The role of the dorsomedial part of the prefrontal cortex serotonergic innervation in rat responses to the aversively conditioned context: Behavioral, biochemical and immunocytochemical studies

Serotonergic neuromodulation of synaptic plasticity

Synaptic plasticity constitutes a fundamental process in the reorganization of neural networks that underlie memory, cognition, emotional responses, and behavioral planning. At the core of this phenomenon lie Hebbian mechanisms, wherein frequent synaptic stimulation induces long-term potentiation (LTP), while less activation leads to long-term depression (LTD). The synaptic reorganization of neuronal networks is regulated by serotonin (5-HT), a neuromodulator capable of modify synaptic plasticity to appropriately respond to mental and behavioral states, such as alertness, attention, concentration, motivation, and mood. Lately, understanding the serotonergic Neuromodulation of synaptic plasticity has become imperative for unraveling its impact on cognitive, emotional, and behavioral functions. Through a comparative analysis across three main forebrain structures-the hippocampus, amygdala, and prefrontal cortex, this review discusses the actions of 5-HT on synaptic plasticity, offering insights into its role as a neuromodulator involved in emotional and cognitive functions. By distinguishing between plastic and metaplastic effects, we provide a comprehensive overview about the mechanisms of 5-HT neuromodulation of synaptic plasticity and associated functions across different brain regions.

Distinct patterns of brain Fos expression in Carioca High- and Low-conditioned Freezing Rats

BACKGROUND

The bidirectional selection of high and low anxiety-like behavior is a valuable tool for understanding the neurocircuits that are responsible for anxiety disorders. Our group developed two breeding lines of rats, known as Carioca High- and Low-conditioned Freezing (CHF and CLF), based on defensive freezing in the contextual fear conditioning paradigm. A random selected line was employed as a control (CTL) comparison group for both CHF and CLF lines of animals. The present study performed Fos immunochemistry to investigate changes in neural activity in different brain structures among CHF and CLF rats when they were exposed to contextual cues that were previously associated with footshock.

RESULTS

The study indicated that CHF rats expressed high Fos expression in the locus coeruleus, periventricular nucleus of the hypothalamus (PVN), and lateral portion of the septal area and low Fos expression in the medial portion of the septal area, dentate gyrus, and prelimbic cortex (PL) compared to CTL animals. CLF rats exhibited a decrease in Fos expression in the PVN, PL, and basolateral nucleus of the amygdala and increase in the cingulate and perirhinal cortices compared to CTL animals.

CONCLUSIONS

Both CHF and CLF rats displayed Fos expression changes key regions of the anxiety brain circuitry. The two bidirectional lines exhibit different pattern of neural activation and inhibition with opposing influences on the PVN, the main structure involved in regulating the hypothalamic-pituitary-adrenal neuroendocrine responses observed in anxiety disorders.

Mild Traumatic Brain Injury with Social Defeat Stress Alters Anxiety, Contextual Fear Extinction, and Limbic Monoamines in Adult Rats

Mild traumatic brain injury (mTBI) produces symptoms similar to those typifying posttraumatic stress disorder (PTSD) in humans. We sought to determine whether a rodent model of stress concurrent with mTBI produces characteristics of PTSD such as impaired contextual fear extinction, while also examining concurrent alterations to limbic monoamine activity in brain regions relevant to fear and anxiety states. Male rats were exposed to social stress or control conditions immediately prior to mTBI induction, and 6 days later were tested either for anxiety-like behavior using the elevated plus maze (EPM), or for contextual fear conditioning and extinction. Brains were collected 24 h after EPM testing, and tissue from various limbic regions analyzed for content of monoamines, their precursors and metabolites using HPLC with electrochemical detection. Either social defeat or mTBI alone decreased time spent in open arms of the EPM, indicating greater anxiety-like behavior. However, this effect was enhanced by the combination of treatments. Further, rats exposed to both social defeat and mTBI exhibited greater freezing within extinction sessions compared to all other groups, suggesting impaired contextual fear extinction. Social defeat combined with mTBI also had greater effects on limbic monoamines than either insult alone, particularly with respect to serotonergic effects associated with anxiety and fear learning. The results suggest social stress concurrent with mTBI produces provides a relevant animal model for studying the prevention and treatment of post-concussive psychobiological outcomes.

Variants of contextual fear conditioning induce differential patterns of Egr-1 activity within the young adult prefrontal cortex

Contextual fear conditioning is a form of associative learning where animals must experience a context before they can associate it with an aversive stimulus. Single-trial contextual fear conditioning (sCFC) and the context preexposure facilitation effect (CPFE) are two variants of CFC where learning about the context is temporally contiguous (sCFC) with or separated (CPFE) from receiving a footshock in that context. Neural activity within CA1 of the dorsal hippocampus (CA1), amygdala (LA), and prefrontal cortex (PFC) may play a critical role when animals learn to associate a context with a footshock (i.e., training). Previous studies from our lab have found that early-growth-response gene 1 (Egr-1), an immediate early gene, exhibits unique patterns of activity within regions of the PFC following training in sCFC and the CPFE of juvenile rats. In the present study, we extended our studies by examining Egr-1 expression in young adult rats to determine (1) if our previous work reflected changes unique to development or extend into adulthood and (2) to contrast expression profiles between sCFC and the CPFE. Rats that learned context fear with sCFC showed increased Egr-1 in the anterior cingulate, orbitofrontal and infralimbic cortices relative to non-associative controls following training, but expression in prelimbic cortex did not differ between fear conditioned and non-associative controls. In contrast, rats trained in the CPFE also showed increased Egr-1 in all the prefrontal cortex regions, including prelimbic cortex. These findings replicate our previous findings in juveniles and suggest that Egr-1 in specific PFC subregions may be uniquely involved in learning context-fear in the CPFE compared to sCFC.

Denial or receipt of expected reward through maternal contact during the neonatal period differentially affect the development of the rat amygdala and program its function in adulthood in a sex-dimorphic way

Early experiences affect brain development and thus adult brain function and behavior. We employed a novel early experience model involving denial (DER) or receipt of expected reward (RER) through maternal contact in a T-maze. Exposure to the DER experience for the first time, on postnatal day 10 (PND10), was stressful for the pups, as assessed by increased corticosterone levels, and was accompanied by enhanced activation of the amygdala, as assessed by c-Fos immunohistochemistry. Re-exposure to the same experience on days 11-13 led to adaptation. Corticosterone levels of the RER pups did not differ on the first and last days of training (PND10 and 13 respectively), while on PND11 and 12 they were lower than those of the CTR. The RER experience did not lead to activation of the amygdala. Males and females exposed as neonates to the DER or RER experience, and controls were tested as adults in the open field task (OF), the elevated plus maze (EPM), and cued and contextual fear conditioning (FC). No group differences were found in the EPM, while in the OF, both male and female DER animals, showed increased rearings, compared to the controls. In the FC, the RER males had increased memory for both context and cued conditioned fear, than either the DER or CTR. On the other hand, the DER males, but not females showed an increased activation, as assessed by c-Fos expression, of the amygdala following fear conditioning. Our results show that the DER early experience programmed the function of the adult amygdala as to render it more sensitive to fearful stimuli. This programming by the DER early experience could be mediated through epigenetic modifications of histones leading to chromatin opening, as indicated by our results showing increased levels of phospho-acetyl-histone-3 in the amygdala of the DER males.

Temporal changes in c-Fos activation patterns induced by conditioned fear

Mechanisms underlying shock-induced conditioned fear - a paradigm frequently used to model posttraumatic stress disorder, PTSD - are usually studied shortly after shocks. Some of the brain regions relevant to conditioned fear were activated in all the c-Fos studies published so far, but the overlap between the activated regions was small across studies. We hypothesized that discrepant findings were due to dynamic neural changes that followed shocks, and a more consistent picture would emerge if consequences were studied after a longer interval. Therefore, we exposed rats to a single session of footshocks and studied their behavioral and neural responses one and 28 days later. The neuronal activation marker c-Fos was studied in 24 brain regions relevant for conditioned fear, e.g. in subdivisions of the prefrontal cortex, hippocampus, amygdala, hypothalamic defensive system, brainstem monoaminergic nuclei and periaqueductal gray. The intensity of conditioned fear (as shown by the duration of contextual freezing) was similar at the two time-points, but the associated neuronal changes were qualitatively different. Surprisingly, however, Multiple Regression Analyses suggested that conditioned fear-induced changes in neuronal activation patterns predicted the duration of freezing with high accuracy at both time points. We suggest that exposure to electric shocks is followed by a period of plasticity where the mechanisms that sustain conditioned fear undergo qualitative changes. Neuronal changes observed 28 days but not 1 day after shocks were consistent with those observed in human studies performed in PTSD patients.

Individual differences in novelty-seeking behavior in rats as a model for psychosocial stress-related mood disorders

Most neuropsychiatric disorders, including stress-related mood disorders, are complex multi-parametric syndromes. Diagnoses are therefore hard to establish and current therapeutic strategies suffer from significant variability in effectiveness, making the understanding of inter-individual variations crucial to unveiling effective new treatments. In rats, such individual differences are observed during exposure to a novel environment, where individuals will exhibit either high or low locomotor activity and can thus be separated into high (HR) and low (LR) responders, respectively. In rodents, a long-lasting, psychosocial, stress-induced depressive state can be triggered by exposure to a social defeat procedure. We therefore analyzed the respective vulnerabilities of HR and LR animals to long-lasting, social defeat-induced behavioral alterations relevant to mood disorders. Two weeks after four daily consecutive social defeat exposures, HR animals exhibit higher anxiety levels, reduced body weight gain, sucrose preference, and a marked social avoidance. LR animals, however, remain unaffected. Moreover, while repeated social defeat exposure induces long-lasting contextual fear memory in both HR and LR animals, only HR individuals exhibit marked freezing behavior four weeks after a single social defeat. Combined, these findings highlight the critical involvement of inter-individual variations in novelty-seeking behavior in the vulnerability to stress-related mood disorders, and uncover a promising model for posttraumatic stress disorder.