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

Abstract

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.

Melanin‐concentrating hormone peptidergic system: Comparative morphology between muroid species

“…Looking at the available data, it becomes apparent that the MCH system is sexually dimorphic, highly plastic, and strongly linked to sexual physiology, both from functional (Messina, Boersma, Overton, & Eckel, ; Murray, Adan, et al, ; Murray et al, ; Santollo & Eckel, ; Wu, Dumalska, Morozova, van den Pol, & Alreja, ) and structural viewpoints (Alvisi, Diniz, Da‐Silva, Bittencourt, & Felicio, ; Chiocchio, Gallardo, Louzan, Gutnisky, & Tramezzani, ; Costa et al, ; Ferreira, Duarte, Diniz, & Bittencourt, ; Gallardo, Chiocchio, & Tramezzani, ; Knollema et al, ; Murray, Baker, Levy, & Wilson, ; Santollo & Eckel, ). Few works, however, have thoroughly examined the dimorphisms between males and females.…”

Regulation and neurochemical identity of melanin‐concentrating hormone neurones in the preoptic area of lactating mice

“…Another study showed that MCH administration into the mPOA of lactating rats suppresses maternal behaviour, including pup retrieval and nest building . MCH may also regulate hormone secretion during lactation, particularly oxytocin . In this sense, mPOA neurones contribute to the appearance of MCH fibres in the internal layer of the median eminence and posterior pituitary gland of lactating rats …”

“…17 MCH may also regulate hormone secretion during lactation, particularly oxytocin. 28 In this sense, mPOA neurones contribute to the appearance of MCH fibres in the internal layer of the median eminence and posterior pituitary gland of lactating rats. 28 One still unknown aspect is how MCH expression is induced in the mPOA during lactation.…”

“…28 In this sense, mPOA neurones contribute to the appearance of MCH fibres in the internal layer of the median eminence and posterior pituitary gland of lactating rats. 28 One still unknown aspect is how MCH expression is induced in the mPOA during lactation. Rondini et al 8 showed that mPOA MCH neurones are innervated by neuropeptides that control energy homeostasis, including agouti-related protein, α-melanocyte-stimulating hormone and CART.…”

The distribution of neuronal primary cilia immunoreactive to melanin-concentrating hormone receptor 1 (MCHR1) in the murine prosencephalon

“…A modulation of sexual function may also be achieved through estrogen receptor neurons of the medial preoptic area and periventricular hypothalamus. The presence of MCHR1 in the medial preoptic area may also be important for the display of maternal behavior (Alachkar et al 2016;Benedetto et al 2014), if the lactation-generated MCH neurons of the medial preoptic area (Knollema et al 1992;Rondini et al 2010;Alvisi et al 2016;Costa et al 2019;Ferreira et al 2017b) release MCH in a paracrine mode. Finally, another major function that may be played by MCH through VT is olfactory integration.…”