There is growing evidence that the antihyperalgesic effects of prostaglandin EP1 receptor antagonists in animal models of inflammatory pain are associated with spinal sites of action (Nakayama et al. 2002). In this study, we have examined the effects of two EP1 receptor antagonists (GW567675 and GW683868) on excitatory (glutamatergic) and inhibitory (glycinergic) synaptic inputs to substantia gelatinosa neurones in vitro.

For purposes of comparison experiments were performed on slices prepared from the spinal cords of two groups of rats. The first group were naive control animals, whereas the second group was composed of rats in which inflammation in the hind paw had been induced with a subcutaneous intraplantar injection of Freund’s complete adjuvant (FCA), delivered approx. 24 h prior to electrophysiological data collection. Animals were humanely killed and spinal cord slices prepared from L4-L6 by standard means. After slice preparation standard whole-cell voltage-clamp recordings were performed at 32 °C on visualised substantia gelatinosa neurones. The holding potential was -60 or -70 mV for EPSCs and 0 mV for IPSCs. Corresponding miniature synaptic currents were isolated in 1 µM TTX. Data were compared using the Kolmogorov-Smirnov test and Student’s t test. Statistical significance was assessed at P < 0.05.

In naive animals, neither GW567675 (1-10 µM) nor GW683868 (1-10 µM) altered the frequency of spontaneous glutamatergic EPSCs or IPSCs. In contrast, in FCA-inflamed animals, the EP1 antagonists significantly increased the frequency of spontaneous EPSCs in 5/10 cells (by 48 ± 14 %) and IPSCs in 6/13 cells (by 67 ± 9 %), respectively. Similarly, analysis of miniature synaptic currents revealed that EP1 antagonism significantly increased mEPSC frequency in 12/17 cells (by 67 ± 9 %) and glycinergic mIPSC frequency in 5/8 cells (by 73 ± 10 %), respectively. These changes in miniature frequency occurred in the absence of changes to the amplitude or kinetics of either the EPSC or IPSC population. The modulation afforded by EP1 antagonism was eliminated in the presence of 10 µM prostaglandin E2.

These data suggest a presynaptic location for prostaglandin EP1 receptors on the terminals of glutamatergic and glycinergic neurones. Following peripheral inflammation, these prostaglandin EP1 receptors become tonically activated by an endogenous ligand(s) which results in decreased synaptic release of both glutamate and glycine in substantia gelatinosa. This effect provides a potential cellular basis for the antihyperalgesic effects of prostaglandin EP1 receptors in inflammatory pain states.