Gabab receptor mediated presynaptic inhibition in laminae III-IV of rat dorsal horn

The neurotransmitter GABA exerts a strong inhibitory control on spinal cord dorsal horn, by acting both at ionotropic (GABAA) and metabotropic (GABAB) receptors. Several electrophysiological studies have shown that GABAB receptors expressed on Adelta and C primary afferents inhibit glutamate release in superficial laminae (I and II). Although GABAB receptors have been localized also on large diameter fibers, projecting to deeper dorsal horn laminae, their function in presynaptic inhibition has not been fully characterized. I performed patch-clamp recordings from lamina III-IV neurons, in spinal cord slices from postnatal rats (P17-P23), to determine GABAB receptor role in modulating glutamate release from low threshold afferent fibers (mainly Abeta). Glutamatergic monosynaptic EPSCs were evoked by stimulating the dorsal root at low intensities and applying the paired pulse protocol. Application of the GABAB receptor agonist baclofen (1 microMol) reversibly inhibited both EPSC peaks (1st peak decrease: 76.3\ub13.1%; 2nd peak: 42.6\ub18.7%, n=9) and increased the paired pulse ratio (PPR) (PPR increase: 147.7\ub120.6%). Baclofen 2.5 microMol produced a stronger inhibition of both peaks (1st peak decrease: 86.2\ub13.1%; 2nd peak: 57.6\ub15.1%, n=7 ) and a larger increase of PPR (288.4\ub160.9%). The effects of baclofen at both concentrations were blocked by the GABAB antagonist CGP 55845 (5 microMol). The intracellular perfusion of the recorded neuron with the G protein inhibitor GDP-beta-S did not alter the inhibitory effects of baclofen, confirming a presynaptic site of action (n=10). In order to assess whether presynaptic GABAB receptors are endogenously activated, I tested the effect of CGP 55845 alone on evoked EPSCs (paired pulse protocol). The GABAB antagonist caused a significant increase of the first EPSC peak in 4 neurons out of 15 tested, accompanied by a smaller increase of the second peak (1st peak increase: 42.5\ub115.9%; 2nd peak: 10.3\ub16.7%). PPR was decreased by CGP 55845 in this group of cells (PPR ctl=0.60\ub10.04; PPR CGP=0.47\ub10.05). These findings indicate that presynaptic GABAB receptors are effective in modulating glutamate release from low threshold A fibers in dorsal horn laminae III-IV and suggest that they could be endogenously activated by GABA