Pentobarbital modulates intrinsic and GABA-receptor conductances in thalamocortical inhibition

We investigated interactions of an anesthetic barbiturate, pentobarbital, with non-ligand gated channels and identified inhibitory synaptic transmission in thalamic neurons. Using whole cell voltage-clamp, current-clamp and single channel recording techniques in rat ventrobasal neurons of slices and dispersed preparations, we determined the mechanisms of pentobarbital actions on ionic currents and inhibitory postsynaptic currents (IPSCs), mediated by aminobutyric acid (GABA). We investigated pentobarbital effects on intrinsic currents using hyperpolarizing voltage commands from rest and tetrodotoxin blockade of action potentials. At concentrations near 8 microM, pentobarbital increased input conductance and induced net outward current, I(PB), at potentials near action potential threshold. The reversal potential of I(PB) was -75 mV, implicating K+ and other ions. Cs+ (3 mM) which inhibits both K+ currents and inward rectifier (Ih), completely blocked IPB, whereas the selective Ih blocker, ZD-7288 (25 microM), or Ba2+ (2 mM) which suppresses only K+ currents, reduced IPB. Pentobarbital inhibited the Ih, consistent with a ZD-7288-induced shift in reversal potential for IPB toward K+ equilibrium potential. Pentobarbital increased the inward K+ rectifier, IKir, and leak current, Ileak. We determined the susceptibility of IPSCs, evoked by reticular stimulation, to antagonism by bicuculline, picrotoxinin and 2-hydroxysaclofen and identified their receptor subclass components. At EC50 = 53 microM, pentobarbital increased the duration of IPSCs. The prolonged IPSC duration during pentobarbital was attributable to enhanced open probability of GABAA channels, because combined with GABA, pentobarbital application increased mean channel open time without affecting channel conductance. At concentrations up to 100 microM, pentobarbital did not directly activate GABAA receptors. The concentration-response relationships for pentobarbital effects on the intrinsic currents and IPSCs overlapped, implying multiple sites of action and possible redundancy in anesthetic mechanisms. This is the first study to show that an i.v. anesthetic modulates the intrinsic currents, Ih, IKir, and Ileak, as well as IPSC time course in the same neurons. These effects likely underlie inhibition in thalamocortical neurons during pentobarbital anesthesia.