Inhibitory Effects of Morphine on Intralaminar Nociceptive Neurons in the Cat Thalamus

Abstract

     We examined the inhibitory effects of morphine administration on the nociceptive neurons in the thalamic relay nuclei of the medial pain pathway in cats anesthetized with urethane and chloralose. Single unit activities were recorded from the thalamus using glass capillary microelectrodes filled with 2% pontamine sky blue in 0.5M sodium acetate. The greater splanchnic nerve (SPL), greater occipital nerve (C2), bilateral maxillary and mandibular canine dental pulps, and midbrain reticular formation (MRF) were prepared for electrical stimulation.
     We electrically stimulated the peripheral nerve and MRF and recorded the response in the nucleus centralis lateralis and nucleus parafascicularis of the thalamic intralaminar nuclei. We also investigated the effects of intravenous administration of morphine on the responses obtained from these neurons. Morphine inhibited the late responses elicited by electrical stimulation of the peripheral nerve. As well, it inhibited the early responses elicited by electrical stimulation of the MRF. Administration of naloxone and cholecystokinin partially reversed these inhibitory effects of morphine.
     These data suggest that morphine acts on the medial pain pathway at the supraspinal level.