Voltage-dependent effects of opioid peptides on hippocampal CA3 pyramidal neurons in vitro

Abstract

Opioid peptides, and especially the dynorphins, have been localized to several circuits in the CA3 hippocampal region, yet electrophysiological studies often find mixed effects of opiates on the excitability of CA3 neurons. Reasoning that these mixed effects might involve voltage-dependent actions, we tested the effect of several opiates on CA3 pyramidal neurons using single-electrode voltage-clamp recording in a slice preparation of rat hippocampus. In most CA3 neurons, the voltage-dependent K+ current known as the M-current (I(M)) was uniquely sensitive to the opioid peptides, with the direction of response dependent upon the opiate type and concentration. Thus, an opiate selective for κ receptors, U-50,488H, significantly augmented I(M). The κ-selective agonists dynorphin A and dynorphin B, which exist in mossy fiber afferents to CA3 pyramidal neurons, also markedly augmented I(M) at low concentrations (20-100 nM). By contrast, dynorphin A at higher concentrations (1-1.5 μM) often reduced I(M). Similarly, several opiates [e.g., D-Ala2,D-Leu5-enkephalin: (DADL), [D-Pen2,5]-enkephalin (DPDPE)] known to act on the δ receptor subtypes reduced the M-current, with partial reversal of this effect by naloxone. Neither the selective μ-receptor agonist [D-Ala2, NMe-Phe1, Gly-ol]-enkephalin (DAMGO) nor the nonopioid fragment of dynorphin, des-Tyr-dynorphin, consistently altered I(M). These opiate effects on I(M) were accompanied by changes in conductance and holding current consistent with their respective effects on I(M). Dynorphin A did not measurably affect the Q-current, a conductance known to contribute to inward rectification in hippocampal pyramidal neurons. The opiate effects on I(M) were not altered by pretreatment with Cs+ (which blocks I(o)) or Ca2+ channel blockers. The opposing effects of the dynorphins (both A and B) and DADL on I(M) were antagonized by naloxone (1-3 μM), and the dynorphin- induced augmentations of I(M) were usually reversed by the κ receptor antagonist norbinaltorphimine. These results suggest that the opiates can have opposing effects on the same voltage-dependent K+ channel type (the M channel) in the rat CA3 pyramidal neuron, with the direction of the response depending on which receptor subtype is activated. These data not only help explain the mixed effects of opiates seen in other studies, but also suggest a potential postsynaptic function for the endogenous opiates contained in the CA3 mossy fibers.