Role of the Anesthetic Determine the Clonidine Effects Mediated by RVLM/-α2-Adrenoceptors on Blood Pressure and Heart Rate in Rats

To examine the hypothesis of a role for α2-adrenoceptors in mediating the mechanism of urethane hypotensive effect whether it's peripheral or central, Wistar rats were anesthetized with urethane or (for comparison) with halothane, to study the influence of urethane that govern the mechanism of central and peripheral α2-adrenoceptors action, on basal BP & HR, and the rise in blood pressure (BP) to the stimulation of caudal pressor area (CPA), when these receptors were either centrally activated by bilateral rostral ventrolateral medulla (RVLM) microinjection of clonidine (30nM), and blockade with any of the clonidine antagonists, yohimbine (500pmol/50nl), and idazoxane (270nM) or yohimbine+idazoxane, or when peripherally activated (of urethane anesthetized rats) by i.v. clonidine (100nmol/kg), which also blockade with idazoxane or yohimbine+idazoxane. The results indicated presence of no anesthetic differences in a partial involvement of α2-receptors-RVLM, vs. a complete involvement of I(1)-imidazole receptors in mediating the hypotensive effects of clonidine. It also indicates α2-/I(1)-receptors synergism in raising the urethane lowering of baseline of SBP to the levels of control or halothane group. In conclusion, the result suggests involvement both of the central and the peripheral α2-adrenoceptors in mediating urethane hypotensive effects.

Novel axonal projection from the caudal end of the ventrolateral medulla to the intermediolateral cell column

We used an optical imaging technique to investigate whether axons of neurons in the caudal end of the ventrolateral medulla (CeVLM), as well as axons of neurons in the rostral ventrolateral medulla (RVLM), project to neurons in the intermediolateral cell column (IML) of the spinal cord. Brain stem-spinal cord preparations from neonatal normotensive Wistar-Kyoto and spontaneously hypertensive rats were stained with a voltage-sensitive dye, and responses to electrical stimulation of the IML at the Th2 level were detected as changes in fluorescence intensity with an optical imaging apparatus (MiCAM-01). The results were as follows: 1) depolarizing responses to IML stimulation during low-Ca high-Mg superfusion were detected on the ventral surface of the medulla at the level of the CeVLM, as well as at the level of the RVLM, 2) depolarizing responses were also detected on cross sections at the level of the CeVLM, and they had a latency of 24.0 ± 5.5 (SD) ms, 3) antidromic action potentials in response to IML stimulation were demonstrated in the CeVLM neurons where optical images were detected, and 4) glutamate application to the CeVLM increased the frequency of excitatory postsynaptic potentials (EPSPs) and induced depolarization of the IML neurons. The optical imaging findings suggested a novel axonal and functional projection from neurons in the CeVLM to the IML. The increase in EPSPs of the IML neurons in response to glutamate application suggests that the CeVLM participates in the regulation of sympathetic nerve activity and blood pressure and may correspond to the caudal pressor area.