Pharmacological studies on the monoaminergic influence on the synthesis and expression of neuropeptide Y and corticotropin releasing factor in rat brain amygdala

Sex differences in aggression: Differential roles of 5-HT2, neuropeptide F and tachykinin

Despite the conserved function of aggression across taxa in obtaining critical resources such as food and mates, serotonin's (5-HT) modulatory role on aggressive behavior appears to be largely inhibitory for vertebrates but stimulatory for invertebrates. However, critical gaps exist in our knowledge of invertebrates that need to be addressed before definitively stating opposing roles for 5-HT and aggression. Specifically, the role of 5-HT receptor subtypes are largely unknown, as is the potential interactive role of 5-HT with other neurochemical systems known to play a critical role in aggression. Similarly, the influence of these systems in driving sex differences in aggressive behavior of invertebrates is not well understood. Here, we investigated these questions by employing complementary approaches in a novel invertebrate model of aggression, the stalk-eyed fly. A combination of altered social conditions, pharmacological manipulation and 5-HT2 receptor knockdown by siRNA revealed an inhibitory role of this receptor subtype on aggression. Additionally, we provide evidence for 5-HT2's involvement in regulating neuropeptide F activity, a suspected inhibitor of aggression. However, this function appears to be stage-specific, altering only the initiation stage of aggressive conflicts. Alternatively, pharmacologically increasing systemic concentrations of 5-HT significantly elevated the expression of the neuropeptide tachykinin, which did not affect contest initiation but instead promoted escalation via production of high intensity aggressive behaviors. Notably, these effects were limited solely to males, with female aggression and neuropeptide expression remaining unaltered by any manipulation that affected 5-HT. Together, these results demonstrate a more nuanced role for 5-HT in modulating aggression in invertebrates, revealing an important interactive role with neuropeptides that is more reminiscent of vertebrates. The sex-differences described here also provide valuable insight into the evolutionary contexts of this complex behavior.

Effect of long-term treatment with classical neuroleptics on NPQ/spexin, kisspeptin and POMC mRNA expression in the male rat amygdala

Neuroleptics modulate the expression level of some regulatory neuropeptides in the brain. However, if these therapeutics influence the peptidergic circuits in the amygdala remains unclear. This study specifies the impact profile of the classical antipsychotic drugs on mRNA expression of the spexin/NPQ, kisspeptin-1 and POMC in the rat amygdala. Animals were treated with haloperidol and chlorpromazine for 28 days prior to transcript quantification via qPCR. Haloperidol and chlorpromazine induced a change in the expression of all neuropeptides analyzed. Both drugs led to the decrease of Kiss-1 expression, whereas in POMC and spexin/NPQ their up-regulation in the amygdala was detected. These modulating effects on may represent alternative, so far unknown mechanisms, of classical antipsychotic drugs triggering pharmacological responses.

Amygdala nuclei critical for emotional learning exhibit unique gene expression patterns

The amygdala is a heterogeneous, medial temporal lobe structure that has been implicated in the formation, expression and extinction of emotional memories. This structure is composed of numerous nuclei that vary in cytoarchitectonics and neural connections. In particular the lateral nucleus of the amygdala (LA), central nucleus of the amygdala (CeA), and the basal (B) nucleus contribute an essential role to emotional learning. However, to date it is still unclear to what extent these nuclei differ at the molecular level. Therefore we have performed whole genome gene expression analysis on these nuclei to gain a better understanding of the molecular differences and similarities among these nuclei. Specifically the LA, CeA and B nuclei were laser microdissected from the rat brain, and total RNA was isolated from these nuclei and subjected to RNA amplification. Amplified RNA was analyzed by whole genome microarray analysis which revealed that 129 genes are differentially expressed among these nuclei. Notably gene expression patterns differed between the CeA nucleus and the LA and B nuclei. However gene expression differences were not considerably different between the LA and B nuclei. Secondary confirmation of numerous genes was performed by in situ hybridization to validate the microarray findings, which also revealed that for many genes, expression differences among these nuclei were consistent with the embryological origins of these nuclei. Knowing the stable gene expression differences among these nuclei will provide novel avenues of investigation into how these nuclei contribute to emotional arousal and emotional learning, and potentially offer new genetic targets to manipulate emotional learning and memory.

Allostasis and addiction: role of the dopamine and corticotropin-releasing factor systems

Allostasis, originally conceptualized to explain persistent morbidity of arousal and autonomic function, is defined as the process of achieving stability through physiological or behavioral change. Two types of biological processes have been proposed to describe the mechanisms underlying allostasis in drug addiction, a within-system adaptation and a between-system adaptation. In the within-system process, the drug elicits an opposing, neutralizing reaction within the same system in which the drug elicits its primary and unconditioned reinforcing actions, while in the between-system process, different neurobiological systems that the one initially activated by the drug are recruited. In this review, we will focus our interest on alterations in the dopaminergic and corticotropin releasing factor systems as within-system and between-system neuroadaptations respectively, that underlie the opponent process to drugs of abuse. We hypothesize that repeated compromised activity in the dopaminergic system and sustained activation of the CRF-CRF1R system with withdrawal episodes may lead to an allostatic load contributing significantly to the transition to drug addiction.

Central dopaminergic circuitry controlling food intake and reward: implications for the regulation of obesity

Prevalence of obesity in the general population has increased in the past 15 years from 15% to 35%. With increasing obesity, the coincident medical and social consequences are becoming more alarming. Control over food intake is crucial for the maintenance of body weight and represents an important target for the treatment of obesity. Central nervous system mechanisms responsible for control of food intake have evolved to sense the nutrient and energy levels in the organism and to coordinate appropriate responses to adjust energy intake and expenditure. This homeostatic system is crucial for maintenance of stable body weight over long periods of time of uneven energy availability. However, not only the caloric and nutritional value of food but also hedonic and emotional aspects of feeding affect food intake. In modern society, the increased availability of highly palatable and rewarding (fat, sweet) food can significantly affect homeostatic balance, resulting in dysregulated food intake. This review will focus on the role of hypothalamic and mesolimbic/mesocortical dopaminergic (DA) circuitry in coding homeostatic and hedonic signals for the regulation of food intake and maintenance of caloric balance. The interaction of dopamine with peripheral and central indices of nutritional status (e.g., leptin, ghrelin, neuropeptide Y), and the susceptibility of the dopamine system to prenatal insults will be discussed. Additionally, the importance of alterations in dopamine signaling that occur coincidently with obesity will be addressed.

Neuropeptide and sigma receptors as novel therapeutic targets for the pharmacotherapy of depression

Among the most prevalent of mental illnesses, depression is increasing in incidence in the Western world. It presents with a wide variety of symptoms that involve both the CNS and the periphery. Multiple pharmacological observations led to the development of the monoamine theory as a biological basis for depression, according to which diminished neurotransmission within the CNS, including that of the dopamine, noradrenaline (norepinephrine) and serotonin systems, is the leading cause of the disorder. Current conventional pharmacological antidepressant therapies, using selective monoamine reuptake inhibitors, tricyclic antidepressants and monoamine oxidase inhibitors, aim to enhance monoaminergic neurotransmission. However, the use of these agents presents severe disadvantages, including a delay in the alleviation of depressive symptoms, significant adverse effects and high frequencies of non-responding patients. Neuroendocrinological data of recent decades reveal that depression and anxiety disorders may occur simultaneously due to hypothalamus-pituitary-adrenal (HPA) axis hyperactivity. As a result, the stress-diathesis model was developed, which attempts to associate genetic and environmental influences in the aetiology of depression. The amygdala and the hippocampus control the activity of the HPA axis in a counter-balancing way, and a plethora of regulatory neuropeptide signalling pathways are involved. Intervention at these molecular targets may lead to alternative antidepressant therapeutic solutions that are expected to overcome the limitations of existing antidepressants. This prospect is based on preclinical evidence from pharmacological and genetic modifications of the action of neuropeptides such as corticotropin-releasing factor, substance P, galanin, vasopressin and neuropeptide Y. The recent synthesis of orally potent non-peptide micromolecules that can selectively bind to various neuropeptide receptors permits the onset of clinical trials to evaluate their efficacy against depression.

Antidepressant-like effects of psoralidin isolated from the seeds of Psoralea Corylifolia in the forced swimming test in mice

The antidepressant-like effects of psoralidin isolated from the seeds of Psoralea corylifolia were investigated in the forced swimming test (FST) in ICR strain of male mice. Psoralidin significantly decreased immobility time and increased swimming behavior without altering climbing behavior in the mouse FST after oral administration for 1 h or 3 consecutive days. Psoralidin did not affect locomotor activity in the open-field test. After a 3-day treatment, psoralidin significantly increased 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels in various brain regions, as well as, changed dopamine (DA) levels in striatum in mice exposed to FST. Psoralidin also ameliorated the elevations in serum corticotropin-releasing factor (CRF), adrenal corticotropin-releasing hormone (ACTH) and corticosterone concentrations induced by swimming stress in mice. These results suggested that psoralidin possessed potent antidepressant-like properties that were mediated via the monoamine neurotransmitter and the hypothalamic-pituitary-adrenal (HPA) axis systems.

Sigma 1 receptor-mediated increase in hippocampal extracellular dopamine contributes to the mechanism of the anticonvulsant action of neuropeptide Y

The potent anticonvulsant properties of neuropeptide Y (NPY) are generally attributed to a Y2 receptor-mediated inhibition of glutamatergic synaptic transmission. Independent studies have shown that NPY increases brain dopamine content, possibly via interaction with sigma 1 receptors. Recently, we showed that increased extracellular hippocampal dopamine attenuates pilocarpine-induced limbic seizures via activation of hippocampal D2 receptors. Our aim in this study was to elucidate the role of increased hippocampal dopamine in the mechanism of the anticonvulsant action of NPY and to investigate the involvement of Y2 and sigma 1 receptors in this process. Limbic seizures were evoked in freely moving rats by intrahippocampal administration of pilocarpine via a microdialysis probe. NPY was administered intracerebroventricularly, intrahippocampally via the microdialysis probe, or coadministered intrahippocampally with the D2 receptor antagonist remoxipride, the Y2 receptor antagonist BIIE0246 or the sigma 1 receptor antagonist BD1047. Changes in hippocampal extracellular dopamine were monitored, and behavioural changes indicative of seizure activity were scored. Intracerebroventricular (10 nmol/3 microL) and intrahippocampal (20-50 microm) NPY administration increased hippocampal dopamine and attenuated pilocarpine-induced seizures. Hippocampal D2 receptor blockade (4 microm remoxipride) reversed the anticonvulsant effect of NPY. Y2 receptor blockade (1 microm BIIE0246) reversed the anticonvulsant effect of NPY but did not prevent NPY-induced increases in hippocampal dopamine. Sigma 1 receptor blockade (10 microm BD1047) abolished NPY-induced increases in hippocampal dopamine and reversed the anticonvulsant effect of NPY. Our results indicate that NPY-induced increases in hippocampal dopamine are mediated via sigma 1 receptors and contribute to the anticonvulsant effect of NPY via increased activation of hippocampal D2 receptors. This novel mechanism of anticonvulsant action of NPY is separate from, and may be complementary to, the well established Y2 receptor-mediated inhibition of hippocampal excitability.

Mechanism of dopamine mediated inhibition of neuropeptide Y release from pheochromocytoma cells (PC12 cells)

In rat pheochromocytoma (PC12) cells the dopamine D(2) receptor agonists apomorphine (APO) and n-propylnorapomorphine (NPA) produced a concentration dependent inhibition of K(+)-evoked neuropeptide Y release (NPY-ir). The effect of APO was blocked by the dopamine D(2)-receptor antagonist, eticlopride, but not the D(1)/D(3) or the D(4)/D(2) antagonists, SCH23390 or clozapine, respectively. The D(1)/D(5) receptor agonist, SKF38393 or the D(3) agonists PD128907 and 7-OH DPAT had no effect. Selective N and L-type voltage gated Ca(2+) channel blockers, omega-conotoxin GVIa (Ctx-GVIa) and nifedipine, respectively, produced a concentration dependent inhibition of NPY-ir release but were not additive with APO. The Ca(2+)/calmodulin-dependent protein kinase (CaM kinase) II inhibitor KN-62 produced a concentration-dependent inhibition of NPY-ir release but the combination of KN-62 and APO produced no further inhibition. PMA-mediated protein kinase C stimulation significantly increased both basal and K(+)-evoked release of NPY-ir, and in the presence of PMA APO had no inhibitory effect. The PKC antagonist, chelerythrine, inhibited K(+)-evoked NPY-ir release but was not additive with APO. Neither forskolin-mediated adenylate cyclase activation and the active cAMP analog Sp-cAMPS, nor the adenylate cyclase inhibitor SQ 22536, and the competitive inhibitor of cAMP-dependent protein kinases Rp-cAMPS, had any significant effect on K(+)-evoked NPY-ir release. This suggests the inhibitory effect of APO on K(+)-evoked release of NPY-ir from PC12 cells is most likely mediated through activation of dopamine D(2) receptors leading to direct inhibition of N and L-type voltage gated Ca(2+) channels, or indirect inhibition of PKC, both of which would reduce [Ca(2+)](i) and inactivate CaM kinase.

Antidepressant-like effect of icariin and its possible mechanism in mice

The behavioral, neurochemical and neuroendocrine effects of icariin isolated from Epimedium brevicornum were investigated in behavioral despair models of KunMing strain of male mice. Icariin was found to significantly shorten immobility time in the forced swimming test (FST) after orally administration for 21 consecutive days. Icarrin also produced a marked reduction in immobility time in the tail suspension test (TST) when administered for at least 7 consecutive days. The preferable antidepressant action by icariin was obtained at 17.5 and 35 mg/kg in the present study. Moreover, it was observed that the stress of FST exposure induced increases in brain monoamine oxidase (MAO) A and B activities, serum corticotropin-releasing factor (CRF) levels, as well as decreases in brain monoamine neurotransmitter levels. Treatment of icariin for 21 consecutive days mainly reversed the above effects in the mouse FST. These results suggested that icarrin possessed potent antidepressant-like properties that were mediated via neurochemical and neuroendocrine systems.

Role of serotonin (5-HT) in the antidepressant-like properties of neuropeptide Y (NPY) in the mouse forced swim test

Neuropeptide Y (NPY) is thought to be implicated in depressive disorders. The mouse forced swim test (FST) is an animal model widely used as a predictor of the efficacy of antidepressant drugs. The present study was undertaken to explore the possible contribution of endogenous serotonin (5-HT) systems in the behavioral effects elicited by NPY in this model. The selective serotonin re-uptake inhibitor (SSRI), fluoxetine, was also tested for comparison. 5-HT was depleted prior to testing by the administration of the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA; 300 mg/kg, i.p., each day for 3 days; control mice received saline-vehicle over the same period). On the fourth day, mice received NPY (3 nmol, I.C.V.), fluoxetine (16 mg/kg, i.p.) or saline injections before testing in the FST. Both NPY and fluoxetine significantly reduced immobility time in saline-treated control animals. Pre-treatment with PCPA significantly blocked the effects of fluoxetine in the FST, confirming the role of endogenous 5-HT. Similarly, pre-treatment with PCPA also significantly attenuated the anti-immobility effects of NPY, thus suggesting a role for 5-HT in the effects of NPY in the FST. Quantitative receptor autoradiography revealed increases in specific [125I][Leu31, Pro34]PYY sites that were sensitive to BIBP3226 (Y1-like sites) in various brain regions. Specific [125I]GR231118 and [125I]PYY(3-36) binding levels were not changed following PCPA treatment, suggesting that depletion of endogenous 5-HT resulted in an apparent increase in the level of Y1 sites in their high-affinity state. Taken together, these results suggest a role for 5-HT-related systems in the antidepressant-like properties of NPY.

The olfactory bulbectomised rat as a model of depression

Bilateral olfactory bulbectomy results in changes in behavior, and in the endocrine, immune and neurotransmitter systems, that simulates many of those seen in patients with major depression. The olfactory system in the rat forms a part of the limbic region in which the amygdala and hippocampus contribute to the emotional and memory components of behavior. However, the loss of olfaction alone, which results from bulbectomy, is not the major factor that contributes to the behavioral abnormalities as peripherally induced anosmia does not cause the same behavioral changes. Thus it would appear that bulbectomy causes a major dysfunction of the cortical-hippocampal-amygdala circuit that underlies the behavioral and other changes. These neuroanatomical areas also seem to be dysfunctional in the patient with major depression. Chronic, but not acute, administration of antidepressants largely corrects most the behavioral, endocrine, immune and neurotransmitter changes that occur following bulbectomy. Thus the olfactory bulbectomized rat is not only a model for detecting antidepressant activity but also one for exploring the inter-relationships between these systems that are also dysfunctional in patients with major depression.

Y1 receptors regulate aggressive behavior by modulating serotonin pathways

Neuropeptide Y (NPY) is pivotal in the coordinated regulation of food intake, growth, and reproduction, ensuring that procreation and growth occur only when food is abundant and allowing for energy conservation when food is scant. Although emotional and behavioral responses from the higher brain are known to be involved in all of these functions, understanding of the coordinated regulation of emotion/behavior and physiological functions is lacking. Here, we show that the NPY system plays a central role in this process because ablation of the Y1 receptor gene leads to a strong increase in territorial aggressive behavior. After exposure to the resident-intruder test, expression of c-fos mRNA in Y1-knockout mice is significantly increased in the medial amygdala, consistent with the activation of centers known to be important in regulating aggressive behavior. Expression of the serotonin [5-hydroxytryptamine (5-HT)] synthesis enzyme tryptophan hydroxylase is significantly reduced in Y1-deficient mice. Importantly, treatment with a 5-HT-1A agonist, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide, abolished the aggressive behavior in Y1-knockout mice. These results suggest that NPY acting through Y1 receptors regulates the 5-HT system, thereby coordinately linking physiological survival mechanisms such as food intake with enabling territorial aggressive behavior.

Adaptive and maladaptive psychobiological responses to severe psychological stress: implications for the discovery of novel pharmacotherapy

Post-traumatic stress disorder (PTSD) is one of the few DSM-IV diagnoses contingent upon a psychosocial stressor. In this context, there is an urgent need to acquire a better understanding of both the adaptive and maladaptive psychobiological responses to traumatic stress. Preclinical investigators have utilized a variety of animal models to identify the behavioral and neurobiological features of the organism's response to stress. However, given the complexity of the healthy and pathological human response to physiological and psychological stress, the extent to which the animal data is immediately transferable to human remains to be fully determined. This review draws upon preclinical and clinical literature to examine the transformation of an adaptive human stress response into a maladaptive and debilitating mental disorder. An integrative psychobiological model for PTSD is presented, linking psychological processes and behavioral patterns with current findings in neurocircuitry, neurochemistry and psychophysiology. The implications of this model for the discovery of novel pharmacological approaches to the treatment of severe psychological distress are discussed.

Modulatory effects of norepinephrine, acting on alpha 1 receptors in the central nucleus of the amygdala, on behavioral and neuroendocrine responses to acute immobilization stress

The central nucleus of the amygdala (CeA) is a component of the limbic fear-anxiety circuit, and has also been implicated in regulation of the hypothalamic-pituitary-adrenal (HPA) stress axis. The CeA receives dense noradrenergic innervation, and is rich in expression of alpha(1)-adrenergic receptors. We hypothesized that norepinephrine (NE), acting on alpha(1) receptors in CeA, may modulate stress-induced anxiety-like behavioral responses and HPA activation. To investigate the role of alpha(1) adrenergic receptors in CeA on stress-induced behavioral reactivity, the alpha(1) antagonist benoxathian was microinjected bilaterally into CeA of male Sprague-Dawley rats, and anxiety-like behavioral responses to acute immobilization stress were measured on the Social Interaction (SI) test and on the Elevated Plus-maze (EPMZ). Benoxathian dose dependently blocked the reduction in SI time induced by immobilization stress, whereas beta-receptor antagonists had no effect, consistent with an absence of beta-receptors in CeA. By contrast, in separate experiments, benoxathian had no effect on stress-induced reduction in open-arm exploratory behavior on the EPMZ, nor on stress-induced plasma ACTH secretion. These results confirm that the SI test and EPMZ measure different aspects of behavioral stress reactivity that can be modulated independently, and likewise, that noradrenergic modulation of behavioral stress reactivity can occur independently of modulation of the HPA axis.

A 6-hydroxydopamine lesion of the mesostriatal dopamine system decreases the expression of corticotropin releasing hormone and neurotensin mRNAs in the amygdala and bed nucleus of the stria terminalis

The mesostriatal dopamine (DA) system is known to play a vital role in extrapyramidal motor responses, and animals with a unilateral 6-hydroxydopamine (6-OHDA) lesion of this system have proved useful in studying the behavioral and neurobiological effects of DA depletion. Less is known about the role of this system in modulating emotional responses, although a number of lines of evidence suggest that dopamine influences emotional behavior. During the course of a study involving rats that had a unilateral 6-OHDA lesion, we discovered a hemispheric asymmetry in the levels of corticotropin releasing hormone (CRH) mRNA in the central nucleus of the amygdala (CEA). The present study was performed in order to determine (1) if the lesion resulted in a decrease in CRH mRNA, and/or if there was upregulation on the intact side, (2) if a similar imbalance in CRH mRNA was observed in other brain regions and (3) if levels of other neuropeptide mRNAs were affected by the lesion. Adult male Sprague-Dawley rats were left unoperated or were pretreated with desipramine and then injected unilaterally with 6-OHDA into the medial forebrain bundle to lesion the ascending mesostriatal DA neurons. Animals were killed 15-31 days following surgery and brain sections processed for CRH, neurotensin and enkephalin mRNAs by in situ hybridization. Levels of CRH and neurotensin mRNAs were decreased on the lesioned side in the CEA and oval nucleus of the BST (BSTov) relative to the intact side and to unoperated controls. Levels of enkephalin mRNA in these regions were not affected by the lesion. These effects appeared specific, because the lesion did not alter CRH mRNA expression in the ventral BST, paraventricular nucleus of the hypothalamus or cortex or neurotensin mRNA expression in the CA1 region of the hippocampus. In contrast, and consistent with previous reports, levels of neurotensin and enkephalin mRNAs were upregulated on the lesioned side of the striatum. This study provides evidence that the mesostriatal DA system regulates CRH and neurotensin mRNA in the BSTov and CEA, suggesting that dopamine may be an important modulator of CRH and neurotensin function within these nuclei. Although the precise mechanisms are not clear, and the involvement of noradrenergic systems cannot be precluded, data are consistent with the idea that dopamine, released in response to a stressful experience for example, interacts with CRH and neurotensin in the extended amygdala to affect emotional responsiveness.