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.
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-gamma-S (G-protein activator) or GDP-beta-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 ( approximately 14%). Oxotremorine-M (OXO-M; 10 microM) produced a reversible slow depolarisation, an increase in R(N) ( approximately 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-gamma-S-loaded cells, R(N) was consistently decreased ( approximately 22%) and spike firing threshold (V(th)) was raised ( approximately 5 mV) after 10 min recording. In approximately 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-gamma-S on R(N), V(th) and I(h) were partly counteracted by adding Ba(2+) (100 microM) to the bathing medium or mimicked by adding baclofen (GABA(B) receptor agonist; 100 microM) to normally-recorded cells. Intracellular GDP-beta-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.
Postlethwaite
Michael
M
Department of Pharmacology, The School of Pharmacy, 29/39 Brunswick Square, London WC1N 1AX, UK.
Constanti
Andrew
A
D000818:Animals; D000831:Animals, Newborn; D001418:Baclofen / Q000494:pharmacology; D001464:Barium / Q000494:pharmacology; D004347:Drug Interactions; D018755:GABA Agonists / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000031:analogs & derivatives* / Q000494:pharmacology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D018721:Muscarinic Agonists / Q000494:pharmacology*; D009431:Neural Conduction / Q000187:drug effects; D009474:Neurons / Q000187:drug effects* / Q000502:physiology; D009833:Olfactory Pathways / Q000166:cytology* / Q000502:physiology; D010095:Oxotremorine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D018408:Patch-Clamp Techniques / Q000379:methods; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors
D018755:GABA Agonists; D018721:Muscarinic Agonists; C034883:guanosine 5'-O-(2-thiotriphosphate); D001464:Barium; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D010095:Oxotremorine; C042743:oxotremorine M; D019204:GTP-Binding Proteins; D001418:Baclofen