Conditioned place aversion produced by microinjections of substance P into the periaqueductal gray of rats

The dorsal periaqueductal gray (DPAG) is one of the main structures involved in the integration of defensive behavior in the brain. In order to investigate the participation of neuropeptides in the generation of aversive states, semicarbazide, a glutamic acid decarboxylase inhibitor, and substance P, an active neuropeptide, were injected into the DPAG and their effects evaluated in the open field and the place conditioning tests. While semicarbazide and substance P both increased locomotor activity only substance P increased grooming in the open field. In the place conditioning procedure similar aversion conditioning was produced by both drugs. These results confirm previous data showing that semicarbazide in the DPAG causes place aversion through reduction of tonic inhibitory mechanisms on neural substrates of aversion. Such mechanisms may include substance P neurons as substance P microinjection into the DPAG also functioned as an unconditioned stimulus in the place aversion test.