Collateral projections from the median raphe nucleus to the medial septum and hippocampus

The Role of Vesicular Glutamate Transporter Type 3 in Social Behavior, with a Focus on the Median Raphe Region

Social behavior is important for our well-being, and its dysfunctions impact several pathological conditions. Although the involvement of glutamate is undeniable, the relevance of vesicular glutamate transporter type 3 (VGluT3), a specific vesicular transporter, in the control of social behavior is not sufficiently explored. Since midbrain median raphe region (MRR) is implicated in social behavior and the nucleus contains high amount of VGluT3+ neurons, we compared the behavior of male VGluT3 knock-out (KO) and VGluT3-Cre mice, the latter after chemogenetic MRR-VGluT3 manipulation. Appropriate control groups were included. Behavioral test battery was used for social behavior (sociability, social discrimination, social interaction, resident intruder test) and possible confounding factors (open field, elevated plus maze, Y-maze tests). Neuronal activation was studied by c-Fos immunohistochemistry. Human relevance was confirmed by VGluT3 gene expression in relevant human brainstem areas. VGluT3 KO mice exhibited increased anxiety, social interest, but also aggressive behavior in anxiogenic environment and impaired social memory. For KO animals, social interaction induced lower cell activation in the anterior cingulate, infralimbic cortex, and medial septum. In turn, excitation of MRR-VGluT3+ neurons was anxiolytic. Inhibition increased social interest 24 h later but decreased mobility and social behavior in aggressive context. Chemogenetic activation increased the number of c-Fos+ neurons only in the MRR. We confirmed the increased anxiety-like behavior and impaired memory of VGluT3 KO strain and revealed increased, but inadequate, social behavior. MRR-VGluT3 neurons regulated mobility and social and anxiety-like behavior in a context-dependent manner. The presence of VGluT3 mRNA on corresponding human brain areas suggests clinical relevance.

Projection-defined median raphe Pet+ subpopulations are diversely implicated in seizure

The raphe nuclei, the primary resource of forebrain 5-HT, play an important but heterogeneous role in regulating subcortical excitabilities. Fundamental circuit organizations of different median raphe (MR) subsystems are far from completely understood. In the present study, using cell-specific viral tracing, Ca2+ fiber photometry and epilepsy model, we map out the forebrain efferent and afferent of different MR Pet+ subpopulations and their divergent roles in epilepsy. We found that PetMR neurons send both collateral and parallel innervations to different downstream regions through different subpopulations. Notably, CA3-projecting PetMR subpopulations are largely distinct from habenula (Hb)-projecting PetMR subpopulations in anatomical distribution and topological organization, while majority of the CA3-projecting PetMR subpopulations are overlapped with the medial septum (MS)-projecting PetMR subpopulations. Further, using Ca2+ fiber photometry, we monitor activities of PetMR neurons in hippocampal-kindling seizure, a classical epilepsy model with pathological mechanisms caused by excitation-inhibition imbalance. We found that soma activities of PetMR neurons are heterogeneous during different periods of generalized seizures. These divergent activities are contributed by different projection-defined PetMR subpopulations, manifesting as increased activities in CA3 but decreased activity in Hb resulting from their upstream differences. Together, our findings provide a novel framework of MR subsystems showing that projection-defined MR Pet+ subpopulations are topologically heterogenous with divergent circuit connections and are diversely implicated in seizures. This may help in the understanding of heterogeneous nature of MR 5-HTergic subsystems and the paradox roles of 5-HTergic systems in epilepsy.

Postnatal development of the relaxin-3 innervation of the rat medial septum

Introduction

The septal area provides a rich innervation to the hippocampus regulating hippocampal excitability to different behavioral states and modulating theta rhythmogenesis. However, little is known about the neurodevelopmental consequences of its alterations during postnatal development. The activity of the septohippocampal system is driven and/or modulated by ascending inputs, including those arising from the nucleus incertus (NI), many of which contain the neuropeptide, relaxin-3 (RLN3).

Methods

We examined at the molecular and cellular level the ontogeny of RLN3 innervation of the septal area in postnatal rat brains.

Results

Up until P13-15 there were only scattered fibers in the septal area, but a dense plexus had appeared by P17 that was extended and consolidated throughout the septal complex by P20. There was a decrease in the level of colocalization of RLN3 and synaptophysin between P15 and P20 that was reversed between P20 and adulthood. Biotinylated 3-kD dextran amine injections into the septum, revealed retrograde labeling present in the brainstem at P10-P13, but a decrease in anterograde fibers in the NI between P10-20. Simultaneously, a differentiation process began during P10-17, resulting in fewer NI neurons double-labeled for serotonin and RLN3.

Discussion

The onset of the RLN3 innervation of the septum complex between P17-20 is correlated with the onset of hippocampal theta rhythm and several learning processes associated with hippocampal function. Together, these data highlight the relevance and need for further analysis of this stage for normal and pathological septohippocampal development.

Clozapine prevented social interaction deficits and reduced c-Fos immunoreactivity expression in several brain areas of rats exposed to acute restraint stress

In the present study, we evaluate the effect of acute restraint stress (15 min) of male Wistar rats on social interaction measurements and c-Fos immunoreactivity (c-Fos-ir) expression, a marker of neuronal activity, in areas involved with the modulation of acute physical restraint in rats, i.e., the paraventricular nucleus of the hypothalamus (PVN), median raphe nucleus (MnR), medial prefrontal cortex (mPFC), cingulate prefrontal cortex (cPFC), nucleus accumbens (NaC), hippocampus (CA3), lateral septum (LS) and medial amygdala (MeA). We considered the hypothesis that restraint stress exposure could promote social withdrawal induced by the activation of the hypothalamic-pituitary-adrenocortical (HPA) axis, and increase c-Fos expression in these limbic forebrain areas investigated. In addition, we investigated whether pretreatment with the atypical antipsychotic clozapine (5 mg/kg; I.P.) could attenuate or block the effects of restraint on these responses. We found that restraint stress induced social withdrawal, and increased c-Fos-ir in these areas, demonstrating that a single 15 min session of physical restraint of rats effectively activated the HPA axis, representing an effective tool for the investigation of neuronal activity in brain regions sensitive to stress. Conversely, pretreatment with clozapine, prevented social withdrawal and reduced c-Fos expression. We suggest that treatment with clozapine exerted a preventive effect in the social interaction deficit, at least in part, by blocking the effect of restraint stress in brain regions that are known to regulate the HPA-axis, including the cerebral cortex, hippocampus, hypothalamus, septum and amygdala. Further experiments will be done to confirm this hypothesis.

A New Player in the Hippocampus: A Review on VGLUT3+ Neurons and Their Role in the Regulation of Hippocampal Activity and Behaviour

Glutamate is the most abundant excitatory amino acid in the central nervous system. Neurons using glutamate as a neurotransmitter can be characterised by vesicular glutamate transporters (VGLUTs). Among the three subtypes, VGLUT3 is unique, co-localising with other "classical" neurotransmitters, such as the inhibitory GABA. Glutamate, manipulated by VGLUT3, can modulate the packaging as well as the release of other neurotransmitters and serve as a retrograde signal through its release from the somata and dendrites. Its contribution to sensory processes (including seeing, hearing, and mechanosensation) is well characterised. However, its involvement in learning and memory can only be assumed based on its prominent hippocampal presence. Although VGLUT3-expressing neurons are detectable in the hippocampus, most of the hippocampal VGLUT3 positivity can be found on nerve terminals, presumably coming from the median raphe. This hippocampal glutamatergic network plays a pivotal role in several important processes (e.g., learning and memory, emotions, epilepsy, cardiovascular regulation). Indirect information from anatomical studies and KO mice strains suggests the contribution of local VGLUT3-positive hippocampal neurons as well as afferentations in these events. However, further studies making use of more specific tools (e.g., Cre-mice, opto- and chemogenetics) are needed to confirm these assumptions.

Neural serotonergic circuits for controlling long-term voluntary alcohol consumption in mice

Alcohol-use-disorders are chronic relapsing illnesses, often co-morbid with anxiety. We have previously shown using the "drinking-in-the-dark" model in mice that the stimulation of the serotonin receptor 1A (5-HT_1A) reduces ethanol binge-drinking behaviour and withdrawal-induced anxiety. The 5-HT_1A receptor is located either on Raphe neurons as autoreceptors, or on target neurons as heteroreceptors. By combining a pharmacological approach with biased agonists targeting the 5-HT_1A auto- or heteroreceptor and a chemogenetic approach (DREADDs), here we identified that ethanol-binge drinking behaviour is dependent on 5-HT_1A autoreceptors and 5-HT neuronal function, with a transition from DRN-dependent regulation of short-term (6 weeks) ethanol intake, to MRN-dependent regulation after longer ethanol exposure (12 weeks). We further identified a serotonergic microcircuit (5-HT^MRN→DG) originating from the MRN and projecting to the dentate gyrus (DG) of the hippocampus, that is specifically affected by, and modulates long-term ethanol consumption. The present study indicates that targeting Raphe nuclei 5-HT_1A autoreceptors with agonists might represent an innovative pharmacotherapeutic strategy to combat alcohol abuse.

The Relationship Between Sleep Quality, Sleep-Related Biomarkers, and Motor Skill Acquisition in People With Multiple Sclerosis: A Pilot Study

OBJECTIVE

Neurorehabilitation that involves learning new motor skills is one of the promising clinical methods for motor recovery in people with multiple sclerosis (PwMS); therefore, factors that influence the acquisition of motor skills in PwMS need to be investigated. Sleep disturbances are common in PwMS; however, no study has investigated the effect of sleep and sleep-related biomarkers on skill acquisition in PwMS. This study aimed to examine the effect of sleep and sleep-related biomarkers on motor acquisition in PwMS.

METHODS

Forty participants with MS and 40 controls were recruited in this study. To assess motor acquisition, each participant was asked to perform a novel game through a virtual reality (VR) system 5 times (blocks). The main outcome measures for each block were the required time to complete the VR game and the recorded errors. The difference in scores between Block 5 and Block 1 for both outcomes were considered to represent motor skill acquisition. Sleep was assessed by self-report using the Pittsburgh Sleep Quality Index (PSQI) and objectively using sleep monitor technology. Serotonin level was assessed using means of enzyme-linked immunosorbent assay using plasma samples.

RESULTS

There were significant positive correlations in both groups between motor skill acquisition and PSQI score. In PwMS, significant negative correlation between motor skill acquisition and sleep efficiency and significant positive correlation between motor skill acquisition and sleep latency were also observed. Interestingly, a significant negative correlation was observed between motor skill acquisition and the plasma serotonin level in both groups. Most of these correlations remained significant after controlling for disease severity, fatigue, baseline performance, and cognitive status.

CONCLUSION

Sleep quality may influence motor skill acquisition in PwMS. Circulatory serotonin level might explain this relationship.

IMPACT

Physical therapists are encouraged to be aware of sleep quality and sleep assessment. Sleep management strategies should be considered when treating PwMS.

5-HT7 receptor activation rescues impaired synaptic plasticity in an autistic-like rat model induced by prenatal VPA exposure

Autism spectrum disorder (ASD) is a severe life-long neuropsychiatric disorder. Alterations and imbalance of several neurochemical systems may be involved in ASD pathophysiology, of them, serotonergic neurotransmission dysfunction and deficiency may underlie behavioral abnormalities associated with ASD. However, the functional importance of serotonergic receptors, particularly 5HT7 receptors in ASD pathology remains poorly defined. Serotonin receptor subtype 7 (5-HT7R) plays a direct regulatory role in the development and also for the mature function of the brain, therefore, further studies are necessary to elucidate the role of these receptors in the etiology of autism. To address this issue, we combined here behavioral, electrophysiological methods to further characterize the contribution of 5-HT7Rs in the prenatal valproic acid (VPA) exposure-induced impairment in synaptic plasticity and their impact on the associated behavioral changes. This may help to unravel the underlying cellular mechanisms involved in ASD and can lead to new treatment and/or prevention therapies based on the role of the serotonergic system for autism. Findings revealed that compared to control, autistic-like offspring showed increased anxiety-like behavior, reduced social interaction, decreased locomotor activity, and impaired identification of the novel object. However, administration of 5-HT7Rs agonist, LP-211, for 7 consecutive days before testing from postnatal day 21 to 27 reversed all behavioral deficits induced by prenatal exposure to VPA in offspring. Also, both short-term depression and long-term potentiation were impaired in the autistic-like pups, but activation of 5-HT7Rs rescued the LTP impairment in the autistic-like group so that there was no significant difference between the two groups. Blockade of 5-HT7Rs caused LTP impairment following HFS in the autistic-like group. Besides, there was a significant difference in LTD induction following SB-269970 application between the control and the autistic-like groups measured at first 10 min following TPS. Moreover, both the number and the size of retrograde fast blue-labelled neurons in the raphe nuclei were reduced. Overall, these results provide for the first time, as far as we know, functional evidence for the restorative role of 5-HT7Rs activation against prenatal VPA exposure induced behavioral deficits and hippocampal synaptic plasticity impairment. Therefore, these receptors could be a potential and promising pharmacotherapy target for the treatment of autism.

Role of 5-HT1A receptors in the ventral hippocampus in the regulation of anxiety- and panic-related defensive behaviors in rats

Changes in 5-HT_1A receptor (5-HT_1AR)-mediated neurotransmission in the hippocampus have been associated with anxiety, depression and in the mode of action of antidepressant drugs. It has been commonly accepted that whereas the dorsal pole of the hippocampus (DH) is involved in cognitive processing, the ventral pole (VH) is associated with emotional regulation. However, to date, only a few studies have directly addressed the role played by VH 5-HT_1ARs in anxiety and panic processing, and their results are conflicting. Here we report that intra-VH administration of the 5-HT_1A receptor agonist 8-OH-DPAT, the endogenous agonist serotonin (5-HT), or the standard anxiolytic benzodiazepine midazolam impaired the acquisition of inhibitory avoidance in the elevated T-maze (ETM) of male Wistar rats, indicating an anxiolytic effect. Conversely, local injection of the 5-HT_1AR antagonist WAY-100635 caused the opposite effect. These results were equally found in the Vogel conflict test. None of these drugs interfered with locomotor activity in the open-field test, nor did they alter the expression of the escape response in the ETM, a defensive behavior associated with panic. Pre-injection of a sub-effective dose of WAY-100635 in the VH blocked the anxiolytic effect of 5-HT or 8-OH-DPAT in the Vogel test, confirming the involvement of 5-HT_1AR for this behavioral effect. The effect in this test was anxiety-selective as none of the drugs affected water consumption or nociception. In conclusion, our results suggest that 5-HT_1ARs in the VH play a tonic inhibitory role in anxiety processing. These receptors, however, are not involved in the regulation of panic-related escape behavior.

The CA1 hippocampal serotonin alterations involved in anxiety-like behavior induced by sciatic nerve injury in rats

OBJECTIVES

Several clinical and experimental studies reported the anxiety as one of the neuropathic pain comorbidities; however, the mechanisms involved in this comorbidity are incompletely cleared. The current study investigated the consequence of pain induced by peripheral neuropathy on the serotonin (5-HT) level of the CA1 region of the hippocampus, which is known as a potential reason, for anxiety associated with neuropathic pain.

METHODS

In this manner, 72 male rats were inconstantly subdivided into three experimental groups as follows: control, sham, and chronic constriction injury (CCI). Neuropathic pain was initiated by the CCI of the sciatic nerve, and then, mechanical allodynia, thermal hyperalgesia, and anxiety-like behavior were evaluated using the von Frey filaments, radiant heat, open field test (OFT), and elevated plus maze (EPM) respectively. To investigate the probable mechanisms, the in vivo extracellular levels of 5-HT were assessed by microdialysis and using reverse-phase high-pressure liquid chromatography (HPLC) in the CA1 region of hippocampus on days 16 and 30 post-CCI.

RESULTS

Our data suggested that CCI caused anxiety-like behavior in OFT and EPM test. 5-HT concentration in the CA1 region of the hippocampus significantly (F=43.8, p=0.000) reduced in CCI rats, when the pain threshold was minimum. Nevertheless, these alterations reversed while the pain threshold innate increased.

CONCLUSIONS

Neuropathic pain, initiated by constriction of the sciatic nerve can induce anxiety-like behavior in rats. This effect accompanies the reduction in 5-HT concentration in the CA1 region of the hippocampus. When the pain spontaneously alleviated, 5-HT level increased and anxiety-like behavior relieved.

Two Functionally Distinct Serotonergic Projections into Hippocampus

Hippocampus receives dense serotonergic input specifically from raphe nuclei. However, what information is carried by this input and its impact on behavior has not been fully elucidated. Here we used in vivo two-photon imaging of activity of hippocampal median raphe projection fibers in behaving male and female mice and identified two distinct populations: one linked to reward delivery and the other to locomotion. Local optogenetic manipulation of these fibers confirmed a functional role for these projections in the modulation of reward-induced behavior. The diverse function of serotonergic inputs suggests a key role in integrating locomotion and reward information into the hippocampal CA1.SIGNIFICANCE STATEMENT Information constantly flows in the hippocampus, but only some of it is captured as a memory. One potential process that discriminates which information should be remembered is concomitance with reward. In this work, we report a neuromodulatory pathway, which delivers reward signal as well as locomotion signal to the hippocampal CA1. We found that the serotonergic system delivers heterogeneous input that may be integrated by the hippocampus to support its mnemonic functions. It is dynamically involved in regulating behavior through interaction with the hippocampus. Our results suggest that the serotonergic system interacts with the hippocampus in a dynamic and behaviorally specific manner to regulate reward-related information processing.

Temporary inactivation of interpeduncular nucleus impairs long but not short term plasticity in the perforant-path dentate gyrus synapses in rats

The interconnectivity of the hippocampus, interpeduncular nucleus (IPN) and several brain structures which are involved in modulating hippocampal theta rhythm activity makes a complicated dynamic network of interconnected regions and highlights the role of IPN in the hippocampal dependent learning and memory. In the present study we aimed to address whether IPN is involved in the perforant path-dentate gyrus (PPDG) short term and long term synaptic plasticity in rats. To silent IPN transiently, lidocaine was injected through the implanted cannula above the IPN. To evaluate short term plasticity, paired pulses stimulation of PPDG synapses were used upon IPN temporary inactivation. Furthermore, long term plasticity was investigated by measuring the induction and maintenance of PPDG synapses long term potentiation (LTP) after high frequency stimulation (HFS) of the mentioned pathway following to IPN inactivation. The results showed that IPN reversible inactivation had no effect on short term plasticity of PPDG synapses. However, IPN inactivation before the PPDG high frequency stimulation could significantly suppress both the population spike (PS) and fEPSP-LTP induction compared to the saline group. Conversely, IPN inactivation had no significant effect on maintenance of both PS-LTP and fEPSP-LTP. All together our study suggests the contribution of IPN in the PPDG synaptic plasticity and excitability of DG granule cells which could be through direct and/or indirect pathways from IPN to the hippocampus.

Antidepressant effects of acupoint stimulation and fluoxetine by increasing dendritic arborization and spine density in CA1 hippocampal neurons of socially isolated rats

Given the importance of depression and the adverse effects of conventional treatment, it is necessary to seek complementary therapies. In a rat model of depression, this study aimed to assess the behavioral and morphological effects of embedding absorbable thread in acupoints (acu-catgut), and compare the results to those of fluoxetine treatment and the corresponding control groups. Therefore, depressive-like behavior was evaluated with the forced swimming test, and dendritic morphology (in the CA1 hippocampal region) with the Golgi-Cox technique and Sholl analysis. After weaning, male Sprague-Dawley rats were housed in social isolation for 8 weeks to induce depressive-like behavior. They were then given a 21-day treatment by stimulating acupoints with acu-catgut (AC) or fluoxetine (FX) (2 mg/kg). Rats were divided into six groups: Control (socially housed), social isolation (SI), SI + AC, SI + Sham (sham embedding of thread), SI + FX and SI + VH (vehicle). Compared to fluoxetine, acu-catgut treatment was more effective in reversing depressive-like behavior elicited by SI. The SI-induced reduction in dendritic length and spine density in hippocampal CA1 pyramidal neurons was attenuated after prolonged treatment with acu-catgut or fluoxetine. Hence, both treatments proved capable of reversing depressive-like alterations caused by SI, likely due to dendritic remodeling in the hippocampus.

Interaction between estradiol and 5-HT1A receptors in the median raphe nucleus on acquisition of aversive information and association to the context in ovariectomized rats

The median raphe nucleus (MRN) is related to stress resistance and defensive responses, a crucial source of serotonergic neurons that project to prosencephalic structures related to stress and anxiety. Estrogen receptors were identified in this mesencephalic structure. It is possible that the estrogen action is related to serotonin effect on somatodendritic 5-HT_1A receptors, inhibiting the function of serotonergic neurons and thus preventing of the stress effect and inducing anxiolysis. So, in order to evaluate these aspects, female Wistar rats were ovariectomized and 21 days later were given a direct microinjection of estradiol benzoate (EB) (1200 ng) into the MRN, preceded by microinjections of saline or WAY100.635 (100 ng), a 5-HT_1A receptor antagonist. Immediately after the two microinjections, the ovariectomized rats were conditioned with an aversive event (foot shock) session in a Skinner box. Twenty-four hours later, they were exposed to the same context in a test session for 5 min for behavioral assessment: freezing, rearing, locomotion, grooming, and autonomic responses (fecal boluses and micturition). EB microinjection in the MRN prior to the exposure of animals to the foot shocks in the conditioning session did not alter their behavior in this session, but neutralized the association of the aversive experience to the context: there was a decrease in the expression of freezing and an increased rearing activity in the test session. This effect was reversed by prior microinjection of WAY100.635. In conclusion, EB acted on serotonergic neurons in the MRN of the ovariectomized rats, impairing the association of the aversive experience to the context, by co-modulating the functionality of somatodendritic 5-HT_1A.

Neuronal Circuitry Mechanisms Regulating Adult Mammalian Neurogenesis

The adult mammalian brain is a dynamic structure, capable of remodeling in response to various physiological and pathological stimuli. One dramatic example of brain plasticity is the birth and subsequent integration of newborn neurons into the existing circuitry. This process, termed adult neurogenesis, recapitulates neural developmental events in two specialized adult brain regions: the lateral ventricles of the forebrain. Recent studies have begun to delineate how the existing neuronal circuits influence the dynamic process of adult neurogenesis, from activation of quiescent neural stem cells (NSCs) to the integration and survival of newborn neurons. Here, we review recent progress toward understanding the circuit-based regulation of adult neurogenesis in the hippocampus and olfactory bulb.

Theta Rhythm of the Hippocampus: Subcortical Control and Functional Significance

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain and has been strongly implicated in mnemonic processes of the hippocampus. We describe (a) ascending brain stem–forebrain systems involved in controlling theta and nontheta (desynchronization) states of the hippocampal electroencephalogram; (b) theta rhythmically discharging cells in several structures of Papez's circuit and their possible functional significance, specifically with respect to head direction cells in this same circuit; and (c) the role of nucleus reuniens of the thalamus as a major interface between the medial prefrontal cortex and hippocampus and as a prominent source of afferent limbic information to the hippocampus. We suggest that the hippocampus receives two main types of input: theta rhythm from ascending brain stem– diencephaloseptal systems and information bearing mainly from thalamocortical/cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it from the entorhinal cortex and nucleus reuniens.

Serotonin's Complex Role in Alcoholism: Implications for Treatment and Future Research

Current pharmacological treatments for alcohol dependence have focused on reducing alcohol consumption, but to date there are few treatments that also address the negative affective symptoms during acute and protracted alcohol withdrawal which are often exacerbated in people with comorbid anxiety and depression. Selective serotonin reuptake inhibitors (SSRIs) are sometimes prescribed to ameliorate these symptoms but can exacerbate anxiety and cravings in a select group of patients. In this critical review, we discuss recent literature describing an association between alcohol dependence, the SERT linked polymorphic region (5-HTTLPR), and pharmacological response to SSRIs. Given the heterogeneity in responsiveness to serotonergic drugs across the spectrum of alcoholic subtypes, we assess the contribution of specific 5-HT circuits to discrete endophenotypes of alcohol dependence. 5-HT circuits play a distinctive role in reward, stress, and executive function which may account for the variation in response to serotonergic drugs. New optogenetic and chemogenetic methods for dissecting 5-HT circuits in alcohol dependence may provide clues leading to more effective pharmacotherapies. Although our current understanding of the role of 5-HT systems in alcohol dependence is incomplete, there is some evidence to suggest that 5-HT3 receptor antagonists are effective in people with the L/L genotype of the 5-HTTLPR polymorphism while SSRIs may be more beneficial to people with the S/L or S/S genotype. Studies that assess the impact of serotonin transporter polymorphisms on 5-HT circuit function and the subsequent development of alcohol use disorders will be an important step forward in treating alcohol dependence.

Hippocampal laminar distribution of tau relates to Alzheimer's disease and age of onset

BACKGROUND

Cerebral deposition of phospho-tau in Alzheimer's disease (AD) occurs with varying patterns within hippocampus. Lamina-specific tau changes in AD may reflect trans-synaptic propagation of phospho-tau along neuroanatomical pathways.

OBJECTIVE

To study patterns of tau deposition within inner (IML) and outer (OML) molecular layers of dentate gyrus and their clinical and neuropathological correlates.

METHODS

98 consecutive autopsied brains from the Columbia University Brain Bank were stained for phospho-tau using AT-8. Staining density was rated as High versus Low within IML and OML. Four patterns were observed among the 98 brains: High IML&OML, n = 44; High OML Only (n = 35); High IML Only (n = 5); and Low IML&OML (n = 14). Demographic, clinical, and neuropathological characteristics of these four groups were compared.

RESULTS

High IML&OML subjects, versus High OML Only, were more likely to fulfill CERAD criteria for Definite AD (93% versus 66%, p < 0.01) and to have higher median Braak stage (6 versus 5, p < 0.01) and earlier mean age of onset (65.9 versus 73.7 y, p = 0.02), with similar symptom duration. Using logistic regression, the association between High IML&OML and AD remained significant after adjustment for demographics but not symptom duration. In the 70 subjects with Definite AD, High IML&OML was associated with younger age of onset (mean difference 3.7 years, 95%CI -6.7 to -0.7, p < 0.01), after adjustment for demographics and symptom duration.

CONCLUSIONS

Phospho-tau pathology, when prominent within both IML and OML, is associated with CERAD diagnosis of Definite AD and earlier age of onset in AD. Laminar patterns of tau deposition reflect regional involvements during disease course.

Median raphe stimulation-induced motor inhibition concurrent with suppression of type 1 and type 2 hippocampal theta

This study investigated behavioral, anatomical and electrophysiological effects produced by electrical stimulation of posterior hypothalamic (PH) or median raphe (MR) nuclei, independently and during combined stimulation of both PH and MR. These three stimulation conditions were applied during spontaneous behavior in an open field and during PH stimulation-induced wheel running, while simultaneously recording hippocampal (HPC) field activity. An additional objective was to determine the effects of MR stimulation on Type 1 movement related theta and Type 2 sensory processing related theta. To achieve the latter, when behavioral studies were completed we studied the same rats under urethane anesthesia and then during urethane anesthesia with the addition of atropine sulfate (ATSO4). Here we demonstrated that electrical stimulation of a localized region of the MR nucleus resulted in a profound inhibition of both spontaneously occurring theta related motor behaviors and the theta related motor behaviors induced by electrical stimulation of the PH nucleus. Furthermore, this motor inhibition occurred concurrently with strong suppression of hippocampal theta field oscillations in the freely moving rat, a condition where the theta recorded is Type 2 sensory processing theta occurring coincidently with Type 1 movement related theta (Bland, 1986). Our results indicate that motor inhibition resulted from stimulation of neurons located in the mid central region of the MR, while stimulation in adjacent regions produced variable responses, including movements and theta activity. The present study provided evidence that the pharmacological basis of the suppression of Type 2 sensory processing HPC theta was cholinergic. However, MR inhibition of PH-induced wheel running was not affected by cholinergic blockade, which blocks Type 2 theta, indicating that MR stimulation-induced motor inhibition also requires the suppression of Type 1 theta.

The ascending median raphe projections are mainly glutamatergic in the mouse forebrain

The median raphe region (MRR) is thought to be serotonergic and plays an important role in the regulation of many cognitive functions. In the hippocampus (HIPP), the MRR exerts a fast excitatory control, partially through glutamatergic transmission, on a subpopulation of GABAergic interneurons that are key regulators of local network activity. However, not all receptors of this connection in the HIPP and in synapses established by MRR in other brain areas are known. Using combined anterograde tracing and immunogold methods, we show that the GluN2A subunit of the NMDA receptor is present in the synapses established by MRR not only in the HIPP, but also in the medial septum (MS) and in the medial prefrontal cortex (mPFC) of the mouse. We estimated similar amounts of NMDA receptors in these synapses established by the MRR and in local adjacent excitatory synapses. Using retrograde tracing and confocal laser scanning microscopy, we found that the majority of the projecting cells of the mouse MRR contain the vesicular glutamate transporter type 3 (vGluT3). Furthermore, using double retrograde tracing, we found that single cells of the MRR can innervate the HIPP and mPFC or the MS and mPFC simultaneously, and these double-projecting cells are also predominantly vGluT3-positive. Our results indicate that the majority of the output of the MRR is glutamatergic and acts through NMDA receptor-containing synapses. This suggests that key forebrain areas receive precisely targeted excitatory input from the MRR, which is able to synchronously modify activity in those regions via individual MRR cells with dual projections.

The median raphe nucleus in anxiety revisited

Although the role of the median raphe nucleus (MRN) in the regulation of anxiety has received less attention than that of the dorsal raphe nucleus (DRN) there is substantial evidence supporting this function. Reported results with different animal models of anxiety in rats show that whereas inactivation of serotonergic neurons in the MRN causes anxiolysis, the stimulation of the same neurons is anxiogenic. In particular, studies using the elevated T-maze comparing serotonergic interventions in the MRN and in the DRN indicate that the former affect only the inhibitory avoidance task, which has been related to generalized anxiety. In contrast, similar operations in the DRN change both the inhibitory avoidance and the one-way escape task, the latter being representative of panic disorder. Simultaneous injections of 5-HT-acting drugs in the MRN and in the dorsal hippocampus (DH) suggest that the MRN-DH pathway mediates the regulatory function of the MRN in anxiety. Overall, the results discussed in this review point to a relevant role of the MRN in the regulation of anxiety, but not panic, through the 5-HT pathway that innervates the DH.

Functional topography of serotonergic systems supports the Deakin/Graeff hypothesis of anxiety and affective disorders

Over 20 years ago, Deakin and Graeff hypothesized about the role of different serotonergic pathways in controlling the behavioral and physiologic responses to aversive stimuli, and how compromise of these pathways could lead to specific symptoms of anxiety and affective disorders. A growing body of evidence suggests these serotonergic pathways arise from topographically organized subpopulations of serotonergic neurons located in the dorsal and median raphe nuclei. We argue that serotonergic neurons in the dorsal/caudal parts of the dorsal raphe nucleus project to forebrain limbic regions involved in stress/conflict anxiety-related processes, which may be relevant for anxiety and affective disorders. Serotonergic neurons in the "lateral wings" of the dorsal raphe nucleus provide inhibitory control over structures controlling fight-or-flight responses. Dysfunction of this pathway could be relevant for panic disorder. Finally, serotonergic neurons in the median raphe nucleus, and the developmentally and functionally-related interfascicular part of the dorsal raphe nucleus, give rise to forebrain limbic projections that are involved in tolerance and coping with aversive stimuli, which could be important for affective disorders like depression. Elucidating the mechanisms through which stress activates these topographically and functionally distinct serotonergic pathways, and how dysfunction of these pathways leads to symptoms of neuropsychiatric disorders, may lead to the development of novel approaches to both the prevention and treatment of anxiety and affective disorders.

Prior cold water swim stress alters immobility in the forced swim test and associated activation of serotonergic neurons in the rat dorsal raphe nucleus

Prior adverse experience alters behavioral responses to subsequent stressors. For example, exposure to a brief swim increases immobility in a subsequent swim test 24h later. In order to determine if qualitative differences (e.g. 19°C versus 25°C) in an initial stressor (15-min swim) impact behavioral, physiological, and associated neural responses in a 5-min, 25°C swim test 24h later, rats were surgically implanted with biotelemetry devices 1 week prior to experimentation then randomly assigned to one of six conditions (Day 1 (15 min)/Day 2 (5 min)): (1) home cage (HC)/HC, (2) HC/25°C swim, (3) 19°C swim/HC, (4) 19°C swim/25°C swim, (5) 25°C swim/HC, (6) 25°C swim/25°C swim. Core body temperature (Tb) was measured on Days 1 and 2 using biotelemetry; behavior was measured on Day 2. Rats were transcardially perfused with fixative 2h following the onset of the swim on Day 2 for analysis of c-Fos expression in midbrain serotonergic neurons. Cold water (19°C) swim on Day 1 reduced Tb, compared to both 25°C swim and HC groups on Day 1, and, relative to rats exposed to HC conditions on Day 1, reduced the hypothermic response to the 25°C swim on Day 2. The 19°C swim on Day 1, relative to HC exposure on Day 1, increased immobility during the 5-min swim on Day 2. Also, 19°C swim, relative to HC conditions, on Day 1 reduced swim (25°C)-induced increases in c-Fos expression in serotonergic neurons within the dorsal and interfascicular parts of the dorsal raphe nucleus. These results suggest that exposure to a 5-min 19°C cold water swim, but not exposure to a 5-min 25°C swim alters physiological, behavioral and serotonergic responses to a subsequent stressor.

Integrative physiology of depression and antidepressant drug action: implications for serotonergic mechanisms of action and novel therapeutic strategies for treatment of depression

Major depressive disorder (MDD) is predicted to be the second leading cause of disability worldwide by the year 2020. Currently available treatments for MDD are suboptimal. Only 50% of MDD patients recover in less than 12 weeks with adequate treatment, and up to 20% of patients will fail to adequately respond to all currently available interventions. Moreover, current treatments come at the cost of significant central nervous system (CNS) side effects, further highlighting the need for more effective treatments with fewer side effects. A greater mechanistic understanding of MDD and the actions of antidepressant drugs would provide opportunities for development of novel therapeutic approaches to treatment. With this aim in mind, we explore the novel, but empirically supported, hypothesis that an evolutionarily ancient thermoafferent pathway, signaling via the spinoparabrachial pathway from serotonergic sensory cells in the skin and other epithelial linings to serotonergic neurons and depression-related circuits in the brain, is dysfunctional in MDD and that antidepressant therapies, including antidepressant drugs and exercise, act by restoring its function.

Distribution and targets of the relaxin-3 innervation of the septal area in the rat

Neural tracing studies have revealed that the rat medial and lateral septum are targeted by ascending projections from the nucleus incertus, a population of tegmental GABA neurons. These neurons express the relaxin-family peptide, relaxin-3, and pharmacological modulation of relaxin-3 receptors in medial septum alters hippocampal theta rhythm and spatial memory. In an effort to better understand the basis of these interactions, we have characterized the distribution of relaxin-3 fibers/terminals in relation to different septal neuron populations identified using established protein markers. Dense relaxin-3 fiber plexuses were observed in regions of medial septum containing hippocampal-projecting choline acetyltransferase (ChAT)-, neuronal nitric oxide synthase (nNOS)-, and parvalbumin (PV)-positive neurons. In lateral septum (LS), relaxin-3 fibers were concentrated in the ventrolateral nucleus of rostral LS and the ventral nucleus of caudal LS, with sparse labeling in the dorsolateral and medial nuclei of rostral LS, dorsal nucleus of caudal LS, and ventral portion nuclei. Relaxin-3 fibers were also observed in the septofimbrial and triangular septal nuclei. In the medial septum, we observed relaxin-3-immunoreactive contacts with ChAT-, PV-, and glutamate decarboxylase-67-positive neurons that projected to hippocampus, and contacts between relaxin-3 terminals and calbindin- and calretinin-positive neurons. Relaxin-3 colocalized with synaptophysin in nerve terminals in all septal areas, and ultrastructural analysis revealed these terminals were symmetrical and contacted spines, somata, dendritic shafts, and occasionally other axonal terminals. These data predict that this GABA/peptidergic projection modulates septohippocampal activity and hippocampal theta rhythm related to exploratory navigation, defensive and ingestive behaviors, and responses to neurogenic stressors.

Swim stress activates serotonergic and nonserotonergic neurons in specific subdivisions of the rat dorsal raphe nucleus in a temperature-dependent manner

Physical (exteroceptive) stimuli and emotional (interoceptive) stimuli are thought to influence stress-related physiologic and behavioral responses through different neural mechanisms. Previous studies have demonstrated that stress-induced activation of brainstem serotonergic systems is influenced by environmental factors such as temperature. In order to further investigate the effects of environmental influences on stress-induced activation of serotonergic systems, we exposed adult male Wistar rats to either home cage control conditions or a 15-min swim in water maintained at 19 °C, 25 °C, or 35 °C and conducted dual immunohistochemical staining for c-Fos, a marker of immediate-early nuclear activation, and tryptophan hydroxylase (TPH), a marker of serotonergic neurons. Changes in core body temperature were documented using biotelemetry. As expected, exposure to cold (19 °C) swim, relative to warm (35 °C) swim, increased c-Fos expression in the external lateral part of the parabrachial nucleus (LPBel), an important part of the spinoparabrachial pathway involved in sensation of cold, cutaneous stimuli, and in serotonergic neurons in the raphe pallidus nucleus (RPa), an important part of the efferent mechanisms controlling thermoregulatory warming responses. In addition, exposure to cold (19 °C) swim, relative to 35 °C swim, increased c-Fos expression in the dorsal raphe nucleus, ventrolateral part/periaqueductal gray (DRVL/VLPAG) and dorsal raphe nucleus, interfascicular part (DRI). Both of these subregions of the dorsal raphe nucleus (DR) have previously been implicated in thermoregulatory responses. Altogether, the data are consistent with the hypothesis that midbrain serotonergic neurons, possibly via activation of afferents to the DR by thermosensitive spinoparabrachial pathways, play a role in integration of physiologic and behavioral responses to interoceptive stress-related cues involved in forced swimming and exteroceptive cues related to cold ambient temperature.

Functional topography of midbrain and pontine serotonergic systems: implications for synaptic regulation of serotonergic circuits

RATIONALE

Dysfunction of serotonergic systems is thought to play an important role in a number of neurological and psychiatric disorders. Recent studies suggest that there is anatomical and functional diversity among serotonergic systems innervating forebrain systems involved in the control of physiologic and behavioral responses, including the control of emotional states.

OBJECTIVE

Here, we highlight the methods that have been used to investigate the heterogeneity of serotonergic systems and review the evidence for the unique anatomical, hodological, and functional properties of topographically organized subpopulations of serotonergic neurons in the midbrain and pontine raphe complex.

CONCLUSION

The emerging understanding of the topographically organized synaptic regulation of brainstem serotonergic systems, the topography of the efferent projections of these systems, and their functional properties, should enable identification of novel therapeutic approaches to treatment of neurological and psychiatric conditions that are associated with dysregulation of serotonergic systems.

Urocortin-1 and -2 double-deficient mice show robust anxiolytic phenotype and modified serotonergic activity in anxiety circuits

The urocortin (Ucn) family of neuropeptides is suggested to be involved in homeostatic coping mechanisms of the central stress response through the activation of corticotropin-releasing factor receptor type 2 (CRFR2). The neuropeptides, Ucn1 and Ucn2, serve as endogenous ligands for the CRFR2, which is highly expressed by the dorsal raphe serotonergic neurons and is suggested to be involved in regulating major component of the central stress response. Here, we describe genetically modified mice in which both Ucn1 and Ucn2 are developmentally deleted. The double knockout mice showed a robust anxiolytic phenotype and altered hypothalamic-pituitary-adrenal axis activity compared with wild-type mice. The significant reduction in anxiety-like behavior observed in these mice was further enhanced after exposure to acute stress, and was correlated with the levels of serotonin and 5-hydroxyindoleacetic acid measured in brain regions associated with anxiety circuits. Thus, we propose that the Ucn/CRFR2 serotonergic system has an important role in regulating homeostatic equilibrium under challenge conditions.

The structure of the dorsal raphe nucleus and its relevance to the regulation of sleep and wakefulness

Serotonergic (5-HT) cells in the rat dorsal raphe nucleus (DRN) appear in topographically organized groups. Based on cellular morphology, expression of other neurotransmitters, afferent and efferent connections and functional properties, 5-HT neurons of the DRN have been grouped into six cell clusters. The subdivisions comprise the rostral, ventral, dorsal, lateral, caudal and interfascicular parts of the DRN. In addition to 5-HT cells, neurons containing γ-aminobutyric acid (GABA), glutamate, dopamine, nitric oxide and the neuropeptides corticotropin-releasing factor, substance P, galanin, cholecystokinin, neurotensin, somatostatin, vasoactive intestinal peptide, neuropeptide Y, thyrotropin-releasing hormone, growth hormone, leu-enkephalin, met-enkephalin and gastrin have been characterized in the DRN. Moreover, numerous brain areas have neurons that project to the DRN and express monoamines (norepinephrine, histamine), amino acids (GABA, glutamate), acetylcholine or neuropeptides (orexin, melanin-concentrating hormone, corticotropin-releasing factor and substance P) that directly or indirectly, through local circuits, regulate the activity of 5-HT cells. The 5-HT cells predominate along the midline of the rostral, dorsal and ventral subdivisions of the DRN and outnumber the non-5-HT cells occurring in the raphe nucleus. The GABAergic and glutamatergic neurons are clustered mainly in the lateral and dorsal subdivisions of the DRN, respectively. The 5-HT(1A) receptor is located on the soma and the dendrites of 5-HT neurons and at postsynaptic sites (outside the DRN). It is expressed, in addition, by non-5-HT cells of the DRN. The 5-HT(1B) receptor is located at presynaptic and postsynaptic sites (outside the boundaries of the DRN). It has been described also in the ventromedial DRN where it is expressed by non-5-HT cells. The 5-HT(2A) and 5-HT(2C) receptors are located within postsynaptic structures. At the level of the DRN the 5-HT(2A) and 5-HT(2C) receptor-containing cells are predominantly GABAergic interneurons and projection neurons. Within the boundaries of the DRN the 5-HT(3) receptor is expressed by, among others, glutamatergic interneurons. 5-HT(7) receptors in the DRN are not localized to serotonergic neurons but, at least in part, to GABAergic cells and terminals. The complex structure of the DRN may have important implications for neural mechanisms underlying 5-HT modulation of wakefulness and REM sleep.

Expression of GDNF transgene in astrocytes improves cognitive deficits in aged rats

Glial cell line-derived neurotrophic factor (GDNF) was assayed for its neurotrophic effects against the neuronal atrophy that causes cognitive deficits in old age. Aged Fisher 344 rats with impairment in the Morris water maze received intrahippocampal injections at the dorsal CA1 area of either a lentiviral vector encoding human GDNF or the same vector encoding human green fluorescent protein as a control. Recombinant lentiviral vectors constructed with human cytomegalovirus promotor and pseudotyped with lyssavirus Mokola glycoprotein specifically transduced the astrocytes in vivo. Astrocyte-secreted GDNF enhanced neuron function as shown by local increases in synthesis of the neurotransmitters acetylcholine, dopamine and serotonin. This neurotrophic effect led to cognitive improvement of the rats as early as 2 weeks after gene transduction. Spatial learning and memory testing showed a significant gain in cognitive abilities due to GDNF exposure, whereas control-transduced rats kept their performance at the chance level. These results confirm the broad spectrum of the neurotrophic action of GDNF and open new gene therapy possibilities for reducing age-related neurodegeneration.

Pathophysiology of involuntary emotional expression disorder

Extensive clinical experience and research suggest that a cortico-limbic-subcortico-thalamic-ponto-cerebellar network plays a significant role in the expression of human emotions. This network includes specific cerebral, cerebellar, and brainstem areas and their multiple projections/pathways, with activity modulated through serotonergic, dopaminergic, glutamatergic, and possibly sigma receptor neurotransmitter systems. Disruptions of regulatory and inhibitory mechanisms in the structure and function of this network likely constitute a pathophysiological basis for the crying and laughing episodes characteristic of involuntary emotional expression disorder. Pharmacologic interventions targeting the neurochemical modulators of the emotional expression systems may afford opportunities for symptom control among persons affected by this disorder.

Extrinsic afferent systems to the dentate gyrus

The dentate gyrus is the first stage of the intrahippocampal, excitatory, trisynaptic loop, and a primary target of the majority of entorhinal afferents that terminate in a laminar fashion on granule cell dendrites and carry sensory information of multiple modalities about the external world. The electric activity of the trisynaptic pathway is controlled mainly by different types of local, GABAergic interneurons, and subcortical and commissural afferents. In this chapter we will outline the origin and postsynaptic targets in the dentate gyrus of chemically identified subcortical inputs. These systems are afferents originating from the medial septum/diagonal band of Broca GABAergic and cholinergic neurons, neurochemically distinct types of neurons located in the supramammillary area, serotonergic fibers from the median raphe, noradrenergic afferents from the pontine nucleus, locus ceruleus, dopamine axons originating in the ventral tegmental area, and the commissural projection system. Because of the physiological implications, these afferents are discussed in the context of the glutamatergic innervation of the dentate gyrus. One common feature of the extrinsic dentate afferent systems is that they originate from a relatively small number of neurons. However, the majority of these afferents are able to exert a powerful control over the electrical activity of the hippocampus. This strong influence is due to the fact that the majority of the extrinsic afferents terminate on a relatively small, but specific, populations of neurons that are able to control large areas of the hippocampal formation.

Microinjection of ritanserin into the dorsal hippocampal CA1 and dentate gyrus decrease nociceptive behavior in adult male rat

Prenatal 5HT depletion causes a significant decrease in the level of nociceptive sensitivity during the second phase of the formalin test behavioral response. These experiments were designed to test whether blocking 5HT2A/2c receptors in the CA1 region of the hippocampus and dentate gyrus would decrease nociceptive behaviors induced by a peripheral noxious stimulus formalin as an animal model of unremitting human being. The 5HT2A/2c receptor antagonist ritanserin (2, 4 and 8 microg/0.5 microl) was injected into the CA1 area and dentate gyrus of behaving rats 5 min before subcutaneous injection of formalin irritant. Nociceptive behaviors in both phases of the formalin test were significantly decreased by ritanserin (4 and 8 microg/0.5 microl) and ritanserin had no effect at 2 microg/0.5 microl. These results support the hypothesis that the hippocampal formation may modify the processing of incoming nociceptive information and that 5HT2A/2c receptor-sensitive mechanisms in the hippocampus may play a role in nociception and/or the expression of related behaviors.

Evidence supporting a role for corticotropin-releasing factor type 2 (CRF2) receptors in the regulation of subpopulations of serotonergic neurons

Corticotropin-releasing factor (CRF)-related peptides can modulate stress-related physiology and behavior. Some of these effects may be mediated via the CRF type 2 (CRF2) receptor on serotonergic neurons in the dorsal raphe nucleus (DR). To determine if the CRF2 receptor agonist urocortin 2 (Ucn 2) increases c-Fos expression in rat DR serotonergic neurons via actions on CRF2 receptors, we gave intracerebroventricular (icv) injections of mouse Ucn 2 after icv injections of either saline or the CRF2 receptor antagonist antisauvagine-30 (ASV-30). Double immunostaining methods for c-Fos and tryptophan hydroxylase revealed that, consistent with previous studies, mouse Ucn 2 increased c-Fos expression in tryptophan hydroxylase immunostained neurons in the middle and caudal parts (-8.18, -8.54, and -9.16 mm bregma) of the dorsal subdivision of the dorsal raphe nucleus 2 h after drug treatment. Pre-treatment with ASV-30 blocked these effects. Mouse Ucn 2 had no effect on c-Fos expression within the median raphe nucleus, consistent with the hypothesis that Ucn 2 has specific actions on an anatomically and functionally distinct subset of serotonergic neurons via activation of CRF2 receptors. These findings are also consistent with the hypothesis that Ucn 2, or another CRF-related neuropeptide acting at CRF2 receptors, modulates physiological and behavioral responses to stress-related stimuli via actions on a specific subset of serotonergic neurons within the dorsal raphe nucleus.

Microinjection of procaine and electrolytic lesion in the ventral tegmental area suppresses hippocampal theta rhythm in urethane-anesthetized rats

The midbrain ventral tegmental area (VTA), a key structure of the mesocorticolimbic system is anatomically connected with the hippocampal formation. In addition mesocortical dopamine was found to influence hippocampus-related memory and hippocampal synaptic plasticity, both being linked to the theta rhythm. Therefore, the aim of the present study was to evaluate the possible role of the VTA in the regulation of the hippocampal theta activity. The study was performed on urethane-anesthetized male Wistar rats in which theta rhythm was evoked by tail pinch. It was found that unilateral, temporal inactivation of the VTA by means of direct procaine injection resulted in bilateral suppression of the hippocampal theta which manifested as a loss of synchronization of hippocampal EEG and respective reduction of the power and also the frequency of the 3-6 Hz theta band. Depression of the power of the 3-6 Hz component of the EEG signal was also seen in spontaneous hippocampal EEG after procaine. The permanent destruction of the VTA by means of unilateral electrocoagulation evoked a long-lasting, mainly ipsilateral depression of the power of the theta with some influence on its frequency. Simultaneously, there was a substantial increase of the power in higher frequency bands indicating decrease of a synchrony of the hippocampal EEG activity. On the basis of these results indicating impairment of synchronization of the hippocampal activity the VTA may be considered as another part of the brainstem theta synchroning system.

Median raphe mediates estrogenic effects to the hippocampus in female rats

Subcortical regions such as the medial septum-diagonal band of Broca and supramammillary area have been shown to mediate indirect oestrogenic effects on hippocampal morphology and function. Here, the role of the median raphe (MR), a serotonergic subcortical structure, is studied. To this end, 17beta-estradiol-filled 30-gauge cannulae were implanted into the MR of female ovariectomized rats; cholesterol-filled cannulae served as controls. After seven days, using unbiased electron microscopic stereological calculations and semiquantitative analysis, the spine synapse density and surface density of glial fibrillary acidic protein-positive astrocyte processes, respectively, were determined in the stratum radiatum of the CA1 region of the hippocampus. Changes in the serotonergic innervation of the hippocampal CA1 region were determined by immunohistochemistry and subsequent morphometric analysis. In the stratum radiatum of the CA1 region, local estradiol application into the MR resulted in a 47% increase in spine synapse density. Simultaneously, the density of glial fibrillary acidic protein-positive fibers decreased by 16%. The density of serotonin (5-HT) innervation of the strata lacunosum moleculare and radiatum of the CA1 region of the hippocampus was reduced in response to estradiol, as shown by a decrease in the length of fibers (27.6 and 48.3% decrease, respectively) and the number of large varicosities (32.5 and 38.8% decrease, respectively). These observations suggest a major role of the MR in mediating oestrogenic effects on the hippocampus and an involvement of the serotonergic system.

5-HT mechanisms of median raphe nucleus in the conditioned freezing caused by light/foot-shock association

We have shown that 5-HT mechanisms of the median raphe nucleus (MRN) are involved in contextual fear-conditioning processes as electrolytic or neurotoxic lesions with N-methyl-D-aspartate (NMDA) or injections of 8-hydroxy-2-(di-n-propilamino)-tetralin (8-OH-DPAT) into this structure inhibit freezing behavior in a contextual fear paradigm. In this work, we extend these studies by analyzing the behavioral responses in a classical fear-conditioning paradigm (light or tone/foot-shock association) in rats with either neurochemical lesion with NMDA or injected with 8-OH-DPAT into the MRN. The animals received NMDA or 8-OH-DPAT or saline microinjections into the MRN and were submitted to conditioning trials in an experimental chamber, where they received 10 foot-shocks (0.6 mA, 1 s, variable interval between 10 and 50 s) paired with tone or light (CS). On the next day, they were tested in a different experimental chamber, with or without CS presentation, where the duration of freezing and the number of rearing episodes were recorded. Light or tone alone caused a significant amount of freezing. NMDA lesions or 8-OH-DPAT injections into the MRN clearly inhibited freezing behavior in rats conditioned to light/foot-shock association, but not in the conditioning sessions with tones. Besides the proposed role in contextual fear conditioning, these results clearly show that MRN is involved in the fear conditioning with light as conditioned stimuli. Distinct neural substrates seem to subserve conditioning fear with acoustic stimuli.