Electrical stimulation of thalamic Nucleus Submedius inhibits responses of spinal dorsal horn neurons to colorectal distension in the rat

In 78 halothane-anesthetized rats, we characterized the responses of single neurons in the dorsal horn of L(6)-S(1) spinal segments to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of focal electrical stimulation of Nucleus Submedius (Sm) on these responses using standard extracellular microelectrode recording techniques. A total of 102 neurons were isolated on the basis of spontaneous activity. Eighty (78%) responded to CRD, of which 70% had excitatory and 30% had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 100 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 73 of the spinal dorsal horn neurons isolated. Fifty-seven (78%) of these neurons (46 CRD-responsive and 11 CRD-nonresponsive) had cutaneous receptive fields, of which 35 (61%) were small and ipsilateral, 14 (25%) were large and ipsilateral, 7 (12%) were large or small and bilateral, and 1 (2%) was small and contralateral. Sixty-one percent of these neurons responded to both noxious and innocuous cutaneous stimulation, 35% responded only to noxious stimulation, and 4% responded only to innocuous stimulation. Electrical stimulation (50-300 microA) of the contralateral Sm produced intensity-dependent attenuation of the CRD-evoked activities of most neurons (18/28 of CRD-excited and 7/12 of CRD-inhibited) tested. Sm stimulation produced facilitation of CRD responses of only one neuron (CRD-inhibited). Sm stimulation had no effects on spontaneous activity. These data indicate that Sm may be involved in the descending inhibitory modulation of visceral nociception at the spinal level.