We have recently shown that neurokinin 1 receptor (NK1r) agonists provoke an increase in GABA release onto pyramidal cells in the entorhinal cortex (EC) in vitro (Stacey et al. 2002). This effect is due to excitation of inhibitory interneurones, and is extremely robust, with low nanomolar concentrations provoking a 3-4-fold increase in frequency of spontaneous inhibitory postsynaptic currents (sIPSCs). In the present study we have examined (1) whether the NK1r-mediated increase in release of GABA might be self-limiting via activation of presynaptic GABA_B autoreceptors and (2) whether such feedback could act to reduce electrically evoked IPSCs (eIPSCs).

Wistar rats were anaesthetized by intramuscular injection of ketamine (120 mg kg^-1) plus xylazine (8 mg kg^-1) and then decapitated. Slices of the EC were prepared and maintained in vitro by conventional techniques. Whole cell patch clamp recordings were made from pyramidal neurones in layer V visually identified using differential interference contrast optics and infra-red video microscopy. The patch pipette solution was Cs⁺-based and contained QX-314 to block postsynaptic GABA_B-receptors. Intra- and extracellular Cl^– concentrations were symmetrical and the extracellular medium contained NBQX and 2-AP5 to block ionotropic glutamate receptors. Under these conditions we recorded pure GABA_A receptor-mediated IPSCs. eIPSCs were electrically elicited at 0.1 Hz via an extracellular electrode placed lateral to the recording site in layer V.

Mean (± S.E.M.) control baseline frequency of sIPSCs was 4.4 ± 1.3 Hz and this was doubled to 8.7 ± 1.6 Hz on addition of the NK1r agonist, GR73632 (50 nM, n = 10). Perfusion with the GABA_B-receptor antagonist, CGP55845 (1 µM), alone had no effect on baseline frequency (4.6 ± 0.6 Hz, n = 10), but addition of GR73632 in the presence of CGP55845 increased sIPSC frequency to 15.5 ± 3.2 Hz, a significant (P > 0.05, paired t test, n = 10) increase compared to the agonist alone. Under control conditions the peak amplitude of eIPSCs (n = 7) was 215.8 ± 45.6 pA. During the increase in release induced by GR73632 the amplitude decreased to 151.6 ± 39.7 pA (i.e. by around 30 %). CGP55845 (1 µM), alone increased eIPSC amplitude to 383.1 ± 72.9 pA. However, in the presence of CGP55845, GR73632 still decreased the amplitude (276.7 ± 62.1 pA) to around the same extent (28 %).

These results indicate that feedback activation of GABA_B receptors does not limit spontaneous GABA release under baseline conditions, but that the increased release provoked by NK1r activation is sufficient to access the presynaptic inhibitory autoreceptor. The increase in amplitude of eIPSCs by CGP55845 indicates that GABA_B receptors were acting to limit the eIPSC amplitude. However, the reduction in the eIPSC amplitude still occurred during NK1r agonist-induced GABA release, suggesting that a mechanism other than feedback via the GABA_B autoreceptor might underlie this effect.

A.E.S. was an MRC Scholar.