Richard Paylor

PKCγ mutant mice exhibit mild deficits in spatial and contextual learning

Abstract We are undertaking a genetic approach to investigate the role that synaptic modulation in the mammalian central nervous system plays in learning and memory and to identify relevant molecular components. We have generated mice deficient in the γ isoform of protein kinase C (PKCγ), an enzyme that has previously been implicated in both long-term potentiation (LTP) and learning and memory. These mice have a modified LTP of synaptic transmission in the hippocampus. We demonstrate that PKCγ-mutant mice can learn to carry out hippocampus-dependent tasks, although mild deficits are evident. Thus, hippocampal CA1 LTP induced by the conventional tetanic stimulation is not essential for the mice to exhibit spatial and contextual learning. Furthermore, the modification of hippocampal synaptic plasticity correlates with the learning deficits we observe.

Brief exposure to an enriched environment improves performance on the Morris water task and increases hippocampal cytosolic protein kinase C activity in young rats

This study was designed to determine whether brief exposure to an enriched environment around the time of weaning would affect learning and memory processes in young rats. In addition, this study sought to determine if experience in an enriched environment would alter hippocampal protein kinase C (PKC) which is thought to be a possible neural substrate that underlies learning and memory processes. Animals were either reared in an enriched environment or standard laboratory cages starting at 15 days old. After 6 (21 days old) or 12 (27 days old) days subjects were either tested in the Morris water task, or had the hippocampus removed for biochemical analysis of PKC activity. Morris water task results showed that compared to laboratory reared controls, the performance of subjects reared in the enriched environment for 12 days, but not 6 days, was improved. In addition, 12 days of exposure to the enriched environment, but not 6 days, produced more cytosolic hippocampal PKC activity. The particulate fraction appeared not to be affected by rearing in the enriched environment. Brief exposure to an enriched environment around weaning, therefore, both improved Morris water task performance and increased hippocampal PKC activity. These outcomes suggest that performance in the Morris water task and hippocampal PKC may be functionally related.

Behavioral dissociations between C57BL/6 and DBA/2 mice on learning and memory tasks: A hippocampal-dysfunction hypothesis

We recently demonstrated that C57BL/6 (C57) mice are capable of spatial learning in the Morris water task whereas DBA/2 (DBA) mice are impaired. There may be a number of reasons why the DBA mice are impaired in the Morris task. The main objective of this study was to understand the nature of the DBA impairment. First, we showed that DBA mice do not have a general learning impairment, since they can solve two different discrimination problems. Using a water-filled plus maze and a modified version of the Morris task, we showed that DBA mice can (1) see distal cues, (2) attend to the extramaze environment, and (3) learn the place of a hidden platform if training allows them to utilize only part of the distal environment. Probe data indicate, however, that in contrast to C57 mice, DBA mice do not form a complete representation of the extra-maze environment. Final experiments demonstrate that DBA mice are also impaired on a task that requires conditional learning but perform as well as C57 mice on a response-tendency task and a nonconditional learning task. The results of these and other studies suggest that learning and memory processes in DBA mice may be disrupted as a result of impaired hippocampal function.

Behavioral assessment of c-fos mutant mice

Induction of the proto-oncogene c-fos has been associated with a number of neural and behavioral responses to acute stimuli. Behavioral characterization of mice containing a mutant c-fos allele created via homologous recombination-based gene targeting was performed to analyze the role of this protein in baseline neurological properties as well as paradigms that require neural adaptive responses. Performance of 9 out of 11 c-fos-deficient animals was impaired in the spatial version of the Morris water task. However, this poor performance in the spatial version of the task was highly correlated to their performance in the non-spatial version of the task which suggests that they have a behavioral impairment that interrupts their ability to perform adequately on both versions of the task with the same proficiency as wild-type and heterozygous litter mates. To examine learning impairments further, a simple left/right discrimination in a T-maze was used. Mutants were not impaired in this simple learning task. These results suggest that c-fos mutants have some behavioral impairments that interfere with evaluation of complex learning on the Morris water task, but because all genotypes could perform a simple discrimination task, it is clear that c-fos is not essential for this simpler form of learning and memory.

Developmental Differences in Place-Learning Performance Between C57BL/6 and DBA/2 Mice Parallel the Ontogeny of Hippocampal Protein Kinase C

This study determined the ontogenic changes in learning and hippocampal protein kinase C (PKC) in C57 and DBA mice. Mice were tested on the visible- or hidden-platform versions of the Morris water task starting at 17, 24, 31, or 60 days of age. Both strains learned to locate the visible platform at all ages. C57 mice learned to solve the hidden-platform task when they were 24 days old, whereas DBA mice never learned to solve this task. Using a [3H]-phorbol ester binding assay, the authors found that both strains had similar amounts of hippocampal PKC at 10 and 17 days of age but that C57 mice had significantly more PKC at 24, 31, and 60 days of age. Immunoblotting results revealed that C57 mice had more gamma-PKC, but not alpha-PKC, than DBA mice. Thus, the development of performance differences in spatial learning between C57 and DBA mice parallels the ontogeny of hippocampal PKC.

Identification of Quantitative Trait Loci Involved in Contextual and Auditory-Cued Fear Conditioning in BXD Recombinant Inbred Strains

Fear conditioning shows associations formed between contextual or auditory stimuli with an unconditioned stimulus. Inbred mouse strains differ in their ability to demonstrate fear conditioning, suggesting at least a partial genetic influence. The present study identified the possible chromosomal loci regulating fear conditioning in BXD recombinant inbred strains using quantitative trait loci (QTL) analysis. Estimates of heritability for all 3 measures of conditioning were about .28. Correlational analyses between genetic markers and strain means identified multiple putative QTLs. The strongest associations were on Chromosomes 1 and 17 for freezing to the context, Chromosome 12 for freezing to an altered context, and Chromosome 1 for freezing to the auditory stimulus. Overlapping QTLs may indicate some common genes that underlie aspects of this learning task.

Cholinergic receptor blockade can impair the rat's performance on both the place learning and cued versions of the Morris water task: The role of age and pool wall brightness

It is known that the administration of a cholinergic receptor blocker impairs the rats performance on the place learning version of the Morris water task. We confirm this finding but in addition report that animals receiving the cholinergic antagonist, scopolamine hydrobromide, are significantly impaired on the cued platform version of the Morris water task. This latter result, however, is dependent on both the age of the subject and the training context. Weanling animals were more impaired on the cued platform task than were adult animals, and the magnitude of the impairment was much larger when animals were trained in a pool with a gray interior wall than when the pool wall was white. Our findings suggest that the influence of cholinergic systems on performance in the Morris task extend beyond their contribution to place learning and memory processes. We suggest that functional central cholinergic systems also contribute to the processes that enable the animal to inhibit behaviors that are incompatible with the requirements of the task.

Enhancement of hippocampally-mediated learning and protein kinase C activity by oxiracetam in learning-impaired DBA/2 mice.

The effects of oxiracetam on hippocampally-mediated learning performance and hippocampal protein kinase C (PKC) were examined in C57BL/6Ibg (C57) and DBA/2Ibg (DBA) mice. C57 and DBA mice were subjected to daily injections of oxiracetam (50 mg/kg i.p.) or vehicle (0.9% saline) for a total of 9 days. C57 and DBA mice were examined on a modified version of the Morris water maze task and the contextual fear conditioning task on the last 5 or 2 days, respectively, of the 9-day treatment schedule. When compared with controls, C57 and DBA oxiracetam-treated mice showed no difference in motor skill capability to perform these complex learning tasks (swim speed or ability to freeze). Hippocampal PKC activity was measured in cytosolic, loosely-bound, and membrane-bound homogenate fractions. Oxiracetam-treated DBA mice demonstrated a significant increase in spatial learning performance as determined by the Morris task. DBA performance was also improved in contextual learning as determined by the fear conditioning task. The increase in spatial learning performance was correlated to an increase in membrane-bound PKC. No substantial improvements in C57 mice were observed on either learning task nor did hippocampal PKC activity change in response to oxiracetam treatment. These data demonstrate that the learning impairment of DBA mice can be reversed by treatment with a nootropic agent and support previous studies suggesting that PKC may be one mechanism of action for oxiracetam.

THE USE OF NULL MUTANT MICE TO STUDY COMPLEX LEARNING AND MEMORY PROCESSES

A number of neural substrates have been proposed to mediate complex learning and memory processes in mammalian organisms. One strategy for testing the involvement of a particular gene in learning and memory is to create a mouse line with a null mutation in that gene. Recently, embryonic stem cell-based gene-targeted homologous recombination techniques have been employed to create a number of such mutant mouse lines that do not express interesting candidate genes. These animals have been examined for impairments in several complex learning paradigms which are known to depend on the integrity of the hippocampus. In this review several complex learning and memory paradigms are described, the techniques to create null mutants are reviewed, and the results of recent studies with null mutants are described. Finally, the limitations for interpretation of behavioral data using null mutants are discussed.

Reducing the temporal demands of the Morris place-learning task fails to ameliorate the place-learning impairment of preweanling rats

Rats less than 20 days old display no evidence of place learning when trained in the Morris (1981) water task. In the present experiment, we evaluated the possibility that this impairment was a result of the immature “temporary memory buffer” that Rawlins (1985) has proposed as being critical for successful place learning. To evaluate this hypothesis, we reduced to a minimum the temporal demands of the Morris place navigation task by training pups of different ages to swim directly to a visible platform in a fixed location. All pups were then tested for place learning on the probe trial with no escape platform in the pool. In spite of being trained with a procedure that greatly reduced the temporal demands of the task, the 19-day-old pups displayed no learning of the platform’s location. Several measures of place learning indicated that 22-day-olds could learn the location of the platform. Our data thus provide no support for the hypothesis that young rats are impaired on place-learning task because of an immature temporary memory buffer.