Serotonin, locomotion, exploration, and place recall in the rat

TIAM2S as a novel regulator for serotonin level enhances brain plasticity and locomotion behavior

TIAM2S, the short form of human T-cell lymphoma invasion and metastasis 2, can have oncogenic effects when aberrantly expressed in the liver or lungs. However, it is also abundant in healthy, non-neoplastic brain tissue, in which its primary function is still unknown. Here, we examined the neurobiological and behavioral significance of human TIAM2S using the human brain protein panels, a human NT2/D1-derived neuronal cell line model (NT2/N), and transgenic mice that overexpress human TIAM2S (TIAM2S-TG). Our data reveal that TIAM2S exists primarily in neurons of the restricted brain areas around the limbic system and in well-differentiated NT2/N cells. Functional studies revealed that TIAM2S has no guanine nucleotide exchange factor (GEF) activity and is mainly located in the nucleus. Furthermore, whole-transcriptome and enrichment analysis with total RNA sequencing revealed that TIAM2S-knockdown (TIAM2S-KD) was strongly associated with the cellular processes of the brain structural development and differentiation, serotonin-related signaling, and the diseases markers representing neurobehavioral developmental disorders. Moreover, TIAM2S-KD cells display decreased neurite outgrowth and reduced serotonin levels. Moreover, TIAM2S overexpressing TG mice show increased number and length of serotonergic fibers at early postnatal stage, results in higher serotonin levels at both the serum and brain regions, and higher neuroplasticity and hyperlocomotion in latter adulthood. Taken together, our results illustrate the non-oncogenic functions of human TIAM2S and demonstrate that TIAM2S is a novel regulator of serotonin level, brain neuroplasticity, and locomotion behavior.

Reorganization of motor cortex and impairment of motor performance induced by hindlimb unloading are partially reversed by cortical IGF-1 administration

Immobilization, bed rest, or sedentary lifestyle, are known to induce a profound impairment in sensorimotor performance. These alterations are due to a combination of peripheral and central factors. Previous data conducted on a rat model of disuse (hindlimb unloading, HU) have shown a profound reorganization of motor cortex and an impairment of motor performance. Recently, our interest was turned towards the role of insulin-like growth factor 1 (IGF-1) in cerebral plasticity since this growth factor is considered as the mediator of beneficial effects of exercise on the central nervous system, and its cortical level is decreased after a 14-day period of HU. In the present study, we attempted to determine whether a chronic subdural administration of IGF-1 in HU rats could prevent deleterious effects of HU on the motor cortex and on motor activity. We demonstrated that HU induces a shrinkage of hindlimb cortical representation and an increase in current threshold to elicit a movement. Administration of IGF-1 in HU rats partially reversed these changes. The functional evaluation revealed that IGF-1 prevents the decrease in spontaneous activity found in HU rats and the changes in hip kinematics during overground locomotion, but had no effect of challenged locomotion (ladder rung walking test). Taken together, these data clearly indicate the implication of IGF-1 in cortical plastic mechanisms and in behavioral alteration induced by a decreased in sensorimotor activity.

Equal effects of typical environmental and specific social enrichment on posttraumatic cognitive functioning after fimbria-fornix transection in rats

Enriched environment (EE) has been shown to have beneficial effects on cognitive recovery after brain injury. Typical EE comprises three components: (i) enlarged living area providing physical activation, (ii) sensory stimulation, and (iii) social stimulation. The present study assessed the specific contribution of the social stimulation. Animals were randomly divided into groups of (1) a typical EE, (2) pure social enrichment (SE), or (3) standard housing (SH) and subjected to either a sham operation or transection of the fimbria-fornix (FF). The effect of these conditions on acquisition of a delayed alternation task in a T-maze was assessed. The sham control groups were not affected by housing conditions. In the lesioned groups, both typical EE and SE improved the task acquisition, compared to SH. A baseline one-hour activity measurement confirmed an equal level of physical activity in the EE and SE groups. After delayed alternation testing, pharmacological challenges (muscarinergic antagonist scopolamine and dopaminergic antagonist SKF-83566) were used to assess cholinergic and dopaminergic contributions to task solution. Scopolamine led to a marked impairment in all groups. SKF-83566 significantly enhanced the performance of the lesioned group subjected to SE. The results demonstrate that housing in a typical as well as atypical EE can enhance cognitive recovery after mechanical injury to the hippocampus. The scopolamine challenge revealed a cholinergic dependency during task performance in all groups, regardless of lesion and housing conditions. The dopaminergic challenge revealed a difference in the neural substrates mediating recovery in the lesioned groups exposed to different types of housing.

Reorganization of the injured brain: implications for studies of the neural substrate of cognition

In the search for a neural substrate of cognitive processes, a frequently utilized method is the scrutiny of post-traumatic symptoms exhibited by individuals suffering focal injury to the brain. For instance, the presence or absence of conscious awareness within a particular domain may, combined with knowledge of which regions of the brain have been injured, provide important data in the search for neural correlates of consciousness. Like all studies addressing the consequences of brain injury, however, such research has to face the fact that in most cases, post-traumatic impairments are accompanied by a "functional recovery" during which symptoms are reduced or eliminated. The apparent contradiction between localization and recovery, respectively, of functions constitutes a problem to almost all aspects of cognitive neuroscience. Several lines of investigation indicate that although the brain remains highly plastic throughout life, the post-traumatic plasticity does not recreate a copy of the neural mechanisms lost to injury. Instead, the uninjured parts of the brain are functionally reorganized in a manner which - in spite of not recreating the basic information processing lost to injury - is able to allow a more or less complete return of the surface phenomena (including manifestations of consciousness) originally impaired by the trauma. A novel model [the Reorganization of Elementary Functions-model] of these processes is presented - and some of its implications discussed relative to studies of the neural substrates of cognition and consciousness.

Post-traumatic functional recovery and reorganization in animal models: a theoretical and methodological challenge

Studies addressing cerebral functional localization face methodological and theoretical problems. Lesion experiments expect that when a functionally specialized structure is missing, its function can be deduced from the resulting impairments. Mostly, however, initial impairments are partially or fully eliminated through functional recovery. Apparently, such a recovery contradicts the notion of functional localization. In order to understand the mechanisms of recovery, improved methodology and a new theoretical framework are required. Insights into the mechanisms of recovery can be achieved by using "challenge" techniques, where functionally recovered individuals are exposed to organic and behavioral challenges, e.g. pharmacological manipulations or additional lesions, as well as modified test situations. Using such methods, a number of principles of functional recovery have emerged. We evaluate some of the available theories of post-traumatic recovery against these principles and find that none of them can account for the principles. Finally, we present a new conceptual framework - the Reorganization of Elementary Functions (REF) model. This model reconceptualizes the term "function", suggests mechanisms of post-traumatic reorganizations, and resolves the contradiction between localization and functional recovery.

Dissociation of the neurochemical and behavioral toxicology of MDMA ('Ecstasy') by citalopram

High or repeated doses of the recreational drug 3,4-methylenedioxymethamphetamine (MDMA, or 'Ecstasy') produce long-lasting deficits in several markers of serotonin (5-HT) system integrity and also alter behavioral function. However, it is not yet clear whether MDMA-induced serotonergic neurotoxicity is responsible for these behavioral changes or whether other mechanisms are involved. The present experiment tested the hypothesis that blocking serotonergic neurotoxicity by pretreatment with the selective 5-HT reuptake inhibitor citalopram will also prevent the behavioral and physiological consequences of an MDMA binge administration. Male, Sprague-Dawley rats (N=67) received MDMA (4 x 10 mg/kg) with or without citalopram (10 mg/kg) pretreatment. Core temperature, ejaculatory response, and body weight were monitored during and immediately following drug treatments. A battery of tests assessing motor, cognitive, exploratory, anxiety, and social behaviors was completed during a 10-week period following MDMA administration. Brain tissue was collected at 1 and 10 weeks after drug treatments for measurement of regional 5-HT transporter binding and (for the 1-week samples) 5-HT and 5-HIAA concentrations. Citalopram pretreatment blocked MDMA-related reductions in aggressive and exploratory behavior measured in the social interaction and hole-board tests respectively. Such pretreatment also had the expected protective effect against MDMA-induced 5-HT neurotoxicity at 1 week following the binge. In contrast, citalopram did not prevent most of the acute effects of MDMA (eg hyperthermia and weight loss), nor did it block the decreased motor activity seen in the binge-treated animals 1 day after dosing. These results suggest that some of the behavioral and physiological consequences of a high-dose MDMA regimen in rats are mediated by mechanisms other than the drug's effects on the serotonergic system. Elucidation of these mechanisms requires further study of the influence of MDMA on other neurotransmitter systems.

Brain serotonin depletion impairs short-term memory, but not long-term memory in rats

Intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) (150 microg; 4.5 microl/ventricle), a serotonergic neurotoxin, significantly decreased spontaneous alternation in Y-maze task and working memory in radial 8 arm-maze task, suggesting effects on short-term memory, without affecting long-term memory, explored by reference memory in radial 8 arm-maze task and step-through latency in multi-trial passive avoidance task. Parachlorophenylalanine (PCPA) (3 days treatment 200 microg, i.c.v.), a serotonin synthesis inhibitor, did not impair step-through-latency in multi-trial passive avoidance task, suggesting no effects on long-term memory. These results suggest that serotonin, among other neurotransmitters, play an important role in cognitive functions, especially short-term memory.

Prefrontal cortex and hippocampus in posttraumatic functional recovery: Spatial delayed alternation by rats subjected to transection of the fimbria–fornix and/or ablation of the prefrontal cortex

Lesions of the prefrontal cortex and the hippocampus often lead to impairment of the same behavioural tasks (e.g., allocentric as well as egocentric spatial orientation and spatial delayed alternation). In case of allocentric and egocentric spatial orientation we have previously found that the two structures mutually contribute to the posttraumatic functional recovery of such tasks. We therefore presently tested the hypothesis that this would even be true in case of spatial delayed alternation. The acquisition of a spatial delayed alternation task in a T-maze was studied in four groups of rats: animals in which the fimbria-fornix had been transected bilaterally, rats who had received bilateral ablations of the anteromedial prefrontal cortex, animals in which both of these structures had been lesioned, and a sham operated control group. All three lesion groups demonstrated an impaired task acquisition. The group given prefrontal cortical lesions in isolation underwent a complete functional recovery. Both of the fimbria-fornix transected groups were significantly impaired even when compared to the group given prefrontal cortical ablations in isolation. The two fimbria-fornix lesioned groups did, however, exhibit levels of functional recovery. The group in which both structures had been lesioned demonstrated a task acquisition, which was significantly inferior to that of the group given fimbria-fornix transections in isolation. After completion of the task acquisition period, all animals were subjected to two behavioural challenges including a session on which the duration of the inter-trial delay was doubled. This expansion of the inter-trial delay rather selectively impaired the task performance of the group given fimbria-fornix transections in isolation. Consequently, both during the acquisition period and in one of the challenges a differentiation of functional recovery was seen between the combined lesioned group and the group given fimbria-fornix lesions only. This indicates that even in case of a spatial delayed alternation task the prefrontal cortex normally contributes significantly to mediation of posttraumatic functional recovery after isolated lesions of the fimbria-fornix. The results are discussed in the context of models of posttraumatic functional recovery.

Dietary restriction protects against chronic-ethanol-induced changes in exploratory behavior in Wistar rats

Chronic ethanol intake causes various types of neural damage and behavioral impairments, probably acting through oxidative stress and excitotoxicity, while dietary restriction is considered by some authors to protect the central nervous system from these kinds of damage. In the present study, a factorial experimental design was used to investigate the effects of chronic ethanol and dietary restriction treatments, associated or not, on Wistar rats' exploratory behavior, spatial memory aspects and cortical and hippocampal acetylcholinesterase (AChE) activity. Dietary restriction lasted for the whole experiment, while ethanol treatment lasted for only 3 weeks. Despite the short ethanol treatment duration, for two behavior categories assessed, moving and rearing, an interaction was observed between the effects of chronic ethanol and dietary restriction. There were no significant differences in AChE activities among the groups. Cerebellar neural nitric oxide synthase (nNOs) activity was measured as a first step to assess oxidative stress. Dietary restriction significantly reduced NO formation. The present results indicate that dietary restriction might exert a protective effect against chronic-ethanol-induced changes in exploratory behavior. It is hypothesized that the mechanisms underlying this protection can involve prevention of oxidative stress.

Inhibitory effect of ghrelin on food intake is mediated by the corticotropin-releasing factor system in neonatal chicks

It is known that, in rats, central and peripheral ghrelin increases food intake mainly through activation of neuropeptide Y (NPY) neurons. In contrast, intracerebroventricular (ICV) injection of ghrelin inhibits food intake in neonatal chicks. We examined the mechanism governing this inhibitory effect in chicks. The ICV injection of ghrelin or corticotropin-releasing factor (CRF), which also inhibits feeding and causes hyperactivity in chicks. Thus, we examined the interaction of ghrelin with CRF and the hypothalamo-pituitary-adrenal (HPA) axis. The ICV injection of ghrelin increased plasma corticosterone levels in a dose-dependent or a time-dependent manner. Co-injection of a CRF receptor antagonist, astressin, attenuated ghrelin-induced plasma corticosterone increase and anorexia. In addition, we also investigated the effect of ghrelin on NPY-induced food intake and on expression of hypothalamic NPY mRNA. Co-injection of ghrelin with NPY inhibited NPY-induced increase in food intake, and the ICV injection of ghrelin did not change NPY mRNA expression. These results indicate that central ghrelin does not interact with NPY as seen in rodents, but instead inhibits food intake by interacting with the endogenous CRF and its receptor.

Place learning and object recognition by rats subjected to transection of the fimbria-fornix and/or ablation of the prefrontal cortex

The acquisition of a water maze-based allocentric place learning task and an exploration based object recognition task were studied in four groups of rats: animals in which the fimbria-fornix had been transected, rats who had received bilateral ablations of the anteromedial prefrontal cortex, animals in which both of these structures had been lesioned, and a sham operated control group. None of the groups showed impairments of object recognition. Ablations of the prefrontal cortex caused a mild impairment in the acquisition of the place learning task. The two fimbria-fornix transected groups exhibited a severe impairment during the acquisition of this task. All groups reached criterion level task performance eventually. All groups were subjected to a number of behavioural and pharmacological challenges in order to elucidate the neural and cognitive mechanisms of this behavioural recovery. During a no-platform session both the fimbria-fornix transected group and the prefrontally ablated group demonstrated a normal preference for the former platform position. The combined lesion group, however, failed to show a similar preference for this position. The outcome of the pharmacological challenges demonstrated that while the task performance of all four groups relied equally on catecholaminergic mediation, only the task solution of the fimbria-fornix transected group was significantly impaired by disturbance of the catecholaminergic systems. The data indicated a high likelihood that prefrontal cortical mechanisms contribute to the recovery of allocentric place learning after fimbria-fornix transections.

Associative and nonassociative learning after chronic imipramine in rats

We investigated effects of 15 daily injections of imipramine (20 mg/kg; in one experiment also 10 and 30 mg/kg). The associative learning types (place learning and object recognition) as well as nonassociative learning (habituation of exploration in an open field and within the object recognition test) were studied. Tests were performed immediately after the final injection (early test) and 24 h after the final injection (late test). The 5-HT(1A), 5-HT(1B/D), 5-HT(2A), beta-adrenergic, D(2) receptors were assayed 24 h after the final injection and the 5-HT(2A) and beta-adrenergic receptors were also measured 60 and 96 h after the final injection. While associative types of learning were impaired in early tests, they remained unaffected in late tests and, while the nonassociative learning (habituation of exploration) remained unaffected in early tests, it was changed in late tests. Measured 24 h after the final injection, imipramine (20 and 30 mg/kg per day) down-regulated the concentration of beta-adrenergic and 5-HT(2A) receptors, while leaving all other measured receptors unaffected. However, only the down-regulation of the 5-HT(2A) receptor outlasted the initial 24-h period after the final injection. On the basis of present and previous results, we interpret the impairment of associative types of learning in early tests as a reflection of anticholinergic effects of imipramine, while the modifications of habituation of exploration in late tests are likely primarily to be mediated by imipramine-provoked regulations of serotonergic receptors.