Effect of GABA _B receptor agonist baclofen on excitatory and inhibitory synapses of ventrolateral periaqueductal gray in rats

Aim: To investigate the possible difference between glutamatergic synapses and GABAergic synapses in the ventrolateral periaqueductal gray (vlPAG) when GABA _B receptor agonist is at nonsaturating concentration or saturating concentration. Methods We employed whole cell patch-clamp recordings on acute PAG slices from adult rats. Results Glutamatergic synapses were less sensitive to baclofen, a putative GABA _B receptor agonist, than that of GABAergic synapses, but only at nonsaturating concentration (0.1 μmol • L ^-1 ). On the contrary, saturating concentration of baclofen (5 μmol • L ^-1 ) inhibited both glutamatergic and GABAergic synapses to a similar ratio. Accordingly, low concentration of baclofen increased, while high concentration decreased the excitability of postsynaptic vlPAG neurons. Conclusions The overall action of baclofen in vlPAG depends on its concentrations. Since vlPAG plays an important role in reducing pain modulation system, the different concentrations of GABA may induce its varying actions on modulation.