Concurrent mapping of brain sites for sensitivity to the direct application of morphine and focal electrical stimulation in the production of antinociception in the rat

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Abstract
The effect of morphine microinjection (5 .mu.g/0.5 .mu.l) and focal electrical stimulation on the animal''s response to radiant heat and noxious pinch was studied concurrently at 117 brain loci extending from the medial thalamus caudally to the periaqueductal gray area (PAG). Three populations of brain sites were discernible based on their responsiveness to focal electrical stimulation and morphine microinjection in the production of antinociception: sites which support stimulation-produced analgesia (SPA, n = 24), sites which were sensitive to the direct application of morphine (n = 8) and sites responsive to both manipulations (n = 8). With few exceptions all morphine sensitive sites were located within the anatomical boundaries of the PAG; sites supporting SPA were located not only within the PAG but also in the brain regions peripheral to this structure. Sites responsive to both manipulations were generally distributed througout the lateral aspect of the posteroventral PAG. Stimulation strength-effect curves for sites subserving SPA were also obtained. No differences were discovered between curves obtained from morphine-sensitive and -insensitive brain loci.