GABA(B) presynaptic inhibition has an in vivo time constant sufficiently rapid to allow modulation at theta frequency.

Cyclical activity of GABAergic interneurons during theta rhythm could mediate phasic changes in strength of glutamatergic synaptic transmission in the hippocampal formation if presynaptic inhibition from activation of GABA(B) receptors is sufficiently rapid to change within a theta cycle. The experiments described here analyzed the time course of GABA(B) modulation using a heterosynaptic depression paradigm in anesthetized rats at physiological temperatures. Heterosynaptic depression of the slope of evoked potentials decayed with a time constant that would allow significant changes in transmission across different phases of the theta cycle. This heterosynaptic depression was significantly reduced by local infusion of the GABA(B) receptor antagonist CGP55845A.