MORPHOLOGICAL AND ELECTROPHYSIOLOGICAL CHARACTERISTICS OF NONCHOLINERGIC BASAL FOREBRAIN NEURONS

Cholinergic neurons in the basal forebrain are? the focus of considera ble interest because they are severely affected in Alzheimer's disease . However, both cholinergic and noncholinergic neurons are intermingle d in this region. The goal of the present study st-as to characterize the morphology and in vivo electrophysiology of noncholinergic basal f orebrain neurons. Neurons in the ventral pallidum and substantia innom inata were recorded extracellularly, labeled juxtacellularly with bioc ytin and characterized for the presence of choline acetyltransferase i mmunoreactivity. Two types of ventral pallidal cells were observed. Ty pe I ventral pallidal neurons had axons that rarely branched near the cell body and tended to have smaller somata and lower spontaneous firi ng rates than did type II ventral pallidal neurons, which displayed ex tensive local axonal arborizations. Subtypes of substantia innominata neurons could not be distinguished based on axonal morphology. These n oncholineregic neurons exhibited local axon arborizations along a cont inuum that varied from no local collaterals to quite extensive arbors. Substantia innominata neurons had lower spontaneous filing rates, mor e variable interspike intervals, and different spontaneous tiring patt erns than did type II ventral pallidal neurons and could be antidromic ally activated from cortex or substantia nigra, indicating that they w ere projection neurons. Ventral pallidal neurons resemble, both morpho logically and electrophysiologically, previously described neurons in the globus pallidus, whereas the substantia innominata neurons bore si milarities to isodendritic neurons of the reticular formation. These r esults demonstrate the heterogeneous nature of noncholinergic neurons in the basal forebrain. (C) 1998 Wiley-Liss, Inc.