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INTRQDUCTION: Opiates are hypothesized to cause a vasodepressor effect by acting on the rostral vantrolateral medulla (RVLM) wherein lie neurons which control sympathetic tone. Using voltammetry which measures catecholamine oxidation currents (CA.OC) generated by adrenergic activity in these neurons, our objective was to demonstrate the in vivo effect of morphine on the RVLM. METHODS: Halothane anaesthetized rats were implanted stereotaxically with carbon-fibre electrodes to monitor CA-OC by differential normal pulse voltammetry. Mean arterial pressure (MAP) was monitored by a femoral arterial line. After stabilization, rats received either intracerebroventricular (icy) morphine 10/zg (n =5) followed 45 minutes later by intravenous (iv) naloxone 1 mg-kg" or icy saline 5/Jl (n=5) followed by iv saline 0.45 ml. Changes in MAP and CA. OC were compared to baseline. RESULTS: The CA-OC was maximally depressed by 39 percent 45 minutes following morphine (Fig 1). Naloxone reversed the effects of morphine and produced a significant 21 percent rebound increase in the CA.OC. MAP was maximally depressed by 21 percent 45 minutes following morphine (Fig 1). This decrease in MAP was reversed by naloxone. Saline treatment had no significant effect on the CA.OC (ANOVA, p=0.321) or MAP (ANOVA, p=.531). Fig 1 40 30 Change 20 from lO Baseline o (percent)-lO-20-3O-40 n CA'OC naloxone 9 MAP r (mean + SEM) 9 , , , Y '* ,p < 0.05; Dunnstt's 0 15 30 45 60 75 90 Time (rain) DISCUSSION: An increase in the CA.OC in the RVLM accompanies induced hypotension possibly by way of a baroreceptor reflex. However, our results suggest that morphine depresses adrenergic neurons in the RVLM to produce a vasodepressor effect. The rebound effect following the antagonism of morphine with iv naloxone is analogous to the observation of excessive sympathetic activity in post-operative patients receiving naloxone. This acute opiate withdrawal phenomenon is possibly mediated through the RVLM and if so, it may be prevented by attenuation of activity in this area. REFERENCES= i.