Time and stimulus specificity in extinction of the conditioned nictitating membrane response in the rabbit (Oryctolagus cuniculus)

The present experiments demonstrated that, in the rabbit nictitating membrane preparation, a conditioned response (CR) can be selectively eliminated in one portion of a conditioned stimulus (CS) while it is still paired with the unconditioned stimulus (US). Rabbits were initially trained with two stimuli (tone, light). Each was paired with the US by using a mixture of two CS-US interstimulus intervals (ISIs): 200 ms and 1,200 ms in Experiment 1; 150 ms and 500 ms in Experiment 2. The CRs showed double peaks, one for each ISI. Subsequently, one CS (A) was trained with only the longer ISI, whereas the other CS (B) continued to be trained with both ISIs. Consequently, the CR peak based on the shorter ISI disappeared for CSA but not for CSB. The later CR peaks during both CSA and CSB were maintained. These results support time-based models of conditioning. Implications for proposed mechanisms of extinction are discussed.     

Effects of a corneal anesthetic on extinction of the classically conditioned nictitating membrane response in the rabbit (Oryctolagus cuniculus)

Rabbits (Oryctolagus cuniculus) were presented with 7 daily sessions of tone-alone training after conditioning. Before the beginning of each of the first 4 extinction sessions, an artificial tear solution or tetracaine hydrochloride was administered to the cornea of rabbits in the control group (n = 6) and experimental group (n = 7), respectively. There were no between-group differences in the percentage of conditioned responses between both groups. However, the amplitude of the conditioned response was notably reduced in the tetracaine group (M = 0.40, SEM +/- 0.216) relative to the control group (M = 1.32, SEM +/- 0.639) early in extinction. Results seem to suggest that although motor output has been found to play an important role in extinction, corneal sensory feedback is not necessary.     

Time-specific extinction and recovery of the rabbit's (Oryctolagus cuniculus) conditioned nictitating membrane response using mixed interstimulus intervals

Extinguishing a conditioned response (CR) has entailed separating the conditioned stimulus (CS) from the unconditioned stimulus (US). This research reveals that elimination of the rabbit nictitating membrane response occurred during continuous CS-US pairings. Initial training contained a mixture of 2 CS-US interstimulus intervals (ISIs), 150 ms and 500 ms. The CRs showed double peaks, one for each ISI. When the 150-ms ISI was removed, its CR peak showed 2 hallmarks of extinction: a decline across sessions and spontaneous recovery between sessions. When a further stage of training was introduced with a distinctive CS using the 150-ms ISI, occasional tests of the original, extinguished CS revealed another hallmark of extinction, specifically, strong recovery of the 150-ms peak. These results support both abstract and cerebellar models of conditioning that encode the CS into a cascade of microstimuli, while challenging theories of extinction that rely on changes in CS processing, US representations, and contextual control.     

Role of extraocular muscles in the rabbit (Oryctolagus cuniculus) nictitating membrane response

The contributions of the extraocular and retractor bulbi muscles to the nictitating membrane response (NMR) were assessed using transections of the abducens nerve or the extraocular muscles. Transection of the six extraocular muscles or the muscle that attaches to the nictitating membrane had no effect on the NMR in otherwise intact animals. Transections of the abducens nerve reduced the NMR by 50%, and transections of the abducens nerve and the extraocular muscles abolished the NMR. These results indicate that the NMR can be effected by the six extraocular muscles, but that these six muscles may play a role that is redundant in the intact animal. The anatomical location of the motoneurons innervating the retractor bulbi is discussed.     

Cholesterol modifies classical conditioning of the rabbit (Oryctolagus cuniculus) nictitating membrane response

Cholesterol plays an important role in synapse formation, receptor function, and synaptic plasticity, and animal studies show that modifying cholesterol may improve learning and memory. Other data show that feeding animals cholesterol can induce beta amyloid accumulation. Rabbits (Oryctolagus cuniculus) fed 2% cholesterol for 8 weeks were given trace conditioning of the nictitating membrane response using a 100-ms tone, a 700-ms trace, and periorbital electrical stimulation or airpuff. Rabbits fed cholesterol showed significant facilitation of trace conditioning to airpuff and conditioning-specific reflex modification to periorbital electrical stimulation and airpuff. The cholesterol-fed rabbits had beta amyloid accumulation in the cortex, but little in the hippocampus. The data suggest cholesterol had facilitative effects that outweighed potential amnesic effects of cortical beta amyloid.     

Efferent neuronal control of the nictitating membrane response in rabbit (Oryctolagus cuniculus): a reexamination

Four Albino rabbits acquired a classically conditioned nictitating membrane response (NMR) to light, tone, and back shock conditioned stimuli using a paraorbital shock applied to the right eye as the unconditioned stimulus. Electrocoagulation lesions were made in the ipsilateral abducens nucleus of the pons in two animals, and the pontine reticular formation in two animals. Reinstitution of conditioning following surgery (retention test) indicated that the lesions failed to eliminate conditioned or unconditioned responses. This result is surprising in light of recent evidence that efferent control of the rabbit NMR derives solely from motoneurons of the ipsilateral abducens nucleus. Alternative hypotheses are considered.     

Disruption of rabbit (Oryctolagus cuniculus) nictitating membrane conditioning by posttrial electrical stimulation of hippocampus

Two experiments were conducted involving classical conditioning of the nictitating membrane response of rabbits with posttrial electrical brain stimulation. Hippocampal posttrial stimulation (PTS) presented immediately after each CS-US paired trial was found to cause massive disruption of conditioning by comparison with performance of either a cortical PTS group, an operated but unstimulated group, or an unoperated control group. The major effect of PTS was to postpone CR emergence. Once CRs began to occur, conditioning seemed to proceed normally regardless of the presence or absence of stimulation. Hippocampal PTS initiated after conditioning had no effect on the elicitation of conditioned responses. The results indicated that hippocampal PTS disrupted consolidation but did not affect either registration of stimuli or execution of unconditioned or conditioned responses. It was concluded that classical conditioning of the rabbit nictitating membrane response is an excellent model neural system for the investigation of neurophysiological aspects of memory.     

Magnitude and timing of nictitating membrane movements during classical conditioning of the rabbit (Oryctolagus cuniculus)

A trial-by-trial, subject-by-subject analysis was conducted to determine whether generation of the conditioned response (CR) occurs on a continuous or all-or-none basis. Three groups of rabbits were trained on different partial reinforcement schedules with the conditioned stimulus presented alone on 10%, 30%, or 50%, respectively, of all trials. Plots of each rabbit's nictitating membrane movements revealed that their magnitude rose in a continuous fashion. Response growth during acquisition followed a sigmoidal curve, and the timing of CR-sized movements was largely stable throughout the experiment. The results are discussed with respect to alternative models of CR generation.     

Conditioning-specific reflex modification of the rabbit (Oryctolagus cuniculus) nictitating membrane response: generality and nature of the phenomenon

Conditioning-specific reflex modification (CRM) occurs when classical conditioning modifies responding to an unconditioned stimulus in the absence of a conditioned stimulus. This form of reflex modification suggests that learning modifies the unconditioned reflex pathway. Rabbit (Oryctolagus cuniculus) nictitating membrane responses to 5 intensities and 3 durations of airpuff (AP) or periorbital electrical stimulation (ES) were monitored before and after conditioning. AP tests detected strong CRM after conditioning with ES and modest levels of CRM after conditioning with AP. After conditioning with AP, ES tests failed to detect CRM. After conditioning with a stronger AP, CRM was again detected by AP tests. CRM is a general phenomenon but is more readily detected after training with a relatively aversive stimulus; thus, it may be a function of level of arousal.     

Reversible inactivations of the cerebellum with muscimol prevent the acquisition and extinction of conditioned nictitating membrane responses in the rabbit

Lesions of the cerebellum severely impair the classically conditioned nictitating membrane response (NMR) in rabbits. Thus, the cerebellum is essential for the production of conditioned responses (CRs), either because it is actively involved in NMR conditioning or because damage to it causes motor or other general deficits. To distinguish between these alternatives, the cerebellum may be inactivated during training. Inactivation of the cerebellum during acquisition training might result in the absence of CRs on initial trials of subsequent training without the neuronal blockade. The blockade may have prevented learning but it may have produced other deficits that require time or further training to overcome. This problem can be addressed by inactivating the cerebellum during extinction training. If inactivation during extinction training results in the immediate production of CRs when training is resumed without the blockade, then it may be concluded that extinction learning was prevented by the blockade — the presence of CRs argues against any deficits not associated with learning. We used muscimol to inactivate the cerebellum and test its involvement in acquisition and extinction of NMR conditioning in the same subjects. We injected muscimol close to the interpositus nucleus of the cerebellum 1 h before each of four daily training sessions of delay conditioning. Almost no CRs were produced in these training sessions — there was little or no acquisition of NMR conditioning during cerebellar inactivation. The subjects were then trained for four daily sessions without injections of muscimol. There were no CRs on initial trials of the first session of retraining, but all subjects produced CRs by the end of this session. The subjects then received four daily sessions of extinction training with muscimol inactivation of the nuclei — no CRs were produced. Extinction training then continued for four daily sessions without muscimol inactivation. On the first of these sessions, all subjects immediately produced high levels of CRs. These responses then extinguished within and between sessions with characteristic beginning-of-session spontaneous recovery. There was little or no extinction of NMR conditioning during cerebellar inactivation. After inactivation, the muscimolinactivated subjects went on to acquire and extinguish NM responses at rates similar to those of appropriate controls. We conclude that cerebellar circuitry is essential for, and actively engaged in, both acquisition and extinction of this simple form of motor learning.     

Inactivation of the central nucleus of the amygdala abolishes conditioning-specific reflex modification of the rabbit (Oryctolagus cuniculus) nictitating membrane response and delays classical conditioning

The central nucleus (CE) of the amygdala has been gaining attention for its importance in the plasticity underlying conditioned emotional responding. Already known for its role in nictitating membrane response (NMR) reflex facilitation, the CE may also be involved in conditioning-specific reflex modification (CRM)--changes in the NMR to the unconditioned stimulus (US) when tested in the absence of the conditioned stimulus following classical conditioning. To examine the CE's role in acquisition and/or expression of CRM, the authors temporarily inactivated the CE of rabbits (Oryctolagus cuniculus) with muscimol during NMR conditioning and/or during US testing. Results show that CRM was abolished by inactivation during US testing but intact following inactivation during NMR conditioning, suggesting that the CE is involved in CRM expression. Also, inactivation during conditioning delayed the development of conditioned NMRs. These findings show that the CE may act as an output center for expression of emotional responding in one situation (CRM) but is involved in facilitating plasticity in another (NMR conditioning). The authors propose that analysis of CRM may be an important corollary to current models for the treatment of posttraumatic stress disorder.     

Morphine's effects on differential serial compound conditioning and reflex modification of the rabbit's (Oryctolagus cuniculus) nictitating membrane response

This study sought to determine the effects of morphine (0, 2, and 5 mg/kg) on (a) differential classical conditioning of the rabbit's (Oryctolagus cuniculus) nictitating membrane response (NMR) to the serial compounds A-X-unconditioned stimulus (US) and B-X-US (Experiment 1) and (b) the reflex modification effects of the compounds and their components (Experiment 2). These experiments determined specifically morphine's effects on the distinctiveness and time course of stimulus representations by examining morphine's dose-response effect on (a) differential responding to A and B and their conditional control over responding to X within the compounds and (b) the unconditioned excitatory effects of the compounds and their components as assessed by their ability to modify the amplitude of the unconditioned NMR. The results of these experiments indicate that morphine, in a dose-dependent manner, can operate to profoundly attenuate the distinctiveness and persistence (short-term memory) of stimulus representations.     

Eyeblink conditioning in the rabbit (Oryctolagus cuniculus) with stimulation of the mystacial vibrissae as a conditioned stimulus.

Eyeblink conditioning is a well-understood paradigm for the study of learning and memory and has been successfully employed with the use of auditory and visual conditioned stimuli (CSs). In this study, vibrotactile stimulation of the mystacial vibrissae was examined as an alternative CS in the rabbit (Oryctolagus cuniculus). The technique is described and acquisition of eyeblink conditioning (EBC) with stimulation of a single row of vibrissae in a delay paradigm is reported. Extinction of EBC with presentation of the CS alone is demonstrated, as well as reacquisition with stimulation of a single whisker. Finally, control experiments ensure that the CS has no auditory components. Ipsilateral presentation of the CS and airpuff is a more effective combination for training than contralateral presentations. Vibrotactile stimulation of the vibrissae as a CS will enable further examination of the neural correlates of learning in a well-characterized sensory system.     

Herpes simplex encephalitis in a domestic rabbit (Oryctolagus cuniculus)

An adult domestic rabbit showing neurological signs was subjected to euthanasia. At necropsy, macroscopical lesions were absent. Histopathologically, extensive lesions were seen, particularly in the cerebral cortex. Non-suppurative meningitis was present and there was lymphocytic and plasmacytic perivascular cuffing in the neuropil. The cerebral cortex showed extensive segmental neuronal and glial necrosis. Within the necrotic areas, large amphophilic intranuclear inclusion bodies were present in neurons and glial cells. Immunohistochemically, neurons and glial cells in the affected areas were labelled by polyclonal antibodies against both herpes simplex virus-1 (HSV-1) and HSV-2. The agent was classified as HSV-1 by polymerase chain reaction analysis. This is only the second reported natural case of herpes simplex infection in a rabbit.     

Conditioning-specific reflex modification of rabbit (Oryctolagus cuniculus) heart rate

Conditioning-specific reflex modification (CRM) describes changes in rabbit (Oryctolagus cuniculus) nictitating membrane responses (NMR) to an unconditioned stimulus (US) when the US is tested by itself after pairings of tone and electrodermal stimulation. Although CRM has been replicated, it is unclear whether it occurs in response systems other than that of the NMR. The authors report that CRM of rabbit heart rate (HR) can occur following HR conditioning. A US that elicits HR acceleration before conditioning can elicit HR deceleration after conditioning. The rabbits' electrocardiograms showed both HR conditioning and HR CRM were correlated with an increased PQ interval--an index of parasympathetic function mediated by the vagus. The data suggest conditioned HR deceleration can generalize from conditioned stimulus to US as a function of conditioning.     

Intestinal lesions resembling paratuberculosis in a wild rabbit (Oryctolagus cuniculus)

One of three feral rabbits from a colony on a hill grazing used to sustain farmed red deer in north-east Scotland, had chronic intestinal lesions resembling paratuberculosis (Johne's disease), but similar lesions were not found in 29 juvenile and adult rabbits killed subsequently on the same ground. Acid- and alcohol-fast bacteria were observed in fixed tissues, but material for culture was not obtained from the affected rabbit, thus the species of Mycobacterium involved was not identified. The farmed deer had experienced recently a severe outbreak of paratuberculosis.     

Cell types of the third ventricle wall of the rabbit (Oryctolagus cuniculus)

A comparative study using optical and electron transmission and scanning microscopes was carried out to determine the cells that comprise the wall of the third ventricle of the rabbit (Oryctolagus cuniculus). Three cell types have been distinguished: type I cells are the most numerous and may present a flat, cubical or cylindrical aspect, with cilia and microvilli at their apical end. Type II cells have an irregular elongated morphology, lack cilia on their luminal end and show functional characteristics of secretory cells. Type III cells are also elongated, and bear irregular protrusions at the apical end. A long process at the basal end of these cells goes deep into the neuropil and becomes related with neurons or blood vessels; these correspond to tanycytes.     

Disrupted conditioned inhibition of the rabbit nictitating membrane response following mesencephalic lesions

Following acquisition of conditioned inhibition (CI) of the nictitating membrane, response, rabbits received radio-frequency lesions of the mesencephalon. The reinforced conditioned stimulus (CS) was light onset and the nonreinforced CS was the same light compounded with a tone. The unconditioned stimulus was electric shock to the paraorbital region of the eye. In accord by an earlier study by Mis [14], lesions disrupted conditioned inhibition by reducing the capacity to suppress conditioned responses on nonreinforced trials; conditioned responding on reinforced trials was unaffected. Lesions of the pretectum in the region of the posterior commissure and of the periaqueductal grey produced the greatest disruption CI, and fiber degeneration from these regions, as assessed by the Fink-Heimer method, projecting in a ventro-anterior direction to the posterior hypothalamus was implicated in CI disruption. This observation, together with indirect evidence that the dorsal noradrenergic bundle is involved in CI, suggested that conditioned inhibitors may act via a brainstem-hypothalamus reward system to antagonize the motivational support of the conditioned response.