Inhibition in spinal cord of nociceptive information by electrical stimulation and morphine microinjection at identical sites in midbrain of the cat

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Abstract
The descending inhibition of noxious heat-evoked spinal neuronal excitation produced by morphine (MOR) and electrical brain stimulation (EBS) given at identical sites in the midbrain was quantitatively studied in the anesthetized cat. Fifty-two dorsal horn units driven by electrical stimulation of the posterior tibial and/or superficial peroneal nerves at A- and C-fiber strength and responding to noxious radiant heating (50 degrees C) of the skin of the foot-or toepads were studied. All units also responded to mechanical skin stimuli and were located primarily in laminae IV-VI of the dorsal horn. MOR (10-20 micrograms) was administered at 23 different sites in the midbrain. MOR attenuated the heat-evoked responses of 14 of 18 dorsal horn units studied to a mean 43% of the control heat-evoked response when administered at 18 sites in and immediately surrounding the periaqueductal gray (PAG). MOR administered at four sites in the PAG failed to significantly attenuate spinal nociceptive responses. The 14 sites where MOR was efficacious were distributed throughout the PAG. The efficacy of the MOR-produced inhibition was not correlated significantly to the distance from the cerebral aqueduct for the 18 PAG sites examined nor was there any difference in the spinal inhibitory effects of MOR, whether administered dorsally or ventrally in the PAG. MOR also failed to affect spinal neuronal heat-evoked responses (n = 5) when administered at five sites in the reticular formation ipsilateral to the PAG. EBS at the same 23 sites where MOR was given and at 34 additional sites in the midbrain attenuated the heat-evoked responses of all but one dorsal horn unit studied (mean maximal inhibition to 35% of control). In the PAG, stimulation ventrally inhibited a significantly greater proportion (13/21, 62%) of heat-evoked spinal neuronal responses to less than or equal to 25% of the control heat-evoked response than did stimulation in the dorsal PAG (5/15, 33%). The efficacy of the stimulation-produced inhibition was not, however, correlated significantly to the distance from the cerebral aqueduct for the total 57 midbrain sites examined, the 23 sites at which MOR was also tested, or only those EBS sites in and surrounding the PAG (n = 44).(ABSTRACT TRUNCATED AT 400 WORDS)

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