198 Brain Research, 514 (1990) 198-205 Elsevier BRES 15375 Hypercapnia depresses nociception: endogenous opioids implicated Gregory D. Gamble and Richard J. Milne Department of Physiology, University of Auckland, (New Zealand) (Accepted 19 September 1989) Key words: Hypercapnia; Nociception; Opioid; Pain; Respiration Hypoventilation produces hypercapnia which can elevate pain thresholds. Hypercapnia is a potent stressor which releases catecholamines and activates the sympathetic nervous system. Some stressors produce analgesia by releasing endogenous opioids. To determine the roles of endogenous opioids and catecholamines in hypercapnic analgesia, we administered CO2 in the inspired gas mixture to conscious rats. CO2 in the range 5-10% elevated tail flick and leg flexion latencies 2- to 3-fold in both intact and spinalised animals. The effects on reflex latencies but not on paCO2 or pHa were blocked by naloxone (2 mg/kg), and were not present in morphine-tolerant animals. The effects were reduced by dexamethasone but were not changed either by adrenalectomy or by systemic guanethidine, propanolol or phentolamine. Hypercapnia delayed the onset of the late phase of behavioural responses to formalin injected into the plantar surface of the hindpaw. We conclude that moderate hypercapnia powerfully depresses flexor withdrawal responses to noxious stimuli, by a mechanism involving release of endogenous opioids but not systemic catecholamines. This effect may account in part for the elevation in pain threshold during hypoventilation. INTRODUCTION Hypoventilation raises the pain threshold in human subjects. This effect is thought to be due to hypercapnia, since breathing 10% 0 2 had little effect upon pain thresholds 39. Moderate hypercapnia depresses monosyn- aptic reflexes, reduces lumbar3'28 and phrenic 17 moto- neuronal excitability and has weak depressant effects on convergent neurones in the spinal cord of the decerebrate spontaneously respiring cat 5. Various in vitro studies have suggested possible mechanisms for these effects 4' 7,32,40; however, the relevance of these findings to pain is not clear and studies in conscious animals have not previously been available. The narcotic action of severe hypercapnia (paCO2 > 100 mm Hg) is thought to be due to changes in cellular metabolism, secondary to reductions in cerebral intracel- lular pH 29. However, it is known that even moderate hypercapnia (paCO2:40 + 8 to 90 + 9 mm Hg) is a potent stressor which releases catecholamines and activates the sympathetic nervous system 29'33'38. Various stressors pro- duce analgesia by releasing endogenous opioids 2°, raising the possibility that the analgesic effect of hypercapnia may also be opioid mediated. The present study was undertaken to determine whether moderate hypercapnia changes the responses to noxious stimuli in the conscious rat, and whether circu- lating catecholamines or endogenous opioids are respon- sible for the anti-nociceptive effects of CO 2. We have approached these questions by using a modified version of the tail flick test 9'25 while administering various concentrations of CO2 in the inspired gas mixture to conscious rats. These results have been presented in abstract form 24. MATERIALS AND METHODS Animals The subjects of this study were 72 female George River-Wistar rats weighing 200-250 g at the beginning of the experiments. The animals were housed 3 to a cage with free access to food and water. All animals had been habituated to the laboratory environmentover a period of at least 1 week 13. Where possible, animals were reused in different experiments. Analgesiometric tests Tail flick test. Animals were restrained with plugs of foam rubber in a clear perspex tube (8 cm x 2.5 cm i.d.) with the tail and legs protruding through openings in the cylinder. A focussed projector bulb was directed in turn to a 2 cm blackened portion of the tail 3 cm from its tip, or to one or other blackened hind paw. The intensity of the stimulus was adjusted to produce withdrawal and tail flick latencies of 3-5 s. Respiratory rates were measured visually immediately before each tail flick test. Formalin test. A 0.05 ml bolus of 10% formal saline was injected into the plantar surface of one hind paw, and the animal placed in a clear perspex chamber. Pain scores were derived for twelve 5-min blocks over a period of 1 h subsequent to the injection, while the chamber was continuously flushed with the appropriate concentra- tion of CO2 in 0 2. Behaviours were rated in terms of the amount of time spent emitting a rateable behaviour during sequential 5 rain periods as follows: 0, weight evenly distributed over both paws; 1, injected paw resting on the ground but bearing no weight; 2, injected paw elevated; 3, injected paw bitten. These scores were weighted and accumulated as described elsewhere6. In a few Correspondence: R.J. Milne, Department of Physiology, University of Auckland, Private Bag, New Zealand. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)