Action of serotonin on the hyperpolarization-activated cation current (Ih) in rat CA1 hippocampal neurons.
We studied the effects of serotonin (5-HT) on hippocampal CA1 pyramidal neurons. In current-clamp mode, 5-HT induced a hyperpolarization and reduction of excitability due to the opening of inward rectifier K+ channels, followed by a late depolarization and partial restoration of excitability. These two components could be dissociated, as in the presence of BaCl2 to block K+ channels, 5-HT induced a depolarization accompanied by a reduction of membrane resistance, whereas in the presence of ZD 7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride], a selective blocker of the hyperpolarization-activated cation current (Ih), 5-HT only hyperpolarized neurons. We then studied the action of 5-HT on Ih in voltage-clamp conditions. 5-HT increased Ih at -90 mV by 29.1 +/- 2.9% and decreased the time constant of activation by 20.1 +/- 1.7% (n = 16), suggesting a shift in the voltage dependence of the current towards more positive potentials; however, the fully activated current measured at -140 mV also increased (by 14.1 +/- 1.7%, n = 14); this increase was blocked by ZD 7288, implying an effect of 5-HT on the maximal conductance of Ih. Both the shift of activation curve and the increase in maximal conductance were confirmed by data obtained with ramp protocols. Perfusion with the membrane-permeable analogue of cAMP, 8-bromoadenosine 3'5'-cyclic monophosphate (8-Br-cAMP), increased Ih both at -90 and -140 mV, although the changes induced were smaller than those due to 5-HT. Our data indicate that 5-HT modulates Ih by shifting its activation curve to more positive voltages and by increasing its maximal conductance, and that this action is likely to contribute to the 5-HT modulation of excitability of CA1 cells.
Gasparini
S
S
Università degli Studi di Milano, Dipartimento di Fisiologia e Biochimica General, Elettrofisiologia, Italy. gaspari@sissa.it
DiFrancesco
D
D
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D004594:Electrophysiology; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D007473:Ion Channels / Q000819:agonists / Q000187:drug effects / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology; D017966:Pyramidal Cells / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D015290:Second Messenger Systems / Q000187:drug effects / Q000502:physiology; D012701:Serotonin / Q000494:pharmacology*
D007473:Ion Channels; D015221:Potassium Channels; D015124:8-Bromo Cyclic Adenosine Monophosphate; D012701:Serotonin