Synaptic interactions in the GABA system during postnatal development in retina

Postnatal development of GABAA receptor beta1, beta2/3, and gamma2 immunoreactivity in the rat retina

GABA (gamma-aminobutyric acid) is the major inhibitory neurotransmitter in the mammalian central nervous system and plays an important role in neuronal physiology during ontogenesis. The distribution of the beta1-, beta2/3-, and gamma2-subunit of the GABAA receptor in the rat retina was studied during postnatal development using immunohistochemical methods. All subunits were found at birth. However, each subunit showed a unique staining pattern with a different local distribution. The immunoreactivity pattern changed during the time course of postnatal development for each of the proteins investigated. A clustered distribution at presumptive synaptic sites as indicated by a punctate staining pattern of the inner plexiform layer was detected as early as the second day of postnatal development. However, diffuse staining of presumptive extrasynaptic sites was found throughout development. The typical adult layering of immunoreactivity into distinctive bands appeared later in development, characteristically in the second postnatal week. The results of the present study suggest that GABAA receptor expression precedes the formation of functional synapses and changes along with cellular differentiation of the rat retina. Developmentally regulated changes in GABAA receptor composition and distribution indicate possible functions for this receptor during retinal ontogeny.

Pharmacologic and therapeutic aspects of the developmentally regulated expression of GABA(A) and GABA(B) receptors: cerebellar granule cells as a model system

Cerebellar granule neurons can be conveniently kept in culture. They constitute a useful model to study regulation of glutamatergic activity, in particular the inhibitory action of GABA (7-aminobutyrate). GABA exerts an inhibitory action on evoked transmitter release acting on both GABA(A) and GABA(B) receptors. The functional properties of these receptors are dependent upon the environment of the neurons during early development in culture as the expression of both receptor subtypes is enhanced by exposure of the neurons to GABA(A) receptor agonists. Thus, the inducible GABA(A) receptors are of low affinity and lack benzodiazepine sensitivity, and the G-protein coupling differs among the native and the inducible GABA(B) receptors. Moreover, the GABA(A) and the GABA(B) receptors are functionally coupled, leading to a disinhibitory action of GABA. Therefore drugs exhibiting selective agonist or antagonist action on subclasses of GABA(A) and GABA(B) may be of potential use as regulators of glutamatergic excitatory activity.

Pharmacological and functional implications of developmentally-regulated changes in GABA(A) receptor subunit expression in the cerebellum

The cerebellum undergoes many morphological, pharmacological, and electrophysiological changes during the first 3 weeks of postnatal development. The purpose of this review is to present the most up to date synopsis of the pharmacological and functional changes in, gamma-aminobutyric acid (GABA) type A receptors during this time of cerebellar maturation. Since most of the diversity in cerebellar, GABA(A) receptor pharmacology lies within the granule cell layer, research groups have focused on this area of the cerebellum to study the developmental changes in GABA(A) receptor subunit expression and the neurodifferentiating factors involved in regulating this expression. Thus, it is important to note that developmental changes in GABA(A) receptor composition and its corresponding pharmacology will be essential for determining the type of GABA-mediated transmission that occurs between neuronal contacts in the neonatal and subsequently in the mature cerebellum.

Analysis of the distribution of glycine and GABA in amacrine cells of the developing rabbit retina: a comparison with the ontogeny of a functional GABA transport system in retinal neurons

The objectives of this study were to (1) determine whether the glycinergic and GABAergic amacrine cells in the developing rabbit retina were neurochemically distinct at birth, (2) determine if the ratio of GABAergic to glycinergic amacrine cells was constant during development, (3) determine whether the capacity to take up a GABA analogue was restricted to GABAergic neurons, and (4) whether initiation of GABA transport into GABAergic neurons preceded the presence of a content of GABA in these neurons. We have used a novel strategy to immunolocalize a non-endogenous GABA analogue, gamma-vinyl GABA, which is taken up into neurons by a GABA transporter. Examination of serial semithin resin-embedded sections of neonatal rabbit retinae that had been immunolabelled for glycine, GABA or gamma-vinyl GABA revealed that at 1 day postnatum, 60% of amacrine cells contain glycine but not GABA and did not accumulate gamma-vinyl GABA, which is similar to the percentage of glycinergic amacrine cells in the adult retina. The vast majority of the remaining amacrine cells contained GABA and many also transported gamma-vinyl GABA; however, a significant number of GABA-containing cells failed to accumulate gamma-vinyl GABA suggesting that possession of a content of GABA did not have to be preceded by, or be concomitant with, the presence of a GABA transport system. By 10 days postnatum, over 99% of GABA-containing amacrine cells also transported gamma-vinyl GABA indicating their functional maturity. Analysis of the horizontal cells revealed no evidence for uptake of gamma-vinyl GABA, but another GABA analogue, diaminobutyric acid, which is a substrate both for the neuron-associated GABA transporter and the glial GABA transporter, was accumulated into some horizontal cells at 21 days postnatum, a time point when these cells also contain endogenous GABA. We conclude that amacrine cells are committed to being GABAergic or glycinergic at, or prior to birth, and that in some amacrine cells, expression of a content of GABA may occur prior to the capacity to transport GABA. Conversely, in some ganglion cells transport of gamma-vinyl GABA may precede a content of GABA.

GABA and GABA-A receptors are maximally expressed in association with cone synaptogenesis in neonatal rabbit retina

Previous studies have shown the cone photoreceptors form reciprocal synapses with horizontal cells during the first week after birth in rabbits. These synapses constitute pioneering elements of the developing outer plexiform layer. We now report that antibodies against the alpha-1 and against the beta-2/3 subunits of the GABA-A receptor label a highly restricted sublamina in the developing outer plexiform layer known to contain nascent cone photoreceptor terminals. Staining is relatively weak at birth, increases to maximal levels between postnatal days 5 and 7, and is significantly reduced in the adult. These results support recent calcium imaging studies which have shown that the activation of GABA-A receptors causes an increase in intracellular free calcium in cones, an effect which is observed only at 3-9 days after birth. The transient expression of GABA-A receptors in this region coincides with the period of peak expression of GABA immunoreactivity in horizontal cells. A direct functional link between GABAergic transmission and cone synaptogenesis is suggested by previous reports that GABA-A receptor antagonists cause disruption of cone synaptogenesis. Together these findings support the notion that GABA functions as a developmental neurotransmitter which is produced by horizontal cells and interacts with developing cone axons in order to facilitate synaptic linkage between these two cells types.

GABA-induced increases in [Ca2+]i in retinal neurons of postnatal rabbits

Previous studies have indicated that gamma-aminobutyric acid (GABA) plays an important trophic role in the synapse formation between horizontal cells and photoreceptors in postnatal rabbit retina. However, the mechanism of the GABA effect has not been identified. Using fluo-3 Ca2+ imaging and confocal laser scanning microscopy we examined the effect of GABA on [Ca2+]i during postnatal retinal development. GABA (100 microM) evoked a fast and transient increase of [Ca2+]i in selected populations of freshly dissociated retinal cells from postnatal rabbits. This increase was apparent on postnatal day 1 and reached a maximum on day 5. Little increase in [Ca2+]i was observed in retinal cells isolated from adult rabbits. GABA receptor antagonists, picrotoxin and bicuculline, significantly reduced the response. The GABAB agonist, baclofen, did not evoke any [Ca2+]i changes. The GABA-induced increase in [Ca2+]i was observed in all retinal layers in neonatal retinal whole-mount explants. In the outer retina, the increase was seen in cone photoreceptors which were specifically labeled with peanut agglutinin (PNA). The GABA-induced increase in [Ca2+]i may provide an important mechanism for regulating cone synaptogenesis in the outer plexiform layer of the postnatal retina.

Differential distribution of GABAA receptor subunits in soma and processes of cerebellar granule cells: effects of maturation and a GABA agonist

Quantitative analysis of the density of alpha 1 and beta 2/3 GABAA receptor subunits was performed at the electron microscope level after indirect pre-embedding immunogold labeling with subunit-specific antibodies of rat cerebellar granule cell cultures grown for 4 or 8 days and in the presence or absence of the GABAA receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4c]pyridin-3-ol (THIP). THIP (150 microM) induced a 2-fold increase in the number of alpha 1 and beta 2/3 subunits in both cell bodies and processes in 4-day-old cultures. Extending the culture period to 8 days led to a polarization of the receptor expression, since the increase in the number of subunits selectively was observed in the processes. Moreover, a general subcellular differentiation of the receptor population was observed in all culture conditions, since the ratio between the two subunits (beta 2/3; alpha 1) was four times higher in cell bodies compared to processes. A detailed analysis of the less mature (4-day-old) cultures revealed the existence of two populations of neurons exhibiting differences in the average number of receptors. During maturation neurons with few receptors developed into cells with a higher density of receptors resulting in a single population of the latter neurons, a process enhanced by exposure to THIP. This may indicate that receptor development is a discontinuous process with individual neurons following different temporal patterns. In double-labeling experiments, a spatially close association of the alpha 1 and beta 2/3 subunits could be seen, but the subunits were more frequently found separated from each other. In spite of the fact that exposure of the neurons to THIP increased the total number of receptor subunits, its presence apparently prevented formation of receptors with this subunit composition. Interestingly, receptor subunit clusters, consisting of alpha 1 alone, were more frequently observed than composite (alpha 1; beta 2/3) clusters. This substantiates the view that receptors not having alpha 1 and beta 2/3 subunits in the same complex may exist.

Modulatory actions of gamma aminobutyric acid (GABA) on GABA type A receptor subunit expression and function

Gamma aminobutyric acid (GABA) is present in the central nervous system (CNS) during very early embryogenesis. It is therefore likely to play a role not only as a neurotransmitter but also as a signal molecule for neuronal differentiation, growth, and development. It has been firmly established that formation of synapses is strengthened by GABA, and the expression of certain subunits of the GABA type A (GABAA) receptor complex is clearly promoted by GABA. This latter effect of GABA may have profound implications for the functional activity of GABAergic synapses since the pharmacological properties of GABAA receptors are governed by the subunit composition of the receptor complex. Dynamic changes in GABAA receptor expression and diversity during development and differentiation may therefore play important roles for the inhibitory potential of the CNS during mature stages.

Early development of GABA-like immunoreactive cells in the retina of turtle embryos

Gamma aminobutyric acid (GABA) is one of the earliest neuroactive substances appearing in the developing central nervous system. The distribution and the time course of the appearance of GABA-like immunoreactivity in the retina of the turtle Emys orbicularis were investigated from embryonic stage 13 to hatching. The first GABA-like immunoreactive cells were observed at stage 14. These cells were located in both the scleral third of the neuroblastic layer and the inner layers of the retina. They were identified as presumptive immature horizontal cells and amacrine cells, respectively. The observation of numerous labelled fibers in the nerve fiber layer suggests that some of the GABA-like immunoreactive cells in the layers were ganglion cells. The development of GABA-like immunoreactive cells followed a gradient of maturation from central to peripheral retina. At hatching, the central retina appeared nearly morphologically mature. In conclusion, GABA is present before the morphofunctional maturation of the retina and this precocious existence supports the idea of its involvement in a neurotrophic role preceding the establishment of synaptic connections and neurotransmitter function.

GABA immunoreactive axons and growth cones in the developing chicken optic nerve and tract

Immunohistochemical studies of the chicken embryo optic tract using an antibody to gamma-aminobutyric acid (GABA) reveal that the tract is initially free of GABA immunoreactive axons. During the second week of incubation, GABA+ axons appear in the tract, chiasm, and optic nerve. The number of GABA+ axons in the optic nerve increases through E18, although few are recognizable after hatching. Detailed staining of GABA+ growth cones confirmed that virtually all the GABA+ axons in the optic nerve were growing toward the retina. Taken together, the findings suggest that the GABA+ axons in the chiasm and nerve are largely a transient extension of the GABA+ optic tract cells, the tectogeniculate projection, or both.

Postnatal development of ganglion cells in the rabbit retina: characterizations with AB5 and GABA antibodies

The use of cell-specific monoclonal antibodies provides a means by which the emergence, differentiation and maturation of retinal neurons can be studied. The present study investigates the labelling of ganglion cells in the developing rabbit retina by a ganglion cell-specific monoclonal antibody, AB5(12,13). AB5 labelling of ganglion cells was observed as early as day postnatal. By 6-8 days postnatal, AB5-labelled ganglion cells had begun differentiating into the various ganglion cell subtypes observed in the adult retina. This differentiation process appeared to continue throughout the first 3 weeks postnatal. The AB5 monoclonal antibody was also used in a double-label paradigm with an anti-gamma-aminobutyric acid (GABA) polyclonal antibody to differentiate the GABAergic ganglion cells from other GABAergic elements in the retina and to study their development. GABAergic ganglion cells were first observed at 3 days postnatal and by 6 days postnatal, it was possible to observe a wide variety of GABAergic ganglion cells ranging from small cells to large alpha-type cells. The appearance of AB5 labelling in ganglion cells at relatively early stages of development suggests that the AB5 monoclonal antibody may be a useful tool for studying the development of ganglion cell structure, distribution, synaptic relationships and neurochemical specificity.

Kainic acid lesioning alters development of the outer plexiform layer in neonatal rabbit retina

The first synaptic relay in the primary visual pathway occurs between terminals of photoreceptors and second-order neurons within the outer plexiform layer of the retina. During development, one of these types of second-order neurons, the type A horizontal cell, differentiates and assumes mature characteristics several days before any other cells ramifying in that synaptic layer. In neonates, horizontal cells appear to be GABAergic during the first 5 days of postnatal life and in addition they also are responsive to kainic acid. We have previously suggested that they may play a pioneering role in the postnatal development of the outer plexiform layer, perhaps providing structural guidance or trophic substances such as GABA, for synaptic development. To test this hypothesis, we first demonstrated that a single intraocular injection of kainic acid within 24 hr of birth results in a permanent and selective loss of type A horizontal cells in the outer retina. Retinas from animals maintained for 5 days postinjection were harvested for analysis of postnatal development of the outer plexiform layer in the absence of horizontal cells. One of the major findings was that kainic acid treatment caused a reversal of the normal complement of photoreceptor cell types, resulting in an abnormally high rod/cone ratio. The distribution of cell processes within the outer plexiform layer was also altered and normal synaptic connections were not made. In spite of these changes in the constituents of the outer plexiform layer, the normal position of the synaptic layer was not affected by the loss of horizontal cells. These results rule out the possibility that horizontal cells provide a structural barrier which is an absolute requirement for establishing the location of the outer plexiform layer. Rather, these cells may be more involved in cell differentiation and synaptogenesis.

Maturational changes in retinal excitatory amino acid receptors

The appearance, kinetics and pharmacological properties of receptors for n-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate (KA), L-glutamate (Glu) and L-aspartate (Asp) was investigated using 3H-ligand binding during the development of chick embryo retina. Receptors for AMPA are maximally concentrated at embryonic day 7 (ED 7) and decline 50% in subsequent days; L-Glu receptors are low until ED 11, and the same is true for Asp and NMDA receptors which increase at ED 14 and 18 respectively. All receptors studied underwent an increase in pharmacological specificity, whereas only AMPA-receptors showed an important change in affinity during ontogeny. Results demonstrate that receptors for excitatory amino acids in the retina suffer maturational changes and suggest that while NMDA and aspartate could interact with the same receptor, AMPA and glutamate seem to bind to different sites.

Neurotrophic effects of GABA in cultures of embryonic chick brain and retina

The present study examined whether GABA treatments would affect the growth and development of embryonic chick cortical and retinal neurons in culture. Incubation of these cells in the presence of 10(-5) M GABA produced several responses. It promoted the proliferation and the differentiation of the neurons studied by affecting the length and branching of the neurites as well as synaptogenesis, as revealed by morphometric measures. At the ultrastructural level, GABA treatment also led to an increased density of neurotubules, rough endoplasmic reticulum (RER), Golgi apparatus, coated vesicles, and other vesicles. These data support the hypothesis that GABA functions as a trophic or regulatory factor of at least certain neuron types.

GABA agonist promoted formation of low affinity GABA receptors on cerebellar granule cells is restricted to early development

The ability of the GABA receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) to promote formation of low affinity GABA receptors on cerebellar granule cells was tested using primary cultures of these neurons. Granule cells were exposed to THIP (150 microM) for 6 hr after, respectively, 4, 7, 10 and 14 days in culture. It was found that THIP treatment of 4- and 7-day-old cultures led to formation of low affinity GABA receptors, whereas such receptors could not be detected after THIP treatment in the older cultures (10 and 14 days) in spite of the fact that these cultured granule cells expressed a high density of high affinity GABA receptors. It is concluded that the ability of THIP to promote formation of low affinity GABA receptors on cerebellar granule cells is restricted to an early developmental period.

Temporal development of GABA agonist induced alterations in ultrastructure and GABA receptor expression in cultured cerebellar granule cells

The temporal development of the effect of THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) on the ultrastructure composition and GABA receptor expression in cerebellar granule cells was investigated by quantitative electron microscopy (morphometric analysis) and GABA binding assays. It was found that the cytoplasmic density of smooth endoplasmic reticulum was decreased, while the cytoplasmic density of rough endoplasmic reticulum, Golgi apparatus, vesicles and coated vesicles was greatly enhanced after exposure of the cells to THIP (150 microM) for only 1 hr. In cerebellar granule cells exposed to THIP (150 microM) for 3 hr low affinity GABA receptors were induced. These findings show that the effect of THIP on the ultrastructure composition and GABA receptor expression in cultured cerebellar granule cells may be interrelated and moreover it is likely that the turn-over of GABA receptors is extremely fast.

Gamma-aminobutyric acid affects the developmental expression of neuron-associated proteins in cerebellar granule cell cultures

The effect of gamma-aminobutyric acid (GABA) on the expression of the neuron-associated D2 and neuron-specific enolase (NSE) was studied during development in culture of cerebellar granule cells. It was found that the presence of GABA during culture development increased the overall protein content. D2 content was also increased but not above the general increase in protein whereas NSE increased above the general level of protein. The presence of GABA in the growth medium also appeared to accelerate the changes in molecular forms of D2 and NSE seen during neuronal development. This suggests that GABA promotes or accelerates the general maturation of neurons, as these two neuron-associated proteins otherwise differ from each other with respect to their subcellular localization and their physiological and biochemical properties.

GABA-agonists induce the formation of low-affinity GABA-receptors on cultured cerebellar granule cells via preexisting high affinity GABA receptors

The kinetics of specific GABA-binding to membranes isolated from cerebellar granule cells, cultured for 12 days from dissociated cerebella of 7-day-old rats was studied using [3H]GABA as the ligand. The granule cells were cultured in the presence of the specific GABA receptor agonist 4, 5, 6, 7-tetrahydroisoxazolo [5,4-c]pyridin-3-ol (THIP, 150 microM) or THIP plus the antagonist bicuculline methobromide (150 microM of each) or in the absence of the agonist or antagonist. Membranes isolated from granule cells cultured in a medium without the GABA agonist revealed a single binding site for GABA with a binding constant (KD) of 7.9 +/- 0.4 nM and a Bmax of 3.42 +/- 0.08 pmol X mg-1 protein. Membranes from cells cultured in the presence of THIP had two binding sites for GABA with KD-values of 6.8 +/- 0.9 nM and 476 +/- 311 nM, respectively. The corresponding Bmax values were 4.41 +/- 0.42 pmol X mg-1 and 5.81 +/- 1.20 pmol X mg-1. The effect of culturing the cells in THIP was antagonized by the simultaneous presence of bicuculline in the culture media, i.e. no significant low-affinity binding for GABA was found on the membranes from granule cells cultured in both THIP and bicuculline. The KD value (14.3 +/- 1.4 nM) for the high affinity binding site was, however, slightly increased compared to the non-treated cells. These findings suggest that the ability of THIP to induce formation of low-affinity GABA receptors is mediated by preexisting high-affinity GABA-receptors on the granule cells.

GABA receptor binding site "induction" in rabbit retina after nipecotic acid treatment: changes during postnatal development

The development of the GABA system in the rabbit retina was studied. The number of high- and low-affinity GABA receptor binding sites increased in a sigmoidal manner, with the curve for the low-affinity sites lagging 2-3 days behind that for the high-affinity sites. The KD for both high- (17.5 nM) and low-affinity (138.0 nM) sites remained constant during development. Treatment of isolated eyecups with the uptake blocker nipecotic acid resulted in an increase in the Bmax for high-affinity sites in developing tissue with the maximum sensitivity around eye opening; mature tissue exhibited a decrease in Bmax. In contrast, a gradual decrease in sensitivity to stimulation of the low-affinity sites occurred. These data indicate that the "trophic" action of GABA is limited to the time when the tissue is developing.

Postnatal development of 3H-GABA-accumulating cells in rabbit retina

Light and electron microscopic autoradiography demonstrates that 3H-GABA is accumulated by horizontal cells in neonatal rabbit retina but not in the adult. A specific population of horizontal cells appears to be mature at birth and they avidly accumulate 3H-GABA during a 15-minute incubation period in vitro. Uptake into horizontal cells is not observed after the fifth postnatal day; 3H-GABA-accumulating horizontal cell bodies and their processes are the first identifiable components that clearly mark the future location of the outer plexiform layer at birth and as such, may be considered pioneering elements. Our observations raise the interesting possibility that the pioneering horizontal cell may provide structural and/or chemical factors necessary for the subsequent development of the outer plexiform layer of the retina. Labeling patterns of other retinal cells also show varying degrees of change during development. A population of amacrine cells accumulate 3H-GABA at birth. These cells show little change in their morphological or 3H-GABA uptake properties from birth to adulthood. Müller cells show weak accumulation of 3H-GABA at birth. Subsequent to this time, labeling of Müller cells is significantly more robust, resulting in Müller cell domination of retinal autoradiographic patterns in more mature retinas. Every cell body in the ganglion cell layer accumulates 3H-GABA at birth. The number of labeled cells declines during postnatal development, resulting in a very limited adult population. We conclude that the ability of retinal cells to accumulate 3H-GABA does not remain constant during postnatal development; rather each cell population displays a unique maturation sequence that results in a dramatic developmental shift in the number and types of GABA-accumulating cells present in the retina.

Neurotransmitter systems in morphologically undifferentiated human Y-79 retinoblastoma cells: studies of GABAergic, glycinergic, and beta-adrenergic systems

Elements of three neurotransmitter systems were investigated in morphologically undifferentiated human Y-79 retinoblastoma cells in suspension culture. Specific gamma-aminobutyric acid (GABA) uptake, GABA binding, and glycine binding were absent from these cells, although the cells had been shown to exhibit an active uptake and release of [3H]glycine. Binding and competition studies using both alpha- and beta-adrenergic ligands indicated the presence of a beta-adrenergic receptor. This finding was confirmed by treatment of the cells with beta-agonists in competition with a beta-antagonist and with an alpha-antagonist; the level of cyclic AMP was competitively stimulated. Therefore, human Y-79 cells in suspension culture contain beta-adrenergic receptors, and not glycinergic or GABAergic ones. Thus, the Y-79 cells may be of use in studying the factors involved in developmental regulation of neurotransmitter function.

Postnatal development of GABA- and glycine-mediated inhibition of feline retinal ganglion cells in the area centralis

Intraretinal iontophoresis in the optically intact eye of adult cats (18-22 weeks of age) and kittens (7-9 weeks of age) under pentobarbitone anaesthesia was performed. Studies were concentrated on retinal ganglion cells of the sustained (X) type in the area centralis under photopic conditions. In both the adult and kitten, gamma-aminobutyric acid (GABA) and muscimol inhibited the visually induced excitation, and bicuculline blocked the visually induced inhibition of on-cells. On the other hand, glycine inhibited the excitation and strychnine blocked the inhibition of off-cells. However, a greater current of GABA (muscimol) and glycine was required to produce total inhibition in kitten's on- and off-cells respectively when compared with the adult's. Furthermore, a smaller current of bicuculline and strychnine was needed to abolish the visually induced inhibition of kitten on- and off-cells respectively when compared with the adult's. In the adult, GABA and glycine did not affect the responses of off- and on-cells respectively, but in the kitten GABA inhibited off-cells and glycine inhibited on-cells to some extent. In neither the adult nor the kitten did bicuculline have any effect upon off-cells or strychnine any effect upon on-cells. Thus, the sustained on- and off-cells in the kitten area centralis exhibit: a reduced selectivity to inhibitory transmitters; a reduced sensitivity to exogenously applied inhibitory transmitter agonists; but a greater sensitivity to inhibitory transmitter antagonists, in comparison with the sustained on- and off-cells in the adult area centralis. The observed differences between the kitten and adult cat in transmitter actions on retinal ganglion cells appear to be analogous to those found in the postnatal development of functional synapses at the neuromuscular junction and sympathetic ganglia.

Metabolism of [3H]nipecotic acid in the rabbit retina

Nipecotic acid has been demonstrated to block the gamma-aminobutyric acid transport systems. Previous studies have shown that the uptake system is the first transmitter-specific parameter to appear during the development of the rabbit retina. Use of these observations has been made to study the influence on the development of gamma-aminobutyric acid receptors of altering the uptake mechanism by treating newborn pups with nipecotic acid to block GABA transport. The present study of the in vivo metabolism of [3H]nipecotic acid in the CNS measured the changes in the levels of [3H]nipecotic acid in both adult and newborn rabbit retinas after injection of the label into the vitreal chamber. It was found that the effective half-life of [3H]nipecotic acid in the vitreous is about 5 h for adult tissue and 3 h for newborn. In contrast, all retinal fractions retained the label longer, the effective half-lives being about 60 h (adult) and 45 h (newborn). Further, no labeled metabolites of nipecotic acid were detected in either adult or newborn tissue. This study gives evidence that the degradation of nipecotic acid in nervous tissue is minimal and suggests that, although the rate of clearance is faster in neonates, the fate of nipecotic acid in vivo may be similar in both adult and newborn tissues.

Benzodiazepines and the developing rat: a critical review

This paper reviews: the development of benzodiazepine binding-sites and the GABA system; the evidence that prenatal exposure to benzodiazepines can cause malformations; other persisting effects of developmental exposure to benzodiazepines; and the behavioral effects of benzodiazepines (and other relevant drugs) in immature animals. The review concentrates on the rat, since fundamental work in other species is scarce. The data on neurochemical development are found to be generally consistent; however, reports that the enhancement of benzodiazepine binding by GABA varies with age are controversial. The physical development of the rat is disturbed only by extremely high doses of benzodiazepines. The evidence for persisting effects after early exposure to benzodiazepines is impressive at first sight, but in most studies, confounding variables have not been eliminated. Startle and some learning tasks are affected by prenatal diazepam; submissiveness is affected by neonatal lorazepam; social behaviour and convulsions are affected by neonatal CGS 8216. Benzodiazepines inhibit chemically-induced seizures in neonatal rats, but the developmental profile of sensitivity to the convulsants is disputed. Benzodiazepines stimulate motor behavior in the neonatal rat.