Membrane properties of cell types within guinea pig basal forebrain nuclei in vitro.

1. Neurons in the nucleus of the diagonal band of Broca (nDBB) and ventral portion of the medial septum (MS) were studied using intracellular recording and single-electrode voltage clamp (SEVC) techniques in an in vitro brain slice preparation. Cell types could be operationally divided into three categories: cells with a slow postspike afterhyperpolarization (SAHP cell, 40%), neurons with a fast AHP (FAHP cells, 53%), and a third cell group recorded infrequently (7% of the cells) that fired in a burst pattern. Double-labeling techniques have shown that SAHP cells stain positively for acetylcholinesterase (AChE) and are presumably cholinergic (22). The present study provides a more detailed analysis of the passive and active membrane properties of SAHP and FAHP types within these forebrain nuclei. 2. SAHP cells were characterized by a postspike afterhyperpolarization (AHP) with an amplitude of 10-20 mV and duration of approximately 600 ms at -65 mV. In the voltage range of -60--70 mV, the AHP decayed as a single exponential function with a time constant of 170 +/- 53 ms (n = 10). However, many neurons at these membrane potentials exhibited an AHP decay that was a multiple exponential function lasting for seconds. The null potential of the SAHP was approximately -90 mV and shifted by 25 mV in 9 mM KCl, a value closely predicted for a potassium (K+) conductance. The SAHP was reversibly blocked by cadmium (Cd2+), suggesting the SAHP was mediated by a calcium (Ca2+)-activated K+ conductance. 3. FAHP cells displayed afterhyperpolarizations of smaller amplitude (5-10 mV) and duration (5-50 ms) that reversed at approximately -85 mV. Elevating extracellular K+ concentration [Ko] to 6 mM shifted the reversal 13 mV more positive. Cd2+ also reduced the AHP in these cells suggesting a second faster Ca2+-activated K+ conductance may be present. 4. Both SAHP and FAHP cells had similar input resistances and resting membrane potentials but markedly different action-potential characteristics. SAHP cells had a spike duration of 1.4 ms and a prominent shoulder on the falling phase of the SAHP cell action potentials that was reduced by Cd2+. In contrast, FAHP cells had an average spike duration of 0.63 ms that was unaffected by Cd2+. 5. The passive electrical cable properties of both cell types were characterized. Equivalent electrotonic length of the dendrites (L) and the dendritic-to-somatic conductance ratio (rho) were calculated for different cell groups. SAHP cells displayed average L values of 0.61, and the average rho was 2.13. Similar values of 0.69 and 2.14 were calculated for L and rho, respectively, in FAHP cells.(ABSTRACT TRUNCATED AT 400 WORDS)