Effects Of 5-HT1A Receptor Activation Research Articles

Activation of 5-HT1A receptor reduces abnormal emotionality in stress-maladaptive mice by alleviating decreased myelin protein in the ventral hippocampus

We previously reported that abnormal emotionality in stress-maladaptive mice was ameliorated by chronic treatment with flesinoxan, a 5-HT1A receptor agonist. Furthermore, the maintenance of hippocampal myelination appeared to contribute to the development of stress adaptation in mice. However, the effects of 5-HT1A receptor activation on myelination under the stress-maladaptive situations and the underlying mechanisms remain unknown. In the present study, we examined using flesinoxan whether activation of 5-HT1A receptor can reduce an abnormal emotional response by acting on oligodendrocytes to preserve myelin proteins in stress-maladaptive mice. Mice were exposed to repeated restraint stress for 4 h/day for 14 days as a stress-maladaptive model. Flesinoxan was given intraperitoneally immediately after the daily exposure to restraint stress. After the final exposure to restraint stress, the emotionality of mice was evaluated by the hole-board test. The expression levels of brain-derived neurotrophic factor (BDNF), phosphorylated-extracellular signal-regulated kinase (p-ERK), phosphorylated-cAMP response element-binding protein (p-CREB), myelin-associated glycoprotein (MAG), myelin basic protein (MBP) and oligodendrocyte transcription factor 2 (olig2) in the hippocampus was assessed by western blotting. Hippocampal oligodendrogenesis were examined by immunohistochemistry. Chronic treatment with flesinoxan suppressed the decrease in head-dipping behaviors in stress-maladaptive mice in the hole-board test. Under this condition, the decreases in MAG and MBP in the hippocampus recovered with increase in BDNF, p-ERK, p-CREB, and olig2. Furthermore, hippocampal oligodendrogenesis in stress-maladaptive mice was promoted by chronic treatment with flesinoxan. These findings suggest that 5-HT1A receptor activation may promote oligodendrogenesis and myelination via an ERK/CREB/BDNF signaling pathway in the hippocampus and reduces abnormal emotionality due to maladaptation to excessive stress.

Medial PFC AMPA receptor and BDNF signaling are required for the rapid and sustained antidepressant-like effects of 5-HT1A receptor stimulation.

We previously reported that the serotonergic system is important for the antidepressant-like effects of ketamine, a non-competitive N-methyl-D-aspartate receptor antagonist, which produces rapid and long-lasting antidepressant effects in patients with major depressive disorder (MDD). In particular, selective stimulation of the 5-HT1A receptor in the medial prefrontal cortex (mPFC), as opposed to the somatic 5-HT1A autoreceptor, has been shown to play a critical role in the antidepressant-like actions of ketamine. However, the detailed mechanisms underlying mPFC 5-HT1A receptor-mediated antidepressant-like effects are not fully understood. Here we examined the involvement of the glutamate AMPA receptor and brain-derived neurotrophic factor (BDNF) in the antidepressant-like effects of 5-HT1A receptor activation in the mPFC. The results show that intra-mPFC infusion of the 5-HT1A receptor agonist 8-OH-DPAT induces rapid and long-lasting antidepressant-like effects in the forced swim, novelty-suppressed feeding, female urine sniffing, and chronic unpredictable stress tests. In addition, the results demonstrate that the antidepressant-like effects of intra-mPFC infusion of 8-OH-DPAT are blocked by co-infusion of an AMPA receptor antagonist or an anti-BDNF neutralizing antibody. In addition, mPFC infusion of 8-OH-DPAT increased the phosphorylation of signaling proteins downstream of BDNF, including mTOR, ERK, 4EBP1, and p70S6K. Finally, selective stimulation of the 5-HT1A receptor increased levels of synaptic proteins and synaptic function in the mPFC. Collectively, these results indicate that selective stimulation of 5-HT1A receptor in the mPFC exerts rapid and sustained antidepressant-like effects via activation of AMPA receptor/BDNF/mTOR signaling in mice, which subsequently increase synaptic function in the mPFC, and provide evidence for the 5-HT1A receptor as a target for the treatment of MDD.

A behavioral mechanistic investigation of the role of 5-HT1A receptors in the mediation of rat maternal behavior

Previous work suggests that 5-HT1A receptors play a special role in rodent maternal aggression, but not in other aspects of maternal care (e.g. pup retrieval and nest building). The present study re-assessed the basic effects of 5-HT1A activation or blockade on various maternal responses in postpartum female rats. We also examined the possible behavioral mechanisms underlying the maternal effects of 5-HT1A. Sprague–Dawley mother rats were injected with a 5-HT1A agonist 8-OH-DPAT (0.1, 0.5 or 1.0 mg/kg, sc), a 5-HT1A antagonist WAY-101405 (0.1, 0.5 or 1.0 mg/kg, sc) or 0.9% saline solution on postpartum days 3, 5, and 7. Maternal behavior was tested 30 min before, 30 min, 120 min, and 240 min after the injection. Acute and repeated 8-OH-DPAT treatment significantly disrupted pup retrieval, pup licking, nursing, and nest building in a dose-dependent fashion, whereas WAY-101405 had no effect at the tested doses. The 5-HT1A receptor specificity of 8-OH-DPAT's action was confirmed as its maternal disruption effect was reversed by pretreatment of WAY-100635 (a highly selective 5-HT1A receptor antagonist). Subsequent pup preference test found that 8-OH-DPAT did not decrease the pup preference over a novel object, thus no inhibition on maternal motivation or maternal affect. The pup separation test and pup retrieval on an elevated plus maze test also failed to find any motivational and motor impairment effect with 8-OH-DPAT. However, 8-OH-DPAT at the maternal disruptive dose did disrupt the prepulse inhibition (a measure of attentional function) of acoustic startle response and enhanced the basal startle response. These findings suggest that stimulation of 5-HT1A receptors by 8-OH-DPAT impairs maternal care by partially interfering with the attentional processing or basal anxiety. More work is needed to further delineate the psychological and neuronal mechanisms underlying the maternal disruptive effect of 5-HT1A receptor activation.

Conditioned fear in low- and high-anxious rats is differentially regulated by cortical subcortical and midbrain 5-HT1A receptors

Interactions between the prelimbic cortex and the basolateral amygdala underlie fear memory processing, mostly through acquiring and consolidating the learning of a conditioned fear. More recently, studies highlighted the role of the dorsal periaqueductal gray (DPAG) in the modulation of learning fear responses. In addition, extensive data in the literature have signaled the importance of serotonin (5-HT) on fear and anxiety. In the present study, the role of 5-HT neurotransmission of the prelimbic cortex, basolateral amygdala or the DPAG on the unconditioned and conditioned fear responses in rats previously selected as low- (LA) or high-anxious (HA) were assessed through local infusions of 5-HT itself (10nmol/0.2μl) or the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT – 0.3μg/0.2μl). Behavioral analysis was conducted using the fear-potentiated startle (FPS) procedure. Dependent variables recorded were the latency and amplitude of the unconditioned startle response and FPS. Our findings suggest that, on the prelimbic cortex, 5-HT modulates the expression of conditioned fear response in HA rats and this modulation is dependent on 5-HT1A receptors. This is not true, however, for the basolateral amygdala or the DPAG. In these regions LA but not HA rats were susceptible to the anxiolytic-like effect of 5-HT1A receptor activation. It is thought that the expression of conditioned fear in HA subjects may be dependent on other 5-HT receptors, as the 5-HT1B subtype, and/or changes in other systems such as the GABA and glutamate neurotransmitters. These results increase our understanding of the rostrocaudal influence of 5-HT on the unconditioned and conditioned fear responses in LA and HA subjects and, to some extent, are in disagreement with the theoretical current that emphasizes the role of 5-HT on anxiety, mainly at the subcortical and midbrain levels.

RGS inhibition at G(alpha)i2 selectively potentiates 5-HT1A-mediated antidepressant effects.

Elevating serotonin (5-HT) levels with selective serotonin reuptake inhibitors (SSRIs) is the most widely used treatment for depression. However, current therapies are ineffective, have delayed benefit, or cause side effects in many patients. Here, we define a mechanism downstream of 5-HT1A receptors that mediates antidepressant-like behavior and is profoundly and selectively enhanced by genetic disruption of regulators of G protein signaling (RGS) activity at G(alpha)i2. Animals rendered insensitive to RGS protein regulation through a mutation in G(alpha)i2 (G184S) exhibited spontaneous antidepressant- and anxiolytic-like behaviors. Mice expressing RGS-insensitive G(alpha)i2 also exhibited increased cortical and hippocampal phosphorylation of glycogen synthase kinase-3beta, a constitutively active proapoptotic kinase that is inhibited through phosphorylation in response to serotonin, SSRIs, and 5-HT1 receptor agonists. Both behavioral and biochemical phenotypes were blocked by treatment with WAY 100635, a 5-HT1A-selective antagonist. RGS-insensitive mice were also 5-10 times more responsive to the antidepressant-like effects of the SSRI fluvoxamine and 5-HT1A-selective agonist 8-hydroxy-2-dipropylaminotetralin. In contrast, the antidepressant potency of agents acting through nonserotonergic mechanisms was unchanged as was 5-HT1A action on body temperature. The findings point to a critical role for endogenous RGS proteins to suppress the antidepressant-like effects of 5-HT1A receptor activation. By selectively enhancing the beneficial effects of serotonin, inhibition of RGS proteins represents a therapeutic approach for the treatment of mood disorders.

Oppositional effects of serotonin receptors 5-HT1a, 2, and 2c in the regulation of adult hippocampal neurogenesis

Serotonin (5-HT) appears to play a major role in controlling adult hippocampal neurogenesis and thereby it is relevant for theories linking failing adult neurogenesis to the pathogenesis of major depression and the mechanisms of action of antidepressants. Serotonergic drugs lacked acute effects on adult neurogenesis in many studies, which suggested a surprisingly long latency phase. Here we report that the selective serotonin reuptake inhibitor fluoxetine, which has no acute effect on precursor cell proliferation, causes the well-described increase in net neurogenesis upon prolonged treatment partly by promoting the survival and maturation of new postmitotic neurons. We hypothesized that this result is the cumulative effect of several 5-HT-dependent events in the course of adult neurogenesis. Thus, we used specific agonists and antagonists to 5-HT1a, 2, and 2c receptor subtypes to analyze their impact on different developmental stages. We found that 5-HT exerts acute and opposing effects on proliferation and survival or differentiation of precursor cells by activating the diverse receptor subtypes on different stages within the neuronal lineage in vivo. This was confirmed in vitro by demonstrating that 5-HT1a receptors are involved in self-renewal of precursor cells, whereas 5-HT2 receptors effect both proliferation and promote neuronal differentiation. We propose that under acute conditions 5-HT2 effects counteract the positive proliferative effect of 5-HT1a receptor activation. However, prolonged 5-HT2c receptor activation fosters an increase in late-stage progenitor cells and early postmitotic neurons, leading to a net increase in adult neurogenesis. Our data indicate that serotonin does not show effect latency in the adult dentate gyrus. Rather, the delayed response to serotonergic drugs with respect to endpoints downstream of the immediate receptor activity is largely due to the initially antagonistic and un-balanced action of different 5-HT receptors.

Mice lacking RGS protein activity at Gαi2 exhibit a 5HT1A receptor‐mediated antidepressant‐like phenotype

Serotonin (5HT) acting through 5HT1 receptors plays an integral role in regulating mood. Multiple lines of evidence suggest 5HT1 receptors preferentially couple to Gαi2 relative to the more abundant Gαo. Therefore, transgenic “knock‐in” mice that express RGS‐insensitive Gαi2 (G184S; RGS‐i) were evaluated in the tail suspension test (TST), a classical assay of antidepressant‐related behavior in rodent models. In the TST, RGS‐i animals exhibited basal antidepressant‐like behavior and were 5–10 times more sensitive to the antidepressant‐like effects of the SSRI fluvoxamine and the 5HT1A‐selective agonist 8‐OH‐DPAT. In contrast, the antidepressant potency of the norepinephrine reuptake inhibitor desipramine was unchanged. Yet, 8‐OH‐DPAT‐mediated hypothermia was unaltered in animals of either genotype. Furthermore, the antidepressant‐like phenotype of RGS‐i animals was completely reversed by treatment with the 5HT1A‐selective antagonist WAY‐100635 without altering spontaneous motility. These data point to a critical role for endogenous RGS protein regulation at Gαi2 in mediating the antidepressant‐like effects of 5HT1A receptor activation. By selectively enhancing the antidepressant effects of serotonin, RGS proteins represent a novel therapeutic target for the treatment of depression. Supported by GM39561 and the Bower, Bennet, & Bennet Endowed Chair Research Award.

Effect of 5-HT1A receptor activation on hypothalamic glucose

The somatodendritic 5-HT1A agonist 8-OH-DPAT reduces serotonergic activity and stimulates feeding in freely feeding rats. Interactions between circulating glucose and 5-HT1A receptor expression related to feeding have been described. The aim of the present microdialysis study was to (1) describe the relation between feeding and glucose in the LH, (2) to investigate if peripherally administered 8-OH-DPAT itself has an effect on extracellular glucose in the lateral hypothalamus (LH) of conscious rats. Baseline glucose concentrations were significantly different in microdialysis samples obtained from food deprived rats compared to freely feeding rats. After re-feeding, a significant rise in glucose levels by 45% was observed in the formerly food deprived rats. In freely feeding rats, 8-OH-DPAT (0.3 mg/kg, i.p.) reduced glucose level in the LH significantly. The effect of 8-OH-DPAT on brain glucose was antagonized by pre-treatment with the 5-HT1A antagonist WAY 100635 (3 mg/kg i.p.) which had no effect on its own. The data indicate, therefore, that the effect of 8-OH-DPAT on hypothalamic glucose is mediated by 5-HT1A receptors. In contrast, the same dose of 8-OH-DPAT proven effective in the brain had no effect on peripheral glucose. Only a very high dose of the 5-HT1A agonist (1.8 mg/kg i.p.) had a hyperglycaemic effect in the periphery. In conclusion, the present results show for the first time, that glucose in the lateral hypothalamus increases with a meal. The data demonstrate furthermore 8-OH-DPAT-induced changes of hypothalamic glucose level, implicating 5-HT1A receptors being involved not only in the control of hypothalamic 5-HT as shown before, but also in the control of hypothalamic glucose.

Influence of the 5-HT1A agonist buspirone on rectal tone and on the perception of rectal distension in man

The 5-HT1A receptor agonist buspirone is an anxiolytic drug which has been reported to be beneficial in the treatment of the irritable bowel syndrome (Friedman 1991). It has been assumed that the central anxiolytic effect was responsible for the therapeutic benefit in these patients. However, the effect of 5-HT1A receptor activation on the tone and the visceral sensitivity of the rectum have not been studied. Aim: To study the effect of buspirone on rectal tone and sensitivity using a barostat. Methods: In 9 healthy subjects (7 males, age 21-30 years), an electronic barostat was used to assess the effect of buspirone on rectal tone and on perception of isobaric rectal distensions. After rectal cleansing with a water enema, a flaccid spherical barostat balloon (max. volume = 500 ml) was introduced into the rectum. After an accommodation period of 30 minutes, graded isobaric distensions (increments of 2 mm Hg of 2 min duration) were performed until the subjects reported discomfort. Volume and pressure changes were recorded and perception was scored by a questionnaire (score 0-6, l=threshold sensation, 4=desire to defecate, 5=discomfort). Subsequently, the pressure was set at Operating Pressure (OP, defined as the lowest pressure needed to reach an intraballoon volume of >30 ml) +2 mm Hg during one hour, with administration of buspirone 10 mg orally after 30 minutes. Afterwards, graded distensions were repeated. Results (mean -+ SEM) were compared by paired t-test and by two-way ANOVA. Results: During baseline isobaric rectal distensions, threshold sensation was reached at a distending pressure of 9.4-+ 1.1 mm Hg and a volume of 96.9 -+ 22.6 ml. Desire to defecate was reached at a distending pressure of 19.9-+ 1.7 mm Hg and a volume of 236 -+41 ml, and discomfort at a distending pressure of 24.4 -+ 1.8 mm Hg and a volume of 307 -+ 40 ml. After buspirone, the thresholds for perception (11.8 -+ 6.4 mm Hg and 181 -+ 44 ml, p < 0.05) or desire to defecate (23.7 -+ 1.8 mm Hg and 296 -+ 31 ml, p < 0.05) were significantly higher. Discomfort thresholds were not altered by buspirone (27.1 -+ 1.8 mm Hg and 316 -+ 30 ml). Administration of buspirone shifted the pressure-volume curve towards significantly higher volumes for the same distending pressures (p < 0.01, ANOVA). The slope of the pressurevolume relation (rectal compliance) was not significantly altered (13.8 -+ 1.8 vs 12.1 -+ 1.3 ml/mm Hg, NS). Conclusions; Administration of buspirone 10 mg orally causes a relaxation of the rectum in man. This is accompanied by a decrease of rectal sensitivity for first perception and desire to defecate during rectal distension. The effects of buspirone on the rectum are likely to contribute to the observed benefit in the treatment of the irritable bowel syndrome. Our findings suggest that 5-HTIA receptor agonists could have therapeutic potential in the treatment of the irritable bowel syndrome.

Central 5-HT1A receptors inhibit adrenocortical secretion.

Serotonin (5-HT) is generally considered to serve a facilitatory role in the regulation of adrenocortical secretion. Numerous studies have shown that administration of 5-HT1A receptor agonists increases plasma corticosterone (CS) concentrations in rats; however, the mechanism has not been established. Rats were prepared with a cannula implanted above the lateral cerebral ventricle, or bilateral cannulae above the hypothalamic paraventricular nuclei (PVN), the site of the perikarya of corticotropin-releasing factor (CRF)-secreting neurons regulating adrenocortical secretion. In sodium pentobarbital-anesthetized rats, intracerebroventricular and intra-PVN administration of 5-HT resulted in a multi-component dose-response curve in plasma CS, whereas administration of 5-HT in conscious animals resulted in low-dose inhibition and higher dose elevation of plasma CS levels. Under pentobarbital anesthesia, central administration of the selective 5-HT1A agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and ipsapirone decreased plasma CS levels, relative to saline-treated control rats, at all doses tested (0.001-20 nmol). In conscious rats, administration of 8-OH-DPAT decreased adrenocortical secretion at lower doses and significantly increased plasma CS concentrations at higher doses. Ipsapirone produced similar but less pronounced effects. In contrast, intraperitoneal injection of 8-OH-DPAT (2 mumol/kg) increased plasma CS concentrations, but this was not prevented by prior intracerebroventricular administration of the 5-HT1A antagonist, NAN-190 (5 nmol). Pentobarbital anesthesia completely blocked the plasma CS response to peripheral administration of 8-OH-DPAT. In view of the adrenocortical activating effects of hypotensive stimuli, we speculate that the well-documented hemodynamic changes following 5-HT1A receptor stimulation may be responsible for the adrenocortical responses observed. Our data demonstrate that low doses of 5-HT1A agonists delivered directly into the CNS decrease adrenocortical secretion. Since intra-PVN injections of 8-OH-DPAT to pentobarbital-anesthetized rats also decreased hypothalamo-pituitary-adrenocortical activity, it appears that a component of the inhibitory effect of 5-HT1A receptor activation is mediated by a direct effect at the level of the PVN, and presumably involves CRF-secreting neurons.

Activation of 5-HT1A receptors expressed in NIH-3T3 cells induces focus formation and potentiates EGF effect on DNA synthesis.

NIH-3T3 fibroblasts have been transfected with human serotonin 5-HT1A receptors. Clonal cell lines expressed between 40 and 500 fmol receptor/mg. 5-HT1A agonists strongly inhibited nonstimulated- as well as forskolin- or isoproterenol-stimulated adenylyl cyclase. The effects of 5-HT1A receptor activation on cell growth were investigated. 5-HT1A agonists accelerated cell division, generated foci, and increased DNA synthesis. The stimulation of [3H]thymidine incorporation was much stronger when tyrosine kinase receptors were activated concomitantly. Cyclic AMP (cAMP) elevating agents inhibited DNA synthesis induced by all mitogens tested. The mitogenic activity of 5-HT1A agonists did not seem to be linked to adenylyl cyclase inhibition because 1) we were not able to measure any decrease in intracellular cAMP levels under the conditions of DNA synthesis assay and 2) 2',5'-dideoxyadenosine, which strongly inhibited adenylyl cyclase, was not mitogenic and did not modify the mitogenic effects of 5-HT1A agonists. Pertussis toxin completely blocked potentiation of epidermal growth factor effect induced by 8-hydroxy-di-(n-propyl)aminotetralin, a 5-HT1A agonist, but only partially blocked the one induced by insulin. In conclusion, in transfected NIH-3T3 cells, transforming and mitogenic effects of 5-HT1A agonists involve a pertussis toxin-sensitive G protein but do not seem to be linked to adenylyl cyclase inhibition.