Myorelaxant effect of baclofen injected to the nucleus accumbens septi.

The GABAergic modulation in the nucleus accumbens septi (NAS) of muscle tone was investigated in rats using behavioral tests. The GABAB receptor agonist baclofen dose-dependently decreased muscle tone in the wire-mesh and bar holding tests both after local injection into the NAS (1.0 and 2.5 micrograms), and after intraperitoneal administration in a dose of 20 mg/kg. In the Wirth's test haloperidol (5 mg/kg i.p.), produced catalepsy, whereas baclofen (20 mg/kg, i.p.) significantly deteriorated rats' performance. Intraaccumbens microinjections of muscimol, midazolam, nicardipine, as well as peripheral injections of haloperidol and midazolam failed to modify muscle tone in the wire-mesh test. These findings argue against the involvement of GABAA receptors, benzodiazepine receptors, as well as dopaminergic- and calcium channel-related mechanisms in the effect of baclofen. Hence, the muscle relaxant effect of baclofen seems to be also mediated through GABAB receptor sites within the NAS.