The effect of morphine on responses of nucleus ventroposterolateralis neurons to colorectal distension in the rat

In 71 halothane-anesthetized rats, we characterized the responses of single neurons in the nucleus ventroposterolateralis (VPL) of the thalamus to a n oxious visceral stimulus (colorectal balloon distension; CRD) and studied t he effects of intravenous morphine on these responses using standard extrac ellular microelectrode recording techniques. One hundred nine neurons were isolated on the basis of spontaneous activity, Sixty-four (59%) responded t o CRD, of which 52 (81%) had excitatory and 12 (19%) had inhibitory respons es. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 80 mmHg. Responses to no xious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studi ed in 95 of the VPL neurons isolated. Eighty-three of these neurons (48 CRD responsive and 35 CRD nonresponsive) (87%) had cutaneous receptive fields, of which 96% were small and contralateral and 4% were large and contralate ral or bilateral. Ninety-four percent of these neurons responded to both no xious and innocuous cutaneous stimulation, and 6% responded to only noxious stimulation. No neurons responded solely to innocuous stimulation. Cumulat ive doses of morphine (0.125, 0.25, 0.5, 1, and 2 mg/kg, i.v) produced stat istically significant dose-dependent attenuation of neuronal responses to C RD, Naloxone (0.4 mg/kg, i.v,) reversed the effects of morphine. Morphine a nd naloxone had no significant effects on spontaneous activity. These data support the involvement of VPL neurons in visceral nociception and are cons istent with a role of VPL in sensory-discriminative aspects of nociception. (C) 1999 Elsevier Science Inc.