Spinal and trigeminal projections to the parabrachial nucleus in the rat: electron-microscopic evidence of a spino-ponto-amygdalian somatosensory pathway.

The fine structure of spinal and trigeminal projections to the parabrachial area (PB) of the rat was studied using either the anterograde transport of a lectin-peroxidase conjugate or the degeneration technique. Two morphologically different types of terminals were observed. Most labeled terminals contained round vesicles (R type) and formed asymmetrical synapses, usually with large dendrites. Others contained pleomorphic vesicles (P type) and usually made symmetrical contacts with large or medium-size dendrites. A double-labeling strategy was used, combining the retrograde labeling of PB neurons with lectin-peroxidase conjugate from the amygdala and the identification of degenerating terminals after lesions of spinal or trigeminal pathways. These experiments demonstrated that spinal and trigeminal terminals contact PB neurons that project to the central nucleus of the amygdala. The role of this spino(trigemino)-ponto-amygdalian pathway is discussed in relation to some aspects of pain.