Effects of single nanoinjections of Met-enkephalin in the minimally anesthetized rat brainstem on trigeminal nuclear neurons: differential effects on spontaneous versus evoked activity and noci-evoked versus innocuously-evoked responses.

Two separate studies were done on two sets of rats (33 rats, 101 cells in Expt. 1; 16 rats, 25 cells in Expt. 2). In the first study trigeminal nuclear neurons were categorized by adequate stimulus and receptive field. A total of 43.6% responded to light touch and noxious (pinch and heat) stimulation; 27.7% responded only to noxious pinch and/or heat; 8.9% responded exclusively to innocuous touch, and the remainder were non-responsive. Ninety percent of the responsive cells showed increased discharge rates in response to noxious stimulation. In the second study, after characterization of response properties, single nanoinjections of Met-enkephalin (ME) were separately made into midbrain periaqueductal gray (PAG) and nucleus reticularis paragigantocellularis (PGC). The effects of ME on spontaneous (SD) and evoked (ED) discharge were assessed, and then in 8 cells, the effects were repeated and successfully challenged with naloxone. The major findings were: (1) ME applied either to PAG or PGC causes naloxone-reversible reduction of noci-evoked ED with no effect on SD; and (2) the responses of neurons to innocuous light touch were unaffected by ME.