Eliyahu Dremencov

ORIGINAL RESEARCH—BASIC SCIENCE: Acute and Repeated Flibanserin Administration in Female Rats Modulates Monoamines Differentially Across Brain Areas: A Microdialysis Study

INTRODUCTION Hypoactive sexual desire disorder (HSDD) is defined as persistent lack of sexual fantasies or desire marked by distress. With a prevalence of 10% it is the most common form of female sexual dysfunction. Recently, the serotonin-1A (5-HT(1A)) receptor agonist and the serotonin-2A (5-HT(2A)) receptor antagonist flibanserin were shown to be safe and efficacious in premenopausal women suffering from HSDD in phase III clinical trials. AIM The current study aims to assess the effect of flibanserin on neurotransmitters serotonin (5-HT), norepinephrine (NE), dopamine (DA), glutamate, and gamma-aminobutyric acid (GABA) in brain areas associated with sexual behavior. METHODS Flibanserin was administered to female Wistar rats (280-350 g). Microdialysis probes were stereotactically inserted into the mPFC, NAC, or MPOA, under isoflurane anesthesia. The extracellular levels of neurotransmitters were assessed in freely moving animals, 24 hours after the surgery. MAIN OUTCOME MEASURES Dialysate levels of DA, NE, and serotonin from medial prefrontal cortex (mPFC), nucleus accumbens (NAC), and hypothalamic medial preoptic area (MPOA) from female rats. RESULTS Acute flibanserin administration decreased 5-HT and increased NE levels in all tested areas. DA was increased in mPFC and MPOA, but not in the NAC. Basal levels of NE in mPFC and NAC and of DA in mPFC were increased upon repeated flibanserin administration, when compared to vehicle-treated animals. The basal levels of 5-HT were not altered by repeated flibanserin administration, but basal DA and NE levels were increased in the mPFC. Glutamate and GABA levels remained unchanged following either repeated or acute flibanserin treatment. CONCLUSIONS Systemic administration of flibanserin to female rats differentially affects the monoamine systems of the brain. This may be the mechanistic underpinning of flibanserins therapeutic efficacy in HSDD, as sexual behavior is controlled by an intricate interplay between stimulatory (catecholaminergic) and inhibitory (serotonergic) systems.

Interaction Between Brain Histamine and Serotonin, Norepinephrine, and Dopamine Systems: In Vivo Microdialysis and Electrophysiology Study

Brain monoamines (serotonin, norepinephrine, dopamine, and histamine) play an important role in emotions, cognition, and pathophysiology and treatment of mental disorders. The interactions between serotonin, norepinephrine, and dopamine were studied in numerous works; however, histamine system received less attention. The aim of this study was to investigate the interactions between histamine and other monoamines, using in vivo microdialysis and electrophysiology. It was found that the inverse agonist of histamine-3 receptors, thioperamide, increased the firing activity of dopamine neurons in the ventral tegmental area. Selective agonist of histamine-3 receptors, immepip, reversed thiperamide-induced stimulation of firing activity of dopamine neurons. The firing rates of serotonin and norpeinephrine neurons were not attenuated by immepip or thioperamide. Thioperamide robustly and significantly increased extracellular concentrations of serotonin, norepinephrine, and dopamine in the rat prefrontal cortex and slightly increased norepinephrine and dopamine levels in the tuberomammillary nucleus of the hypothalamus. It can be concluded that histamine stimulates serotonin, norepinephrine, and dopamine transmission in the brain. Modulation of firing of dopamine neurons is a key element in functional interactions between histamine and other monoamines. Antagonists of histamine-3 receptors, because of their potential ability to stimulate monoamine neurotransmission, might be beneficial in the treatment of mental disorders.

The role of cortical and hypothalamic histamine-3 receptors in the modulation of central histamine neurotransmission: an in vivo electrophysiology and microdialysis study

The current study aimed to investigate the effect of histamine‐3 (H3) receptors, expressed in the tuberomammillary nucleus (TMN) of the hypothalamus and in the prefrontal cortex (PFC), on histamine neurotransmission in the rat brain. The firing activity of histamine neurons in the TMN was measured using in vivo extracellular single‐unit electrophysiology, under propofol anesthesia. Extracellular histamine levels were determined using the dual (PFC and TMN) probe microdialysis, in freely‐moving animals. Histamine levels in dialysates were determined using high‐performance liquid chromatography (HPLC) and fluorescence detection. It was found that systemic administration of the selective H3‐agonist, immepip, decreases, and the reverse H3/H4‐agonist, thioperamide, increases the firing activity of histamine neurons in the TMN and the release of histamine in TMN and PFC. Local perfusion of immepip into the TMN increased, and thioperamide decreased, histamine levels in the TMN but not in the PFC. Local perfusion of immepip into the PFC, however, decreased extracellular histamine levels in both TMN and PFC. It can be concluded that brain H3 receptors, and especially those expressed in the PFC, play an important role in the autoregulation of histamine neurotransmission. It is possible that H3 receptors in the PFC are expressed on pyramidal neurons projecting to the TMN, and activation of these receptors diminishes glutamate excitatory input from PFC to the TMN. As the brain histamine system has a role in pathophysiology of psychotic, affective, cognitive, sleep and eating disorders, H3 receptors are potential targets for future CNS medications.

Upregulation of the dorsal raphe nucleus-prefrontal cortex serotonin system by chronic treatment with escitalopram in hyposerotonergic Wistar-Kyoto rats

Wistar-Kyoto (WKY) rats are sensitive to chronic stressors and exhibit depression-like behavior. Dorsal raphe nucleus (DRN) serotonin (5-HT) neurons projecting to the prefrontal cortex (PFC) comprise the important neurocircuitry underlying the pathophysiology of depression. To evaluate the DRN-PFC 5-HT system in WKY rats, we examined the effects of escitalopram (ESCIT) on the extracellular 5-HT level in comparison with Wistar rats using dual-probe microdialysis. The basal levels of 5-HT in the DRN, but not in the PFC, in WKY rats was reduced as low as 30% of Wistar rats. Responses of 5-HT in the DRN and PFC to ESCIT administered systemically and locally were attenuated in WKY rats. Feedback inhibition of DRN 5-HT release induced by ESCIT into the PFC was also attenuated in WKY rats. Chronic ESCIT induced upregulation of the DRN-PFC 5-HT system in WKY rats, with increases in basal 5-HT in the DRN, responsiveness to ESCIT in the DRN and PFC, and feedback inhibition, whereas downregulation of these effects was induced in Wistar rats. Thus, the WKY rat is an animal model of depression with low activity of the DRN-PFC 5HT system. The finding that chronic ESCIT upregulates the 5-HT system in hyposerotonergic WKY rats may contribute to improved understanding of mechanisms of action of antidepressants, especially in depression with 5-HT deficiency.

Serotonin-2C antagonism augments the effect of citalopram on serotonin and dopamine levels in the ventral tegmental area and nucleus accumbens.

Many patients with major depression do not respond to selective serotonin reuptake inhibitors (SSRIs). Lack of response could be due to inhibition of dopamine (DA) release by serotonin (5-HT) through 5-HT2C receptors. Combining an SSRI with a 5-HT2C antagonist may result in improved efficacy by causing simultaneous increases of 5-HT and DA. In order to test this augmentation strategy, male Wistar rats were treated (s.c.) with an acute dose of the SSRI citalopram (Cit, 5 mg/kg), the 5-HT2C antagonist SB 242084 (SB, 2 mg/kg), or Cit + SB, and the effect on 5-HT and DA release in the nucleus accumbens (NAcc) was assessed by microdialysis. In a separate experiment, animals were treated with vehicle, Cit (20 mg/kg/d), SB (2 mg/kg/d) or Cit + SB for a period of 2 days (s.c.), and the impact on the release of 5-HT and DA in the ventral tegmental area (VTA) and NAcc was studied. On the day of microdialysis, 5-HT2C receptor sensitivity was assessed with an SB challenge. Acutely administered Cit + SB increased 5-HT release in the NAcc more than Cit alone. SB alone increased DA release in the NAcc (not in the VTA), but when administered together with Cit, this effect was abolished. A 2-day treatment with Cit or Cit + SB increased 5-HT release in both VTA and NAcc. Combining Cit with SB augmented the effect of Cit in the VTA. DA release in VTA and NAcc was only significantly increased after 2-days of treatment with Cit + SB. In conclusion, Cit + SB had synergistic effects on 5-HT and DA release after 2-days of treatment, probably related to a decreased tonic inhibition of DA release via 5-HT2C receptors. Regional differences occur and future studies should elucidate if this augmentation strategy is beneficial at the behavioral level.

Effect of Physical Exercise and Acute Escitalopram on the Excitability of Brain Monoamine Neurons: In Vivo Electrophysiological Study in Rats

Abstract Background The antidepressant effect of physical exercise has been reported in several clinical and animal studies. Since serotonin, norepinephrine, and dopamine play a central role in depression, it is possible that the beneficial effects of physical exercise are mediated via monoamine pathways. This study investigates the effects of voluntary wheel running on the excitability of monoamine neurons. Materials and Methods Male Sprague-Dawley rats were used in the study. Voluntary wheel running (VWR) rats were housed in individual cages with free access to a running wheel, while control animals were housed in standard laboratory cages. After three weeks, the rats were anesthetized, and in vivo electrophysiological recordings were taken from dorsal raphe nucleus serotonin neurons, locus coeruleus norepinephrine neurons, and ventral tegmental dopamine neurons. Results VWR stimulated activity in serotonin, but not in norepinephrine or dopamine neurons. Subsequently, acute administration of the selective serotonin reuptake inhibitor escitalopram in control rats led to complete suppression of serotonin neurons; this suppression was reversed by subsequent administration of selective antagonist of serotonin-1A receptors, WAY100135. Escitalopram induced only partial inhibition of serotonin neurons in the VWR rats while WAY100135 increased the firing activity of serotonin neurons above the baseline value. Conclusions The beneficial effect of physical exercise on mood is mediated, at least in part, via activation of serotonin neurons. Physical exercise can potentiate the response to selective serotonin reuptake inhibitors by increasing the basal firing activity and diminishing selective serotonin reuptake inhibitor-induced inhibition of serotonin neurons.

Oxazepam and temazepam attenuate paroxetine-induced elevation of serotonin levels in guinea-pig hippocampus

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are used as a first-line treatment in depression. However, many depressed patients are also treated with benzodiazepines to alleviate increased anxiety and sleep disturbances normally associated with depression. Since benzodiazepines inhibit 5-HT neuronal firing activity, they might attenuate SSRI-induced increase in extracellular 5-HT levels. This study aimed to assess, using in-vivo microdialysis, the effects of the benzodiazepines oxazepam or temazepan on the SSRI paroxetine-induced 5-HT increase in the hippocampus of freely moving guinea-pigs. It was found that the acute systemic administration of paroxetine increased extracellular 5-HT levels. Pre-administration of oxazepam or temazepam significantly diminished the paroxetine-induced elevation of extracellular 5-HT levels (from 350% to 200% of baseline). It was concluded that benzodiazepines attenuate the ability of SSRIs to elevate hippocampal 5-HT levels. Thus, co-administration of benzodiazepines might affect the therapeutic efficacy of SSRI treatment.

Purinergic regulation of brain catecholamine neurotransmission: In vivo electrophysiology and microdialysis study in rats

It was previously reported that adenosine-2A (A2A) receptors interact with dopamine-2 (D2) receptors on a molecular level. The aim of the current study was to investigate the functional output of this interaction. In vivo microdialysis was used to assess the effects of an antagonist of A2A receptors, ZM 241385, and an antagonist of D2 receptors haloperidol, either alone or in combination, on brain catecholamine levels. It was found that ZM 241385 did not alter catecholamine levels by its own, but potentiated haloperidol-induced dopamine and norepinephrine release in the nucleus accumbens and prefrontal cortex, respectively. In vivo electrophysiology was used to assess the effect of an agonist (CGS 216820) and an antagonist (ZM 241385) of A2A receptors on the excitability of dopamine and norepinephrine neurons. It was found that CGS 216820 dose-dependently inhibited dopamine and norepinephrine neurons and ZM 241385 reversed this inhibition. In conclusion, those A2A receptors modulate brain catecholamine transmission, and this modulation is mediated, at least in part, via the regulation of excitability of norepinephrine and dopamine neurons. The ability of antagonists of A2A receptors to potentiate the effect of haloperidol on brain norepinephrine and dopamine levels may enhance its clinical efficacy as an antipsychotic drug.

The Role of 5-HT2C Receptors in the Pathophysiology and Treatment of Depression

There is considerable evidence supporting the involvement of serotonin-2C (5-HT2C) receptors in the pathophysiology of depression. First, 5-HT2C receptors play an important role in the regulation of monoamine transmission. Second, abnormalities in expression and functioning of 5-HT2C receptors have been observed in depressed patients and in animal models of depression. Third, a large body of evidence indicates that 5-HT2C receptors mediate certain behavioral functions that are disturbed in depression. Finally, some compounds acting on 5-HT2C receptors are effective in the treatment of depression. This chapter aims to summarize these four lines of evidence for the involvement of 5-HT2C receptors in pathophysiology and treatment of depression.

Role of Serotonin-2A Receptors in Pathophysiology and Treatment of Depression

This chapter aims to summarize the up-to-day evidence-based biomedical knowledge on serotonin-2A (5-HT2A) receptors and their role in pathophysiology and treatment of central nervous system (CNS) disorders, with a primary focus on depression. The first paragraph provides a brief introduction to serotonin (5-HT) system and 5-HT receptors, focusing on serotonin-2 (5-HT2) family and 5-HT2A receptor specifically. The second paragraph is focused on molecular genetics of 5-HT2A receptors, polymorphism of 5-HT2A receptor (5HT2AR) gene, 5HT2AR gene epigenetic mechanisms, such as DNA methylation, and post-translational modifications of 5HT2AR messenger ribonucleic acid (mRNA), such as alternative splicing. The molecular and cellular pharmacology and physiology of 5-HT2A receptors in normal and pathological conditions are discussed in the third paragraph. The 5-HT2A receptors-acting ligands are addresses. The fourth paragraph describes the role of 5-HT receptors in the interaction between 5-HT and other neurotransmitter systems in health and in CNS disorders. The fifth and the final paragraph specifically deals with the role of 5-HT2A receptor in pathophysiology and treatment of depression, focusing on the 5-HT2A receptor expressed in the hippocampus.