Differential distribution of melanin-concentrating hormone (MCH)- and hypocretin (Hcrt)-immunoreactive neurons projecting to the mesopontine cholinergic complex in the rat.

Hypocretin (Hcrt or orexin) and melanin-concentrating hormone (MCH) containing neurons are located in the hypothalamus and are implicated in the regulation of feeding behavior, energy homeostasis, and sleep-wake cycle. MCH and Hcrt are not co-localized within the same neuron, but these neurons project widely throughout the brain, especially to brain regions regulating arousal. Recent data indicate that HCRT and MCH neurons located medially with respect to the fornix have a differential projection pattern compared to those located lateral to the fornix. To further elucidate the projection of these neurons in the present study we use retrograde tracing methods combined with double immunofluorescence to determine the differential distribution of Hcrt- and MCH-immunoreactive neurons projecting to the pedunculopontine tegmental (PPTg) or laterodorsal tegmental (LDTg) nuclei. In rats where the retrograde tracer was confined to the PPTg/LDTg we found that there were more MCH neurons projecting to these targets compared to HCRT neurons (P<0.01). When the retrograde tracer was confined to the PPTg, there were more retrogradely labeled MCH neurons lateral to the fornix compared to MCH neurons in the medial LH subdivision (P<0.05). On the average, only about 4.5% of MCH neurons versus 6.1% of HCRT neurons project to PPTg/LDTg. Thus, very few of the MCH or HCRT neurons project to these arousal populations. Although there were significantly more MCH neurons projecting to the mesopontine cholinergic arousal zone compared to the HCRT neurons, the HCRT neurons also exert an indirect influence via the tuberomammillary nucleus. Based on the present and previous (Hong and Lee, 2011) observations, we suggest that both MCH and HCRT neurons exert a potent influence on the PPTg/LDTg, which might play an important role in arousal.