No abstract

Major depression is one of the most common affective disorders caused by schizophrenia. The administration of N-methyl-D-aspartate receptor antagonists, such as ketamine, can reproduce the negative and affective symptoms of this disorder in animals. Preclinical studies have shown that 5-HT6 receptor (5-HT6R) agonists and antagonists have a considerable antipsychotic response. The aim of the present study was to evaluate the effect of an acute treatment with an agonist, E-6837, and an antagonist, SB-271046, of 5-HT6R on the immobility induced in mice by a subchronic ketamine regimen (5 days; 10 mg/kg/day, intraperitoneal). Repeated ketamine administration alone increased the immobility time in the forced-swimming test and the tail-suspension test. E-6837 at 10 and 20 mg/kg caused a significant reduction of immobility in the tail-suspension test and forced-swimming test, respectively. Interestingly, SB-271046 (10 mg/kg) also elicited an antidepressant-like effect in both tests. The current findings suggest an important role for these 5-HT6R ligands as mood modulators. However, it is necessary to explore the physiological mechanisms involved in this process in greater detail.

Ketamine is an anesthetic agent that antagonizes N-methyl-d-aspartate receptors, inducing psychotic-like symptoms in healthy humans and animals. This agent has been used as a pharmacological tool for studying biochemical and physiological mechanisms underlying the clinical manifestations of schizophrenia. The main goal of this study was to evaluate the effect of repeated injections of ketamine (5 and 10 mg/kg, i.p., daily for 5 days) on recognition memory and neuronal morphology in ICR-CD1 mice. This treatment induced recognition memory impairment in the novel object recognition test and a decrease in dendritic spines density in both dorsal striatum and CA1-hippocampus. Sholl analysis showed that both ketamine doses decrease the dendritic arborization in ventromedial prefrontal cortex, dorsal striatum, and CA1-hippocampus. Finally, dendritic spines morphology was modified by both doses; that is, an increase of the filipodia-type spines (10 mg/kg) and a reduction of the mushroom-type spines (5 and 10 mg/kg) was observed in the ventromedial prefrontal cortex. In the dorsal striatum, the low dose of ketamine induced an increase in long thin spines and a decrease of mushroom spines. Interestingly, in CA1-hippocampus, there was an increase in the mushrooms type spines (5 mg/kg). Current findings suggest that the subchronic blockade of N-methyl-d-aspartate receptor changes the neuronal plasticity of several brain regions putatively related to recognition memory impairment.

Some types of schizophrenia have been associated with repetitive movements lacking specific purpose, also known as stereotyped behavior. Dopamine agonists (D2) and noncompetitive N-methyl-D-aspartate receptor antagonists (e.g. ketamine) have been administered in rodent models to induce stereotyped behavior that resembles some motor symptoms of schizophrenia. Recently, a relationship has been found between 5-HT6 receptors (5-HT6Rs) and dopaminergic activity. The present study evaluates the effect of ketamine (5 and 10 mg/kg), alone and in combination with the 5-HT6R agonist E-6837, on the climbing behavior of male mice. Ketamine was administered with an acute (1 day) and subchronic (5 day) scheme. Later, these doses and schemes were combined with an acute scheme of E-6837 (5 and 10 mg/kg). With both the acute and the subchronic schemes, ketamine increased climbing behavior at a dose of 10 mg/kg, and this effect was reversed by E-6837 (at 5 and 10 mg/kg). The present results suggest that there is an interaction between N-methyl-D-aspartate and 5-HT6 receptors in the regulation of climbing behavior. Further research is necessary to provide more evidence on this interaction.