The Journal of Neurophysiology Vol. 85 No. 5 May 2001, pp. 1986-1997
Copyright ©2001 by the American Physiological Society

Medium Afterhyperpolarization and Firing Pattern Modulation in Interneurons of Stratum Radiatum in the CA3 Hippocampal Region

Nataa Savi,¹ Paola Pedarzani,² and Marina Sciancalepore¹

 ¹Neuroscience Programme and Istituto Nazionale Fisica della Materia Unit, International School for Advanced Studies (SISSA), 34014 Trieste, Italy; and  ²Max-Planck Institut für experimentelle Medizin, Department of Molecular Biology of Neuronal Signals, 37075 Gottingen, Germany

Savi, Nataa, Paola Pedarzani, and Marina Sciancalepore. Medium Afterhyperpolarization and Firing Pattern Modulation in Interneurons of Stratum Radiatum in the CA3 Hippocampal Region. J. Neurophysiol. 85: 1986-1997, 2001. Stratum (st.) radiatum interneurons represent a heterogeneous class of hippocampal cells with as yet poorly characterized physiological properties. Intracellular staining with biocytin, in situ hybridization, and patch-clamp recording have been combined to investigate the morphological and electrophysiological properties of these cells in the CA3 hippocampal region in young rats [postnatal days 10 to 21 (P10-21)]. Labeled cells presented a heterogeneous morphology with various soma shapes, often found multipolar, and dendritic arborizations confined to st. radiatum. The passive membrane properties of these st. radiatum interneurons showed instead no significant differences between P10 and P21. Low resting potential, high-input resistance, and short time constants characterized CA3 st. radiatum interneurons, which were silent at rest. Action potentials, elicited by brief current pulses, were lower and shorter than in pyramidal cells and followed by a Ca²⁺-dependent medium-duration afterhyperpolarizing potential (mAHP). Prolonged depolarizing current injection generated trains of action potentials that fired at constant frequency after a slight accommodation. The maximum steady-state firing rate was 31 ± 4 (SD) Hz. Hyperpolarizing current pulses revealed a prominent inward rectification characterized by a "sag," followed by a depolarizing rebound that triggered action potentials. Sag and anodal brake excitation were blocked by Cs⁺, suggesting that they were mediated by a hyperpolarization-activated cation conductance (I_h). In the presence of tetrodotoxin and tetraethylammonium, biphasic tail currents were elicited in voltage clamp after a depolarizing step inducing Ca²⁺ influx. Tail currents presented a fast Ca²⁺-activated and apamin-sensitive component (I_AHP) and were further reduced by carbachol. The presence of I_AHP was consistent with the high expression level of the apamin-sensitive SK2 subunit transcript in CA3 st. radiatum interneurons as detected by in situ hybridization. Different pharmacological agents were shown to affect the afterhyperpolarizing potential as well as the firing properties of st. radiatum interneurons. Exposure to Ca²⁺-free solutions mainly affected the late phase of repolarization and strongly reduced the mAHP. The mAHP was also attenuated by carbachol and by apamin, suggesting it to be partly mediated by I_AHP. Reduction of the mAHP increased the interneuron firing frequency. In conclusion, st. radiatum interneurons of CA3 hippocampal region represent a class of nonpyramidal cells with action potentials followed by an AHP of relatively short duration, partially generated by apamin and carbachol-sensitive conductances involved in the regulation of the cell firing rate.