Antagonism of pentobarbitone-induced respiratory depression by naloxone in rats

Effect of chronic resistive loading on hypoxic ventilatory responsiveness

Depression of ventilation mediated by endogenous opioids has been observed acutely after resistive airway loading. We evaluated the effects of chronically increased airway resistance on hypoxic ventilatory responsiveness shortly after load imposition and 6 wk later. A circumferential tracheal band was placed in 200-g rats, tripling tracheal resistance. Sham surgery was performed in controls. Ventilation and the ventilatory response to hypoxia were measured by using barometric plethysmography at 2 days and 6 wk postsurgery in unanesthetized rats during exposure to room air and to 12% O2-5% CO2-balance N2. Trials were performed with and without naloxone (1 mg/kg i.p.). Room air arterial blood gases demonstrated hypercapnia with normoxia in obstructed rats at 2 days and 6 wk postsurgery. During hypoxia, a 30-Torr fall in PO2 occurred with no change in PCO2. Hypoxic ventilatory responsiveness was suppressed in obstructed rats at 2 days postloading. Naloxone partially reversed this suppression. However, hypoxic responsiveness at 6 wk was not different from control levels. Naloxone had a small effect on ventilatory pattern at this time with no overall effect on hypoxic responsiveness. This was in contrast to previously demonstrated long-term suppression of CO2 sensitivity in this model, which was partially reversible by naloxone only during the immediate period after load imposition. Endogenous opioids apparently modulate ventilatory control acutely after load imposition. Their effect wanes with time despite persistence of depressed CO2 sensitivity.

Differential effects of hypothermia upon blood acid-base state and blood gases in sodium pentobarbital and urethane anaesthetised rats

1. The effects of two anaesthetics, sodium pentobarbital and urethane, and the effects of anaesthesia-associated hypothermia on acid-base status and blood gases were studied in rats without assisted ventilation. 2. Manipulation of conscious rats produces a progressive increase in arterial lactate associated with slight hyperventilation. 3. Sodium pentobarbital anaesthesia produces mild respiratory acidosis accompanied by increase in lactate arterial values. Urethane anaesthesia leads to partially compensated metabolic acidosis. 4. Hypothermia reduces metabolic acidosis and hypercapnia induced by sodium pentobarbital anaesthesia. No difference between hypothermic and normothermic values was observed in urethane anaesthesia.

Postnatal development of opioid regulation of micturition in the kitten

Endogenous opioids tonically regulate micturition in adult mammals. The present study sought to determine if opioids regulate micturition in neonatal kittens. Naloxone (up to 2 mg/kg given i.p. or i.v. to unanesthetized/ketamine-anesthetized or chloralose-anesthetized kittens, respectively), an opioid receptor antagonist, produced no effects in unanesthetized, ketamine-anesthetized, or chloralose-anesthetized kittens that had been prepared for bladder pressure recording, until 3 weeks of age. This indicates that endogenous opioids are not tonically regulating micturition in neonatal kittens. From 3 weeks up to at least 6 weeks of age, naloxone (100 micrograms/kg i.p. or i.v.) weakly facilitated bladder activity by transiently increasing the amplitude and/or duration of bladder contractions, but no effects on frequency of contractions was recorded. Morphine (up to 2 mg/kg given i.p. or i.v. to unanesthetized/ketamine-anesthetized or chloralose-anesthetized kittens, respectively), an opioid agonist, did not inhibit bladder contractions in unanesthetized or ketamine-anesthetized neonatal kittens, but it did inhibit (at a threshold dose of 100 micrograms/kg) and completely abolished (at a dose of 300 micrograms/kg) bladder activity in chloralose-anesthetized kittens in a dose-dependent, naloxone-reversible manner. Surprisingly, following morphine administration to unanesthetized or ketamine-anesthetized neonatal kittens, naloxone now produced an adult-like enhancement of bladder activity. These latter results indicate that opioid receptors, whose inhibitory effects are anesthetic-dependent, are present along the micturition reflex pathway in neonates. Immunohistochemical studies of the sacral spinal cord revealed that opioid peptides are distributed similarly in neonatal and adult cats.

Opiate detoxification under general anesthesia by large doses of naloxone

For opiate detoxification 6 volunteer opiate addicts were intravenously administered 10 mg naloxone within one hour while under barbiturate anesthesia. During administration of naloxone none of the patients demonstrated significant changes in the hemodynamic parameters of heart rate, mean arterial pressure, cardiac index, peripheral resistance or in the oxygen saturation. After patients awoke from anesthesia, they experienced no or only minimal withdrawal symptoms. Possible explanations for the suppression of withdrawal symptoms are discussed.

Naloxone accelerates the rate of ventilatory acclimatization to hypoxia in awake rats

During ventilatory acclimatization to hypoxia in rats, PaCO2 progressively falls from about 40 torr in normoxia (PIO2 approximately equal to 150 torr) to a new steady-state at about 23 torr in chronic hypoxia (24 or more hours at PIO2 approximately equal to 90 torr). In acute (20 or 60 minutes) hypoxia naloxone treatment caused a hyperventilation greater than that caused by acute hypoxia alone. Following 20 minutes hypoxia, naloxone treated rats had a PaCO2 = 28.6 +/- 0.7 torr (mean +/- 95% confidence limits) which was significantly lower (P less than .001) than the saline treated PaCO2 = 31.0 +/- 0.6 torr. In contrast, in normoxia and at 24 hour hypoxia and at 20 minute return to normoxia following 24 hours hypoxia, naloxone treatment had no effect on PaCO2. We conclude that in the rat about one third of the ventilatory acclimatization to hypoxia is due to a progressively decreasing endogenous opioid-like inhibition of ventilation.