Role of serotonin 1A receptors in the median raphe nucleus on the behavioral consequences of forced swim stress

Behavioral Consequences of Hippocampal 5-HT7 Receptors Blockade in Stressed Rats

Serotonin (5-HT) has long been involved in response to stress and its effect may be, in part, mediated by 5-HT1a and 5-HT7 receptor subtypes in different brain structures. Both pre- and post-synaptic activation of 5-HT1a receptor, respectively, in the rat median raphe nucleus (MnRN) and hippocampus, lead to adaptation to acute inescapable stressors such as restraint and forced swim. 5-HT7 receptor (5HT7r), a stimulatory G-protein coupled receptor, has also been investigated as a possible candidate for mediating stress response. In the MnRN, activation of 5-HT7r has antidepressant effects, while in the hippocampus, 5HT7r mRNA expression is increased after exposure to restraint stress, but the functional significance of these receptors remains to be determined. Therefore, the aim of this study was to investigate whether blockade of hippocampal 5HT7r would prevent and/or attenuate the behavioral effects of stress. Male adult Wistar rats with bilateral cannulas aimed at the dorsal hippocampus were restrained for 2 h and tested in the elevated plus maze (EPM) 24 h later. SB 258741 (3 nmoles/0.5 μL/side; selective 5HT7r antagonist) was administered bilateraly into the hippocampus according to the experimental protocol: immediately before or after stress, or 24 h after it (immediately before the test). In a second experiment, rats were exposed to 15 min. of forced swim, and tested 24 h later. Intra-hippocampal treatment was performed as described for the restraint stress protocol. We found that blockade of hippocampal 5-HT7r immediately after, but not before, the exposure to restraint or forced swim attenuated stress-induced behavioral changes. Similar results were obtained when SB was administered before the test in previously stressed rats. Our data suggest that activation of hippocampal 5-HT7r is crucial for the consolidation and retrieval of aversive stimulus-related memories, such as those caused by a stressful experience, probably through mechanisms involving stress-induced changes in 5-HT7r expression.

Pharmacogenetic excitation of the median raphe region affects social and depressive-like behavior and core body temperature in male mice

AIMS

Median raphe region (MRR) is an important bottom-up regulatory center for various behaviors as well as vegetative functions, but detailed descriptions and links between the two are still largely unexplored.

METHODS

Pharmacogenetics was used to study the role of MRR in social (sociability, social interaction, resident intruder test) and emotional behavior (forced swim test) parallel with some vegetative changes (biotelemetry: core body temperature). Additionally, to validate pharmacogenetics, the effect of clozapine-N-oxide (CNO), the ligand of the artificial receptor, was studied by measuring (i) serum and brainstem concentrations of CNO and clozapine; (ii) MRR stimulation induced neurotransmitter release in hippocampus; (iii) CNO induced changes in body temperature and locomotor activity.

KEY FINDINGS

MRR stimulation decreased locomotion, increased friendly social behavior in the resident intruder test and enhanced depressive-like behavior. The latter was accompanied by diminished decrease in core body temperature. Thirty minutes after CNO injection clozapine was predominant in the brainstem. Nonetheless, peripheral CNO injection was able to induce glutamate release in the hippocampus. CNO had no immediate (<30 min) or chronic (repeated injections) effect on the body temperature or locomotion.

SIGNIFICANCE

We confirmed the role of MRR in locomotion, social and depressive-like behavior. Most interestingly, only depressive-like behavior was accompanied by changed body temperature regulation, which was also observed in human depressive disorders previously. This indicates clinical relevance of our findings. Despite low penetration, CNO acts centrally, but does not influence the examined basic parameters, being suitable for repeated behavioral testing.

Attenuation of stress-induced behavioral changes by activation of serotonin type 7 receptors in the median raphe nucleus of rats

BACKGROUND

Exposure to stressful aversive situations induces physiological and behavioral changes. Serotonin has been suggested to mediate such changes, as well as adaptation to stressful events. Serotoninergic projections arising from the median raphe nucleus to the dorsal hippocampus have been suggested to promote adaptation to chronic aversive stimuli. Such pathway may involve serotonin type 1a receptor-mediated neurotransmission. However, the serotonin 7 receptor can also be found in the median raphe nucleus and may be involved in mechanisms underlying response to stress.

AIMS

In this work we sought to investigate if activation of serotonin type 7 receptors would attenuate stress-induced deficits in different animal models of depression.

METHODS

Male Wistar rats with a guide-cannula aimed to the median raphe nucleus were submitted to restraint or forced swim stress and were tested in an elevated plus maze or forced swim test, respectively, 24 h later. SB 258741 (serotonin type 7 receptor antagonist) and/or LP 44 (serotonin type 7 receptor agonist) were administered intra-median raphe nucleus immediately before or after exposure to stress or before test. Control groups received intra-median raphe nucleus treatment 24 h or immediately before test in the elevated plus maze or forced swim test.

RESULTS

LP 44 attenuated restraint-induced exploratory deficits independently of the moment it was administered. Similar results were observed in the forced swim test, with the exception on post-stress condition. These effects on adaptation to stress induced by serotonin type 7 receptor activation were prevented by previous treatment with SB 258741.

CONCLUSIONS

Our data support the idea that activation of median raphe nucleus serotonin 7 receptor is important to the development of adaptation to stress.

Exploring the impact of trauma type and extent of exposure on posttraumatic alterations in 5-HT1A expression

The long-term behavioral, psychological, and neurobiological effects of exposure to potentially traumatic events vary within the human population. Studies conducted on trauma-exposed human subjects suggest that differences in trauma type and extent of exposure combine to affect development, maintenance, and treatment of a variety of psychiatric syndromes. The serotonin 1-A receptor (5-HT1A) is an inhibitory G protein-coupled serotonin receptor encoded by the HTR1A gene that plays a role in regulating serotonin release, physiological stress responding, and emotional behavior. Studies from the preclinical and human literature suggest that dysfunctional expression of 5-HT1A is associated with a multitude of psychiatric symptoms commonly seen in trauma-exposed individuals. Here, we synthesize the literature, including numerous preclinical studies, examining differences in alterations in 5-HT1A expression following trauma exposure. Collectively, these findings suggest that the impact of trauma exposure on 5-HT1A expression is dependent, in part, on trauma type and extent of exposure. Furthermore, preclinical and human studies suggest that this observation likely applies to additional molecular targets and may help explain variation in trauma-induced changes in behavior and treatment responsivity. In order to understand the neurobiological impact of trauma, including the impact on 5-HT1A expression, it is crucial to consider both trauma type and extent of exposure.

Depressive-like state sensitizes 5-HT1A and 5-HT1B auto-receptors in the dorsal raphe nucleus sub-system

Dorsal raphe (DR) and median raphe (MR) 5-HT neurons are two distinct sub-systems known to be regulated by 5-HT_1A and 5-HT_1B auto-receptors. Whether the auto-receptors in each sub-system are functionally altered in depressive-like state remains unknown. The present study is aimed to study a specific circuit (DR-ventral hippocampus and MR-dorsal hippocampus) within each sub-system to investigate changes in receptor sensitivity in the pathogenesis of depression. A mouse model of depression was developed through the social defeat paradigm, and was then treated with fluoxetine (FLX). 5-HT_1A auto-receptor in the neuronal cell body (DR or MR) and 5-HT_1B auto-receptor in the axonal terminal (ventral or dorsal hippocampus) were directly targeted by local perfusion of antagonists (5-HT_1A: WAY100635; 5-HT_1B: GR127935) through reverse microdialysis. Time courses of dialysate 5-HT measured at the axonal terminal were subsequently determined for each circuit. At baseline, 5-HT_1A and 5-HT_1B antagonists dose-dependently increased dialysate 5-HT, with sub-circuit specificity. In the depressive-like state, greater increases in dialysate 5-HT were observed only in the DR-ventral hippocampus circuit following local delivery of both antagonists, which were then fully restored following the FLX treatment. In contrast, no changes were observed in the MR-dorsal hippocampus circuit. Our results demonstrate differential changes in sensitivities of 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus and MR-dorsal hippocampus circuits. 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus circuit are sensitized in the depressive-like state. Taken together, these results suggest that the DR sub-system maybe the neural substrate mediating depressive phenotypes.

Ethanol withdrawal induces anxiety-like effects: Role of nitric oxide synthase in the dorsal raphe nucleus of rats

Nitric oxide (NO) mediated transmission in the dorsal raphe nucleus (DRN) has been shown to be involved in the modulation of anxiety-like behaviors. We investigated whether inhibition of nitric oxide synthase (NOS) in the DRN would prevent anxiety-like behavior induced by ethanol withdrawal. Male Wistar rats were treated with ethanol 2-6% (v/v) for a period of 21 days. Ethanol withdrawal was induced by abrupt discontinuation of the treatment. Experiments were performed 48 h after ethanol discontinuation. Rats with a guide cannula aimed at the DRN received intra-DRN injections of the non-selective NOS inhibitor NG-nitro-l-arginine methyl ester (l-NAME), selective neuronal NOS (nNOS) inhibitor N(ω)-propyl-l-arginine (NPLA), or selective inhibitor of inducible NOS (iNOS) N-([3-(aminomethyl)phenyl] methyl) ethanimidamidedihydrochloride (1400W). Five minutes later, the animals were tested in the elevated plus maze (EPM). Plasma ethanol levels were determined by gas chromatography. There was a reduction in plasma ethanol levels 48 h after ethanol withdrawal. Rats from the ethanol withdrawal group showed decreased exploration of the open arms of the EPM with no change in the exploration of enclosed arms. Intra-DRN treatment with l-NAME (100 nmoles/0.2 μL) and 1400W (1 nmol/0.2 μL), but not NPLA (10 nmoles/0.2 μL) in the DRN attenuated the decrease in the exploration of the open arms of the EPM induced by ethanol withdrawal. The major new finding of the present study is that iNOS in the DRN plays a role in the anxiety-like behavior induced by ethanol withdrawal.

Activity of Raphé Serotonergic Neurons Controls Emotional Behaviors

Despite the well-established role of serotonin signaling in mood regulation, causal relationships between serotonergic neuronal activity and behavior remain poorly understood. Using a pharmacogenetic approach, we find that selectively increasing serotonergic neuronal activity in wild-type mice is anxiogenic and reduces floating in the forced-swim test, whereas inhibition has no effect on the same measures. In a developmental mouse model of altered emotional behavior, increased anxiety and depression-like behaviors correlate with reduced dorsal raphé and increased median raphé serotonergic activity. These mice display blunted responses to serotonergic stimulation and behavioral rescues through serotonergic inhibition. Furthermore, we identify opposing consequences of dorsal versus median raphé serotonergic neuron inhibition on floating behavior, together suggesting that median raphé hyperactivity increases anxiety, whereas a low dorsal/median raphé serotonergic activity ratio increases depression-like behavior. Thus, we find a critical role of serotonergic neuronal activity in emotional regulation and uncover opposing roles of median and dorsal raphé function.