Preprohypocretin (orexin) and prolactin-like immunoreactivity are coexpressed by neurons of the rat lateral hypothalamic area.

Recently, two new neuropeptides named hypocretins I and II (Hcrt) have been described in the rat lateral hypothalamus. The distribution of the neurons expressing these new peptides is similar to that of neurons described in the literature as prolactin (PRL) immunoreactive neurons and specifically labeled by an antiserum (AS) raised against ovine prolactin (oPRL). In this study, we report that every neuron labeled by immunohistochemistry or in situ hybridization for Hcrt is also labeled by the oPRL-AS, and conversely. Furthermore, the labeling of the oPRL-AS is inhibited when this AS is pre-incubated with the 104-109 fragment of the preproHcrt, thus indicating that the oPRL-AS is recognizing an epitope carried by this particular fragment of the preproHcrt. Our previous experimental work on these neurons showed that they may be involved in energy metabolism and water balance, which is in agreement with the current literature about Hcrt functions.