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A wide body of evidence suggests that 5-HT7 receptors are implicated in a variety of central nervous system functions, including control of learning and memory processes. According to recent preclinical data, the selective blockade of these receptors may be a potential target for cognitive improvement in schizophrenia. The first aim of the present study was to evaluate the effects of the selective 5-HT7 receptor antagonist, SB-269970, and the antipsychotic drug with a high affinity for 5-HT7 receptors, amisulpride, on ketamine-induced deficits in attentional set-shifting and novel object recognition tasks in rats. Because the role of 5-HT7 receptor blockade in ameliorating positive and negative symptoms of schizophrenia remains equivocal, the second aim of these experiments was to examine the effectiveness of SB-269970 and amisulpride in reversing ketamine-induced deficits in prepulse inhibition of the startle reflex and in social interaction test in rats. The study revealed that acute administration of SB-269970 (1 mg/kg) or amisulpride (3 mg/kg) ameliorated ketamine-induced cognitive inflexibility and novel object recognition deficit in rats. Both compounds were also effective in attenuating ketamine-evoked disruption of social interactions. In contrast, neither SB-269970 nor amisulpride affected ketamine-disrupted prepulse inhibition or 50 kHz USVs accompanying social behaviour. In conclusion, antagonism of 5-HT7 receptors may represent a useful pharmacological approach in the treatment of cognitive deficits and some negative symptoms of schizophrenia.

Modulators of the serotonin 5-HT6 receptor (5-HT6R) offer a promising strategy for the treatment of the cognitive deficits that are associated with dementia and Alzheimer's disease. Herein, we report the design, synthesis, and characterization of a novel class of 5-HT6R antagonists that is based on the 1H-pyrrolo[3,2-c]quinoline core. The most active compounds exhibited comparable binding affinity to the reference compound, SB-742457, and markedly improved selectivity. Lead optimization led to the identification of (S)-1-[(3-chlorophenyl)sulfonyl]-4-(pyrrolidine-3-yl-amino)-1H-pyrrolo[3,2-c]quinoline (14) (Ki = 3 nM and Kb = 0.41 nM). Pharmacological characterization of the 5-HT6R's constitutive activity at Gs signaling revealed that 14 behaved as a neutral antagonist, while SB-742457 was classified as an inverse agonist. Both compounds 14 and SB-742457 reversed phencyclidine-induced memory deficits and displayed distinct procognitive properties in cognitively unimpaired animals (3 mg/kg) in NOR tasks. Compounds 14 and SB-742457 were also active in the Vogel test, yet the anxiolytic effect of 14 was 2-fold higher (MED = 3 mg/kg). Moreover, 14 produced, in a 3-fold higher dose (MED = 10 mg/kg), antidepressant-like effects that were similar to those produced by SB-742457 (MED = 3 mg/kg). Together, these data suggest that the 4-(pyrrolidine-3-yl-amino)-1H-pyrrolo[3,2-c]quinoline scaffold is an attractive molecular framework for the development of procognitive agents. The results are promising enough to warrant further detailed mechanistic studies on the therapeutic potential of 5-HT6R antagonists and inverse agonists for the treatment of cognitive decline and depression/anxiety symptoms that are comorbidities of Alzheimer's disease.

These data indicate that ketamine neither produces enduring antidepressant-like effects in rodents nor does it display antidepressant-like behavioral or neurochemical effects after chronic treatment.

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Rationale and objectivesCognitive deficits, including an impaired ability to shift perceptual attentional set, belong to the core features of schizophrenia, are associated with prefrontal cortical dysfunctions, and may involve glutamate NMDA receptors. Although phencyclidine disturbs cognitive flexibility, little is known about the effects of ketamine and other NMDA antagonists that differ in receptor subunit selectivity, particularly in the mouse species.MethodsAt different times following the administration of ketamine, the NMDA NR2B-subtype specific antagonist Ro 25-6981, or the atypical antipsychotic sertindole, male C57Bl/6J mice were investigated in a modified version of attentional set-shifting task (ASST).ResultsSpecific extra-dimensional shift (EDS) deficit was observed in all control mice. As revealed by the increased number of trials, time and errors to reach criterion, ketamine at 10 or 20 mg/kg given 50 min prior to sessions, but not at 10 mg/kg given 3 or 24 h prior to sessions, further worsened the EDS performance. Sertindole (2.5 mg/kg) prevented ketamine-induced cognitive inflexibility, although it did not affect ASST performance when given alone. In contrast to ketamine, Ro 25-6981 at 10 but not 3 mg/kg, reduced the number of trials and errors to criterion, suggesting a facilitation of cognitive flexibility. Finally, as revealed by the number of trials and time to criterion measures, Ro 25-6981 (10 mg/kg) administration to ketamine (10 mg/kg)-pretreated mice inhibited ketamine-induced cognitive inflexibility.ConclusionThe present study provides an improved and reliable mouse ASST protocol and confirms and extends previous findings demonstrating that NR2B subunit-selective antagonists improve cognitive processes.

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