THE EFFECT OF MORPHINE ON RESPONSES OF MEDIODORSAL THALAMIC NUCLEI AND NUCLEUS SUBMEDIUS NEURONS TO COLORECTAL DISTENSION IN THE RAT

In halothane-anesthetized rats, we characterized the responses of sing le neurons in the nuclei of medial thalamus (MT), specifically the med iodorsal thalamic nucleus (MD) and the nucleus submedius (Sm), to a no xious visceral stimulus (colorectal balloon distension, CRD), and stud ied the effects of intravenous morphine (Mor) on these responses using standard extracellular microelectrode recording techniques. 62 MD and 46 Sm neurons were isolated on the basis of spontaneous activity. 47 of the MD neurons (76%) responded to CRD, of which 70% had excitatory and 30% had inhibitory responses. 38 of the Sm neurons (83%) responded to CRD, of which 89% had excitatory and 11% had inhibitory responses. Responses of MD and Sm neurons excited by CRD were related significan tly to distension pressure (20-100 mmHg), with maximum excitation occu rring at 60 and 100 mmHg, respectively. MD neurons inhibited by CRD al so had graded responses to graded CRD, with maximum inhibition occurri ng at 80 mmHg. The responses to noxious (pinch, heat) and nonnoxious ( tap, brush) cutaneous stimuli were studied in 59 of the MD and 44 of t he Sm neurons isolated. 22 of the MD neurons (37%) studied had cutaneo us receptive fields, of which 59% were large and bilateral, 41% were s mall and usually contralateral receptive fields. 55% of these neurons were nociceptive-specific, 45% responded to both noxious and nonnoxiou s cutaneous stimulation. 29 of the Sm neurons (66%) studied had cutane ous receptive fields, of which 72% were large and usually bilateral, 1 4% were small and bilateral, 14% were small and contralateral receptiv e fields. 90% of these neurons were nociceptive-specific, 10% responde d to both noxious and nonnoxious stimulation. No MD or Sm neurons resp onded exclusively to nonnoxious cutaneous stimulation. Mor (0.125, 0.2 5, 0.5 and 1 mg/kg IV) attenuated MD and Sm neuronal excitatory respon ses to CRD in a dose-dependent fashion, abolishing evoked activity wit h a dose of 0.5 mg/kg (p < 0.05) and 1 mg/kg (p < 0.05), respectively. Naloxone (0.4 mg/kg IV) reversed the effects of Mor. Mor and naloxone had no effects on spontaneous activity. These data support the involv ement of MD and Sm neurons in visceral nociception, and are consistent with a role of Sm in affective-motivational, and MD in both sensory-d iscriminative and affective-motivational aspects of nociception. (C) 1 998 Elsevier Science B.V.