Galanin-containing axons synapse on tyrosine hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus of the rat.

Prolactin (PRL) secretion by the anterior pituitary gland is dependent upon the tonic inhibitory influence of the tuberoinfundibular dopaminergic (TIDA) neuronal system. TIDA neurons, in turn, are regulated by various afferent neuronal systems. To support the concept that the recently-discovered neuropeptide, galanin (GAL), is one of the neurotransmitter/neuromodulator substances which might synaptically regulate the function of the TIDA system, immunocytochemical double-labeling studies were carried out in the hypothalamic arcuate nucleus (AN) of the male rat. The analysis of light microscopic preparations revealed the overlapping of GALergic and dopaminergic (detected by tyrosine hydroxylase immunoreactivity) neuronal elements in both the dorsomedial and ventrolateral parts of the AN. TH-containing perikarya and dendrites were contacted by varicose GAL-IR axons in these regions. The electron microscopic studies of ultrathin sections demonstrated axosomatic and axodendritic synapses between GALergic axons and TH-IR neurons. These findings support the view that GAL may modulate PRL release, acting as a neurotransmitter/neuromodulator in synaptic afferents to the TIDA system.