Regional specificity of changes in [3H]Leu-enkephalin binding associated with warm water swimming in mice.

Warm water swimming produces in mice an opiate-like antinociceptive response. Immediately following a 3 min swim, mice were sacrificed by immersion in liquid nitrogen and three brain areas dissected from frozen tissue: basal ganglia (#1), periventricular, thalamic and ventral areas (#2), and medulla-brainstem (#3). A 37% reduction in exogenous [3H]Leu-enkephalin binding was observed in section #2. When section #2 was further divided, a 41% reduction was observed in an area containing opiate receptor contributions from interpeduncular nucleus, thalamus, hypothalamus and amygdala. These results are consistent with the association of opiate receptors in these regions with swim-induced antinociception.