THE EFFECT OF MORPHINE ON RESPONSES OF VENTROLATERAL ORBITAL CORTEX (VLO) NEURONS TO COLORECTAL DISTENSION IN THE RAT

In 49 halothane-anesthetized rats, we characterized the responses of s ingle neurons in the ventrolateral orbital cortex (VLO) to a noxious v isceral stimulus (colorectal balloon distension, CRD), and studied the effects of intravenous morphine on these responses using standard ext racellular microelectrode recording techniques. One hundred and four n eurons were isolated on the basis of spontaneous activity. Fifty-seven (55%) responded to CRD, of which 32% had excitatory and 68% had inhib itory responses. Neurons showed tendencies toward graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibit ion occurring at 80 or 100 mmHg, respectively. Responses to noxious (p inch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 80 of the VLO neurons isolated. Thirty-three (41%) of these neurons (21 CRD-responsive and 12 CRD-nonresponsive) had cutaneous receptive fields, of which 79% were large and bilateral, 18% were small and bila teral, 3% were small and ipsilateral. Ninety-four percent of these neu rons responded only to noxious cutaneous stimulation, 6% responded to both noxious and innocuous stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.0625, 0.125 an d 0.25 mg/kg i.v.) produced statistically significant dose-dependent a ttenuation of neuronal responses to CRD. Naloxone (0.4 mg/kg i.v.) rev ersed the effects of morphine. Morphine and naloxone had no significan t effects on spontaneous activity. These data support the involvement of VLO neurons in visceral nociception. (C) 1998 Elsevier Science B.V. All rights reserved.