Regionally specific alterations in the low-affinity GABAA receptor following perinatal exposure to diazepam

Seizures and Antiepileptic Drugs: Does Exposure Alter Normal Brain Development?

Seizures and antiepileptic drugs (AEDs) affect brain development and have long-term neurological consequences. The specific molecular and cellular changes, the precise timing of their influence during brain development, and the full extent of the long-term consequences of seizures and AEDs exposure have not been established. This review critically assesses both the basic and clinical science literature on the effects of seizures and AEDs on the developing brain and finds that evidence exists to support the hypothesis that both seizures and antiepileptic drugs influence a variety of biological process, at specific times during development, which alter long-term cognition and epilepsy susceptibility. More research, both clinical and experimental, is needed before changes in current clinical practice, based on the scientific data, can be recommended.

Perinatal allopregnanolone influences prefrontal cortex structure, connectivity and behavior in adult rats

Cortical neurosteroid levels vary dramatically across development; during the second week of life elevated levels of allopregnanolone are associated with decreased GABA(A) receptor function. Since GABA(A) receptor modulation plays a role in proliferative regulation in developing neocortex, it is possible that endogenous neurosteroids such as allopregnanolone, acting through GABA(A) receptors, modulate cortical development. We augmented normally low levels with exogenous administration of allopregnanolone (10 mg/kg) during the first week of rodent life. The localization of parvalbumin-labeled cells was markedly altered; the ratio of cell number in the deep (layers V-VI) vs. superficial (layers I-III) layers of adult prefrontal cortex increased two-fold in rats administered allopregnanolone on postnatal day 1 or 5. The mechanism underlying these anatomical changes likely involves GABA(A) receptors because similar changes in interneuron placement were observed after neonatal benzodiazepine administration. Measures of mature cortical function were also altered after neonatal neurosteroid administration, including [(3)H]MK-801 binding, prepulse inhibition and amphetamine-induced locomotor activity. Moreover, neonatal allopregnanolone administration increases the number of parvalbumin-expressing neurons in medial dorsal nucleus of the thalamus while the total neuron number is decreased. These findings suggest that connectivity between the medial dorsal nucleus of the thalamus and prefrontal cortex is likely altered by neonatal neurosteroid administration and may result in a disinhibited frontal cortex. Disinhibition in the prefrontal cortex is associated with behavioral changes relevant to human psychosis and developmental disorders. If neurosteroids play a role in normal development of prefrontal/medial dorsal patency as suggested by these studies, then alterations in neurosteroid levels may contribute to abnormal neurodevelopment.

Is the brain hormonally imprintable?

Hormonal imprinting develops at the first encounter between the target hormone and its developing receptor in the perinatal critical period. This determines the binding and response capacity of the receptor-signal transduction system and hormone production of cells for life. Molecules similar to the hormone and excess or absence of the target hormone cause faulty imprinting with lifelong consequences. Prenatal or neonatal imprinting with opiates, other drugs and prenatal stress have harmful consequences on the adult brain. Perinatal imprinting with endorphin or serotonin decreases the serotonin level of the brain while increasing sexual activity and (as in the case of endorphin) aggression. Endorphin or serotonin antagonist treatment at weaning (late imprinting) also significantly reduces the serotonin content of the brain. Backed by literary data, these observations are discussed, and the possible consequences of medical treatments are shown. The paper concludes that an excess of molecules produced by the brain itself can provoke perinatal imprinting, and it points to the possibility of late imprinting of the brain by receptor level acting agents, including a brain product (endorphin).

Reversal of prenatal diazepam-induced deficit in a spatial-object learning task by brief, periodic maternal separation in adult rats

In the rat, prenatal exposure to diazepam (DZ) induces a permanent reduction in GABA/BZ receptor (R) function and behavioural abnormalities. Environmental modifications during early stages of life can influence brain development and induce neurobiological and behavioural changes throughout adulthood. Indeed, a subtle, periodic, postnatal manipulation increases GABA/BZ R activity and produces facilitatory effects on neuroendocrine and behavioural responses. We here investigated the impact of prenatal treatment with DZ on learning performance in adult 3- and 8-month-old male rats and the influence of a brief, periodic maternal separation on the effects exerted by prenatal DZ exposure. Learning performance was examined employing a non-aversive spatial, visual and/or tactile task, the "Can test". Behavioural reactivity, emotional state and fear/anxiety-driven behaviour were also examined using open field (OF), acoustic startle reflex (ASR) and elevated plus-maze (EPM) tests. A single daily injection of DZ (1.5mg/kg, s.c.), over gestational days (GD) 14-20, induced, in an age-independent manner, a severe deficit in learning performance, a decrease in locomotor and explorative activity and an increase in peak amplitude in the ASR. Furthermore, anxiety-driven behaviour in EPM was disrupted. Daily maternal separation for 15 min over postnatal days 2-21 exerted opposite effects in all the paradigms examined. Prenatally DZ-exposed maternal separated rats, in contrast to respective non-separated rats, showed an improvement in learning performance, a decrease in emotionality and a normalization of the exploratory behaviour in EPM. These results suggest that a greater maternal care, induced by separation, can serve as a source for the developing brain to enhance neuronal plasticity and to prevent the behavioural abnormalities induced by prenatal DZ exposure.

GABA(A) receptor sites in the developing human foetus

GABA(A) receptor sites were characterised in cerebral cortex tissue samples from deceased neurologically normal infants who had come to autopsy during the third trimester of pregnancy. Pharmacological parameters were obtained from homogenate binding studies which utilised the 'central-type' benzodiazepine ligands [3H]diazepam and [3H]flunitrazepam, and from the GABA activation of [3H]diazepam binding. It was found that the two radioligands behaved differently during development. The affinity of [3H]flunitrazepam for its binding site did not vary significantly between preparations, whereas the [3H]diazepam K(D) showed marked regional and developmental variations: infant tissues showed a distinctly lower affinity than adults for this ligand. The density of [3H]flunitrazepam binding sites increased approximately 35% during the third trimester to reach adult levels by term, whereas [3H]diazepam binding capacity declined slightly but steadily throughout development. The GABA activation of [3H]diazepam binding was less efficient early in the trimester, in that the affinity of the agonist was significantly lower, though it rose to adult levels by term. The strength of the enhancement response increased to adult levels over the same time-frame. The results strongly suggest that the subunit composition of cortical GABA(A) sites changes significantly during this important developmental stage.

Prenatal diazepam exposure functionally alters the GABA(A) receptor that modulates [3H]noradrenaline release from rat hippocampal synaptosomes

In rats, exposure to diazepam (DZ) during the last week of gestation is associated with behavioral alterations (in some cases sexually dimorphic) that appear when the animals reach adulthood. This study was conducted to evaluate the effects of prenatal DZ exposure on the function of the gamma-aminobutyric (GABA)(A) receptor complex. The method used - perfusion of rat hippocampal nerve terminals labeled with [3H]noradrenaline (NA) - allowed us to evaluate the effects of DZ on a specific native GABA(A) receptor subtype which is located on hippocampal noradrenergic nerve endings and mediates the release of NA. Muscimol stimulated synaptosomal release of [3H]NA in a concentration-dependent manner; maximal stimulation (50%) was achieved with a concentration of 30 microM, and the ED(50) was 1.7 microM. The effect of muscimol was potentiated by the positive allosteric modulators DZ and 3alpha-pregnan-5alpha-ol-20-one (3alpha,5alpha-P; allopregnanolone), which displayed similar maximal effects and affinities. In the presence of DZ (0.1 microM), muscimol stimulated the release of [3H]NA with an ED(50) of 0.5 microM; in the presence of 3alpha,5alpha-P (0.1 microM), the ED(50) of muscimol was 0.3 microM. Prenatal DZ exposure did not modify the concentration-effect curve for muscimol, but it did abolish the potentiating effects of DZ and 3alpha,5alpha-P. These findings demonstrate that prenatal exposure to DZ produces functional modifications of the GABA(A) receptor subtype we investigated. This effect may be related to the relative contributions of the various protein subunits that compose the GABA(A) receptor complex. Exposure to DZ while the GABA(A) receptors are developing might influence the expression of these subunits, giving rise to a receptor that can be activated by muscimol but is not susceptible to allosteric modulation by DZ or 3alpha,5alpha-P.

Regional variations in the effects of chronic ethanol administration on GABA(A) receptor expression: potential mechanisms

Gamma-aminobutyric acid type A (GABA(A)) receptors in brain adapt to chronic ethanol exposure via changes in receptor function and subunit expression. The present review summarizes currently available data regarding changes in GABA(A) receptor subunit mRNA and peptide expression. Data are presented from various different brain regions and the variations between specific brain regions used to draw conclusions about mechanisms that may underlie GABA(A) receptor adaptations during chronic ethanol exposure. In the whole cerebral cortex, chronic ethanol exposure leads to a reduction of GABA(A) receptor alpha1 subunit mRNA and peptide levels and a near equivalent increase in alpha4 subunit mRNA and peptide levels. This observation is the primary support for the hypothesis that altered receptor composition is a mechanism for GABA(A) receptor adaptation produced by chronic ethanol exposure. However, other brain regions do not display similar patterns of subunit changes. Moreover, subregions within cortex (prefrontal, cingulate, parietal, motor, and piriform) exhibit patterns of changes in subunit expression that differ from whole cortex. Therefore, regional differences in GABA(A) receptor subunit expression are evident following chronic ethanol administration, thus suggesting that multiple mechanisms contribute to the regulation of GABA(A) receptor expression. These mechanisms may include the involvement of other neurotransmitter systems, endogenous steroids and second or third messenger cross-talk.

Effects of repeated amphetamine treatment on regional GABAA receptor binding

The present study examined the effects of repeated exposure to amphetamine on GABAA receptor binding in cortical and subcortical areas. The goal of the study was to determine whether changes in specific binding were related to behavioral sensitization. Animals were exposed to either saline (0.3 ml, s.c.; n=12) or d-amphetamine (2.5 mg/kg, s.c.; n=12) for 6 consecutive days and sacrificed after a 14-day withdrawal period. Differences in GABAA receptor binding in these two groups of animals were assessed using the GABAA receptor antagonist [3H]SR 95531. To verify that the preceding treatment regimen led to the development of behavioral sensitization, a separate set of animals (n=8/group) was exposed to the same regimen and challenged with d-amphetamine (2.5 mg/kg, s.c.) after the 14-day withdrawal period. As expected, preexposure to amphetamine led to the development of amphetamine sensitization. There were no differences in GABAA receptor binding in animals preexposed to saline and amphetamine in the prefrontal cortex, caudate-putamen, hypothalamus, or cerebellum. These findings do not provide support for the idea that changes in GABAA receptor binding in the medial prefrontal cortex or various subcortical areas are related to the development of behavioral sensitization.

Novelty-associated locomotion: correlation with cortical and sub-cortical GABAA receptor binding

The present study was designed to determine whether variability in GABA (eta-aminobutyric acid)A receptor binding in cortical and subcortical brain regions was correlated with locomotor activity in a novel environment. Twenty four animals were rated for locomotor activity in a novel circular runway. Eight days later, locomotor activity was assessed following 1.5 mg/kg amphetamine sulfate (i.p.). After four to six days, animals were killed and samples were pooled in groups of four animals ranked according to novely locomotor score, and specific binding of the GABAA receptor antagonist [2-(3'-carboxy-2'-propyl)-3-amino-6-p-methoxy phenylpyridazinium bromide] ([3H]SR95531) was determined. Significant negative correlations were seen between specific ([3H]SR95531) binding and novelty induced locomotion in the cingulate and prefrontal cortices, and in the ventral pallidum. A near-significant negative correlation was seen in the striatum. Correlation coefficients between locomotion scores in the novel environment and specific [3H]SR95531 binding were: cingulate cortex, R = -0.91, P = 0.012; prefrontal cortex, R = -0.85, P = 0.032; ventral pallidum, R = -0.85, P = 0.030; striatum, R = -0.73, P = 0.097; and nucleus accumbens, R = -0.09, P = 0.85. The positive correlation between novelty- and amphetamine-induced locomotion was also quite high (R = 0.95, P = 0.004). These results are discussed in terms of their relevance to potential biochemical correlates of drug abuse vulnerability.

Alterations in GABAA receptor binding in the prefrontal cortex following exposure to chronic stress

The present study was designed to examine the effects of chronic stress on GABAA receptor binding. Animals were randomly assigned to either a control, acute, or chronic stress condition and changes in specific binding were assessed using the GABAA receptor antagonist [3H]SR 95531. Exposure to chronic restraint stress led to a significant reduction in GABAA receptor binding in the prefrontal cortex. Alterations in specific binding were not observed in the cerebellum, caudate-putamen, hippocampus, or cingulate cortex however, suggesting that the effects of chronic stress may be regionally specific. Exposure to acute restraint did not lead to a significant alteration in [3H]SR 95531 binding in any brain region examined.

Short- and long-term effects of neonatal diazepam exposure on local cerebral glucose utilization in the rat

The short- and long-term consequences of a neonatal exposure to diazepam (DZP) on the postnatal changes in local cerebral metabolic rates for glucose (LCMRglcs) were studied by the quantitative autoradiographic [14C]2-deoxyglucose method in a total number of 66 brain structures of freely moving rats. Rat pups received a daily subcutaneous injection of 10 mg/kg DZP, of the dissolution vehicle or of saline from postnatal day 2 (P2) to 21 (P21). The animals were studied at 4 ages, P10, P14, P21 and P60. DZP induced a decrease in LCMRglcs which was restricted to 13 areas at P10, mainly sensory and limbic regions. At P14, the treatment had significant metabolic effects on 48 structures belonging to all functional systems. By P21, 23 brain areas were still affected by the treatment, mainly sensory, limbic and motor areas. At P60, i.e. at about 40 days after the end of drug exposure, LCMRglcs still decreased in 14 brain regions which were mainly sensory and limbic structures. The structures most sensitive to both short- and long-term consequences of the anticonvulsant treatment are mammillary body, limbic cortices and sensory regions. The dissolution vehicle increased LCMRglcs in a few brain regions at P14 and P60, whereas it decreased metabolic levels in 5 brain regions at P21. The results of the present study show that the brain appears to be particularly vulnerable to the treatment at P14, period of active brain growth, whereas by P21, the drug is actively metabolized and a tolerance to the treatment may occur. The long-term effects of the treatment are in good accordance with the well-known effects of DZP on anxiety, sedation and memory. The structures most sensitive to early neonatal DZP exposure are the mammillary body, limbic cortices and sensory regions that all contain a high density of benzodiazepine binding sites.

Perinatal diazepam exposure: alterations in exploratory behavior and mesolimbic dopamine turnover

Perinatal exposure to diazepam has been shown to lead to alterations in motor activity and exploratory behavior in neonatal animals. Exploratory and locomotor behavior have been associated with changes in mesotelencephalic dopamine function. We have therefore examined the effects of perinatal diazepam administration on both exploratory behavior and mesotelencephalic dopamine turnover in the adult rat. Animals exposed to the benzodiazepine during the perinatal period engaged in significantly less exploratory behavior than did control subjects. The diazepam-induced alterations in behavior were developmentally specific: decreased exploratory behavior was observed at 90, but not 60, days of age. At 90 days of age, specific changes in dopamine turnover in diazepam-treated animals were restricted to mesolimbic (nucleus accumbens and ventral tegmental area) sites; alterations in dopamine turnover were not seen in other mesotelencephalic sites examined. The findings indicate that perinatal exposure to benzodiazepines leads to behavioral changes that are present in adulthood. These changes in exploratory behavior may be associated with alterations in mesolimbic dopamine function.