Dynorphin Selectively Augments the M-Current in Hippocampal CA1 Neurons by an Opiate Receptor Mechanism

Abstract

Most electrophysiological studies of opioids on hippocampal principal neurons have found indirect actions, usually through interneurons. However, our laboratory recently found reciprocal alteration of the voltage-dependent K+current, known as the M-current ( I M), by κ and δ opioid agonists in CA3 pyramidal neurons. Recent ultrastructural studies have revealed postsynaptic δ opiate receptors on dendrites and cell bodies of CA1 and CA3 hippocampal pyramidal neurons (HPNs). Reasoning that previous electrophysiological studies may have overlooked voltage-dependent postsynaptic effects of the opioids in CA1, we reevaluated their role in CA1 HPNs using the rat hippocampal slice preparation for intracellular current- and voltage-clamp recording. None of the δ and μ receptor-selective opioids tested, including {d-Pen2,5}-enkephalin (DPDPE), {d-Ala2}-deltorphin II (deltorphin), {d-Ala2, NMe-Phe4, Gly-ol}-enkephalin (DAMGO), and {d-Ala2,d-Leu5} enkephalin (DADLE), altered membrane properties such as I M or Ca2+-dependent spikes in CA1 HPNs. The nonopioid, Des-Tyr-dynorphin (D-T-dyn), also had no effect. By contrast, dynorphin A (1–17) markedly increased I M at low concentrations and caused an outward current at depolarized membrane potentials. The opioid antagonist naloxone and the κ receptor antagonist nor-binaltorphimine (nBNI) blocked the I M effect. However, the κ-selective agonists U69,593 and U50,488h did not significantly alter I M amplitudes when averaged over all cells tested, although occasional cells showed an I M increase with U50,488h. Our results suggest that dynorphin A postsynaptically modulates the excitability of CA1 HPNs through opiate receptors linked to voltage-dependent K+ channels. These findings also provide pharmacological evidence for a functional κ opiate receptor subtype in rat CA1 HPNs but leave unanswered questions on the role of δ receptors in CA1 HPNs.