Analysis of the long-term actions of gabapentin and pregabalin in dorsal root ganglia and substantia gelatinosa

The alpha 2 delta-ligands pregabalin (PGB) and gabapentin (GBP) are used to treat neuropathic pain. We used whole cell recording to study their long-term effects on substantia gelatinosa and dorsal root ganglion (DRG) neurons. Spinal cord slices were prepared from embryonic day 13 rat embryos and maintained in organotypic culture for >5 wk (neuronal age equivalent to young adult rats). Exposure of similarly aged DRG neurons (dissociated and cultured from postnatal day 19 rats) to GBP or PGB for 5-6 days attenuated high-voltage-activated calcium channel currents (HVA I-Ca). Strong effects were seen in medium-sized and in small isolectin B-4-negative (IB4-) DRG neurons, whereas large neurons and small neurons that bound isolectin B-4 (IB4+) were hardly affected. GBP (100 mu M) or PGB (10 mu M) were less effective than 20 mu M Mn2+ in suppression of HVA I-Ca in small DRG neurons. By contrast, 5-6 days of exposure to these alpha 2 delta-ligands was more effective than 20 mu M Mn2+ in reducing spontaneous excitatory postsynaptic currents at synapses in substantia gelatinosa. Spinal actions of gabapentinoids cannot therefore be ascribed to decreased expression of HVA Ca2+ channels in primary afferent nerve terminals. In substantia gelatinosa, 5-6 days of exposure to PGB was more effective in inhibiting excitatory synaptic drive to putative excitatory neurons than to putative inhibitory neurons. Although spontaneous inhibitory postsynaptic currents were also attenuated, the overall long-term effect of alpha 2 delta-ligands was to decrease network excitability as monitored by confocal Ca2+ imaging. We suggest that selective actions of alpha 2 delta-ligands on populations of DRG neurons may predict their selective attenuation of excitatory transmission onto excitatory vs. inhibitory neurons in substantia gelatinosa.