Bh. Manning et al., MORPHINE ANALGESIA IN THE FORMALIN TEST - EVIDENCE FOR FOREBRAIN AND MIDBRAIN SITES OF ACTION, Neuroscience, 63(1), 1994, pp. 289-294
A mapping study was performed to determine where in the rat brain morp
hine acts to produce analgesia in the formalin test, which is an anima
l model of prolonged pain associated with tissue injury. A single dose
(5 nmol) of morphine was bilaterally microinjected into a wide range
of brain areas throughout the midbrain and forebrain. Strong analgesia
was elicited from the posterior hypothalamic area, the periaqueductal
gray and ventral tegmental area. Other sites from which analgesia was
elicited were the nucleus accumbens and a few sites in the retrorubra
l field and caudate-putamen, Analgesia from the periaqueductal gray or
nucleus accumbens was accompanied by decreased locomotor activity and
catalepsy, whereas analgesia from the posterior hypothalamic area or
ventral tegmentum was accompanied by a noticeable increase in locomoto
r activity and rearing. Morphine into various thalamic nuclei had no e
ffect. These results indicate that the primary sites of action of morp
hine in the formalin test are probably the posterior hypothalamic area
and periaqueductal gray, with an additional contribution from regions
innervated by tegmental dopamine cells.