Involvement of the midbrain reticular formation in self-injurious behavior, stereotyped behavior, and analgesia induced by intranigral microinjection of muscimol.

Bilateral microinjection of muscimol (60 ng), a gamma-aminobutyric acid (GABA) agonist, into the central region of the substantia nigra (pars reticulata) produced self-injurious behavior (SIB), stereotyped behavior and analgesic-like effects in rats. Bilateral electrolytic lesions of the midbrain reticular formation ventrolateral to the periaqueductal gray matter completely blocked the SIB but had little effect on stereotyped behavior produced by intranigral muscimol. Lesions of the midbrain reticular formation reduced the antinociceptive effect of intranigral muscimol on the tail-flick but not on the hot-plate test. Bilateral microinjection of muscimol (10-100 ng) into the midbrain reticular formation produced intense stereotyped behavior and had an analgesic-like effect on the hot-plate test but not on the tail-flick test. Stereotyped behavior appeared to interfere with the paw-lick response on the hot-plate test. These data suggest that the antinociceptive effect of intranigral muscimol on the tail-flick test is mediated by fibers that project to or pass through the midbrain reticular formation and that analgesia may play an important role in muscimol-induced SIB. The midbrain reticular formation does not appear to be involved in the stereotyped behavior produced by intranigral muscimol.