Electrophysiological and molecular effects of bilateral deep brain stimulation of the medial forebrain bundle in a rodent model of depression.

BACKGROUND: Deep Brain Stimulation (DBS) of the Medial Forebrain Bundle (MFB) induces antidepressant effects both clinically and pre-clinically. However, the acute electrophysiological changes induced by MFB DBS remain unknown. OBJECTIVE: The study investigated acute mfb DBS effects on neuronal oscillations in distinct neuronal populations implicated in the pathophysiology of depression. METHODS: The Flinders Sensitive Line (FSL) rodent depression model and Sprague-Dawley (SD) controls were used in the study. Recording electrodes were implanted unilaterally in the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), ventral tegmental area (VTA); DBS electrodes were implanted bilaterally in the mfb. The FSL Stim and SD Stim received bilateral mfb DBS, whereas the FSL Sham and SD Shams were not stimulated. Local field potentials (LFPs) from all areas were recorded at baseline, during, and post stimulation. Neuronal oscillations were analyzed. RESULTS: mfb DBS induced 1) a significant increase of low gamma (30-45 Hz) oscillations in the mPFC uniquely in FSLs; 2) a significant increase of low gamma oscillations in the NAc and VTA in SDs and FSLs; and 3) an increase in the expression of Gad1 in the mPFC of FSL and SDs, while only increasing the expression in the NAc of FSLs. CONCLUSION: mfb DBS differentially affected neuronal oscillations in the mPFC, NAc and VTA across SD and FSL rats. Low gamma oscillations rose significantly in the mPFC of FSL rats. Molecular analysis points to a mechanism involving GABAergic interneurons as they regulate low gamma oscillations.

Olfactory discrimination and memory deficits in the Flinders Sensitive Line rodent model of depression.

Major Depressive Disorder (MDD) is a heterogeneous psychiatric disorder with broad symptomatic manifestations. The current study examined, for the first time, olfactory memory and discrimination in the Flinders Sensitive Line (FSL) rodent model of depression. Male FSL rats and controls were trained on an Olfactory Discrimination (OD) and a Social Interaction (SI) test. On the OD test, the FSL and controls performed similarly at the shortest inter-trial interval (5min), however, with extended delay of 30min, the FSLs had a recall and odour discrimination deficit. At the longest delay (60min) both groups performed poorly. The FSL rats i.) had a deficit in olfactory discrimination suggesting impairment in olfactory memory and recall; ii.) were less likely to socialize with unfamiliar rats. The data suggests that FSL animals have an impaired olfactory information processing capacity.

Long-term characterization of the Flinders Sensitive Line rodent model of human depression: Behavioral and PET evidence of a dysfunctional entorhinal cortex.

The etiology of depression is unknown but has been associated with dysregulation of neuronal activity at numerous loci on the limbic-cortical circuitry. The Flinders Sensitive Line (FSL) is a validated rodent model of human depression with spontaneously emerging behavioral and physiological phenotype, however, the durability and robustness of the phenotypes have not been described. The objective of the current study was to evaluate longitudinal dynamics of the depressive-like symptoms in this animal model. FSL and control rats of both genders were assessed over 8 months, characterizing their performance at different time points on motor, sensorimotor and complex learning/memory based tasks. Changes over time in physiological parameters, such as corticosterone and blood glucose levels, were monitored. Regional glucose metabolism, used as a marker of neuronal activity, was assessed at different time points using F18-FDG Positron Emission Tomography (PET). Results show that certain deficits at 2-3 months--on tests such as the Elevated Plus Maze, Object Recognition, and the Forced Swim Test--were transitory and the phenotype was no longer present when re-testing at 6-7 months of age. However, a stable impairment was detected on a learning and memory task, particularly indicating dysfunction in retention of spatial information. Furthermore, at multiple time points, the PET scan indicated a significate bilateral, hypo-metabolism in the temporal lobes in the FSL rats compared to healthy controls. The data suggests possible alterations of entorhinal cortex metabolism concomitant with specific behavioral changes and supports the importance of understanding the dynamics and the time and gender dependence of the phenotypes present.

Microstructural effects of a neuro-modulating drug evaluated by diffusion tensor imaging.

In a longitudinal mouse study we evaluated whether diffusion tensor imaging (DTI) can monitor microstructural changes after administration of the neuromodulating drug EPO and whether erythropoietin (EPO) has an effect on cognitive performance. Twelve mice (2 groups with 6 mice each) were scanned in a 7T Bruker Biospin animal scanner with a highly resolved DTI sequence before and 16 days after intraperitoneal injections of EPO or saline. All mice underwent behavioral testing (Morris water maze) and histologic evaluation of hippocampal and corpus callosum cell proliferation and oligodendrogenesis. Whole brain DTI analysis showed significant Trace, RD and AD decrease within the dentate gyrus, subiculum, primary motor, somatosensory, and supplementary somatosensory areas and FA increase in the hippocampus, corpus callosum, and fimbria fornix in EPO treated mice only. ROI-based DTI analysis showed significant Trace and RD decrease and FA increase only in the corpus callosum of EPO treated mice, whereas in the dentate gyrus significant Trace, RD, and AD decrease occurred in both, EPO- and control-group. Behavioral tests showed that EPO treated mice performed better and learned faster than controls. Histologically, the number of BrdU-positive nuclei and optical density of DCX-labeled juvenile neurons significantly increased within the dentate gyrus, corpus callosum and fimbria fornix and the number of NG2-positive oligodendrocyte progenitors in corpus callosum and fimbria fornix, respectively. In conclusion we were able to monitor microstructural changes with DTI and showed EPO treatment-related alterations correlating with enhanced dentate gyrus and corpus callosum cell proliferation and better learning capabilities.

Ventral tegmental area dopaminergic lesion-induced depressive phenotype in the rat is reversed by deep brain stimulation of the medial forebrain bundle.

DBS of the medial forebrain bundle (MFB) has been investigated clinically in major depressive disorder patients with rapid and long-term reduction of symptoms. In the context of chronic bilateral high frequency deep brain stimulation (DBS) of the MFB, the current study looked at the impact of lesioning the ascending dopaminergic pathway at the level of the ventral tegmental area (VTA). Sprague-Dawley female rats were given bilateral injection of 6-OHDA into the VTA (VTA-lx group) or were left unlesioned (control group). Later, all animals received bilateral microelectrode implantation into the MFB followed by chronic continuous stimulation for 3 weeks. Behavioral tests were performed as baseline and following MFB-DBS, along with histological analysis. Pre-stimulation baseline testing of the VTA-lx animals indicated depressive-like phenotype in comparison with controls. Response to MFB-DBS varied according to (i) the degree of dopaminergic depletion: animals with severe mesocorticolimbic dopamine depletion did not, whilst those with mild dopamine loss responded well to stimulation; (ii) environmental conditions and the nature of the behavioral tests, e.g., stressful vs non-stressful situations. Neuromodulation-induced c-fos expression in the prelimbic frontal cortex and nucleus accumbens was also dependent upon integrity of the dopaminergic ascending projections. Our results confirm a potential role for dopamine in symptom relief observed in clinical MFB-DBS. Although mechanisms are not fully understood, the data suggests that the rescue of depressive phenotype in rodents can work via both dopamine-dependent and independent mechanisms. Further investigations concerning the network of depression using neuromodulation platforms in animal models might give insight into genesis and treatment of major depression disorder.

Training specificity, graft development and graft-mediated functional recovery in a rodent model of Huntington's disease.

Neuronal function and morphology are affected by the environment and the behavioral experience. Here we report on the effects of differential training protocols on the development and the functional recovery mediated by intrastriatal striatal grafts. Rats were trained exclusively on the left or the right paw to perform on the skilled staircase task before being lesioned unilaterally in the dorsal striatum with quinolinic acid. E15 whole ganglionic eminence suspension grafts were implanted into the lesioned striatum. Subsequent testing probed unilateral performance of the affected contralateral paw, as well as bilateral performance. The grafted animals were initially as impaired as the lesioned, but partially recovered their performance with additional training. Grafted animals with appropriate previous experience initially performed better on the staircase test, but the advantage was transient. Furthermore, the grafted animals performed better with their affected paw under forced choice than under conditions when both paws were simultaneously probed. Improvements of the grafted animals were also observed on tests of forelimb akinesia and asymmetry. Morphological data suggest that the training conditions influenced the development specifically of striatal-like, but not of non-striatal like, neurones within the grafts. The grafts were smaller containing less striatal-like neurones in animals that were trained on the contralateral side prior to lesioning and grafting. The results support the hypothesis that unilateral training sensitizes the striatum that subserves the motor learning, leading to exacerbated excitotoxic lesions and to an environment less conducive for graft development.

Differential effects of learning on neurogenesis: learning increases or decreases the number of newly born cells depending on their birth date.

The hippocampal formation, to which new neurons are added on a daily basis throughout life, is important in spatial learning. Surviving de novo produced cells integrate into the functional circuitry, where they can influence both normal and pathological behaviors. In this study, we examined the effect of the water-maze (a hippocampal-dependent spatial task) on neurogenesis. Learning in this task can be divided into two phases, an early phase during which performance improves rapidly, and a late phase during which asymptotic levels of performance are reached. Here we demonstrate that the late phase of learning has a multifaceted effect on neurogenesis depending on the birth date of new neurons. The number of newly born cells increased contingently with the late phase and a large proportion of these cells survived for at least 4 weeks and differentiated into neurons. In contrast, late-phase learning decreased the number of newly born cells produced during the early phase. This decline in neurogenesis was positively correlated with performance in the water-maze. Thus, rats with the highest de novo cell number were less able to acquire and use spatial information than those with low numbers of new cells. These results show that learning has a complex effect on hippocampal neurogenesis, and reveals a novel mechanism through which neurogenesis may influence normal and pathological behaviors.

Influence of environment on the efficacy of intrastriatal dopaminergic grafts.

Functional recovery is influenced by experience. The aim of the present work was to examine the effects of "enriched" environment (EE) versus an "impoverished" environment on the anatomical and functional integration of intrastriatal dopaminergic grafts. These influences were studied using a paradigm where grafting was performed before the dopamine-depleting lesion. Dopaminergic grafts were implanted into the left neostriatum of adult male rats. In the enriched group, grafted rats were housed collectively and were trained on different behavioral tests following grafting. In contrast, impoverished grafted rats were housed individually and not further manipulated. Ten weeks after grafting, the mesotelencephalic dopaminergic pathway was destroyed unilaterally to the grafted side and different behaviors were followed for 7 months. Grafting prior to lesioning had no prophylactic effects on the performance as the graft did not prevent the onset of the lesion-induced impairments. However, under EE conditions, a graft effect was manifested in the reduction of drug-induced rotation and on the indices of bias as tested by a spatial alternation test. No positive graft effects were observed in the skilled paw reaching test. Grafted rats raised under impoverished conditions performed in a fashion indistinguishable from the control lesioned animals on most measures of behavior. A beneficial effect of EE conditions was observed on survival of TH-positive neurons within the grafts. The results suggest that survival of grafted neurons, and the reduction of the magnitude of particular behavioral impairments, can be optimized by increasing the complexity of the subject's environment.

Striatal lesions produce distinctive impairments in reaction time performance in two different operant chambers.

The dorsal striatum plays a crucial role in mediating voluntary movement. Excitotoxic striatal lesions in rats have previously been shown to impair the initiation but not the execution of movement in a choice reaction time task in an automated lateralised nose-poke apparatus (the "nine-hole box"). Conversely, when a conceptually similar reaction time task has been applied in a conventional operant chamber (or "Skinner box"), striatal lesions have been seen to impair the execution rather than the initiation of the lateralised movement. The present study was undertaken to compare directly these two results by training the same group of rats to perform a choice reaction time task in the two chambers and then comparing the effects of a unilateral excitotoxic striatal lesion in both chambers in parallel. Particular attention was paid to adopting similar parameters and contingencies in the control of the task in the two test chambers. After striatal lesions, the rats showed predominantly contralateral impairments in both tasks. However, they showed a deficit in reaction time in the nine-hole box but an apparent deficit in response execution in the Skinner box. This finding confirms the previous studies and indicates that differences in outcome are not simply attributable to procedural differences in the lesions, training conditions or tasks parameters. Rather, the pattern of reaction time deficit after striatal lesions depends critically on the apparatus used and the precise response requirements for each task.

Striatal grafts alleviate deficits in response execution in a lateralised reaction time task.

It has been reported that homotopic neural transplants can ameliorate behavioural impairments induced by striatal lesions in a reaction time (RT) task. In the present study we seek to replicate and extend this observation in a new lateralised choice RT task based on the conventional Skinner box apparatus. Rats were trained to make rapid lateralised lever press responses to a visual stimulus presented on either the left or the right side of the animal. The RTs required to initiate and execute correct responses were recorded, along with other accuracy and performance indices. Following unilateral lesions of the dorsal striatum, the rats exhibited an increased number of error trials, a bias to respond towards the ipsilateral side, a decreased accuracy on the contralateral side, and an increase of the execution time to respond correctly to contralateral stimuli. Striatal grafts alleviated the lateralised response deficits, prevented the development of lateral disparity, and restored the speed of responding back to pre-lesion levels. Control grafts of cortical tissues also increased task accuracy and reduced the ipsilateral bias in responding, but were without effect on the RT deficit.

Unilateral striatal lesions impair response execution on a lateralised choice reaction time task.

A novel lateralised reaction time task is described and used to evaluate the effects of D-amphetamine injections and unilateral dorso-striatal lesions in rats. The task involves a two-lever Skinner box adaptation of the nine-hole box visual choice reaction time task first developed by Carli et al. D-Amphetamine had a dose dependent effect on nearly all aspects of task performance. Low and the intermediate doses of D-amphetamine speeded reaction time and movement time, and abolished the delay-dependent pattern or responding in the task. The highest dose of amphetamine disrupted the animals' ability to perform reliably, the task contingencies. Unilateral lesions in the dorsal neostriatum resulted in an increase of error trials, produced a bias to respond towards the ipsilateral side, and decreased the accuracy of responding to contralateral stimuli. The overall mean reaction time to contralateral stimuli was not influenced by the lesions, but the movement time was increased selectively when responding to contralateral stimuli. The data suggest that striatal activation by amphetamine increases motor readiness, which can enhance reaction time performance at the cost of increased errors due to anticipation of cue presentation, in particular at long holding delays. Conversely, striatal lesions induce lateralised defects in executive, rather than sensory, processes, and impair the animals' ability to execute movement towards the contralateral side.

Bilateral striatal lesions impair retention of an operant test of short-term memory.

It has been previously shown that lesions of the dorsal striatum can disrupt performance on a variety of cognitive tasks related to prefrontal cortex function. In order to extend these studies, we have compared the effects of bilateral striatal lesions on retention of an operant test of short-term memory using a delayed matching to position task. Rats were initially pretrained on the matching task. Then, one group of animals received ibotenic acid lesions in the dorsal striatum and a second group received sham lesions. The striatal lesions induced marked deficits on delayed matching performance when the rats were retested one week following surgery. The delay-independent pattern of deficit observed does not suggest a primary impairment in short-term memory function. The lesion induced deficits were temporary, with recovery seen in the lesion group back to a control level of performance in 2 additional test sessions 5 and 10 weeks post-lesion. A follow up study investigating the time course of the deficit suggests that the recovery seen in performance measures cannot be attributed to the elapse of time per se, but rather is due to re-learning. In addition to the performance deficits in the operant task, the lesioned rats exhibited marked nocturnal locomotor hyperactivity. The results indicate that bilateral striatal lesions induce marked deficits in retention of the delayed matching to position task. They suggest a key role for the neostriatum in the execution of pre-learned responses, but do not exclude the possibility of additional involvement in short-term memory functions under certain testing conditions.

The effects of bilateral striatal lesions on the acquisition of an operant test of short term memory.

It has been previously shown that lesions of the dorsal striatum can disrupt performance on a variety of cognitive tasks related to frontal cortex function. In order to extend these studies, we have investigated the effects of bilateral striatal lesions on the acquisition of an operant test of short term memory in the delayed non-matching to position paradigm. The animals received either ibotenic acid or saline control injections into the dorsal striatum prior to training on the non-matching task. Striatal lesions retarded acquisition of the task, although with further training the lesioned rats achieved a similar level of asymptotic performance to the control animals. The lesioned rats also exhibited marked nocturnal locomotor hyperactivity when tested under conditions of food deprivation, but not when tested satiated. The results indicate that bilateral striatal lesions induce mild deficits in the acquisition of the discrimination rules involved in performance of the delayed non-matching to position task. The present study does not support a role for the neostriatum in the specific mediation of short term memory in a operant DNMTP test.