Sodium pentobarbital: sensory and associative effects in classical conditioning of the rabbit nictitating membrane response

Four experiments were conducted to determine the effects of sodium pentobarbital (0, 3, 9, and 15 mg/kg) on the acquisition of the rabbit's classically conditioned nictitating membrane response (NMR) and to determine the locus of the drug's effects on sensory, motor, associative, and nonassociative processes. In experiment 1, classical conditioning of the NMR was accomplished by pairing tone and light conditioned stimuli (CSs) with paraorbital shock as the unconditioned stimulus (US). The experiment revealed that pentobarbital retarded the acquisition of conditioned responses (CRs) to both tone and light CSs. Experiment 2, employing unpaired CS, UCS presentations, indicated small but significant drug effects on NMR base rate and nonassociative NMRs to the CS. Experiment 3 revealed no significant drug effect on the psychophysical functions relating UCS intensity to UCR frequency or amplitude, nor on the UCS intensity threshold for eliciting UCRs. On the other hand, in experiment 4, the drug significantly impaired CR frequency over an extended range of CS intensities and raised CS intensity threshold. It was concluded that pentobarbital's attenuation of CS intensity also operated to impair CR acquisition.     

Sensory and associative effects of LSD on classical appetitive conditioning of the rabbit jaw movement response

Three experiments were conducted to determine the effects of LSD (30 nmol/kg) in classical appetitive conditioning of the rabbit jaw movement response (JMR). In experiment 1, LSD significantly enhanced the acquisition of conditioned responses (CRs). The performance of control groups receiving unpaired presentations of the conditioned stimulus (CS) and unconditioned stimulus (UCS) demonstrated that LSD's enhancing effect on conditioning could not be attributed to an elevation in baseline responding, sensitization, or pseudoconditioning. Accordingly, experiments 2 and 3 were conducted to determine whether LSD's enhancement of conditioning could have arisen from its altering the sensory processing of either the CS or UCS, or botj. In experiment 2, LSD was found to have no significant effect on the functions relating UCS magnitude to the frequency, amplitude, or number of sinusoidal peaks comprising each unconditioned response (UCR). In contrast, experiment 3 revealed that LSD significantly enhanced the frequency of CRs to an extended range of CS intensities and lowered the CS intensity threshold. It was concluded that the enhancing effect of LSD on the acquisition of CRs is attributable, at least in part, to the drug's enhancement of the sensory processing of the CS.     

Effects of morphine and LSD on the classically conditioned nictitating membrane response

Two experiments were carried out to determine the effects of LSD and morphine on the unconditioned nictitating membrane response of the rabbit elicited by 5 intensities of a 100 msec puff of air directed at the cornea, and on the acquisition of conditioned responses to a tone and light conditioned stimulus using the air-puff as an unconditioned stimulus. In Experiment 1, LSD tartrate (0.013 mg/kg) had no effect of the frequency, amplitude, magnitude or latency of the unconditioned response. However, LSD significantly enhanced the rate of acquisition of conditioned responses to both tone and light conditioned stimuli. In Experiment 2, morphine sulfate (5 mg/kg) had no effect on the frequency, amplitude, magnitude or latency of the unconditioned response, but significantly retarded the acquisition of conditioned responses to both tone and light conditioned stimuli. The results indicated that the enhancement of acquisition produced by LSD and the retardation of acquisition produced by morphine were not due to effects of the drugs on either the sensory processing of the air-puff unconditioned stimulus or on the motoric expression of the unconditioned response.     

Behavioral effects of morphine and naloxone following chronic morphine administration

The effects of morphine, naloxone, and combinations of these drugs were examined in squirrel monkeys under shock-postponement schedules. In the absence of a lever press, shocks could be presented every 4s, and each response postponed shock for 20s. Acutely, morphine (0.10–3.00 mg/kg) produced not only overall response-rate decreases, but also increases in the number of shocks, whereas naloxone (0.10–30.00 mg/kg) had little effect on responding. When given in combination with morphine, several doses of naloxone antagonized the rate-reducing and shock-increasing effects of morphine. Daily administration of morphine resulted in a substantial decrease in the number of shocks received and a moderate attenuation of the rate-decreasing effects of morphine (tolerance). Lower doses substituted for the fixed daily dose resulted in a smaller effect on behavior than under acute administration. Naloxone given in combination with the daily morphine dose or substituted for the daily administration of morphine, produced effects similar to those seen prior to chronic drugging. Thus, behavioral effects of naloxone were not altered even though tolerance to morphine was observed. Larger doses of naloxone continued to antagonize the effects of morphine for at least 24h. No signs of physical dependence were noted when naloxone was administered or when administration of morphine ended.     

Expression of morphine-conditioned hyperactivity is attenuated by naloxone and pimozide

The present experiments assessed whether morphine-conditioned hyperactivity could be attenuated by either the opiate antagonist naloxone or the dopamine antagonist pimozide. Both of these antagonists were shown to block the unconditioned hyperactivity induced by 2 mg/kg morphine (Experiment 1). Rats were then conditioned by pairing this dose of morphine repeatedly with a distinctive environment (Experiment 2). Following several drug-environment pairings, rats displayed a hyperactive conditioned response (CR) when exposed to the environment in the absence of the drug. CR expression was counteracted by 1 mg/kg naloxone and was attenuated by pimozide (0.25, 0.33, and 0.4 mg/kg) in a dose-related manner. These findings suggest that the unconditioned and conditioned hyperactive responses produced by morphine may involve similar neuropharmacologic substrates.     

Effects of MDA on classical conditioning of the rabbit nictitating membrane response

In Experiment 1, classical conditioning of the rabbit's nictitating membrane response (NMR) was accomplished by pairing tone and light conditioned stimuli (CSs) with a shock unconditioned stimulus (UCS). MDA impaired the acquisition of conditioned responses (CR) to a tone-CS, while significantly enhancing CR acquisition to a light-CS. Experiment 2, employing explicitly unpaired CS, UCS training, revealed no reliable effects of MDA upon nonassociative processes. Subsequent efforts determined if MDA's CR acquisition effects resulted from alterations in sensory processing of the CS, UCS, and/or UCR motor functioning. Specifically, it was determined that MDA: (a) increased the tone-CS intensity threshold for eliciting CRs (Experiment 3); (b) attenuated the tone-induced reflex modification of the unconditioned NMR (Experiment 4); and (c) enhanced UCR frequency at varying UCS intensities (Experiment 5). It was concluded that MDA's effect upon CR acquisition reflected the drug's effect upon CS and UCS/UCR processing and thereby altered the ability of these components of conditioning to enter into associative learning.     

Effects of morphine,ethylketocyclazocine,N-allylnormetazocine and naloxone on locomotor activity in the rabbit

Locomotor activity was studied in the rabbit following injections of morphine, ethylketocyclazocine andN-allylnormetazocine. All three drugs produced only depression of activity. The opioid antagonist naloxone antagonized the effects of both morphine and ethylketocyclazocine. Naloxone (0.1 mg/kg) did not antagonize the effects ofN-allylnormetazocine. Naloxone alone depressed locomotor activity at doses above 0.3 mg/kg. This effect of naloxone was partially antagonized by 0.1 mg/kg ethylketocyclazocine, but not by 0.1 mg/kg morphine. The GABA agonist muscimol (0.1 and 1.0 mg/kg) also did not antagonize the effect of naloxone on locomotor activity. Finally, amphetamine did not produce a great deal of locomotor activation in the rabbit, which may indicate that increasing activity in the rabbit by drug intervention may be inherently difficult. These results indicate that the opioids have effects in the rabbit that are clearly different from those observed in rodents, where morphine andN-allylnormetazocine have been reported to produce locomotor activation, and naloxone typically has little effect. In addition, the effects of the opioids on locomotor activity were clearly distinguishable from their effects on learning in the rabbit. While morphine and ethylketocyclazocine were approximately equipotent in depressing locomotor activity, morphine is much less potent than ethylketocyclazocine in retarding acquisition of the classically conditioned nictitating membrane response in the rabbit.     

Asymmetrical effects of morphine and naloxone on reward mechanisms

Rats with bilaterally implanted lateral hypothalamic electrodes were tested daily for self-stimulation to each side of the brain, and rotation (circling behavior) was recorded concomitantly. All rats rotated in a preferred direction regardless of the side of the brain stimulated and all rats had asymmetries in self-stimulation sensitivity related to the direction of rotation. Morphine increased rotation and lowered self-stimulation thresholds at low doses (e.g., 2.5 mg/kg) and decreased rotation and raised self-stimulation thresholds at high doses (e.g., 20.0 mg/kg). The changes in self-stimulation thresholds preferentially occurred on opposite sides of the brain, i.e., the low-dose decrease in thresholds was greater in the normally less sensitive side of the brain whereas the high-dose increase in thresholds was greater in the normally more sensitive side of the brain. Naloxone produced no changes in rates of rotation but did elicit small changes in self-stimulation that varied with the side of the brain, i.e., dose-related decreases in thresholds occurred in the normally more sensitive side of the brain whereas dose-related increases in thresholds occurred in the normally less sensitive side of the brain. Subsequently rats were tested in a choice procedure providing concurrent access to rewarding stimulation of either side of the brain; currents were titrated such that, under baseline conditions, rats continually alternated between self-stimulating one side of the brain or the other. Morphine induced a preference for the less sensitive side of the brain that was comparable in magnitude at all doses and independent of its biphasic effects on rates of responding. Naloxone induced a dose-related preference for the more sensitive side of the brain while not altering rates of responding. Naloxone (1.0 mg/kg) also completely antagonized the effects of all doses of morphine. The results are discussed in terms of lateralized actions mediating the discriminable effects of reinforcing drugs.     

Discriminative response control by naloxone in morphine pretreated rats

In an operant procedure using a lever press response 12 male, hooded rats were trained to discriminate 1.25 mg/kg naloxone from a saline injection. On certain days, according to a counterbalanced training schedule, naloxone was administered 8 h after 40 mg/kg morphine and 10 min prior to a trial in which food was available on an FR10 schedule from one of two levers in a dual lever operant chamber. On other days saline was administered 10 min prior to a trial in which food was made available by pressing the other lever. After criterion performance for acquisition of the discrimination had been reached, tests were carried out to determine its nature. Discrimination of naloxone was dose-dependent and was significantly diminished when naloxone was administered 36 h after morphine. Partial generalization of cyclazocine with naloxone was observed. Spontaneous withdrawal from morphine, tested during trials preceeded by an injection of saline instead of naloxone at various time intervals after morphine, did not generalize with the naloxone discriminative stimulus.     

The stinging response of the honeybee: Effects of morphine,naloxone and some opioid peptides

Changes in responsiveness for the stinging reaction of honeybees fixed in a holder after receiving 3 electrical shocks delivered with 1 min interval, was registered and used as measurement for the effect of 2 μl of different solutions injected. Every shock consisted of a train of pulses of 1 msec each, delivered for 2 sec at a frequency of 100 Hz. Injection of morphine-HCl (50 to 200 n-moles/bee) produced a dose dependent reduction of the honeybee stinging response to the electrical shocks. The morphine dose that produced a 50% inhibition of the response (D₅₀) was 148 n-moles/bee (927 μg/g), i.e., a value far greater than that reported for vertebrates in behavioral test of analgesia. Naloxone 1.1 μg/g produces a significant reduction of morphine D₅₀ effect and at 4–5 μg/g, a full disinhibition. Thus, whereas the D₅₀ of morphine for honeybees is far greater than that for vertebrates, the doses of naloxone that antagonize morphine are similar for bees and vertebrates. Possible explanations of this difference are mentioned. Injections of met-enkephalin, leu-enkephalin, kyotorphin and (D-Ala²) methionine-enkephalinamide, given in doses of 200 n-moles/bee, an amount greater than that of the morphine D₅₀, exhibited no effect on the stinging response.     

Brain acetylcholine in morphine pellet implanted rats given naloxone

Adult male rats were implanted with intraventricular (ivt.) brain cannulae for injection of 5 g of acetylseco-hemicholinium-3 (acetylseco HC-3) as a means of studying acetylcholine (ACh) utilization during morphine withdrawal. Animals were made dependent by implanting s.c. two 75 mg morphine base pellets 24 hrs apart. On the 4th day animals were given 10 mg/kg of naloxone i.p. and/or 5 g acetylseco HC-3 ivt. and sacrificed by decapitation at various times. The brains were removed and assayed for ACh using a pyrolysis gas Chromatographie procedure. Total brain ACh before or after acetylseco-HC-3 was not altered at 5, 30, 60 and 120 but was decreased at 10 min after naloxone. These results are in sharp contrast to our previous data of enhanced brain ACh utilization in withdrawn rats made dependent to morphine by several weeks of twice daily injections. It is apparent that short term morphine pellet administration does not produce the marked neurochemical and behavioral changes of long term morphine injections.Supported in part by grant DA 00830, USPHS.     

Behavioral effects of morphine, levorphanol, dextrorphan and naloxone in the frog Rana pipiens

Systemic morphine induces explosive motor behavior and generalized muscular rigidity in frogs. Naloxone does not reverse either of these effects of morphine but at high doses causes muscular flaccidity and unresponsiveness to stimulation. Intraspinal morphine induces rigidity, but not explosive motor behavior, and this action is blocked by naloxone. Behavioral effects are seen rarely after intraspinal levorphanol (rigidity) and never after intraspinal dextrorphan or naloxone. In contrast to systemic morphine and naloxone, systemic levorphanol and dextrorphan are lethal to frogs at high doses.     

Effect of naloxone and morphine on various forms of memory in the rat: Possible role of endogenous opiate mechanisms in memory consolidation

Naloxone, morphine, and morphine plus naloxone were injected IP into rats immediately after training in several different behavioral tasks, and their effect on retention was evaluated in test sessions carried out 1 or 7 days later. Naloxone (0.4 mg/kg) enhanced and morphine (1.0 mg/kg) depressed retention for a standard shuttle avoidance task acquired with contiguous tone-footshock presentations. Retention of this behavior is known to result from the independent operation of four memory channels (Izquierdo and Elisabetsky, 1978). In consequence, the effect of naloxone on each of those channels was investigated. The channel which stores responses acquired through an avoidance mode and makes them available for retrieval in the same mode, was facilitate by as little as 0.2 mg/kg of the drug. The other channels were sensitive only to 0.4 or 0.8 mg/kg naloxone. Retention of the habituation of a rearing response to a tone from one day to the next was also enhanced by naloxone (0.8 mg/kg) and depressed by morphine (1.0 mg/kg), and, again, there was an antagonistic interaction between the two drugs. The results suggest a role for endogenous opiate substances as general inhibitory modulators of memory consolidation, point to a differential sensitivity of the various memory channels to naloxone (therefore, presumably to endogenous opiate modulation), and demonstrate that the post-training effect of naloxone and morphine on memory (or the role of endogenous opiates thus suggested) does not depend on the presence of foot shocks in the training session to become manifest.     

Disruption of trace conditioning of the nictitating membrane response in rabbits by central cholinergic blockade

 Central muscarinic cholinergic involvement in classical conditioning of eyeblink responses was determined in trace and delay paradigms. Rabbits were trained on a trace procedure in which a 250-ms tone conditioned stimulus (CS) and a 100-ms air-puff unconditioned stimulus (UCS) were presented with a 500-ms trace interval. Each training session day consisted of ten tone alone, ten air-puff alone and 80 paired CS-UCS trials. Scopolamine hydrochloride at doses of 0.03 and 0.1 mg/0.5 ml per kg, SC dose-dependently disrupted acquisition of conditioned responses. Rabbits that were treated with scopolamine and failed to learn showed a gradual increase in conditioned responses during an additional training period with saline injections and no transfer from earlier training. Scopolamine methyl bromide, which does not appreciably cross the blood-brain barrier, showed no effects in the trace conditioning paradigm at a dose of 0.1 mg/kg, SC, indicating central cholinergic blockade is responsible for the suppressive effect of scopolamine. Scopolamine hydrochloride at a dose of 0.1 mg/kg, SC did not block acquisition in the delay procedure with a 250-ms inter-stimulus interval, although the rate of acquisition was somewhat reduced by the drug. These data are the first to demonstrate that classical conditioning of the eyeblink response in the trace procedure is highly sensitive to central cholinergic deficits. Received: 16 August 1996 / Final version: 14 January 1997     

Contribution of associative and nonassociative processes to the development of morphine tolerance

The contribution of associative and nonassociative processes to the development of tolerance to the analgesic effects of morphine in rats was investigated in two experiments. Associative contingencies were manipulated by administering a series of moderately high morphine doses (20 mg/kg) either explicitly paired or explicitly unpaired with a distinctive context. During distinctive context exposures, animals were placed for 60 min in plastic boxes located in a room adjacent to the colony room. The distinctiveness of this environment was enhanced by the presence of white noise and a pine scent. Nonassociative processes were manipulated by administering the morphine at either a very short (6 h) or relatively long (96 h) interdose-interval (IDI). Analgesia was measured on a tail-flick test. At the 96 h IDI, tolerance, as indexed by shifts in dose-response curves, was controlled primarily by associative processes. Associative control over tolerance at the long IDI was evident at an immediate test (experiment 1) and was retained for a 30 day interval (experiment 2). In contrast, tolerance that developed at the 6 h IDI was not influenced by associative contingencies at the immediate test (experiment 1) and showed no retention over a 30 day interval (experiment 2). These data suggest that tolerance that developed at the short IDI was nonassociative. Overall, the results indicate that conditions conducive to the development of non-associative tolerance disrupt the acquisition of associative tolerance. Hypotheses regarding the absence of associative effects at the short IDI are reviewed. Methodological implications of these results for evaluations of associative and nonassociative morphine tolerance are also discussed.     

Cross-tolerance of associative and nonassociative morphine tolerance in the rat with mu- and kappa-specific opioids

The present study examined the cross-tolerance profiles of associatively and nonassociatively morphine-tolerant rats with analgesia produced by morphine and fentanyl (mu-receptor agonists) and U50,488H (a kappa-receptor agonist). Subjects were given a series of eight morphine injections either paired or unpaired with a distinctive environment and then tested for tolerance using the tail-flick method. Evidence was found that nonassociative morphine tolerance, which was produced using a 6-h interdose interval (IDI), was receptor-specific, i.e. cross-tolerant with analgesia produced by mu-specific, but not kappa-specific drugs. Nonassociative tolerance was characterized by a shift to the right in dose-response curves of 0.32 log units in morphine-tested animals and 0.28 log units in fentanyl-tested animals. Conversely, associative morphine tolerance, which was produced using a 96-h IDI, evidenced a lack of receptor specificity by showing cross-tolerance to the analgesic effects of U50,488H. Associative tolerance was characterized by shifts of 0.42 log units in morphine-tested animals, 0.34 log units in fentanyl-tested animals, and 0.39 log units in U50,488H-tested animals. These results were interpreted as suggesting the mechanisms responsible for associative tolerance differ from those producing nonassociative tolerance. This conclusion is problematic for theories of learned tolerance that assume a unitary set of mechanisms subserving associative and nonassociative tolerance.     

Blockade of the discriminative effects of morphine in the rat by naltrexone and naloxone

The capacity of the specific narcotic antagonists naltrexone and naloxone to block the discriminative effects produced by morphine in the rat were evaluated using a two-choice, discrete-trial avoidance task. The antagonists produced a dose-dependent and time-dependent blockade of morphine's effects as measured by responding on the morphine-appropriate choice lever. Naltrexone and naloxone were equipotent when given subcutaneously concomitantly with subcutaneously administered morphine. However, when the antagonists were administered orally at 0, 2, 4 or 8 h prior to s.c. morphine, naltrexone was more potent that naloxone at every time point and had a duration of action at least twice that of oral naloxone. The discriminitive effects of the narcotic analgesics morphine and methadone were also compared after oral and subcutaneous administration. Both drugs produced dose-related discriminative effects and were one-tenth as potent by the oral as by the subcutaneous route of administration. These results suggest that the discriminative effects produced by morphine in the rat can provide an animal model for the quantitative evaluation of the narcotic antagonist properties of drugs that might be considered for use in narcotic antagonist maintenance programs for the treatment of narcotic addiction.Predoctoral trainee of the Public Health Service Graduate Pharmacology Training Grant 2T1 GM179. This project represents partial fulfillment of the degree of Doctor of Philosophy at Emory UniversityRecipient of Research Scientist Development Award K02 DA00008     

Enkephalin,morphine, and naloxone in tardive dyskinesia

Eight psychiatric patients with tardive dyskinesia (TD) were treated with single doses of the synthetic met-enkephalin analogue FK 33-824 (1, 2, and 3 mg IM) morphine (10 mg SC) and naloxone, an opiate receptor antagonist (0.8 mg IM). The drug effects were assessed by blind evaluation of randomly sequenced videotapes made before and during treatment. FK 33-824 (1,2, and 3 mg IM) slightly reduced TD (P<0.05) and increased preexisting bradykinesia. The effect on TD, however, was pronounced only in patients concurrently treated with neuroleptics in relatively high doses. Morphine had a similar although weaker antihyperkinetic effect, whereas naloxone had no effect. Side effects of FK 33-824 included dizziness, heaviness in the extremities, slurred speech, and dryness of mouth. Morphine caused drowsiness, dizziness, ataxia, and nausea, and naloxone had no side effects. The results do not point to a primary role of enkephalin in the pathophysiology of TD, but enkephalin may interact with dopamine functions and potentiate some of the effects of neuroleptic drugs.     

Effect of morphine and naloxone on intestinal transit in mice

Morphine caused a dose-dependent slowing of the rate of intestinal transit in mice. This inhibitory effect of morphine was antagonised by naloxone administration. Pretreatment with a single dose of morphine did not induce any detectable tolerance to the inhibitory effect of a second dose of morphine given 5 h later. However, naloxone was more effective in antagonising this inhibitory effect of morphine-pretreated mice than in saline-pretreated animals. Molecular sieve morphine pellet implantation for 24 h induced detectable tolerance to the inhibitory effect of morphine administered 3 h after removal of the pellet. In addition, the antagonistic effect of naloxone was also augmented when compared with blank pellet-implanted control animals. The present study has shown that the enhanced naloxone potency against the inhibitory effect of morphine on intestinal transit was observable before the development of overt tolerance, and that tolerance to the effect of morphine on the small intestine could be induced by implantation of a molecular sieve morphine pellet for 24 h.     

Effects of cocaine on conditioning of the rabbit nictitating membrane response

Two experiments were conducted to determine cocaine's (0, 1, 3, and 6 mg/kg) effects on associative, nonassociative, and motor processes in classical conditioning of the rabbit's nictitating membrane response (NMR). In Experiment 1, acquisition training consisted of tone- and light-conditioned stimuli (CSs) each paired on separate trials with a shock unconditioned stimulus (UCS). Cocaine injected prior to each session significantly impaired acquisition of conditioned responses (CRs). In Experiment 2, rabbits received cocaine injections prior to each training session involving explicitly unpaired CS-alone and UCS-alone presentations. Cocaine had no significant effects upon: base rate of NMRs; frequency of NMRs during presentations of the CSs; and frequency, amplitude, and latency of the UCRs. Consequently, cocaine's impairment of CR acquisition could not be attributed to its effects upon the nonassociative processes of base rate, sensitization, and pseudoconditioning, nor upon the sensory processing of the UCS and/or motor functioning of the UCR. Rather, cocaine's effects upon CR acquisition were mediated by the drug's effect upon associative processes. It appears likely that the drug affected the ability of the CS to enter into the associative conditioning process.