The α₂-adrenergic antagonist idazoxan counteracts prepulse inhibition deficits caused by amphetamine or dizocilpine in rats

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

Effects of Manipulation of Noradrenergic Activities on the Expression of Dopaminergic Phenotypes in Aged Rat Brains

This study investigated the effects of the pharmacological manipulation of noradrenergic activities on dopaminergic phenotypes in aged rats. Results showed that the administration of L-threo-3,4-dihydroxyphenylserine (L-DOPS) for 21 days significantly increased the expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the striatum and substantia nigra (SN) of 23-month-old rats. Furthermore, this treatment significantly increased norepinephrine/DA concentrations in the striatum and caused a deficit of sensorimotor gating as measured by prepulse inhibition (PPI). Next, old rats were injected with the α2-adrenoceptor antagonist 2-methoxy idazoxan or β2-adrenoceptor agonist salmeterol for 21 days. Both drugs produced similar changes of TH and DAT in the striatum and SN. Moreover, treatments with L-DOPS, 2-methoxy idazoxan, or salmeterol significantly increased the protein levels of phosphorylated Akt in rat striatum and SN. However, although a combination of 2-methoxy idazoxan and salmeterol resulted in a deficit of PPI in these rats, the administration of 2-methoxy idazoxan alone showed an opposite behavioral change. The in vitro experiments revealed that treatments with norepinephrine markedly increased mRNAs and proteins of ATF2 and CBP/p300 and reduced mRNA and proteins of HDAC2 and HDAC5 in MN9D cells. A ChIP assay showed that norepinephrine significantly increased CBP/p300 binding or reduced HDAC2 and HDAC5 binding on the TH promoter. The present results indicate that facilitating noradrenergic activity in the brain can improve the functions of dopaminergic neurons in aged animals. While this improvement may have biochemically therapeutic indication for the status involving the degeneration of dopaminergic neurons, it may not definitely include behavioral improvements, as indicated by using 2-methoxy idazoxan only.

Therapeutic Potential of Selectively Targeting the α2C-Adrenoceptor in Cognition, Depression, and Schizophrenia-New Developments and Future Perspective

α_2A- and α_2C-adrenoceptors (ARs) are the primary α₂-AR subtypes involved in central nervous system (CNS) function. These receptors are implicated in the pathophysiology of psychiatric illness, particularly those associated with affective, psychotic, and cognitive symptoms. Indeed, non-selective α₂-AR blockade is proposed to contribute toward antidepressant (e.g., mirtazapine) and atypical antipsychotic (e.g., clozapine) drug action. Both α_2C- and α_2A-AR share autoreceptor functions to exert negative feedback control on noradrenaline (NA) release, with α_2C-AR heteroreceptors regulating non-noradrenergic transmission (e.g., serotonin, dopamine). While the α_2A-AR is widely distributed throughout the CNS, α_2C-AR expression is more restricted, suggesting the possibility of significant differences in how these two receptor subtypes modulate regional neurotransmission. However, the α_2C-AR plays a more prominent role during states of low endogenous NA activity, while the α_2A-AR is relatively more engaged during states of high noradrenergic tone. Although augmentation of conventional antidepressant and antipsychotic therapy with non-selective α₂-AR antagonists may improve therapeutic outcome, animal studies report distinct yet often opposing roles for the α_2A- and α_2C-ARs on behavioral markers of mood and cognition, implying that non-selective α₂-AR antagonism may compromise therapeutic utility both in terms of efficacy and side-effect liability. Recently, several highly selective α_2C-AR antagonists have been identified that have allowed deeper investigation into the function and utility of the α_2C-AR. ORM-13070 is a useful positron emission tomography ligand, ORM-10921 has demonstrated antipsychotic, antidepressant, and pro-cognitive actions in animals, while ORM-12741 is in clinical development for the treatment of cognitive dysfunction and neuropsychiatric symptoms in Alzheimer's disease. This review will emphasize the importance and relevance of the α_2C-AR as a neuropsychiatric drug target in major depression, schizophrenia, and associated cognitive deficits. In addition, we will present new prospects and future directions of investigation.

Investigation of the role of alpha-2 adrenergic receptors on prepulse inhibition of acoustic startle reflex in rats

OBJECTIVES

Alpha-2 adrenergic receptors target several behavioral functions. These receptors may connect with the brain pathways mediating sensorimotor gating system that associate with psychoses, and the literature that investigate the relationship between alpha-2 receptors and sensorimotor gating system is very limited and some results are controversial. Thus, we aimed to investigate the role of alpha-2 receptors on prepulse inhibition (PPI) of acoustic startle reflex which is a measure of sensorimotor gating.

EXPERIMENTAL DESIGN

Adult male Wistar rats were subjects. PPI was measured as the per cent inhibition of the startle reflex produced by a startling pulse stimulus. The average PPI levels were used in the further analyses. Clonidine (0.03-1 mg/kg), an agonist of alpha-2 receptors, idazoxan (10 mg/kg), an antagonist alpha-2 receptors, and saline were injected to rats intraperitoneally. PPI was evaluated at two different startle intensity levels (78 and 86 dB, respectively).

PRINCIPAL OBSERVATIONS

Treatments produced some significant changes on PPI of startle reflex at all two levels of startle intensity. While clonidine (0.06, 0.25, 0.5, and 1 mg/kg) disrupted significantly PPI, idazoxan (10 mg/kg) did not produce any significant effect on PPI. However, pretreatment with idazoxan reversed significantly clonidine-induced disruption of PPI. Neither idazoxan (10 mg/kg) nor clonidine (1 mg/kg) produces any significant change on locomotor activity in naive rats.

CONCLUSION

Because idazoxan and clonidine also act through imidazoline receptors, our results suggest that alpha-2 and/or imidazoline receptors are associated with PPI of acoustic startle reflex in rats. Stimulation of these receptors may cause sensorimotor gating disturbances.

Effects of ziprasidone, SCH23390 and SB277011 on spatial memory in the Morris water maze test in naive and MK-801 treated mice

Introduction: Patients with schizophrenia have cognitive dysfunctions; positive psychotic symptoms are the primary purposes for schizophrenia treatment. Improvements in cognitive function should be a characteristic of all newly developed drugs for the treatment of schizophreniawith dementia. Thus,we investigated the effects of the second-generation antipsychotic ziprasidone, dopamine D1 antagonist SCH-23390 and dopamine D3 antagonist SB-277011 on spatial learning and memory. Materials and methods: Male inbred mice were used. The effects of ziprasidone, SCH-23390 and SB-277011 were investigated using the Morris water maze test. Results: Ziprasidone (0.5 and 1mg/kg), SCH-23390 (0.05 and 0.1 mg/kg) and SB-277011 (10 and 20 mg/kg) had no effect on the time spent in the target quadrant in naive mice.MK-801 (0.1mg/kg) significantly decreased the time spent in the target quadrant. The time spent in the target quadrant was significantly prolonged by Ziprasidone (0.5 and 1 mg/kg) and SCH-23390 (0.1 mg/kg), but not with SB-277011 (20 mg/kg) in MK-801-treated mice. Ziprasidone (0.5 and 1mg/kg), SCH-23390 (0.05 and 0.1 mg/kg) and SB-277011 (10 and 20 mg/kg) had no effect on themean distance to the platformin naivemice.MK-801 significantly increased themean distance to the platform. Ziprasidone (1 mg/kg) and SCH-23390 (0.1 mg/kg) significantly decreased the mean distance to the platform in MK-801-treated mice, but SB-277011 (20 mg/kg) didn't. MK-801 significantly increased the total distance moved. Ziprasidone (0.5 and 1 mg/kg), SCH-23390 (0.05 and 0.1 mg/kg) and SB-277011 (10 and 20 mg/kg) had no effect on the total distance moved in naive mice. Ziprasidone (1 mg/kg) and SCH-23390 (0.1 mg/kg) significantly decreased the total distance moved in MK-801-treated mice, but SB-277011 (20 mg/kg) didn't. Conclusions: The second-generation antipsychotic drug ziprasidone and D1 antagonist SCH23390, but not the D3 antagonist SB277011, might be clinically useful for the treatment of cognitive impairments in patients with schizophrenia.

Role of nicotinic receptors in the lateral habenula in the attenuation of amphetamine-induced prepulse inhibition deficits of the acoustic startle response in rats

RATIONALE

Prepulse inhibition (PPI) refers to the reduction of the startle response magnitude when a startling stimulus is closely preceded by a weak stimulus. PPI is commonly used to measure sensorimotor gating. In rats, the PPI reduction induced by the dopamine agonist apomorphine can be reversed by systemic administration of nicotine. A high concentration of nicotinic receptors is found in the lateral habenula (LHb), an epithalamic structure with efferent projections to brain regions involved in the modulation of PPI, which has been shown to regulate the activity of midbrain dopamine neurons.

OBJECTIVES

The prospective role of nicotinic receptors in the LHb in the regulation of PPI was assessed in this study, using different pharmacological models of sensorimotor gating deficits.

METHODS

Interactions between systemic amphetamine and haloperidol and intra-LHb infusions of mecamylamine (10 μg/side) or nicotine (30 μg/side) on PPI were analyzed in Experiments 1 and 2. Intra-LHb infusions of different nicotine doses (25, and 50 μg/side) and their interactions with systemic administration of amphetamine or dizocilpine on PPI were examined in Experiments 3 and 4.

RESULTS

Infusions of nicotine into the LHb dose-dependently attenuated amphetamine-induced PPI deficits but had no effect on PPI disruptions caused by dizocilpine. Intra-LHb mecamylamine infusions did not affect PPI nor interact with dopaminergic manipulations.

CONCLUSIONS

These results are congruent with previous reports of systemic nicotine effects on PPI, suggesting a role of the LHb in the attenuation of sensorimotor gating deficits caused by the hyperactivity of dopamine systems.

Effects of the nicotinic α7 receptor partial agonist GTS-21 on NMDA-glutamatergic receptor related deficits in sensorimotor gating and recognition memory in rats

RATIONALE

Disturbances in information processing and cognitive function are key features of schizophrenia. Nicotinic α7 acetylcholine receptors (α7-nAChR) are involved in sensory gating and cognition, thereby representing a viable therapeutic strategy.

OBJECTIVES AND METHODS

We investigated the effects of GTS-21, an α7-nAChR partial agonist, on prepulse inhibition (PPI) of acoustic startle in two pharmacologic impairment models in Wistar male rats: NMDA-glutamate receptor antagonism by MK-801 and dopamine receptor agonism by apomorphine. The cognitive effects of GTS-21 were assessed using the object recognition task (ORT) at short (3 h) and long (48 h) delays in Sprague-Dawley male rats. Pharmacological specificity was assessed by methyllycaconitine (MLA) coadministration with GTS-21.

RESULTS

In the PPI task, GTS-21 (1-10 mg/kg) alone did not alter the PPI response or startle amplitude. Coadministration of GTS-21 with MK-801 (0.1 mg/kg) or apomorphine (0.5 mg/kg) abolished the pharmacologic-induced PPI impairment as did the antipsychotics clozapine (5.0 mg/kg) and haloperidol (0.3 mg/kg). MK-801 alone increased startle amplitude which was blocked by GTS-21. In the ORT, GTS-21 (0.1-10 mg/kg) reversed the MK-801 (0.08 mg/kg)-induced memory deficit at the 3 h delay and enhanced memory at the 48 h delay, an effect abolished by MLA (0.313-5 mg/kg).

CONCLUSIONS

The results extend our preclinical pharmacological understanding of GTS-21 to include the ability of GTS-21 to modulate NMDA-glutamate receptor function, in vivo. Given the role of NMDA-glutamate receptor involvement in schizophrenia, α7-nAChR agonists may represent a novel treatment strategy for the pathophysiological deficits of schizophrenia and other psychiatric disorders.

The noradrenergic projection from the locus coeruleus to the cochlear root neurons in rats

The cochlear root neurons (CRNs) are key components of the primary acoustic startle circuit; mediating auditory alert and escape behaviors in rats. They receive a great variety of inputs which serve to elicit and modulate the acoustic startle reflex (ASR). Recently, our group has suggested that CRNs receive inputs from the locus coeruleus (LC), a noradrenergic nucleus which participates in attention and alertness. Here, we map the efferent projection patterns of LC neurons and confirm the existence of the LC-CRN projection using both anterograde and retrograde tract tracers. Our results show that each LC projects to the CRNs of both sides with a clear ipsilateral predominance. The LC axons terminate as small endings distributed preferentially on the cell body and primary dendrites of CRNs. Using light and confocal microscopy, we show a strong immunoreactivity for tyrosine hydroxylase and dopamine β-hydroxylase in these terminals, indicating noradrenaline release. We further studied the noradrenergic system using gene expression analysis (RT-qPCR) and immunohistochemistry to detect specific noradrenergic receptor subunits in the cochlear nerve root. Our results indicate that CRNs contain a noradrenergic receptor profile sufficient to modulate the ASR, and also show important gender-specific differences in their gene expression. 3D reconstruction analysis confirms the presence of sexual dimorphism in the density and distribution of LC neurons. Our study describes a coerulean noradrenergic projection to the CRNs that might contribute to neural processes underlying sensory gating of the ASR, and also provides an explanation for the gender differences observed in the behavioral paradigm.

Differential effects of the antidepressant mirtazapine on amphetamine- and dizocilpine-induced PPI deficits

Prepulse inhibition (PPI) refers to the decrease in motor startle response to salient sensory stimuli (pulses) when they are closely preceded in time by another more modest sensory stimulus (prepulse). PPI deficits can be induced by stimulation of dopamine receptors (e.g., amphetamine or apomorphine) or blockade of NMDA glutamate receptors (e.g., dizocilpine or PCP). Previously we found that antagonists of α(2)-noradrenergic and H(1)-histaminergic receptors significantly attenuate PPI impairments caused by amphetamine or dizocilpine. In the current study we assessed the effects of the antidepressant mirtazapine, which has combined antagonist effects at α(2)-noradrenergic, H(1)-histaminergic and 5-HT serotonergic receptors, on amphetamine- and dizocilpine-induced PPI deficits. In Experiment 1, rats were tested for PPI of the startle response to a tactile air-puff stimulus after auditory prepulses of three different intensities. Drug treatments consisted of combinations of amphetamine (0 and 1mg/kg) and mirtazapine (0, 0.5, 1, 2, and 5mg/kg), with all rats receiving all drug doses and combinations with different counterbalanced orders. In Experiment 2, a different group of rats was tested with drug treatments consisting of combinations of dizocilpine (0 and 0.05 mg/kg) and mirtazapine (0, 0.5, 1, 2, and 5 mg/kg). In Experiment 1 amphetamine (1 mg/kg) significantly reduced PPI whereas mirtazapine caused the opposite effect, with the highest dose of mirtazapine (5 mg/kg) effectively reversing the amphetamine-induced PPI deficit. In Experiment 2 dizocilpine (0.05 mg/kg) significantly reduced PPI, but mirtazapine did not have a significant effect on the inhibition of the startle response. These results indicate that the potential beneficial effects of combined α-adrenergic, 5-HT, and H(1) receptor blockade in counteracting PPI deficits may be associated to cases of sensorimotor gating disorders mediated by dopamine, but not necessarily to NMDA glutamate-induced PPI impairments.