Behavioral actions of baclofen in the rat ventromedial thalamic nucleus: antagonism by delta-aminovalerate.

Baclofen, an agonist at GABAB receptors injected locally into the rat ventromedial thalamic nucleus (VM) induced catalepsy in a dose-dependent, stereospecific and site-specific way. Baclofen-induced catalepsy was accompanied by tonic activity in the electromyogram (EMG) recorded from the gastrocnemius muscle, which is considered to be a measure of limb rigidity. delta-Aminovalerate (delta-AVA) coadministered with baclofen into the VM prevented the development of both catalepsy and limb rigidity. delta-AVA injected alone into the VM was devoid of depressant or stimulating behavioral effects. In contrast to delta-AVA, bicuculline, a GABAA antagonist, failed to antagonize baclofen-induced catalepsy. On the other hand, catalepsy due to local injection of a GABAA agonist, muscimol into the VM was antagonized by bicuculline, but not by delta-AVA. The present results suggest that delta-AVA may be useful as an antagonist of central pharmacological actions of baclofen.