Evidence for the involvement of G-proteins in the generation of the slow poststimulus afterdepolarisation (sADP) induced by muscarinic receptor activation in rat olfactory cortical neurones in vitro

Abstract

The involvement of G-proteins in generating the slow poststimulus afterdepolarising potential (sADP) induced by muscarinic receptor activation in immature (P10–20) rat olfactory cortical brain slice neurones was investigated under whole-cell patch clamp, using GTP-γ-S (G-protein activator) or GDP-β-S (G-protein blocker)-filled electrodes. In control experiments using K methylsulphate electrodes, cell resting potential ( V m) and spike firing properties were unaffected over 10–15 min recording, although input resistance ( R N) was slightly increased (∼14%). Oxotremorine-M (OXO-M; 10 μM) produced a reversible slow depolarisation, an increase in R N (∼90%) and induction of a slow poststimulus inward tail current ( I ADP) (measured under voltage clamp at −60 mV) that was sustained during drug exposure (up to 15 min); the amplitude of slow inward rectifier ( I h) currents activated from −50 mV were also apparently increased. By contrast, in GTP-γ-S-loaded cells, R N was consistently decreased (∼22%) and spike firing threshold ( V th) was raised (∼5 mV) after 10 min recording. In ∼60% of loaded cells, a persistent muscarinic slow inward current and I ADP were induced by OXO-M; I h relaxation amplitude was also significantly decreased. The effects of GTP-γ-S on R N, V th and I h were partly counteracted by adding Ba 2+ (100 μM) to the bathing medium or mimicked by adding baclofen (GABA B receptor agonist; 100 μM) to normally-recorded cells. Intracellular GDP-β-S (up to 30 min) had no effect on cell membrane properties or I h, but irreversibly blocked the muscarinic slow inward current and I ADP induced by OXO-M. We conclude that both muscarinic responses require G-protein-linked transduction mechanisms for their generation.