Distribution of Orexigenic Orexin‐A, Anorexigenic CRF and MCH at the Levels of Preoptic Area and Hypothalamus in the Conger Eel Brain

The somal distributions of orexigenic orexin-A, anorexigenic corticotropin-releasing factor (CRF) and melanin-concentrating hormone (MCH) were immunohistochemically investigated in the whitespotted conger eel (Conger myriaster) brain, with particular focus on the preoptic nuclei and hypothalamus. Immunoreactive somata of these three neuroactive peptides were categorised into large (around 30 μm in diameter) and small (around 10 μm in diameter) types. Large-type orexin-A-, CRF- and MCH-immunoreactive (ir) somata were present in the magnocellular preoptic nucleus. Large-type MCH-ir somata were also seen in the lateral tuberal nucleus (TL). Fibres of these large-type somata may penetrate the neurointermediate lobe of the pituitary. In contrast, the small-type immunoreactive somata of these three neuroactive peptides showed differential distributions in the hypothalamus. Small-type orexin-A-ir somata were mainly seen in the paraventricular organ. CRF-ir somata were seen in the posterior parvocellular preoptic nucleus, around the dorsal and ventral areas of the anterior lateral recesses, and around the dorsal area of the posterior lateral recesses. MCH-ir somata were seen in the TL and around the medial region of the posterior lateral recesses. Based on the hypothalamic differential localisations of these peptides, of which association with feeding behaviour is suggested, the presence of intra-hypothalamic functional localisation related to feeding is likely.