Conditioned behavioral and physiological changes associated with injections of a narcotic antagonist in morphine-dependent monkeys

Environmental stimuli which are repeatedly associated with the nalorphine-induced withdrawal syndrome in morphine-dependent monkeys acquire the ability to produce a variety of conditioned behavioral and physiological responses. Morphine-dependent rhesus monkeys were studied under a fixed-ratio schedule where every tenth lever press produced a food pellet. After several pairings of a stimulus (light or tone) with intravenous injection of a dose of nalorphine which produced an immediate and severe withdrawal syndrome, onset of the stimulus alone produced conditioned suppression of lever pressing, heartrate decrease, vomiting and salivation. Conditioned suppression of responding and conditioned heart-rate changes persisted in post-dependent monkeys for one to four months after termination of chronic morphine treatment. No conditioned electrocardiogram, respiration or temperature changes were ever seen. A second group of morphine-dependent rhesus monkeys was studied under a schedule where every lever press produced an intravenous injection of morphine. After 10 pairings of a light with the intravenous injection of a dose of nalorphine which produced marked withdrawal signs and increased responding for morphine, presentation of the light and injection of sahne produced conditioned increases in responding for morphine. A third group of morphine-dependent rhesus monkeys was studied under a schedule where every nth lever press (n = 1 to 10) terminated a stimulus light associated with periodic injections of nalorphine or naloxone; lever-press responding was engendered and subsequently maintained. Thus, stimuli associated with the nalorphineor naloxone-induced withdrawal syndrome can either suppress, enhance or maintain behavior depending on the schedule conditions.     

Associative regulation of Pavlovian fear conditioning: unconditional stimulus intensity, incentive shifts, and latent inhibition.

Conditional stimuli (CS) associated with painful unconditional stimuli (US) produce a naloxone-reversible analgesia. The analgesia serves as a negative-feedback regulation of fear conditioning that can account for the impact of US intensity and CS predictiveness on Pavlovian fear conditioning. In Experiment 1 training under naloxone produced learning curves that approached the same high asymptote despite US intensity. Shifting drug treatment during acquisition had effects that paralleled US intensity shifts. In Experiment 3 naloxone reversed Hall-Pearce (1979) negative transfer using a contextual CS, indicating that conditional analgesia acquired during the CS-weak-footshock phase retards acquisition in the CS-strong-footshock phase. Experiment 5 used a tone CS in both a latent-inhibition and a negative-transfer procedure. Only negative transfer was blocked by naloxone. Therefore, negative transfer but not latent inhibition is mediated by a reduction of US processing.     

Tail-pinch hyperalgesia and analgesia: Test-specific opioid and nonopioid actions

Reactivity to noxious stimuli in rats is altered following acute exposure to tail-pinch. However, while our laboratory has reported that tail-pinch produces hyperalgesia as measured by the flinch-jump test and attenuates analgesic responses following morphine and cold-water swims, others have found that tail-pinch elicits an opioid-sensitive analgesia on the hot plate test and a nonopioid-sensitive analgesia on the writhing test. The first experiment of the present study examined whether tail-pinch altered responses on two somatic pain tests and showed that tail-pinch significantly decreased both jump thresholds and tail-flick latencies. In assessing whether tail-pinch hyperalgesia on the jump test was mediated by endogenous opioids, the second experiment indicated that low (0.1 and 0.5 mg/kg) doses of naloxone eliminated tail-pinch hyperalgesia by selectively lowering control thresholds and a high (10 mg/kg) dose of naloxone eliminated tail-pinch hyperalgesia by selectively increasing thresholds following tail-pinch. Further, the third experiment showed that morphine-tolerant rats (10 mg/kg morphine daily over 14 days) did not exhibit tail-pinch hyperalgesia on the 15th day, an effect attributable to a selective lowering of control thresholds. The fourth experiment was a direct replication of the observation that tail-pinch produces analgesia on the writhing test which is not antagonized by naloxone. These results demonstrate that the pain test employed and the amount of prior tail-pinch experience are critical variables in determining the direction of tail-pinch effects upon pain perception in rats.     

吗啡对不同距离条件下大鼠信号追踪和目标追踪的影响

条件刺激短暂呈现并消失后, 奖赏立即呈现, 多次匹配后诱导出动物对条件刺激(信号追踪)或奖赏呈现装置如食盒(目标追踪)的接近。条件刺激与食盒间的距离是影响信号/目标追踪反应和损害联结学习的重要变量, 成瘾药物能够增加奖赏的诱因动机, 进而增加个体的奖赏寻求行为。距离能否通过损害联结学习而减弱成瘾药物的动机放大作用尚未见到报道。本实验采用autoshaping模型, 考察8、30和60 cm距离条件下吗啡处理对大鼠信号追踪和目标追踪的影响。结果发现：(1)信号追踪随距离增加而减少, 目标追踪对距离不敏感。(2)急性吗啡处理减少8、30和60 cm条件下信号追踪而增加8和60 cm条件下目标追踪, 慢性吗啡处理在8和30 cm条件下减少信号追踪增加目标追踪; 消退检测中, 吗啡前暴露减少8和60 cm条件下信号追踪而增加60 cm条件下目标追踪。(3)辨别反转学习中, 吗啡前暴露使30和60 cm条件下的大鼠偏爱旧信号、辨别力受损, 减少8、30和60 cm条件下大鼠对新信号的接触。这些结果提示, 距离较少影响吗啡的信号追踪抑制作用和目标追踪增强效应, 而易化吗啡前暴露对反转学习的损害。说明距离是易化成瘾药物对联结学习不利影响而非反转其动机放大作用的重要因素。     

Effects of morphine and naloxone upon the reactions of rats to a heat stressor

Five experiments used rats to examine the conditioned hypoalgesia induced by exposure to a heated floor. Experiments 1 and 2 demonstrated that this hypoalgesia is mediated by non-opioid mechanisms of pain control, as evidenced by insensitivity to the opioid antagonist naloxone and by the absence of cross-tolerance with the opioid agonist morphine. Although non-opioid in nature, the acquisition of conditioned hypoalgesia was facilitated by naloxone and impaired by morphine (Experiments 3 and 4). These effects did not appear to be due to an opioid regulation of pain. (1) Pairing morphine with the heated floor attenuated acquisition in drug-tolerant rats. (2) This attenuation by morphine was removed when naloxone was given after exposure to the heated floor. (3) Conditioning was facilitated when naloxone was given after exposure to the heated floor (Experiment 5). The results were discussed in terms of an opioid regulation of (a) surprise, (b) arousal of an aversive motivational system, and (c) the affective component of pain.     

Role of conditioned reinforcers in the initiation,maintenance and extinction of drug-seeking behavior

The development of a secondary reinforcer as a result of associating a neutral stimulus (buzzer) with intravenous (IV) doses of morphine was studied in rats. Secondary reinforcement developed in the absence of physical dependence and followed the association of the stimulus with either response-contingent or non-contingent injections of morphine. Strength of the conditioned reinforcer, measured in terms of responding on a lever for the stimulus plus infusion of saline solution, was proportional to the unit dosage of morphine employed in pairings of buzzer and drug. When extinction of the lever-press response for IV morphine was conducted (by substituting saline for morphine solution) in the absence of the conditioned reinforcing stimulus, it was seen later that the stimulus could still elicit lever responses, until it too had been present for a sufficient interval of non-reinforced responding. Similarly, extinction of the response for morphine by blocking its action with naloxone in the absence of the stimulus did not eliminate the conditioned reinforcement. Another study showed that a passive, subcutaneous (SC) dose of morphine served to maintain lever-pressing on a contingency of buzzer plus sahne infusion. Furthermore, the stimuli resulting from the presence of morphine (after a SC injection) were able to reinstate the lever-responding with only the buzzer-saline contingency when such responses had previously been extinguished. Moreover, it was shown thatd-amphetamine could restore responding under the same conditions, and that morphine could also do so for rats in which the primary reinforcer had beend-amphetamine. It is suggested that animal data such as these show that procedures designed for the elimination of human drug-taking behavior must take into account secondary reinforcers as well as the primary reinforcer(s).     

Conditioned increases in locomotor activity produced with morphine as an unconditioned stimulus, and the relation of conditioning to acute morphine effect and tolerance

Rats administered 5 mg/kg morphine SO4, through subcutaneously implanted catheters, during each of several daily sessions in an open field showed a progressive increase in locomotor activity measured in the open field prior to each morphine administration. Since the increases in activity were not observed in rats given morphine in a different environment (home cage) and saline in the open field, it is concluded that the increases were due to conditioning. In addition, the increases in activity were retained over a 7-day rest period; they were also produced when a second opiate (5 microgram/kg etorphine HCl) was substituted for morphine, were not seen when 2 mg/kg naloxone HCl (ip) was administered during treatment, and were present in rats showing tolerance to opiate-produced hypoactivity. Morphine's direct effect on activity is believed to have a biphasic dose-response curve; therefore, the relation of dose to conditioning was also studied. Increases in activity were the only conditioned behaviors observed; they were present only at the higher doses (16, 4, and 1 vs. .25, .065, and 0 mg/kg), and they occurred whether or not the dose was associated with unconditioned hypoactivity. The discussion deals with the relation of conditioning and morphine tolerance, the question of whether the unconditioned stimulus of morphine conditioning is a compensatory or a direct effect of morphine, and the similarity of conditioned increases in activity produced by morphine and by other stimuli that are reinforcing.     

Associative morphine tolerance in the rat: examinations of compensatory responding and cross-tolerance with stress-induced analgesia

The results of three experiments designed to examine the processes subserving associative tolerance to morphine's analgesic effects, as assessed by the tail-flick test, are reported. The experiments indicated that associative tolerance in male Holtzman rats was not subserved by conditioned compensatory responses but was cross-tolerant with an apparently nonopioid form of stress-induced analgesia (SIA). Experiment 1 showed that rats tested for morphine analgesia in a distinctive context that had been paired previously with morphine injections (5 mg/kg) were more tolerant than animals that had had this context explicitly unpaired with a series of morphine injections or animals that were drug-naive. There was no evidence of a conditioned compensatory response of hyperalgesia in animals given a saline injection in the presence of environmental stimuli that had been previously paired with morphine, even under test conditions designed to minimize the possibility of floor effects that might have obscured the detection of drug-compensatory hyperalgesia. Experiment 2 demonstrated that the footshock procedures used for stress induction produced an SIA that was not attenuated by naloxone (10 mg/kg). Experiment 3 replicated the associative tolerance phenomenon of Experiment 1 and again failed to find evidence of conditioned compensatory responses. It was found that animals exposed to footshock in the context that had been associated with morphine administration developed significantly less SIA than control animals. The relevance of these findings for associative models of drug tolerance is discussed.     

Anterograde amnesia for Pavlovian fear conditioning and the role of one-trial overshadowing: effects of preconditioning exposures to morphine in the rat

Four experiments studied anterograde deficits in Pavlovian fear conditioning following prolonged exposure to the mu-opioid receptor agonist morphine. Injections of morphine produced temporally graded anterograde amnesia characterized by deficits in contextual and conditioned-stimulus (CS) conditioning 1 or 7 days and selective impairment in CS conditioning 21 days after last injection. This anterograde deficit in conditioning did not recover across a retention interval, was absent when rats were tested immediately after conditioning, and required the presence of an auditory CS. These results suggest that anterograde deficits in Pavlovian fear conditioning emerged from differences in susceptibility to 1-trial overshadowing of context by CS.     

Transfer across unconditioned stimuli in serial feature discrimination training

The transfer of conditioned modulation across conditioned stimuli (CS) and unconditioned stimuli (US) was examined in 3 experiments that used Pavlovian appetitive training procedures with rats. In Experiment 1, after training in a positive patterning discrimination (X-->A+/X-/A-), X increased conditioned responding elicited by another trained-then-extinguished CS as long as that CS had been trained with the same US as was used in discrimination training. In Experiment 2, after training with a feature-negative discrimination (X-->A-/A+), X inhibited conditioned responding elicited by another trained-then-extinguished CS as long as that CS had been trained with the same US. Experiments 1 and 2 used a between-groups design, whereas Experiment 3 used a more powerful within-groups design. In Experiment 3, rats were trained in a feature-positive discrimination (X-->A+/A-). In transfer tests, X increased conditioned responding elicited by another CS trained then extinguished with the same US from training. This increase was greater than the X increased conditioned responding elicited by another CS trained then extinguished with a different US from training. The results supported the suggestion that features trained in serial discrimination tasks influence behavior indirectly by transiently raising or lowering the threshold for activation of the US representations by its target stimuli and by any other stimuli that may be associated with that US. Other interpretations of the findings were also considered.     

Conditioning of striatal dopamine metabolism with methadone,morphine or bulbocapnine as an unconditioned stimulus

Psychotropic drugs such as methadone, morphine and bulbocapnine produce increments in dopamine metabolism as an unconditioned reflex. When a buzzer noise is used as a conditioned stimulus (CS) with these drugs as unconditioned stimuli, the buzzer CS acquires the properties of the drugs in increasing dopamine metabolism. These results suggest that the brain, like other visceral organs, can be conditioned in terms of neurotransmitter release or metabolism.     

Tolerance to the antinociceptive effect of intrathecal morphine in intact and chronic spinal rats

The antinociceptive effect of daily acute intrathecal morphine injections on the tail-flick withdrawal response was compared in Intact rats and rats that were spinally transected 3 to 4 weeks prior to morphine administration (Spinal rats). Spinal rats became tolerant to repeated intrathecal injections of 5, 15, or 30 micrograms of morphine within 3 days. Intact rats were not tolerant to these doses after 4 daily injections. Spinal rats, made tolerant to repeated intrathecal morphine injections, were not tolerant to a subcutaneous injection of 6.0 mg/kg of morphine. These data suggest that, in intact animals, tolerance to the antinociceptive effect of spinal morphine is modulated by descending, supraspinal input.     

Some effects of exposure to a heat stressor upon the rat's subsequent reactions to that stressor

In six experiments, rats were placed on a heated plate on two occasions (test and retest), and the latencies with which they licked their paws were taken as an index of their sensitivity to nociceptive stimulation. Experiment 1 provided evidence that rats were selectively analgesic on retest depending upon the intensity of the heat experienced on the initial test. Experiment 2 showed that this analgesia was recruited rapidly and was long-lasting, as it was observed when retest was scheduled as early as 0.2 and as late as 48 h after the initial exposure to the hot-plate. Experiment 3 documented a role for conditioning processes, as this analgesia was removed by an extinction-like procedure conducted between test and retest. Experiments 4 and 5 provided evidence for a non-opioid mechanism of pain control, because the analgesia was not diminished by the opioid antagonist, naloxone, nor by a history of morphine injections. These experiments also revealed that the analgesia observed on retest was enhanced and reduced when rats were given naloxone and morphine, respectively, on the initial test. Finally, Experiment 6 showed that the analgesia on retest summated with that produced by morphine. The results were taken to mean that the hot-plate assay for analgesia can itself activate endogenous mechanisms of pain control, and some speculations were offered as to how this occurred.     

Startle responding and context conditioning

The possibility that acoustic startle stimuli could support a conditional response (freezing) to contextual stimuli was investigated. Rats were exposed to three acoustic startle stimuli on the first day, and one on the second day. On day 1, 20 rats received naloxone pretreatment and another 20 received saline (placebo) pretreatment. Half of each group received a high-intensity acoustic stimulus, the other half a low-intensity acoustic stimulus. Both the higher stimulus intensity and the naloxone pretreatment led to greater freezing behavior during the 3-minute test period before the single startle stimulus on day 2. These findings support the notion that increased actual or perceived intensity of the acoustic startle stimulus increases conditioning to contextual stimuli as indexed by freezing behavior.     

Selective antagonism of the error-increasing effect of morphine by naloxone in a repeated-acquisition task.

Pigeons acquired a different four-response chain each session by responding sequentially on three keys in the presence of four colors. The response chain was maintained by food presentation under a fixed-ratio schedule. Errors produced a brief timeout but did not reset the chain. When either morphine or naloxone was administered alone, the overall response rate decreased with increasing doses. The rate-decreasing effect was accompanied by an increase in percent errors with morphine but not with naloxone. Both effects of morphine were antagonized by doses of naloxone that were ineffective when given alone. The antagonism was selective in that naloxone (3 mg/kg) completely blocked the error-increasing effect but not the rate-decreasing effect of the higher doses of morphine. The view that naloxone is a specific narcotic antagonist was supported by the finding that naloxone failed to antagonize the rate-decreasing and error-increasing effects of d-amphetamine, pentobarbital, and phencyclidine.     

Pavlovian second-order conditioned analgesia

Three experiments with rat subjects assessed conditioned analgesia in a Pavlovian second-order conditioning procedure by using inhibition of responding to thermal stimulation as an index of pain sensitivity. In Experiment 1, rats receiving second-order conditioning showed longer response latencies during a test of pain sensitivity in the presence of the second-order conditioned stimulus (CS) than rats receiving appropriate control procedures. Experiment 2 found that extinction of the first-order CS had no effect on established second-order conditioned analgesia. Experiment 3 evaluated the effects of post second-order conditioning pairings of morphine and the shock unconditioned stimulus (US). Rats receiving paired morphine-shock presentations showed significantly shorter response latencies during a hot-plate test of pain sensitivity in the presence of the second-order CS than did groups of rats receiving various control procedures; second-order analgesia was attenuated. These data extend the associative account of conditioned analgesia to second-order conditioning situations and are discussed in terms of the mediation of both first- and second-order analgesia by an association between the CS and a representation or expectancy of the US, which may directly activate endogenous pain inhibition systems.     

Effects of an infusion of morphine into the accumbens nucleus upon acquisition of hypoalgesic and fear responses in rats exposed to a heat stressor

Four experiments examined the effects of an infusion of morphine into the accumbens nucleus upon the aversive conditioning that can occur in rats exposed to the heated floor of a hot-plate apparatus. An infusion of morphine into the accumbens nucleus but not into the caudoputamen or into the prefrontal cortex impaired the acquisition of a conditioned hypoalgesic (Experiment 1) and fear (Experiment 4) response. This impairment was dose-dependent (Experiment 2) and mediated by opioid receptors in the accumbens nucleus, because it was removed by a systemic (Experiment 3a) or by an accumbal (Experiment 3b) infusion of naloxone. The results were attributed to an antagonism between the reinforcement process for aversive conditioning and the appetitive properties of an accumbal infusion of morphine.     

The effects of adenosine receptor agonists and antagonists on morphine state-dependent memory of passive avoidance

Pretraining administration of morphine (5 mg/kg, intraperitonically) in a step-down passive avoidance task led to state-dependent learning with impaired retrieval on the test day that was dose-dependently restored by pretest administration of morphine (0.5, 1, 3, and 5 mg/kg). This restoration was reversible by pretest naloxone administration. Pretest administration of adenosine receptor antagonists theophylline or 8-phenyltheophylline (8-PT) did not alter morphine-induced amnesia. However, both the antagonists inhibited the restoration of memory by pretest morphine (5 mg/kg). Adenosine A(1) receptor agonists N(6)-cyclohexyladenosine (CHA) or N(6)-phenylisopropyladenosine (R-PIA) only at the higher doses used, and adenosine A(2) receptor agonist 5'-N-ethylcarboxaminoadenosine (NECA), at all doses used, decreased morphine-induced amnesia in a dose-dependent manner. Pretest administration of low doses of CHA, R-PIA, or NECA significantly showed additive effects with low dose pretest morphine (1 mg/kg) in restoring memory. The promnestic effects of high-dose CHA and R-PIA were inhibited by theophylline or 8-PT but not by naloxone. The additive effects of low-dose CHA or R-PIA and morphine were inhibited by theophylline, 8-PT, or a higher dose of naloxone. The promnestic effect of NECA and its additive effect with low-dose morphine were both inhibited by theophylline and naloxone but not by 8-PT. It is concluded that activation of the adenosinergic system, through both A(1) and A(2) receptors, can reverse morphine-induced amnesia and is involved in morphine state of memory.     

Pontine stimulation overcomes developmental limitations in the neural mechanisms of eyeblink conditioning

Pontine neuronal activation during auditory stimuli increases ontogenetically between postnatal days (P) P17 and P24 in rats. Pontine neurons are an essential component of the conditioned stimulus (CS) pathway for eyeblink conditioning, providing mossy fiber input to the cerebellum. Here we examined whether the developmental limitation in pontine responsiveness to a CS in P17 rats could be overcome by direct stimulation of the CS pathway. Eyeblink conditioning was established in infant rats on P17-P18 and P24-P25 using pontine stimulation as a CS. There were no significant age-related differences in the rate or level of conditioning. Eyeblink conditioned responses established with the stimulation CS were abolished by inactivation of the ipsilateral cerebellar nuclei and overlying cortex in both age groups. The findings suggest that developmental changes in the CS pathway play an important role in the ontogeny of eyeblink conditioning.     

Pavlovian inhibitory conditioning and tolerance to pentobarbital-induced hypothermia in rats

In this experiment we investigated inhibitory Pavlovian conditioning in the development of tolerance to pentobarbital-induced hypothermia. During an initial phase, one group of rats (discrimination group) received training in which, on alternate days, one conditional stimulus (CS+) was associated with administration of 30 mg/kg pentobarbital, and a different conditional stimulus (CS-) was associated with administration of physiological saline. During the phase, control groups received either exposure to both CSs but not the drug or to the drug but no CSs or to neither the CSs nor the drug. Subsequently, half the rats in each group received injections of pentobarbital in the presence of one of the CSs and the remaining half in the presence of the other CS. Rats from the discrimination group injected with pentobarbital in the presence of CS+ displayed the most tolerance (i.e., smallest drug effect), whereas rats from the discrimination group injected with pentobarbital in the presence of CS- displayed the least tolerance (i.e., greatest drug effect). The attenuation of tolerance seen in rats of the discrimination group injected in the presence of CS- provides evidence of inhibitory Pavlovian conditioning. Additional evidence of inhibitory conditioning was provided by the fact that CS2 enhanced the hypothermic effects of pentobarbital in the discrimination group, whereas CS1 attenuated these effects. Implications of the results for the nature of inhibitory conditioning are discussed.