ANTIHISTAMINE-MEDIATED INHIBITION OF GIRK 1/2 CHANNELS

Antihistamines are widely used to treat allergic reactions. In particular, first-generation antihistamines are capable of crossing the blood-brain barrier, and are linked to severe neurological side effects, including seizures and worsening of epilepsy, at high doses. While second-generation antihistamines inhibit G protein-gated inwardly rectifying K⁺ (GIRK) channels, it remains unknown whether first-generation antihistamines, can also interact with GIRK channels, which are involved in regulation of neuronal excitability. To address this question, we investigated whether first generation antihistamines, including the commonly used diphenhydramine, inhibit GIRK1/2 channels using whole-cell patch-clamp recordings. Additionally, we evaluated structurally related derivatives of diphenhydramine to identify critical chemical determinants for GIRK inhibition. To elucidate the molecular basis of this interaction, we performed structural modeling and docking to identify candidate binding residues within the GIRK1/2 channels, followed by site-directed mutagenesis and functional characterization. Our results demonstrate that first-generation antihistamines inhibit GIRK1/2 channels at concentrations relevant to overdose. Structure-activity analysis enabled us to identify critical chemical properties required for channel inhibition. Mutagenesis studies revealed a ring of phenylalanine residues at the gate of GIRK1/2 channels to be essential for drug-channel interaction. These findings provide evidence for a potential mechanism underlying the neurological side effects observed following exposure to high doses of first-generation antihistamines.