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The rostral ventrolateral medulla is the primary site of action for clonidine, a centrally acting antihypertensive. In the rostral ventrolateral medulla, clonidine binds not only to alpha-2 adrenergic receptors but also to specific imidazole sites. In order to determine whether a putative imidazole receptor mediates the hypotensive action of clonidine, a series of compounds was tested 1) in vitro for binding affinity at imidazole and alpha-2 sites and 2) in vivo for ability to lower arterial pressure and heart rate when microinjected directly into the rostral ventrolateral medulla. Hypotensive potency was correlated with affinity at imidazole sites (r = 0.84), but not with alpha-2 affinity (r = -0.05). The bradycardia elicited by this series of compounds also correlated with affinity at imidazole receptors (r = 0.89), but not with alpha-2 affinity (r = 0.10). Furthermore, the imidazole idazoxan selectively reversed the fall in arterial pressure elicited by clonidine, whereas SKF-86466, an alpha-2 antagonist which is not an imidazole, failed to attenuate clonidine's action. An imidazole receptor in the rostral ventrolateral medulla appears to mediate the central hypotensive actions of clonidine and related centrally acting imidazoles and may participate in the regulation of arterial pressure and heart rate.

L-Glutamate (in 0.1 microliter of 0.9% NaCl) was injected via multibarrelled glass micropipettes into the ventrolateral medulla of urethane-anesthetized rats. This area, explored stereotaxically, extended from 2.2 mm rostral (near trapezoid bodies) to 0.4 mm caudal, with respect to the obex, 2.4 mm lateral to the mid-line on each side and 2.4 mm deep from the ventral surface of the medulla. Two types of responses were elicited by injection of L-glutamate. One type was a dose-related increase in arterial pressure and heart rate; these responses were elicited from the lateral portion of nucleus reticularis gigantocellularis, the medial aspect of nucleus reticularis parvocellularis and the dorsal-lateral reticular nucleus. The second type of response was a dose-related fall in arterial pressure with no change in heart rate; this response was localized caudal to pressure areas in the caudal ventrolateral part of nucleus reticularis gigantocellularis, ventrolateral nucleus reticularis ventralis, nucleus ambiguous and the A1 region. Glutamic acid diethylester (GDEE), an antagonist of L-glutamate, blocked all the cardiovascular effects of L-glutamate. These results indicate the presence of receptors for glutamate in the pressor and depressor areas. Glutamic acid diethylester caused a fall in blood pressure when injected on its own into pressor sites suggesting the existence of a glutaminergic input to the pressor sites. Inhibition of neuronal activity in pressor sites produced by microinjection of muscimol (a potent neuroinhibitory analogue of GABA) caused a decrease in blood pressure. On the other hand, pressor responses resulted following similar inhibition in the depressor sites. These results indicate that the pressor and depressor sites identified in the ventral medulla of the rat may have an important role to play in central cardiovascular regulation.

In urethane-anesthetized rats, vasodepressor neuron pools were located bilaterally in and adjacent to the A1 area of the ventrolateral medulla by injecting the neuroexcitatory amino acid, L-glutamate. Ventrolateral vasodepressor areas included the caudalateral part of the nucleus reticularis gigantocellularis, the rostrolateral part of the nucleus reticularis ventralis, and the dorsal nucleus reticularis lateralis. In the ventrolateral vasodepressor areas L-glutamate elicited a transient fall in blood pressure (BP) and heart rate (HR). The opiate agonist (D-ala2-met5)-enkephalinamide (DAME) was used to stimulate opiate receptors in vasodepressor sites, identified with L-glutamate. In these sites, bilateral injections (0.1 microliter/site) of DAME caused a dose-related (2.5-500.0 ng) increase in blood pressure and heart rate, as well as exaggeration of the response to occlusion of the carotid. The effects of DAME on blood pressure were completely abolished by alpha-adrenergic blockade (phentolamine, 2 mg/kg, i.v.) and all effects of DAME were reversed by the administration of naloxone HCl (1 mg/kg, i.v.). Naloxone reversal was accompanied by an unexpected "rebound" hypertension. Saline had no significant effects when injected, or administered intravenously, in the absence or presence of DAME. It was concluded that stimulation of opiate receptors in the ventrolateral vasodepressor areas activated sympathetic outflow. An enkephalinergic system in this area of the brain stem may serve to modulate blood pressure, heart rate and cardiovascular reflexes.

The role of cholinergic mechanisms in opioid-induced respiratory depression was investigated in isoflurane-anesthetized rats. In these animals, the i.v. administration of fentanyl, a clinically useful potent opioid analgesic/anesthetic, induced a prolonged period of apnea and subsequent rise in the end-tidal CO2, hypotension and bradycardia. The centrally acting anticholinesterase, physostigmine, significantly decreased fentanyl-induced respiratory and circulatory depression. The beneficial respiratory and circulatory effects of physostigmine could be abolished by the administration of muscarinic antagonists, e.g., scopolamine hydrobromide or atropine sulfate. Inhibition of inspiratory phrenic nerve activity by fentanyl and restoration of this activity by the subsequent i.v. administration of physostigmine indicates that the origin of this opioid-cholinergic interaction is the central nervous system. Similar effects on phrenic nerve discharge were observed when the systemic administration of fentanyl was followed by microinjection of physostigmine or carbachol in the rostral ventrolateral medulla. In conclusion, opioid-induced respiratory depression after systemic administration is obtunded greatly by facilitation of muscarinic mechanisms in the ventrolateral medulla. Inhibition of a cholinergic link in the central chemosensor may underlie opioid-induced respiratory depression. Manipulation of this cholinergic link could lead to the use and development of analgesics devoid of respiratory depression.

Physostigmine, a choline-esterase inhibitor, is known to elevate endogenous levels of acetylcholine. Intravenously administered physostigmine causes a rise in blood pressure via its action in the central nervous system. Exact site of this action of physostigmine is not known. In this paper, it was demonstrated that microinjections of tetrodotoxin (a fast sodium channel blocker), lidocaine (a local anesthetic) and scopolamine (a cholinergic muscarinic receptor blocker) into the rostral ventrolateral medullary pressor area abolished the pressor action of intravenously administered physostigmine. These results demonstrate that the rostral ventrolateral medulla is the site of action of intravenously administered physostigmine and this action is mediated via cholinergic muscarinic receptors.

Clonidine acts within the rostral ventrolateral medulla to lower arterial pressure. The receptor mechanism for this action is unknown. In the rostral ventrolateral medulla clonidine binds not only to the alpha 2-adrenergic receptor but also to a novel class of sites which are specific for imidazolines and imidazoles and are distinct from adrenergic or histaminergic receptors. In order to distinguish whether a putative imidazole receptor or the alpha 2-receptor mediates the clonidine hypotensive response, a series of clonidine analogues were tested (1) in radioligand binding assays of their affinity at imidazole and alpha 2-adrenergic receptors, and (2) by microinjection into the rostral ventrolateral medulla of anaesthetized rats to measure their capacity to lower arterial pressure. The hypotensive response elicited by the test agents was strongly correlated with affinity at imidazole sites (r = 0.92) but not with alpha 2-adrenergic affinity (r = 0.36). An imidazole receptor in the rostral ventrolateral medulla may participate in the hypotensive action of clonidine.

In the present study, the gamma-aminobutyric acid receptor antagonist, bicuculline methiodide (BMI), was used to explore the role of endogenous medullary GABAergic mechanisms in the neural control of circulation. In urethane-anesthetized rats, microinjections of the neuroexcitatory amino acid, L-glutamate (150-300 ng/site) were used to functionally identify rostral ventrolateral vasopressor neuron pools (VLPA) and caudal ventrolateral vasodepressor neuron pools (VLDA). The bilateral microinjection of BMI into the ventrolateral vasodepressor neuron pool caused a dose-related (0.1-100.0 ng/site) decrease in blood pressure (BP), pulse pressure and heart rate (HR). Maximum decreases in BP and HR were 51 +/- 2 mm Hg and 245 +/- 9 beats/min, respectively. In the VLPA, BMI caused a dose-related (0.1-100.0 ng/site) increase in BP, HR and pulse pressure. Maximum increases in BP and HR were 88 +/- 3 mm Hg and 73 +/- 4 beats/min, respectively. In the VLPA, BMI also caused a 40% increase in the carotid artery occlusion response and a 48% reduction in the aortic depressor nerve response. It seems plausible that GABAergic mechanisms in both the caudal and rostral ventrolateral medulla are tonically involved with the maintenance and reflex regulation of vasomotor activity. Medullary gamma-aminobutyric acid may provide a reciprocal inhibition between rostral vasopressor and caudal vasodepressor neuron pools.

The cardiovascular effects of a GABAergic agonist (muscimol) and antagonist (bicuculline methiodide) were determined after microinjection (100 nl) into ventrolateral vasodepressor and vasopressor neuron pools. In urethane-anesthetized rats, these neuron pools were identified bilaterally by observing pressor or depressor responses after the microinjection of L-glutamate (333 ng). The bilateral microinjection of muscimol (0.1-10 ng/site) into vasodepressor sites caused a dose-related (0.1-10.0 ng/site) increase in the blood pressure, heart rate and pulse pressure. In ventrolateral vasopressor sites, muscimol (0.1-10.0 ng/site) caused a marked fall in blood pressure, heart rate and pulse pressure. The depressor response elicited by aortic nerve stimulation (2 V, 25 HZ, 3 msec) was blocked and reversed by muscimol microinjections into vasodepressor sites. In these sites, muscimol also caused a similar reversal of the depressor response induced by stimulation of pulmonary C-fibers (J-receptors) after the right atrial administration of phenyldiguanide (30 micrograms/kg). All the effects of muscimol were blocked or reversed by the bilateral microinjection of bicuculline methiodide into the vasodepressor and vasopressor sites. These results demonstrate the importance of ventrolateral vasodepressor and vasopressor neuron pools in the maintenance and reflex regulation of blood pressure and suggest that intrinsic GABAergic systems may serve to modulate activity in these neuron pools.

Regional vascular resistance changes were determined following chemical excitation and inhibition of the rostral vasopressor (RVLM) and caudal vasodepressor (CVLM) areas in the ventrolateral medulla. Mesenteric, renal and hindquarter vascular resistances were assessed in paralyzed and artificially ventilated urethane-anesthetized rats instrumented with pulsed-Doppler flow probes. Microinjection of the excitatory amino acid L-glutamate in the RVLM elicited a significant dose-related transient increase in blood pressure, heart rate and resistance of mesenteric, renal and hindquarter vascular beds. A similar dose-related hemodynamic profile was obtained following microinjection of muscimol, a GABAmimetic, in the CVLM. In contrast, significant dose-related decrease in blood pressure, heart rate and resistance in mesenteric and hindquarter vascular beds was observed following glutamate-induced excitation of the CVLM and muscimol-induced inhibition of the RVLM. Changes in renal vascular resistance were inconsistent in this second hemodynamic profile. Intravenous administration of the alpha 1 adrenergic antagonist, prazosin, abolished all of the hemodynamic effects elicited by excitation of the RVLM except the tachycardia. Intravenous atropine methylnitrate blocked the bradycardia associated with excitation of the CVLM but did not alter the vascular resistance changes. These results indicate that the changes in heart rate did not contribute significantly to the resistance profiles described. The changes in vascular resistance elicited by excitation and inhibition of the RVLM were correlated with increase and decrease in the greater splanchnic nerve activity, respectively. In conclusion, neuron pools in the RVLM and CVLM exert differential effects upon resistance in different vascular beds via changes in sympathetic outflow.(ABSTRACT TRUNCATED AT 250 WORDS)

The cardiovascular effects associated with the microinjection (100 nl) of carbachol, physostigmine and atropine into the pressor area in the ventrolateral medulla (VLPA) were studied. In urethane anesthetized rats, VLPAs were functionally identified bilaterally by microinjection of the neuroexcitatory amino acid L-glutamate (300 ng/site). L-Glutamate microinjections into the VLPA cause a transient rise in blood pressure (BP) and heart rate (HR). The bilateral microinjection of carbachol into the VLPA caused a prolonged dose-related increase in BP and HR in the dose range of 0.8 to 400.0 ng/site. At higher doses (1-10 micrograms), carbachol caused a decrease in BP and HR; indicative of depolarization blockade. Acetylcholine esterase inhibition by physostigmine microinjections in the VLPA caused cardiovascular effects similar to those observed with carbachol. The hypertensive responses evoked by muscarinic receptor stimulation in the VLPA were mediated by increasing sympathetic outflow. The pathway by which cholinergic receptor stimulation in the VLPA activates vasomotor outflow appears to be entirely descending as coronal knife cuts at the level of the trapezoid body failed to alter the hypertensive responses. Pressor responses elicited by both physostigmine and carbachol were reversed completely by the i.v. administration of atropine sulfate (0.5-3 mg/kg i.v.). Muscarinic receptor blockade in the VLPA after atropine sulfate microinjection caused a dose-related (0.4-15.0 micrograms/site) fall in the BP and HR suggesting that cholinergic mechanisms in the VLPA are tonically active.