Pharmacological characterization of GABA(B)-mediated responses in the CA1 region of the rat hippocampal slice

Abstract

It is generally accepted that the bicuculline-resistant responses to GABA are mediated through the activation of GABA(B) receptors that mediate a slow IPSP. However, a number of reported observations are difficult to reconcile with this model. Specifically, GABA(B) antagonists only partially block bicuculline-resistant GABA responses, and both 4-aminopyridine (4-AP) and carbachol have been reported to block responses to the selective GABA(B) agonist baclofen, but not GABA itself. Thus, it has been argued that baclofen and GABA increase potassium conductance through separate receptor mechanisms. This suggestion is not easily reconcilable with the postulated physiological role of GABA(B) receptors in mediating the slow IPSP. We have addressed these discrepancies by using the new GABA(B) antagonists 2-hydroxy-saclofen (2-OH- SAC) and CGP 35348 in the presence of the GABA uptake inhibitor SKF 89976A. The weak antagonism of 2-OH-SAC against the bicuculline-resistant GABA response was improved when the GABA uptake was inhibited with SKF 89976A, allowing for the application of lower GABA concentrations. Under these circumstances, 2-OH-SAC and CGP 35348 strongly antagonized GABA and baclofen responses, but did not have any effect on outward currents evoked by 5-HT. The slow IPSP evoked in the presence of glutamate antagonists was reversibly inhibited by CGP 35348 (IC50 = : 14 μM), without affecting the fast IPSP. Carbachol (0.3-20 μM) had no effect on outward currents evoked by either baclofen or GABA. 4-AP (5 μM to 1 mM), despite causing a large increase in cell excitability, did not change baclofen responses. Higher concentrations of 4-AP (5 mM) induced inward current, and reduced both baclofen and GABA outward currents to a similar extent. The strong presynaptic inhibitory effect of baclofen and GABA on EPSPs was completely blocked by CGP 35348. The fact that the post- and presynaptic effects of both baclofen and GABA have similar pharmacological characteristics suggests that both substances act through the same receptor mechanism. The present findings are entirely consistent with the proposal that the slow IPSP is generated by synaptically released GABA acting on GABA(B) receptors.