Appetitive and aversive sensory preconditioning in rats is impaired by disruption of the postrhinal cortex.

Episodic memory involves binding stimuli and/or events together in time and place. Furthermore, memories become more complex when new experiences influence the meaning of stimuli within the original memory. Thus collectively, complex episodic memory formation and maintenance involves processes such as encoding, storage, retrieval, updating and reconsolidation, which can be studied using animal models of higher-order conditioning. In the present study aversive and appetitive sensory preconditioning paradigms were used to test the hypothesis that the postrhinal cortex (POR), which is a component of the hippocampal memory system, is involved in higher-order conditioning. Drawing on the known role of the POR in contextual learning, Experiment 1 employed a four-phase sensory preconditioning task that involved fear learning and context discrimination in rats with or without permanent lesions of the POR. In parallel, to examine POR function during higher-order conditioning in the absence of a particular spatial arrangement, Experiments 2 and 3 used a three-phase sensory preconditioning paradigm involving phasic stimuli. In Experiment 2, bilateral lesions of the POR were made and in Experiment 3, a chemogenetic approach was used to temporarily inactivate POR neurons during each phase of the paradigm. Evidence of successful sensory preconditioning was observed in sham rats which, during the critical context discrimination test, demonstrated higher levels of freezing behavior when re-exposed to the paired versus the unpaired context, whereas POR-lesioned rats did not. Data from the appetitive sensory preconditioning paradigm also confirmed the hypothesis in that during the critical auditory discrimination test, sham rats showed greater food cup responding following presentations of the paired compared to the unpaired auditory stimulus, whereas POR-lesioned rats did not. Lastly, in Experiment 3, when the POR was inactivated only during preconditioning or only during conditioning, discrimination during the critical auditory test was impaired. Thus, regardless of whether stimulus-stimulus associations were formed between static or phasic stimuli or whether revaluation of the paired stimulus occurred through association with an aversive or an appetitive unconditioned stimulus, the effects were the same; POR lesions disrupted the ability to use higher-order conditioned stimuli to guide prospective behavior.

Electromyographic Evidence of Excessive Achilles Tendon Elongation During Isometric Contractions After Achilles Tendon Repair.

BACKGROUND: Weakness in end-range plantarflexion has been demonstrated after Achilles tendon repair and may be because of excessive tendon elongation. The mean frequency (MNF) of surface electromyogram (EMG) data during isometric maximum voluntary contraction (MVC) increases with muscle fiber shortening. HYPOTHESIS: During isometric plantarflexion, MNF during MVCs will be higher on the involved side compared with the uninvolved side after Achilles tendon repair because of excessive tendon elongation and greater muscle fiber shortening. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: Isometric plantarflexion MVC torque was measured at 20° and 10° dorsiflexion, neutral, and 10° and 20° plantarflexion in 17 patients (15 men, 2 women; mean age, 39 ± 9 years) at a mean 43 ± 26 months after surgery. Surface EMG signals were recorded during strength tests. MNF was calculated from fast Fourier transforms of medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL) EMG signals. RESULTS: Patients had marked weakness on the involved side versus the uninvolved side in 20° plantarflexion (deficit, 28% ± 18%; P < .001) but no significant weakness in 20° dorsiflexion (deficit, 8% ± 15%; P = .195). MNF increased when moving from dorsiflexion to plantarflexion (P < .001), but overall, it was not different between the involved and uninvolved sides (P = .195). However, differences in MNF between the involved and uninvolved sides were apparent in patients with marked weakness. At 10° plantarflexion, 8 of 17 patients had marked weakness (>20% deficit). MNF at 10° plantarflexion was significantly higher on the involved side versus the uninvolved side in patients with weakness, but this was not apparent in patients with no weakness (side by group, P = .012). Mean MNF at 10° plantarflexion across the 3 muscles was 13% higher on the involved side versus the uninvolved side in patients with weakness (P = .012) versus 3% lower in patients with no weakness (P = .522). CONCLUSION: Higher MNF on the involved side versus the uninvolved side in patients with significant plantarflexion weakness is consistent with greater muscle fiber shortening. This indicates that weakness was primarily because of excessive lengthening of the repaired Achilles tendon. If weakness was simply because of atrophy, a lower MNF would have been expected and patients would have had weakness throughout the range of motion. Surgical and rehabilitative strategies are needed to prevent excessive tendon elongation and weakness in end-range plantarflexion after Achilles repair.

Loss of the Intellectual Disability and Autism Gene Cc2d1a and Its Homolog Cc2d1b Differentially Affect Spatial Memory, Anxiety, and Hyperactivity.

Hundreds of genes are mutated in non-syndromic intellectual disability (ID) and autism spectrum disorder (ASD), with each gene often involved in only a handful of cases. Such heterogeneity can be daunting, but rare recessive loss of function (LOF) mutations can be a good starting point to provide insight into the mechanisms of neurodevelopmental disease. Biallelic LOF mutations in the signaling scaffold CC2D1A cause a rare form of autosomal recessive ID, sometimes associated with ASD and seizures. In parallel, we recently reported that Cc2d1a-deficient mice present with cognitive and social deficits, hyperactivity and anxiety. In Drosophila, loss of the only ortholog of Cc2d1a, lgd, is embryonically lethal, while in vertebrates, Cc2d1a has a homolog Cc2d1b which appears to be compensating, indicating that Cc2d1a and Cc2d1b have a redundant function in humans and mice. Here, we generate an allelic series of Cc2d1a and Cc2d1b LOF to determine the relative role of these genes during behavioral development. We generated Cc2d1b knockout (KO), Cc2d1a/1b double heterozygous and double KO mice, then performed behavioral studies to analyze learning and memory, social interactions, anxiety, and hyperactivity. We found that Cc2d1a and Cc2d1b have partially overlapping roles. Overall, loss of Cc2d1b is less severe than loss of Cc2d1a, only leading to cognitive deficits, while Cc2d1a/1b double heterozygous animals are similar to Cc2d1a-deficient mice. These results will help us better understand the deficits in individuals with CC2D1A mutations, suggesting that recessive CC2D1B mutations and trans-heterozygous CC2D1A and CC2D1B mutations could also contribute to the genetics of ID.

Putting fear in context: Elucidating the role of the retrosplenial cortex in context discrimination in rats.

The retrosplenial cortex (RSC), which receives visuo-spatial sensory input and interacts with numerous hippocampal memory system structures, has a well-established role in contextual learning and memory. While it has been demonstrated that RSC function is necessary to learn to recognize a single environment that is directly paired with an aversive event, the role of the RSC in discriminating between two different contexts remains largely unknown. To address this, first order (Experiment 1) and higher order (Experiment 2) fear conditioning paradigms were conducted with sham and RSC-lesioned rats. In Experiment 1 rats were exposed to one context in which shock was delivered and to a second, distinct context without shock. Their ability to discriminate between the contexts was assessed during a re-exposure test. In a second experiment, a new cohort of RSC-lesioned rats was exposed to two contexts made distinct through visual, olfactory and auditory stimuli. In a subsequent conditioning phase, the salience of one of the auditory stimuli was paired to an aversive footshock while the other was not. Similar to Experiment 1, freezing behavior was analyzed upon re-exposure to the contexts in the absence of both the auditory cue and the footshock. The results revealed that RSC is not necessary for rats to use contextual information to successfully discriminate between two contexts under the relatively simple demands involved in this first order conditioning paradigm but that context discrimination is impaired when the processing of complex and/or ambiguous contextual stimuli is required to select appropriate behavioral responses.

A Method for Remotely Silencing Neural Activity in Rodents During Discrete Phases of Learning.

This protocol describes how to temporarily and remotely silence neuronal activity in discrete brain regions while animals are engaged in learning and memory tasks. The approach combines pharmacogenetics (Designer-Receptors-Exclusively-Activated-by-Designer-Drugs) with a behavioral paradigm (sensory preconditioning) that is designed to distinguish between different forms of learning. Specifically, viral-mediated delivery is used to express a genetically modified inhibitory G-protein coupled receptor (the Designer Receptor) into a discrete brain region in the rodent. Three weeks later, when designer receptor expression levels are high, a pharmacological agent (the Designer Drug) is administered systemically 30 min prior to a specific behavioral session. The drug has affinity for the designer receptor and thus results in inhibition of neurons that express the designer receptor, but is otherwise biologically inert. The brain region remains silenced for 2-5 hr (depending on the dose and route of administration). Upon completion of the behavioral paradigm, brain tissue is assessed for correct placement and receptor expression. This approach is particularly useful for determining the contribution of individual brain regions to specific components of behavior and can be used across any number of behavioral paradigms.