Agmatine-attenuated cognitive and social deficits in subchronic MK-801 model of schizophrenia in rats

Pharmacological profile of agmatine: An in-depth overview

Agmatine, a naturally occurring polyamine derived from arginine via arginine decarboxylase, has been shown to play multifaceted roles in the mammalian body, impacting a wide range of physiological and pathological processes. This comprehensive review delineates the significant insights into agmatine's pharmacological profile, emphasizing its structure and metabolism, neurotransmission and regulation, and pharmacokinetics and function. Agmatine's biosynthesis is highly conserved across species, highlighting its fundamental role in cellular functions. In the brain, comparable to established neurotransmitters, agmatine acts as a neuromodulator, influencing the regulation, metabolism, and reabsorption of neurotransmitters that are key to mood disorders, learning, cognition, and the management of anxiety and depression. Beyond its neuromodulatory functions, agmatine exhibits protective effects across various cellular and systemic contexts, including neuroprotection, nephroprotection, cardioprotection, and cytoprotection, suggesting a broad therapeutic potential. The review explores agmatine's interaction with multiple receptor systems, including NMDA, α2-adrenoceptors, and imidazoline receptors, elucidating its role in enhancing cell viability, neuronal protection, and synaptic plasticity. Such interactions underpin agmatine's potential in treating neurological diseases and mood disorders, among other conditions. Furthermore, agmatine's pharmacokinetics, including its absorption, distribution, metabolism, and excretion, are discussed, underlining the complexity of its action and the potential for therapeutic application. The safety and efficacy of agmatine supplementation, demonstrated through various animal and human studies, affirm its potential as a beneficial therapeutic agent. Conclusively, the diverse physiological and therapeutic effects of agmatine, spanning neurotransmission, protection against cellular damage, and modulation of various receptor pathways, position it as a promising candidate for further research and clinical application. This review underscores the imperative for continued exploration into agmatine's mechanisms of action and its potential in pharmacology and medicine, promising advances in the treatment of numerous conditions.

Chronic MK-801 administration provokes persistent deficits in social memory in the prairie vole (Microtus ochrogaster): A potential animal model for social deficits of schizophrenia

The prairie vole (Microtus ochrogaster) is a rodent species that has been used extensively to study biological aspects of human social bonding. Nevertheless, this species has not been studied in the context of modeling social deficits characteristic of schizophrenia. Building on studies in rodents that show that sub-chronic administration of an NMDA receptor antagonist induces persistent behavioral and neurological characteristics of schizophrenia, we administered MK-801 (0.2 mg/kg, daily, for 7 days) or physiological saline to young adult (45 days old) virgin male voles. At 69 days of age, we paired these males with virgin females. 24 h later, we assessed the males' social investigation of each female across the first 5 min of a three-hour preference test, and side-by-side contact with each female during the last hour of the test. Unlike saline-treated males, MK-801-treated males did not preferentially investigate the unfamiliar female, indicating a deficit in social memory. Although males of both groups preferentially spent time with their female partner, regression analysis revealed that deficits in social memory predicted lower partner preference in MK-801-treated males. We interpret these results in the context of recent studies of the natural history of the prairie vole as well as in the context of cognitive deficits in schizophrenia and propose that the social component of episodic memory might influence an individual's capacity to form and maintain long-term social bonds.

Evaluation of acute and chronic nociception in subchronically administered MK-801-induced rat model of schizophrenia

Patients diagnosed with schizophrenia have been reported to exhibit atypically low pain sensitivity and to vary in their experience of chronic pain. To the best of our knowledge, there has yet to be an animal study that provides information concerning the relationship between models of schizophrenia and pain. In the present study, we investigated several distinct nociceptive behaviors in a translational rat model of schizophrenia (0. 5 mg/kg MK-801, twice a day for 7 days followed by a 7-day washout period). The presence of the expected cognitive deficit was confirmed with novel object recognition (NOR) paradigm prior to nociception testing. MK-801-treated rats with lack of novelty interest in NOR testing showed: hyposensitivity to thermal and mechanical stimuli; short-term hypoalgesia followed by augmented hyperalgesia in response to formalin-induced spontaneous nociception and increased thermal and mechanical hyperalgesia in the complete Freund's adjuvant (CFA) induced chronic pain model. In conclusion, MK-801 induced antinociception effects for thermal stimuli in rats that were consistent with the decreased pain sensitivity observed in schizophrenia patients. Additionally, the amplified biphasic response exhibited by the MK-801 group in the formalin-induced spontaneous nociception test affirms the suitability of the test as a model of acute to delayed pain transition.

Behavioral Consequences of a Combination of Gad1 Haplodeficiency and Adolescent Exposure to an NMDA Receptor Antagonist in Long-Evans Rats

Glutamate decarboxylase 67-kDa isoform (GAD67), which is encoded by the GAD1 gene, is one of the key enzymes that produce GABA. The reduced expression of GAD67 has been linked to the pathophysiology of schizophrenia. Additionally, the excitatory glutamatergic system plays an important role in the development of this disorder. Animal model studies have revealed that chronic blockade of NMDA-type glutamate receptors can cause GABAergic dysfunction and long-lasting behavioral abnormalities. Based on these findings, we speculated that Gad1 haplodeficiency combined with chronic NMDA receptor blockade would lead to larger behavioral consequences relevant to schizophrenia in a rat model. In this study, we administered an NMDAR antagonist, MK-801 (0.2 mg/kg), to CRISPR/Cas9-generated Gad1^+/− rats during adolescence to test this hypothesis. The MK-801 treated Gad1^+/− rats showed a shorter duration in each rearing episode in the open field test than the saline-treated Gad1^+/+ rats. In contrast, immobility in the forced swim test was increased and fear extinction was impaired in Gad1^+/− rats irrespective of MK-801 treatment. Interestingly, the time spent in the center region of the elevated plus-maze was significantly affected only in the saline-treated Gad1^+/− rats. Additionally, the MK-801-induced impairment of the social novelty preference was not observed in Gad1^+/− rats. These results suggest that the synergistic and additive effects of Gad1 haplodeficiency and NMDA receptor blockade during adolescence on the pathogenesis of schizophrenia may be more limited than expected. Findings from this study also imply that these two factors mainly affect negative or affective symptoms, rather than positive symptoms.

α7 nicotinic receptor agonist and positive allosteric modulators differently improved schizophrenia-like cognitive deficits in male rats

The majority of schizophrenia patients have cognitive deficits as a separate symptom cluster independent of positive or negative symptoms. Current medicines, unfortunately, cannot provide clear benefits for cognitive symptoms in patients. Recent findings showed decreased α7 nicotinic acetylcholine receptor (nAChR) expressions in subjects with schizophrenia. α7 nAChR full/partial agonists and positive allosteric modulators (PAMs) may be valuable drug candidates to treat cognitive deficits of disease. This study comparatively investigated the effect of α7 nAChR agonist (A-582941), type I PAM (CCMI), type II PAM (PNU-120596), and the antipsychotic drug (clozapine) on behavioral, molecular, and immunohistochemical parameters in a subchronic MK-801 model of schizophrenia in male rats. Novel object recognition (NOR) and Morris water maze (MWM) tests were performed to evaluate recognition and spatial memories, respectively. Gene and protein expressions of parvalbumin, glutamic acid decarboxylase-67 (GAD67), and α7 nAChR were examined in the rats' hippocampal tissue. The subchronic MK-801 administration produced cognitive deficits in the NOR and MWM tests. It also decreased the protein and gene expressions of parvalbumin, GAD67, and α7 nAChR in the hippocampus. Clozapine, A-582941, and PNU-120596 but not CCMI increased the parvalbumin and α7 nAChR expressions and provided benefits in recognition memory. Interestingly, clozapine and CCMI restored the MK-801 induced deficits on GAD1 expression and spatial memory while A-582941 and PNU-120596 were ineffective. These results indicated that α7 nAChR agonist, type I and type II PAMs may provide benefits in different types of cognitive deficits rather than a complete treatment in schizophrenia.

NMDAR Hypofunction Animal Models of Schizophrenia

The N-methyl-d-aspartate receptor (NMDAR) hypofunction hypothesis has been proposed to help understand the etiology and pathophysiology of schizophrenia. This hypothesis was based on early observations that NMDAR antagonists could induce a full range of symptoms of schizophrenia in normal human subjects. Accumulating evidence in humans and animal studies points to NMDAR hypofunctionality as a convergence point for various symptoms of schizophrenia. Here we review animal models of NMDAR hypofunction generated by pharmacological and genetic approaches, and how they relate to the pathophysiology of schizophrenia. In addition, we discuss the limitations of animal models of NMDAR hypofunction and their potential utility for therapeutic applications.