Brainstem afferents to the magnocellular basal forebrain studied by axonal transport, immunohistochemistry, and electrophysiology in the rat

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Abstract
Brainstem afferents to the magnocellular basal forebrain were studied by using tract tracing, immunohistochemistry and extracellular recordings in the rat. WGA‐HRP injections into the horizontal limb of the diagonal band (HDB) and the magnocellular preoptic area (MgPA) retrogradely labelled many neurons in the pedunculopontine and laterodorsal tegmental nuclei, dorsal raphe nucleus, and ventral tegmental area. Areas with moderate numbers of retrogradely labelled neurons included the median raphe nucleus, and area lateral to the medial longitudinal fasciculus in the pons, the locus ceruleus, and the medial parabrachial nucleus. A few labelled neurons were seen in the substantia nigra pars compacta, mesencephalic and pontine reticular formation, a midline area in the pontine central gray, lateral parabrachial nucleus, raphe magnus, prepositus hypoglossal nucleus, nucleus of the solitary tract, and ventrolateral medulla. A similar but not identical distribution of labelled neurons was seen following WGA‐HRP injections into the nucleus basalis magnocellularis. The possible neurotransmitter content of some of these afferents to the HDB/MgPA was examined by combining retrograde Fluoro‐Gold labelling and immunofluorescence. In the mesopontine tegmentum, many retrogradely labelled neurons were immunoreactive for choline acetyltransferase. In the dorsal raphe nucleus, some retrogradely labelled neurons were positive for serotonin and some for tyrosine hydroxylase (TH); however, the majority of retrogradely labelled neurons in this region were not immunoreactive for either marker. The ventral tegmental area, substantia nigra pars compacta, and locus ceruleus contained retrogradely labelled neurons which were also immunoreactive for TH. Of the retrogradely labelled neurons occasionally observed in the nucleus of the solitary tract, prepositus hypoglossal nucleus, and ventrolateral medulla, some were immunoreactive for either TH or phenylethanolamine‐N‐methyltransferase. To characterize functionally some of these brainstem afferents, extracellular recordings were made from antidromically identified cortically projecting neurons, mostly located in the HDB and MgPA. In agreement with most previous studies, about half (48%) of these neurons were spontaneously active. Electrical stimulation in the vicinity of the pedunculopontine tegmerital and dorsal raphe nuclei elicited either excitatory or inhibitory responses in 21% (13/62) of the cortically projecting neurons. These findings indicate that neurons in the magnocellular basal fore‐brain receive afferents from widely distributed brainstem neurons that contain acetylcholine, serotonin, dopamine, noradrenaline, adrenaline, and some unknown neurotransmitters. Through their actions on magnocellular basal forebrain neurons, these brainstem neurons may indirectly regulate the activity of neurons throughout the cerebral cortex.