Effects of light- and dark-rearing on the postnatal development of GABA receptor sites in the chick optic lobe

Developmental pattern of NADPH-diaphorase positive neurons in chick optic tectum is sensitive to changes in visual stimulation

The chick retinotectal system is a suitable model to investigate the mechanisms involved in the establishment of synaptic connections in whose refinement nitric oxide was implicated. The purpose of this work was to describe the developmental pattern of the nitric oxide synthase (NOS)-positive neurons as well as to determine if it is sensitive to changes in visual stimulation. The NADPH-diaphorase histochemical method was used to describe and quantify NOS neurons in normally stimulated and subnormally stimulated chickens. Nine types of NOS neurons were identified; seven of them express NOS until adulthood, while two of them show only a transient expression. The developmental pattern of NOS neurons follows the process of laminar segregation. It can be divided into three phases. The first includes the onset of NOS expression in periventricular neurons and the formation of a deep network of NOS fibers during early development. These neurons do not show any significant change in subnormally stimulated animals. The second phase includes the appearance of two transient NOS populations of bipolar neurons that occupy the intermediate layers during the optic fibers ingrowth. One of them significantly changes in subnormally stimulated chicks. The third phase occurs when the transitory expression of bipolar neurons decreases. It includes NOS expression in six neuronal populations that innervate the superficial retinorecipient layers. Most of these cells suffer plastic changes in subnormally stimulated chicks. The diversity of neuronal types with regard to their morphology, location, and sensitivity to visual stimulation strongly suggests that they serve different functions.

Acute hypoxic hypoxia transiently reduces GABA(A) binding site number in developing chick optic lobe

The Central Nervous System is known to be critically affected in the prenatal-perinatal period by hypoxic-ischemic insults, which produce several disorders such as loss of neural projections, increased susceptibility to seizures, apoptosis and an imbalance in normal activity of glutamatergic and GABAergic neurones, resulting in acute cell excitotoxicity. The aim of the present work was to establish a chick embryo model of normobaric acute hypoxic hypoxia as well as to evaluate modifications in GABA(A) receptor complex from chick optic lobe, that may result from this injury. Fertile chicken (Gallus gallus domesticus) eggs from White Leghorn were incubated and at embryonic days (ED) 12 to 18, subjected to a stream of 8%O(2)/92%N(2) during1 h, and then were either returned to their shelves in the incubator for recovery, or immediately processed for biochemical studies. Hypoxic treatment produced a significant age dependent reduction in GABA binding sites showing the greatest decrease at the earliest stages studied (ED12-ED16). Saturation curves of GABA binding performed at ED12 showed a decrease in B(max), (control, 5.48+/-0.20, hypoxic, 3.90+/-0.39 pmol/mg protein), but no significant change in K(d). Following 48 h in normoxic atmosphere post-hypoxia reduction in [3H]GABA binding was reversed. Pharmacological properties of GABA(A) receptor at ED12 showed that positive allosteric modulation effects of the steroid 3alpha-hydroxy-5alpha-pregnan-20-one and the barbiturate pentobarbital sodium were enhanced by the treatment. This model of acute prenatal hypoxic hypoxia produced marked alterations in inhibitory CNS neurotransmission that proved reversible and age dependent.

Comparative modulation by 3 alpha,5 alpha and 3 beta,5 beta neurosteroids of GABA binding sites during avian central nervous system development

Neurosteroids are endogenous Central Nervous System (CNS) compounds which act mainly by allosteric modulation of the GABAA receptor complex. The presence of a 3 alpha-hydroxyl group and a 5 alpha-hydrogen atom have been found to be essential structural requirements for biological activity in mammals. In the present work we report the enhancing activity on [3H]GABA binding to its receptor sites in chick optic lobe produced by progesterone metabolites 3 alpha-hydroxy,5 alpha-pregnan-20-one (3 alpha,5 alpha-P) and 3 beta-hydroxy,5 beta-pregnan-20-one (3 beta,5 beta-P). Both steroids were found able to enhance [3H]GABA binding along ontogeny, displaying a similar profile at early developmental stages, while in adulthood 3 alpha,5 alpha-P had greater potency (EC50 0.22 microM) and enhancing effect (Emax: 122%). In adult synaptic membranes, the two compounds displayed a complex interaction with the GABAA receptor, disclosed by a Schild plot with slope below one and an incomplete displacement of 3 alpha,5 alpha-P by its 3 beta,5 beta isomer. Such complexity could be related to the steroidogenic profile in avian CNS, with 5 alpha-reduced progesterone metabolites present since early development, while 3 alpha,5 alpha-P is found only in adulthood. Bearing in mind differences between avian and mammalian steroidogenic profiles and the relevance of 5 beta-steroids in early avian development, we propose that 3 beta,5 beta-P, instead of the classical potent 3 alpha,5 alpha-steroids, may be the endogenous modulator of GABAergic activity in developing avian brain.

Development of serotonergic innervation of the chick embryo tectum opticum

This paper describes the development of the serotonergic innervation of the chick tectum opticum as revealed by an immunohistochemical methodology. The development of this innervation was previously described simply as the formation of an irregular network of serotonergic fibers that gradually invades the organ and increases in density. Our results show that the developmental pattern of serotonergic innervation differs significantly through the distinct tectal layers and that it progresses through a characteristic temporospatial pattern related to the lamination process. These findings support the idea that the concept of laminar segregation can be applied to describe the development of the serotonergic innervation. On the other hand, it is clear that the existence of a typical ordered developmental pattern of innervation makes it possible to detect embryonic or post-hatching alterations. Thus, the tectal serotonergic innervation could be used as a suitable model to investigate possible plastic changes in experimental conditions.

Ontogeny and distribution of GABAA receptors in rat brainstem and rostral brain regions

Previous studies from our laboratory and others have shown that there are major age-related differences in brainstem neuronal function. Since GABAA receptors are major targets for GABA-mediated inhibitory modulation and play a key role in regulating cardiorespiratory function, especially during O2 deprivation, we examined differences in GABAA receptor density and distribution during postnatal development. Using quantitative receptor autoradiography, the present study was performed to examine the postnatal expression of GABAA receptors in the rat brainstem and rostral brain areas at five ages, i.e. postnatal day 1 (P1), P5, P10, P21 and P120. Ten-micrometer brain sections at different brain levels were labelled with [3H]muscimol in Tris-citrate buffer. We found that (i) GABAA receptors appeared very early in almost all the brainstem as well as rostral areas; (ii) at P1, the brainstem had a higher GABAA receptor binding density than rostral areas and its density peaked at P5 or P10; and (iii) receptor densities of the cerebellum and rostral brain areas such as cortex, thalamus and dentate gyrus increased with age, especially between P10 and P21, but most other subcortical areas like caudate-putamen and hippocampal CA1 area did not increase remarkably after birth. We conclude that: (i) GABAA receptors exist in most brain areas at birth; (ii) there are several patterns of postnatal development of GABAA receptors in the CNS with dramatic differences between the brainstem and cortex; (iii) brainstem functions rely more on GABAA receptors in early postnatal life than at more mature stages. We speculate that GABAA receptors develop earlier in phylogenetically older structures (such as brainstem) than in newer brain regions (such as cortex).

The postnatal development of benzodiazepine receptor sites in the chick optic lobe is modulated by environmental lighting

The present paper describes the ability of benzodiazepine receptor sites to undergo light mediated-plastic changes during the early postnatal development of the chick optic lobe. The postnatal development pattern of these receptors was studied under different levels of light stimulation, i.e. normal-, light-and dark-rearing. At hatching the specific binding of [3H]Flunitrazepam was 0.23 +/- 0.01 pmol/mg protein. The developmental profile shows a sharp and transient peak of receptor overexpression between the 1st and the 2nd postnatal day in three experimental groups. Between the 2nd and the 6th day significant differences were found between the three groups, being this difference maximal during the peak of overexpression. In fact, on the 2nd day the specific [3H]Flunitrazepam binding showed an increase of 17% (P < 0.0005) and a decrease of 34% (P < 0.0005) for light- and dark-reared animals as compared with normally-reared ones. The changes in receptor density were transient since from the 6th day onward they gradually disappeared, being almost identical in the three groups by the day 15. At this moment the number of benzodiazepine receptor sites stabilized at the adult level. Scatchard analysis at the 2nd postnatal day revealed that the differences observed in the high affinity benzodiazepine binding sites between the three groups were due to modifications in the total number of binding sites while the affinity remained unchanged. The maximal number of binding sites were: 2.76 +/- 0.03, 3.40 +/- 0.01 and 1.46 +/- 0.11 pmol/mg protein in normally-, light- and dark-reared chicks, respectively; while the apparent dissociation constants were unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAA receptors in auditory brainstem nuclei of the chick during development and after cochlea removal

The presence of GABAA receptors (GABARs) in auditory brainstem nuclei of the chick was determined by immunocytochemical (ICC) and receptor autoradiographic techniques. A monoclonal antibody to the GABAR/benzodiazepine/chloride channel complex and radiolabeled ligand binding using [3H]-muscimol, a GABA agonist, revealed labeling in nucleus magnocellularis (NM), nucleus laminaris (NL), nucleus angularis (NA), and the superior olive (SO) in both posthatch and embryonic chicks. GABAR-immunoreactivity (GABAR-I), as well as [3H]-muscimol binding, appear homogeneous throughout these nuclei at all ages studied. During development, GABAR-I is first observed in these nuclei around embryonic day 13 (E13). GABAR-I, which appears heavier in embryos than in posthatch chicks, becomes less intense with age in all 4 nuclei. Levels of receptor binding are also greater in embryos compared to posthatch chicks. [3H]-Muscimol binding is consistently greatest in SO followed by that in NL. NM and NA exhibit the least amount of binding at all ages studied. [3H]-Muscimol binding decreases in auditory brainstem nuclei as a function of age. Two days after unilateral cochlea removal, there is an apparent increase in GABAR-I in the ipsilateral NM compared to controls. This, however, may be the result of a decrease in the cross-sectional area of NM neurons as a result of de-afferentation (Born and Rubel, 1985). In contrast, there is a 28% decrease in [3H]-muscimol binding in the ipsilateral NM compared to controls probably reflecting the 30% reduction in the number of NM neurons due to cochlea removal (Born and Rubel, 1985). Fourteen days after cochlea removal, there is still a small, but not significant, decrease in [3H]-muscimol binding in the ipsilateral NM. In the contralateral NM, GABAR-I is less intense compared to that in the ipsilateral NM and controls. Additionally, there is a slight but insignificant decrease in [3H]-muscimol binding compared to that in controls 2 days after cochlea removal. After 14 days survival, however, the average binding is similar to that in controls. Thus, cochlea removal appears to transiently decrease the number of GABARs in the ipsilateral NM and may have a similar, but not as dramatic, effect in the contralateral NM. These GABARs are most likely to be postsynaptic, that is, located on NM neurons.

Light-dependent development and asymmetry of visual projections

The projections from thalamus to visual Wulst in chicks are asymmetrical and their development is determined by exposure to light just before and after hatching. The asymmetry results from the orientation of the embryo in the egg, the left eye being occluded and not the right. We have shown that this asymmetry can be eliminated by incubating the embryos in darkness or light so that both eyes receive the same stimulation.

Benzodiazepine receptor sites in the chick optic lobe: development and pharmacological characterization

To investigate the interaction between gamma-aminobutyric acid (GABA) and benzodiazepine (BZD) receptor sites during development, the time-course of appearance of flunitrazepam (FNZ) binding sites and their pharmacological characterization were studied in developing chick optic lobe. At the earliest stage examined, embryonic day (Ed) 12, the receptor density was 30.9% (0.05 +/- 0.01 pmol/mg protein) of that found in the chick optic lobes of adult chicks. The adult value was achieved on Ed 16 (0.16 +/- 0.01 pmol/mg protein). After this stage there was a sharp and transient increase in specific [3H]FNZ binding of about two-fold reaching a maximal value between hatching and the postnatal day (pnd) 2 (0.33 +/- 0.01 pmol/mg protein). Scatchard analysis at different stages of development revealed the presence of a single population of specific FNZ binding sites. The increase in [3H]FNZ binding during development was due to a large number of binding sites while their affinity remained unchanged. Competition experiments in the chick optic lobe revealed that the order of potency for displacement of specific [3H]FNZ binding paralleled the pharmacological potency of the BZDs tested. The IC50s for clonazepam, flunitrazepam, Ro 15-1788 and chlordiazepoxide were 3.02, 4.30, 0.32, and 4778.64 nM respectively. Ro 5-4864, a potent inhibitor of BZD binding to peripheral tissues, had no effect on specific [3H]FNZ binding indicating that only central BZD binding sites are present in the chick optic lobe. The peak of maximal expression of BZD receptor sites precedes in 5-6 days the peak of GABA receptor sites indicating a precocious development of BZD receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a simple visual pattern on the early postnatal development of GABA receptor sites in the chick optic lobe

It is known that manipulation of the visual environment results in changes in the developmental pattern of several neurotransmitter receptors and that the GABA receptor shows a high degree of plasticity in differential illumination experiments. In the present paper we investigated whether exposure to a visual pattern has a developmental effect on GABA receptor expression during early postnatal life. Two groups of newly hatched chicks were used: one was exposed to a simple and specific visual pattern and the other was deprived of any visual pattern. GABA receptors at each developmental stage were determined by binding experiments performed in a crude membrane fraction. Saturation studies were carried out in a fraction enriched in synaptic membranes. The developmental pattern of both high and low affinity GABA binding sites was affected by the visual pattern. This effect displays its maximal expression by the end of the first postnatal week. The modification in receptor expression was due to a change in the receptor density while the affinity was not affected. The change in receptor density induced by the presence of a visual pattern was highest at the end of the first postnatal week suggesting that at that time there is a brief period of higher plasticity for GABA receptor expression in the visual system than at other times. Our results also suggest that variations in GABA receptor density could be instrumental in adaptative changes in the visual system in response to variations in the environmental stimulation.

Glutamic acid decarboxylase in different areas of the developing chick central nervous system

The temporal course of the development of GAD activity in GABAergic neurons was studied in the chick retina, optic lobe and cerebellum. The developmental pattern of GAD activity was similar in the three areas studied, showing typical sigmoideal curves, which reached a maximal value at the 3rd post-hatching day. Kinetic studies during development revealed that Km remained unchanged while Vmax increased 3-fold in the retina (48.99 +/- 0.84 nmol/hr/mg protein), almost 4-fold in the optic lobe (162.77 +/- 4.32 nmol/hr/mg protein) and 3.5 fold in the cerebellum (69.30 +/- 1.26 nmol/hr/mg protein). The developmental pattern of GAD activity in homogenates of the three areas studied from dark-reared and light-reared chicks with respect to normal light-dark cycle animals showed no significant differences. These results indicate that the increase in GAD activity during development are not due to a change in the affinity for its substrate but rather to changes in the concentration of the enzyme. The developmental pattern of GAD activity in the chick visual system was not affected by environmental conditions suggesting that the developmental profile is light-independent.