Projection from median raphe to mesencephalic trigeminal nucleus

Objectives: It is not well understood whether the mesencephalic trigeminal nucleus (mesV) receives projections from the various brainstem structures. We here report a projection from the median and paramedian raphe nuclei to the mesV in the rat.

Methods: A retrograde tracer Fast Blue (FB) was first injected into the mesV to identify the distribution of neurons projecting to the mesV. An anterograde tracer biotinylated dextran amine (BDA) was then injected into the median and paramedian raphe nuclei to reveal synaptic contact on the mesV neurons. The tracers were stereotaxically injected through a glass micropipette (internal tip diameter:25-35µm) using a pressure system (PicospritzerII).

Results: After injections of FB into the mesV, retrogradely labeled neurons were distributed in the raphe nuclei and the adjacent reticular formation. Following restricted injections of BDA into the median and paramedian raphe nuclei at the lower end of the midbrain, anterogradely labeled axon terminals were found on the cell bodies of the mesV.

Conclusion: The results show the existence of direct projection from the median raphe nuclei to the mesV. Since the mesV transmits proprioceptive sensory signals from the muscle spindles and periodontal ligament, we suggest that the serotonergic axons from median raphe nuclei modulate the proprioceptive signals at the level of primary afferent cell bodies in the mesV.

Supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan(Open Research Center subsidy (H18) to S.S. and Grant-in-Aid for Scientific Research(C)18591912 to H.N.)