Inhibition of hyperpolarization-activated cation currents by phencyclidine and some sigma ligands in rat hippocampal CA1 pyramidal neurons in vitro

Low doses of dextromethorphan attenuate morphine-induced rewarding via the sigma-1 receptor at ventral tegmental area in rats

Chronic use of morphine causes rewarding and behavioral sensitization, which may lead to the development of psychological craving. In our previous study, we found that a widely used antitussive dextromethorphan (known as a low affinity NMDA receptor antagonist), at doses of 10-20 mg/kg (i.p.), effectively decreased morphine rewarding in rats. In this study, we further investigated the effects and mechanisms of low doses of DM (μg/kg range) on morphine rewarding and behavioral sensitization. A conditioned place preference test was used to determine the rewarding and a locomotor activity test was used to determine the behavioral sensitization induced by the drug(s) in rats. When a low dose of DM (3 or 10 μg/kg, i.p.) was co-administered with morphine (5 mg/kg, s.c.), the rewarding effect, but not behavioral sensitization, induced by morphine was inhibited. The inhibiting effect of DM could be blocked by systemically administering a sigma-1 receptor antagonist, BD1047 (3 mg/kg, i.p.). When BD1047 (5 nmole/site) was locally given at the VTA, it also blocked the effects of a low dose of DM in inhibiting morphine rewarding. Our findings suggest that the activation of the sigma-1 receptor at the VTA may be involved in the mechanism of low doses of DM in inhibiting the morphine rewarding effect and the possibility of using extremely low doses of DM in treatment of opioid addiction in clinics.

NMDA receptor hypofunction phase couples independent γ-oscillations in the rat visual cortex

Hallucinations, a hallmark of psychosis, can be induced by the psychotomimetic N-methyl-D-aspartic acid (NMDA) receptor antagonists, ketamine and phencyclidine (PCP), and are associated with hypersynchronization in the γ-frequency band, but it is unknown how reduced interneuron activation associated with NMDA receptor hypofunction can cause hypersynchronization or distorted perception. Low-frequency γ-oscillations (LFγ) and high-frequency γ-oscillations (HFγ) serve different aspects of perception. In this study, we test whether ketamine and PCP affect the interactions between HFγ and LFγ in the rat visual cortex in vitro. In slices of the rat visual cortex, kainate and carbachol induced LFγ (∼ 34 Hz at 32°C) in layer V and HFγ (∼ 54 Hz) in layer III of the same cortical column. In controls, HFγ and LFγ were independent, and pyramidal neurons recorded in layer III were entrained by HFγ, but not by LFγ. Sub-anesthetic concentrations of ketamine selectively decelerated HFγ by 22 Hz (EC(50)=2.7 μM), to match the frequency of LFγ in layer V. This caused phase coupling of the two γ-oscillations, increased spatial coherence in layer III, and entrained the firing of layer III pyramidal neurons by LFγ in layer V. PCP similarly decelerated HFγ by 22 Hz (EC(50)=0.16 μM), causing cross-layer phase coupling of γ-oscillations. Selective NMDA receptor antagonism, selective NR2B subunit-containing receptor antagonism, and reduced D-serine levels caused a similar selective deceleration of HFγ, whereas increasing NMDA receptor activation through exogenous NMDA, D-serine, or mGluR group 1 agonism selectively accelerated HFγ. The NMDA receptor hypofunction-induced phase coupling of the normally independent γ-generating networks is likely to cause abnormal cross-layer interactions, which may distort perceptions due to aberrant matching of top-down information with bottom-up information. If decelerated HFγ and subsequent cross-layer synchronization also underlie pathological psychosis, acceleration of HFγ could be the target for improved antipsychotic therapy.