Changes of GABA(A) receptor binding and subunit mRNA level in rat brain by infusion of subtoxic dose of MK-801

Vortioxetine Treatment Reverses Subchronic PCP Treatment-Induced Cognitive Impairments: A Potential Role for Serotonin Receptor-Mediated Regulation of GABA Neurotransmission

Major depressive disorder (MDD) is associated with cognitive impairments that may contribute to poor functional outcomes. Clinical data suggests that the multimodal antidepressant vortioxetine attenuates some cognitive impairments in MDD patients, but the mechanistic basis for these improvements is unclear. One theory suggests that vortioxetine improves cognition by suppressing γ-amino butyric acid (GABA)ergic neurotransmission, thereby increasing glutamatergic activation. Vortioxetine’s effects on cognition, GABA and glutamate neurotransmission have been supported in separate experiments, but no empirical work has directly connected vortioxetine’s cognitive effects to those on GABA and glutamate neurotransmission. In this paper, we attempt to bridge this gap by evaluating vortioxetine’s effects in the subchronic PCP (subPCP) model, which induces impaired cognitive function and altered GABA and glutamate neurotransmission. We demonstrate that acute or subchronic vortioxetine treatment attenuated subPCP-induced deficits in attentional set shifting (AST) performance, and that the selective 5-HT₃ receptor antagonist ondansetron or the 5-HT reuptake inhibitor escitalopram could mimic this effect. Furthermore, acute vortioxetine treatment reversed subPCP-induced object recognition (OR) deficits in rats, while subchronic vortioxetine reversed subPCP-induced Object Recognition and object placement impairments in mice. Finally, subPCP treatment reduced GABA_B receptor expression in a manner that was insensitive to vortioxetine treatment, and subchronic vortioxetine treatment alone, but not in combination with subPCP, significantly increased GABA’s affinity for the GABA_A receptor. These data suggest that vortioxetine reverses cognitive impairments in a model associated with altered GABA and glutamate neurotransmission, further supporting the hypothesis that vortioxetine’s GABAergic and glutamatergic effects are relevant for cognitive function.

Traumatic brain injury and the effects of diazepam, diltiazem, and MK-801 on GABA-A receptor subunit expression in rat hippocampus

Background

Excitatory amino acid release and subsequent biochemical cascades following traumatic brain injury (TBI) have been well documented, especially glutamate-related excitotoxicity. The effects of TBI on the essential functions of inhibitory GABA-A receptors, however, are poorly understood.

Methods

We used Western blot procedures to test whether in vivo TBI in rat altered the protein expression of hippocampal GABA-A receptor subunits α1, α2, α3, α5, β3, and γ2 at 3 h, 6 h, 24 h, and 7 days post-injuy. We then used pre-injury injections of MK-801 to block calcium influx through the NMDA receptor, diltiazem to block L-type voltage-gated calcium influx, or diazepam to enhance chloride conductance, and re-examined the protein expressions of α1, α2, α3, and γ2, all of which were altered by TBI in the first study and all of which are important constituents in benzodiazepine-sensitive GABA-A receptors.

Results

Western blot analysis revealed no injury-induced alterations in protein expression for GABA-A receptor α2 or α5 subunits at any time point post-injury. Significant time-dependent changes in α1, α3, β3, and γ2 protein expression. The pattern of alterations to GABA-A subunits was nearly identical after diltiazem and diazepam treatment, and MK-801 normalized expression of all subunits 24 hours post-TBI.

Conclusions

These studies are the first to demonstrate that GABA-A receptor subunit expression is altered by TBI in vivo, and these alterations may be driven by calcium-mediated cascades in hippocampal neurons. Changes in GABA-A receptors in the hippocampus after TBI may have far-reaching consequences considering their essential importance in maintaining inhibitory balance and their extensive impact on neuronal function.

Altered markers of tonic inhibition in the dorsolateral prefrontal cortex of subjects with schizophrenia

OBJECTIVE

Cognitive impairments in schizophrenia are associated with lower expression of markers of gamma-aminobutyric acid (GABA) synthesis in the prefrontal cortex. The effects of GABA are mediated by GABA(A) receptors that mediate either phasic or tonic inhibition. The authors assessed the expression of GABA(A) receptor alpha4 and delta subunits, which coassemble to form receptors mediating tonic inhibition, in schizophrenia.

METHOD

The authors used in situ hybridization to quantify expression patterns of GABA(A) receptor alpha4 and delta subunits in prefrontal cortex from 23 matched pairs of schizophrenia and comparison subjects.

RESULTS

Levels of delta mRNA were significantly lower in schizophrenia subjects regardless of medication use, whereas alpha4 mRNA levels were lower only in subjects with schizophrenia receiving certain medications at the time of death. To understand the nature of this unexpected dissociation between alpha4 and delta subunit expression in schizophrenia, the authors used similar methods to quantify alpha4 and delta mRNA levels in multiple animal models. During postnatal development of monkey prefrontal cortex, levels of alpha4 mRNA decreased, whereas delta mRNA levels increased. In addition, delta mRNA levels, but not alpha4 mRNA levels, were lower in the medial frontal cortex of mice with a genetic deletion of the GABA(A) receptor alpha1 subunit, and neither delta nor alpha4 mRNA levels were altered in rodent models of altered excitatory neurotransmission.

CONCLUSIONS

Since GABA(A) receptor alpha1 subunits also have lower mRNA levels in schizophrenia, show increased expression with age in monkey prefrontal cortex, and can coassemble with delta subunits to form functional GABA(A) receptors, lower delta mRNA levels in schizophrenia might reflect a reduced number of alpha(1)beta(x)delta GABA(A) receptors that could contribute to deficient tonic inhibition and prefrontal cortical dysfunction in schizophrenia.

Construction and implantation of a microinfusion system for sustained delivery of neuroactive agents

Sustained delivery of neuroactive agents is widely used in neuroscience, but poses many technical challenges. It is necessary to deliver the agent with high precision while minimizing localized trauma and inflammation. Also, the ability to customize the system to accommodate animals of different species and sizes is desirable. This video presentation demonstrates the construction of an infusion system that can be fitted to any particular research animal. The delivery microcannula diameter is approximately 10-fold smaller than most infusion cannulas presently used. This translates into enhanced accuracy and reduced trauma to the brain region under study. The delivery cannula can also be sculpted to fit the contour of the surface of the animal's skull, thereby allowing closure of the scalp incision neatly over the infusion system, precluding the need for a skull-mounted pedestal, reducing risk of infection, and ensuring a greater level of comfort to the animal. The system is assembled in an air-free environment and requires the researcher to fashion glass micropipettes with a heat source. These construction methods require special skills that are best acquired, if not in person, using video instruction.

Construction and implantation of a microinfusion system for sustained delivery of neuroactive agents

Experimental protocols used for chronic infusion of neuroactive agents within regions of the brain often utilize a mini-osmotic pump system. Agents are commonly delivered via a stainless steel cannula with a diameter of 0.30 mm or greater. Systems utilizing a cannula of this caliber may impose trauma to the area of interest resulting in architectural damage, thereby compromising structural integrity and normal functioning. As neuroscience inquiry becomes more sophisticated, investigation of brain structures and circuitry requires improved levels of accuracy and higher resolution. We have developed a method for the preparation and implantation of a chronic infusion system within the brain utilizing a borosilicate microcannula with a tip diameter of 50 microm. This technique reduces damage to the local environment and diminishes reactive gliosis at the site of infusion. The configuration of the microinfusion system is also able to conform to the surface of the animal's skull, precluding the need for large cranial pedestals, and thus facilitating closure of the scalp incision and reducing the risk of infection. We demonstrate reliable sustained delivery of a dye having a representative molecular weight using an in vitro model and in vivo studies in rats.

The effects of glutamate receptor antagonists on cerebellar granule cell survival and development

N-Methyl-d-aspartate (NMDA) receptor stimulation promotes neuronal survival and differentiation under both in vitro and in vivo conditions. We studied the effects of various NMDA receptor antagonists acting at different NMDA receptor binding sites and non-NMDA receptor antagonists on the development and survival of cerebellar granule cell (CGC) culture. Only three of the drugs tested induced neurotoxicity-MK-801 (non-competitive NMDA channel blocking antagonist), ifenprodil (an antagonist of the NR2B site and polyamine site of the NMDA receptor) and L-701.324 (full antagonist at glycine site), while CGP-37849 (a competitive NMDA antagonist), (+)-HA-966 (a partial agonist of the glycine site of the NMDA receptor), and NBQX (a competitively acting AMPA receptor antagonist) were not toxic at any concentration (1-100 microM) used. Among these drugs, only MK-801 was toxic for the immature CGC on second day in vitro (2DIV), and toxicity was diminished parallel to the neuronal maturation. In more mature neurons (7DIV), MK-801 demonstrated some neuroprotection, which diminished spontaneously occurring neuronal death in culture. Neither NMDA nor glutamate were able to prevent the neurotoxic effect of MK-801 at 2DIV. MK-801, ifenprodil and L-701.324 induced DNA fragmentation on 2DIV in CGC culture measured by the TUNEL method. The BOC-D-FMK, the universal caspase inhibitor, completely reversed MK-801-induced DNA fragmentation, suggesting an apoptotic pathway of MK-801-induced cell death. Neurite outgrowth as a characteristic feature of the development of CGC was diminished after treatment with MK-801, ifenprodil and L-701.324. In conclusion, the results of the present study demonstrate that only nonselective channel blocker MK-801 decreases cell viability, induces apoptosis and inhibits neurite outgrowth of CGC in a development-dependent manner.

AMPA/kainate receptor-mediated up-regulation of GABAA receptor δ subunit mRNA expression in cultured rat cerebellar granule cells is dependent on NMDA receptor activation

We have studied the effects of AMPA/kainate receptor agonists on GABA(A) receptor subunit mRNA expression in vitro in cultured rat cerebellar granule cells (CGCs). Kainate (KA) (100 microM) and high K(+) (25 mM) dramatically up-regulated delta subunit mRNA expression to 500-700% of that in control cells grown in low K(+) (5 mM). KA or high K(+) had no effect on the expression of the other major GABA(A) receptor subunits alpha1, alpha6, beta2, beta3 or gamma2. Up-regulation of delta mRNA was also detected with the AMPA receptor-selective agonist CPW-399 and to a lesser extent with the KA receptor-selective agonist ATPA. AMPA/kainate receptor-selective antagonist DNQX completely inhibited KA-, CPW-399- and ATPA-induced delta mRNA up-regulation indicating that the effects were mediated via AMPA and KA receptor activation. NMDA receptor-selective antagonist MK-801 inhibited 76% of the KA- and 57% of the CPW-399-induced delta up-regulation suggesting that KA and CPW-399 treatments may induce glutamate release resulting in NMDA receptor activation, and subsequently to delta mRNA up-regulation. In CGCs, delta subunit is a component of extrasynaptic alpha6betadelta receptors that mediate tonic inhibition. Up-regulation of delta during prolonged glutamate receptor activation or cell membrane depolarization may be a mechanism to increase tonic inhibition to counteract excessive excitation.

NMDA receptor antagonism disrupts the development of morphine analgesic tolerance in male, but not female C57BL/6J mice

Multiple studies demonstrate that coadministration of N-methyl-D-aspartate (NMDA) receptor antagonists with the opioid agonist morphine attenuates the development of analgesic tolerance. Sex differences in the effects of noncompetitive, but not competitive NMDA receptor antagonists on acute morphine analgesia, have been reported in mice, yet the role of sex in modulation of morphine tolerance by NMDA receptor antagonists has yet to be addressed. Therefore, we tested whether there is a sex difference in the effect of NMDA receptor antagonists on the development of morphine analgesic tolerance in C57BL/6J mice. Acutely, at a dose required to affect morphine tolerance in male mice, the noncompetitive NMDA receptor antagonist dizocilpine (MK-801) prolonged morphine analgesia similarly in both sexes in the hot plate and tail withdrawal assays. In the hot plate assay, coadministration of MK-801 or the competitive antagonist 3-(2-carboxpiperazin-4-yl)propyl-1-phosphanoic acid (CPP) with morphine attenuated the development of tolerance in male mice, while having no effect in females. Like normal and sham females, ovariectomized mice were similarly insensitive to the attenuation of morphine tolerance by MK-801 in the hot plate assay. Surprisingly, in the tail withdrawal assay, MK-801 facilitated the development of morphine-induced hyperalgesia and tolerance in males but not females. The results demonstrate that male mice are more sensitive to modulation of nociception and morphine analgesia after repeated coadministration of NMDA receptor antagonists. Furthermore, the underlying mechanisms are likely to be different from those mediating the sex difference in the modulation of acute morphine analgesia that has previously been reported.

Selective reduction of γ-aminobutyric acid type A receptor δ subunit mRNA levels by MK-801 in rat dentate gyrus

The influence of excitatory blockade elicited by uncompetitive N-methyl-D-aspartate (NMDA)/glutamate receptor antagonists on inhibitory GABAergic systems is not well understood. Adult male rats were injected i.p. with a single dose of the prototypical uncompetitive antagonist MK-801 (0.2-10 mg/kg) and in situ hybridization was performed to measure mRNA levels of gamma-aminobutyric acid type A (GABAA) receptor subunits (alpha1-6, beta1-3, gamma1-3, delta, and theta). A significant decrease in delta subunit mRNA levels, that reached approximately 70% of saline-treated values, was observed in the hippocampal dentate gyrus following MK-801 administration. Other subunits did not display statistically significant alterations. These data demonstrate selective actions on GABAA receptor subunit levels that result from blockade of excitation by MK-801.

Changes of GABA(A) receptor binding and subunit mRNA level in rat brain by infusion of NOS inhibitor

In the present study, we have investigated the effects of prolonged inhibition of nitric oxide synthase (NOS) by infusion of NOS inhibitor, L-nitroarginine, to examine the pentobarbital-induced sleep, modulation of GABA(A) receptor binding, and GABA(A) receptor subunit mRNA level in rat brain. Pre-treatment with L-nitroarginine 30 min before pentobarbital treatment (60 mg/kg, i.p.) significantly increased the duration of sleep in rats. However, the duration of pentobarbital-induced sleep was shortened by the prolonged infusion of L-nitroarginine into ventricle. We have investigated the effect of NOS inhibitor on GABA(A) receptor binding characteristics in discrete areas of brain regions by using autoradiographic and in situ hybridization techniques. Rats were infused with L-nitroarginine (10, 100 pmol/10 microl/h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps. The levels of [(3)H]muscimol and [(3)H]flunitrazepam binding were markedly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, there was no change in the level of [(35)S]TBPS binding. The levels of beta2-subunit were elevated in the cortex, brainstem, and cerebellar granule layers. By contrast, the levels of beta3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in L-nitroarginine-infused rats. Following L-nitroarginine treatment, the levels of alpha6- and delta-subunits which were strictly localized to the cerebellum, were not changed in the cerebellar granule layer. These results show that the prolonged inhibition of NOS by L-nitroarginine-infusion markedly elevates [(3)H]muscimol and [(3)H]flunitrazepam binding throughout the brain, and alters GABA(A) receptor subunit mRNA levels in different directions. Chronic inhibition of NO generation has differential effects on the various expressions of GABA(A) receptor subunits. These suggest the involvement of different regulatory mechanisms for the NO-induced expression of GABA(A) receptor.

The bindings of [3H]muscimol and [3H]flunitrazapam are elevated in discrete brain regions of butorphanol-withdrawal rats

We have investigated the effects of continuous infusion of butorphanol on the modulation of GABA(A) receptor binding. Butorphanol was infused continuously into intracerebroventricle (ICV) at a constant rate of 26 nmol/microl/h for 3 days, and the withdrawal from opioid was rendered 7 h after the cessation of infusion. The GABA(A) receptor bindings in rat brain slices were analyzed by quantitative autoradiography using [3H]muscimol and [3H]flunitrazepam. In the rats withdrawn from butorphanol, the levels of [3H]muscimol binding were significantly elevated in cortex, thalamus, and part of the hippocampus. The levels of [3H]flunitrazepam binding were elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, brainstem, and cerebellum in the rats withdrawn from butorphanol. The levels of binding of either [3H]muscimol or [3H]flunitrazepam were not changed in the rats tolerant to butorphanol. However, the activity of GABAergic neuron was not found to have been modulated by butorphanol withdrawal, because the level of glutamic acid decarboxylase was not changed markedly either in rats that were tolerant to or withdrawn from butorphanol by Western blot and immunohistochemical data. These results suggest that the withdrawal from butorphanol infusion markedly elevates the binding of [3H]muscimol and [3H]flunitrazepam throughout the brain in a region-specific manner, and that the regulatory mechanisms in butorphanol tolerance and withdrawal may be different.

Modulation of the levels of NMDA receptor subunit mRNA and the bindings of [3H]MK-801 in rat brain by chronic infusion of subtoxic dose of MK-801

The effects of continuous infusion of NMDA receptor antagonist MK-801 on the modulation of NMDA receptor subunits NR1, NR2A, NR2B, and NR2C were investigated by using in situ hybridization study. Differential assembly of NMDA receptor subunits determines their functional characteristics. Continuous intracerebroventricular (i.c.v.) infusion with MK-801 (1 pmol/10 microl/h) for 7 days resulted in significant modulations in the NR1, NR2A, and NR2B mRNA levels without producing stereotypic motor syndromes. The levels of NR1 mRNA were significantly increased (9-20%) in the cerebral cortex, striatum, septum, and CA1 of hippocampus in MK-801-infused rats. The levels of NR2A mRNA were significantly decreased (11-16%) in the CA3 and dentate gyrus of hippocampus in MK-801-infused rats. In contrast to NR2A, NR2B subunit mRNA levels were increased (10-14%) in the cerebral cortex, caudate putamen, and thalamus. However, no changes of NR2C subunits in cerebellar granule layer were observed. Using quantitative ligand autoradiography, the binding of NMDA receptor ligand [3H]MK-801 was increased (12-25%) significantly in almost all brain regions except in the thalamus and cerebellum after 7 days infusion with MK-801. These results suggest that region-specific changes of NMDA receptor subunit mRNA and [3H]MK-801 binding are involved in the MK-801-infused adult rats.