Stimulus-dependent, reciprocal up- and downregulation of glutamic acid decarboxylase and Ca2+/calmodulin-dependent protein kinase II gene expression in rat cerebral cortex

Effective connectivity relates seizure outcome to electrode placement in responsive neurostimulation

Responsive neurostimulation is a closed-loop neuromodulation therapy for drug resistant focal epilepsy. Responsive neurostimulation electrodes are placed near ictal onset zones so as to enable detection of epileptiform activity and deliver electrical stimulation. There is no standard approach for determining the optimal placement of responsive neurostimulation electrodes. Clinicians make this determination based on presurgical tests, such as MRI, EEG, magnetoencephalography, ictal single-photon emission computed tomography and intracranial EEG. Currently functional connectivity measures are not being used in determining the placement of responsive neurostimulation electrodes. Cortico-cortical evoked potentials are a measure of effective functional connectivity. Cortico-cortical evoked potentials are generated by direct single-pulse electrical stimulation and can be used to investigate cortico-cortical connections in vivo. We hypothesized that the presence of high amplitude cortico-cortical evoked potentials, recorded during intracranial EEG monitoring, near the eventual responsive neurostimulation contact sites is predictive of better outcomes from its therapy. We retrospectively reviewed 12 patients in whom cortico-cortical evoked potentials were obtained during stereoelectroencephalography evaluation and subsequently underwent responsive neurostimulation therapy. We studied the relationship between cortico-cortical evoked potentials, the eventual responsive neurostimulation electrode locations and seizure reduction. Directional connectivity indicated by cortico-cortical evoked potentials can categorize stereoelectroencephalography electrodes as either receiver nodes/in-degree (an area of greater inward connectivity) or projection nodes/out-degree (greater outward connectivity). The follow-up period for seizure reduction ranged from 1.3-4.8 years (median 2.7) after responsive neurostimulation therapy started. Stereoelectroencephalography electrodes closest to the eventual responsive neurostimulation contact site tended to show larger in-degree cortico-cortical evoked potentials, especially for the early latency cortico-cortical evoked potentials period (10-60 ms period) in six out of 12 patients. Stereoelectroencephalography electrodes closest to the responsive neurostimulation contacts (≤5 mm) also had greater significant out-degree in the early cortico-cortical evoked potentials latency period than those further away (≥10 mm) (P < 0.05). Additionally, significant correlation was noted between in-degree cortico-cortical evoked potentials and greater seizure reduction with responsive neurostimulation therapy at its most effective period (P < 0.05). These findings suggest that functional connectivity determined by cortico-cortical evoked potentials may provide additional information that could help guide the optimal placement of responsive neurostimulation electrodes.

Acute effects of brain-responsive neurostimulation in drug-resistant partial onset epilepsy

OBJECTIVE

Understanding the acute effects of responsive stimulation (AERS) based on intracranial EEG (iEEG) recordings in ambulatory patients with drug-resistant partial epilepsy, and correlating these with changes in clinical seizure frequency, may help clinicians more efficiently optimize responsive stimulation settings.

METHODS

In patients implanted with the NeuroPace® RNS® System, acute changes in iEEG spectral power following active and sham stimulation periods were quantified and compared within individual iEEG channels. Additionally, acute stimulation-induced acute iEEG changes were compared within iEEG channels before and after patients experienced substantial reductions in clinical seizure frequency.

RESULTS

Responsive stimulation resulted in a 20.7% relative decrease in spectral power in the 2-4 second window following active stimulation, compared to sham stimulation. On several detection channels, the AERS features changed when clinical outcomes improved but were relatively stable otherwise. AERS change direction associated with clinical improvement was generally consistent within detection channels.

CONCLUSIONS

In this retrospective analysis, patients with drug-resistant partial epilepsy treated with direct brain-responsive neurostimulation showed an acute stimulation related reduction in iEEG spectral power that was associated with reductions in clinical seizure frequency.

SIGNIFICANCE

Identifying favorable stimulation related changes in iEEG activity could help physicians to more rapidly optimize stimulation settings for each patient.

Odorant Sensory Input Modulates DNA Secondary Structure Formation and Heterogeneous Ribonucleoprotein Recruitment on the Tyrosine Hydroxylase and Glutamic Acid Decarboxylase 1 Promoters in the Olfactory Bulb

Adaptation of neural circuits to changes in sensory input can modify several cellular processes within neurons, including neurotransmitter biosynthesis levels. For a subset of olfactory bulb interneurons, activity-dependent changes in GABA are reflected by corresponding changes in Glutamate decarboxylase 1 (Gad1) expression levels. Mechanisms regulating Gad1 promoter activity are poorly understood, but here we show that a conserved G:C-rich region in the mouse Gad1 proximal promoter region both recruits heterogeneous nuclear ribonucleoproteins (hnRNPs) that facilitate transcription and forms single-stranded DNA secondary structures associated with transcriptional repression. This promoter architecture and function is shared with Tyrosine hydroxylase (Th), which is also modulated by odorant-dependent activity in the olfactory bulb. This study shows that the balance between DNA secondary structure formation and hnRNP binding on the mouse Th and Gad1 promoters in the olfactory bulb is responsive to changes in odorant-dependent sensory input. These findings reveal that Th and Gad1 share a novel transcription regulatory mechanism that facilitates sensory input-dependent regulation of dopamine and GABA expression.SIGNIFICANCE STATEMENT Adaptation of neural circuits to changes in sensory input can modify several cellular processes within neurons, including neurotransmitter biosynthesis levels. This study shows that transcription of genes encoding rate-limiting enzymes for GABA and dopamine biosynthesis (Gad1 and Th, respectively) in the mammalian olfactory bulb is regulated by G:C-rich regions that both recruit heterogeneous nuclear ribonucleoproteins (hnRNPs) to facilitate transcription and form single-stranded DNA secondary structures associated with repression. hnRNP binding and formation of DNA secondary structure on the Th and Gad1 promoters are mutually exclusive, and odorant sensory input levels regulate the balance between these regulatory features. These findings reveal that Th and Gad1 share a transcription regulatory mechanism that facilitates odorant-dependent regulation of dopamine and GABA expression levels.

Intracranial stimulation for children with epilepsy

OBJECTIVES

To evaluate the efficacy of intracranial stimulation to treat refractory epilepsy in children.

METHODS

This is a retrospective analysis of a pilot study on all 8 children who had intracranial electrical stimulation for the investigation and treatment of refractory epilepsy at King's College Hospital between 2014 and 2015. Five children (one with temporal lobe epilepsy and four with frontal lobe epilepsy) had subacute cortical stimulation (SCS) for a period of 20-161 h during intracranial video-telemetry. Efficacy of stimulation was evaluated by counting interictal discharges and seizures. Two children had thalamic deep brain stimulation (DBS) of the centromedian nucleus (one with idiopathic generalized epilepsy, one with presumed symptomatic generalized epilepsy), and one child on the anterior nucleus (right fronto-temporal epilepsy). The incidence of interictal discharges was evaluated visually and quantified automatically.

RESULTS

Among the three children with DBS, two had >60% improvement in seizure frequency and severity and one had no improvement. Among the five children with SCS, four showed improvement in seizure frequency (>50%) and one chid did not show improvement. Procedures were well tolerated by children.

CONCLUSION

Cortical and thalamic stimulation appear to be effective and well tolerated in children with refractory epilepsy. SCS can be used to identify the focus and predict the effects of resective surgery or chronic cortical stimulation. Further larger studies are necessary.

Neuronal activity controls the development of interneurons in the somatosensory cortex

BACKGROUND

Neuronal activity in cortical areas regulates neurodevelopment by interacting with defined genetic programs to shape the mature central nervous system. Electrical activity is conveyed to sensory cortical areas via intracortical and thalamocortical neurons, and includes oscillatory patterns that have been measured across cortical regions.

OBJECTIVE

In this work, we review the most recent findings about how electrical activity shapes the developmental assembly of functional circuitry in the somatosensory cortex, with an emphasis on interneuron maturation and integration. We include studies on the effect of various neurotransmitters and on the influence of thalamocortical afferent activity on circuit development. We additionally reviewed studies describing network activity patterns.

METHODS

We conducted an extensive literature search using both the PubMed and Google Scholar search engines. The following keywords were used in various iterations: "interneuron", "somatosensory", "development", "activity", "network patterns", "thalamocortical", "NMDA receptor", "plasticity". We additionally selected papers known to us from past reading, and those recommended to us by reviewers and members of our lab.

RESULTS

We reviewed a total of 132 articles that focused on the role of activity in interneuronal migration, maturation, and circuit development, as well as the source of electrical inputs and patterns of cortical activity in the somatosensory cortex. 79 of these papers included in this timely review were written between 2007 and 2016.

CONCLUSIONS

Neuronal activity shapes the developmental assembly of functional circuitry in the somatosensory cortical interneurons. This activity impacts nearly every aspect of development and acquisition of mature neuronal characteristics, and may contribute to changing phenotypes, altered transmitter expression, and plasticity in the adult. Progressively changing oscillatory network patterns contribute to this activity in the early postnatal period, although a direct requirement for specific patterns and origins of activity remains to be demonstrated.

Critical review of the responsive neurostimulator system for epilepsy

Patients with medically refractory epilepsy have historically had few effective treatment options. Electrical brain stimulation for seizures has been studied for decades and ongoing technological refinements have made possible the development of an implantable electrical brain stimulator. The NeuroPace responsive neurostimulator was recently approved by the FDA for clinical use and the initial reports are encouraging. This device continually monitors brain activity and delivers an electric stimulus when abnormal activity is detected. Early reports of efficacy suggest that the device is well tolerated and offers a reduction in seizure frequency by approximately half at 2 years.

GABAergic interneurons shape the functional maturation of the cortex

From early embryonic development to adulthood, GABA release participates in the construction of the mammalian cerebral cortex. The maturation of GABAergic neurotransmission is a protracted process which takes place in discrete steps and results from the dynamic interaction between developmentally directed gene expression and brain activity. During the course of development, GABAergic interneurons contribute to key aspects of the functional maturation of the cortex in different ways, from exerting a trophic role to pacing immature neural networks. In this review, we provide an overview of the maturation of GABAergic neurotransmission and discuss the role of GABAergic interneurons in cortical wiring, plasticity, and network activity during pre- and postnatal development. We also discuss psychiatric diseases that may be considered at least in part developmental disorders of the GABAergic system.

Age-related changes in rostral basal forebrain cholinergic and GABAergic projection neurons: relationship with spatial impairment

Both cholinergic and GABAergic projections from the rostral basal forebrain contribute to hippocampal function and mnemonic abilities. While dysfunction of cholinergic neurons has been heavily implicated in age-related memory decline, significantly less is known regarding how age-related changes in codistributed GABAergic projection neurons contribute to a decline in hippocampal-dependent spatial learning. In the current study, confocal stereology was used to quantify cholinergic (choline acetyltransferase [ChAT] immunopositive) neurons, GABAergic projection (glutamic decarboxylase 67 [GAD67] immunopositive) neurons, and total (neuronal nuclei [NeuN] immunopositive) neurons in the rostral basal forebrain of young and aged rats that were first characterized on a spatial learning task. ChAT immunopositive neurons were significantly but modestly reduced in aged rats. Although ChAT immunopositive neuron number was strongly correlated with spatial learning abilities among young rats, the reduction of ChAT immunopositive neurons was not associated with impaired spatial learning in aged rats. In contrast, the number of GAD67 immunopositive neurons was robustly and selectively elevated in aged rats that exhibited impaired spatial learning. Interestingly, the total number of rostral basal forebrain neurons was comparable in young and aged rats, regardless of their cognitive status. These data demonstrate differential effects of age on phenotypically distinct rostral basal forebrain projection neurons, and implicate dysregulated cholinergic and GABAergic septohippocampal circuitry in age-related mnemonic decline.

Activity-dependent regulation of inhibition via GAD67

Persistent alterations in network activity trigger compensatory changes in excitation and inhibition that restore neuronal firing rate to an optimal range. One example of such synaptic homeostasis is the downregulation of inhibitory transmission by chronic inactivity, in part through the reduction of vesicular transmitter content. The enzyme glutamic acid decarboxylase 67 (GAD67) is critical for GABA synthesis, but its involvement in homeostatic plasticity is unclear. We explored the role of GAD67 in activity-dependent synaptic plasticity using a mouse line (Gad1(-/-)) in which GAD67 expression is disrupted by genomic insertion of the green fluorescent protein (GFP). Homozygous deletion of Gad1 significantly reduced miniature inhibitory postsynaptic current (mIPSC) amplitudes and GABA levels in cultured hippocampal neurons. The fractional block of mIPSC amplitude by a low affinity, competitive GABA(A) receptor antagonist was higher in GAD67-lacking neurons, suggesting that GABA concentration in the synaptic cleft is lower in knockout animals. Chronic suppression of activity by the application of tetrodotoxin (TTX) reduced mIPSC amplitudes and the levels of GAD67 and GABA. Moreover, TTX reduced GFP levels in interneurons, suggesting that GAD67 gene expression is a key regulatory target of activity. These in vitro experiments were corroborated by in vivo studies in which olfactory deprivation reduced mIPSC amplitudes and GFP levels in glomerular neurons in the olfactory bulb. Importantly, TTX-induced downregulation of mIPSC was attenuated in Gad1(-/-) neurons. Altogether, these findings indicate that activity-driven expression of GAD67 critically controls GABA synthesis and, thus, vesicular filling of the transmitter.

Atlas of transgenic Tet-Off Ca2+/calmodulin-dependent protein kinase II and prion protein promoter activity in the mouse brain

Conditional transgenic mouse models are important tools for investigations of neurodegenerative diseases and evaluation of potential therapeutic interventions. A popular conditional transgenic system is the binary tetracycline-responsive gene (Tet-Off) system, in which the expression of the gene of interest depends on a tetracycline-regulatable transactivator (tTA) under the control of a specific promoter construct. The most frequently used Tet-Off promoter mouse lines are the Ca(2+)/calmodulin-dependent protein kinase II (CamKII) and prion protein (PrP) promoter lines, respectively. To target the regulated gene of interest to relevant brain regions, a priori knowledge about the spatial distribution of the regulated gene expression in the brain is important. Such distribution patterns can be investigated using double transgenic mice in which the promoter construct regulates a LacZ reporter gene encoding the marker β-galactosidase which can be histologically detected using its substrate X-gal. We have previously published an atlas showing the brain-wide expression mediated by the Tet-Off PrP promoter mouse line, but the distribution of activity in the Tet-Off CamKII promoter mouse line is less well known. To compare promoter activity distributions in these two Tet-Off mouse lines, we have developed an online digital atlas tailored for side-by-side comparison of histological section images. The atlas provides a comprehensive list of brain regions containing X-gal labeling and an interactive dual image viewer tool for panning and zooming of corresponding section images. Comparison of spatial expression patterns between the two lines show considerable regional and cellular differences, relevant in context of generation and analysis of inducible models based on these two tetracycline responsive promoter mouse lines.

Toward rational design of electrical stimulation strategies for epilepsy control

Electrical stimulation is emerging as a viable alternative for patients with epilepsy whose seizures are not alleviated by drugs or surgery. Its attractions are temporal and spatial specificity of action, flexibility of waveform parameters and timing, and the perception that its effects are reversible unlike resective surgery. However, despite significant advances in our understanding of mechanisms of neural electrical stimulation, clinical electrotherapy for seizures relies heavily on empirical tuning of parameters and protocols. We highlight concurrent treatment goals with potentially conflicting design constraints that must be resolved when formulating rational strategies for epilepsy electrotherapy, namely, seizure reduction versus cognitive impairment, stimulation efficacy versus tissue safety, and mechanistic insight versus clinical pragmatism. First, treatment markers, objectives, and metrics relevant to electrical stimulation for epilepsy are discussed from a clinical perspective. Then the experimental perspective is presented, with the biophysical mechanisms and modalities of open-loop electrical stimulation, and the potential benefits of closed-loop control for epilepsy.

Activity-dependent development of inhibitory synapses and innervation pattern: role of GABA signalling and beyond

GABA-mediated synaptic inhibition is crucial in neural circuit operations. The development of GABAergic inhibitory synapses and innervation pattern in mammalian neocortex is a prolonged process, extending well into the postnatal period, and is regulated by neural activity and experience. Accumulating evidence supports the hypothesis that GABA signalling acts beyond synaptic transmission and regulates inhibitory synapse development; in other words, similar to glutamate signalling at developing excitatory synapses, GABA may coordinate pre- and post-synaptic maturation at inhibitory synapses. These findings raise numerous questions regarding the underlying mechanisms, including the role of GABA receptors and their link to synaptic adhesion molecules. Since synapse formation is a crucial component of axon growth, GABA signalling may also shape the axon arbor and innervation pattern of inhibitory neurons. A mechanism unique to GABAergic neurons is activity-dependent GABA synthesis, largely mediated through activity-regulated transcription of the rate-limiting enzyme GAD67. Such cell-wide as well as synaptic regulation of GABA signalling may constitute a mechanism by which input levels and patterns onto GABAergic neurons shape their innervation pattern during circuit development.

Development of GABA innervation in the cerebral and cerebellar cortices

In many areas of the vertebrate brain, such as the cerebral and cerebellar cortices, neural circuits rely on inhibition mediated by GABA (gamma-aminobutyric acid) to shape the spatiotemporal patterns of electrical signalling. The richness and subtlety of inhibition are achieved by diverse classes of interneurons that are endowed with distinct physiological properties. In addition, the axons of interneurons display highly characteristic and class-specific geometry and innervation patterns, and thereby distribute their output to discrete spatial domains, cell types and subcellular compartments in neural networks. The cellular and molecular mechanisms that specify and modify inhibitory innervation patterns are only just beginning to be understood.

GAD67-mediated GABA synthesis and signaling regulate inhibitory synaptic innervation in the visual cortex

The development of GABAergic inhibitory circuits is shaped by neural activity, but the underlying mechanisms are unclear. Here, we demonstrate a novel function of GABA in regulating GABAergic innervation in the adolescent brain, when GABA is mainly known as an inhibitory transmitter. Conditional knockdown of the rate-limiting synthetic enzyme GAD67 in basket interneurons in adolescent visual cortex resulted in cell autonomous deficits in axon branching, perisomatic synapse formation around pyramidal neurons, and complexity of the innervation fields; the same manipulation had little influence on the subsequent maintenance of perisomatic synapses. These effects of GABA deficiency were rescued by suppressing GABA reuptake and by GABA receptor agonists. Germline knockdown of GAD67 but not GAD65 showed similar deficits, suggesting a specific role of GAD67 in the maturation of perisomatic innervation. Since intracellular GABA levels are modulated by neuronal activity, our results implicate GAD67-mediated GABA synthesis in activity-dependent regulation of inhibitory innervation patterns.

Cell-specific expression of wild-type MeCP2 in mouse models of Rett syndrome yields insight about pathogenesis

Rett syndrome (RTT), a leading cause of mental retardation with autistic features in females, is caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). RTT is characterized by a diverse set of neurological features that includes cognitive, motor, behavioral and autonomic disturbances. The diverse features suggest that specific neurons contribute to particular phenotypes and raise the question whether restoring MeCP2 function in a cell-specific manner will rescue some of the phenotypes seen in RTT. To address this, we generated transgenic mice expressing inducible MeCP2 under the control of the brain-specific promoters calcium/calmodulin-dependent protein kinase II (CamKII) or neuron-specific enolase (Eno2) and bred them onto mouse models lacking functional MeCP2. Expression of normal MeCP2 in either CamKII or Eno2 distribution was unable to prevent the appearance of most of the phenotypes of the RTT mouse models. These results suggest that most RTT phenotypes are caused either by disruption of complex neural networks involving neurons throughout the brain or by disruption of the function of specific neurons outside of the broad CamKII or Eno2 distribution.

Automated seizure abatement in humans using electrical stimulation

The need for novel, efficacious, antiseizure therapies is widely acknowledged. This study investigates in humans the feasibility, safety, and efficacy of high-frequency electrical stimulation (HFES; 100-500 Hz) triggered by automated seizure detections. Eight patients were enrolled in this study, which consisted of a control and an experimental phase. HFES was delivered directly to the epileptogenic zone (local closed-loop) in four patients and indirectly, through anterior thalami (remote closed-loop), to the other four patients for every other automated seizure detection made by a validated algorithm. Interphase (control vs experimental phase) and intraphase (stimulated vs nonstimulated) comparisons of clinical seizure rate and relative severity (clinical and electrographic) were performed, and differences were assessed using effect size. Patients were deemed "responders" if seizure rate was reduced by at least 50%; the remaining patients were deemed "nonresponders." All patients completed the study; rescue medications were not required. There were 1,491 HFESs (0.2% triggered after-discharges). Mean change in seizure rate in the local closed-loop group was -55.5% (-100 to +36.8%); three of four responders had a mean change of -86% (-100 to -58.8%). In the remote closed-loop, the mean change of seizure rate was -40.8% (-72.9 to +1.4%); two of four responders had a mean change of -74.3% (-75.6 to -72.9%). Mean effect size was zero in the local closed-loop (responders: beneficial and medium to large in magnitude) and negligible in the remote closed-loop group (responders: beneficial and medium to large). HFES effects on epileptogenic tissue were immediate and also outlasted the stimulation period. This study demonstrates the feasibility and short-term safety of automated HFES for seizure blockage, and also raises the possibility that it may be beneficial in pharmaco-resistant epilepsies.

Antidepressants activate CaMKII in neuron cell body by Thr286 phosphorylation

CaM kinase II, a regulator of synaptic plasticity, is implicated in pathophysiology and pharmacology of psychiatric disorders. Chronic treatment with antidepressants desipramine and reboxetine up-regulated CaM kinase II in neuronal cell bodies of hippocampus. mRNA/protein expression for alphaCaM kinase II was unchanged, whereas Thr phosphorylation was increased in pyramidal/granular cell bodies, suggesting that increased phosphorylation is responsible for kinase activation. Short-term treatment of neuronal cultures with reboxetine reduced kinase activation in a concentration-dependent manner. The short-term inhibitory effect of reboxetine suggests that kinase up-regulation during antidepressant drug treatment is an adaptive response compensating for initial functional down-regulation.

N-methyl-D-aspartate receptor dependent transcriptional regulation of two calcium/calmodulin-dependent protein kinase type II isoforms in rodent cerebral cortex

Alpha Calcium/calmodulin-dependent protein kinase type II (CaMKII-alpha) expression is regulated in an activity-dependent manner, but it is not known whether other CaMKII isoforms (beta, delta, and gamma) are similarly regulated. We examined the activity-dependent regulation of these CaMKII isoforms in vivo, using a model of generalized seizures caused by i.p. injection of kainic acid. Following seizure induction, CaMKII-alpha expression was downregulated and CaMKII-delta expression upregulated while CaMKII-beta and CaMKII-gamma expression was unaffected. A transient downregulation in CaMKII-alpha and a transient increase in CaMKII-delta occurred throughout neocortex in the same temporal order. Although CaMKII-alpha mRNA was decreased by seizure activity, the less abundant, alternatively spliced, CaMKII-alpha33 mRNA was unaffected. Organotypic cortical slice cultures treated with bicuculline and 4-aminopyridine to induce seizure activity also showed a downregulation of CaMKII-alpha mRNA and an upregulation of CaMKII-delta mRNA. Prior exposure to tetrodotoxin prevented the changes in CaMKII-alpha and CaMKII-delta mRNA regulation and this was mimicked by D-L-2-amino-5-phosphonovaleric acid, but not by 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline, suggesting that CaMKII-alpha and CaMKII-delta mRNA expression is regulated in an N-methyl-D-aspartate receptor-dependent manner. Regulation was also transcription dependent. Blocking transcription with actinomycin-D prevented activity-dependent changes in CaMKII-alpha and CaMKII-delta mRNA, but produced opposite effects on basal transcription, resulting in more stabilized CaMKII-alpha mRNA and less stabilized CaMKII-delta mRNA. These results reveal unique patterns of seizure-induced alterations in CaMKII mRNAs. Activity-dependent changes in subunit composition could, therefore, differentially influence the functional attributes of the CaMKII holoenzyme.

Neuronal activity and neurotrophic factors regulate GAD-65/67 mRNA and protein expression in organotypic cultures of rat visual cortex

Environmental factors are known to regulate the molecular differentiation of neocortical interneurons. Their class-defining transmitter synthetic enzymes are the glutamic acid decarboxylases (GAD); yet, fairly little is known about the developmental regulation of transcription and translation of the GAD-65/67 isoforms. We have characterized the role of neuronal activity, neurotrophins and afferent systems for GAD-65/67 expression in visual cortex in organotypic cultures (OTC) compared with in vivo in order to identify cortex-intrinsic regulatory mechanisms. Spontaneously active OTC prepared at postnatal day 0 displayed from 10 days in vitro (DIV) onwards 12-14% GAD-65/GAD-67 neurons similar to in vivo. However, GAD-65 mRNA was higher, whereas GAD-67 protein was lower, than in vivo. During the first week neurotrophins increased whereas the Trk receptor inhibitor K252a and MEK inhibitors decreased both GAD mRNAs and proteins. After 10 DIV GAD expression no longer depended on neurotrophin signalling. Activity-deprived OTC revealed only 6% GAD-67 neurons and mRNA and protein were reduced by 50%. GAD-65 mRNA was less reduced, but protein was reduced by half, suggesting translational regulation. Upon recovery of activity GAD mRNAs, cell numbers, and both proteins quickly returned to normal and these 'adult' levels were resistant to late-onset deprivation. In 20 DIV activity-deprived OTC, only neurotrophin 4 increased GAD-65/67 mRNAs, rescued the percentage of GAD-67 neurons and increased both proteins in a TrkB-dependent manner. Activity deprivation had thus shifted the period of neurotrophin sensitivity to older ages. The results suggested neuronal activity as a major regulator differentially affecting transcription and translation of the GAD isoforms. The early presence of neuronal activity promoted the GAD expression in OTC to a neurotrophin-independent state suggesting that neurotrophins play a context-dependent role.

Effects of chronic salicylate on GABAergic activity in rat inferior colliculus

It is well accepted that salicylate ototoxicity results in reversible tinnitus in humans. Salicylate-induced tinnitus may be an example of plasticity of the central auditory system and could potentially serve as a model to further understand mechanisms of tinnitus generation. This study examined levels of glutamic acid decarboxylase (GAD) and the binding characteristics of the GABA(A) receptor in auditory brainstem structures of Long-Evans rats chronically treated with salicylate. Western blotting revealed a significant 63% (P<0.008) elevation of GAD levels in the inferior colliculus (IC) of salicylate-treated subjects. This occurred in subjects demonstrating behavioral evidence of tinnitus. Muscimol saturation analysis was indicative of a salicylate-related increase in receptor affinity. Linear regression of [(3)H]muscimol saturation analysis data revealed a significant (P<0.05) reduction in K(d) values in whole IC (-48%), as well as in the central nucleus of IC (CIC, -58%) and combined external and dorsal cortex of IC (E/DCIC, -46%). The number of GABA(A) binding sites (B(max)) were also significantly (P<0.05) decreased. These changes were observed only in central auditory structures. This suggests that GAD expression and GABA(A) receptor binding characteristics may be altered with chronic exposure to sodium salicylate and these changes may represent aberrant plasticity clinically experienced as tinnitus.

GAD levels and muscimol binding in rat inferior colliculus following acoustic trauma

Pharmacological studies of the inferior colliculus (IC) suggest that the inhibitory amino acid neurotransmitter gamma-aminobutyric acid (GABA) plays an important role in shaping responses to simple and complex acoustic stimuli. Several models of auditory dysfunction, including age-related hearing loss, tinnitus, and peripheral deafferentation, suggest an alteration of normal GABA neurotransmission in central auditory pathways. The present study attempts to further characterize noise-induced changes in GABA markers in the IC. Four groups (unexposed control, 0 h post-exposure, 42 h post-exposure, and 30 days post-exposure) of 3-month-old male Fischer 344 rats were exposed to a high intensity sound (12 kHz, 106 dB) for 10 h. Observed hair cell damage was primarily confined to the basal half of the cochlea. There was a significant decrease in glutamic acid decarboxylase (GAD(65)) immunoreactivity in the IC membrane fraction compared to controls (P<0.05) at 0 h (-41%) and 42 h (-28%) post-exposure, with complete recovery by 30 days post-exposure (P>0.98). Observed decreases in cytosolic levels of GAD(65) were not significant. Quantitative muscimol receptor binding revealed a significant increase (+20%) in IC 30 days after sound exposure (P<0.05). These data suggest that changes in GABA neurotransmission occur in the IC of animals exposed to intense sound. Additional studies are needed to determine whether these changes are a result of protective/compensatory mechanisms or merely peripheral differentiation, as well as whether these changes preserve or diminish central auditory system function.

Detection of glutamate decarboxylase isoforms in rat inferior colliculus following acoustic exposure

The inferior colliculus is a central auditory structure which serves as a site for the integration of ascending and descending auditory information. Changes in central auditory structures may occur with acoustic exposure, which cannot be explained by alterations in cochlear function alone. Rats were exposed to a 10-kHz tone at 100 dB SPL for 9 h. Auditory brainstem response measures showed an initial 25-30-dB threshold shift across all tested frequencies. By 30 days post-exposure, thresholds for clicks and most frequencies returned to near control levels; however, thresholds remained elevated at 10 and 20 kHz. Inner hair cell loss was confined to apical and basal ends of the cochlea, and did not exceed 20%. Inferior colliculus levels of the two isoforms of the GABA synthetic enzyme glutamate decarboxylase (65,000 and 67,000 mol. wt forms) were measured immediately post-exposure (0 h) and at two and 30 days post-exposure using quantitative immunocytochemical and western blotting techniques. Zero-hour measures revealed a significant increase in the level of glutamate decarboxylase (mol. wt 67,000) protein (118%), as well as in the optical density (35%) of immunolabeled cells. By 30 days post-exposure, inferior colliculus protein levels of both glutamate decarboxylase isoforms were significantly below unexposed controls (39% and 21% for the 65,000 and 67,000 mol. wt forms, respectively). These studies describe increased markers for GABA immediately following acoustic exposure, followed by a decline to below control levels from two to 30 days post-exposure. It remains to be determined whether noise trauma-induced changes in glutamate decarboxylase levels in the inferior colliculus reflect protective up-regulation in response to intense stimulation, followed by the establishment of new neurotransmitter equilibrium levels.

Cell- and lamina-specific expression and activity-dependent regulation of type II calcium/calmodulin-dependent protein kinase isoforms in monkey visual cortex

In situ hybridization histochemistry and immunocytochemistry were used to study localization and activity-dependent regulation of alpha, beta, gamma, and delta isoforms of type II calcium/calmodulin-dependent protein kinase (CaMKII) and their mRNAs in areas 17 and 18 of normal and monocularly deprived adult macaques. CaMKII-alpha is expressed overall at levels three to four times higher than that of CaMKII-beta and at least 15 times higher than that of CaMKII-gamma and -delta. All isoforms are expressed primarily in pyramidal cells of both areas, especially those of layers II-III, IVA (in area 17), and VI, but are also expressed in nonpyramidal, non-GABAergic cells of layer IV of both areas and in interstitial neurons of the white matter. CaMKII-alpha and -beta are colocalized, suggesting the formation of heteromers. There was no evidence of expression in neuroglial cells. Each isoform has a unique pattern of laminar and sublaminar distribution, but cortical layers or sublayers enriched for one isoform do not correlate with layers receiving inputs only from isoform-specific layers of the lateral geniculate nucleus. CaMKII-alpha and -beta mRNA and protein levels in layer IVC of area 17 are subject to activity-dependent regulation, with brief periods of monocular deprivation caused by intraocular injections of tetrodotoxin leading to a 30% increase in CaMKII-alpha mRNA and a comparable decrease in CaMKII-beta mRNA in deprived ocular dominance columns, especially of layer IVCbeta. Expression in other layers and expression of CaMKII-gamma and delta were unaffected. Changes occurring in layer IVC may influence the formation of heteromers and protect supragranular layers from CaMKII-dependent plasticity in the adult.

Estradiol increases dendritic spine density by reducing GABA neurotransmission in hippocampal neurons

We have previously shown that estradiol causes a twofold rise in dendritic spine density in cultured rat hippocampal neurons, as it does in vivo. More recently, estrogen receptors have been localized to aspiny inhibitory hippocampal interneurons, indicating that their effect on spiny pyramidal neurons may be indirect. We therefore examined the possibility that estradiol affects spine density by regulating inhibition in cultured hippocampal interneurons. Immunocytochemically, estrogen receptors were found to be co-localized with glutamate decarboxylase (GAD)-positive neurons (approximately 21% of total neurons in the culture). Exposure of cultures to estradiol for 1 d caused a marked decrease (up to 80%) in the GAD content of the interneurons, measured both by immunohistochemistry and Western blotting. Also, the number of GAD-positive neurons in the cultures decreased to 12% of the total cell population. Moreover, GABAergic miniature IPSCs were reduced in both size and frequency by estradiol, whereas miniature EPSCs increased in frequency. We then mimicked the proposed effects of estradiol by blocking GABA synthesis with mercaptopropionic acid (MA). Cultures treated with MA expressed a dose-dependent decrease in GABA immunostaining that mimicked that seen with estradiol. MA-treated cultures displayed a significant 50% increase in dendritic spine density over controls, similar to that produced by estradiol. These results indicate that estradiol decreases GABAergic inhibition in the hippocampus, which appears to effectively increase the excitatory drive on pyramidal cells, and thus may provide a mechanism for formation of new dendritic spines.

Differential and time-dependent changes in gene expression for type II calcium/calmodulin-dependent protein kinase, 67 kDa glutamic acid decarboxylase, and glutamate receptor subunits in tetanus toxin-induced focal epilepsy

To study potential molecular mechanisms of epileptogenesis in the neocortex, the motor cortex of rats was injected with tetanus toxin (TT), and gene expression for 67 kDa glutamic acid decarboxylase (GAD-67), type II calcium/calmodulin-dependent protein kinase (CaMKII), NMDA receptor subunit 1 (NR1), and AMPA receptor subunit 2 (GluR2) was investigated by in situ hybridization histochemistry. Injections of 20-35 ng TT induced recurrent seizures after a postoperative period ranging from 4 to 13 d. A majority of rats perfused 5-7 d after TT injection showed altered gene expression, but the changes varied in their areal extent, ranging from most neocortical areas on the injected side in some rats to mainly the frontoparietal cortex or the motor cortex in others. Epileptic rats perfused 14 d after TT injection showed a focus of increased GAD-67 and NR1, and of decreased alpha-CaMKII and GluR2 mRNA levels at the injection site. A zone of cortex surrounding the focus showed changes in alpha-CaMKII, GAD-67, and NR1 mRNA levels that were reciprocal to those in the focus. The results suggest that TT-induced seizure activity initially spread to a variable extent but was gradually restricted 2-3 d after seizure onset. The focus and the surround showing reciprocal changes in gene expression are thought to correspond to the electrophysiologically identified epileptic focus and inhibitory surround, respectively. The findings suggest that lateral inhibition between neighboring cortical regions will be affected and contribute to a neurochemical segregation of an epileptic focus from surrounding cortex.