In the present study we show that central administration of NPS can significantly attenuate MK-801-induced neuropathological and neurochemical changes in rat retrosplenial cortex. NPS also significantly improved MK-801-induced PPI deficits in mice. It has been reported that clozapine can attenuate neuropathological changes (Farber et al., 1996
; Hashimoto et al., 2000
) as well as PPI deficits (Bakshi et al., 1994
; Geyer and Ellenbroek, 2003
) produced by administration of MK-801. Our present data demonstrate that in these paradigms NPS shows a pharmacological profile similar to that of clozapine. Since these paradigms have been validated as models for antipsychotic drug effects under conditions of hypo-glutamatergic neurotransmission, our current data indicate that NPS may have potential atypical antipsychotic activity that might be useful for treatment of schizophrenia.
Although the mechanisms and neuroanatomical substrates underlying the ability of NPS to alleviate MK-801-induced PPI deficits are currently unknown, it is possible that the enhanced arousal produced by NPS could lead to improved behavioral attention that might be required during the acoustic startle reflex. Indeed, studies in human subjects have demonstrated that directed attention can enhance PPI (Heekeren et al., 2004
). An alternative hypothesis to explain the observed effects of NPS on MK-801-induced neuropathological, neurochemical and behavioral changes can be deduced from the abundant co-localization of NPS and glutamate in NPS-positive neurons of the LC area, implying co-release of both transmitters under certain conditions. The present study indicates that under conditions of attenuated neurotransmission at NMDA-type glutamate receptors, NPS is able to overcome the specific deficits or effects produced by the NMDA receptor antagonist MK-801. This observation suggests that under normal conditions NPS might act as a co-transmitter at glutamatergic synapses with potentially synergistic or potentiating functions. Such a functional interaction of NPS receptors and glutamate receptors in postsynaptic neurons might lead to enhanced glutamatergic neurotransmission. Similar interactions have been described for dopamine D1 receptors and NMDA-type glutamate receptors (Lee et al., 2002
) or between prolactin-releasing peptide receptors and GluR2 or GluR3 AMPA receptors (Lin et al., 2001
). Obviously, further neuroanatomical and physiological investigations are needed to support this hypothesis.
The major limitation in psychosis research is the lack of a comprehensive animal model that resembles all aspects of the human disorder. Therefore, scientists have developed separate models for specific dysfunctions of the syndrome that can be validated for pharmacological intervention. Sensory-motor gating deficits are commonly found in schizophrenic patients but are also observed in Huntington's disease, Tourette's syndrome or obsessive-compulsive disorder patients. In the animal model, disruption of sensory-motor gating is induced pharmacologically (either using glutamate antagonists, dopamine agonists or serotonin agonists) or by developmental manipulations (e.g. isolation rearing). The different models of disrupted PPI display different sensitivity to pharmacological treatment and a number of inconsistencies regarding the effect of the prototypical antipsychotic clozapine have been reported from animal studies (reviewed in Geyer et al., 2001
). However, it is generally accepted that the ability of a drug to reverse PPI disruption has predictive validity as a potential antipsychotic effect. We used male ddY mice for PPI experiments because this species and strain has shown robust acoustic startle with reliable prepulse inhibition (Furuya et al., 1999
; Tohmi et al., 2005
; Sakaue et al., 2003
, Zhang et al., 2007
). Furthermore, these mice display significant disruption of PPI after MK-801 administration (Sakaue et al., 2003
; Zhang et al., 2007
) and other competitive NMDA receptor antagonist (Sakaue et al., 2003
). On the other hand, it has been shown earlier that female Sprague-Dawley rats are significantly more sensitive to the neurotoxic effects of NMDA antagonists than male rats (Fix et al., 1995
; Auer 1996
). Also, most studies showing NMDA antagonist-induced ACh release in the retrosplenial cortex have used female Sprague-Dawley rats (Kim et al., 1999
; Farber et al., 2002
) and we, therefore, chose the same gender and strain of rats for our neuropathology and microdialysis studies. The neurobiological basis for the documented differences across species, strain or gender in the different experimental models is currently unknown but certainly rewards further research.
The neuropathological changes produced by NMDA receptor antagonists are thought to be mediated by a complex polysynaptic mechanism involving the interference of GABAergic disinhibition resulting in excessive release of ACh (Olney and Farber, 1995
; Farber et al., 2002
). It has been reported that systemic administration of MK-801 (0.5 mg/kg) significantly increases the extracellular levels of ACh in the rat retrosplenial cortex, and that the α2
-adrenergic agonist clonidine (Kim et al., 1999
), the metabotropic glutamate receptor mGluR type II agonist LY 379268 (Okamura et al., 2003
), and adenosine A1 receptor agonists (Okamura et al., 2004
) significantly suppress both MK-801-induced neuronal vacuolization and MK-801-induced ACh release in rat retrosplenial cortex, suggesting that excessive release of ACh by systemic administration of MK-801 may partly contribute to the neuropathological damages in this region. In this study, we found that central injection of NPS significantly attenuates the increase of ACh levels and neurotoxicity in rat retrosplenial cortex after administration of MK-801. This effect might be mediated by NPS receptors that are particularly expressed in neurons of the retrosplenial cortex and anterior thalamic nuclei (Xu et al., 2004
). Interestingly, it has been reported that bilateral injection of MK-801 (5, 10, 15, 20 μg/μL per side) into the anterior thalamus induced the expression of heat shock protein HSP-70, a reversible marker of neuronal injury, in pyramidal neurons in deep layer III of the rat retrosplenial cortex (Tomitaka et al., 2000
), suggesting that the circuit from anterior thalamus to retrosplenial cortex is implicated in the neuropathological changes observed after administration of MK-801. It is thus likely that inhibitory projections to the retrosplenial cortex from anterior thalamic nuclei that express NPS receptors may play a role in the neuroprotective effects of NPS. Alternatively, it is possible that NPS may modulate glutamatergic neurotransmission directly at the postsynaptic level as discussed above. In the current study we have also investigated the specificity of NPS-induced neuroprotective effects in the retrosplenial cortex by co-administration of the NPSR antagonist SHA 68. However, SHA 68 did not fully antagonize the protective effects of NPS in this assay. Such an incomplete reversal of NPS-induced effects by SHA 68 has been observed previously for NPS-induced hyperlocomotion (Okamura et al., 2008
) and may be caused by limited availability of SHA 68 at NPSR due to its lipophilic character.
A number of studies demonstrated that NMDA receptor antagonists disrupt PPI in rodents, mimicking the clinically observed PPI deficits in schizophrenia patients and supporting the role of glutamatergic systems in the neural circuitry of sensorimotor gating (Geyer et al., 2001
). Most importantly, clozapine improves MK-801-induced PPI deficits in rodents (Geyer and Ellenbroek, 2003
) and PPI deficits in schizophrenic patients (Oranje et al., 2002
), suggesting that NMDA receptor antagonist-induced PPI deficits may serve as a model for sensorimotor gating deficits in schizophrenia. Therefore, the potency to restore MK-801-induced PPI deficits has been used as one of the most reliable screening assays for atypical antipsychotics (Geyer and Ellenbroek, 2003
). Our results show that NPS significantly improved MK-801-induced PPI deficits in mice similar to clozapine, suggesting that activation of NPS receptors might have antipsychotic-like effects. It should be noted that psychostimulants such as cocaine or amphetamine enhance PPI disruption, although less robust than dopamine D2 receptor agonists or NMDA receptor antagonists (Geyer et al., 2001
). Therefore, the present data support the hypothesis that NPS might not be a typical psychostimulant although some of its effects, such as stimulation of locomotor responses and suppression of sleep, resemble the pharmacological profile of the classical stimulants.
In conclusion, the present results suggest that activation of NPS receptors elicits protective effects against neurotoxicity and PPI deficits produced by the NMDA receptor antagonist MK-801. The ability of NPS to alleviate MK-801-induced neuropathological, neurochemical and behavioral symptoms across three experimental models in two different species suggests that the NPS receptor might be an interesting target for development of novel antipsychotic drugs.