Projections of GABAergic and cholinergic basal forebrain and GABAergic preoptic‐anterior hypothalamic neurons to the posterior lateral hypothalamus of the rat

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Abstract
Within the basal forebrain, γ-aminobutyric acid (GABA)-synthesizing neurons are codistributed with acetylcholine-synthesizing neurons (Gritti et al. [1993] J. Comp. Neurol. 329:438–457), which constitute one of the major forebrain sources of subcortical afferents to the cerebral cortex. In the present study, descending projections of the GABAergic and cholinergic neurons were investigated to the lateral posterior hypothalamus (LHp) through which the medial forebrain bundle passes and where another major forebrain source of subcortical afferents is situated. Retrograde transport of cholera toxin b subunit (CT) from the LHp was combined with immunohistochemical staining for glutamic acid decarboxylase (GAD) and choline acetyl transferase (ChAT) using a sequential peroxidase-antiperoxidase (PAP) technique. A relatively large number of GAD+ neurons (estimated at ∼ 6,200), which represented > 15% of the total population of GAD+ cells in the basal forebrain (estimated at ∼ 39,000), were retrogradely labeled from the LHp. These cells were distributed through the basal forebrain cell groups, where ChAT+ cells are also located, including the medial septum and diagonal band nuclei, the magnocellular preoptic nucleus, and the substantia innominata, with few cells in the globus pallidus. In these same nuclei, a small number of ChAT+ cells were retrogradely labeled (estimated at ∼ 800), which represented only a small percentage ( 20%) of the total population (estimated at ∼ 30,000) from the LHp. The retrogradely labeled GAD+ neurons were distributed in continuity with those in the basal forebrain through the lateral preoptic area, medial preoptic area, bed nucleus of the stria terminalis, and anterior and dorsal hypothalamic areas. Of the large number of cells that project to the LHp in the basal forebrain and preoptic-anterior hypothalamic regions (estimated at ∼ 66,000), the GAD+ neurons represented a significant proportion (> 15%) and the ChAT+ neurons a very small proportion (∼ 2%). The relative magnitude of the GABAergic projection suggests that it may represent an important inhibitory influence of the descending efferent output from the basal forebrain and preoptic-anterior hypothalamic regions. This influence could suppress the activity of neurons in the posterior lateral hypothalamus that are involved in establishing the vegetative, somatomotor, and electrocortical components of the waking state and behaviors.