Anatomical basis for cannabinoid-induced antinociception as revealed by intracerebral microinjections

Cannabinoids suppress behavioral and neurophysiological responses to noxious stimuli in rodents when administered systemically. The purpose of this study was to extend previous studies of the site of cannabinoid analgesia. Rats were tested in the tail flick test before and after microinjections of the cannabinoid agonist WIN55, 212-2 (5 microg) into one of 17 different brain regions. WIN55,212-2 significantly elevated tail-flick latencies when injected into the amygdala, the lateral posterior and submedius regions of the thalamus, the superior colliculus and the noradrenergic A5 region. By contrast, pain behavior was unaffected by microinjections of the cannabinoid into the other 11 areas examined (prefrontal cortex, nucleus accumbens, lateral hypothalamus, substantia nigra, cuneiform nucleus, anterior pretectal, intralaminar, parafasicular, posterior, thalamic nuclei, as well as the ventral medial, ventral lateral nuclei in the posterior thalamus).