Nociceptive projection from tooth pulp to the lateral hypothalamus in rats

The response of the rat lateral hypothalamic (LHA) neurons to tooth pulp electrical stimulation and the sensory projection pathway from the incisor pulp to the LHA were studied by electrophysiology and histology. 1) LHA neurons that responded to contralateral lower incisor pulp stimulation were found in the lateral part of the LHA. These neurons also responded to intensive tail pinch, but not to innocuous stimuli nor to applied glucose. 2) Histological study after injection of WGA-HRP into the lateral part of the LHA revealed many retrogradely labeled neurons in the ventral part of the periventricular gray (PVG) in the mesencephalon. 3) The PVG neurons responded antidromically to LHA stimulation and to contralateral lower incisor pulp stimulation. 4) After injection of WGA-HRP into the ventral portion of the PVG, many labeled cells were found in the contralateral subnucleus caudalis in the spinal tract of the trigeminal nucleus (NTST) where termination of the pulpal afferent was previously reported. It is thus suggested that the PVG is the most likely site of transmission relay of nociceptive inputs from incisors to the LHA.