Identification of sensory relay nuclei in brainstem that projecting to the dorsal raphe nucleus

Abstract

Objective To explore the distribution and contact pattern of sensory relay nuclei in the brainstem that projecting to the dorsal raphe nucleus (DRN) by using neural circuit tracing technique, so as to investigate the morphological basis of sensory information regulating wakefulness/sleep. Methods Retrograde transporting tracing fluorescent particles RetroBeads were injected into the DRN of adult male C57BL/6 mice (n=3), and the expression of RetroBeads was observed in the sensory relay nuclei of the brainstem. Furthermore, AAV-CaMKIIα-EGFP and RetroBeads were injected into the parabrachial nucleus (PBN) of another 3 mice to observe the distribution of the fiber terminals in DRN and the sensory inputs from the brainstem. Results By the injection of RetroBeads into the DRN, we identified the sensory relay nuclei projecting to the DRN. These nuclei included the superior olivary nucleus, rostral solitary tract nucleus, caudal solitary tract nucleus, parvicellular part of medial vestibular nucleus and magnocellular part of medial vestibular nucleus, accounting for 3.11%, 2.95%, 10.40%, 10.45% and 1.75% of the total brainstem inputs of the DRN, respectively. In addition, there were a large number of RetroBeads labeled neurons in the PBN, accounting for 71.33% of the total input of DRN. Meanwhile, AAV-CaMKIIα-EGFP mediated anterograde tracing showed that the PBN neurons sent dense fiber projections to the DRN. PBN received the inputs form the trigeminal sensory nucleus and spinal tract nucleus, which relay pain, temperature and touch sensation of the head and face. They accounted for 6.14% and 49.86% of the total brainstem input of PBN. The rostral solitary tract nucleus, caudal solitary tract nucleus and parvicellular part of medial vestibular nucleus also projected to the PBN, accounting for 11.67%, 20.53% and 8.94% of the total brainstem input of PBN, respectively. In addition, a few labeled neurons were found in the inferior olivary nucleus, accounting for 2.14% of the total brainstem input of PBN. Conclusion The DRN receives the direct inputs from the superior olivary nucleus, solitary tract nucleus, vestibular nucleus group and PBN. At the same time, the DRN also indirectly receives the inputs from the solitary tract nucleus, inferior olivary nucleus, vestibular nucleus group, trigeminal sensory nucleus and trigeminal spinal nucleus through the PBN.

Keywords

• dorsal raphe nucleus
• parabrachial nucleus
• sensory relay nuclei
• neural circuit tracing