The role of nociceptinergic transmission in the nucleus tractus solitarii (NTS) in the central modulation of cardiovascular activity was investigated in
pentobarbital-anesthetized and conscious rats. Pharmacological activation of
nociceptin receptors with a unilateral injection of synthetic
nociceptin into the NTS, wherein injection of
L-glutamate (1 nmol) caused typical depressor responses, elevated blood pressure and heart rate (HR) in most of the anesthetized rats. The elevation of blood pressure and HR by
nociceptin was dose-dependent (0.04, 0.2, and 1 nmol) with a threshold dose of 0.2 nmol. At 1 nmol, changes in blood pressure and HR were evident at 5 min, and remained for 45 min after injection. Pretreatment with the selective
nociceptin receptor antagonist
nocistatin (1 nmol) into the NTS abolished the
nociceptin-induced
hypertension and
tachycardia. In contrast, the nonselective
opioid receptor antagonist naloxone (5 nmol) did not modify the cardiovascular responses to
nociceptin. Intra-NTS injection of
nocistatin (0.04 and 1 nmol) and
naloxone alone had no significant effect on baseline blood pressure and HR. In chronically cannulated and conscious rats, similar pressor and tachycardic responses were induced by intra-NTS injection of 1 nmol of
nociceptin. However, changes in blood pressure and HR were rapid, and quickly returned to normal levels within 10 min. These data suggest that the newly discovered nociceptinergic transmission in the NTS has a powerful influence on peripheral hemodynamic activity. This influence is inhibitory and may not be tonically active under normal physiological conditions. Moreover, the cardiovascular responses to exogenous
nociceptin were mediated through activation of specific
nociceptin receptors rather than typical
naloxone-sensitive
opioid receptors.