The Reciprocal Connections Of Endomorphin 1-and Endomorphin 2-containing Neurons Between The Hypothalamus And Nucleus Tractus Solitarii In The Rat

Endomorphin 1 (EM1) and endomorphin 2 (EM2) have been identified to be endogenous ligands with high affinity and selectivity for the μ-opioid receptors (MOR). In the central nervous system, the EM-like immunoreactive (EM-IR) cell bodies are present both in the hypothalamus and the nucleus of the solitary tract (NTS); there are also many EM-IR fibers in these two regions. On the other hand, the hypothalamus is the center for controlling many homeostatic processes and the NTS is the main target of the primary sensory information termination for visceral and taste inputs in the brainstem. To investigate whether the EM1- and EM2-IR neurons in the hypothalamus and NTS projected reciprocally to the two regions, the endomorphin 1- and endomorphin 2-immunofluorescence histochemistry combined with fluorescent retrograde labeling method was used. After injection of FG into the NTS, FG retrogradely labeled neurons were present bilaterally with a strong ipsilateral predominance in the hypothalamic regions, such as medial and lateral parvicellular portion of the paraventricular nucleus, posterior...