Analysis of the functional role of the hyperpolarization-activated cation cunert within the centaral nervous system.

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This study elucidated the membrane properties, morphology and chemosensitivities of area postrema neurons using a patch-clamp technique in rat brain slices. Results are summarized as below.1)The activation of the hyperpolarization-activated cation current (I_h) was identified in approximately 62% of area postrema neurons tested. Extracellular Cs^+ ions (2 mM) and ZD7288 (100 mM), a potent selective I_h channel antagonist, blocked I_h and induced a membrane potential hyperpolarization, suggesting the sustained activation of I_h near the resting potential and a contribution from I_h to membrane potential maintenance at more depolarized levels. ZD7288 decreased the spontaneous discharge rate by prolonging the slow depolarization between two spikes, with minimal effect on the amplitude of the afterhyperpolarization or action potential waveforms. I_h stabilized the latency of rebound action potentials. I_h was weakly activated by external 8-bromoadenosine 3,5, cyclic monophosphate (1 mM) or … More forskolin (50-100 mM), indicating that the I_h channel subtypes in area postrema cells could be modulated by intracellular cAMP. Our findings indicate that I_h contributes to the subthreshold membrane and firing properties of rat area postrema neurons and may regulate their resting membrane potential and firing patterns.2)mEPSCs and evoked EPSCs were completely blocked in media containing the non-NMDA ionotropic receptor antagonist, CNQX (10 μM), indicating that EPSC were glutamate events.3)Excitatory responses to the bath application of nicotine (50 μM) were found in approximately 78% (35/45) of all cells tested. Responsive cells included both the cells that display the hyperpolarization-activated cation current (Ih) and cells that do not display Ih. An inhibitory effect of nicotine was never seen. We conclude that nicotinic receptors in the rat area postrema can excite cells via (1)a direct post- and/or extrasynaptic mechanism ; and (2)an indirect enhancement of glutamate release.4)The bath application of serotonin (5-HT, 50 μM) or phenylbiguanide (PBA, 50 μM, a potent 5-HT_3 receptor agonist) increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) or miniature EPSCs (mEPSCs) in 35 of 83 neurons (42%). These increases occurred in all electrophysiological cell classes. No cells exhibited a decrease in EPSC frequency. These results suggest that glutamate release is increased in the area postrema by presynaptic 5-HT_3 receptor activation. Further, we present evidence that 5-HT_3 receptor activation may be able to directly release glutamate from terminals, by-passing a requirement for voltage-dependent calcium entry into terminals. Such a mechanism may contribute to the chemosensitive function of area postrema neurons.5)Although propofol suppressed Ih of area postrema neurons in a dose-dependent manner that was similar to what we observed for the hippocampal CA1 neurons, the IC_<50>, for I_h in area postrema neurons (38 mM) was more than six times less than that found for hippocampal CA1 neurons (235 mM). We conclude that rat area postrema neurons are exquisitely sensitive to propofol. Given that reductions of Ih are associated with decreased excitability in neurons, we believe that the known antiemetic effects of propofol anesthesia are at least partly a result of a direct action on area postrema neurons to lower their excitability. Less