Effects of morphine and midazolam on reactivity to peripheral noxious and central aversive stimuli

Electrical stimulation of the mesencephalic tectum elicits behavioral and autonomic responses similar to those following peripheral noxious stimulation. Benzodiazepine and opioid compounds attenuate escape behavior induced by electrical stimulation of the dorsal periaqueductal gray (PAG) and deep layers of the superior colliculus (SC). The present study determines if microinjections of midazolam and morphine applied to these PAG-SC sites affect both responsiveness to peripheral noxious stimulation and to aversive PAG-SC stimulation. Both aversive brain stimulation or foot-shocks applied at threshold intensities caused running or jumps concomitant with increases in mean arterial blood pressure (MBP) and heart rate (HR). Microinjection of both drugs attenuated the behavioral reaction and increases in MBP and HR induced by mesencephalic tectum stimulation, while attenuating only the increase in heart rate induced by peripheral painful stimulation. These results suggest that the neural substrates of the behavioral and autonomic effects of stimulating the mesencephalic tectum and peripheral nociceptors are different although they may partially overlap.