Effects of inducible nitric oxide synthase blockade within the periaqueductal gray on cardiovascular responses during mechanical, heat, and cold nociception.

We have examined the role of inducible nitric oxide synthase (iNOS) within the dorsolateral periaqueductal gray mater (dlPAG) on cardiovascular responses during mechanical, thermal, and cold nociception in anesthetized rats. Mechanical stimulus was applied by a unilateral hindpaw pinch for 10 s that increased mean arterial pressure (MAP) and heart rate (HR). Bilateral microdialysis of a selective iNOS inhibitor, aminoguanidine (AGN; 10 µM), into the dlPAG for 30 min augmented MAP and HR responses during a mechanical stimulation. The cardiovascular responses recovered following discontinuation of the drug. Heat stimulus was generated by immersing one hindpaw metatarsus in a water bath at 52°C for 10 s, and this increased MAP and HR. Administration of AGN into the PAG potentiated these cardiovascular responses. Cardiovascular responses recovered following discontinuation of the drug. In contrast, application of a cold stimulus by immersing one hindpaw at 10°C for 10 s resulted in depressor and bradycardic responses. A second cold stimulus resulted in a response that was not significantly different from that prior to or after recovery from the AGN infusion. These results demonstrate that iNOS within the dlPAG plays a differential role in modulating cardiovascular responses during mechanical-, heat-, and cold-mediated nociception.