The Journal of Neurophysiology Vol. 80 No. 4 October 1998, pp. 1653-1669
Copyright ©1998 The American Physiological Society

Morphological and Membrane Properties of Rat Magnocellular Basal Forebrain Neurons Maintained in Culture

J. A. Sim and T.G.J. Allen

Department of Pharmacology, University College London, London WC1E 6BT, United Kingdom

Sim, J. A. and T.G.J. Allen. Morphological and membrane properties of rat magnocellular basal forebrain neurons maintained in culture. J. Neurophysiol. 80: 1653-1669, 1998. Morphological and electrophysiological characteristics of magnocellular neurons from basal forebrain nuclei of postnatal rats (11-14 days old) were examined in dissociated cell culture. Neurons were maintained in culture for periods of 5-27 days, and 95% of magnocellular (>23 µm diam) neurons stained positive with acetylcholinesterase histochemistry. With the use of phase contrast microscopy, four morphological subtypes of magnocellular neurons could be distinguished according to the shape of their soma and pattern of dendritic branching. Corresponding passive and active membrane properties were investigated with the use of whole cell configuration of the patch-clamp technique. Neurons of all cell types displayed a prominent (6-39 mV; 6.7-50 ms duration) spike afterdepolarization (ADP), which in some cells reached firing threshold. The ADP was voltage dependent, increasing in amplitude and decreasing in duration with membrane hyperpolarization with an apparent reversal potential of 59 ± 2.3 (SE) mV. Elevating [Ca²⁺]_o (2.5-5.0 mM) or prolonging spike repolarization with 10 mM tetraethylammonium (TEA) or 1 mM 4-aminopyridine (4-AP), potentiated the ADP while it was inhibited by reducing [Ca²⁺]_o (2.5-1 mM) or superfusion with Cd²⁺ (100 µM). The ADP was selectively inhibited by amiloride (0.1-0.3 mM or Ni²⁺ 10 µM) but unaffected by nifedipine (3 µM), -conotoxin GVIA (100 nM) or -agatoxin IVA (200 nM), indicating that Ca²⁺ entry was through T-type Ca²⁺ channels. After inhibition of the ADP with amiloride (300 µM), depolarization to less than 65 mV revealed a spike afterhyperpolarization (AHP) with both fast and slow components that could be inhibited by 4-AP (1 mM) and Cd²⁺ (100 µM), respectively. In all cell types, current-voltage relationships exhibited inward rectification at hyperpolarized potentials E_K (approximately 90 mV). Application of Cs⁺ (0.1-1 mM) or Ba²⁺ (1-10 µM) selectively inhibited inward rectification but had no effect on resting potential or cell excitability. At higher concentrations, Ba²⁺ (>10 µM) also inhibited an outward current tonically active at resting potential (V_H 70 mV), which under current-clamp conditions resulted in small membrane depolarization (3-10 mV) and an increase in cell excitability. Depolarizing voltage commands from prepulse potential of 90 mV (V_H 70 mV) in the presence of tetrodotoxin (0.5 µM) and Cd²⁺ (100 µM) to potentials between 40 and +40 mV cause voltage activation of both transient A-type and sustained delayed rectifier-type outward currents, which could be selectively inhibited by 4-AP (0.3-3 mM) and TEA (1-3 mM), respectively. These results show that, although acetylcholinesterase-positive magnocellular basal forebrain neurons exhibit considerable morphological heterogeneity, they have very similar and characteristic electrophysiological properties.

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