Effects of baclofen and lorazepam on interhemispheric inhibition in humans

Transcallosal projections within the corpus callosum allow for the transfer of information between bilateral primary motor cortices (M1). Interhemispheric inhibition (IHI) is one approach to probe transcallosal communication using transcranial magnetic stimulation (TMS) whereby the motor evoked potential (MEP) elicited by TMS is reduced in amplitude when preceded by a conditioning TMS pulse delivered to the opposite M1. In a double-blinded, placebo-controlled study, 24 participants received 2.5 mg of lorazepam (GABA_A agonist), 50 mg of baclofen (GABA_B agonist) and a placebo in separate sessions. Short-interval interhemispheric inhibition (SIHI) and long-interval interhemispheric inhibition (LIHI) recorded from the first dorsal interosseous muscle were quantified before and at peak plasma concentration following drug ingestion. Baclofen significantly reduced SIHI and LIHI while lorazepam had no effect. These findings advance our understanding of the pharmacological basis of transcallosal inhibition in humans.