Characterisation and origins of melanin-concentrating hormone immunoreactive fibres of the posterior lobe of the pituitary and median eminence during lactation in the Long-Evans rat.

Although the melanin-concentrating hormone (MCH) and its coding mRNA are predominantly found in the tuberal hypothalamus, there is detectable synthesis of MCH in the preoptic hypothalamus exclusively in lactating dams, suggesting a participation of MCH in the alterations that take place after parturition. Also implicated in the dam physiology is oxytocin, a neurohormone released from the posterior pituitary that is necessary for milk ejection. Because the projection fields from oxytocin-immunoreactive (-IR) neurones and the mediobasal preoptic hypothalamus overlap and MCH-IR neurones are found in proximity to oxytocin neurones, we investigated the spatial relationship between MCH and oxytocin fibres. Accordingly, we employed multiple immunohistochemistry labelling for MCH and oxytocin for light and electron microscopy techniques, in addition to i.v. tracer injection combined with in situ hybridisation to identify MCH neurones that project to neurosecretory areas. As described for other strains, lactating Long-Evans dams also display immunoreactivity for MCH in the preoptic hypothalamus on days 12 and 19 of lactation. The appearance of these neurones is contemporaneous with an increase in MCH-IR fibres in both the internal layer of the median eminence and the posterior pituitary. In both regions, MCH- and oxytocin-IR fibres were found in great proximity, although there was no evidence for synaptic interaction between these two populations at the ultrastructural level. The tracer injection revealed that only mediobasal preoptic MCH neurones project to the posterior pituitary, suggesting a neuroendocrine-modulatory role for this population. When taken together, the results obtained in the present study indicate that neuroplasticity events at the mediobasal preoptic hypothalamus that occur during late lactation may be part of a neuroendocrinology control loop involving both MCH and oxytocin.

Hypothalamic melanin-concentrating hormone projections to the septo-hippocampal complex in the rat.

Melanin-concentrating hormone (MCH) and neuropeptide glutamic acid-isoleucine (NEI) are expressed in neurons that are located mainly in the hypothalamus and project widely throughout the rat central nervous system. One of the main targets of melanin-concentrating hormone is the hippocampal formation, although the exact origin of the projections is unknown. By using injections of the retrograde tracer True Blue into the hippocampus, together with immunohistochemical analysis, we observed retrogradely labeled melanin-concentrating hormone-containing neurons in the lateral hypothalamic area, incerto-hypothalamic area, perifornical area, the periventricular nucleus of the hypothalamus, and in the internuclear area (between the dorsomedial and ventromedial nuclei of the hypothalamus), as well as a few retrogradely labeled and melanin-concentrating hormone-immunoreactive cells in the supramammillary nucleus. The afferents from the lateral hypothalamic area were confirmed using injection of the anterograde tracer biotinylated dextran amine, which enabled us to use histochemical analysis in order to visualize fibers and terminals in the hippocampal formation. In the medial septal nucleus, we found cholinergic neurons that are also putatively innervated by melanin-concentrating hormone immunoreactive fibers and project to the hippocampal formation. Finally, using two different protocols for immunoperoxidase, we were able to show GABAergic basket cells presumably innervated by melanin-concentrating hormone-immunoreactive fibers in the hippocampal formation. On the basis of the data collected herein, we hypothesize that the MCH/NEI projections from hypothalamic nuclei participate in spatial memory and learning through direct and indirect pathways. These pathways would enable the animal to organize its exploratory behavior during foraging.