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