Alejandro Múnera

Histone deacetylase inhibitors improve learning consolidation in young and in KA-induced-neurodegeneration and SAMP-8-mutant mice.

Histone deacetylases (HDAC) are enzymes that maintain chromatin in a condensate state, related with absence of transcription. We have studied the role of HDAC on learning and memory processes. Both eyeblink classical conditioning (EBCC) and object recognition memory (ORM) induced an increase in histone H3 acetylation (Ac-H3). Systemic treatment with HDAC inhibitors improved cognitive processes in EBCC and in ORM tests. Immunohistochemistry and gene expression analyses indicated that administration of HDAC inhibitors decreased the stimulation threshold for Ac-H3, and gene expression to reach the levels required for learning and memory. Finally, we evaluated the effect of systemic administration of HDAC inhibitors to mice models of neurodegeneration and aging. HDAC inhibitors reversed learning and consolidation deficits in ORM in these models. These results point out HDAC inhibitors as candidate agents for the palliative treatment of learning and memory impairments in aging and in neurodegenerative disorders.

Characterizing spatial extinction in an abbreviated version of the Barnes maze

Adult male Wistar rats were trained to find an escape box in the Barnes maze in order to characterize the extinction process of a learned spatial preference. To do so, once they had fully acquired the spatial task, they were repeatedly exposed to the maze without the escape box. Multiple behavioral measurements (grouped into motor skill and spatial preference indicators) were followed up throughout the complete training process. Animals gained efficiency in finding the escape box during acquisition, as indicated by the reduction in the time spent escaping from the maze, the number of errors, the length of the traveled path, and by the increase in exploration accuracy and execution speed. When their retention and preference were tested 24h later, all the subjects retained their enhanced performance efficiency and accuracy and displayed a clear-cut preference for the escape hole and its adjacent holes. Almost all motor skill indicators followed an inverse, though not monotonic, pattern during the extinction training, returning to basal levels after three trials without escape box, displaying a transient relapse during the fifth extinction trial. Preference indicators also followed a reverse pattern; however, it took seven trials for them to return to basal levels, relapsing during the eighth extinction trial. The abbreviated Barnes maze acquisition, evaluation, and extinction procedures described herein are useful tools for evaluating the effects of behavioral and/or pharmacological treatment on different stages of spatial memory, and could also be used for studying the neurophysiological and neurobiological underpinnings of this kind of memory.

Vibrissal paralysis unveils a preference for textural rather than positional novelty in the one-trial object recognition task in rats

In order to explore the role of active whisking in object novelty detection, the performance of rats having bilateral vibrissal paralysis was compared to that of non-lesioned animals in three modified versions of the one-trial object recognition task performed in the dark. Vibrissal paralysis was induced by crushing the buccal and mandibular branches of the facial nerve. Lesioned animals were not different from non-lesioned ones in terms of weight-gain, locomotive activity, motivation to explore, and ability to become habituated to a given environment. Only lesioned animals were unable to discriminate a change in object texture as novelty cue in the first task, designed to test textural novelty detection. In the second task, designed to test positional novelty detection, both lesioned and non-lesioned subjects were able to discriminate a change in object position as novelty cue. In the third task, designed to force the subjects to choose between two conflicting novelty cues (texture and position), non-lesioned subjects displayed a clear-cut preference for textural novelty while subjects having bilateral vibrissal paralysis preferred positional novelty. According to these results, active whisking is necessary for textural, but not for positional novelty detection. Moreover, these results indicate that textural novelty in non-lesioned animals seems to overcome positional novelty if these are in competition in an object recognition memory task.

PERIPHERAL FACIAL NERVE LESIONS INDUCE CHANGES IN THE FIRING PROPERTIES OF PRIMARY MOTOR CORTEX LAYER 5 PYRAMIDAL CELLS

Facial nerve lesions elicit long-lasting changes in vibrissal primary motor cortex (M1) muscular representation in rodents. Reorganization of cortical representation has been attributed to potentiation of preexisting horizontal connections coming from neighboring muscle representation. However, changes in layer 5 pyramidal neuron activity induced by facial nerve lesion have not yet been explored. To do so, the effect of irreversible facial nerve injury on electrophysiological properties of layer 5 pyramidal neurons was characterized. Twenty-four adult male Wistar rats were randomly subjected to two experimental treatments: either surgical transection of mandibular and buccal branches of the facial nerve (n=18) or sham surgery (n=6). Unitary and population activity of vibrissal M1 layer 5 pyramidal neurons recorded in vivo under general anesthesia was compared between sham-operated and facial nerve-injured animals. Injured animals were allowed either one (n=6), three (n=6), or five (n=6) weeks recovery before recording in order to characterize the evolution of changes in electrophysiological activity. As compared to control, facial nerve-injured animals displayed the following sustained and significant changes in spontaneous activity: increased basal firing frequency, decreased spike-associated local field oscillation amplitude, and decreased spontaneous theta burst firing frequency. Significant changes in evoked-activity with whisker pad stimulation included: increased short latency population spike amplitude, decreased long latency population oscillations amplitude and frequency, and decreased peak frequency during evoked single-unit burst firing. Taken together, such changes demonstrate that peripheral facial nerve lesions induce robust and sustained changes of layer 5 pyramidal neurons in vibrissal motor cortex.

Vibrissal paralysis produces increased corticosterone levels and impairment of spatial memory retrieval

Graphical abstract Figure. No Caption available. HighlightsVibrissal paralysis was induced by bilateral facial nerve lesion in rats.Injured and control rats were trained and tested in a spatial memory task.Vibrissal paralysis induced retrieval but not acquisition impairment.Corticosterone response to training or testing was higher in injured rats.Paralysis‐induced stress response potentiation may cause retrieval impairment. ABSTRACT This research was aimed at establishing how the absence of active whisking in rats affects acquisition and recovery of spatial memory. The mystacial vibrissae were irreversibly paralyzed by cutting the facial nerve’s mandibular and buccal branches bilaterally in the facial nerve lesion group (N = 14); control animals were submitted to sham‐surgery (N = 15). Sham‐operated (N = 11) and facial nerve‐lesioned (N = 10) animals were trained (one session, eight acquisition trials) and tested 24 h later in a circular Barnes maze. It was found that facial nerve lesioned‐animals adequately acquired the spatial task, but had impaired recovery of it when tested 24 h after training as compared to control ones. Plasma corticosterone levels were measured after memory testing in four randomly chosen animals of each trained group and after a single training trial in the maze in additional facial nerve‐lesioned (N = 4) and sham‐operated animals (N = 4). Significant differences respecting the elevation of corticosterone concentration after either a single training trial or memory testing indicated that stress response was enhanced in facial nerve‐lesioned animals as compared to control ones. Increased corticosterone levels during training and testing might have elicited the observed whisker paralysis‐induced spatial memory retrieval impairment.

Layer 5 Pyramidal Neurons’ Dendritic Remodeling and Increased Microglial Density in Primary Motor Cortex in a Murine Model of Facial Paralysis

This work was aimed at characterizing structural changes in primary motor cortex layer 5 pyramidal neurons and their relationship with microglial density induced by facial nerve lesion using a murine facial paralysis model. Adult transgenic mice, expressing green fluorescent protein in microglia and yellow fluorescent protein in projecting neurons, were submitted to either unilateral section of the facial nerve or sham surgery. Injured animals were sacrificed either 1 or 3weeks after surgery. Two-photon excitation microscopy was then used for evaluating both layer 5 pyramidal neurons and microglia in vibrissal primary motor cortex (vM1). It was found that facial nerve lesion induced long-lasting changes in the dendritic morphology of vM1 layer 5 pyramidal neurons and in their surrounding microglia. Dendritic arborization of the pyramidal cells underwent overall shrinkage. Apical dendrites suffered transient shortening while basal dendrites displayed sustained shortening. Moreover, dendrites suffered transient spine pruning. Significantly higher microglial cell density was found surrounding vM1 layer 5 pyramidal neurons after facial nerve lesion with morphological bias towards the activated phenotype. These results suggest that facial nerve lesions elicit active dendrite remodeling due to pyramidal neuron and microglia interaction, which could be the pathophysiological underpinning of some neuropathic motor sequelae in humans.

[Peripheral facial nerve lesion induced long-term dendritic retraction in pyramidal cortico-facial neurons].

INTRODUCTION Little evidence is available concerning the morphological modifications of motor cortex neurons associated with peripheral nerve injuries, and the consequences of those injuries on post lesion functional recovery. OBJECTIVE Dendritic branching of cortico-facial neurons was characterized with respect to the effects of irreversible facial nerve injury. MATERIALS AND METHODS Twenty-four adult male rats were distributed into four groups: sham (no lesion surgery), and dendritic assessment at 1, 3 and 5 weeks post surgery. Eighteen lesion animals underwent surgical transection of the mandibular and buccal branches of the facial nerve. Dendritic branching was examined by contralateral primary motor cortex slices stained with the Golgi-Cox technique. Layer V pyramidal (cortico-facial) neurons from sham and injured animals were reconstructed and their dendritic branching was compared using Sholl analysis. RESULTS Animals with facial nerve lesions displayed persistent vibrissal paralysis throughout the five week observation period. Compared with control animal neurons, cortico-facial pyramidal neurons of surgically injured animals displayed shrinkage of their dendritic branches at statistically significant levels. This shrinkage persisted for at least five weeks after facial nerve injury. DISCUSSION Irreversible facial motoneuron axonal damage induced persistent dendritic arborization shrinkage in contralateral cortico-facial neurons. This morphological reorganization may be the physiological basis of functional sequelae observed in peripheral facial palsy patients.

Overtraining modifies spatial memory susceptibility to corticosterone administration

HighlightsMale rats were either trained or overtrained in a spatial memory task in Barnes maze.Memory test and extinction were done after corticosterone or vehicle administration.Overtraining enhanced memory retrieval and made it resistant to corticosterone.Neither training intensity nor corticosterone affected extinction acquisition.Training intensity and corticosterone have opposite effects on extinction retrieval. ABSTRACT Even though the effects of overtraining and glucocorticoids on different phases of spatial memory are known, the interaction between these factors on the retrieval and extinction of spatial memory has not yet been described. Adult male Wistar rats received eight training trials per day in the Barnes maze for either one or two days. Twenty‐four hours after the last training trial they were randomly assigned for receiving an intraperitoneal vehicle or corticosterone injection (0.125 or 0.5 mg/kg) and ten minutes later they were given a memory test, followed by seven extinction trials. Extinction retention was evaluated twenty‐four hours after extinction. The second training session did not provoke significant changes regarding escape latency nor weighted errors, thereby showing that overtraining had been obtained. The overtrained animals performed better than the trained ones during the retrieval test. Corticosterone administration did not affect the overtrained animals’ performance; by contrast, only the lower dose impaired trained animals’ retrieval. Overtrained subjects acquired extinction more rapidly than those which received just one session, but corticosterone did not significantly modify extinction. However, whilst the spatial task remained extinguished in trained animals during the extinction retrieval test, spontaneous recovery occurred in overtrained animals. Such training intensity effects on extinction retrieval were reverted by corticosterone. Overall, these results suggested that overtraining modified the susceptibility of spatial memory’s trace to the effects of corticosterone on retrieval and extinction.