354 Possible conduction block by mu opioid in primary afferent fibers in the spinal dorsal horn

Abstract

We have recently shown that morphine withdrawal sensitizes the neural substrates of fear in the midbrain tectum structures—the dorsal periaqueductal gray (dPAG) and inferior colliculus (IC). In the present study, we investigated the role of μ- and κ-opioid receptors in the mediation of these effects. Periadolescent rats chronically treated with morphine (10 mg/kg; s.c.) twice daily for 10 days were implanted with an electrode glued to a guide-cannula into the dPAG or the IC. Forty-eight hours after the interruption of this treatment, the effects of intra-dPAG or intra-IC microinjections of [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO; 0.6 and 1 nmol/0.2 μl) – a selective μ-receptor agonist – or nor-binaltorphimine (BNI; 2.5 and 5 μg/0.2 μl) – a selective κ-receptor antagonist with tardive action – on the freezing and escape thresholds determined by electrical stimulation of the dPAG and the IC were examined. For both structures, morphine withdrawal produced pro-aversive effects. DAMGO and BNI had antiaversive effects when injected into the dPAG and IC of non-dependent rats. In morphine-withdrawn rats, only BNI continued to promote antiaversive effects in both structures. Whereas DAMGO lost its antiaversive efficacy when injected into the dPAG, only its highest dose promoted antiaversive effects in the IC of morphine-withdrawn rats, suggesting the development of an apparent tolerance. Thus, the enhanced reactivity of the midbrain tectum in morphine-withdrawn periadolescent rats may be due, at least partially, to an impairment of the inhibitory influence of mechanisms mediated by μ-receptors on the neural substrates of fear in this region.