Self-administration of the GABAA agonist muscimol into the medial septum: dependence on dopaminergic mechanisms

The role of serotonergic and catecholaminergic systems for possible antidepressant activity of apigenin

Background and objective

Although, the anti-depressant like effects of apigenin (APG) are documented in the literature, the underlying mechanism for exerting such an effect is still not clear. In this research, an attempt was made to determine the possible role of APG for antidepressant activity through serotonergic and catecholaminergic systems using standardized animal models.

Materials and methods

The antidepressant property of APG was determine by involving tail suspension (TST) and modified forced swimming tests (MFST). The effect of APG was evaluated at 25 and 50 mg/kg. In mechanistic models, animals were pretreated with catecholaminergic and serotonergic antagonists prior to administration of APG. The results obtained were statistically analyzed to determine the level of significance.

Results

The period of immobility in both models (TST and MFST) was significantly reduced by APG (25 and 50 mg/kg). The best therapetuic dose of APG (50 mg/kg) was selected for the mechanistic study. The anti-immobility effect of APG declined to a significant extent upon pretreatment with catecholaminergic antagonists (α-methyl-para-tyrosine methyl ester; SCH 23390; sulpiride; phentolamine) and serotonergic inhibitors (p-clorophenylalanine-methyl-ester; ondansetron) in both TST and MFST models. The antidepressant benefits of apigenin were only modestly reversed when rats were given propranolol.

Conclusions

The findings suggest that APG's antidepressant effect is mediated by the α-adrenergic, dopaminergic and 5-HT3 serotonergic receptors.

The Role of the Medial Septum-Associated Networks in Controlling Locomotion and Motivation to Move

The Medial Septum and diagonal Band of Broca (MSDB) was initially studied for its role in locomotion. However, the last several decades were focussed on its intriguing function in theta rhythm generation. Early studies relied on electrical stimulation, lesions and pharmacological manipulation, and reported an inconclusive picture regarding the role of the MSDB circuits. Recent studies using more specific methodologies have started to elucidate the differential role of the MSDB's specific cell populations in controlling both theta rhythm and behaviour. In particular, a novel theory is emerging showing that different MSDB's cell populations project to different brain regions and control distinct aspects of behaviour. While the majority of these behaviours involve movement, increasing evidence suggests that MSDB-related networks govern the motivational aspect of actions, rather than locomotion per se. Here, we review the literature that links MSDB, theta activity, and locomotion and propose open questions, future directions, and methods that could be employed to elucidate the diverse roles of the MSDB-associated networks.

Pharmacological Screening of Mangifera indica Seeds for Antidepressant-like Action Along with a Mechanistic Study

Depression is emerging as a major global issue. There are several antidepressants available in the market, but their efficacy is usually unpredictable. Therefore, there is a need to find an alternative therapeutic agent with better therapeutic efficacy and availability. In the current investigation, the antidepressant-like action of the aqueous methanolic extract of Mangifera indica seeds (25, 50, and 100 mg/kg) was evaluated by two predictive models like the tail suspension test and forced swimming test along with the determination of the mechanism of action working behind this action. The results of the acute treatment with the extract show a dose-dependent reduction in the duration of immobility in both models. The antidepressant-like action of the extract (100 mg/kg) was blocked by the administration of p-chlorophenyl alanine, α-methyl-p-tyrosine, prazosin, and sulpiride while remaining unaffected with propranolol. In contrast, the administration of d-serine along with the extract (a full agonist of glycine/N-methyl-d-aspartate, NMDA, receptors) diminished the anti-immobility action. The administration of the extract along with nitro-l-arginine-methyl ester synergizes into the anti-immobility action of the extract, and intake of l-arginine remained unable to effect this action, whereas sildenafil blocks the effect. The antidepressant-like action of the extract is probably due to the involvement of serotonergic and adrenergic (mainly α receptors are involved) systems, an NMDA receptor complex, and the nitric oxide pathway.

β4-Nicotinic Receptors Are Critically Involved in Reward-Related Behaviors and Self-Regulation of Nicotine Reinforcement

Nicotine addiction, through smoking, is the principal cause of preventable mortality worldwide. Human genome-wide association studies have linked polymorphisms in the CHRNA5-CHRNA3-CHRNB4 gene cluster, coding for the α5, α3, and β4 nicotinic acetylcholine receptor (nAChR) subunits, to nicotine addiction. β4*nAChRs have been implicated in nicotine withdrawal, aversion, and reinforcement. Here we show that β4*nAChRs also are involved in non-nicotine-mediated responses that may predispose to addiction-related behaviors. β4 knock-out (KO) male mice show increased novelty-induced locomotor activity, lower baseline anxiety, and motivational deficits in operant conditioning for palatable food rewards and in reward-based Go/No-go tasks. To further explore reward deficits we used intracranial self-administration (ICSA) by directly injecting nicotine into the ventral tegmental area (VTA) in mice. We found that, at low nicotine doses, β4KO self-administer less than wild-type (WT) mice. Conversely, at high nicotine doses, this was reversed and β4KO self-administered more than WT mice, whereas β4-overexpressing mice avoided nicotine injections. Viral expression of β4 subunits in medial habenula (MHb), interpeduncular nucleus (IPN), and VTA of β4KO mice revealed dose- and region-dependent differences: β4*nAChRs in the VTA potentiated nicotine-mediated rewarding effects at all doses, whereas β4*nAChRs in the MHb-IPN pathway, limited VTA-ICSA at high nicotine doses. Together, our findings indicate that the lack of functional β4*nAChRs result in deficits in reward sensitivity including increased ICSA at high doses of nicotine that is restored by re-expression of β4*nAChRs in the MHb-IPN. These data indicate that β4 is a critical modulator of reward-related behaviors.SIGNIFICANCE STATEMENT Human genetic studies have provided strong evidence for a relationship between variants in the CHRNA5-CHRNA3-CHRNB4 gene cluster and nicotine addiction. Yet, little is known about the role of β4 nicotinic acetylcholine receptor (nAChR) subunit encoded by this cluster. We investigated the implication of β4*nAChRs in anxiety-, food reward- and nicotine reward-related behaviors. Deletion of the β4 subunit gene resulted in an addiction-related phenotype characterized by low anxiety, high novelty-induced response, lack of sensitivity to palatable food rewards and increased intracranial nicotine self-administration at high doses. Lentiviral vector-induced re-expression of the β4 subunit into either the MHb or IPN restored a "stop" signal on nicotine self-administration. These results suggest that β4*nAChRs provide a promising novel drug target for smoking cessation.

Antidepressant-like effect of gallic acid in mice: Dual involvement of serotonergic and catecholaminergic systems

AIMS

This study was planned to examine the antidepressant potency of gallic acid (30 and 60mg/kg), a phenolic acid widely distributed in nature, together with its possible underlying monoaminergic mechanisms.

MAIN METHODS

Antidepressant-like activity was assessed using the tail suspension (TST) and the modified forced swimming tests (MFST). Locomotor activity was evaluated in an activity cage.

KEY FINDINGS

Administration of gallic acid at 60mg/kg reduced the immobility duration of mice in both the TST and MFST without any changes in the locomotor activity. The anti-immobility effect observed in the TST was abolished with pre-treatment of p-chlorophenylalanine methyl ester (an inhibitor of serotonin synthesis; 100mg/kg i.p. administered for 4-consecutive days), ketanserin (a 5-HT2A/2C antagonist; 1mg/kg i.p.), ondansetron (a 5-HT3 antagonist; 0.3mg/kg i.p.), α-methyl-para-tyrosine methyl ester (an inhibitor of catecholamine synthesis; 100mg/kg i.p.), phentolamine (non-selective alpha-adrenoceptor antagonist; 5mg/kg i.p.), SCH 23390 (a dopamine D1 antagonist; 0.05mg/kg s.c.), and sulpiride (a dopamine D2/D3 antagonist; 50mg/kg i.p.). However, NAN 190 (a 5-HT1A antagonist; 0.5mg/kg i.p.) and propranolol (a non-selective β-adrenoceptor antagonist; 5mg/kg i.p.) pre-treatments were ineffective at reversing the antidepressant-like effects of gallic acid.

SIGNIFICANCE

The results of the present study indicate that gallic acid seems to have a dual mechanism of action by increasing not only serotonin but also catecholamine levels in synaptic clefts of the central nervous system. Further alpha adrenergic, 5-HT2A/2C and 5-HT3 serotonergic, and D1, D2, and D3 dopaminergic receptors also seem to be involved in this antidepressant-like activity.

Corticosterone and dopamine D2/D3 receptors mediate the motivation for voluntary wheel running in C57BL/6J mice

Physical exercise can improve cognition but whether this is related to motivation levels is unknown. Voluntary wheel running is a rewarding activity proposed as a model of motivation to exercise. To question the potential effects of exercise motivation on subsequent behaviour, we used a pharmacological approach targeting some reward mechanisms. The stress hormone corticosterone has rewarding effects mediated by activation of low affinity glucocorticoid receptors (GR). To investigate whether corticosterone synthesis motivates exercise via activation of GRs and subsequently, impacts on behaviour, we treated C57BL/6J mice acutely with the inhibitor of corticosterone synthesis metyrapone (35mg/kg) or repeatedly with the GR antagonist mifepristone (30mg/kg) prior to 1-h running wheel sessions. To investigate whether reducing motivation to exercise impacts on behaviour, we antagonised running-induced dopamine D2/D3 receptors activation with sulpiride (25 or 50mg/kg) and assessed locomotor, anxiety-related and memory performance after 20 running sessions over 4 weeks. We found that corticosterone synthesis contributes to running levels, but the maintenance of running behaviour was not mediated by activation of GRs. Intermittent exercise was not associated with changes in behavioural or cognitive performance. The persistent reduction in exercise levels triggered by sulpiride also had limited impact on behavioural performance, although the level of performance for some behaviours was related to the level of exercise. Altogether, these findings indicate that corticosterone and dopamine D2/D3 receptor activation contribute to the motivation for wheel running, but suggest that motivation for exercise is not a sufficient factor to alter behaviour in healthy mice.

Alcohol consumption increases locomotion in an open field and induces Fos-immunoreactivity in reward and approach/withdrawal-related neurocircuitries

Drug addiction is a chronically relapsing disorder characterized by compulsion to seek and take the drug, loss of control in limiting intake and, eventually, the emergence of a negative emotional state when access to the drug is prevented. Both dopamine and corticotropin-releasing factor (CRF)-mediated systems seem to play important roles in the modulation of alcohol abuse and dependence. The present study investigated the effects of alcohol consumption on anxiety and locomotor parameters and on the activation of dopamine and CRF-innervated brain regions. Male Wistar rats were given a choice of two bottles for 31 days, one containing water and the other a solution of saccharin + alcohol. Control animals only received water and a solution of 0.2% saccharin. On the 31st day, animals were tested in the elevated plus-maze and open field, and euthanized immediately after the behavioral tests. An independent group of animals was treated with ethanol and used to measure blood ethanol concentration. Results showed that alcohol intake did not alter behavioral measurements in the plus-maze, but increased the number of crossings in the open field, an index of locomotor activity. Additionally, alcohol intake increased Fos-immunoreactivity (Fos-ir) in the prefrontal cortex, in the shell region of the nucleus accumbens, in the medial and central amygdala, in the bed nucleus of the stria terminalis, in the septal region, and in the paraventricular and dorsomedial hypothalamus, structures that have been linked to reward and to approach/withdrawal behavior. These observations might be relevant to a better understanding of the behavioral and physiological alterations that follow alcohol consumption.

Glycine blocks long-term potentiation of GABAergic synapses in the ventral tegmental area

The mesocorticolimbic dopamine system, originating in the ventral tegmental area (VTA) is normally constrained by GABA-mediated synaptic inhibition. Accumulating evidence indicates that long-term potentiation of GABAergic synapses (LTPGABA) in VTA dopamine neurons plays an important role in the actions of drugs of abuse, including ethanol. We previously showed that a single infusion of glycine into the VTA of rats strongly reduces ethanol intake for 24h. In the current study, we examined the effect of glycine on the electrophysiological activities of putative dopamine VTA neurons in midbrain slices from ethanol-naïve rats. We report here that a 15-min exposure to 10 μM glycine prevented trains of high-frequency stimulation (HFS) from producing LTPGABA, which was rescued by the glycine receptor (GlyR) antagonist strychnine. Glycine also concentration-dependently decreased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs). By contrast, glycine pretreatment did not prevent potentiation of inhibitory postsynaptic currents (IPSCs) during a continuous exposure to the nitric oxide (NO) donor, SNAP (S-nitroso-N-acetylpenicillamine), or a brief exposure to 10 μM glycine and 10 μM NMDA (N-methyl-D-aspartate), an agonist of NMDA-type glutamate receptors. Thus, the blockade of LTPGABA by glycine is probably resulted from suppressing glutamate release by activating the GlyRs on the glutamatergic terminals. This effect of glycine may contribute to the reduction in ethanol intake induced by intra-VTA glycine observed in vivo.

Enlarged cavum septum pellucidum as a neurodevelopmental marker in adolescent-onset opiate dependence

Objective

Adolescent-onset exposure to highly addictive substances such as opiates may induce far-reaching deleterious effects on later mental and physical health. However, little is known about the neurodevelopmental basis for adolescent-onset opiate dependence. Here we examined whether having an abnormally large cavum septum pellucidum (CSP), a putative marker of limbic structural maldevelopment, is associated with opiate dependence particularly beginning in adolescence.

Method

The overall length of the CSP and the prevalence of abnormal enlargement of the CSP were assessed and compared in 65 opiate-dependent subjects (41 adolescent-onset opiate users and 24 adult-onset opiate users) and 67 healthy subjects.

Results

Opiate-dependent subjects showed a greater prevalence of abnormal CSP enlargement relative to healthy subjects (odds ratio [OR]=3.64, p=0.034). The overall CSP length of adolescent-onset opiate-dependent subjects was greater, as compared not only with healthy subjects (F_1,104=11.03, p=0.001) but also with those who began opiate use during adulthood (F_1,61=4.43, p=0.039).

Conclusions

The current findings provide the first evidence that abnormal CSP enlargement, which reflects limbic system dysgenesis of neurodevelopmental origin, may be linked to later development of opiate dependence. In addition, a greater CSP length, which indicates more severe limbic abnormalities, appears to confer higher risk for earlier onset of opiate use.

Neurocircuitry of drug reward

In recent years, neuroscientists have produced profound conceptual and mechanistic advances on the neurocircuitry of reward and substance use disorders. Here, we will provide a brief review of intracranial drug self-administration and optogenetic self-stimulation studies that identified brain regions and neurotransmitter systems involved in drug- and reward-related behaviors. Also discussed is a theoretical framework that helps to understand the functional properties of the circuitry involved in these behaviors. The circuitry appears to be homeostatically regulated and mediate anticipatory processes that regulate behavioral interaction with the environment in response to salient stimuli. That is, abused drugs or, at least, some may act on basic motivation and mood processes, regulating behavior-environment interaction. Optogenetics and related technologies have begun to uncover detailed circuit mechanisms linking key brain regions in which abused drugs act for rewarding effects. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Anti-depressant-like effect of vitexin in BALB/c mice and evidence for the involvement of monoaminergic mechanisms

The present study was designed to investigate the putative effect of vitexin, a flavone C-glucoside present in some drugs, medicinal plants and nutraceuticals, on the central nervous system. Vitexin (10-30 mg/kg) did not show significant alterations in the behaviour of mice tested in hole-board, plus-maze or activity cage tests. However, immobility time of the mice significantly reduced by vitexin administrations in both the tail-suspension and modified forced swimming tests. The anti-immobility effect of vitexin in the tail-suspension test was reversed with α-methyl-para-tyrosine methyl ester (AMPT, an inhibitor of catecholamine synthesis, 100mg/kg, i.p.), yohimbine (an α(2)-adrenoceptor antagonist, 1mg/kg, i.p.), NAN 190 (a 5-HT(1A) antagonist, 0.5mg/kg, i.p.), SCH 23390 (a dopamine D(1) antagonist, 0.05 mg/kg, s.c.) and sulpiride (a dopamine D(2)/D(3) antagonist, 50mg/kg, i.p.). The same effect was not reversed, however, by p-chlorophenylalanine methyl ester (PCPA; an inhibitor of serotonin synthesis 100mg/kg, i.p., administered for 4 consecutive days), ketanserin (a 5-HT(2A/2C) antagonist, 1-4 mg/kg, i.p.), ondansetron (a 5-HT(3) antagonist, 0.1-0.4 mg/kg, i.p.), prazosin (an α(1)-adrenoceptor antagonist, 1-4 mg/kg, i.p.), or propranolol (a non-selective β-adrenoceptor antagonist, 5-20mg/kg, i.p.). These results suggest that the anti-depressant-like effect of vitexin is mediated through an increase in catecholamine levels in the synaptic cleft as well as through interactions with the serotonergic 5-HT(1A), noradrenergic α(2), and dopaminergic D(1), D(2), and D(3) receptors. To our knowledge, this is the first study to show findings that indicate an anti-depressant-like effect of vitexin and its underlying mechanisms.

GABA neurons of the VTA drive conditioned place aversion

Salient but aversive stimuli inhibit the majority of dopamine (DA) neurons in the ventral tegmental area (VTA) and cause conditioned place aversion (CPA). The cellular mechanism underlying DA neuron inhibition has not been investigated and the causal link to behavior remains elusive. Here, we show that GABA neurons of the VTA inhibit DA neurons through neurotransmission at GABA(A) receptors. We also observe that GABA neurons increase their firing in response to a footshock and provide evidence that driving GABA neurons with optogenetic effectors is sufficient to affect behavior. Taken together, our data demonstrate that synaptic inhibition of DA neurons drives place aversion.

Functional correlation between GABAergic and dopaminergic systems of dorsal hippocampus and ventral tegmental area in passive avoidance learning in rats

The aim of the present study was to investigate the existence of possible functional correlation between GABA-A and dopamine (DA) receptors of the dorsal hippocampus and the ventral tegmental area (VTA) in passive avoidance learning. Two guide cannulas were stereotaxically implanted in the CA1 region of the dorsal hippocampus and the VTA of male Wistar rats. In order to measure memory retrieval, the animals were trained in a step-through type passive avoidance task and tested 24 h after training. Post-training intra-CA1 administration of a GABA-A receptor agonist, muscimol (0.01-0.02 μg/rat) dose-dependently impaired memory retrieval. Post-training intra-VTA administration of SCH23390 (a dopamine D1 receptor antagonist; 0.1-0.8 μg/rat) or sulpiride (a D2 receptor antagonist; 0.5-1.5 μg/rat) decreased the inhibitory effect of muscimol (0.02 μg/rat, intra-CA1) on memory retrieval. Intra-VTA administration of the same doses of SCH23390, but not sulpiride, decreased the step-through latencies. On the other hand, post-training administration of muscimol (0.02 μg/rat) into the VTA inhibited memory retrieval. The administration of SCH23390 (0.01-0.2 μg/rat) or sulpiride (0.1-1 μg/rat) into the CA1 region, immediately after training, had no effect on memory retrieval. Furthermore, the amnesic effect of intra-VTA administration of muscimol was significantly decreased by intra-CA1 administration of sulpiride (0.5 and 1 μg/rat, intra-CA1), but not SCH23390. The practical conclusion is that the relationship between the hippocampus and the VTA may regulate memory formation in passive avoidance learning. Also, the correlation between the hippocampus and VTA by a dopaminergic system may be involved in mediating muscimol-induced amnesia.

Brain reward circuitry beyond the mesolimbic dopamine system: a neurobiological theory

Reductionist attempts to dissect complex mechanisms into simpler elements are necessary, but not sufficient for understanding how biological properties like reward emerge out of neuronal activity. Recent studies on intracranial self-administration of neurochemicals (drugs) found that rats learn to self-administer various drugs into the mesolimbic dopamine structures-the posterior ventral tegmental area, medial shell nucleus accumbens and medial olfactory tubercle. In addition, studies found roles of non-dopaminergic mechanisms of the supramammillary, rostromedial tegmental and midbrain raphe nuclei in reward. To explain intracranial self-administration and related effects of various drug manipulations, I outlined a neurobiological theory claiming that there is an intrinsic central process that coordinates various selective functions (including perceptual, visceral, and reinforcement processes) into a global function of approach. Further, this coordinating process for approach arises from interactions between brain structures including those structures mentioned above and their closely linked regions: the medial prefrontal cortex, septal area, ventral pallidum, bed nucleus of stria terminalis, preoptic area, lateral hypothalamic areas, lateral habenula, periaqueductal gray, laterodorsal tegmental nucleus and parabrachical area.

Administration of the GABAA receptor antagonist picrotoxin into rat supramammillary nucleus induces c-Fos in reward-related brain structures. Supramammillary picrotoxin and c-Fos expression

BACKGROUND

Picrotoxin blocks GABAA receptors, whose activation typically inhibits neuronal firing activity. We recently found that rats learn to selectively self-administer picrotoxin or bicuculline, another GABAA receptor antagonist, into the supramammillary nucleus (SuM), a posterior hypothalamic structure localized anterior to the ventral tegmental area. Other drugs such as nicotine or the excitatory amino acid AMPA are also self-administered into the SuM. The SuM appears to be functionally linked with the mesolimbic dopamine system and is closely connected with other brain structures that are implicated in motivational processes, including the prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus. Here, we hypothesized that these brain structures are activated by picrotoxin injections into the SuM.

RESULTS

Picrotoxin administration into the SuM markedly facilitated locomotion and rearing. Further, it increased c-Fos expression in this region, suggesting blockade of tonic inhibition and thus the disinhibition of local neurons. This manipulation also increased c-Fos expression in structures including the ventral tegmental area, medial shell of the nucleus accumbens, medial prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus.

CONCLUSIONS

Picrotoxin administration into the SuM appears to disinhibit local neurons and recruits activation of brain structures associated with motivational processes, including the mesolimbic dopamine system, prefrontal cortex, septal area, preoptic area, lateral hypothalamic area and dorsal raphe nucleus. These regions may be involved in mediating positive motivational effects triggered by intra-SuM picrotoxin.

Ketamine-induced deficit of auditory gating in the hippocampus of rats is alleviated by medial septal inactivation and antipsychotic drugs

RATIONALE

Gating of sensory responses is impaired in schizophrenic patients and animal models of schizophrenia. Ketamine, an N-methyl-D-aspartate receptor antagonist, is known to induce schizophrenic-like symptoms including sensory gating deficits in humans.

OBJECTIVE

This study aims to investigate the mechanisms underlying ketamine's effect on gating of auditory evoked potentials in the hippocampus of freely moving rats.

METHODS

Gating was measured by the ratio of the test-click response (T) to the conditioning-click response (C), or T/C, with T and C measured as peak amplitudes.

RESULTS

Ketamine (1, 3, or 6 mg/kg s.c.) injection dose-dependently increased T/C ratio as compared to saline injection (s.c.). T/C ratio was 0.48 +/- 0.05 after saline injection and 0.73 +/- 0.17 after ketamine (3 mg/kg s.c.) injection. The increase in T/C ratio after ketamine was blocked by prior inactivation of the medial septum with GABA(A) receptor agonist muscimol or by systemic administration of antipsychotic drugs, including chlorpromazine (5 mg/kg i.p.), haloperidol (1 mg/kg i.p.), or clozapine (7.5 mg/kg i.p.). Infusion of muscimol into the medial septum or injection of an antipsychotic drug alone did not affect the T/C ratio. However, rats with selective lesion of the septohippocampal cholinergic neurons by 192 IgG-saporin showed a significantly higher T/C ratio (0.86 +/- 0.10) than sham lesion rats (0.26 +/- 0.07), and ketamine did not further increase T/C ratio in rats with septohippocampal cholinergic neuron lesion.

CONCLUSIONS

Ketamine's disruption of hippocampal auditory gating was normalized by inactivation of the medial septum; in addition, septal cholinergic neurons participate in normal auditory gating.