Paired-housing selectively facilitates within-session extinction of avoidance behavior, and increases c-Fos expression in the medial prefrontal cortex, in anxiety vulnerable Wistar-Kyoto rats.

OBJECTIVE: The perseveration of avoidance behavior, even in the absence of once threatening stimuli, is a key feature of anxiety and related psychiatric conditions. This phenomenon can be observed in the Wistar-Kyoto (WKY) rat which, in comparison to outbred controls, demonstrates impaired extinction of avoidance behavior. Also characteristic of the WKY rat is abnormalities of the neurocircuitry and neuroplasticity of the medial prefrontal cortex (mPFC). One means of reducing physiological responses to anxiety, and conditioned fear, in social species is the presence of a conspecific animal. The current study investigates whether or not pair-housed WKY rats would show facilitated extinction of avoidance in comparison to individual-housed WKY rats, and whether or not pair-housing influences mPFC activation during lever-press avoidance. METHODS: Male WKY rats were assigned to individual-housed and pair-housed conditions. Rats were trained in lever-press avoidance. Each session of lever-press avoidance consisted of 20 trials, where pressing a lever in response to a warning tone prevented foot-shocks. Rats received 12 acquisition sessions over 4weeks; followed by 6 extinction sessions over 2weeks, where foot-shocks ceased to be delivered. Brains were harvested 90min after trials 1 and 10 of extinction sessions 1 and 6, and mPFC sections underwent c-Fos staining as a measure of activation. RESULTS: Pair-housed rats showed facilitated lever-press avoidance extinction rates, but the main cause for this overall difference was a selective facilitation of within-session extinction. Similar to individual-housed rats, pair-housed rats continued to avoid during trial 1 of extinction even when the avoidance responding had been significantly reduced by the end of the previous session. Pair-housed rats sacrificed on trial 1 showed greater c-Fos expression in the anterior cingulate cortex and prelimbic cortex subregions of the mPFC compared individual-housed rats sacrificed on trial 1. CONCLUSION: This data shows pair-housing to facilitate the extinction of avoidance, and to influence activity of the mPFC, in WKY rats. Despite this environmental manipulation, the pair-housed WKY rats continued to show avoidance responding on trial 1 of extinction sessions. This demonstrates that within-session extinction can be dissociated from between-session extinction-resistance in WKY rats. Furthermore, it suggests the individual-housing of WKY rats selectively slows within-session extinction, possibly by reducing neuronal activity of the mPFC during the testing situation.

Dysfunction in amygdala-prefrontal plasticity and extinction-resistant avoidance: A model for anxiety disorder vulnerability.

Individuals exhibiting an anxiety disorder are believed to possess an innate vulnerability that makes them susceptible to the disorder. Anxiety disorders are also associated with abnormalities in the interconnected brain regions of the amygdala and prefrontal cortex (PFC). However, the link between anxiety vulnerability and amygdala-PFC dysfunction is currently unclear. Accordingly, the present study sought to determine if innate dysfunction within the amygdala to PFC projection underlies the susceptibility to develop anxiety-like behavior, using an anxiety vulnerable rodent model. The inbred Wistar Kyoto (WKY) rat was used to model vulnerability, as this strain naturally expresses extinction-resistant avoidance; a behavior that models the symptom of avoidance present in anxiety disorders. Synaptic plasticity was assessed within the projection from the basolateral nucleus of the amygdala (BLA) to the prelimbic cortical subdivision of the PFC in WKY and Sprague Dawley (SD) rats. While WKY rats exhibited normal paired-pulse plasticity, they did not maintain long-term potentiation (LTP) as SD rats. Thus, impaired plasticity within the BLA-PL cortex projection may contribute to extinction resistant avoidance of WKY, as lesions of the PL cortex in SD rats impaired extinction of avoidance similar to WKY rats. Treatment with d-cycloserine to reverse the impaired LTP in WKY rats was unsuccessful. The lack of LTP in WKY rats was associated with a significant reduction of NMDA receptors containing NR2A subunits in the PL cortex. Thus, dysfunction in amygdala-PFC plasticity is innate in anxiety vulnerable rats and may promote extinction-resistant avoidance by disrupting communication between the amygdala and prefrontal cortex.

Altered activity of the medial prefrontal cortex and amygdala during acquisition and extinction of an active avoidance task.

Altered medial prefrontal cortex (mPFC) and amygdala function is associated with anxiety-related disorders. While the mPFC-amygdala pathway has a clear role in fear conditioning, these structures are also involved in active avoidance. Given that avoidance perseveration represents a core symptom of anxiety disorders, the neural substrate of avoidance, especially its extinction, requires better understanding. The present study was designed to investigate the activity, particularly, inhibitory neuronal activity in mPFC and amygdala during acquisition and extinction of lever-press avoidance in rats. Neural activity was examined in the mPFC, intercalated cell clusters (ITCs) lateral (LA), basal (BA) and central (CeA) amygdala, at various time points during acquisition and extinction, using induction of the immediate early gene product, c-Fos. Neural activity was greater in the mPFC, LA, BA, and ITC during the extinction phase as compared to the acquisition phase. In contrast, the CeA was the only region that was more activated during acquisition than during extinction. Our results indicate inhibitory neurons are more activated during late phase of acquisition and extinction in the mPFC and LA, suggesting the dynamic involvement of inhibitory circuits in the development and extinction of avoidance response. Together, these data start to identify the key brain regions important in active avoidance behavior, areas that could be associated with avoidance perseveration in anxiety disorders.

Avoidance expression in rats as a function of signal-shock interval: strain and sex differences.

Inbred Wistar Kyoto (WKY) rats express inhibited temperament, increased sensitivity to stress, and exaggerated expressions of avoidance. A long-standing observation for lever press escape/avoidance learning in rats is the duration of the warning signal (WS) determines whether avoidance is expressed over escape. Outbred female Sprague-Dawley (SD) rats trained with a 10-s WS efficiently escaped, but failed to exhibit avoidance; avoidance was exhibited to a high degree with WSs longer than 20-s. We examined this longstanding WS duration function and extended it to male SD and male and female WKY rats. A cross-over design with two WS durations (10 or 60 s) was employed. Rats were trained (20 trials/session) in four phases: acquisition (10 sessions), extinction (10 sessions), re-acquisition (8 sessions) and re-extinction (8 sessions). Consistent with the literature, female and male SD rats failed to express avoidance to an appreciable degree with a 10-s WS. When these rats were switched to a 60-s WS, performance levels in the initial session of training resembled the peak performance of rats trained with a 60-s WS. Therefore, the avoidance relationship was acquired, but not expressed at 10-s WS. Further, poor avoidance at 10-s does not adversely affect expression at 60-s. Failure to express avoidance with a 10-s WS likely reflects contrasting reinforcement value of avoidance, not a reduction in the amount of time available to respond or competing responses. In contrast, WKY rats exhibited robust avoidance with a 10-s WS, which was most apparent in female WKY rats. Exaggerated expression of avoidances by WKY rats, especially female rats, further confirms this inbred strain as a model of anxiety vulnerability.

ITI-Signals and Prelimbic Cortex Facilitate Avoidance Acquisition and Reduce Avoidance Latencies, Respectively, in Male WKY Rats.

As a model of anxiety disorder vulnerability, male Wistar-Kyoto (WKY) rats acquire lever-press avoidance behavior more readily than outbred Sprague-Dawley rats, and their acquisition is enhanced by the presence of a discrete signal presented during the inter-trial intervals (ITIs), suggesting that it is perceived as a safety signal. A series of experiments were conducted to determine if this is the case. Additional experiments investigated if the avoidance facilitation relies upon processing through medial prefrontal cortex (mPFC). The results suggest that the ITI-signal facilitates acquisition during the early stages of the avoidance acquisition process, when the rats are initially acquiring escape behavior and then transitioning to avoidance behavior. Post-avoidance introduction of the visual ITI-signal into other associative learning tasks failed to confirm that the visual stimulus had acquired the properties of a conditioned inhibitor. Shortening the signal from the entirety of the 3 min ITI to only the first 5 s of the 3 min ITI slowed acquisition during the first four sessions, suggesting the flashing light (FL) is not functioning as a feedback signal. The prelimbic (PL) cortex showed greater activation during the period of training when the transition from escape responding to avoidance responding occurs. Only combined PL + infralimbic cortex lesions modestly slowed avoidance acquisition, but PL-cortex lesions slowed avoidance response latencies. Thus, the FL ITI-signal is not likely perceived as a safety signal nor is it serving as a feedback signal. The functional role of the PL-cortex appears to be to increase the drive toward responding to the threat of the warning signal. Hence, avoidance susceptibility displayed by male WKY rats may be driven, in part, both by external stimuli (ITI signal) as well as by enhanced threat recognition to the warning signal via the PL cortex.

Effects of psychotropic agents on extinction of lever-press avoidance in a rat model of anxiety vulnerability.

Avoidance and its perseveration represent key features of anxiety disorders. Both pharmacological and behavioral approaches (i.e., anxiolytics and extinction therapy) have been utilized to modulate avoidance behavior in patients. However, the outcome has not always been desirable. Part of the reason is attributed to the diverse neuropathology of anxiety disorders. Here, we investigated the effect of psychotropic drugs that target various monoamine systems on extinction of avoidance behavior using lever-press avoidance task. Here, we used the Wistar-Kyoto (WKY) rat, a unique rat model that exhibits facilitated avoidance and extinction resistance along with malfunction of the dopamine (DA) system. Sprague Dawley (SD) and WKY rats were trained to acquire lever-press avoidance. WKY rats acquired avoidance faster and to a higher level compared to SD rats. During pharmacological treatment, bupropion and desipramine (DES) significantly reduced avoidance response selectively in WKY rats. However, after the discontinuation of drug treatment, only those WKY rats that were previously treated with DES exhibited lower avoidance response compared to the control group. In contrast, none of the psychotropic drugs facilitated avoidance extinction in SD rats. Instead, DES impaired avoidance extinction and increased non-reinforced response in SD rats. Interestingly, paroxetine, a widely used antidepressant and anxiolytic, exhibited the weakest effect in WKY rats and no effects at all in SD rats. Thus, our data suggest that malfunctions in brain catecholamine system could be one of the underlying etiologies of anxiety-like behavior, particularly avoidance perseveration. Furthermore, pharmacological manipulation targeting DA and norepinephrine may be more effective to facilitate extinction learning in this strain. The data from the present study may shed light on new pharmacological approaches to treat patients with anxiety disorders who are not responding to serotonin re-uptake inhibitors.

Avoidance as expectancy in rats: sex and strain differences in acquisition.

Avoidance is a core feature of anxiety disorders and factors which increase avoidance expression or its resistance represent a source of vulnerability for anxiety disorders. Outbred female Sprague Dawley (SD) rats and inbred male and female Wistar-Kyoto (WKY) rats expressing behaviorally inhibited (BI) temperament learn avoidance faster than male SD rats. The training protocol used in these studies had a longstanding interpretive flaw: a lever-press had two outcomes, termination of the warning signal (WS) and prevention of foot shock. To disambiguate between these two explanations, we conducted an experiment in which: (a) a lever-press terminated the WS and prevented shock, and (b) a lever-press only prevented shock, but did not influence the duration of the WS. Thus, a 2 × 2 × 2 (Strain × Sex × Training) design was employed to assess the degree to which the response contingency of the WS termination influenced acquisition. Male and female SD and WKY rats were matched on acoustic startle reactivity within strain and sex and randomly assigned to the training procedures. In addition, we assessed whether the degree of avoidance acquisition affected estrus cycling in female rats. Consistent with earlier work, avoidance performance of female rats was generally superior to males and WKY rats were superior to SD rats. Moreover, female SD and male WKY rats were roughly equivalent. Female sex and BI temperament were confirmed as vulnerability factors in faster acquisition of avoidance behavior. Avoidance acquisition disrupted estrus cycling with female WKY rats recovering faster than female SD rats. Although termination of the WS appears to be reinforcing, male and female WKY rats still achieved a high degree (greater than 80% asymptotic performance) of avoidance in the absence of the WS termination contingency. Such disambiguation will facilitate determination of the neurobiological basis for avoidance learning and its extinction.

The role of the hippocampus in avoidance learning and anxiety vulnerability.

The hippocampus has been implicated in anxiety disorders and post-traumatic stress disorder (PTSD); human studies suggest that a dysfunctional hippocampus may be a vulnerability factor for the development of PTSD. In the current study, we examined the effect of hippocampal damage in avoidance learning, as avoidance is a core symptom of all anxiety disorders. First, the effect of hippocampal damage on avoidance learning was investigated in outbred Sprague Dawley (SD) rats. Second, the function of the hippocampus in Wistar-Kyoto (WKY) rats was compared to SD rats. The WKY rat is an animal model of behavioral inhibition, a risk factor for anxiety, and demonstrates abnormal avoidance learning, marked by facilitated avoidance acquisition and resistance to extinction. The results of the current study indicate that hippocampal damage in SD rats leads to impaired extinction of avoidance learning similar to WKY rats. Furthermore, WKY rats have reduced hippocampal volume and impaired hippocampal synaptic plasticity as compared to SD rats. These results suggest that hippocampal dysfunction enhances the development of persistent avoidance responding and, thus, may confer vulnerability to the development of anxiety disorders and PTSD.

Activation of extracellular signal-regulated kinase (ERK) and ΔFosB in emotion-associated neural circuitry after asymptotic levels of active avoidance behavior are attained.

Avoidance susceptibility may constitute a vulnerability to develop anxiety disorders, and Wistar-Kyoto (WKY) rats exhibit unique features in their acquisition of avoidance behavior that appear to promote susceptibility to this form of learning, namely the absence of the commonly observed "warm-up" effect. The present study sought to determine if strain differences in acquired avoidance behavior, between WKY and Sprague Dawley rats, could be attributed to differences in dopamine-related plasticity, represented by extracellular signal-regulated kinase (ERK) activity, and prolonged neuronal activation, represented by ΔFosB accumulation, in three key areas of the brain: the medial prefrontal cortex (mPFC), dorsal striatum (DS), and basolateral amygdala (BLA). Consistent with earlier work, WKY rats exhibited a higher level of asymptotic performance of avoidance behavior, which included an absence of warm-up in the first few trials of later training sessions, and they exhibited more non-reinforced anticipatory responses in the single minute prior to the initiation of the first warning signal presentation of each training session. In the brain, phosyphorylated ERK2 (pERK2) activation was higher in avoidance trained rats in both the mPFC and DS, although the difference in DS was mostly observed in WKY rats. Avoidance-training was associated with higher levels of ΔFosB expression in the mPFC of SD rats, but not WKY rats. The strain differences in pERK2 activation in the DS and ΔFosB levels in the mPFC may underlie the strain-specific differences observed in warm-up, the emission of non-reinforced anticipatory responses, and general differences in asymptotic performance of active avoidance behavior. The mPFC and DS require further study as potential neural targets for understanding avoidance susceptibility and, as a result, anxiety vulnerability.

Vulnerability factors in anxiety: Strain and sex differences in the use of signals associated with non-threat during the acquisition and extinction of active-avoidance behavior.

Rats that exhibit a behaviorally inhibited temperament acquire active-avoidance behaviors quicker, and extinguish them slower, than normal outbred rats. Here we explored the contribution of stimuli that signal periods of non-threat (i.e. safety signals) in the process of acquiring active-avoidance behavior. Utilizing a discrete lever-press escape-avoidance protocol, outbred Sprague-Dawley (SD) rats and inbred, behaviorally inhibited, Wistar-Kyoto (WKY) rats were tested under conditions where a flashing light was either presented or not during periods of non-threat (the inter-trial interval, ITI). For males, we found the absence of the ITI-signal slowed the acquisition of avoidance behavior selectively in WKY rats. However, extinction of the avoidance behavior was not influenced by training with or without the ITI-signal; WKY males extinguished slower than SD males. For females, the presence of the ITI-signal did not affect acquisition in either strain. However, after training with the ITI-signal, females of both strains extinguished quicker in its absence than in its presence. In order to determine if facilitated acquisition of avoidance learning in male WKY rats was due to a paradigm-independent influence of the visual stimulus used as ITI-signal upon associative learning, we conducted eyeblink conditioning in the presence or absence of a similar visual stimulus. No differences in acquisition, as a function of this visual stimulus, were observed within the male WKY rats, but, as was observed in avoidance learning, male WKY rats extinguished slower than male SD rats. Thus, avoidance susceptibility for male WKY rats may be tied both to the presence of non-threat signals as well as a resistance to extinguish Pavlovian-conditioned associations. Female susceptibility to resist extinguishing avoidant behavior is discussed with respect to the possible role of stimuli serving as occasion setters for threat contexts.

Avoidance perseveration during extinction training in Wistar-Kyoto rats: an interaction of innate vulnerability and stressor intensity.

Given that avoidance is a core feature of anxiety disorders, Wistar-Kyoto (WKY) rats may be a good model of anxiety vulnerability for their hypersensitivity to stress and trait behavioral inhibition. Here, we examined the influence of strain and shock intensity on avoidance acquisition and extinction. Accordingly, we trained WKY and Sprague-Dawley (SD) rats in lever-press avoidance using either 1.0-mA or 2.0-mA foot-shock. After extinction, neuronal activation was visualized by c-Fos for overall activity and parvalbumin immunoreactivity for gamma-aminobutyric acid (GABA) neuron in brain areas linked to anxiety (medial prefrontal cortex and amygdala). Consistent with earlier work, WKY rats acquired lever-press avoidance faster and to a greater extent than SD rats. However, the intensity of foot shock did not differentially affect acquisition. Although there were no differences during extinction in SD rats, avoidance responses of WKY rats trained with the higher foot shock perseverated during extinction compared to those WKY rats trained with lower foot shock intensity or SD rats. WKY rats trained with 2.0-mA shock exhibited less GABAergic activation in the basolateral amygdala after extinction. These findings suggest that inhibitory modulation in amygdala is important to ensure successful extinction learning. Deficits in avoidance extinction secondary to lower GABAergic activation in baslolateral amygdala may contribute to anxiety vulnerability in this animal model of inhibited temperament.

Damage of GABAergic neurons in the medial septum impairs spatial working memory and extinction of active avoidance: effects on proactive interference.

The medial septum and diagonal band (MSDB) are important in spatial learning and memory. On the basis of the excitotoxic damage of GABAergic MSDB neurons, we have recently suggested a role for these neurons in controlling proactive interference. Our study sought to test this hypothesis in different behavioral procedures using a new GABAergic immunotoxin. GABA-transporter-saporin (GAT1-SAP) was administered into the MSDB of male Sprague-Dawley rats. Following surgery, rats were trained in a reference memory water maze procedure for 5 days, followed by a working memory (delayed match to position) water maze procedure. Other rats were trained in a lever-press avoidance procedure after intraseptal GAT1-SAP or sham surgery. Intraseptal GAT1-SAP extensively damaged GABAergic neurons while sparing most cholinergic MSDB neurons. Rats treated with GAT1-SAP were not impaired in acquiring a spatial reference memory, learning the location of the escape platform as rapidly as sham rats. In contrast, GAT1-SAP rats were slower than sham rats to learn the platform location in a delayed match to position procedure, in which the platform location was changed every day. Moreover, GAT1-SAP rats returned to previous platform locations more often than sham rats. In the active avoidance procedure, intraseptal GAT1-SAP impaired extinction but not acquisition of the avoidance response. Using a different neurotoxin and behavioral procedures than previous studies, the results of this study paint a similar picture that GABAergic MSDB neurons are important for controlling proactive interference.

Classical and instrumental conditioning of eyeblink responses in Wistar-Kyoto and Sprague-Dawley rats.

Wistar-Kyoto (WKY) rats, an animal model of anxiety vulnerability, acquire lever-press avoidance faster than outbred Sprague-Dawley (SD) rats. Faster avoidance acquisition may reflect an inherent ability to acquire cue-outcome associations, response-outcome associations or both. To evaluate cue-outcome learning, acquisition of classically conditioned eyeblink response was compared in SD and WKY rats using a delay-type paradigm (500-ms conditioned stimulus (CS) coterminating with a 10-ms unconditional stimulus (US)). WKY rats demonstrated enhanced classical conditioning, with both faster acquisition and greater asymptotic performance in delay-type training than SD rats. To evaluate response-outcome learning, separate SD and WKY rats were given control over US delivery through imposition of an omission contingency into delay-type training (emitting a conditioned response (CR) prevented delivery of the US). The schedule of US delivery derived by these rats became the training regimen for a separate group of SD and WKY rats, yoked within strain. In SD rats, no differences in acquisition were detected between those given control over US delivery and those trained with the same partial reinforcement schedule. Acquisition rates of those WKY rats with control exceeded those trained with a yoked-schedule of US presentation. Collectively, WKY rats exhibit enhanced classical conditioning and sensitivity to schedules of reinforcement compared to outbred SD rats. Anxiety vulnerability, in particular inhibited temperament, may be traced to active processes in the prediction and control of aversive events.

Deficient proactive interference of eyeblink conditioning in Wistar-Kyoto rats.

Wistar-Kyoto (WKY) rats exhibit behavioral inhibition and model anxiety vulnerability. Although WKY rats exhibit faster active avoidance acquisition, simple associative learning or the influence of proactive interference (PI) has not been adequately assessed in this strain. Therefore, we assessed eyeblink conditioning and PI in WKY and outbred Sprague-Dawley (SD) rats. Rats were pre-exposed to either the experimental context, the conditioned stimulus (CS), the unconditional stimulus (US), or the CS & US in an explicitly unpaired (EUP) manner, to examine latent inhibition (LI), US pre-exposure effect, or learned irrelevance (LIRR), respectively. Immediately following pre-exposures, rats were trained in a delay-type paradigm (500 ms CS coterminating with a 10-ms US) for one session. During training SD rats exhibited LI and inhibition from US pre-exposures without evidence of LIRR. PI was less evident in WKY rats; LI was absent in WKY rats. Even in the context of reduced PI to CS-alone and US-alone pre-exposures, LIRR was not apparent in WKY rats. The more normal acquisition rates exhibited by WKY rats, under conditions which degrade performance in SD rats, increases the overall likelihood for WKY rats to acquire defensive responses. Enhanced acquisition of defensive responses is a means by which anxiety vulnerability (e.g., behavioral inhibition) is translated to anxiety psychopathology.

Vulnerability factors in anxiety determined through differences in active-avoidance behavior.

The risk for developing anxiety disorders is greater in females and those individuals exhibiting a behaviorally inhibited temperament. Growth of behavioral avoidance in people is a significant predictor of symptom severity in anxiety disorders, including post-traumatic stress disorder. Using an animal model, our lab is examining how the process of learning avoidant behavior may lead certain individuals to develop anxiety. Here we examined whether the known vulnerabilities of female sex and behaviorally inhibited temperament have individual or additive effects upon the acquisition of an active-avoidance response. A discrete trial lever-press escape-avoidance protocol was used to examine the acquisition of behavioral avoidance in male and female Sprague-Dawley (SD) rats and behaviorally inhibited inbred Wistar-Kyoto (WKY) rats. Overall, WKY rats of both sexes were indistinguishable in their behavior during the acquisition of an active-avoidance response, exhibiting quicker acquisition of reinforced responses both between and within session compared to SD rats. Further WKY rats emitted more non-reinforced responses than SD rats. Sex differences were evident in SD rats in both the acquisition of the reinforced response and the emission of non-reinforced responses, with SD females acquiring the response quicker and emitting more non-reinforced responses following lever presses that led to an escape from shock. As vulnerability factors, behavioral inhibition and female sex were each associated with more prevalent reinforced and non-reinforced avoidant behavior, but an additive effect of these 2 factors was not observed. These data illustrate the importance of genetics (both strain and sex) in the assessment and modeling of anxiety vulnerability through the acquisition of active-avoidance responses and the persistence of emitting those responses in periods of non-reinforcement.

Estrus cycle stage modifies the presentation of stress-induced startle suppression in female Sprague-Dawley rats.

Tailshock stress causes transient reductions in startle reactivity, associative learning and open field activity in female rats in an ovarian hormone dependent manner. Others have shown estrogen modulation of associative learning by testing across the estrus cycle and pharmacological manipulations. Here we tested whether stress-induced suppression of startle reactivity can be attributed to circulating ovarian hormones. Female rats were tracked across the estrus cycle and subjected to the stressor (2 h periodic tailshock) the morning of diestrus, proestrus, estrus, or metestrus. Startle reactivity was tested 2 h following the cessation of the tailshock. Using a multi-stimulus protocol, we determined there were differences in startle sensitivity and responsivity. Following stressor exposure, estrus females exhibited reduced startle responsivity. In contrast, diestrus females exhibited increased sensitivity to the lowest acoustic stimulus. The results are discussed with respect to ovarian hormone regulation of the immune system and sensory reactivity during and following trauma that may lead to different abnormal behaviors in females in the wake of traumatic stress.

Preclinical investigation of the functional effects of memantine and memantine combined with galantamine or donepezil.

Combinations of drugs approved to treat Alzheimer's disease (AD) were tested in older rabbits with delay eyeblink classical conditioning, a form of associative learning severely impaired in AD. In Experiment 1 (n=49 rabbits), low doses (0.1, 0.5, 1.0, and 0.0 (vehicle) mg/kg) of memantine (Namenda) were tested. These three doses neither improved nor impaired acquisition at a statistically significant level. The 0.5 mg/kg dose had the greatest effect numerically and did not cause sensitization or habituation in explicitly unpaired controls. In Experiment 2 (n=56), doses of galantamine (Razadyne; 3.0 mg/kg) and donepezil (Aricept; 0.75 mg/kg) that had comparable magnitudes of cholinesterase inhibition were tested alone and in combination with 0.5 mg/kg memantine. Older rabbits treated with galantamine and with galantamine+memantine learned significantly better than vehicle-treated rabbits, but adding memantine did not improve learning over galantamine alone. Older rabbits treated with donepezil or a combination of memantine and donepezil did not learn significantly better than rabbits treated with vehicle. Galantamine has two mechanisms of action: mild cholinesterase inhibition and allosteric modulation of nicotinic acetylcholine receptors (nAChRs). When equated for cholinesterase inhibition, galantamine had significant efficacy in the eyeblink conditioning model system, but donepezil did not, indicating that modulation of nAChRs may be the mechanism that significantly ameliorates learning deficits in this model. In the absence of AD neuropathology in older rabbits, memantine had no efficacy alone or in combination with the other drugs.

Cholinergic overstimulation supports conditioned-facilitated startle but not conditioned hyperalgesia.

Pyridostigmine bromide (PB), a peripheral cholinesterase inhibitor, has been shown to support odor-potentiated startle responding in rats. Here we conducted 2 sets of experiments that further characterize aspects of this learned association. First we conducted experiments designed to further characterize the learning parameters of the odor-PB association that leads to startle facilitation weeks later. We found that an acute injection of PB causes an increase in startle reactivity that lasts less than 2 h. This is evidence for PB's direct action on the startle response as an enhancing agent. We also delineated the duration of the conditioned enhancement to less than 4 weeks. Second, we conducted similar studies but substituted a nociceptive paw-lick response (thermal pain reflex) for the startle reflex. PB did not have an unconditional action upon the latency to paw-lick to a 48.5 degrees C heated plate nor did any subsequent changes in paw-lick occur in the presence of the previously paired odor. These results suggest that the actions of PB, as an unconditional stimulus, are limited to specific behaviors. Future work examining this compound as a source of conditioned symptoms (as in the case of Gulf War Illness) should focus on those symptoms that are directly influenced by peripheral cholinergic activity.

Strain differences in the distribution of dopamine (DA-2 and DA-3) receptor sites in rat brain.

The dopamine (DA) pathway mediates numerous neuronal functions which are implicated in psychiatric disorders. Previously, our lab investigated the status of the dopamine transporter in the Wistar-Kyoto rat, a purported rodent model of depressive behavior, and reported significant alterations in transporter binding sites in several brain regions when compared to control rat strains. Given that DA-2 and DA-3 receptors belong to the same class of DA receptors, are co-localized in the mesolimbic and nigrostriatal regions of the brain and function as autoreceptors, this study mapped the distribution of central DA-2 and DA-3 receptors in Wistar-Kyoto and Wistar rats. The results indicated that while the binding of 125I-sulpride to DA-2 receptors was higher in the nucleus accumbens (shell) and ventral tegmental area, it was lower in the nucleus accumbens (core), caudate putamen and hypothalamus in Wistar-Kyoto compared to Wistar rats. In contrast, the binding of 125I-sulpride to DA-3 receptors was higher in the caudate putamen, nucleus accumbens (shell and core) and islands of Calleja in Wistar-Kyoto compared to Wistar rats. Given that DA-2 like receptors in the ventral tegmental area function as autoreceptors, it is possible that the greater inhibitory effects exerted by DA-2 and DA-3 receptors in Wistar-Kyoto rats may lead to a net deficit in DA levels in areas receiving projection from this cell body area.

Antidepressant drug induced alterations in binding to central dopamine transporter sites in the Wistar Kyoto rat strain.

The Wistar Kyoto (WKY) rat has been proposed as an animal model of depressive behavior. Exposing WKY rats to stress stimulation produces symptoms such as anhedonia, psychomotor retardation, ambivalence and negative memory bias. Given the role of the mesolimbic dopamine (DA) system in cognitive, emotional and motivational behaviors, we previously examined the distribution of DA transporter (DAT) sites in the brains of WKY compared to Wistar (WIS) and Sprague-Dawley (S-D) rats. WKY rats exhibited significant differences in DAT binding sites in the cell body as well as mesolimbic areas compared to the other strains. It was reasoned that these differences may lead to altered synaptic levels of DA in specific brain regions thus contributing to the behavioral differences observed in this rat strain. Thus, the present study examined whether repeated treatment with antidepressant drugs that block the uptake of DA (nomifensine and bupropion) would modify [3H]-GBR12935 binding to DAT sites in WKY rats compared to WIS and S-D rats. The results indicate that while nomifensine and bupropion increased the binding of [3H]-GBR12935 to DAT sites in the mesocorticolimbic regions in WKY rats, these drugs increased the binding of [3H]-GBR12935 to DAT sites in the cell body areas in WIS rats but not in S-D and WKY rats. The data from this study suggest that antidepressant induced alterations in DAT sites in the mesocorticolimbic brain regions may play a role in the behavioral improvement seen in WKY rats, as measured by the Open Field Test (OFT) and the Porsolt Forced Swim Test (FST).