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Although intrathecal administration of baclofen, a selective GABA(B)-receptor agonist, is known to have an antinociceptive effect on various pain models, the role of presynaptic GABA(B) receptors in antinociception is not well characterized. In the present study, the action of baclofen on primary afferent-evoked glutamatergic excitatory transmission was examined in substantia gelatinosa (SG) neurons of an adult rat spinal cord slice with an attached dorsal root, prepared from the lumbar segment, by use of the blind whole-cell patch-clamp technique. Under the condition where a postsynaptic action of baclofen was inhibited, baclofen (1 microM) reduced the amplitudes of excitatory postsynaptic currents (EPSCs; V(H)=-70 mV) which were monosynaptically evoked by stimulating primary-afferent C- and/or Adelta-fibers and which were remarkably depressed by CNQX (10 microM). The identification of the C-fiber or Adelta-fiber EPSC was based on antidromic action potentials recorded from neurons of isolated dorsal root ganglia. The C-fiber EPSC was depressed in peak amplitude by baclofen (1 microM) to a larger extent than the Adelta-fiber EPSC (20 and 45% of control, respectively). Each of the baclofen actions was suppressed by a selective GABA(B)-receptor antagonist, CGP 35348 (50 microM). Baclofen (1 microM) did not affect a response of SG neurons to bath-applied AMPA (10 microM). These results indicate that baclofen inhibits the release of L-glutamate from Adelta and C primary-afferent terminals in the SG through the activation of GABA(B) receptor; this action is more effective to C-fiber than Adelta-fiber transmission. Considering that the SG is the main part of termination of Adelta- and C-fibers transmitting nociceptive information, the present finding would account for at least a part of the inhibitory action of baclofen on pain transmission.

The observations suggest that GABAergic interneurons possess somatodendritic alpha1 receptors, and activation of these receptors excites inhibitory interneurons. The alpha1 actions reported herein may contribute to the analgesic action of intrathecally administered phenylephrine.

Phasic and tonic inhibitions are two types of inhibitory activities involved in inhibitory processing in the CNS. In the spinal cord dorsal horn, phasic inhibition is mediated by both GABAergic and glycinergic inhibitory postsynaptic currents. In contrast to phasic inhibitory currents, using patchclamp recording technique on spinal cord slices prepared from adult mice we revealed that tonic inhibitory currents were mediated by GABA A receptors but not by glycine receptors in dorsal horn lamina II region. We found that there was a linear relationship (r = 0.85) between the amplitude of tonic inhibitory currents and the frequency of GABAergic inhibitory postsynaptic currents. Analysis of charge transfer showed that the charges carried by tonic inhibitory currents were about 6 times of charges carried by phasic inhibitory currents. The prominent charge transfer by tonic inhibitory currents and their synaptic activity dependency suggest a significant role of tonic inhibition in sensory processing.

Midazolam augmented both the duration of GABA-mediated synaptic current and the amplitude of GABA-induced current by acting on the GABA(A)-benzodiazepine receptor in substantia gelatinosa neurons; this would increase the inhibitory GABAergic transmission. This may be a possible mechanism for antinociception by midazolam.

Midazolam reduced excitatory synaptic transmission by acting on the gamma-aminobutyric acid type A/benzodiazepine receptor in interneurons, leading to a decrease in the excitability of spinal dorsal horn neurons. This may be a possible mechanism for the antinociception by midazolam in the spinal cord.

Isoflurane augments gamma-aminobutyric acid-mediated inhibitory transmission, leading to a decrease in the excitability of spinal dorsal horn neurons. This may be a possible mechanism for the antinociceptive effect of isoflurane in the spinal cord.