Age-dependent regulation of GABAA receptors in neocortex

Implications for treatment: GABAA receptors in aging, Down syndrome and Alzheimer's disease

In addition to progressive dementia, Alzheimer's disease (AD) is characterized by increased incidence of seizure activity. Although originally discounted as a secondary process occurring as a result of neurodegeneration, more recent data suggest that alterations in excitatory-inhibitory (E/I) balance occur in AD and may be a primary mechanism contributing AD cognitive decline. In this study, we discuss relevant research and reports on the GABA(A) receptor in developmental disorders, such as Down syndrome, in healthy aging, and highlight documented aberrations in the GABAergic system in AD. Stressing the importance of understanding the subunit composition of individual GABA(A) receptors, investigations demonstrate alterations of particular GABA(A) receptor subunits in AD, but overall sparing of the GABAergic system. In this study, we review experimental data on the GABAergic system in the pathobiology of AD and discuss relevant therapeutic implications. When developing AD therapeutics that modulate GABA it is important to consider how E/I balance impacts AD pathogenesis and the relationship between seizure activity and cognitive decline.

The ongoing balance of cortical excitation and inhibition during early development

Two papers recently published in Nature propose that the balance between excitation and inhibition is important for the maturation of cortical function. Their conclusions however, are contradictory; one study suggests that balance is established before hearing onset, whereas the other proposes that balance is established after hearing onset. We carefully examined the data and found that the differences between the two groups are less dramatic than they first appear. Despite their methodological differences, both studies provide evidence that an ongoing balance between cortical excitation/inhibition accounts for the maturation and refinement of cortical function during early development.

GABAAreceptors in aging and Alzheimer’s disease

In this article we present a comprehensive review of relevant research and reports on the GABA(A) receptor in the aged and Alzheimer's disease (AD) brain. In comparison to glutamatergic and cholinergic systems, the GABAergic system is relatively spared in AD, but the precise mechanisms underlying differential vulnerability are not well understood. Using several methods, investigations demonstrate that despite resistance of the GABAergic system to neurodegeneration, particular subunits of the GABA(A) receptor are altered with age and AD, which can induce compensatory increases in GABA(A) receptor subunits within surrounding cells. We conclude that although aging- and disease-related changes in GABA(A) receptor subunits may be modest, the mechanisms that compensate for these changes may alter the pharmacokinetic and physiological properties of the receptor. It is therefore crucial to understand the subunit composition of individual GABA(A) receptors in the diseased brain when developing therapeutics that act at these receptors.

Age-related alterations in GABAA receptor subunits in the nonhuman primate hippocampus

Pharmacological studies have documented that altered drug responses, particularly to benzodiazepines, are common in elderly populations. While numerous factors may contribute to changes in drug response, age-related alterations in the molecular composition of GABA(A) receptors may be a key factor in regulating these responses. We employed quantitative densitometry to examine the cytological features and density of highly prevalent hippocampal GABA(A) receptor subunits (alpha1 and beta2/3) in young and aged rhesus monkeys. alpha1 and beta2/3 subunit immunostaining was differentially distributed throughout the hippocampus. In addition, beta2/3 immunolabeling in aged monkeys was characterized by marked intersubject variability in labeling intensity, with dramatic reductions present in 3 of 5 samples. alpha1 immunolabeling in aged monkeys was significantly reduced in the CA2 and CA3 subregions, and in hilus/polymorphic layer of the dentate gyrus. Collectively, our findings demonstrate that not only are GABA(A) receptor subunits differentially distributed throughout the hippocampus, but they are also differentially altered with increased age--changes that may have an important impact on the binding properties of GABA(A) receptor pharmacological agents.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Effects of bilateral olfactory bulbectomy on N-methyl-D-aspartate receptor function: autoradiographic and behavioral studies in the rat

Rat bilateral olfactory bulbectomy (OBX) serves as a useful model in the study of depression and the mechanisms of action of antidepressant treatments. Considering the evidence of NMDA receptors involvement in depression, the present study was undertaken in order to investigate the time-course effects of OBX on the NMDA receptor function. Following bilateral olfactory bulbectomy, rats display an increase in locomotor activity and changes in other types of behavior in a novel environment. Autoradiographic experiments using the noncompetitive NMDA antagonist [(125)I]-iodo-MK-801 as the labeling agent showed that this increase in behavioral activities corresponds to a decrease in [(125)I]-iodo-MK-801 binding in a number of brain regions. In most regions, this reduction reached significance by the third week following OBX. However, in some cortical areas-a nucleus of the thalamus (AV) and one of the amygdala (LA)-this reduction was already significant in the first or second week following OBX and lasted throughout the 4 weeks of the study. We also compared the behavioral modifications induced by a challenge injection of MK-801 (0.2 mg/kg i.p.) in OBX and sham-operated rats. This challenge is known to induce hyperlocomotion and a number of stereotypies in naive rats. These effects were drastically reduced in OBX as compared to sham-operated rats. These data are consistent with the above-mentioned decrease in cerebral binding of MK-801 to NMDA receptors.

Stress and neurosteroids in adult and aged rats

The progesterone derivative 3 alpha-hydroxy-5 alpha-pregnan-20 one (allopregnanolone/AP) and the deoxycorticosterone derivative 3 alpha-21-dihydroxy-5 alpha- pregnan-20 one (allotetra-hydrodeoxycorticosterone/THDOC) are endogenous neuroactive steroids endowed with neuromodulatory actions in the central nervous system. Their best-characterized membrane-receptor-dependent action consists in the amplification of GABA-gated chloride currents mediated by specific interactions with the GABAA receptor complex, which appears responsible for the pharmacological effects (anxiolytic, anticonvulsant, hypnotic/anaesthetic) of exogenously administered AP and THDOC. Several acute stress paradigms and different negative allosteric modulators (isoniazid and FG 7142) of GABAA receptors time dependently increase brain and plasma concentrations of AP and THDOC only in intact or sham-operated but not in adrenalectomized-orchiectomized rats. These results suggest that acute stress and inhibitors of GABAA receptors increase the brain and plasma neurosteroid concentrations via a reduction of the inhibitory action exerted by GABA on the hypothalamic-pituitary-adrenal axis. The comparison between the time course of the changes in GABAA receptor function and of their behavioral correlates (proconflict behavior) and that of the changes of endogenous neuroactive steroids are consistent with the view that AP and THDOC may play a role in restoring the GABAergic tone to prestress conditions, by limiting the duration and the extent of its stress-induced reduction. The acute stress-elicited increase of AP and THDOC is observed in adult as well as in aged rats, which show a reduced basal GABAergic transmission and a greater response to the effect of stress in terms of their brain cortical neuroactive steroid concentrations than adult rats.

Afferent regulation of glycine receptor distribution in the gerbil LSO

Synaptic activity plays an important role in many aspects ofneuronal development, particularly the expression of proteins. In this study, the influence of inhibitory and excitatory afferents on the development of glycine receptor density in the lateral superior olive (LSO) of Mongolian gerbils was investigated. Afferent activity was manipulated by removing one or both cochleas at postnatal day 7, prior to the onset of sound-evoked responses. Due to the anatomy of the LSO, these manipulations result in either excitatory denervation, inhibitory denervation, or both. The density of glycine receptors in the LSO was determined at 21 days postnatal. Glycine receptors were either labeled with tritiated strychnine (3H-SN) or with an antibody directed against gephyrin, a protein closely associated with the receptor complex. Antibody binding was used to quantify the differential glycine receptor density between the medial limb (high frequency area) and the lateral limb (low frequency area) of the LSO. 3H-SN was used to quantify the amount of glycine receptors in each part of the LSO in control and experimental animals. In addition, changes in neuron density and neuron cross-sectional area were quantified following cochlear ablations. In control animals, the amount of glycine receptors is about 2- to 3-fold higher in the high-frequency than in the low-frequency region. In bilaterally ablated animals, the same density of glycine receptors was measured in the high- and low-frequency region. Unilateral ablations had no significant effect on glycine receptor distribution, either ipsi- or contralateral to the ablation. The neuron cross-sectional area decreased about 30% in the ipsilateral LSO of unilaterally ablated animals and in bilaterally ablated animals. However, alterations of soma density and cross-sectional area were similar in the high- and low-frequency projection region. These results suggest that the distribution of glycine receptors is only changed when excitatory and inhibitory afferents have been denervated.

GABAA receptor subunit expression changes in the rat cerebellum and cerebral cortex during aging

Significant aging-related decreased expression of various GABAAR subunit mRNAs (alpha 1, gamma 2, beta 2, beta 3 and sigma) was found in both cerebellum and cerebral cortex using quantitative dot blot and in situ hybridization techniques. Contrary to the other subunits, the alpha 6 mRNA expression was significantly increased in the aged cerebellum. Parallel age-related changes in protein expression for gamma 2 and beta 2/3 (decrease) and alpha 6 (increase) were revealed in cerebellum by quantitative immunocytochemistry. However, no significant changes in alpha 1 protein expression nor in the number or affinity of [3H]zolpidem binding sites were detected in cerebellum even though alpha 1 mRNA expression was significantly decreased in the aged rat. Age-related increased expression of alpha 6 mRNA and protein in the cerebellum was accompanied by no significant changes in the number of diazepam-insensitive [3H]Ro15-4513 binding sites. In the cerebral cortex, no changes in the protein expression of the main GABAA receptor subunits (alpha 1, gamma 2 and beta 2/3) were observed which contrasted with the age-related decreased expression of the corresponding mRNAs. No significant changes in the number or affinity of [3H]zolpidem binding sites were observed in the cerebral cortex. Thus, age-related changes in the mRNA expression of a particular subunit does not necessarily lead to similar changes in protein or assembly into mature GABAA receptors. The results reveal the existence of complex regulatory mechanisms of GABAA receptor expression, at the transcriptional, translational and post-translational and/or assembly levels, which vary with the subunit and brain area.

Aging-related subunit expression changes of the GABAA receptor in the rat hippocampus

Aging-related changes in the subunit expression of some hippocampal GABAA receptors have been found. Quantitative in situ hybridization has revealed that alpha 1, subunit messenger RNA expression was significantly increased in the hippocampus (34%) of old rats. The largest increases were observed in the dentate gyrus (76%) and in the CA1 field (30%). Quantitative immunocytochemistry also showed increased protein expression of the alpha 1 subunit in the dentate gyrus (19%) and CA1 (14%) of old rats. The increased alpha 1 messenger RNA and protein expression led to increased proportions of assembled GABAA receptors that contained alpha 1 subunits, as revealed by quantitative immunoprecipitation of (3H)flunitrazepam and (3H)muscimol binding. In contrast, there were no significant changes in the expression of beta 2, beta 3 and total gamma 2 (gamma 2S + gamma 2L) subunits, although a slightly increased expression of gamma 2L peptide was detected in the hippocampus proper (7%), but not in the dentate gyrus. The results are consistent with the notion that in the rat hippocampus there is an aging-related change in the subunit composition of some GABAA receptors.

The origin of synaptic neuroplasticity: crucial molecules or a dynamical cascade?

What is neuroplasticity and what are its origins? These questions have been the subject of intense theoretical and experimental research in the neurosciences for decades. Basically, the term neuroplasticity refers to the ability of neurons to alter some functional property in response to alterations in input. Traditional definitions, however, are often imprecise and restricted to particular 'model' neural systems. In the present article we will consider several of the most widely studied models of synaptic-level neuroplasticity including alterations in response properties of two types of invertebrate sensory neurons, long-term potentiation (LTP) in mammalian hippocampus and cortex, and ocular dominance shifts in cat visual cortex. While many other forms of neuroplasticity have been studied, these examples typify the diversity of the subject, as well as illustrate our contention that no unitary model of the phenomena is possible for all conditions. This last point is of particular importance for the mammalian literature, since many hypotheses concerning the mechanism(s) underlying the initiation of neuroplasticity have proposed a single crucial molecular element as the primary causal agent. A closer examination of these various hypotheses, in concert to several examples from the invertebrate literature, leads, however, to the conclusion that synaptic neuroplasticity must arise from a series of inter-related molecular events of a particular form, a cascade, in which individual elements may differ radically from system to system. We next provide an overview of our studies of age-dependent regulation of excitatory and inhibitory ionotropic neurotransmitter receptor populations in cortex in response to agonist and depolarizing stimulation. We provide evidence that such regulation for ionotropic receptors is under the control of ionically driven receptor kinase and phosphatase activity which is also age-dependent in function. These data provide the basis for a cascade model of receptor regulation. Based on this qualitative model, we describe a quantitative computer simulation of certain age-dependent stages in the receptor regulatory cascade which may interact to produce LTP-like effects. While such a model is not exclusive, it nevertheless provides a demonstration that elements in the proposed cascade may comprise the necessary and sufficient conditions for some forms of neuroplasticity. We also propose some of the principles underlying our model as a means of unifying much of the diverse phenomenology reported in the literature. Finally, we make a series of explicit predictions which are testable with current experimental techniques.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative autoradiographic studies of the effects of bilateral olfactory bulbectomy in the rat brain: central- and peripheral-type benzodiazepine receptors

We investigated the discrete regional effects of bilateral olfactory bulbectomy on central- and peripheral-type benzodiazepine receptors in rat brains at weekly intervals until one month after bulb ablation. Persistent increases in [3H]flunitrazepam binding to central benzodiazepine receptors were observed in the cingulum (27%) and in the frontal (15%) and parietal (14%) cortices. Progressive increases in central benzodiazepine receptors, reaching statistical significance four weeks after olfactory bulbectomy, were observed in the ventromedial thalamic nucleus (35%), the lateral hypothalamic region (22%), the basolateral amygdaloid nucleus (23%) and substantia nigra (25%). Persistent major increases (between four- and six-fold) in [3H]PK-11195 [eH]1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide binding to peripheral-type benzodiazepine receptors were observed in all anterior olfactory nuclei. Similarly, throughout the time period studied, [3H]PK-11195 binding densities were increased two- to three-fold in the piriform cortex and lateral olfactory tract. These observations confirm the usefulness of [3H]PK-11195 binding as a marker of neuronal insult in the brain. Moreover, the persistent regional increases in [3H]flunitrazepam binding to central-type benzodiazepine receptors suggest that GABAergic transmission is altered following olfactory bulb ablation.

Age-dependent expression, phosphorylation and function of neurotransmitter receptors: pharmacological implications

In recent years, a number of experimental controversies have arisen in the pharmacological sciences literature. Two possibly related examples concern the role of phosphorylation in receptor regulation and the occurrence of paradoxical effects of neurally active drugs in patients. In this article, Ruth Lanius and colleagues review these two issues and suggest that some of the recently reported age-dependent aspects of neurotransmitter receptor regulation and function may explain drug action. An appreciation of the dynamic interactions involved in receptor regulation, especially during development, may offer novel perspectives on neural function.

gamma-Aminobutyric acidA receptor regulation by a chloride-dependent kinase and a sodium-dependent phosphatase

gamma-Aminobutyric acidA (GABAA) receptors are linked to ion channels which mediate many aspects of neural inhibition. Although the effects of phosphorylation on GABAA receptor function have been widely studied, the actual role of phosphorylation in the regulation of these receptors still remains controversial. In recent reports, we have described the effects of phosphorylating/dephosphorylating enzymes on the regulation of GABAA receptors in a rat cortical slice preparation (Shaw et al., Mol. Neuropharmacol., 2 (1992) 297-302; Shaw and Lanius, Dev. Brain Res., 70 (1992) 153-161; Pasqualotto et al., Neuroreport, 4 (1993) 447-450) and predicted that ionic co-factors are involved in mediating the regulation of GABAA receptors by kinases and phosphatases. In the present report, the effects of chloride, sodium, potassium, and calcium were examined alone and in the presence of cAMP-dependent protein kinase (protein kinase A) or alkaline phosphatase. The results showed a decrease in [3H]SR 95531 (GABAA receptor antagonist) binding after incubation with chloride alone; this decrease was further enhanced in the presence of protein kinase A. Both effects could be blocked by a protein kinase A inhibitor. Conversely, an increase in [3H]SR 95531 binding was observed after incubation with sodium alone; this increase was further enhanced in the presence of alkaline phosphatase. In both cases these increases in binding could be blocked by sodium orthovanadate, a phosphatase inhibitor. Potassium was ineffective under all conditions; calcium showed enzyme-independent effects at low concentrations only. These results suggest the existence of a novel chloride-dependent protein kinase which may have significant sequence homology to protein kinase A, and a novel sodium-dependent phosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)

High-affinity kainate binding sites in living slices of rat neocortex: characterization and regulation

We have characterized a high-affinity kainate binding site in in vitro living rat neocortical slices using [3H]kainate. [3H]Kainate labelled at least two binding sites, the higher affinity site with a Kd of 7.1 nM and a Bmax of 71.2 fmol/mg protein. This high-affinity binding site showed a pharmacology consistent with a kainate receptor with competition by kainate and domoic acid, as well as the (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate antagonist 6-cyano-2,3-dihydroxy-7-nitroquinoxaline. Increases in cellular depolarization induced by 2-h preincubations in veratridine and glutamate led to a significant 55% average decrease in [3H]kainate binding in adult cortex. Similarly, preincubation in kainate led to a significant average 26% decrease in binding. In both instances, Eadie-Hofstee analysis of saturation binding data revealed that the decreased binding reflected changes in receptor number. At different postnatal ages, increases in cellular depolarization significantly decreased binding (< 20 days postnatal age, -86%; > 60 days, -48%). Kainate treatment also significantly decreased binding at all ages (-64% at < 20 days; > 60 days, -18%), with significant differences noted between ages. These age-dependent effects are unlike those previously described for either N-methyl-D-aspartate [Lanius and Shaw (1992) Anat. Rec. 232, 54(A)] or (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate high affinity receptors [Shaw and Lanius (1992) Devl Brain Res. 68, 225-233].(ABSTRACT TRUNCATED AT 250 WORDS)

Reversible kinase and phosphatase regulation of brain amino acid receptors in postnatal development

In a previous study [Shaw, C., Pasqualotto, B. and Lanius, R.A., Mol. Neuropharmacol., in press] we have shown that phosphorylation and dephosphorylation actions of protein kinase and alkaline phosphatase lead to decreases or increases in the number of GABAA and AMPA receptors in adult rat neocortex. Using the same in vitro cortical slice preparation, we have now examined the role of these enzymes in regulating GABAA and AMPA receptors at different stages of postnatal development. GABAA receptors were labelled with [3H]SR95531 [Shaw, C. and Scarth, B.A., Mol. Brain Res., 11 (1991) 273-282]; AMPA receptors were labelled with [3H]CNQX [Lanius, R.A. and Shaw, C., Mol. Brain Res., 15 (1992) 256-262]. At postnatal day 14, GABAA receptors showed a decrease in binding in response to alkaline phosphatase treatment as opposed to an increase in binding observed in response to protein kinase treatment. Similar effects were observed for AMPA receptors at 20 days of age. The direction of regulation following the enzyme treatments were opposite to those observed in the adult cortex for both receptor populations. These fundamental changes in the enzymatic nature of regulation for such key inhibitory and excitatory receptor populations in cortex may signal an important role for age-dependent kinases and phosphatases in the events leading to modifications in neuronal function during postnatal development.

Characterization and regulation of a high affinity [3H]CNQX labelled AMPA receptor in rat neocortex

We have characterized a high affinity site of the alpha-amino-3 hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor in in vitro living slices of adult rat neocortex using [3H]CNQX, and AMPA antagonist. [3H]CNQX labelled multiple binding sites with a Bmax of a high affinity site of approximately 470 fmol/mg protein and an apparent Kd of 11.3 nM. The high affinity site of the AMPA receptor could be down-regulated (36%) by 2 h preincubations in quisqualate, an AMPA agonist. Increases in electrical activity induced by a combination of veratridine and glutamate also led to an average decrease of the high affinity AMPA receptor number of 17%. In addition, preincubations with muscimol, a GABAA agonist, as well as glutamate agonists kainate and N-methyl-D-aspartate (NMDA) led to an average increase in high affinity AMPA receptor number of 17%, 14%, and 37%, respectively. The present results show that a ligand-gated high affinity AMPA receptor can be regulated by agonist stimulation as well as changes in neural activity.

Cortical AMPA receptors: age-dependent regulation by cellular depolarization and agonist stimulation

We have recently shown that a high-affinity AMPA receptor labelled with the antagonist [3H]CNQX can be regulated in a 'living' cortical slice preparation by agonist stimulation or changes in electrical activity (Lanius, R.A. and Shaw, C. (1992) Anat. Rec., in press). Based on a study of GABAA receptors (Shaw, C. and Scarth, B.A. (1992) Mol. Brain Res., in press), which showed age-dependent changes in regulation, we have now investigated the regulation of high-affinity AMPA receptors in neocortex at different stages in postnatal development. The results show that regulation by agonist stimulation and increases in bioelectric activity are age-dependent in amount and, in the latter case, in direction. Agonist stimulation using quisqualate resulted in a significant receptor down-regulation of approximately 7% at ages less than 20 days postnatal; in adult rats quisqualate led to a significant 23% decrease. Changes in bioelectric activity induced by a combination of veratridine and glutamate showed a significant increase in AMPA receptor number of 16% at ages less than 20 days, whereas such treatment resulted in a significant 18% decrease in adult rats. The present data reveal a near mirror-image to the effects of veratridine and glutamate and agonist on GABAA receptors in the same preparation, but with a temporal mismatch in the amount and direction of regulation. We speculate that the age-dependent differences in direction of regulation for the receptor populations which serve key excitatory and inhibitory functions in cortex may provide a molecular basis for the gradual decline of neuronal plasticity during the critical period.