Paralemniscal reticular formation: response of cells to a noxious stimulus.

Extracellular single unit recordings were made in the paralemniscal reticular formation in adult male rats. A majority of the cells studied were characterized as nociceptive because a noxious stimulus evoked a change (either an increase or decrease) in their spontaneous firing rates. Norepinephrine (NE) administered microiontophoretically usually mimicked the response to the noxious stimulus (foot pinch). After a neuron had been characterized with respect to its response to NE and the noxious stimulus, horseradish peroxidase (HRP) was iontophoretically ejected from the micropipette. Following iontophoresis of HRP into the paralemniscal reticular formation, retrograde and orthograde labeling was observed in the periaqueductal gray and the nucleus raphe magnus. These data support a possible role of the paralemniscal reticular formation in an endogenous analgesic system.