Short Duration Waveforms Recorded Extracellularly from Freely Moving Rats are Representative of Axonal Activity

While extracellular somatic action potentials from freely moving rats have been well characterized, axonal activity has not. We report direct extracellular tetrode recordings of putative axons whose principal feature is a short duration waveform (SDW) with an average peak-trough length less than 179 μs. While SDW recordings using tetrodes have previously been treated as questionable or classified as cells, we hypothesize that they are representative of axonal activity. These waveforms have significantly shorter duration than somatic action potentials, are triphasic and are therefore similar to classic descriptions of microelectrode recordings in white matter and of in vitro action potential propagation along axons. We describe SDWs recorded from pure white-matter tracts including the alveus and corpus callosum. Recordings of several SDWs in the alveus exhibit grid-like firing patterns suggesting these axons carry spatial information from entorhinal cortical neurons. Finally, we locally injected the GABAA agonist Muscimol into layer CA1 of the hippocampus while simultaneously recording somatic activity and SDWs on the same tetrodes. The persistent activity of SDWs during Muscimol inactivation of somatic action potentials indicates that SDWs are representative of action potential propagation along axons projecting from more distal somata. This characterization is important as it illustrates the dangers of exclusively using spike duration as the sole determinant of unit type, particularly in the case of interneurons whose peak-trough times overlap with SDWs. It may also allow future studies to explore how axonal projections from disparate brain regions integrate spatial information in the hippocampus, and provide a basis for studying the effects of pharmaceutical agents on signal transmission in axons, and ultimately to aid in defining the potential role of axons in cognition

Enrichment and Training Improve Cognition in Rats with Cortical Malformations

Children with malformations of cortical development (MCD) frequently have associated cognitive impairments which reduce quality of life. We hypothesized that cognitive deficits associated with MCD can be improved with environmental manipulation or additional training. The E17 methylazoxymethanol acetate (MAM) exposure model bears many anatomical hallmarks seen in human MCDs as well as similar behavioral and cognitive deficits. We divided control and MAM exposed Sprague-Dawley rats into enriched and non-enriched groups and tested performance in the Morris water maze. Another group similarly divided underwent sociability testing and also underwent Magnetic Resonance Imaging (MRI) scans pre and post enrichment. A third group of control and MAM rats without enrichment were trained until they reached criterion on the place avoidance task. MAM rats had impaired performance on spatial tasks and enrichment improved performance of both control and MAM animals. Although MAM rats did not have a deficit in sociability they showed similar improvement with enrichment as controls. MRI revealed a whole brain volume decrease with MAM exposure, and an increase in both MAM and control enriched volumes in comparison to non-enriched animals. In the place avoidance task, MAM rats required approximately 3 times as long to reach criterion as control animals, but with additional training were able to reach control performance. Environmental manipulation and additional training can improve cognition in a rodent MCD model. We therefore suggest that patients with MCD may benefit from appropriate alterations in educational strategies, social interaction and environment. These factors should be considered in therapeutic strategies

The genetic architecture of the human cerebral cortex

The cerebral cortex underlies our complex cognitive capabilities, yet little is known about the specific genetic loci that influence human cortical structure. To identify genetic variants that affect cortical structure, we conducted a genome-wide association meta-analysis of brain magnetic resonance imaging data from 51,665 individuals. We analyzed the surface area and average thickness of the whole cortex and 34 regions with known functional specializations. We identified 199 significant loci and found significant enrichment for loci influencing total surface area within regulatory elements that are active during prenatal cortical development, supporting the radial unit hypothesis. Loci that affect regional surface area cluster near genes in Wnt signaling pathways, which influence progenitor expansion and areal identity. Variation in cortical structure is genetically correlated with cognitive function, Parkinson's disease, insomnia, depression, neuroticism, and attention deficit hyperactivity disorder

MRI data of MAM and control, enriched and non-enriched.

<p>At P22 MAM rats had significantly smaller total brain volume (<b>A</b>, p<0.001) and hippocampal volume (<b>B</b>, p<0.001). At P150, animals had undergone 128 days of either enrichment or non-enrichment. The effect of MAM was present in both total brain volume (<b>C</b>, p=0.015) and hippocampal volume (<b>D</b>, p<0.001). Enrichment increased total brain volume (<b>C</b>, p=0.024) and hippocampal volume (<b>D</b>, p=0.026) in both control and MAM animals. Error bars represent standard error.</p

Morris water maze performance data for control and MAM treated rats.

<p>(<b>A</b>) Survival analysis representing the fraction of animals that have found the platform during a trial, capped a 120 seconds. The effect of day has been modeled in. The solid black line represents MAM non-enriched, the dotted black line represents MAM enriched, the solid grey line represents control non-enriched, and the dotted grey line represents control enriched. Controls were shown to outperform MAM animals (p<0.001), and enriched animals outperformed their non-enriched counterparts (p<0.026). (<b>B</b>) Mean number of body rotations, averaged per day. Higher numbers of rotations indicate less certainty in the where the platform is located. The solid grey bar is control non-enriched, the diagonally segmented grey bar is control enriched, the black solid bar is MAM non-enriched and the diagonally segmented black bar is MAM enriched. Controls had fewer body rotations than MAM (p<0.001), and enriched animals had fewer rotations than non-enriched (p<0.024). (<b>C</b>) Mean path efficiency per day. Path efficiency indicated the directness of the animal’s path to the platform, higher values representing a more direct path. Controls outperformed MAM (p<0.001), and enriched animals outperformed non-enriched (p<0.007). </p

Control and MAM treated animal’s data from the place avoidance task.

<p>Control and MAM had similar levels of exploration of the two local cues (white and black) as measured by (<b>A</b>) nose pokes and (<b>B</b>) time spent at the cue. The controls (grey line) decrease entrances into the zone (<b>C</b>) and number of shocks (<b>D</b>) far more quickly than MAM (black line), but both decreased the number of entrances (p<0.001) and shocks (p<0.001) over time. (<b>E</b>) Latency to first entrance is a measure of how long an animal takes to enter the shock zone from the start of the session. MAM has significantly shorter latencies (p<0.001). Inset graphs show number of entrances, shocks, and latency for the probe session. In the probe, the black and white local cues were transposed and the shock was turned off, but entries and “shocks” that would have been delivered were still counted. This allowed us to infer the rat's strategy toward shock avoidance. Avoidance of the original shock quadrant would indicate place learning, where the location of the shock zone is defined by a configuration of multiple cues. Avoidance of the black cue would indicate stimulus-response learning, where the black cue card was associated with shock. Control and MAM both largely avoided the original shock zone rather than the black local cue.</p

Data from the three chambered social choice test with control non-enriched, control enriched, MAM non-enriched and MAM enriched.

<p>(<b>A</b>) Control non-enriched (solid grey bar) and MAM non-enriched (solid black bar) spent less time with the novel rat than their enriched counterparts (striped bars) (p<0.006). (<b>B</b>) Non-enriched animals spent more time with the novel object than enriched animals (p<0.001). (<b>C</b>) While MAM did not significantly affect time spent in the chamber with the novel rat or novel object, there was a significant effect on time spent in the center chamber (p<0.001). (<b>D</b>) When the data was examined as a ratio of time spent with novel rat over time spent with novel object, the enrichment effect remained and the trend for a MAM effect was gone. Error bars represent standard error.</p

Analysis of the movement of control and MAM rats during the place avoidance test.

<p><b>A-F</b> are dwell-time maps for individual animals where lighter colors indicate more time spent in that area. During habituation, both control (<b>A</b>) and MAM (<b>B</b>) explored the perimeter of the arena. In the sessions in which the animals met criterion, controls (<b>C</b>) avoided in the region directly opposite the shock zone (arc). MAM (<b>D</b>) avoided to the left of the shock zone where there was increased risk of shock. (<b>E</b>-<b>F</b>) The avoidance behavior did not change when the local cues where transposed. Control animals spent more time in the zone opposite the shock zone (p<0.001(<b>G</b>)), while MAM animals spent more time in the zone closest to the shock zone (left) (p<0.001(<b>H</b>)) throughout the sessions. </p

Diagrams of the behavioral set ups used in this manuscript.

<p>(<b>A</b>) Morris water maze. (<b>B</b>) Place avoidance arena. (<b>C</b>) Three chambered social choice test.</p