Extensive spatial training does not negate age differences in response latency.

Previously, Nippak et al. [Nippak, P.M.D., Chan, A.D.F., Campbell, Z., Muggenburg, B., Head, E., Ikeda-Douglas, C., Murphy, H., Cotman, C.W., Milgram, N.W., 2003. Response latency in the canine: mental ability or mental strategy? Behav. Neurosci. 117 (5), 1066-1075] reported that young dogs respond significantly slower than aged dogs during the acquisition of a three-component delayed non-match to position (3-DNMP) task. Thus, we examined how age influences response latency (RL) when animals are trained extensively on the 3-DNMP task. Animals were separated into two groups based on their task sophistication. The first group comprised young (N=5) and aged (N=10) dogs that received extensive spatial training on a two-component delayed non-match to position task (2-DNMP) before 3-DNMP testing, while the second group of young (N=8) and aged (N=11) animals received extensive training on a variety of other non-spatial cognitive tasks between each 3-DNMP test period. RL age differences were absent following extensive 3-DNMP testing; however, other age-dependent performance differences emerged: all young animals learned the task and displayed RL slowing and superior response accuracy (RA) on the center-incorrect (CI) subtest, while several aged animals failed to learn the task and displayed no RL or RA subtest variations even when they acquired the task. Toates's [Toates, F., 1998. The interaction of cognitive and stimulus-response processes in the control of behaviour, Neurosci. Biobehav. Rev. 22 (1), 59-83] theory of RL and mental strategy was proposed to explain these age differences in response strategies: the fast-responding aged animals utilized stimulus-response strategies, while the slow-responding young animals adopted cognitive strategies, a specific requirement for solving the CI subtest.

Changes in phosphorylation of the NMDA receptor in the rat hippocampus induced by status epilepticus.

Systemic administration of pilocarpine preceded by lithium induces status epilepticus (SE) that results in neurodegeneration and may lead to the development of spontaneous recurrent seizures. We investigated the effect of Li/pilocarpine-induced SE on phosphorylation of the NMDA receptor in rat hippocampus. Phosphorylation of NR1 by PKC on Ser890 was decreased to 45% of control values immediately following 1 h of SE. During the first 3 h following the termination of SE, phosphorylation of Ser890 increased 4-fold before declining to control values by 24 h. Phosphorylation of NR1 by PKA was also depressed relative to controls immediately following SE and transiently increased above control values upon the termination of SE. SE was accompanied by a general increase in tyrosine phosphorylation of hippocampal proteins that lasted for several hours following the termination of seizures. Tyrosine phosphorylation of the NR2A and NR2B subunits of the NMDAR increased 3-4-fold over control values during SE, continued to increase during the first hour following SE and then declined to control levels by 24 h. SE resulted in the activation of Src and Pyk2 associated with the postsynaptic apparatus, suggesting a role for these enzymes in the SE-induced increase in tyrosine phosphorylation. Changes in phosphorylation of the NMDA receptor may play a role in the pathophysiological consequences of SE.

Learning ability in aged beagle dogs is preserved by behavioral enrichment and dietary fortification: a two-year longitudinal study.

The effectiveness of two interventions, dietary fortification with antioxidants and a program of behavioral enrichment, was assessed in a longitudinal study of cognitive aging in beagle dogs. A baseline protocol of cognitive testing was used to select four cognitively equivalent groups: control food-control experience (C-C), control food-enriched experience (C-E), antioxidant fortified food-control experience (A-C), and antioxidant fortified food-enriched experience(A-E). We also included two groups of young behaviorally enriched dogs, one receiving the control food and the other the fortified food. Discrimination learning and reversal was assessed after one year of treatment with a size discrimination task, and again after two years with a black/white discrimination task. The four aged groups were comparable at baseline. At one and two years, the aged combined treatment group showed more accurate learning than the other aged groups. Discrimination learning was significantly improved by behavioral enrichment. Reversal learning was improved by both behavioral enrichment and dietary fortification. By contrast, the fortified food had no effect on the young dogs. These results suggest that behavioral enrichment or dietary fortification with antioxidants over a long-duration can slow age-dependent cognitive decline, and that the two treatments together are more effective than either alone in older dogs.

Landmark discrimination learning in the dog: effects of age, an antioxidant fortified food, and cognitive strategy.

The landmark discrimination learning test can be used to assess the ability to utilize allocentric spatial information to locate targets. The present experiments examined the role of various factors on performance of a landmark discrimination learning task in beagle dogs. Experiments 1 and 2 looked at the effects of age and food composition. Experiments 3 and 4 were aimed at characterizing the cognitive strategies used in performance on this task and in long-term retention. Cognitively equivalent groups of old and young dogs were placed into either a test group maintained on food enriched with a broad-spectrum of antioxidants and mitochondrial cofactors, or a control group maintained on a complete and balanced food formulated for adult dogs. Following a wash-in period, the dogs were tested on a series of problems, in which reward was obtained when the animal responded selectively to the object closest to a thin wooden block, which served as a landmark. In Experiment 1, dogs were first trained to respond to a landmark placed directly on top of coaster, landmark 0 (L0). In the next phase of testing, the landmark was moved at successively greater distances (1, 4 or 10 cm) away from the reward object. Learning varied as a function of age group, food group, and task. The young dogs learned all of the tasks more quickly than the old dogs. The aged dogs on the enriched food learned L0 significantly more rapidly than aged dogs on control food. A higher proportion of dogs on the enriched food learned the task, when the distance was increased to 1cm. Experiment 2 showed that accuracy decreased with increased distance between the reward object and landmark, and this effect was greater in old animals. Experiment 3 showed stability of performance, despite using a novel landmark, and new locations, indicating that dogs learned the landmark concept. Experiment 4 found age impaired long-term retention of the landmark task. These results indicate that allocentric spatial learning is impaired in an age-dependent manner in dogs, and that age also affects performance when the distance between the landmark and target is increased. In addition, these results both support a role of oxidative damage in the development of age-associated cognitive dysfunction and indicate that short-term administration of a food enriched with supplemental antioxidants and mitochondrial cofactors can partially reverse the deleterious effects of aging on cognition.

Dietary enrichment counteracts age-associated cognitive dysfunction in canines.

Advanced age is accompanied by cognitive decline indicative of central nervous system dysfunction. One possibly critical causal factor is oxidative stress. Accordingly, we studied the effects of dietary antioxidants and age in a canine model of aging that parallels the key features of cognitive decline and neuropathology in humans. Old and young animals were placed on either a standard control food, or a food enriched with a broad spectrum of antioxidants and mitochondrial enzymatic cofactors. After 6 months of treatment, the animals were tested on four increasingly difficult oddity discrimination learning problems. The old animals learned more slowly than the young, making significantly more errors. However, this age-associated decline was reduced in the animals fed the enriched food, particularly on the more difficult tasks. These results indicate that maintenance on foods fortified with complex mixtures of antioxidants can partially counteract the deleterious effects of aging on cognition.

Seizure activity results in increased tyrosine phosphorylation of the N-methyl-D-aspartate receptor in the hippocampus.

Systemic administration of kainic acid (KA) induces status epilepticus (SE) that causes neurodegeneration and may subsequently lead to spontaneous recurrent seizures. We investigated the effects of KA-induced SE on tyrosine phosphorylation and solubility properties of the NMDA receptor. Following 1 h of SE, total protein tyrosine phosphorylation was elevated in both the hippocampus and frontal cortex relative to controls. Tyrosine phosphorylation of the NMDA receptor subunits NR2A and NR2B was also enhanced following SE. Animals that received KA but did not develop SE, did not exhibit increased tyrosine phosphorylation. SE resulted in a decrease in the solubility of NMDA receptor subunits and of PSD-95 in 1% deoxycholate. In contrast, the detergent solubility of AMPA and kainate receptors was not affected. These findings demonstrate that SE alters tyrosine phosphorylation of the NMDA receptor, and indicate that the interaction of the NMDA receptor with other components of the NMDA receptor complex are altered as a consequence of seizure activity.

Use of a delayed non-matching to position task to model age-dependent cognitive decline in the dog.

Spatial learning and memory in young and old dogs was studied in a series of experiments using a delayed non-matching to position (DNMP) paradigm. Past research from our laboratory has suggested that aged dogs perform more poorly on a version of the DNMP task compared to young dogs [Head et al., Spatial learning and memory as a function of age in the dog, Behav. Neurosci. 1995;109(5):851-585]. We have now extended these findings by testing a large number of dogs on three different variations of the DNMP paradigm to evaluate different aspects of spatial learning and memory. Our results indicate that: (1) aged dogs show impaired spatial learning compared to young dogs, (2) aged dogs display spatial working memory deficits compared to young dogs, (3) young dogs have a greater maximum working spatial memory capacity than old dogs and (4) we can use the DNMP paradigm to cognitively categorize different subsets of aged dogs. These data indicate that the DNMP paradigm can serve as a valuable tool to evaluate age-dependent cognitive dysfunction in the canine.

Development of a protocol for studying object recognition memory in the dog.

1. Dogs had considerable difficulty learning a delayed-non-matching-to sample task at a short delay (approximately 5 seconds) for an extended period (900 trials). Only 3 of 19 dogs met the learning criterion. 2. Acquisition on the DNMS task was markedly improved when a pause was introduced on presentation of the stimulus objects, when the objects were approximately 30 cm from the dog; eleven of 16 dogs learned the task within 600 trials. 3. Dogs learned the task more rapidly at 20 and 30 second delays than at 10-second delays. This indicates a transfer of learning. 4. Dogs that did learn the task were able to perform at accuracy greater than 85% at delays of 150 and 200 seconds. At a 5-minute delay, performance was at 75%. 5. When the animals were switched to a repeated object paradigm, accuracy markedly declined. 6. The improved performance produced by introduction of the pause is attributable to: (1) presenting the object at a distance longer than the dogs' near point, and (2) allowing increased processing time.

Selective loss of early suppression in the dentate gyrus precedes kainic acid induced electrographic seizures.

The role of inhibitory and facilitatory processes in the induction of seizures was studied in a kainic acid (KA) model of epilepsy. The dentate gyrus (DG) response to paired-pulse stimulation of the perforant path (PP) was monitored prior to and immediately following the initial KA induced afterdischarge (AD) in rats chronically prepared with stimulation recording electrodes. The subjects received a 1-h program of stimulation consisting of repeated sequences of pulse pairs at a short (20-30 ms), intermediate (45-90 ms), and long (200-300 ms) interpulse interval (IPIs). The stimulation program was administered both under control conditions and immediately following systemic injection of KA. During the control condition, stable suppression of population spike measures was obtained at the short (early phase) and long (late phase) IPIs, while facilitation was observed at the intermediate IPI. Administration of KA resulted in a progressive loss of suppression prior to the initial AD at the short IPI; neither facilitation nor the late phase of suppression were significantly affected. The early phase decreased further following the initial discharge. Since the early phase most likely reflects recurrent inhibition, these results provide evidence that inhibitory loss precedes the occurrence of KA induced AD, and that this inhibitory loss is increased further following the initial evoked AD. A use-dependent disinhibition is one possible explanation for the change in responsiveness that precedes the AD. This disinhibition could result from a depressed response at GABA-A receptors, an increased responsiveness at GABA-B receptors or possibly both.

Development of spontaneous seizures over extended electrical kindling. I. Electrographic, behavioral, and transfer kindling correlates.

The present study was aimed at evaluating an extended kindling model of spontaneous epilepsy. Behavioral and electrographic responses to repeated kindling of either the perforant path or amygdala were monitored for up to 300 trials. Kindling initially led to generalized convulsions equivalent to the level 5 seizure on the rating scale developed by Racine. The evoked seizures became progressively more complex with additional kindling, which was described by a 10-stage classification system. The highest stage (stage 10) was achieved when the kindling stimulation evoked two or more bouts of level 5 seizures combined with running and jumping fits. These more complex seizures developed over the course of amygdala, but not perforant path kindling. Electrographic seizures from both the amygdala and dentate gyrus increased in duration and amplitude during the early phase of kindling, but did not correlate with motor seizure development beyond level 5. During the late phase of kindling, the dentate gyrus afterdischarge amplitude decreased and became dissociated from the behavioral seizures. Manifestations of spontaneously recurring seizures were seen in the majority of animals, but spontaneous seizures of level 4 or greater were observed in only five rats. The second part of this study examined kindling transfer effects, the efficacy of kindling a new site after the completion of the initial (in this case extended) kindling protocol. The effect depended on both primary and secondary site location. When the amygdala served as primary site, perforant path transfer was complete in some animals but absent in others. No transfer occurred in the opposite direction, from the perforant path to the amygdala. Finally, transfer effects in the dentate gyrus, which was tested as tertiary site, were complete. Previous studies have found weaker transfer effects in the dentate when kindling to the standard stage 5 level.

Effects of extended electrical kindling on exploratory behavior and spatial learning.

Short-term electrical kindling, a widely used experimental model of epilepsy, appears to have little effect on behavior. The effects of extended kindling are largely unknown. Rats implanted with kindling electrodes in amygdala (AM) or perforant path (PP) received 300 kindling trials over approximately 7 months, and were tested in the Morris watermaze after a 7-10 day recovery period. Kindled animals were impaired during the initial training on hidden-platform acquisition, but not in retention of platform location. No deficits were found in acquiring a new hidden-platform location, latency to reach a visible-platform, or in swim speed. Open-field activity showed a sustained increase when tested during kindling, but only a transient increase when tested following suspension of kindling. Similar results were obtained for both AM and PP kindled animals. Hence, long-term kindling of both of these sites produced behavioral changes that were transient in nature. Further, these results also indicate that propagation of seizure activity from remote sites can alter hippocampally-mediated or related behavior.