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.

Effects of endothelial NOS antagonism within the periaqueductal gray on cardiovascular responses and neurotransmission during mechanical, heat, and cold nociception.

Nitric oxide (NO) is synthesized from L-arginine using NO synthase (NOS) enzyme that exists as 3 isoforms: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). We examined the role of eNOS within the dorsolateral periaqueductal gray mater (dlPAG) on cardiovascular responses along with glutamate and GABA concentrations during mechanical-, heat-, and cold-induced nociception in anesthetized rats. Mechanical stimulus was applied by a 10-second hindpaw pinch that increased mean arterial pressure (MAP) and heart rate (HR). Bilateral microdialysis of a selective eNOS antagonist, L-N(5)-(1-iminoethyl)ornithine (L-NIO; 10 microM), into the dlPAG had no effect on MAP or HR during a mechanical stimulation. Heat stimulus was generated by immersing a hindpaw metatarsus in a water-bath at 52 degrees C for 10 s which increased glutamate, GABA, MAP and HR. Administration of L-NIO into the dlPAG augmented cardiovascular responses and glutamate increase, but attenuated GABA changes during the heat stimulus. In contrast, application of a cold stimulus by immersing the hindpaw at 10 degrees C for 10 s resulted in decreases in MAP, HR, and glutamate. However, there was an increase in GABA concentration. Following microdialysis of L-NIO into the dlPAG, the responses to the cold stimulus was reversed i.e., the cold stimulus induced pressor and tachycardic responses, augmented glutamate, and attenuated GABA levels. These results demonstrate that eNOS within the dlPAG plays a differential role on the cardiovascular system during heat- and cold-mediated nociception via modulating glutamatergic/GABAergic neurotransmission. However, the mechanical stimulation had no effect on cardiovascular responses following eNOS antagonism within the dlPAG.

Modulation of cardiovascular responses and neurotransmission during peripheral nociception following nNOS antagonism within the periaqueductal gray.

Nitric oxide (NO) within the dorsal periaqueductal gray matter (dPAG) attenuated cardiovascular responses and changes in the concentrations of glutamate during both mechanical and thermal nociceptive stimulation [Ishide, T., Amer, A., Maher, T.J., Ally, A., 2005. Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli. Neurosci. Res. 51, 93-103]. Nitric oxide is synthesized from l-arginine via the enzyme, NO synthase (NOS), which exists in 3 isoforms: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). In this study, we examined the role of nNOS within the dPAG on cardiovascular responses and extracellular glutamate and GABA concentrations during mechanical and thermal nociception in anesthetized rats. The noxious mechanical stimulus was applied by a bilateral hindpaw pinch for 5 s that increased mean arterial pressure (MAP) and heart rate (HR) by 24+/-4 mm Hg and 41+/-7 bpm, respectively (n=10). Extracellular glutamate levels within the dPAG increased by 10.7+/-1.3 ng/mul while GABA concentrations decreased by 1.9+/-0.5 ng/microl. Bilateral microdialysis of a selective nNOS antagonist, 1-(2-trifluoromethylphenyl)-imidazole (TRIM; 10.0 microM), into the dPAG had no effect on MAP, HR, glutamate and GABA values (P>0.05) during a mechanical stimulation. In a separate set of experiments, a noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in a water-bath at 52 degrees C for 5 s (n=10). Glutamate, MAP, and HR increased by 14.6+/-2 ng/microl, 45+/-6 mm Hg, and 47+/-7 bpm, while GABA decreased by 2.1+/-0.6 ng/microl. Administration of TRIM into the dPAG significantly enhanced the cardiovascular responses and glutamate increases (P<0.05) but further attenuated GABA changes (P<0.05) during subsequent thermal nociception. These results demonstrate that nNOS within the dPAG plays a differential role in modulating cardiovascular responses and glutamatergic/GABAergic neurotransmission during thermal and mechanical nociception.

Medullary monoamines and NMDA-receptor regulation of cardiovascular responses during peripheral nociceptive stimuli.

We have previously reported that AMPA-receptor blockade within the rostral ventrolateral medulla (RVLM) attenuates cardiovascular responses and extracellular concentrations of glutamate during mechanical, but not during thermal stimulation [Gray, T., Lewis III, E., Maher, T.J., Ally, A., 2001. AMPA-receptor blockade within the RVLM modulates cardiovascular responses via glutamate during peripheral stimuli. Pharmacol. Res. 43, 47-54]. In this study, we examined the role of NMDA-receptor blockade within the RVLM on cardiovascular responses and release of biogenic monoamines (serotonin [5HT], dopamine [DA], and norepinephrine [NE]) during both mechanical and thermal nociception using anesthetized Sprague-Dawley rats. Both mechanical and thermal stimulation have been shown to activate peripheral Adelta and C-fiber polymodal nociceptors. Noxious mechanical stimuli were induced by applying a pinch to alternate hindpaw for 5s while the noxious thermal stimuli involved immersion of the metatarsus of alternate hindpaw in a water bath at a temperature of 52 degrees C for 5 s. Mechanical stimulation increased mean arterial pressure (MAP), heart rate (HR), extracellular fluid 5HT, and DA concentrations (n=10). However, extracellular levels of NE were decreased within the RVLM. Furthermore, NMDA-receptor blockade with a competitive antagonist, AP-7 (200 nM), within the RVLM attenuated the cardiovascular responses and changes in 5HT and DA, but had no effect on NE levels. The thermal stimulation elicited similar increases in MAP and HR, however, extracellular levels of 5HT or DA did not change. Concentrations of NE were decreased during a thermal stimulation similar to the levels observed following mechanical stimuli. In contrast to mechanical stimuli, bilateral administration of AP-7 (200-1 mM) into the RVLM had no effect on cardiovascular responses, 5HT, DA or NE concentrations during a thermal stimulation. These results show that NMDA receptors within the RVLM most likely play a role in modulating cardiovascular responses by altering 5HT and DA concentrations within the RVLM during mechanical but not thermal nociception. Overall, the present study delineates the NMDA-receptor mediated central integrative mechanisms within the RVLM that coordinate processing of sensory impulses arising from peripheral noxious stimulation.