Pain modulation in the adrenergically stimulated area postrema in the alert guinea pig

The effect of chemical stimulation of the area postrema (AP) with noradrenalin (NOR) and its blocking agents was studied in alert guinea pigs submitted to noxious peripheral stimuli (electric shock). The animal's motor defense and vocalizing responses during and after nociceptive stimulation were studied. Topical administration of NOR in the AP at a concentration of 150 microgram/microliter decreased the vocalizing and movement responses with both threshold and overthreshold stimulation. Phentolamine (5 microgram/microliter) elicited the opposite response, i.e. increase in vocalization and movements. Propranolol elicited a brief reduction of the threshold response. These results suggest an involvement of the AP in the mechanism of nociceptive stimulation through alpha-adrenergic mediation. The inhibition of the response to the noxious stimulus may be the consequence of a vagal activation triggered by the AP.