Reciprocal connections between the periaqueductal gray matter and other somatosensory regions of the cat midbrain: a possible mechanism of pain inhibition.

Lectin-conjugated horseradish peroxidase was injected or implanted in crystalline form into various parts of the periaqueductal gray matter (PAG) in the cat. After varying survival periods, the animals were fixed and the mesencephalon was sectioned and incubated for HRP histochemistry. Outside PAG, labelled cells and terminal labelling were observed in the cuneiform, parabrachial and intercollicular nuclei, in the deep and intermediate gray layers of the superior colliculus, in the anterior and posterior pretectal nuclei and in the nucleus of Darkschewitsch. This study has shown that the region of PAG that is known to receive heavy ascending somatosensory input from the spinal cord and to be part of descending pain-inhibiting systems, also has reciprocal connections with other somatosensory areas of the midbrain. The results are discussed in relation to nociception and nociceptive inhibiting mechanisms.