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Abstract
Several lines of evidence support a role for pituitary adenylate cyclase-activating
polypeptide (PACAP) in the regulation of energy balance. In the present study, we
have used fluorescent in situ hybridization and immunohistochemistry to investigate
in detail the cellular localization and chemical content of PACAP mRNA- and peptide-containing
neuronal cell bodies in the mediobasal hypothalamus of the rat. PACAP mRNA-containing
cell bodies were demonstrated in high numbers in the ventromedial hypothalamic nucleus
(VMH) and in lower numbers in the arcuate nucleus (Arc). In colchicine-treated rats,
PACAP immunoreactivity was demonstrated in many cell bodies of the VMH and several
cell bodies of the ARC. Double-labeling revealed that PACAP immunoreactivity was present
in approximately 20% of pro-opiomelanocortin (POMC) neurons in the ventrolateral Arc
as shown by presence of alpha-melanocyte-stimulating hormone (alpha-MSH), but not
in agouti-related peptide (AgRP)-containing neurons in the ventromedial aspect of
the Arc. PACAP immunoreactivity was also colocalized with the vesicular acetylcholine
transporter (VAChT; a marker for cholinergic neurons) in Arc POMC neurons. Brainstem
POMC neurons in the commissural part of the solitary tract nucleus were devoid of
PACAP immunoreactivity. However, several VAChT-positive neurons in the dorsal motor
nucleus of the vagus nerve were also PACAP immunoreactive, whereas VAChT-positive
neurons of the motor nucleus of the hypoglossal nerve were PACAP-negative. The results
show presence of PACAP with alpha-MSH in a subpopulation of hypothalamic POMC neurons
and point further to the neurochemical heterogeneity of hypothalamic, but not brainstem,
POMC neurons.