramidal neurons, using the patch clamp technique on hippocampal slices. In the presence of the NMDA antagonist D-AP5, PACAP (10 nM) reduced the amplitude of excitatory postsynaptic currents (EPSCs) evoked in CA1 pyramidal neurons by stimulation of Schaffer collaterals. Following a paired-pulse stimulation protocol, the paired-pulse ratio was unaffected in most neurons, suggesting that the AMPA-mediated EPSC was modulated by PACAP mainly at a postsynaptic level. PACAP also modulated the currents induced on CA1 pyramidal neurons by applications of either glutamate or AMPA. The effects of PACAP were dose-dependent: at a 0.5 nM dose, PACAP increased AMPA-mediated current; such effect was blocked by PACAP 6-38, a selective antagonist of PAC1 receptors. The enhancement of AMPA-mediated current by PACAP 0.5 nM was abolished when cAMPS-Rp, a PKA inhibitor, was added to the intracellular solution. At a 10 nM concentration, PACAP reduced AMPA-mediated current; such effect was not blocked by PACAP 6-38. The inhibitory effect of 10 nM PACAP was mimicked by Bay 55-9837 (a selective agonist of VPAC2 receptors), persisted in the presence of intracellular BAPTA and was abolished by intracellular cAMPS-Rp. Stimulation-evoked EPSCs in CA1 neurons were significantly reduced following application of the PAC1 antagonist PACAP 6-38; this result indicates that PAC1 receptors in the CA1 region are tonically activated by endogenous PACAP and enhance CA3-CA1 synaptic transmission. Our results show that PACAP differentially modulates AMPA receptor-mediated current in CA1 pyramidal neurons by activation of PAC1 and VPAC2 receptors, both involving the cAMP/PKA pathway; the functional significance will be discussed in light of the multiple effects exerted by PACAP on the CA3-CA1 synapse at different levels.