Type I and II metabotropic glutamate receptor agonists and antagonists evoke cardiovascular effects after intrathecal administration in conscious rats

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Abstract
1. In the present study, the role of metabotropic glutamate receptors (mGluRs) in central cardiovascular regulation in conscious rats was examined. To this end, agonists and antagonists for type I and II mGluRs were administered intrathecally, and the temporal changes in blood pressure and heart rate were recorded. 2. L-glutamate (1 micromol) and the prototypical mGluR agonist (1S,3R)-ACPD (0.1 and 0. 3 micromol) both increased mean arterial pressure (MAP) and heart rate (HR), implicating functional mGluRs in the spinal cord. The type I mGluR agonist DHPG (0.01 - 0.1 micromol) evoked increases in MAP (max=25+/-5 mmHg) and HR (max=88+/-23 beats min-1). The duration of action, but not the maximum effects, were dose-related and ranged from approximately 10 min to 90 min for MAP and HR, respectively. 3. The type I/II mGluR agonist CCG-1 (0.1 and 0. 3 micromol) caused smaller, variable increases in MAP and HR of intermediate duration (5 - 20 min), whereas the type II MGluR agonist APDC (0.1 and 1.0 micromol) caused marked, but transient (3 - 5 min), pressor and tachycardic responses. The highest doses of DHPG and CCG-1, but not APDC, also evoked behavioural responses similar to a spontaneous nociceptive behavioural effect reported previously. 4. The type I and II mGluR antagonists (AIDA and LY307452, respectively) were also given approximately 5 min before the administration of the respective type I and II mGluR agonists (DHPG and APDC). Both compounds caused pressor and tachycardic responses, with the effect of AIDA, but not LY307452, returning to control levels before mGluR agonist administration. AIDA significantly attenuated the overall cardiovascular effects of DHPG, while LY307452 significantly attenuated the overall cardiovascular effects of APDC. 5. These results indicate that functional type I and II mGluRs exist in the spinal cord, and that their activation evokes prolonged cardiovascular effects.