Inactivation of the interpositus nucleus blocks the acquisition of conditioned responses and timing changes in conditioning-specific reflex modification of the rabbit eyeblink response

Conditioning-specific reflex modification (CRM) of the rabbit eyeblink response is an associative phenomenon characterized by increases in the frequency, size, and peak latency of the reflexive unconditioned eyeblink response (UR) when the periorbital shock unconditioned stimulus (US) is presented alone following conditioning, particularly to lower intensity USs that produced minimal responding prior to conditioning. Previous work has shown that CRM shares many commonalities with the conditioned eyeblink response (CR) including a similar response topography, suggesting the two may share similar neural substrates. The following study examined the hypothesis that the interpositus nucleus (IP) of the cerebellum, an essential part of the neural circuitry of eyeblink conditioning, is also required for the acquisition of CRM. Tests for CRM occurred following delay conditioning under muscimol inactivation of the IP and also after additional conditioning without IP inactivation. Results showed that IP inactivation blocked acquisition of CRs and the timing aspect of CRM but did not prevent increases in UR amplitude and area. Following the cessation of inactivation, CRs and CRM latency changes developed similarly to controls with intact IP functioning, but with some indication that CRs may have been facilitated in muscimol rabbits. In conclusion, CRM timing and CRs both likely require the development of plasticity in the IP, but other associative UR changes may involve non-cerebellar structures interacting with the eyeblink conditioning circuitry, a strong candidate being the amygdala, which is also likely involved in the facilitation of conditioning. Other candidates worth consideration include the cerebellar cortex, prefrontal and motor cortices.

Sex differences in a rabbit eyeblink conditioning model of PTSD

We have developed a rabbit model of posttraumatic stress disorder (PTSD) which recapitulates several core features of PTSD, particularly hyperarousal and conditioned responding to trauma-associated cues. The work conducted with this model has all been done in male rabbits and, given sex differences in PTSD prevalence, it is important to expand our animal model of PTSD to include female rabbits to determine if they develop core features of PTSD, and if those core features can be treated. This is particularly important because, contrary to human studies, nearly all animal studies have found that males are consistently more vulnerable to various forms of acute and chronic stress than females. Using eyeblink conditioning in which we paired tone with a brief periorbital shock, we found that although both male and female rabbits acquired identical levels of conditioning, females showed more hyperarousal after conditioning but seemed to respond somewhat better to treatment.

Delayed unpaired extinction as a treatment for hyperarousal of the rabbit nictitating membrane response and its implications for treating PTSD

Treatment for PTSD (Post-traumatic stress disorder) is rarely available immediately after trauma and often delayed for weeks or months after an event. In a rabbit eyeblink conditioning model of PTSD, we have previously shown that presentations of a tone conditioned stimulus (CS) and shock unconditioned stimulus (US) in an explicitly unpaired manner known as unpaired extinction is effective in reducing CS responding and US hyperarousal even if shock intensity is reduced eight-fold and elicits only minimal responding. Here we determined if delayed delivery of unpaired extinction would still be effective in extinguishing hyperarousal. Rabbits were tested for sensitivity to shock before CS-US pairings and after six days of unpaired extinction presented a day, a week or a month after CS-US pairings. Hyperarousal was extinguished a day and a week after conditioning but not after a month suggesting a significant delay in "treatment" can make hyperarousal persist. We next assessed if this persistence of hyperarousal was associative by comparing rabbits given CS-US pairings to those given explicitly unpaired CS and US presentations, measuring hyperarousal a day and a month later, followed by unpaired extinction and hyperarousal assessment. After four weeks, there was an increase in responding for all rabbits but only rabbits receiving CS-US pairings showed a significant increase in associatively-mediated hyperarousal. Importantly, both paired and unpaired groups showed increased levels of responding after unpaired extinction suggesting treatment delayed for too long may no longer be effective and could cause generalized hyperarousal.

Propranolol produces short-term facilitation of extinction in a rabbit model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a learning-based anxiety disorder with significant public health challenges due to difficulties in treating the complex, multiple symptomology. We have developed an animal model of PTSD, based on Pavlovian eyeblink conditioning in rabbits, that addresses two key features: conditioned responses (CRs) to cues associated with an aversive event and a form of conditioned hyperarousal referred to as conditioning-specific reflex modification (CRM). We have found previously that unpaired extinction is ideal for reducing both CRs and CRM simultaneously and shows sensitivity to systemic serotonergic and glutamatergic manipulations. The following study aimed to extend our work to examine the role of the noradrenergic system, dysregulation of which is strongly implicated as part of the neurobiology of PTSD and which may also play a role in the balance shift from fear reconsolidation to extinction during treatment. The goal of the following two studies was to examine whether the β-adrenergic receptor antagonist propranolol combined with either a full or brief course of unpaired extinction treatment could enhance extinction of CRs and/or CRM. Results showed a within-session facilitation of propranolol on extinction of CRs, particularly during the first extinction session, and a short-term enhancement of extinction of CRM when extinction treatment was brief. However, neither benefit translated to long-term extinction retention for the majority of subjects. Findings suggest that propranolol may provide the most therapeutic benefit in situations of high arousal early in treatment, which may be more important for future patient compliance rather than long-term treatment outcomes.

Grouping subjects based on conditioning criteria reveals differences in acquisition rates and in strength of conditioning-specific reflex modification

Averaging behavioral data such as the nictitating membrane response (NMR) across subjects can conceal important individual and group differences. Analyses were conducted of NMR data from rabbits that were grouped based on the point during NMR conditioning when subjects produced 8 conditioned responses (CR) in a set of 10 trials. This resulted in five groups (Early Day 1, Late Day 1, Early Day 2, Late Day 2, Early Day 3) in which group differences in CR acquisition rates were found. Percent (%) CRs were not found to increase monotonically and between-session differences in % CR were found. Conditioning-specific reflex modification (CRM) of the NMR is a type of enhanced reflexive responding of the NMR that is detected when the unconditioned stimulus (US) is presented in the absence of the conditioned stimulus (CS) following paired classical conditioning. CRM occurred in some subjects in all five groups. Subjects from both the group that was fastest and the group that was slowest to reach the learning criterion had unconditioned response (UR) topographies following NMR conditioning that strongly resembled the CR-UR response sequence elicited during NMR conditioning. This finding was most pronounced when the US duration used to assess CRM was equivalent to that used during NMR conditioning, further evidence to support the hypothesis that CRM is a CR that has generalized from the CS to the US. While grouping data based on conditioning criteria did not facilitate identifying individuals more predisposed to exhibiting CRM, strong CRM only occurred in the groups that reached the conditioning criterion the fastest.

Classical Conditioning and Modification of the Rabbit's (Oryctolagus Cuniculus) Unconditioned Nictitating Membrane Response

A fundamental tenet of behavior is that a reflex is automatic, unconscious, involuntary, and relatively invariant. However, we have discovered that a reflex can change dramatically as a function of classical conditioning, and this change can be demonstrated independently of the conditioned stimulus. We have termed this phenomenon conditioning-specific reflex modification (CRM). Although the behavioral laws and neural substrates of nonassociative reflex changes have been identified, the behavioral laws and neural substrates of CRM are only now being revealed. For example, CRM is similar to classical conditioning in that (a) it is a function of both the strength of conditioning and (b) the strength of the unconditioned stimulus, (c) it can be extinguished, and (d) it can be generalized from one unconditioned stimulus to another. Preliminary analysis suggests that CRM may have some features in common with post-traumatic stress disorder and may provide insights into treatment of the disorder.

Incubation of conditioning-specific reflex modification: implications for Post Traumatic Stress Disorder

Incubation of fear has been used to account for the delayed manifestation of symptoms of fear and anxiety including the delayed onset of Post Traumatic Stress Disorder (PTSD). We have shown the utility of classical conditioning-specific modification of the rabbit nictitating membrane response (NMR) as a model of PTSD. This modification includes an exaggeration in the size and a change in the timing of the unconditioned NMR after several days of classical conditioning. To assess the effects of incubation on conditioning-specific modification, we measured changes in responding as a function of the time between classical conditioning and NMR testing. After just one day of classical conditioning resulting in modest levels of learning, increases in response size were an inverted-U shaped function of days of incubation with little if any change occurring one and ten days after training but significant change occurring after six days. The incubation effect persisted for a week. An unpaired control group showed no change in the size of the response confirming the incubation effect was associative. The results bear a striking resemblance to symptoms of PTSD that do not always occur immediately after trauma and become exacerbated over time and then persist. They point to a window when incubation can exacerbate symptoms and speak to the vulnerability of re-experiencing trauma too soon. This could be a serious problem for military or emergency personnel recalled to combat or a disaster site without sufficient time to deal with the effects of their initial experiences.

Unpaired extinction: implications for treating post-traumatic stress disorder

Extinction of fear is important for treating stress-related conditions particularly post-traumatic stress disorder (PTSD). Although traditional extinction presents the feared stimulus by itself, there is evidence from both clinical and basic research that repeatedly presenting the feared stimulus by itself does not prevent fear from returning. This renewal or relapse can be "thwarted" by unpaired extinction-presentations of the feared stimulus and the event producing the fear. However, no matter how effective standard unpaired extinction may be in the laboratory, repeated presentation of a traumatic event is untenable. To make an unpaired extinction procedure more clinically relevant, we classically conditioned the rabbit nictitating membrane response using electrical stimulation or air puff as the unconditioned stimulus and then during unpaired extinction reduced both the intensity of the unconditioned stimulus and the days of unpaired stimulus presentations. We found unpaired extinction reduced conditioned and exaggerated unconditioned responding (an animal analog of PTSD called conditioning-specific reflex modification) and could be accomplished with a weak unconditioned stimulus as long as extended presentations were used. Surprisingly, brief presentations of a weak unconditioned stimulus or extended presentations of a strong one made the exaggerated responses stronger. One implication is that brief treatment may not just be ineffectual; it may heighten the symptoms of PTSD. Another implication is that using strong stimuli may also heighten those symptoms.

Classical conditioning of the rabbit's nictitating membrane response is a function of the duration of dietary cholesterol

Modifying dietary cholesterol may improve learning and memory but very high cholesterol can cause pathophysiology and death. Rabbits fed 2% cholesterol for 8, 10 or 12 weeks with 0.12 ppm copper added to distilled water and rabbits fed a normal diet without copper added to distilled water (0 weeks) were given a difficult trace classical conditioning task and an easy delay conditioning task pairing tone with corneal air puff. The majority of cholesterol-fed rabbits survived the deleterious effects of the diet but survival was an inverse function of the diet duration. Compared to controls, the level of classical conditioning and conditioning-specific reflex modification were an inverted "U"-shaped function of diet duration. Highest levels of responding occurred in rabbits on cholesterol for 10 weeks and trace conditioning was negatively correlated with the number of hippocampal beta-amyloid-positive neurons. Rabbits on the diet for 12 weeks responded at levels comparable to controls. The data provide support for the idea that dietary cholesterol may facilitate learning and memory but there is an eventual trade off with pathophysiological consequences of the diet.

High dietary cholesterol facilitates classical conditioning of the rabbit's nictitating membrane response

Studies have shown that modifying dietary cholesterol may improve learning and that serum cholesterol levels can be positively correlated with cognitive performance. Rabbits fed a 0, 0.5, 1 or 2% cholesterol diet for eight weeks and 0.12 ppm copper added to their drinking water received trace and then delay classical conditioning pairing tone with corneal air puff during which movement of the nictitating membrane (NM) across the eye was monitored. We found that the level of classical conditioning and conditioning-specific reflex modification (CRM) as well as the number of beta amyloid-labeled neurons in the cortex and hippocampus were a function of the concentration of cholesterol in the diet. The data provide support for the idea that dietary cholesterol may facilitate learning and memory.

Conditioning-specific reflex modification of the rabbit (Oryctolagus cuniculus) nictitating membrane response is sensitive to context

Conditioning-specific reflex modification occurs when an unconditioned response is modified in the absence of the conditioned stimulus as a result of pairings of the conditioned stimulus and an unconditioned stimulus. In two experiments, we assessed conditioning-specific reflex modification in either a novel context (Experiment 1) or a context different from, but equally familiar in relation to, the training context (Experiment 2). Conditioning-specific reflex modification did not demonstrate sensitivity to a novel context but did demonstrate sensitivity to a change in familiar context. The data cannot be explained by unconditioned stimulus preexposure, overtraining, or context insensitivity. The results suggest that conditioning-specific reflex modification models normal stress and may be used to evaluate theories of and treatments for posttraumatic stress disorder.

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.

Heart rate changes during conditioning-specific reflex modification of the rabbit's (Oryctolagus cuniculus) nictitating membrane response

Conditioning-specific reflex modification (CRM) of the rabbit's nictitating membrane response (NMR) involves changes in responding to an unconditioned stimulus (US) when the US is tested in the absence of the conditioned stimulus. Previous experiments have shown that CRM is a function of the type and intensity of the aversive US used during classical conditioning. As a result, it has been suggested that CRM may be mediated, at least in part, by the aversiveness of the US. Here, we show that by using a moderately intense electrical pulse to the skin as a US, CRM of the rabbit NMR is accompanied by an increase in heart rate. The largest changes in heart rate occur at US intensities that produce the strongest levels of CRM. The heart rate data show that there may be an increased emotional/arousal component to the US that is correlated with CRM and support the use of CRM as a potential model for posttraumatic stress disorder.

The effects of amygdala lesions on hippocampal activity and classical eyeblink conditioning in rats

The hippocampus and the amygdala have long been associated with memory, emotion, and motivated behaviors. Although the role of these two brain areas in learning a simple, discrete motor response has been well studied, a definitive theory concerning their functions remains elusive. The present experiment involved selective lesions of the central nucleus (CE) or the basolateral nucleus (BA) of the amygdala in rats followed by single-unit analyses of hippocampal CA1 subfield activity during classical eye blink conditioning. Removal of CE or BA adversely affected the development of conditioned responding. Differences between groups in the patterns of hippocampal activity were observed. Similar to previous rabbit studies, hippocampal activity recorded from sham rats showed that CA1 cells became active during the CS-US period as conditioning progressed with activity especially prevalent just prior to US onset. Increased activity over training was seen during the CS-US interval in CE-lesioned rats, but the pattern differed from control rats-uniform excitation was seen across the entire CS-US period. BA-lesioned rats initially showed uniform CS-US period activation in early phases of training, but then showed patterns of hippocampal activity that resembled control rats in later stages of conditioning. The data suggest that the amygdala may play a modulatory role in the acquisition of conditioned eye blink responses and also in the formation of learning-related activity in the hippocampus.

Recovery of the rabbit's conditioned nictitating membrane response without direct reinforcement after extinction

Three experiments demonstrated that, following the extinction of an established conditioned stimulus (CSA--e.g., tone), the pairing of a novel, cross-modal stimulus (CSB--e.g., light) with the unconditioned stimulus (US) results in strong recovery of responding to the extinguished CSA. Experiment 1 demonstrated that the recovery of responding to CSA is not the result of US reinstatement but is attributable to pairings of CSB with the US. Experiment 2 demonstrated that the recovery of responding is specific to CSA and is not the result of cross-modal generalization. Experiment 3 revealed that a large number of CSB-US pairings in Stage 1 significantly reduced the amount of recovery to CSA during subsequent CSB-US trials. Experiment 3 also provided unexpected evidence of cross-modal secondary extinction. The extinction and subsequent recovery of responding seen in the present experiments is discussed with respect to possible contributions from contextual associations, CS processing, US processing, conditioned response expression, and layered excitatory associations.

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.

Trace amounts of copper in water induce beta-amyloid plaques and learning deficits in a rabbit model of Alzheimer's disease

Despite the crucial role played by cholesterol and copper in nutrition and normal brain function, recent evidence indicates that they may both be important factors in the etiology of Alzheimer's disease (AD). Here we provide critical evidence for the role of cholesterol and copper in AD by showing that the addition of trace amounts of copper (0.12 ppm) to water given to cholesterol-fed rabbits can induce beta-amyloid (Abeta) accumulation, including senile plaque-like structures in the hippocampus and temporal lobe, and can significantly retard the ability of rabbits to learn a difficult trace conditioning task. The Abeta deposits do not affect the ability of rabbits to detect or respond to the training stimuli nor to learn a simpler delay conditioning task. Trace amounts of copper in drinking water may influence clearance of Abeta from the brain at the level of the interface between the blood and cerebrovasculature and combined with high cholesterol may be a key component to the accumulation of Abeta in the brain, having a significant impact on learning and memory. Cholesterol-fed rabbits have at least 12 pathological markers seen in AD, suggesting that the cholesterol-fed rabbit is a good animal model for studying AD.

Conditioning-specific reflex modification of the rabbit (Oryctolagus cuniculus) nictitating membrane response: US intensity effects

Conditioning-specific reflex modification (CRM) of the rabbit's nictitating membrane response (NMR) describes changes in responding to an unconditioned stimulus (US) when the rabbit is tested in the absence of the conditioned stimulus. Specifically, after at least 3 days of tone-electrical stimulation pairings, responses to the US increase in size, especially at intensities weaker than the training intensity. CRM is similar to classical conditioning in that it is a function of the strength of conditioning, it can be extinguished, and it can be generalized from one stimulus to another. To compare CRM and classical conditioning further, we conducted three experiments to examine the effects of US intensity (1.0, 2.0, and 4.0 mA) on CRM. CRM was weak following conditioning with 1.0 mA and extremely strong following conditioning with 2.0 mA and 4.0 mA. The data suggest that CRM is a function of US intensity and have implications for posttraumatic stress disorder, a disorder potentially modeled by CRM.