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Abstract
Gonadotropin-releasing hormone-synthesizing neurons represent the final common pathway
in the hypothalamic regulation of reproduction and their secretory activity is influenced
by a variety of neurotransmitters and neuromodulators acting centrally in synaptic
afferents to gonadotropin-releasing hormone neurons. The present study examined the
anatomical relationship of cholinergic neuronal pathways and gonadotropin-releasing
hormone neurons of the preoptic area. The immunocytochemical detection of choline
acetyltransferase or vesicular acetylcholine transporter revealed a fine network of
cholinergic fibers in this region. At the light microscopic level, the cholinergic
axons formed appositions to the gonadotropin-releasing hormone immunoreactive cell
bodies and dendrites. Results of electron microscopic studies confirmed the absence
of glial interpositions in many of these neuronal contacts. Classical cholinergic
synapses, which belonged to the asymmetric category, were only observed rarely on
gonadotropin-releasing hormone neurons. The lack of synaptic density in most contacts
corroborates previous observations on the cholinergic system elsewhere in the brain.
Further, it suggests a dominantly non-synaptic route also in this cholinergic neuronal
communication. This study provides direct neuromorphological evidence for the involvement
of the cholinergic system in the afferent neuronal regulation of gonadotropin-releasing
hormone neurons. The sources of cholinergic afferents and the receptorial mechanisms
underlying this interaction will require further clarification.