Gabaa receptors modulate synaptic transmission mediated by Abeta fibers in lamina III of rat dorsal horn

GABAA receptors contribute to depolarization of primary afferent fibers in the spinal cord following the stimulation of primary afferent fibers. This suggests that GABAA receptors are expressed at primary afferent terminals, where they could modulate glutamate release either by causing membrane depolarization or by shunting. We have investigated the effect of presynaptic GABAA receptor activation on synaptic transmission in the dorsal horn by recording Abeta fiber mediated EPSCs from lamina III neurons in rat spinal cord slices. The monosynaptic nature of the EPSCs was assessed by stimulating the attached dorsal root at 20 Hz and low intensity. Exogenous activation of GABAA receptors by 2 microMol muscimol caused depression of the EPSCS amplitudes and changed paired pulse ratio (PPR) in all cells tested (PPR increase:77.9±17%, n=12). Similar results were obtained from both unidentified and NK1 receptor expressing neurons. Application of muscimol at different concentrations (ranging from 0.5 to 5 microMol showed a dose-dependent increase of PPR. Muscimol also caused an increase of miniature EPSC frequency in lamina III neurons (mean frequency: 2.9±0.7 Hz in control, 6.5±1.5 Hz in muscimol, n=7), while mEPSC amplitude was not affected. Application of 2 microMol muscimol in the presence of 10 microMol bumetanide, a blocker of the chloride transporter NKCC1 in primary afferent fibers, still caused an increase of the evoked EPSC PPR, although the effect was significantly reduced compared to control (PPR increase in bumetanide: 24.3±7%, n=12). In order to assess whether presynaptic GABAA receptors are endogenously activated , we tested the effect of the GABAA antagonist gabazine (10 µM) on Abeta fiber mediated EPSCs in lamina III neurons, evoked by repetitive stimulation (4 pulses at 10-50 Hz). Gabazine application increased the second peak (138.4±38%, n=17), while the other peaks were unaffected or slightly decreased. Our results suggest that functional GABAA receptors are expressed on Abeta fiber terminals, where they depress synaptic transmission during repetitive stimulation. This mechanism of presynaptic inhibition could be impaired during chronic pain, leading to the facilitation of Abeta mediated synaptic transmission and to the generation of allodynia.