Anterior Cingulate Cortex Modulates Visceral Pain as Measured by Visceromotor Responses in Viscerally Hypersensitive Rats

Background & Aims: We have identified that the anterior cingulate cortex (ACC) neurons are responsive to colorectal distention (CRD) and shown that sensitization of ACC neurons occurs in viscerally hypersensitive rats. However, the role of the ACC in pain response has not been clearly defined. We aimed to determine if ACC neuron activation enhances visceral pain in viscerally hypersensitive rats and to identify the receptor involved in facilitation of visceral pain. Methods: The nociceptive response (visceromotor response [VMR]) to CRD was recorded in normal and viscerally hypersensitive rats induced by colonic anaphylaxis. The ACC was stimulated electrically, and ACC lesions were generated with ibotenic acid. l-glutamate, α-amino-3-hydroxy-5-methyl-isoxozole propionic acid receptor antagonist cyanonitroquinoxaline dione, and N-methyl-d-aspartate receptor antagonist aminophosphonopentanoic acid were microinjected into the rostral ACC. Results: Electrical stimulation of the rostral ACC enhanced the VMR to CRD in normal rats. ACC lesions caused a decrease in the VMR in viscerally hypersensitive rats but had no effect in normal rats. ACC microinjection of 2 mmol/L glutamate increased the VMR to CRD (10 mm Hg) in viscerally hypersensitive rats, and 20 mmol/L glutamate induced a more potent VMR in viscerally hypersensitive than in normal rats. Cyanonitroquinoxaline dione did not affect the VMR in either group. Aminophosphonopentanoic acid significantly suppressed the VMR in viscerally hypersensitive rats but not in normal rats. Conclusions: The ACC plays a critical role in the modulation of visceral pain responses in viscerally hypersensitive rats. This process appears to be mediated by enhanced activities of glutamate N-methyl-d-aspartate receptors.