Differential potentiative effects of GABA receptor agonists in the production of antinociception induced by morphine and beta-endorphin administered intrathecally in the mouse

The effect of muscimol or baclofen injected intrathecally (i.t.) on the inh ibition of the tail-flick response induced by morphine and R-endorphin admi nistered i.t. was studied in ICR mice. The i.t. injection of muscimol (100 ng) or baclofen (10 np) alone did not affect the basal inhibition of the ta il-flick response. Morphine (0.2 mu g) and beta-endorphin (0.1 mu g) caused only slight inhibition of the tail-flick response. Baclofen, but not musci mol, injected i.t. enhanced the inhibition of the tail-flick response induc ed by i.t. administered morphine. Both muscimol and baclofen injected i.t, significantly enhanced i.t. injected beta-endorphin-induced inhibition of t he tail-flick response. Our results suggest that the GABA(B), but not GABA( A), receptors located in the spinal cord appear to be involved in enhancing the inhibition of the tail-flick response induced by morphine administered spinally. In addition, both GABA(A) and GABA(B) receptors are involved in enhancing the inhibition of the tail-flick response induced by beta-endorph in administered i.t. (C) 1999 Elsevier Science Inc.