Local and remote modifications of nociceptive sensitivity during carrageenin-induced inflammation in the rat

The modifications of the threshold for vocalization induced by pressure on the paws (both hind paws and both forepaws) were monitored at different times (15 min-96 h) following intraplantar injection of the polysaccharide carrageenin in the rat. During the first 2 h following the carrageenin injection, a decrease in vocalization threshold was observed not only for the right, injected hind paw, but also, in some rats, on paws distant from the inflamed plantar region, especially the right forepaw. This hyperalgesic effect was suppressed by locally administered Xylocaine into the right hind paw. During the 4 days following the injection, the number of rats hyperalgesic in the injected paw progressively declined. Twenty-four hours after the carrageenin injection, only a few rats still presented a clear hyperalgesia in the non-injected paws.

Noradrenaline hyperalgesia is mediated through interaction with sympathetic postganglionic neurone terminals rather than activation of primary afferent nociceptors

In hyperalgesic states, observed commonly as a major symptom of tissue inflammation or after central or peripheral nerve injury, non-noxious stimuli produce pain and noxious stimuli are perceived as more painful than usual. The mechanisms underlying the generation of hyperalgesia are not known. In patients with causalgia (burning pain and severe hyperalgesia after a nerve injury) activation of sympathetic post-ganglionic neurones or application of noradrenaline to painful skin exacerbates pain and hyperalgesia while sympathectomy may afford complete relief. One suggestion is that noradrenaline released from sympathetic post-ganglionic neurons increases the discharge of damaged small-diameter afferents by a direct action on the primary afferents. Here we present a new model for noradrenaline-sensitive hyperalgesia and demonstrate that the site of action of noradrenaline is not on the primary afferents but rather is presynaptic on the sympathetic post-ganglionic terminals.

Hyperalgesia following ischaemia of the rat's tail

We have investigated the effects of ischaemia on responses to a subsequent noxious stimulus in rats. Tail flick latencies to a noxious thermal stimulus were determined by immersing the tail in water at temperatures ranging from 39 to 49 degrees C. We then produced ischaemia by occluding the blood supply to the tail; ischaemia was terminated at the first signs of an escape response. Tail flick latencies were recorded immediately after termination of ischaemia and at 30 min intervals for another 2 h. Each rat acted as its own control. Tail flick latency decreased after ischaemia; we found a decrease of about 39% immediately after ischaemia, at immersion temperatures above 39 degrees C. The duration of the hyperalgesia increased with increasing water temperatures. Thus noxious ischaemia of the rat tail induced hyperalgesia to subsequent noxious thermal stimuli. The hyperalgesia could have arisen through either central or peripheral mechanisms.

Hyperalgesia mediated by peripheral opiate receptors in the rat

Behavioural experiments were undertaken to investigate the possible functional significance of opiate receptors located at peripheral endings of primary sensory neurons. The responses of animals to noxious chemical stimuli applied to the ear (ear scratch test) were measured after local pretreatment of these areas with etorphine. Local etorphine administration produced a low dose hyperalgesia and high dose analgesia. Local as opposed to systemic effects of etorphine were inferred from the absence of effects on the contralateral vehicle-treated ear. Systemic administration of naloxone or of a quaternary opiate antagonist (MRZ 2663-BR), which is relatively ineffective in crossing the blood-brain barrier, blocked the low dose hyperalgesic effect of etorphine in the ear scratch test. As a test for the putative hyperalgesic function of peripheral sensory nerve opiate receptors, neonatal rats were treated with capsaicin (50 mg/kg s.c.) to destroy specifically the subpopulation of primary sensory neurons on which the peripheral opiate receptors are thought to be located, without markedly altering pain thresholds. As adults, these neonatally treated rats showed potentiated analgesic responses to systemic morphine, as would be predicted by central 'analgesic' opiate receptors now acting without opposition from peripheral 'hyperalgesic' opiate receptors. These findings suggest that opiate receptors on primary sensory neurons may mediate hyperalgesic functions and that endogenous opioids might normally play a role in the peripheral induction of irritation, inflammation and pain reactions.

Hyperalgesia induced by altered glycinergic activity at the spinal cord

Glycine or its receptor antagonist, strychnine, were administered perispinally to investigate their effect on nociceptive responses elicited by activation of various cutaneous receptors. Strychnine produced dose-dependent sensory and motor disturbances; 1 and 5 micrograms doses were sub-convulsive, eliciting recurrent episodes of coordinated grooming, scratching and biting at the skin, which persisted for approximately 10 minutes post-injection; higher doses (25 and 100 micrograms) increased the intensity and duration of these effects, and produced convulsive motor seizures. Motor disturbances were not elicited by glycine (5, 25, 100 and 400 micrograms). Strychnine treated rats, at all doses, vocalized consistently in response to light cutaneous stimulation; a significant proportion of glycine treated rats also vocalized, but were not as sensitive to mild stimulation. Skin hyperalgesia persisted for at least 30 minutes in both strychnine and glycine treated rats. Both strychnine and glycine significantly reduced vocalization thresholds to tail shock. However, no clear effect on tail flick latency was observed following either strychnine or glycine. These results indicate that glycinergic neurons contribute to the tonic regulation of nociceptive input at the spinal cord.

[Role of tricyclic antidepressants in the central regulation of hyperalgesia and stress analgesia]

The effect of hypophysectomy (HE) on pain thresholds was studied in female noninbred rats. Hyperalgesia was observed after HE since the first till the sixth day of the observation period. Droperidol (1 mg/kg i.p.) and amitryptyline (5 mg/kg i.p.) produced hyperalgesia in sham-operated rats, which was potentiated in hypophysectomized animals. In rats taken into the experiment 3 days after operation, no increase in the pain threshold was recordable during the 30-minute painful stress, and poststress autoanalgesia did not develop subsequently. The opposing data were obtained in sham-operated animals. On intraperitoneal administration of phentanyl (25 micrograms/kg) after the 30-minute painful stress hypophysectomized rats did not manifest any potentiation of its analgesic effect in contradistinction to sham-operated animals. Simultaneous administration of phentanyl at the same dose and melipramine (5 mg/kg i.p.) produced considerable potentiation of analgesia if administered after stress. In hypophysectomized rats, that effect was somewhat reduced.

Relevance of prostaglandins in migraine

Prostaglandins (PG), particularly PGE, may be linked to the pathophysiology of migraine in several important ways. PGE1 may "simulate" a migraine attack in healthy volunteers. PGE may be elevated in patients with migraine. In animal experiments and in human infusions, PGEs cause vasodilation and hyperalgesia, both typical reactions of inflammation. The view that vascular headache is an "inflammatory reaction" allows the best concept concerning the local role of PGs and the effectiveness of PG-inhibitors in the treatment of migraine. The local role of PGs may provide a common denominator in several hormonal, neural and other influences on vessels. The common triggers of a migraine attack like menstruation, alcohol and stress influence the PG-system and even the dietary reactions, hormonal influences, sleep and reserpine have some connections with the PG-system. A local role for PGs does not diminish the importance of other pathophysiological mechanisms operating during an attack. On the contrary, PGs may fill in gaps in our understanding of how the overt pain of attacks is produced.

Reflex neurogenic inflammation. I. Contribution of the peripheral nervous system to spatially remote inflammatory responses that follow injury

Recent studies of the mechanism of neurogenic inflammation have focused on the contribution of neuropeptides released from peripheral terminals of primary afferent sensory neurons. In this study we addressed the contribution of humoral and neural factors to the hyperalgesia and swelling that are produced contralateral to an injured hindpaw, a phenomenon which we refer to as reflex neurogenic inflammation. The contralateral inflammatory response develops gradually, over a period of hours, and shows no tachyphylaxis with repeated application of the same stimulus. Denervation of either limb significantly attenuated the contralateral responses. Selective lesions of small-diameter, presumed nociceptive afferent fibers with capsaicin, or of sympathetic postganglionic efferents by immunosympathectomy, also reduced swelling and hyperalgesia of the uninjured paw. Interruption of venous circulation to the injured limb by vein ligation did not alter the response in the contralateral paw. Taken together, these data suggest that reflex neurogenic inflammation is neurally mediated, via connections across the spinal cord.

Evidence for different mechanisms of primary and secondary hyperalgesia following heat injury to the glabrous skin

Characteristics of primary (within the area of injury) and secondary (outside the area of injury) hyperalgesia were determined after a heat injury applied to the glabrous skin of the hand in 8 human volunteers. The heat injury consisted of two burns (53 degrees C, 30 s) applied over an area 7.5 mm in diameter separated (centre to centre) by a 2 cm interval. Following the injury, a zone of hyperalgesia to mechanical stimuli measuring 20.1 +/- 3.6 cm2 (mean +/- SEM) developed in an area surrounding and including the burns. Within this zone, the pain threshold for mechanical stimuli decreased significantly by a similar amount for all areas tested (12.0 +/- 1.1 bars to 5.2 +/- 0.5 bars). Hyperalgesia to heat occurred only within the area of the burns. The heat pain threshold decreased and total ratings of heat pain increased significantly. In contrast, there was decreased pain to heat stimuli between the two burn sites, and no change in painfulness of the heat stimuli at other areas within the zone of hyperalgesia to mechanical stimuli. Particularly notable was the coexistence of hypalgesia to heat stimuli and hyperalgesia to mechanical stimuli in the uninjured region between the two burn sites. These results indicate that the characteristics of primary and secondary hyperalgesia differ and also suggest that the mechanism for hyperalgesia to mechanical and thermal stimuli differ.

Manipulation and pain tolerance. A controlled study of the effect of spinal manipulation on paraspinal cutaneous pain tolerance levels

The response of paraspinal cutaneous pain tolerance levels to spinal manipulation has not been studied in an experimental model. This paper proposes such a model of pain tolerance measurement and describes the results of a controlled study of 50 assymptomatic subjects. The group receiving a spinal manipulation demonstrated a 140% increase in local cutaneous pain tolerance levels which was statistically significant (p less than 0.05). This is consistent with previous hypotheses regarding the mode of action of manipulation in the relief of spinal pain.

Time-intensity profiles of cutaneous pain in normal and hyperalgesic skin: a comparison with C-fiber nociceptor activities in monkey and human

Contributions of evoked discharge in nociceptors with C-fibers to the temporal profiles of magnitude judgments of pain by humans were determined for heat stimulations of the skin before and after the development of hyperalgesia (increased sensitivity to pain) produced by a mild heat injury. Human subjects continuously rated the magnitude of pain evoked by short-duration heat stimuli of 39-51 degrees C delivered to the hairy skin of the arm or leg (calf or foot) before and after the development of hyperalgesia produced by a conditioning stimulus (CS) of either 50 degrees C for 100 s or 48 degrees C for 360 s. During heat stimulations of the leg in humans, magnitude judgments of pain were obtained simultaneously with recordings of evoked discharges in single C-fiber mechanoheat (CMH) nociceptive afferent fibers. Seven fibers were studied before and after the CS. In other experiments, magnitude ratings of pain evoked by heat stimulations of the arm were compared with heat-evoked discharges in 21 CMH nociceptive afferent fibers innervating the hairy skin of the wrist or hand in anesthetized monkeys. From CMH responses obtained in each species, median response latencies were calculated as well as poststimulus time (PST) histograms--the latter plotting mean frequency of discharge versus time during each stimulus. In these analyses, the times of action potentials in CMHs were calculated as they would occur at entry to the lumbar or cervical spinal cord in humans, taking into account the temporal dispersion that should occur because of differing conduction velocities. These results were then compared with response latencies for pain and the temporal profiles of pain ratings made by individual subjects. Comparisons were made for data obtained before the CS (normal skin) and those obtained 10 min after the CS in heat-sensitized (hyperalgesic) skin. For normal skin, PST histograms of mean frequencies of discharge were similar for CMHs with similar heat thresholds (41-43 degrees) in the anesthetized monkey and the awake human. Despite minor discrepancies, there were similarities in the changes in these histograms for monkey and human CMHs following heat sensitization after the CS. It was concluded that CMHs in monkeys and humans have similar response magnitudes and temporal profiles of response to heat. The major differences in the temporal profiles of CMH responses and human pain ratings were the latencies at which CMH responses and pain ratings began, reached maximum, and ended.(ABSTRACT TRUNCATED AT 400 WORDS)

Spread of saphenous somatotopic projection map in spinal cord and hypersensitivity of the foot after chronic sciatic denervation in adult rat

The left sciatic nerve was cut and ligated in adult rats (chronic denervation). Twenty-one days later the right sciatic nerve was cut and ligated (acute denervation). The somatotopic maps of the surviving intact saphenous nerves (left and right) were compared on day 21 by recording from single interneurons in the dorsal horn of the spinal cord. On the acute side saphenous mediated natural responses were observed only as far caudally as L3, while no natural responses were found in L4 and 5 (this silent zone in L4 and 5 had previously been sciatic territory). In contrast, after chronic sciatic denervation, L4 and L5 were not silent to natural stimulation as the saphenous natural responses had spread into the sciatic territory. Saphenous inputs always won the sciatic territory in L4 and L5 over competing thigh afferents after chronic sciatic denervation. Electrical stimulation of the saphenous nerve on the acute side produced unit responses all the way down to S1 (with no silent areas in L4 and 5). These electrically evoked unit responses in L4 to S1 of the acute side were called 'long range' pathways. There were no differences in 'long range' electrical responses in the acutely and chronically denervated cord. The caudal boundary for electrically evoked saphenous responses was S1 on both sides of the cord, and post stimulus histograms of unit responses were not statistically different on the two sides. Thus after chronic sciatic denervation natural responses mediated by saphenous spread caudally into sciatic territory, but electrically evoked responses did not change. Behaviorally, there was the expected spread of saphenous mediated responses from the medial toe and foot to more lateral regions after chronic sciatic denervation. Unexpectedly, there was an increase in sensitivity (hyperalgesia) of the medial toe and foot. We postulate that this increased sensitivity might be mediated by the spread of saphenous projections into L4 and L5 of the cord after chronic sciatic denervation. Perhaps post traumatic neuralgia in humans could be due to increased number of spinal cord cells responding to stimulation of the receptive field of the surviving nerve.

Peripheral neural correlates of magnitude of cutaneous pain and hyperalgesia: simultaneous recordings in humans of sensory judgments of pain and evoked responses in nociceptors with C-fibers

The peripheral neuronal correlates of heat pain elicited from normal skin and from skin made hyperalgesic following a mild heat injury were studied by simultaneously recording, in humans, evoked responses in C mechanoheat (CMH) nociceptors and the magnitude estimations of pain obtained from the same subjects. Subjects made continuous magnitude ratings of pain elicited by short-duration stimuli of 39-51 degrees C delivered to the hairy skin of the calf or foot before and at varying intervals of time after a heat injury induced by a conditioning stimulus (CS) of 50 degrees C, 100 s or 48 degrees C, 360 s. The stimuli were applied with a thermode pressed against the nociceptor's receptive field. For heat stimulations of normal skin, that is, uninjured skin, pain thresholds in 14 experiments with nine subjects ranged from 41 to 49 degrees C, whereas response thresholds for most of the 14 CMH nociceptors were 41 degrees C (in two cases, 43 degrees C). The latter suggested that spatial summation of input from many nociceptors was necessary at pain threshold. An intensity-response function was obtained for each CMH by relating the total number of nerve impulses evoked per stimulus to stimulus temperature. A corresponding magnitude scaling function for pain was obtained by relating the maximum rating of pain elicited by each stimulus to stimulus temperature. The relation between the subject's scaling function and the intensity-response function of his CMH nociceptor varied somewhat from one experiment to the next, regardless of whether the results were obtained from the same or from different subjects. However, when averages were computed for all 14 tests, there was a near linear relationship between the mean number of impulses elicited in the CMHs and the median ratings of pain, over the range of 45-51 degrees C. It was concluded that the magnitude of heat pain sensation was more closely related to the magnitude of response in a population of CMH nociceptors than in any individual nociceptor. At 0.5 min after the CS, the pain thresholds of most subjects were elevated, and the magnitude ratings of pain elicited by supra-threshold stimuli were lower than pre-CS values (hypoalgesia). Corresponding changes were seen in the increased thresholds and decreased responses (fatigue) of most CMHs. By 5-10 min after the CS, the pain thresholds of most subjects were lower, and their magnitude ratings of suprathreshold stimuli were greater than pre-CS values (hyperalgesia).(ABSTRACT TRUNCATED AT 400 WORDS)

Information processing in cutaneous nociceptors in relation to sensations of pain

The phenomena of fatigue and sensitization have been observed in the responses to heat of A-fiber and C-fiber mechanoheat nociceptors (AMHs and CMHs, respectively). After heat stimulation that is sufficiently intense to sensitize AMHs, CMHs within the stimulated area become fatigued, which suggests a dominant role of AMHs in contributing to cutaneous hyperalgesia under these conditions. Mild heat injuries, on the other hand, not sufficiently intense to sensitize AMHs, result in a characteristic time course in development of sensitization of CMHs that is matched by similar changes in pain threshold and in magnitude ratings of pain. Although available evidence does not suggest a simple relation between the shapes of psychophysical magnitude scaling functions and the average responses of nociceptors, it does support the conclusions that CMHs encode the intensity of painful heating of skin and that the alterations in responses of CMHs after mild heat injury contribute to the observed changes in scaling functions during the development of hyperalgesia.

Nerve pathophysiology and mechanisms of pain in causalgia

In contrast to sensory endings in skin, muscle, etc., afferents in the mid-course of intact nerves are normally incapable of generating impulses upon slow or prolonged depolarization. However, after various types of nerve injury, including complete nerve section and local demyelination, an ectopic pacemaker capability develops. One peculiarity of such abnormal differentiated sites is chemosensitivity to alpha-adrenergic agonists and to sympathetic efferents discharge. Such ectopic chemosensitivity may well be involved in the etiology of paraesthesias and pain in reflex sympathetic dystrophies including causalgia. Specifically, it is proposed that the fundamental cause of these conditions is the development of abnormal electrogenic membrane properties in the region of demyelination and sprout outgrowth. These abnormal properties presumably include the appearance of excess inward current conductances and ectopic alpha-adrenergic receptors. Catecholamines released from sympathetic efferents in the area of injury locally depolarize damaged sensory fibers, and because of the abnormal electrogenic properties of these fibers, an abnormal afferent discharge is generated.

Hyperalgesia induced by non-noxious stress in the rat

Stress-induced analgesia is well known but the reverse phenomenon is poorly documented. In this work, hyperalgesia is described following stressful but non-noxious manipulations consisting of inescapable holding or exposure to a novel environment. Hypophysectomy (HX) and dexamethasone enhanced 'holding' hyperalgesia. In contrast, 'novelty' hyperalgesia was reduced by HX and not modified by dexamethasone. So, pituitary factors may respectively compensate and take part in stress hyperalgesia. Thus pain can be modulated in opposite directions by different types of stimulation: hyperalgesia would predominate after moderate, anxiogenic stress and analgesia after noxious and/or intense stress.

Myelinated nociceptive afferents account for the hyperalgesia that follows a burn to the hand

Monkeys and human subjects were exposed to a series of thermal stimuli before and after a 53 degrees C, 30-second burn to the glabrous skin of the hand. The responses of C- and A-fiber nociceptive afferents in the monkeys and subjective responses by the humans were compared. The burn resulted in increased sensitivity of the A fibers, decreased sensitivity of the C fibers, and increased pain sensibility (hyperalgesia) in the human subjects.

Prostaglandin hyperalgesia, IV: a metabolic process

Prostaglandin E2, prostacyclin and Db-cAMP injected into the rat paw induce hyperalgesia. This hyperalgesic effect of the prostaglandins but not of Db-CAMP was blocked by pre-treatment of the animals with cycloheximide. Prostaglandin hyperalgesia thus seems to be dependent on the triggering of some metabolic process which enhances the effects of physical or chemical stimuli.

Neurogram recording during acute inflammation in the rat hind paw

Electrophysiological recording was conducted using the intact tibiotarsal nerve of the left hind leg of adult male rats before and after subplantar injections of three different phlogistic agents or saline vehicle in the left hind paw. In three-hour recordings, peak neural activity as activated by 5 g of pressure to the injected paw was reached in 90 min (dextran), 120 min (brewer's yeast) and 150 min (carrageenan). The least neural hyperactivity was seen with dextran, while yeast and carrageenan produced comparably high degrees of hyperactivity. Neurograms were also studied in rats receiving orally administered prototype anti-inflammatory agents or chlorpromazine 1 hr before carrageenan was injected pedally. Chlorpromazine HCl pretreatment (100 mg/kg) exerted the greatest protective effect with the nadir of neural activity seen at 90-120 min. The nadir for phenylbutazone (100 mg/kg) was at 60-120 min, indomethacin (10 mg/kg) at 60-90 min and aspirin (300 mg/kg) at 45-90 min. Despite these temporal differences, the protective effects of these three anti-inflammatory agents were statistically equivalent at the doses tested. Hydrocortisone alcohol (20 mg/kg) provided a significant reduction in hyperactivity at 30-60 min but the duration was much shorter than the other agents which showed protective effects through to the end of the 180-min observation period.

"Prespondylosis" and some pain syndromes following denervation supersensitivity

Pain is determined by the neurologic properties of receptor organs, neurons, and their interconnections. These may become supersensitive or hyperreactive following denervation (Cannon's Law). A common cause of denervation in the peripheral nervous system is neuropathy or radiculopathy as a sequel to spondylosis. Spondylosis in its early stage may be "asymptomatic" or painless and hency unsuspected, because small-diameter pain fibers may not initially be involved despite the attenuation of the other component fibers of the nerve. The term "prespondylosis" is introduced here to describe this presently unrecognized phase of insidious attrition to the other functions of the nerve, especially the trophic aspect. It is postulated that many diverse pain syndromes of apparently unrelated causation may be attributed to abnormal noxious input into the central nervous system from supersensitive receptor organs (nociceptors) and hyperreactive control systems at internuncial pools. Furthermore, trauma to a healthy nerve is usually painless or only briefly painful, unless there is preexisting neuropathy. Some pain syndromes in muscle (eg, trigger points and myofascial pain syndromes) and nerve (eg, causalgia and diabetic neuropathy) that may be related to denervation are discussed.

Hyperalgesic effects of divalent cations and antinociceptive effects of a calcium chelator in naive and morphine-dependent mice

Using a modified tail-flick procedure we have found a highly significant hyperalgesic response in narcotic dependent mice peaking 12 hr following the removal of chronically implanted morphine pellets. Withdrawal-induced hyperalgesia correlated well with other signs of opiate withdrawal behavior. Intracerebroventricular injections of CaCl2, MnCl2 further enhanced the hyperalgesic response in morphine-dependent mice; morphine-dependent mice were more than twice as sensitive to calcium-induced hyperalgesia as placebo-treated controls. On the other hand, i.c.v. injections of the calcium-specific chelator EGTA produced highly significant, dose-dependent antinociceptive responses in both morphine-dependent and control mice, but morphine dependent mice were only half as sensitive to EGTA-induced analgesia as controls. Withdrawal hyperalgesia and EGTA analgesia may be directly related to changes in brain localization of calcium that have been reported previously; it is concluded that morphine and EGTA-induced analgesia may be associated with a calcium depleted state within a relatively small calcium pool of the nerve cell and that opiate withdrawal hyperalgesia is a sensitive measure of narcotic dependency that is likely associated with an increased Ca++ content in the same region.

Analgesic activity of diflunisal [MK-647; 5-(2,4-difluorophenyl)salicylic acid] in rats with hyperalgesia induced by Freund's adjuvant

A method is described for testing analgesia for narcotic or nonnarcotic drugs in rats injected with Freund's adjuvant in the tail, by manipulation of the tail the day after injection, or of the feet after the development of adjuvant arthritis. The method is responsive to a behavioral depressant or an anti-inflammatory steroid. Diflunisal (MK-647; 5-(2,4-difluorophenyl)salicylic acid] exhibited activity in this assay after oral administration with potency about 25 times greater than that of aspirin, about 3 times that of glafenine and twice that of zomepirac. The onset of activity was within a 1/2 hour for narcotic analgesics but required about an hour for non-narcotic compounds. With the latter, the peak of activity was not attained until 2 to 4 hr, depending on the compound. The peak for diflunisal was delayed until the 3rd or 4th hour, but the onset of action was more prompt and the duration greater as the dose was increased. [14C]Diflunisal was concentrated to some extent in the inflamed tissue after adjuvant injection. Peak levels both in plasma and tissue appeared about 2 hr before peak analgesic effect. Repeated administration of large doses produced neither tolerance nor sensitization to the analgesic action of diflunisal. Naloxone and naltrexone did not antagonize the action of the compound, but when morphine and diflunisal were given together, the overall effect was enhanced.

Hyperalgesia induced by naloxone follows diurnal rhythm in responsivity to painful stimuli

A diurnal rhythm was observed in the responsiveness of mice to nociceptive stimuli and in the hyperalgesic activity of endogenous opioid peptides and may partly account for previous controversy over the direct action of naloxone in opiate-naive animals.