Sensory and associative effects of morphine and naloxone in classical conditioning of the rabbit nictitating membrane response

In Experiment I, classical conditioning of the rabbit's nictitating membrane response was accomplished by the pairing of tone and light conditioned stimuli with a shock unconditioned stimulus applied to the paraorbital region of the head. Morphine (5 mg/kg) significantly retarded the acquisition of conditioned responses to both conditioned stimuli. Moreover, morphine had no effect on nonassociative responding (baseline responding or responding to tone and light stimuli) or on the latency and amplitude of the unconditioned response elicited by shock during the explicitly unpaired presentations of tone, light and shock stimuli. The retardant effect of morphine on acquisition of conditioned responses was blocked by naloxone (1 mg/kg). In Experiment II, morphine (0.2–10 mg/kg) had no effect on the intensity threshold of the shock unconditioned stimulus for elicitation of unconditioned responses or on the latencies of the elicited responses. However, morphine (5 and 10 mg/kg) did produce a small but significant decrease in the amplitude of unconditioned responses elicited by the two highest shock intensities employed (3 and 4 mA). This latter effect of morphine was completely blocked by naloxone (1 mg/kg). In Experiment III, morphine (5 mg/kg) blocked the sensory processing of a tone conditioned stimulus, in previously trained animals, as measured by a significant (24 dB) elevation in the intensity threshold of the conditioned stimulus for elicitation of conditioned responses and an increase in the latency of the elicited response. Naloxone (1 mg/kg) completely blocked the effects of morphine on the sensory processing of the tone-conditioned stimulus. The retardant effect of morphine on the acquisition of conditioned responses appears to be primarily due to an action on opioid receptors that produces a decrease in the sensory processing of the conditioned stimulus.     

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 LSD-25 on classical trace conditioning

This study examined the effects of LSD-25 on the excitatory properties of auditory conditioned stimuli as a function of the interstimulus interval. The rabbit's eyeblink response was conditioned using a discriminative trace procedure by the pairing of a 500-msec auditory conditioned stimulus with a 100-msec shock unconditioned stimulus at intervals of 1000, 2000, 4000 and 8000 msec. Animals were able to acquire conditioned responses across all intervals. They then received doses of 35 or 85 micrograms/kg of LSD-25 prior to additional conditioning sessions. LSD-25 produced an increase in the magnitude of conditioned responses to both the positive and negative conditioned stimuli at all interstimulus intervals. It was concluded that LSD did not alter discriminative conditioning but rather enhanced the excitatory properties of both positive and negative conditioned stimuli.     

Effect of LSD on acquisition, maintenance, extinction and differentiation of conditioned responses

Three experiments were conducted to compare the effects of LSD (30 nmol/kg) on the acquisition, maintenance, extinction and differentiation of the rabbit's classically conditioned nictitating membrane response. LSD significantly enhanced the acquisition of conditioned responses to tone and light conditioned stimuli as compared with vehicle injected controls (Experiments 1 and 2), but had no detectable effect on differential conditioning in Experiment 3. The conditioned responses acquired under LSD in Experiments 1 and 2 exhibited some unusual features in that: they were more rapidly extinguished under continued injections of LSD; they demonstrated a significant decrement when animals were switched from LSD to vehicle during maintenance; and they were virtually eliminated when animals were switched from LSD to vehicle during extinction. In contrast, conditioned responses acquired under saline injections in Experiments 1 and 2 were not affected when animals were switched to LSD injections during either maintenance or extinction. These results of Experiments 1 and 2 were interpreted as indicating that LSD produces an asymmetrical state-dependent learning.     

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.     

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     

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.     

The effects of drugs on conditioning and habituation to arousal stimuli in animals

Summary The effects of a number of drugs on conditioned and unconditioned arousal responses (behavioural and electroencephalographic) produced by auditory stimuli in cats are reported. Positive conditioning was achieved by pairing certain auditory stimuli with a painful stimulus (electric shock).Chlorpromazine increased thresholds for both conditioned and unconditioned stimuli and eventually blocked arousal responses completely. Reserpine, which had a delayed effect, caused only a slight rise in the conditioned response but blocked the unconditioned response although this latter effect may have been in part due to habituation.Amphetamine caused a fall in the threshold for unconditioned arousal responses but did not change that for conditioned responses. However, these thresholds could no longer be assessed when doses which produced full alerting were used.LSD 25 also caused a fall in the threshold for arousal to unconditioned stimulus and no change in the conditioned response, but it restored the response to a stimulus which had previously been habituated.The results are discussed in relation to the hypothesis for the sites of action of these drugs in the brain which has been expounded previously.This work has been sponsored by the Office of Scientific Research of the Air Research and Development Command, United States Air Force, Contract No. AF 61 (514)-1184.     

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.     

A method for evaluating visual impairments using operant behavior in mice

A behavioral method, using a shuttle box (one of the conditioned behaviors) for assessing visual function in mice, was investigated. Normal ICR mice were trained to avoid the unconditioned stimulus (electric footshock) during the presentation of conditioned stimuli (tone and light). After acquisition of avoidance response, normal ICR mice were presented randomly tone or light as the conditioned stimulus. The percent of avoidance in the presentation of light was decreased suddenly, but that of tone was not. However, the decrease of avoidance response to light stimulus was recovered by following trainings. The mice, which had acquired the condition response to tone and light stimulus alone, were treated in their cornea with 4N NaOH. The decrease of percent of avoidance in the mice with alkali-treated cornea after differentiation of conditioned stimuli did not recover following training. These results suggested that visual impairment of mice can be detected by this method. Therefore, we attempted to detect the visual impairments of dominant (Cts) and recessive (cac) hereditary cataract mice, hereditary retina degenerated mice (C3H) and first filial generation mice of C3H, F1 [(ICR X C3H) F1]. The acquisition curves of both Cts and cac cataract mice were similar to that of normal ICR mice, but those of C3H and F1 were similar to that of ICR mice treated with alkali. Moreover, changes in recovery %, which was calculated from avoidance response in the presentation of light alone after differentiation of conditioned stimuli, corresponded to the above results. These findings demonstrated that the visual impairment of Cts and cac cataract mice is weak, while that of C3H and F1 mice is strong.     

Studies on interactions between conditioned and unconditioned behavioural responses to apomorphine in rats

Summary Interactions between the direct (unconditioned) behavioural effects apomorphine and its conditioned effects after pairing with previously neutral stimuli were studied. Rats were injected once daily for 3–12 times, with apomorphine (2.0 mg/kg or 0.5 mg/kg or 0.07 mg/kg s.c. the dose kept constant in each series), in the presence of defined environmental stimuli (a wire cage in association with an acoustic and an olfactory stimulus) as conditional stimuli. The two larger doses produced stereotyped sniffing, licking, and gnawing, the smallest dose akinesia, ptosis, yawning and penile erections. During the conditioning phase, the drug produced most of the effects with increasing intensity and in the case of the stereotypies, there also was a shift to higher scores of stereotypy, with a reduced latency in onset of the signs. On the test day, 1 day after the last administration of apomorphine, the conditioned rats as well as pseudoconditioned controls were treated with a test dose of apomorphine in the presence of the conditional stimuli. Pseudoconditioned rats had been treated with the same pharmacological schedule of apomorphine and had the same familiarity with the stimuli, but both were kept separate. A test dose of 0.5 mg/kg of apomorphine produced stereotypies with a significantly higher score and shorter latency in onset in conditioned than in pseudoconditioned rats. Rats conditioned with the lowest dose showed a significantly longer total duration and a shorter latency in onset of akinesia and ptosis. In rats conditioned with the largest dose (2.0 mg/kg), administration of the lowest dose on the test day produced no stereotypies; neither the akinesia nor the ptosis were different between conditioned and pseudoconditioned rats, but yawning occurred with a higher frequency and a shorter latency in pseudoconditioned rats. When rats were conditioned with the lowest dose and tested with 0.5 mg/kg, the level of stereotypies was identical in both groups of rats, whereas akinesia and ptosis were not observed. Yawning and penile erections occurred more frequently, but for short periods only, in conditioned rats.The results showed that apomorphine-induced stereotypies, akinesia and ptosis could be conditioned, and the conditioned effects mimicked the unconditioned responses, which depended on the dose. Conditioned and unconditioned signs of an increased dopaminergic neurotransmission, observed after large doses of apomorphine, thus acted in a synergistic way; the same applied to conditioned and unconditioned signs observed after a small dose and were perhaps due to a decreased dopaminergic transmission. In contrast, when conditioned and unconditioned signs acted in a mutually antagonistic way (increased vs. decreased dopaminergic transmission), the unconditioned signs predominated. Send offprint requests to K. Kuschinsky at the above address     

Morphine-conditioned analgesia using a taste cue: dissociation of taste aversion and analgesia

The present study examined the ability of a taste cue to serve as a conditioned stimulus (CS) for conditioning the analgesic effect of morphine. Rats were given three pairings of a taste CS with a morphine unconditioned stimulus (US). As expected, there was a decrease in CS intake across repeated pairings, indicating that a conditioned taste aversion was obtained. More important, presentation of the CS alone also increased paw-lick latencies on a hot plate test (either 50°C or 54°C hot plate), suggesting that an analgesic conditioned response (CR) was obtained. The dose of morphine required to produce conditioned analgesia was higher than the dose of morphine required to produce conditioned taste aversion. Using 15 mg/kg morphine, however, both conditioned taste aversion and conditioned analgesia were present when the morphine US was given immediately following CS intake, but not when given 6 h following CS intake. In contrast to morphine, pairing a taste CS with lithium produced a conditioned taste aversion without any conditioned analgesic response. These results indicate that acquisition of an analgesic CR is not the result of stress induced by an aversion to the taste CS.     

Conditioned slowing of stomach emptying produced by Pavlovian pairings of a drug CS or a place with lithium chloride

Lithium chloride, in common with other drugs with emetic effects, prolongs stomach emptying. In different experiments, a drug state induced by a low dose of pentobarbital in Experiment 1 and morphine in Experiment 2 or a distinctive place (Experiments 3, 4) was the conditioned stimulus paired with lithium chloride as the unconditioned stimulus. In each case, Pavlovian conditioning occurred and the conditioned response mimicked lithium's unconditioned effect on stomach emptying.     

Effect of hippocampal lesion on acquisition of conditioned avoidance behaviour in albino rats

Control, hippocampal and Sham control male albino rats weighing 150-200 gms were trained for acquisition of conditioned avoidance behavioural response using escape avoidance apparatus. Parameters like rate of performance, error scores, conditioned stimulus latency and unconditioned stimulus latency were studied. It was observed that there was a facilitation in the behavioural response with less error scores in hippocampal animals as compared to Sham control and control groups. Our observations are similar to those of Douglas and Pribram and Douglas and it is concluded that the hippocampus acts as a gate restricting the range of stimuli to which an intact animal attends.     

A unique enhancement of associative learning produced by methylenedioxyamphetamine

The effects of methylenedioxyamphetamine (MDA) on classical conditioning of the rabbit's nictitating membrane response were assessed in four experiments. Experiment 1 established a dose-effect curve for MDA doses of 0, 1, 3, and 10μmol/kg. Both the 3 and 10μmol/kg doses significantly enhanced the rate of learning. Experiment 2 established that the 10μmol/kg dose of MDA had no effect on non-associative determinants of responding to the conditioned stimulus (CS). However, this dose of MDA apparently sensitized the response to the unconditioned stimulus (US). Experiments 3 and 4 assessed the effects of the 10μmol/kg dose of MDA on sensory processing of the CS and US, respectively. MDA had no effect on the intensity thresholds for eliciting responses to either of these stimuli. Thus, the enhanced rate of acquisition observed in experiment 1 cannot be attributed to an increase in nonassociative responding to the CS or to enhanced sensory processing of the CS or US. MDA apparently enhanced the rate of acquisition in experiment 1 by facilitating the association between the CS and US. These findings are unique in that no other hallucinogen we have examined has enhanced acquisition without also affecting either non-associative responding or sensory processing of the CS and US.     

The relative salience of morphine and contextual cues as conditioned stimuli

Two experiments were conducted to delineate further the properties of conditioning when morphine is used as a conditioned stimulus (CS) in the conditioned suppression of drinking paradigm. Experiment 1 used a test for overshadowing designed to compare the relative salience of contextual cues (metal box) and morphine induced cues (6 mg/kg, IP) as CSs when each was paired with a foot shock unconditioned stimulus (US) in water deprived rats. Six groups (six rats each) were exposed to conditioning procedures during which the conditioning context was present 19 h (groups 1 and 2), 90 min (groups 3 and 4), or 5 min (groups 5 and 6) before shock onset, and morphine (in groups 1, 3, and 5) or saline (in groups 2, 4, and 6) was injected 10 min before shock. Subsequently, the magnitude of suppression of drinking in response to morphine, to the metal box, and to morphine plus the metal box was measured. Only group 1 (19 h group) suppressed drinking in response to morphine, while groups 3–6 suppressed drinking whenever tested in the metal box. The results indicate that morphine cues acted as a CS that elicited suppression of drinking in group 1, and that contextual cues present up to 90 min before morphine cues overshadowed morphine. Experiment 2 showed that expression of the conditioned response to morphine was blocked by naloxone.     

Acute effects of morphine and chlorpromazine on the acquisition of shuttle box conditioned avoidance response

Morphine sulfate, 0.25–24.0 mg/kg, or chlorpromazine hydrochloride, 0.0625–4.0 mg/kg, were administered subcutaneously to naive rats 30 min prior to the start of massed-trials conditioned avoidance response (CAR) testing. The graded doses of both drugs were applied in each of three CAR task difficulty levels created by manipulation of the duration of conditioned and unconditioned stimuli, intertrial interval and shock intensity. Chlorpromazine, in a dose-related manner, caused a decrement in CAR acquisition in all tasks. Morphine, in comparison, produced a biphasic dose response. For a given task difficulty, low doses of morphine enhanced acquisition, whereas higher doses inhibited acquisition. With increasing task difficulty, relatively larger doses of morphine were required to inhibit or facilitate acquisition of CAR. These results emphasize the need to consider not only drug dosage levels, but also the interaction of task difficulty in the application of drugs in learning paradigms.     

Effects of opioid antagonists on unconditioned and conditioned hyperactivity to morphine

In a series of experiments, the ability of selective mu- (beta-funaltrexamine, beta-FNA), delta- (naltrindole, nalt) and kappa- (nor-binaltorphimine, nor-BNI) opioid receptor antagonists to attenuate the unconditioned and conditioned hyperactive effects of morphine was examined. For comparison, the nonselective opioid receptor antagonist naloxone (nalx) was also examined. Locomotor activity served as the behavioral measure. Experiment 1 found that doses of 1 and 4, but not 16 mg/kg, of morphine effectively produced conditioned hyperactivity (CH). Experiments 2a-d found that beta-FNA, nalt, nor-BNI and nalx, respectively, attenuated unconditioned morphine-induced hyperactivity. Experiments 3a-c, however, found that none of the selective antagonists, given individually, attenuated CH. In contrast, nalx did attenuate CH (Experiment 3d). Collectively results suggest that the unconditioned and conditioned hyperactive responses to morphine are mediated by different receptor systems and that activation of multiple opioid-receptor subtypes mediate expression of CH.     

Pizotifen (BC-105) attenuates orienting and Pavlovian heart rate conditioning in rabbits

The cardiac component of the orienting reflex (OR) was elicited in rabbits by 75 dB, 4-sec duration tones of either 304 or 1216 Hz. The conditioned cardiac response was also studied using the same tones and paraorbital electric shock as conditioned and unconditioned stimuli, respectively, using a differential Pavlovian conditioning paradigm. Subcutaneous injections of the central 5-HT antagonist pizotifen (BC-105), the peripheral 5-HT antagonist xylamidine, the central 5-HT agonist d-lysergic acid diethylamide (LSD), and LSD in conjunction with BC-105 were administered 15 min prior to behavioral assessment. Both the heart rate (HR) conditioned response (CR) and the OR consisted of bradycardia. BC-105 attenuated, but xylamidine had no effect on, OR habituation. LSD reduced the magnitude of the OR, an effect which was blocked by BC-105. BC-105 also produced a dose-related attenuation of the bradycardiac HR CR; however, xylamidine had no effect on HR conditioning, suggesting that the attenuation of the HR CR by BC-105 was central rather than peripheral in origin. LSD potentiated the bradycardiac HR CR, but BC-105 in conjunction with LSD attenuated this response. These results suggest that central 5-HT neurons may modulate the magnitude of bradycardiac responses during orienting and aversive Pavlovian conditioning.