Effects of 8-OHDPAT administration into the dorsal raphe nucleus and dorsal hippocampus on fear behavior and regional brain monoamines distribution in rats

Adaptive changes in serotonin metabolism preserve normal behavior in mice with reduced TPH2 activity

Polymorphisms in the TPH2 gene coding for the serotonin synthesizing enzyme in the brain are considered as risk factors associated with depression and anxiety in humans. However, whether a certain variation in the TPH2 gene leads to decreased brain serotonin production and development of psychological abnormalities remains unresolved. We generated a new mouse model, carrying one Tph2-null allele and one Tph21473G-allele, coding for a hypoactive form of the enzyme. We tested these mice along with C57BL/6 mice (Tph2C/C), congenic C57BL/6 mice homozygous for the Tph21473G-allele (Tph2G/G), and heterozygous Tph2-deficient mice (Tph2C/-) for anxiety- and depression-like behavior, and evaluated brain serotonin metabolism and 5-HT1AR signaling by high-performance liquid chromatography and quantitative autoradiography, respectively. Progressive reduction in TPH2 activity had no effect on emotional behavior, and only slightly affected brain serotonin levels. However, serotonin degradation rate was drastically decreased in mice with reduced TPH2 activity, thereby compensating for the lowered rate of serotonin production in these mice. In addition, the hypothermic response to the 5-HT1AR agonist, 8-OH-DPAT, was attenuated in mice with reduced serotonin production. In contrast, 5-HT1A autoreceptor density and G-protein coupling were not changed in mice with gradual decrease in central serotonin. Taken together, these data suggest that in conditions of reduced serotonin production lowered serotonin degradation rate contributes to the maintenance of brain serotonin at levels sufficient for adequate behavior responses. These findings reveal that decreased TPH2 activity cannot be considered a reliable predisposition factor for impaired emotional behavior.

Vertebrate cone opsins enable sustained and highly sensitive rapid control of Gi/o signaling in anxiety circuitry

G protein-coupled receptors (GPCRs) coupling to Gi/o signaling pathways are involved in the control of important physiological functions, which are difficult to investigate because of the limitation of tools to control the signaling pathway with precise kinetics and specificity. We established two vertebrate cone opsins, short- and long-wavelength opsin, for long-lasting and repetitive activation of Gi/o signaling pathways in vitro and in vivo. We demonstrate for both opsins the repetitive fast, membrane-delimited, ultra light-sensitive, and wavelength-dependent activation of the Gi/o pathway in HEK cells. We also show repetitive control of Gi/o pathway activation in 5-HT1A receptor domains in the dorsal raphe nucleus (DRN) in brain slices and in vivo, which is sufficient to modulate anxiety behavior in mice. Thus, vertebrate cone opsins represent a class of tools for understanding the role of Gi/o-coupled GPCRs in health and disease.

Early-life stress induces persistent alterations in 5-HT1A receptor and serotonin transporter mRNA expression in the adult rat brain

Early-life experience plays a major role in the stress response throughout life. Neonatal maternal separation (MS) is an animal model of depression with an altered serotonergic response. We hypothesize that this alteration may be caused by differences in 5-HT1A receptor and serotonin transporter (SERT) mRNA expression in brain areas involved in the control of emotions, memory, and fear as well as in regions controlling the central serotonergic tone. To test this, Sprague-Dawley rats were subjected to MS for 3 h daily during postnatal days 2-12. As control, age matched rats were non-separated (NS) from their dams. When animals reached adulthood (11-13 weeks) brain was extracted and mRNA expression of 5-HT1A receptor in amygdala, hippocampus and dorsal raphé nucleus (DRN) and SERT in the DRN was analyzed through in situ hybridisation. Densitometric analysis revealed that MS increased 5-HT1A receptor mRNA expression in the amygdala, and reduced its expression in the DRN, but no changes were observed in the hippocampus in comparison to NS controls. Also, MS reduced SERT mRNA expression in the DRN when compared to NS rats. These results suggest that early-life stress induces persistent changes in 5-HT1A receptor and SERT mRNA expression in key brain regions involved in the development of stress-related psychiatric disorders. The reduction in SERT mRNA indicates an alteration that is in line with clinical findings such as polymorphic variants in individuals with higher risk of depression. These data may help to understand how early-life stress contributes to the development of mood disorders in adulthood.

Infusion of galanin into the mid-caudal portion of the dorsal raphe nucleus has an anxiolytic effect on rats in the elevated T-maze

Galanin and 5-HT coexist in dorsal raphe nucleus (DRN) neurons. Microinjection of galanin into the DRN reduces the firing rate of serotonin neurons. Serotonergic neurons projecting from the DRN to the amygdala facilitate learned anxiety producing an anxiogenic effect, while those projecting from the periaqueductal grey affect innate anxiety producing a panicolytic effect. We tested the hypothesis that injection of galanin into rat DRN would induce anxiolytic/panicogenic effects in the elevated T-maze (ETM), a model that allows for the evaluation of both of these effects. Galanin infusion into the mid-caudal DRN, but not into the rostral DRN, impaired inhibitory avoidance, suggesting an anxiolytic effect. The effective dose of galanin (0.3 nmol) did not modify locomotor activity in the open field. Contrary to expectations, microinjection of galanin into the DRN did not facilitate the latency of one-way escape in the ETM. Pretreatment with a galanin antagonist, M40, attenuated galanin-induced impairment of inhibitory avoidance. The results show that microinjection of a low dose of galanin only into the mid-caudal DRN has an anxiolytic effect. This effect seems to be mediated, at least in part, by galanin receptors. Further investigation is necessary to identify the receptor subtypes and the DRN subregion involved in the anxiolytic effect of galanin.

Exploring the role of 5-HT1A receptors in the regulation of prepulse inhibition in mice: implications for cross-species comparisons

Prepulse inhibition (PPI) is a model of sensorimotor gating, a sensory filtering mechanism which is disrupted in schizophrenia. Here, investigation of the role of the serotonin-1A (5-HT(1A)) receptor in the regulation of PPI in two mouse strains, C57Bl/6 and Balb/c, was used to address findings in the PPI literature on species and mouse strain differences that question the usefulness of PPI as a cross-species preclinical test. Although the full 5-HT(1A) receptor agonist, 8-OH-DPAT, induced markedly different strain-specific responses in PPI, other selective 5-HT(1A) receptor ligands with partial agonist or antagonist activity elicited similar effects across strains. Pretreatment with the serotonin precursor, 5-HTP, to increase serotonergic activity in the brain, unmasked a decrease in PPI caused by 8-OH-DPAT in C57Bl/6 mice. Pretreatment with the serotonin synthesis inhibitor, PCPA, to decrease serotonergic activity in the brain, unmasked an 8-OH-DPAT-induced increase in PPI in this strain. These studies show that the strain-dependent involvement of 5-HT(1A) receptors in PPI can be modulated by the type of 5-HT(1A) ligand used, or increasing or decreasing serotonin levels in the brain. These results help to clarify some of the mouse strain and species differences in PPI regulation and strengthen its usefulness as a cross-species measure of sensorimotor gating.

Regulation of dorsal raphe nucleus function by serotonin autoreceptors: a behavioral perspective

Neurotransmission by serotonin (5-HT) is tightly regulated by several autoreceptors that fine-tune serotonergic neurotransmission through negative feedback inhibition at the cell bodies (predominantly 5-HT(1A)) or at the axon terminals (predominantly 5-HT(1B)); however, more subtle roles for 5-HT(1D) and 5-HT(2B) autoreceptors have also been detected. This review provides an overview of 5-HT autoreceptors, focusing on their contribution in animal behavioral models of stress and emotion. Experiments targeting 5-HT autoreceptors in awake, behaving animals have generally shown that increasing autoreceptor feedback is anxiolytic and rewarding, while enhanced 5-HT function is aversive and anxiogenic; however, the role of serotonergic activity in behavioral models of helplessness is more complex. The prevailing model suggests that 5-HT autoreceptors become desensitized in response to stress exposure and antidepressant administration, two seemingly opposite manipulations. Thus there are still unresolved questions regarding the role of these receptors-and serotonin in general-in normal and pathological states.

The effects of Rhodiola rosea extract on 5-HT level, cell proliferation and quantity of neurons at cerebral hippocampus of depressive rats

The purpose of this study was to investigate the effects of Rhodiola rosea extract and depression on the serotonin (5-HT) level, cell proliferation and quantity of neurons at cerebral hippocampus of depressive rats induced by Chronic Mild Stress (CMS). Seventy male Sprague-Dawley rats were divided into seven groups (10 per group): normal control group, untreated depressive rat model group, negative control group, positive control group, low dosage Rhodiola rosea extract (1.5g/kg) group, medium dosage Rhodiola rosea extract (3g/kg) group and high dosage Rhodiola rosea extract (6g/kg) group. After the depressive rats induced by CMS had received Rhodiola rosea extract for 3 weeks, the 5-HT levels at cerebral hippocampus were detected by high performance liquid chromatography. Bromodeoxyuridine (BrdU) was injected in vivo to label the proliferating cells at hippocampus, and morphometry was used to count the hippocampal neurons. The results showed that the 5-HT level of the three experimental groups had recovered to normal status. The immunohistochemistry of hippocampus BrdU positive cells had returned to the normal level in the group of depressive rats with low dosage Rhodiola rosea extract. In conclusion the results demonstrated that Rhodiola rosea extract could improve 5-HT level in hippocampus in depressive rats, and low dosage Rhodiola rosea could induce neural stem cell proliferation at hippocampus to return to normal level, repairing the injured neurons at hippocampus.

5-HT1A receptors in the dorsal hippocampus mediate the anxiogenic effect induced by the stimulation of 5-HT neurons in the median raphe nucleus

We evaluated the involvement of dorsal hippocampus (DH) 5-HT1A receptors in the mediation of the behavioral effects caused by the pharmacological manipulation of 5-HT neurons in the median raphe nucleus (MRN). To this end, we used the rat elevated T-maze test of anxiety. The results showed that intra-DH injection of the 5-HT1A/7 agonist 8-OH-DPAT facilitated inhibitory avoidance, an anxiogenic effect, without affecting escape. Microinjection of the 5-HT1A antagonist WAY-100635 was ineffective. In the elevated T-maze, inhibitory avoidance and escape have been related to generalized anxiety and panic disorders, respectively. Intra-MRN administration of the excitatory amino acid kainic acid, which non-selectively stimulates 5-HT neurons in this brain area facilitated inhibitory avoidance and impaired escape performance, but also affected locomotion. Intra-MRN injection of WAY-100635, which has a disinhibitory effect on the activity of 5-HT neurons in this midbrain area, only facilitated inhibitory avoidance. Pre-administration of WAY-100635 into the DH blocked the behavioral effect of intra-MRN injection of WAY-100635, but not of kainic acid. These results indicate that DH 5-HT1A receptors mediate the anxiogenic effect induced by the selective stimulation of 5-HT neurons in the MRN.

Acquisition and extinction of a conditioned passive avoidance reflex in mice with genetic knockout of monoamine oxidase A

We report here the results obtained from comparative analysis of learning and the dynamics of extinction of a conditioned passive avoidance response in mice with genetic knockout of monoamine oxidase A (MAO A) and the progenitor line C3H. Mice of both lines acquired the conditioned passive avoidance reaction efficiently. Mice with genetic knockout of MAO A were characterized by prolonged retention of reproduction of the memory trace, as compared with rapid extinction in C3H mice. Smaller numbers of transfers, and vertical rearings on days 7-13 and the numbers of glances into and rom the dark sector on days 11-13 of extinction in MAO A-knockout mice appear to reflect their more marked fear reactions when confronted with the "dangerous" sector, along with increased anxiety, these facilitating longer-lasting retention of the memory trace.

Metabolism and Receptor Binding of Serotonin in Brain Structures During Performance of a Conditioned Passive Avoidance Response

The levels of serotonin and its metabolite 5-hydroxyindoleacetic acid, monoamine oxidase activity, and the specific binding of the radioligand [3H]serotonin were measured in the prefrontal cortex, striatum, amygdaloid complex, hippocampus, and periacqueductal gray matter of the midbrain in rats at different time points after training to a conditioned passive avoidance reaction. Changes in serotoninergic activity were found to be characteristic only for the process of reproducing the conditioned reaction. The metabolism and serotonin receptor binding in these brain structures did not change immediately after the training period or one day after this, or in conditions of failure to reproduce the reaction because of amnesia, or in untrained animals. The involvement of the brain serotoninergic system in the process of performing the conditioned reaction was found to demonstrate a spatial-structural selectivity: the metabolism and receptor binding of serotonin changed in the amygdaloid complex, periacqueductal gray matter, and the striatum, while no changes were seen in the hippocampus or prefrontal cortex. All three brain structures showed decreases in [3H]serotonin receptor binding of. Serotonin levels did not change, though the amygdaloid complex and periacqueductal gray matter showed increases in oxidative deamination of serotonin and increases in the active transport of the metabolite, while the striatum showed decreases in serotonin catabolism. The differences in the catabolism of this neurotransmitter suggest that the decrease in serotonin receptor binding in these brain structures depends on different synaptic processes--presynaptic in the striatum and postsynaptic in the amygdaloid complex and periacqueductal gray matter. It is concluded that the decrease in the functional activity of serotoninergic transmission in the amygdaloid complex and periacqueductal gray matter is one of the mechanisms involved in activation of the emotiogenic system triggering the process of reproduction of the memory trace.

Serotonergic neurons in the median raphe nucleus regulate inhibitory avoidance but not escape behavior in the rat elevated T-maze test of anxiety

RATIONALE

A wealth of evidence supports the involvement of the serotonergic neurons of the median raphe nucleus (MRN) in anxiety. However, it is presently unclear whether serotonergic pathways arising from this nucleus play distinguishing regulatory roles in defensive behaviors that have been associated with specific subtypes of anxiety disorders.

OBJECTIVES

To evaluate the role of the MRN serotonergic neurons in the regulation of two defensive behaviors, inhibitory avoidance and escape, which have been related, respectively, to generalized anxiety and panic disorders.

METHODS

Male Wistar rats were submitted to the elevated T-maze test of anxiety after intra-MRN administration of drugs that either non-selectively or selectively change the activity of the serotonergic neurons.

RESULTS

Intra-MRN injection of FG 7142 (0.04 and 0.08 nmol) and kainic acid (0.03 and 0.06 nmol), drugs that non-selectively stimulate the MRN serotonergic neurons, facilitated inhibitory avoidance acquisition, but impaired escape performance. Microinjection of muscimol (0.11 and 0.22 nmol), a compound that non-selectively inhibits the activity of the MRN serotonergic neurons, impaired inhibitory avoidance and facilitated escape performance. Both kainic acid and muscimol also changed rat locomotion in the open-field test. Intra-MRN injection of 8-OH-DPAT (0.6-15 nmol) and WAY-100635 (0.18-0.74 nmol), respectively an agonist and an antagonist of somatodendritic 5-HT(1A) receptors located on serotonergic neurons of the MRN, only affected inhibitory avoidance-while the former inhibited the acquisition of this behavior, the latter facilitated it.

CONCLUSION

MRN serotonergic neurons seem to be selectively involved in the regulation of inhibitory avoidance in the elevated T-maze. This result supports the proposal that 5-HT pathways departing from this nucleus play an important role in anxiety processing, with implications for pathologies such as generalized anxiety disorder.

Microinjection of urocortin 2 into the dorsal raphe nucleus activates serotonergic neurons and increases extracellular serotonin in the basolateral amygdala

The intra dorsal raphe nucleus (DRN) administration of corticotropin releasing hormone (CRF) inhibits serotonergic (5-HT) activity in this structure, an effect blocked by antagonists selective for the type 1 CRF receptor (CRF1). The DRN has a high density of the type 2 receptor (CRF2), and so the present experiments explored the impact of CRF2 activation within the DRN on 5-HT function. The intra-DRN administration of the selective CRF2 agonist urocortin 2 (Ucn 2) dose dependently increased 5-HT efflux in the basolateral amygdala, a projection region of the DRN. Intra-DRN Ucn 2 also increased c-fos expression in labeled 5-HT neurons. Both of these effects of Ucn 2 were completely blocked by intra-DRN antisauvagine-30 (ASV-30), a relatively selective CRF2 antagonist. These data suggest that CRF1 and CRF2 activation within the DRN affect 5-HT neurons in opponent fashion. Implications of these results for understanding the behavioral effects of CRF and other CRF-like ligands are discussed.

Involvement of the brain serotonergic system in the locomotor stimulant effects of chlorpheniramine in Wistar rats: implication of postsynaptic 5-HT1A receptors

Antihistamines, such as chlorpheniramine (CPA), are lipophilic agents which readily cross the blood-brain barrier, producing sedation in 10-25% of users. However, with excessive doses instead of sedation a stimulating action has been reported. The aim of the present study was to investigate the influence of CPA on the locomotor activity of the rat in relation to its effects on brain biogenic monoamines. Wistar rats were given CPA (40 mg/kg, i.p.) and locomotor activity was measured in a photocell cage. Body temperature was also monitored. In addition, in three brain subregions (striatum, hypothalamus, and midbrain), the levels of 5-HT, NA, DA, as well as their metabolites, were determined by HPLC. Soon after injection, CPA produced a significant increase in locomotor activity, while a hypothermic response was also induced. In striatum and hypothalamus, which are known to be rich in postsynaptic 5-HT1A receptors, we found a significant time-dependent increase of 5-HT, correlated with the clearly enhanced locomotor activity of the animals. On the contrary, in midbrain, where presynaptic 5-HT1A receptors are dominating, no changes could be detected in 5-HT. In all three brain regions measured, 5-HIAA levels were decreased. The levels of the other brain monoamines were only marginally affected. In support of a role in receptor specificity, pretreatment with the 5-HT1A receptor agonist 8-OH-DPAT (1.25 mg/kg, i.p., two times) or with the 5-HT(1A/B) receptor antagonist pindolol (30 mg/kg, i.p., two times), enhanced or blocked, respectively, the hyperlocomotion induced by CPA. These findings suggest that the central serotonergic system may play a key role in the locomotor stimulant effects of CPA in the rat. Moreover, this behavioral component of CPA seems to be primarily mediated via the postsynaptic 5-HT1A receptors.

5-HT1A agonists: alcohol drinking in rats and squirrel monkeys

RATIONALE

Increased alcohol intake after administration of low doses of 5-HT(1A )agonists is thought to be due to a reduction in 5-HT impulse flow due to activation of 5-HT(1A) somatodendritic receptors, whereas decreased alcohol drinking found after administration of higher doses of 5-HT(1A) agonists may be mediated by action at postsynaptic 5-HT(1A) receptors.

OBJECTIVE

This study compares Long-Evans rats and squirrel monkeys to examine the hypothesis that low doses of the 5-HT(1A) selective agonists, 8-OH-DPAT and alnespirone, will preferentially increase, and at higher doses decrease alcohol drinking, and whether these effects can be antagonized by WAY 100635.

METHODS

Male Long-Evans rats were induced to drink from two bottles, one containing a solution of 10% ethanol and 1% sucrose (w/v), the other containing an equally preferred concentration of sucrose. Squirrel monkeys also drank from two bottles, one containing a solution of 2% ethanol and 15% sucrose (w/v), the other, water.

RESULTS

In rats, low doses of both 8-OH-DPAT (0.018-0.03 mg/kg) and alnespirone (0.3-3.0 mg/kg) increased alcohol drinking by ca. 100% without altering sucrose intake. The highest dose of 8-OH-DPAT (0.1 mg/kg) suppressed intake of both solutions without significant motor impairment. Pretreatment with WAY 100635 (0.1 mg/kg), shifted the entire dose-effect curve of 8-OH-DPAT to the right, and antagonized the effects of the 0.56 mg/kg dose of alnespirone. In the monkeys, administration of both agonists dose-dependently decreased alcohol intake and were behaviorally sedative.

CONCLUSIONS

These results support the hypothesis that in rats, 5-HT(1A) receptor stimulation activates somatodendritic receptors at lower doses and postsynaptic receptors at higher doses, each with opposite effects on alcohol intake. The absence of such biphasic dose-effect curves in monkeys suggests a different function of 5-HT(1A) somatodendritic receptors in rats and monkeys, at least with regard to alcohol drinking.

Estrous cycle and food availability affect feeding induced by amygdala 5-HT receptor blockade

We have recently reported that bilateral infusions of the 5-HT receptor antagonist metergoline (MET) into the posterior basolateral amygdala (pBLA) elicit feeding in female rats tested at mid-light cycle. The present study was performed to determine whether (1) testing at two different phases of the estrous cycle, and/or (2) the palatability of the food might modify this effect. Subjects were 18 adult females with bilateral pBLA cannulae. Following familiarization with Froot Loops cereal, a within-subjects design tested all animals for 1- and 2-h food intake under 2 Drug (0.3 nmol MET vs. Vehicle), 2 Estrous Cycle (diestrus vs. estrus) and 2 Food (lab chow vs. Froot Loops) conditions. Rats weighed more at diestrus than at proestrus (P<.05) or estrus (P<.005). Multivariate analyses of variance (MANOVAs) revealed a preference for Froot Loops over lab chow (P<.0001). MET increased feeding regardless of food type (P<.0001). Rats ate more Froot Loops (P<.01), but not lab chow, at diestrus vs. estrus. A three-way interaction (P<.05) showed rats ate more during the first hour in estrus than in diestrus to lab chow but not Froot Loops. These data suggest pBLA MET differentially affects feeding over the estrous cycle depending on the palatability of food available.