Effects of topical baclofen on C fibre-evoked neuronal activity in the rat dorsal horn

Baclofen appears to be an agonist for the bicuculline-insensitive γ-aminobutyrate B receptors associated with C fibre terminals in the dorsal horn of the spinal cord. We have tested the effect of baclofen (applied intrathecally onto the spinal cord) on the A and C fibre-evoked responses of convergent/ multireceptive neurones in the halothane-anaesthetized rat. l-Baclofen produced a dose-dependent inhibition of the C fibre- and pinch-evoked activity of these neurones which persisted for 2 h whilst the A fibre and tactile activities were little changed. The C fibre-evoked ( × 3 threshold) responses were markedly or completely inhibited 10 min after doses of between 0.25 and 30 μg of l-baclofen ( n =21) with 0.05 μg causing a 48% ( n = 3) and 0.01 μg a 28% inhibition ( n = 3). d-Baclofen (30 μg), the inactive isomer, produced no significant changes in activity ( n = 10). Bicuculline (60 μg) applied intrathecally before ( n = 7) or after ( n = 8) l-baclofen did not reverse the inhibitions. Intravenous baclofen (–3 mg/kg) also produced neuronal inhibitions similar to the effects of intrathecal injection. The results suggest that γ-aminobutyrate B receptors may exert a presynaptic control of C fibre afferents in the dorsal horn following intrathecal administration in the rat.