Kristina Reid

Synergistic Interaction Between α2-adrenergic Agonists and Benzodiazepines in Rats

Both α2-adrenergic agonists and benzodiazepines exert anxiolytic and sedative effects when administered as prcoperative medications. Clinical effects achieved with a combination of drugs, representative of these classes of compounds, is greater than that which could be expected from a simple additiv

Pertussis toxin-mediated ribosylation of G proteins blocks the hypnotic response to an α2-agonist in the locus coeruleus of the rat

Biologic responses mediated by adrenoceptors are transduced by a receptor-effector mechanism that involves a guanine nucleotide binding protein (G protein). Recently, we determined that the transduction mechanism for the hypnotic response to dexmedetomidine, a highly selective alpha 2-agonist, is located in the locus coeruleus (LC) of the rat. In this study, we examined the role of pertussis toxin-sensitive (PTX) G proteins in the LC for the hypnotic response to dexmedetomidine. The LC of rats were stereotactically cannulated and treated with PTX, 0.34 micrograms, or vehicle. Five days later, the hypnotic response to dexmedetomidine, 7 micrograms into the LC or 50 micrograms.kg-1 IP, was tested. On the following day, the LC was harvested and assayed to determine whether the G proteins had been ribosylated by pretreatment with PTX in vivo. Quantitative immunoblotting of G0 alpha, Gi alpha 1,2, and Gi alpha 3, the alpha-subunit of three PTX-sensitive proteins, was also performed. In vivo treatment with PTX into the LC blocked the hypnotic response to LC-administered dexmedetomidine and, to a lesser extent, IP-administered dexmedetomidine. The in vivo PTX treatment effectively ribosylated the G proteins. No alteration in the amount of the different species of PTX-sensitive alpha-subunit was produced by in vivo PTX treatment. These data suggest a pivotal role for PTX-sensitive G proteins in the LC in the hypnotic response to alpha 2-agonists in the rat.

Sympatholytic and Minimum Anesthetic Concentration-sparing Responses are Preserved in Rats Rendered Tolerant to the Hypnotic and Analgesic Action of Dexmedetomidine, a Selective α2-adrenergic Agonist

Background The development of tolerance to the sympatholytic and anesthetic-reducing effects of alpha2 agonists after prolonged administration of dexmedetomidine and how the number of available alpha sub 2 adrenoceptors affects these dexmedetomidine-induced responses was studied. Methods The sympatholytic action of acute and chronic (3 and 10 micro gram *symbol* kg sup -1 *symbol* h sup -1 for 7 days) dexmedetomidine, was assessed by the decrease in norepinephrine turnover in the locus coeruleus and hippocampus. The anesthetic-reducing effect of chronic (7 days) dexmedetomidine (5 and 10 micro gram *symbol* kg sup -1 *symbol* h sup -1) was studied by determining the minimum alveolar concentration (MAC) for halothane that prevented rats from responding to a supramaximal noxious stimulus of dexmedetomidine (10 or 30 micro gram *symbol* kg sup -1), doses in the steep part of the dose-response curve. The receptor reserve for the norepinephrine turnover and anesthetic-sparing responses to dexmedetomidine was delineated with 0.3-1.0 mg *symbol* kg sup -1 N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, an irreversible alkylating agent. Results After chronic administration of dexmedetomidine at both doses, acute dexmedetomidine significantly decreased norepinephrine turnover in the hippocampus and locus coeruleus. The baseline minimum anesthetic concentration (MAC) and the MAC-sparing effect to acutely administered dexmedetomidine were preserved after chronic dexmedetomidine treatment. In the N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline experiments, the dexmedetomidine-induced norepinephrine turnover effect required less than 20% and greater than 4% alpha2 adrenoceptor availability in the locus coeruleus and the dexmedetomidine induced MAC-sparing effect required less than 40% and greater than 20% alpha2 adrenoceptor availability in the locus coeruleus. Conclusion Tolerance does not develop for either the sympatholytic or MAC-sparing actions of dexmedetomidine, although it is present for the hypnotic response. The durable quality of the sympatholytic and MAC-sparing responses to dexmedetomidine after chronic treatment is explained by a comparatively larger receptor reserve than is needed for the hypnotic and analgesic responses, which are blunted by the same drug treatment regimen.

chronic Desipramine Treatment Desensitizes the Rat to Anesthetic and Antinociceptive Effects of the α2-adrenergic Agonist Dexmedetomidine

INTRODUCTION The effects of long-term administration of the tricyclic antidepressant agent desipramine on the hypnotic, antinociceptive, anesthetic-sparing, and central norepinephrine turnover suppressant action of short-term dexmedetomidine, a highly selective alpha2-adrenergic agonist, were studied in rats. METHODS Rats were given a 3- or 4-week course of twice daily administration of desipramine, 10 mg/kg, or saline. The effect of a hypnotic dose of dexmedetomidine, 250 microg/kg given intraperitoneally, on the duration of loss of righting reflex was determined. The tail flick latency response was determined before and after 50 microg/kg dexmedetomidine. The minimum anesthetic concentration of halothane and the central norepinephrine turnover rate were determined before and after administration of 30 microg/kg dexmedetomidine. Changes in the affinity and density of the alpha2-adrenergic receptor in locus coeruleus and spinal cord also were determined. RESULTS Treatment with desipramine decreased dexmedetomidine-induced loss of righting reflex duration by 67% and eliminated the antinociceptive effect of dexmedetomidine. Dexmedetomidine produced a 55% decrease in minimum anesthetic concentration in the control group but no reduction in desipramine-treated rats. Desipramine did not change the receptor density or binding affinity of alpha2 receptors at the site for hypnotic (locus coeruleus) or antinociceptive (spinal cord) responses. No decrement in the central norepinephrine turnover rate was noted in the locus coeruleus of dexmedetomidine after 3 weeks of treatment with desipramine. The alpha1-adrenergic antagonist prazosin at 1 or 5 mg/kg completely (minimum anesthetic concentration reduction), almost completely (antinociceptive), or partially (hypnotic) restored responsiveness to normal. CONCLUSIONS These data indicate that treatment with desipramine induces hyporesponsiveness to the hypnotic, analgesic, and minimum anesthetic concentration-reducing, but not to the suppression of central norepinephrine turnover, properties of dexmedetomidine. The hyporesponsiveness appears to involve an alpha1-adrenergic mechanism.

Are cholinergic pathways involved in the anesthetic response to α2 agonists

1. We investigated whether change in neuronal activity in cholinergic pathways mediates the anesthetic effect of the α2 agonist, dexmedetomidine, by determining whether physostigmine, a cholinesterase inhibitor, could antagonize the hypnotic response to dexmedetomidine in the rat and whether dexmedetomidine decreases the release of acetylcholine (ACh) in the thalamus in vivo. 2. Physostigmine did not significantly change the duration of the hypnotic response to dexmedetomidine. There was no significant change in thalamic ACh release after administration of dexmedetomidine. Therefore, α2-adrenergic agonists produce their anesthetic effect through mechanisms which do not involve alteration of the activity of the brainstem cholinergic nuclei.

Chronic treatment with dexmedetomidine desensitizes α2-adrenergic signal transduction

Abstract Tolerance to the hypnotic response was induced in rats by chronically infusing dexmedetomidine, a novel α 2 -adrenergic agonist. The α 2 -adrenocepter affinity for dexmedtomidine and para -iodoclonidine was significantly reduced in tolerant rats, while B max was uncharged. The ability of pertussis toxin (PTX) to ribosylate guanine nucleotide regulatory proteins (G proteins) ex vivo was reduced in tolerant rats; the quantity of PTX-sensitive G proteins was unchanged. Forskolin-stimulated adenylyl cyclase was less sensitive to inhibition by dexmedetomidine in the tolerant rats; however, acute intraperitoneal injection of dexmedetomidine still reduced cyclic adenosine monophosphate levels in tolerant rats. Both the decrease in ribosylation and the lower α 2 -adrenoceptor binding affinity may reflect a decrease in the ability of the G protein to couple to the α 2 adrenoceptors in the loecus coeruleus of tolerant rats. In this state, the α 2 adrenoceptors are less capable of transducing the effector response (inhibition of adenylyl cyclase).

Sedative but not analgesic α2 agonist tolerance is blocked by NMDA receptor and nitric oxide synthase inhibitors

BackgroundStudies show that the sedative and analgesic effects of &agr;2 adrenergic agonists decrease over time, which is a form of synaptic plasticity referred to as tolerance. Because both the N-methyl-d-aspartate (NMDA) receptor complex and nitric oxide synthase are pivotal for some forms of synaptic plasticity, their role in tolerance to the hypnotic and analgesic effects of &agr;2 agonists was investigated. MethodsAfter institutional approval, rats were made tolerant to the hypnotic or analgesic action of an &agr;2 agonist, dexmedetomidine. The hypnotic response to dexmedetomidine was assessed by the duration of loss of righting reflex, and the analgesic response to dexmedetomidine was assessed by the tail-flick assay. In separate cohorts, either the NMDA receptors or nitric oxide synthase was antagonized by coadministration of MK-801, ketamine, or NO2-arginine, respectively, during induction of tolerance. In a separate series of experiments, after tolerance was induced, the hypnotic and analgesic responses to dexmedetomidine were assessed in the presence of acutely administered MK-801 or NO2-arginine. ResultsInduction of tolerance to the hypnotic effect of dexmedetomidine is blocked by coadministration of MK-801, ketamine, and NO2-arginine. However, after tolerance developed, acute administration of MK-801, ketamine, or NO2-arginine did not prevent the expression of tolerance. Coadministration of MK-801 or NO2-arginine neither prevents the development nor reverses the expression of tolerance to the analgesic action of dexmedetomidine. ConclusionThe underlying processes responsible for the development of tolerance to the hypnotic and analgesic actions of systemically administered &agr;2 agonists were different, with only the sedative tolerance involving the NMDA receptor and nitric oxide synthase system.