Inhibitory regulation of excitatory transmitter release is likely to be an important role for GABA in the spinal cord. In keeping with this, systemically administered selective inhibitors of the neuronal uptake site GAT-1 has been reported to be antinociceptive in rodent models of acute pain (Ipponi et al., 1999). However, it is unclear what contribution a spinal site of action makes to this effect. Previously using in vivo microdialysis, we have shown that local inhibition of GAT-1 by the selective inhibitor NO-711 in the anaesthetised rat dorsal spinal cord increases extracellular GABA levels and reduces the K⁺-evoked release of glutamate (Smith et al., 2002). Here, in order to confirm this latter finding as a receptor mediated effect, we have co-administered selective antagonists for GABA_A and GABA_B receptors with NO-711 to determine the receptor subtype(s) involved.

A microdialysis probe was implanted within the dorsal spinal cord as described by Whitehead et al., (2001). Male Wistar rats (250-350g, Charles Rivers U.K. Ltd) were used and anaesthetised under halothane/N₂0 throughout. The probe was perfused with an artificial extracellular fluid (aECF) at 2 µl.min^-1. Following a 120 min recovery period, 10 min samples were collected and assayed for aspartate (ASP), glutamate (GLU), glycine (GLY) and GABA content by HPLC coupled to fluorescence detection. Efflux was evoked by perfusion with aECF containing 45 mM K⁺ for 6 min during the fifth (S1) and eleventh (S2) samples. NO-711 (300 µM) and/or the GABA_A antagonist bicuculline (100 µM), or the GABA_B antagonist SCH 50911 (100 µM) were included in the perfusate for 30 min preceding S2. S1 and S2 values were determined as the difference from mean efflux in the preceding 3 samples and S2/S1 ratios calculated.

Extending our initial report (Smith et al., 2002), high K⁺ -evoked release of both ASP and GLU but not GLY or GABA were significantly reduced in the presence of NO-711, see Table 1. Co-perfusion of bicuculline prevented the NO-711 induced reduction in the S2/S1 ratio of both ASP and GLU. In contrast, the effect of NO-711 on evoked ASP, but not GLU, was prevented by SCH 50911.

Table 1: S2/S1 ratios of high K⁺-evoked excitatory amino acid release in the presence or absence of GABA_A or GABA_B antagonists

Mean ± s. e. mean, n in parenthesis, *P<0.05, one way ANOVA with post hoc Dunnetts test compared to control.

Antagonism of either GABA_A or GABA_B receptors prevented the effect of NO-711 on evoked ASP release, suggesting both receptors are involved in the control of ASP release within the dorsal spinal cord. In contrast, the effect of NO-711 on GLU release was only blocked by GABA_A antagonism indicating this receptor alone mediates GABAergic control. This provides evidence for differential GABAergic regulation on excitatory amino acid neurotransmission within the spinal cord dorsal horn

.Ipponi A. et al., (1999) Eur. J. Pharmacol. 368, 205-211.
Smith C.G.S. et al., (2002) Brit. J. Pharmacol. Suppl. 135, 95P.
Whitehead K.J. et al., (2001) Brain Res. 910, 192-194.