Loss of the sustained antidepressant-like effect of (2R,6R)-hydroxynorketamine in NMDA receptor GluN2D subunit knockout mice.

Ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, has attracted attention for its acute and sustained antidepressant effects in patients with depression. Hydroxynorketamine (HNK), a metabolite of ketamine, exerts antidepressant effects without exerting ketamine's side effects and has attracted much attention in recent years. However, the detailed pharmacological mechanism of action of HNK remains unclear. We previously showed that the GluN2D NMDA receptor subunit is important for sustained antidepressant-like effects of (R)-ketamine. Therefore, we investigated whether the GluN2D subunit is involved in antidepressant-like effects of (2R,6R)-HNK and (2S,6S)-HNK. Treatment with (2R,6R)-HNK but not (2S,6S)-HNK exerted acute and sustained antidepressant-like effects in the tail-suspension test in wildtype mice. Interestingly, sustained antidepressant-like effects of (2R,6R)-HNK were abolished in GluN2D-knockout mice, whereas acute antidepressant-like effects were maintained in GluN2D-knockout mice. When expression levels of GluN2A and GluN2B subunits were evaluated, a decrease in GluN2B protein expression in the nucleus accumbens was found in stressed wildtype mice but not in stressed GluN2D-knockout mice. These results suggest that the GluN2D subunit and possibly the GluN2B subunit are involved in the sustained antidepressant-like effect of (2R,6R)-HNK.

Absence of methamphetamine-induced conditioned place preference in weaver mutant mice.

AIMS: G protein-activated inwardly rectifying potassium (GIRK) channels are related to rewarding effects of addictive drugs. The GIRK2 subunit is thought to play key roles in the reward system. Weaver mutant mice exhibit abnormal GIRK2 function and different behaviors that are caused by several addictive substances compared with wild-type mice. However, mechanisms of reward-related alterations in weaver mutant mice remain unclear. The present study investigated changes in the rewarding effects of methamphetamine (METH) in weaver mutant mice. METHODS: The rewarding effects of METH (4.0 mg/kg) were investigated using the conditioned place preference (CPP) paradigm. Extracellular dopamine level in the nucleus accumbens (NAc) was measured by in vivo microdialysis. To identify brain regions that were associated with these changes in rewarding effects, METH-induced alterations of Fos expression were investigated by immunohistochemical analysis. RESULTS: Weaver mutant mice exhibited a significant decrease in METH-induced CPP and dopamine release in the NAc. Methamphetamine significantly increased Fos expression in the posterior NAc (pNAc) shell in wild-type but not in weaver mutant mice. CONCLUSIONS: Methamphetamine did not induce rewarding effects in weaver mutant mice. The pNAc shell exhibited a significant difference in neuronal activity between wild-type and weaver mutant mice. The present results suggest that the absence of METH-induced CPP in weaver mutant mice is probably related to an innate reduction of dopamine and decreased neural activity in the pNAc shell that is partially attributable to the change of GIRK channel function. GIRK channels, especially those containing the GIRK2 subunit, appear to be involved in METH dependence.

Cognitive Impairment That Is Induced by (R)-Ketamine Is Abolished in NMDA GluN2D Receptor Subunit Knockout Mice.

Although the N-methyl-D-aspartate receptor antagonist ketamine has attracted attention because of its rapid and sustained antidepressant effects in depressed patients, its side effects have raised some concerns. Ketamine is a racemic mixture of equal amounts of the enantiomers (R)-ketamine and (S)-ketamine. The neural mechanisms that underlie the differential effects of these enantiomers remain unclear. We investigated cognitive impairment that was induced by ketamine and its enantiomers in N-methyl-D-aspartate GluN2D receptor subunit knockout (GluN2D-KO) mice. In the novel object recognition test, (RS)-ketamine and (S)-ketamine caused cognitive impairment in both wild-type and GluN2D-KO mice, whereas (R)-ketamine induced such cognitive impairment only in wild-type mice. The present results suggest that the GluN2D subunit plays an important role in cognitive impairment that is induced by (R)-ketamine, whereas this subunit does not appear to be involved in cognitive impairment that is induced by (RS)-ketamine or (S)-ketamine.

Reward-enhancing effect of methylphenidate is abolished in dopamine transporter knockout mice: A model of attention-deficit/hyperactivity disorder.

AIM: Attention-deficit/hyperactivity disorder is a heterogeneous neurobiological disorder that is characterized by inattention, impulsivity, and an increase in motor activity. Although methylphenidate has been used as a medication for decades, unknown is whether methylphenidate treatment can cause drug dependence in patients with attention-deficit/hyperactivity disorder. This study investigated the reward-enhancing effects of methylphenidate using intracranial self-stimulation in an animal model of attention-deficit/hyperactivity disorder, dopamine transporter knockout mice. METHODS: For the intracranial self-stimulation procedures, the mice were trained to nosepoke to receive direct electrical stimulation via an electrode that was implanted in the lateral hypothalamus. After the acquisition of nosepoke responding for intracranial self-stimulation, the effects of methylphenidate on intracranial self-stimulation were investigated. RESULTS: In the progressive-ratio procedure, dopamine transporter knockout mice exhibited an increase in intracranial self-stimulation compared with wild-type mice. Treatment with 5 and 10 mg/kg methylphenidate increased intracranial self-stimulation responding in wild-type mice. Methylphenidate at the same doses did not affect intracranial self-stimulation responding in dopamine transporter knockout mice. We then investigated the effects of high-dose methylphenidate (60 mg/kg) in a rate-frequency procedure. High-dose methylphenidate significantly decreased intracranial self-stimulation responding in both wild-type and dopamine transporter knockout mice. CONCLUSIONS: These results suggest that low-dose methylphenidate alters the reward system (ie, increases intracranial self-stimulation responding) in wild-type mice via dopamine transporter inhibition, whereas dopamine transporter knockout mice do not exhibit such alterations. High-dose methylphenidate appears to suppress intracranial self-stimulation responding not through dopamine transporter inhibition but rather through other mechanisms. These results support the possibility that methylphenidate treatment for attention-deficit/hyperactivity disorder does not increase the risk of drug dependence, in attention-deficit/hyperactivity disorder patients with dopamine transporter dysfunction.

Role of NMDA receptor GluN2D subunit in the antidepressant effects of enantiomers of ketamine.

We investigated the rapid and sustained antidepressant effects of enantiomers of ketamine in N-methyl-d-aspartate (NMDA) receptor GluN2D subunit knockout (GluN2D-KO) mice. Intraperitoneal administration of ketamine or its enantiomers 10 min before the tail-suspension test exerted significant antidepressant effects on restraint stress-induced depression in both wildtype and GluN2D-KO mice. The antidepressant effects of (RS)-ketamine and (S)-ketamine were sustained 96 h after the injection in both wildtype and GluN2D-KO mice, but such sustained antidepressant effects of (R)-ketamine were only observed in wildtype mice. These data suggest that the GluN2D subunit is critical for the sustained antidepressant effects of (R)-ketamine.

Implication of mGlu5 receptor in the enhancement of morphine-induced hyperlocomotion under chronic treatment with zolpidem.

Long-term exposure to zolpidem induces drug dependence, and it is well known that the balance between the GABAergic and glutamatergic systems plays a critical role in maintaining the neuronal network. In the present study, we investigated the interaction between GABAA receptor α1 subunit and mGlu5 receptor in the limbic forebrain including the N.Acc. after treatment with zolpidem for 7 days. mGlu5 receptor protein levels were significantly increased after treatment with zolpidem for 7 days, and this change was accompanied by the up-regulation of phospholipase Cß1 and calcium/calmodulin-dependent protein kinase IIα, which are downstream of mGlu5 receptor in the limbic forebrain. To confirm that mGlu5 receptor is directly involved in dopamine-related behavior in mice following chronic treatment with zolpidem, we measured morphine-induced hyperlocomotion after chronic treatment with zolpidem in the presence or absence of an mGlu5 receptor antagonist. Although chronic treatment with zolpidem significantly enhanced morphine-induced hyperlocomotion, this enhancement of morphine-induced hyperlocomotion was suppressed by treating it with the mGlu5 receptor antagonist MPEP. These results suggest that chronic treatment with zolpidem caused neural plasticity in response to activation of the mesolimbic dopaminergic system accompanied by an increase in mGlu5 receptor.