Midbrain periaqueductal gray neurons with substance P- or enkephalin-like immunoreactivity send projection fibers to the nucleus accumbens in the rat.

Employing a combination of retrograde tracing and immunocytochemistry, midbrain periaqueductal gray (PAG) neurons with substance P- or leucine-enkephalin-like immunoreactivity in the rat were found to send projection fibers to the nucleus accumbens bilaterally with an ipsilateral dominance. These neurons were most frequently seen in the ventrolateral and ventral parts of the medial PAG subdivisions at all rostrocaudal levels of PAG.