Nociceptive excited and inhibited neurons within the pedunculopontine tegmental nucleus and cuneiform nucleus

The pedunculopontine tegmental nucleus (PPTg) is defined by its collection of cholinergic neurons surrounding the lateral portion of the superior cerebellar peduncle at the midbrain pontine junction. Antinociceptive functions have been attributed to the PPTg since electrical stimulation as well as injection of cholinergic agonists in this area produces analgesia. Nociceptive neurons have also been reported in the vicinity of the PPTg and cuneiform nucleus (CN). However, specific histochemical localization of nociceptive modulatory neurons has not been determined. Thus, the goal of this study was to classify neurons according to their response to a noxious stimulus and map their location based on staining of the cholinergic neurons in the PPTg. Extracellular microelectrode recordings were conducted in 19 male Sprague-Dawley rats under light halothane anesthesia. For each neuron identified, a series of noxious tail pinches were administered. The electrode tracts were marked with ionophoresis of pontamine blue. The location of 112 recorded neurons was determined on sections stained with NADPH diaphorase to identify the cholinergic boundaries of the PPTg. Neurons were classified into one of three cell types based on their consistent response to a noxious tail pinch (excited, inhibited, and non-responsive). Tail pinch excited neurons (n=16), inhibited neurons (n=10) and non-responsive neurons (n=23) were mapped within the cholinergic boundaries of the PPTg. Excited (n=9), inhibited (n=10) and non-responsive neurons (n=10) were also found more dorsally within the cuneiform nucleus. Thus, this study localizes nociception-responsive neurons to the region of the largely cholinergic PPTg, as well as the noncholinergic cuneiform nucleus.