The Mechanisms Of GABA_B Receptors Modulating Synaptic Transmission In Midbrain Periaqueductal Gray

The periaqueductal gray（PAG）in the midbrain is one of the important structures involved in the so called “descending pain inhibition system”.Among the subdivisions of PAG,its ventrolateral part（vl PAG）is particularly well-characterized because it plays a pivotal role in regulating nociception at the spinal cord level.PAG sends projection either via rostral ventromedial medulla（RVM）or directly to the spinal cord dorsal horn.In the 1960’s,some Chinese scientists reported that activation of PAG induces analgesia in experimental animals,and the subsequent clinic applications support the idea of PAG-induced analgesia in humans.With the development of methods in neuroscience for morphological and physiological studies,it has been well characterized that PAG expresses multiple neurotransmitters,including excitatory glutamate（Glu）and inhibitory γ-aminobutyric acid（GABA）.The released Glu activates postsynaptic glutamate receptors including N-methyl-D-aspartic acid（NMDA）receptors,kainic acid（KA）receptors and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid（AMPA）receptors as well.On the other hand,released GABA activates postsynaptic GABAA receptors and postsynaptic GABA_B receptors,as well as ‘outsynaptic’ GABA_B receptors.The GABAA receptors are virtually chloride channels that permit Cl^–.GABA_B receptors are a metabotropic type of GABA receptors that couple to intracellular G_i/q proteins.It has been well characterized that GABAA receptors mediates ‘fast’,while GABA_B receptors mediates ‘slow’ inhibitory responses,respectively.Morphological data indicate that GABA_B receptors are expressed on both pre-and postsynaptic vl PAG on both excitatory synapses（containing Glu）and inhibitory synapses（containing GABA）.Behavioral study reported that microinjection of a specific GABA_B receptor agonist,baclofen,induces either analgesia or anti-analgesia,depending on different concentrations.In vitro electrophysiological recordings show that at presynaptic loci,activation of presynaptic GABA_B receptors decreases Glu or GABA release by blocking Ca²⁺ channels.On the other hand,at the postsynaptic,GABA_B receptor activation induces a G protein-coupled inwardly rectifying K+（GIRK）channel-mediated “slow” membrane current.However,the mechanisms for GABA_B receptors in PAG remain largely unknown.In the present study,employing tight-seal,whole cell patch-clamp recordings on the acute PAG in vitro slices from adult rats,we report that:（1）The presynaptic and postsynaptic GABA_B receptors on PAG show different sensitivity to baclofen,a specific agonist for GABA_B receptors.When the responses of presynaptic GABA_B receptors and their postsynaptic counterparts were simultaneously monitored,we found that baclofen exhibits more potency and efficacy at GABA_B presynaptic versus postsynaptic receptors,and this sensitivity bias might contribute to synapse homeostasis and implicate a novel pharmacotherapy.（2）Using nipecotic acid（NPA）,as transportable blocker for all type of GABA transporters to elevate GABA concentrations around presynaptic,the accumulated GABA strengthens GABA_B receptor-mediated presynaptic inhibition on both excitatory and inhibitory synapses.The data suggest that PAG neurons experience GABA_B receptor-mediated inhibition determined by GABA_B transporters and the accumulated GABA may indicate a novel therapeutic way.And（3）,since different baclofen induces different behavioral responses,we thus seek the concentration-response relationship by using ‘saturated concentration’（5 μM）and ‘non-saturated concentration’（0.1 μM）baclofen on PAG.We found that glutamatergic synapses are less sensitive to baclofen than to that of GABAergic synapses,but only in non-saturated concentrations.On the contrary,saturating concentration of baclofen inhibits both glutamatergic and GABAergic synapses to a similar ratio.Accordingly,low concentration of baclofen increases,while high concentration decreases the excitability of postsynaptic vl PAG neurons.The data lead to a hypothesis that overall action of baclofen in the vl PAG depends on it concentration and this difference may underlie the different behavioral responses of PAG baclofen microinjection.In summary,our present data give direct electrophysiological evidence that GABA_B receptors in PAG show different sensitivity at presynaptic and postsynaptic loci,accumulated GABA activates presynaptic GABA_B receptors and inhibits both excitatory and inhibitory synapses and furthermore,different concentration of baclofen induces different action at a single neuron level.Taken together,our present study gives our further understandings for GABA_B receptor roles in PAG and may contribute to discover more therapeutic ways for pain.