Theta oscillations in the hippocampus

Theta oscillations represent the "on-line" state of the hippocampus. The extracellular currents underlying theta waves are generated mainly by the entorhinal input, CA3 (Schaffer) collaterals, and voltage-dependent Ca(2+) currents in pyramidal cell dendrites. The rhythm is believed to be critical for temporal coding/decoding of active neuronal ensembles and the modification of synaptic weights. Nevertheless, numerous critical issues regarding both the generation of theta oscillations and their functional significance remain challenges for future research.

The role of childhood trauma in the neurobiology of mood and anxiety disorders: preclinical and clinical studies

Epidemiologic studies indicate that children exposed to early adverse experiences are at increased risk for the development of depression, anxiety disorders, or both. Persistent sensitization of central nervous system (CNS) circuits as a consequence of early life stress, which are integrally involved in the regulation of stress and emotion, may represent the underlying biological substrate of an increased vulnerability to subsequent stress as well as to the development of depression and anxiety. A number of preclinical studies suggest that early life stress induces long-lived hyper(re)activity of corticotropin-releasing factor (CRF) systems as well as alterations in other neurotransmitter systems, resulting in increased stress responsiveness. Many of the findings from these preclinical studies are comparable to findings in adult patients with mood and anxiety disorders. Emerging evidence from clinical studies suggests that exposure to early life stress is associated with neurobiological changes in children and adults, which may underlie the increased risk of psychopathology. Current research is focused on strategies to prevent or reverse the detrimental effects of early life stress on the CNS. The identification of the neurobiological substrates of early adverse experience is of paramount importance for the development of novel treatments for children, adolescents, and adults.

Steroid hormone metabolites are barbiturate-like modulators of the GABA receptor

Two metabolites of the steroid hormones progesterone and deoxycorticosterone, 3 alpha-hydroxy-5 alpha-dihydroprogesterone and 3 alpha, 5 alpha-tetrahydrodeoxycorticosterone, are potent barbiturate-like ligands of the gamma-aminobutyric acid (GABA) receptor-chloride ion channel complex. At concentrations between 10(-7) and 10(-5)M both steroids inhibited binding of the convulsant t-butylbicyclophosphorothionate to the GABA-receptor complex and increased the binding of the benzodiazepine flunitrazepam; they also stimulated chloride uptake (as measured by uptake of 36Cl-) into isolated brain vesicles, and potentiated the inhibitory actions of GABA in cultured rat hippocampal and spinal cord neurons. These data may explain the ability of certain steroid hormones to rapidly alter neuronal excitability and may provide a mechanism for the anesthetic and hypnotic actions of naturally occurring and synthetic anesthetic steroids.

Variations on an inhibitory theme: phasic and tonic activation of GABA(A) receptors

The proper functioning of the adult mammalian brain relies on the orchestrated regulation of neural activity by a diverse population of GABA (gamma-aminobutyric acid)-releasing neurons. Until recently, our appreciation of GABA-mediated inhibition focused predominantly on the GABA(A) (GABA type A) receptors located at synaptic contacts, which are activated in a transient or 'phasic' manner by GABA that is released from synaptic vesicles. However, there is growing evidence that low concentrations of ambient GABA can persistently activate certain subtypes of GABA(A) receptor, which are often remote from synapses, to generate a 'tonic' conductance. In this review, we consider the distinct roles of synaptic and extrasynaptic GABA receptor subtypes in the control of neuronal excitability.

Nongenomic transmission across generations of maternal behavior and stress responses in the rat

In the rat, variations in maternal care appear to influence the development of behavioral and endocrine responses to stress in the offspring. The results of cross-fostering studies reported here provide evidence for (i) a causal relationship between maternal behavior and stress reactivity in the offspring and (ii) the transmission of such individual differences in maternal behavior from one generation of females to the next. Moreover, an environmental manipulation imposed during early development that alters maternal behavior can then affect the pattern of transmission in subsequent generations. Taken together, these findings indicate that variations in maternal care can serve as the basis for a nongenomic behavioral transmission of individual differences in stress reactivity across generations.

Methods for drug discovery: development of potent, selective, orally effective cholecystokinin antagonists

3-(Acylamino)-5-phenyl-2H-1,4-benzodiazepines, antagonists of the peptide hormone cholecystokinin (CCK), are described. Developed by reasoned modification of the known anxiolytic benzodiazepines, these compounds provide highly potent, orally effective ligands selective for peripheral (CCK-A) receptors, with binding affinities approaching or equaling that of the natural ligand CCK-8. The distinction between CCK-A receptors on the one hand and CNS (CCK-B), gastrin, and central benzodiazepine receptors on the other is demonstrated by using the structure-activity profiles of the new compounds. Details of the binding of these agents to CCK-A receptors are examined, and the method of development of these compounds is discussed in terms of its relevance to the general problem of drug discovery.

GABA(A) receptors: immunocytochemical distribution of 13 subunits in the adult rat brain

GABA(A) receptors are ligand-operated chloride channels assembled from five subunits in a heteropentameric manner. Using immunocytochemistry, we investigated the distribution of GABA(A) receptor subunits deriving from 13 different genes (alpha1-alpha6, beta1-beta3, gamma1-gamma3 and delta) in the adult rat brain. Subunit alpha1-, beta1-, beta2-, beta3- and gamma2-immunoreactivities were found throughout the brain, although differences in their distribution were observed. Subunit alpha2-, alpha3-, alpha4-, alpha5-, alpha6-, gamma1- and delta-immunoreactivities were more confined to certain brain areas. Thus, alpha2-subunit-immunoreactivity was preferentially located in forebrain areas and the cerebellum. Subunit alpha6-immunoreactivity was only present in granule cells of the cerebellum and the cochlear nucleus, and subunit gamma1-immunoreactivity was preferentially located in the central and medial amygdaloid nuclei, in pallidal areas, the substantia nigra pars reticulata and the inferior olive. The alpha5-subunit-immunoreactivity was strongest in Ammon's horn, the olfactory bulb and hypothalamus. In contrast, alpha4-subunit-immunoreactivity was detected in the thalamus, dentate gyrus, olfactory tubercle and basal ganglia. Subunit alpha3-immunoreactivity was observed in the glomerular and external plexiform layers of the olfactory bulb, in the inner layers of the cerebral cortex, the reticular thalamic nucleus, the zonal and superficial layers of the superior colliculus, the amygdala and cranial nerve nuclei. Only faint subunit gamma3-immunoreactivity was detected in most areas; it was darkest in midbrain and pontine nuclei. Subunit delta-immunoreactivity was frequently co-distributed with alpha4 subunit-immunoreactivity, e.g. in the thalamus, striatum, outer layers of the cortex and dentate molecular layer. Striking examples of complementary distribution of certain subunit-immunoreactivities were observed. Thus, subunit alpha2-, alpha4-, beta1-, beta3- and delta-immunoreactivities were considerably more concentrated in the neostriatum than in the pallidum and entopeduncular nucleus. In contrast, labeling for the alpha1-, beta2-, gamma1- and gamma2-subunits prevailed in the pallidum compared to the striatum. With the exception of the reticular thalamic nucleus, which was prominently stained for subunits alpha3, beta1, beta3 and gamma2, most thalamic nuclei were rich in alpha1-, alpha4-, beta2- and delta-immunoreactivities. Whereas the dorsal lateral geniculate nucleus was strongly immunoreactive for subunits alpha4, beta2 and delta, the ventral lateral geniculate nucleus was predominantly labeled for subunits alpha2, alpha3, beta1, beta3 and gamma2; subunit alpha1- and alpha5-immunoreactivities were about equally distributed in both areas. In most hypothalamic areas, immunoreactivities for subunits alpha1, alpha2, beta1, beta2 and beta3 were observed. In the supraoptic nucleus, staining of conspicuous dendritic networks with subunit alpha1, alpha2, beta2, and gamma2 antibodies was contrasted by perykarya labeled for alpha5-, beta1- and delta-immunoreactivities. Among all brain regions, the median emminence was most heavily labeled for subunit beta2-immunoreactivity. In most pontine and cranial nerve nuclei and in the medulla, only subunit alpha1-, beta2- and gamma2-immunoreactivities were strong, whereas the inferior olive was significantly labeled only for subunits beta1, gamma1 and gamma2. In this study, a highly heterogeneous distribution of 13 different GABA(A) receptor subunit-immunoreactivities was observed. This distribution and the apparently typical patterns of co-distribution of these GABA(A) receptor subunits support the assumption of multiple, differently assembled GABA(A) receptor subtypes and their heterogeneous distribution within the adult rat brain.

Ethanol-induced apoptotic neurodegeneration and fetal alcohol syndrome

The deleterious effects of ethanol on the developing human brain are poorly understood. Here it is reported that ethanol, acting by a dual mechanism [blockade of N-methyl-D-aspartate (NMDA) glutamate receptors and excessive activation of GABA(A) receptors], triggers widespread apoptotic neurodegeneration in the developing rat forebrain. Vulnerability coincides with the period of synaptogenesis, which in humans extends from the sixth month of gestation to several years after birth. During this period, transient ethanol exposure can delete millions of neurons from the developing brain. This can explain the reduced brain mass and neurobehavioral disturbances associated with human fetal alcohol syndrome.

Maternal care during infancy regulates the development of neural systems mediating the expression of fearfulness in the rat

The mothers of infant rats show individual differences in the frequency of licking/grooming and arched-back nursing (LG-ABN) of pups that contribute to the development of individual differences in behavioral responses to stress. As adults, the offspring of mothers that exhibited high levels of LG-ABN showed substantially reduced behavioral fearfulness in response to novelty compared with the offspring of low LG-ABN mothers. In addition, the adult offspring of the high LG-ABN mothers showed significantly (i) increased central benzodiazepine receptor density in the central, lateral, and basolateral nuclei of the amygdala as well as in the locus ceruleus, (ii) increased alpha2 adrenoreceptor density in the locus ceruleus, and (iii) decreased corticotropin-releasing hormone (CRH) receptor density in the locus ceruleus. The expression of fear and anxiety is regulated by a neural circuitry that includes the activation of ascending noradrenergic projections from the locus ceruleus to the forebrain structures. Considering the importance of the amygdala, notably the anxiogenic influence of CRH projections from the amygdala to the locus ceruleus, as well as the anxiolytic actions of benzodiazepines, for the expression of behavioral responses to stress, these findings suggest that maternal care during infancy serves to "program" behavioral responses to stress in the offspring by altering the development of the neural systems that mediate fearfulness.

The mouse light/dark box test

The light/dark test is based on the innate aversion of rodents to brightly illuminated areas and on the spontaneous exploratory behaviour of rodents in response to mild stressors, that is, novel environment and light. The test apparatus consists of a small dark safe compartment (one third) and a large illuminated aversive compartment (two thirds). The test was developed with male mice. The strain, weight and age may be crucial factors. The extent to which an anxiolytic compound can facilitate exploratory activity depends on the baseline level in the control group. Differences between the type and severity of external stressors might account for the variable results reported by different laboratories. The light/dark test may be useful to predict anxiolytic-like or anxiogenic-like activity in mice. Transitions have been reported to be an index of activity-exploration because of habituation over time, and the time spent in each compartment to be a reflection of aversion. Classic anxiolytics (benzodiazepines) as well as the newer anxiolytic-like compounds (e.g. serotonergic drugs or drugs acting on neuropeptide receptors) can be detected using this paradigm. It has the advantages of being quick and easy to use, without requiring the prior training of animals.

Neurosteroids: endogenous regulators of the GABAA receptor

GABA(A) (gamma-aminobutyric acid type A) receptors mediate most of the 'fast' synaptic inhibition in the mammalian brain and are targeted by many clinically important drugs. Certain naturally occurring pregnane steroids can potently and specifically enhance GABA(A) receptor function in a nongenomic (direct) manner, and consequently have anxiolytic, analgesic, anticonvulsant, sedative, hypnotic and anaesthetic properties. These steroids not only act as remote endocrine messengers, but also can be synthesized in the brain, where they modify neuronal activity locally by modulating GABA(A) receptor function. Such 'neurosteroids' can influence mood and behaviour in various physiological and pathophysiological situations, and might contribute to the behavioural effects of psychoactive drugs.

Benzodiazepine actions mediated by specific gamma-aminobutyric acid(A) receptor subtypes

GABA(A) (gamma-aminobutyric acid(A)) receptors are molecular substrates for the regulation of vigilance, anxiety, muscle tension, epileptogenic activity and memory functions, which is evident from the spectrum of actions elicited by clinically effective drugs acting at their modulatory benzodiazepine-binding site. Here we show, by introducing a histidine-to-arginine point mutation at position 101 of the murine alpha1-subunit gene, that alpha1-type GABA(A) receptors, which are mainly expressed in cortical areas and thalamus, are rendered insensitive to allosteric modulation by benzodiazepine-site ligands, whilst regulation by the physiological neurotransmitter gamma-aminobutyric acid is preserved. alpha1(H101R) mice failed to show the sedative, amnesic and partly the anticonvulsant action of diazepam. In contrast, the anxiolytic-like, myorelaxant, motor-impairing and ethanol-potentiating effects were fully retained, and are attributed to the nonmutated GABA(A) receptors found in the limbic system (alpha2, alpha5), in monoaminergic neurons (alpha3) and in motoneurons (alpha2, alpha5). Thus, benzodiazepine-induced behavioural responses are mediated by specific GABA(A) receptor subtypes in distinct neuronal circuits, which is of interest for drug design.

Psychobiological mechanisms of resilience and vulnerability: implications for successful adaptation to extreme stress

OBJECTIVE

Most research on the effects of severe psychological stress has focused on stress-related psychopathology. Here, the author develops psychobiological models of resilience to extreme stress.

METHOD

An integrative model of resilience and vulnerability that encompasses the neurochemical response patterns to acute stress and the neural mechanisms mediating reward, fear conditioning and extinction, and social behavior is proposed.

RESULTS

Eleven possible neurochemical, neuropeptide, and hormonal mediators of the psychobiological response to extreme stress were identified and related to resilience or vulnerability. The neural mechanisms of reward and motivation (hedonia, optimism, and learned helpfulness), fear responsiveness (effective behaviors despite fear), and adaptive social behavior (altruism, bonding, and teamwork) were found to be relevant to the character traits associated with resilience.

CONCLUSIONS

The opportunity now exists to bring to bear the full power of advances in our understanding of the neurobiological basis of behavior to facilitate the discoveries needed to predict, prevent, and treat stress-related psychopathology.

Enforcement of temporal fidelity in pyramidal cells by somatic feed-forward inhibition

The temporal resolution of neuronal integration depends on the time window within which excitatory inputs summate to reach the threshold for spike generation. Here, we show that in rat hippocampal pyramidal cells this window is very narrow (less than 2 milliseconds). This narrowness results from the short delay with which disynaptic feed-forward inhibition follows monosynaptic excitation. Simultaneous somatic and dendritic recordings indicate that feed-forward inhibition is much stronger in the soma than in the dendrites, resulting in a broader integration window in the latter compartment. Thus, the subcellular partitioning of feed-forward inhibition enforces precise coincidence detection in the soma, while allowing dendrites to sum incoming activity over broader time windows.

Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects

Caffeine is the most widely consumed central-nervous-system stimulant. Three main mechanisms of action of caffeine on the central nervous system have been described. Mobilization of intracellular calcium and inhibition of specific phosphodiesterases only occur at high non-physiological concentrations of caffeine. The only likely mechanism of action of the methylxanthine is the antagonism at the level of adenosine receptors. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons. The methylxanthine induces dose-response increases in locomotor activity in animals. Its psychostimulant action on man is, however, often subtle and not very easy to detect. The effects of caffeine on learning, memory, performance and coordination are rather related to the methylxanthine action on arousal, vigilance and fatigue. Caffeine exerts obvious effects on anxiety and sleep which vary according to individual sensitivity to the methylxanthine. However, children in general do not appear more sensitive to methylxanthine effects than adults. The central nervous system does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported.

International Union of Pharmacology. LXX. Subtypes of gamma-aminobutyric acid(A) receptors: classification on the basis of subunit composition, pharmacology, and function. Update

In this review we attempt to summarize experimental evidence on the existence of defined native GABA(A) receptor subtypes and to produce a list of receptors that actually seem to exist according to current knowledge. This will serve to update the most recent classification of GABA(A) receptors (Pharmacol Rev 50:291-313, 1998) approved by the Nomenclature Committee of the International Union of Pharmacology. GABA(A) receptors are chloride channels that mediate the major form of fast inhibitory neurotransmission in the central nervous system. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. GABA(A) receptors are assembled from a family of 19 homologous subunit gene products and form numerous, mostly hetero-oligomeric, pentamers. Such receptor subtypes with properties that depend on subunit composition vary in topography and ontogeny, in cellular and subcellular localization, in their role in brain circuits and behaviors, in their mechanisms of regulation, and in their pharmacology. We propose several criteria, which can be applied to all the members of the LGIC superfamily, for including a receptor subtype on a list of native hetero-oligomeric subtypes. With these criteria, we develop a working GABA(A) receptor list, which currently includes 26 members, but will undoubtedly be modified and grow as information expands. The list is divided into three categories of native receptor subtypes: "identified," "existence with high probability," and "tentative."

Trans-synaptic shift in anion gradient in spinal lamina I neurons as a mechanism of neuropathic pain

Modern pain-control theory predicts that a loss of inhibition (disinhibition) in the dorsal horn of the spinal cord is a crucial substrate for chronic pain syndromes. However, the nature of the mechanisms that underlie such disinhibition has remained controversial. Here we present evidence for a novel mechanism of disinhibition following peripheral nerve injury. It involves a trans-synaptic reduction in the expression of the potassium-chloride exporter KCC2, and the consequent disruption of anion homeostasis in neurons of lamina I of the superficial dorsal horn, one of the main spinal nociceptive output pathways. In our experiments, the resulting shift in the transmembrane anion gradient caused normally inhibitory anionic synaptic currents to be excitatory, substantially driving up the net excitability of lamina I neurons. Local blockade or knock-down of the spinal KCC2 exporter in intact rats markedly reduced the nociceptive threshold, confirming that the reported disruption of anion homeostasis in lamina I neurons was sufficient to cause neuropathic pain.

In-vivo measurement of activated microglia in dementia

BACKGROUND

Activated microglia have a key role in the brain's immune response to neuronal degeneration. The transition of microglia from the normal resting state to the activated state is associated with an increased expression of receptors known as peripheral benzodiazepine binding sites, which are abundant on cells of mononuclear phagocyte lineage. We used brain imaging to study expression of these sites in healthy individuals and patients with Alzheimer's disease.

METHODS

We studied 15 normal individuals (age 32-80 years), eight patients with Alzheimer's disease, and one patient with minimal cognitive impairment. Quantitative in-vivo measurements of glial activation were obtained with positron emission tomography (PET) and carbon-11-labelled (R)-PK11195, a specific ligand for the peripheral benzodiazepine binding site.

FINDINGS

In normal individuals, regional [11C](R)-PK11195 binding did not significantly change with age, except in the thalamus, where an age-dependent increase was found. By contrast, patients with Alzheimer's disease showed significantly increased regional [11C](R)-PK11195 binding in the entorhinal, temporoparietal, and cingulate cortex.

INTERPRETATION

In-vivo detection of increased [11C](R)-PK11195 binding in Alzheimer-type dementia, including mild and early forms, suggests that microglial activation is an early event in the pathogenesis of the disease.

Local GABA circuit control of experience-dependent plasticity in developing visual cortex

Sensory experience in early life shapes the mammalian brain. An impairment in the activity-dependent refinement of functional connections within developing visual cortex was identified here in a mouse model. Gene-targeted disruption of one isoform of glutamic acid decarboxylase prevented the competitive loss of responsiveness to an eye briefly deprived of vision, without affecting cooperative mechanisms of synapse modification in vitro. Selective, use-dependent enhancement of fast intracortical inhibitory transmission with benzodiazepines restored plasticity in vivo, rescuing the genetic defect. Specific networks of inhibitory interneurons intrinsic to visual cortex may detect perturbations in sensory input to drive experience-dependent plasticity during development.

Subunit composition, distribution and function of GABA(A) receptor subtypes

GABA(A) receptors are the major inhibitory neurotransmitter receptors in the brain and are the site of action of many clinically important drugs. These receptors are composed of five subunits that can belong to eight different subunit classes. Depending on their subunit composition, these receptors exhibit distinct pharmacological and electrophysiological properties. Recent studies on recombinant and native GABA(A) receptors suggest the existence of far more receptor subtypes than previously assumed. Thus, receptors composed of one, two, three, four, or five different subunits might exist in the brain. Studies on the regional, cellular and subcellular distribution of GABA(A) receptor subunits, and on the co-localization of these subunits at the light and electron microscopic level for the first time provide information on the distribution of GABA(A) receptor subtypes in the brain. These studies will have to be complemented by electrophysiological and pharmacological studies on the respective recombinant and native receptors to finally identify the receptor subtypes present in the brain. The distinct cellular and subcellular location of individual receptor subtypes suggests that they exhibit specific functions in the brain that can be selectively modulated by subtype specific drugs. This conclusion is supported by the recent demonstration that different GABA(A) receptor subtypes mediate different effects of benzodiazepines. Together, these results should cause a revival of GABA(A) receptor research and strongly stimulate the development of drugs with a higher selectivity for alpha2-, alpha3-, or alpha5-subunit-containing receptor subtypes. Such drugs might exhibit quite selective clinical effects.

Sedative but not anxiolytic properties of benzodiazepines are mediated by the GABA(A) receptor alpha1 subtype

Inhibitory neurotransmission in the brain is largely mediated by GABA(A) receptors. Potentiation of GABA receptor activation through an allosteric benzodiazepine (BZ) site produces the sedative, anxiolytic, muscle relaxant, anticonvulsant and cognition-impairing effects of clinically used BZs such as diazepam. We created genetically modified mice (alpha1 H101R) with a diazepam-insensitive alpha1 subtype and a selective BZ site ligand, L-838,417, to explore GABA(A) receptor subtypes mediating specific physiological effects. These two complimentary approaches revealed that the alpha1 subtype mediated the sedative, but not the anxiolytic effects of benzodiazepines. This finding suggests ways to improve anxiolytics and to develop drugs for other neurological disorders based on their specificity for GABA(A) receptor subtypes in distinct neuronal circuits.

Stress-induced elevations of gamma-aminobutyric acid type A receptor-active steroids in the rat brain

A 3 alpha-hydroxy A-ring-reduced metabolite of progesterone, 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone), and one of deoxycorticosterone (DOC), 3 alpha,21-dihydroxy-5 alpha-pregnan-20- one (allotetrahydroDOC), are among the most potent known ligands of gamma-aminobutyric acid (GABA) receptors designated GABAA in the central nervous system. With specific radioimmunoassays, rapid (less than 5 min) and robust (4- to 20-fold) increases of allopregnanolone and allotetrahydroDOC were detected in the brain (cerebral cortex and hypothalamus) and in plasma of rats after exposure to ambient temperature swin stress. Neither steroid was detectable in the plasma of adrenalectomized rats either before or after swim stress. However, allopregnanolone, but not allotetrahydroDOC, was still present in the cerebral cortex (greater than 3 ng/g) after adrenalectomy. These data demonstrate the presence of allopregnanolone and allotetrahydroDOC in brain and show that acute stress results in a rapid increase of these neuroactive steroids to levels known to modulate GABAA receptor function.

Molecular and neuronal substrate for the selective attenuation of anxiety

Benzodiazepine tranquilizers are used in the treatment of anxiety disorders. To identify the molecular and neuronal target mediating the anxiolytic action of benzodiazepines, we generated and analyzed two mouse lines in which the alpha2 or alpha3 GABAA (gamma-aminobutyric acid type A) receptors, respectively, were rendered insensitive to diazepam by a knock-in point mutation. The anxiolytic action of diazepam was absent in mice with the alpha2(H101R) point mutation but present in mice with the alpha3(H126R) point mutation. These findings indicate that the anxiolytic effect of benzodiazepine drugs is mediated by alpha2 GABAA receptors, which are largely expressed in the limbic system, but not by alpha3 GABAA receptors, which predominate in the reticular activating system.

GABAA and GABAB receptor site distribution in the rat central nervous system

An autoradiographic procedure has been used to determine the quantitative distributions of gamma-aminobutyric acid (GABAA and GABAB) receptor subtypes in rat brain. Although the concentrations of both receptor binding sites were similar in some brain regions GABAA sites generally outnumbered GABAB sites. The highest concentration of GABAA sites were detected in the frontal cortex, the granule cell layer of the cerebellum, the olfactory bulb and the thalamic medial geniculate. The highest concentration of GABAB sites occurred in the molecular layer of the cerebellum, the interpeduncular nucleus, frontal cortex, anterior olfactory nucleus and thalamic nuclei. In addition the globus pallidus, temporal cortex, lateral posterior thalamus, superior colliculus, pontine nucleus, raphe magnus, spinal trigeminal tract and substantia gelatinosa contained significantly more GABAB sites than GABAA sites. The physiological and pharmacological significance of this heterogeneity has yet to be determined.

Functional requirement for class I MHC in CNS development and plasticity

Class I major histocompatibility complex (class I MHC) molecules, known to be important for immune responses to antigen, are expressed also by neurons that undergo activity-dependent, long-term structural and synaptic modifications. Here, we show that in mice genetically deficient for cell surface class I MHC or for a class I MHC receptor component, CD3zeta, refinement of connections between retina and central targets during development is incomplete. In the hippocampus of adult mutants, N-methyl-D-aspartate receptor-dependent long-term potentiation (LTP) is enhanced, and long-term depression (LTD) is absent. Specific class I MHC messenger RNAs are expressed by distinct mosaics of neurons, reflecting a potential for diverse neuronal functions. These results demonstrate an important role for these molecules in the activity-dependent remodeling and plasticity of connections in the developing and mature mammalian central nervous system (CNS).