The ontogeny of [3H]muscimol binding sites in the C57BL/6J mouse cerebellum

Autoradiographic evidence of delta-opioid receptor downregulation after prenatal stress in offspring rat brain

In order to visualize neuroanatomical alterations in specific brain regions, light microscopy autoradiography was carried out on offsprings (postnatal day 10) from female rats stressed in different periods of gestation and controls. Group 1 was subjected to restraint stress from day 2 to 6; group 2, from day 7 to 11; group 3, from day 12 to 16; group 4 from day 2 to 16. Group 2 showed decreases in delta-opioid receptor density in different hypothalamic regions. The decrease in delta-opioid receptor density was less marked in groups 1 and 3 whereas there was no modification in group 4. Present data suggest that the prenatal stress induces a downregulation of delta-receptors in different hypothalamic regions.

Neurons derived from P19 embryonal carcinoma cells develop responses to excitatory and inhibitory neurotransmitters

Cells of the P19 line of embryonal carcinoma cells differentiate into neurons, astrocytes and oligodendrocytes following treatment with retinoic acid. The neurons from these differentiating P19 cultures synthesize a pattern of neurotransmitters that resembles that of neurons of the forebrain. We treated P19 cells with retinoic acid and then implanted them into the striatum of adult rats. After times ranging from 1 to 15 weeks post-implantation, brain slices containing the implanted tissue were prepared and used for intracellular recording of electrical activity and responsiveness to application of neurotransmitters. Within 2 weeks of implantation, the P19-derived neurons had developed responsiveness to the excitatory neurotransmitter glutamate and the inhibitory transmitters gamma-aminobutyric acid and glycine. These neurons also exhibited spontaneous synaptic potentials. The responses to glutamate appear to be mediated by N-methyl-D-aspartic acid as well as non-N-methyl-D-aspartic acid receptor subtypes. Gamma-aminobutyric acid evoked bicuculline-sensitive depolarizing responses in the younger grafts and biphasic depolarizing/hyperpolarizing responses in older ones. Responses to glycine were strychnine sensitive and also showed age-related changes from depolarizing to biphasic character. Synaptic potentials in the younger grafts were exclusively depolarizing, but in older ones both depolarizing and hyperpolarizing events were observed. The synaptic potentials appear to arise from synaptic connections between P19-derived neurons within the grafts. Many of the features of P19-derived neurons are similar to those of neurons in the developing forebrain.

GABAA receptor subunit mRNA expression in the weaver cerebellum: modulation by cell-cell interactions

Recent studies have suggested that the developmental expression of GABA(A) receptor subunit mRNAs in cerebellar Purkinje neurons is modulated by cell--cell interactions [correction of interacactions]. In this population, the levels of mRNAs encoding the alpha1, beta2, and gamma2 subunits increase simultaneously during the second week of postnatal ontogeny, a period temporally coincident with cerebellar maturation and synapse formation. To determine the importance of cell--cell interactions in modulating receptor gene expression, the levels of GABA(A) receptor subunit mRNAs in Purkinje neurons of weaver mice and littermate controls were examined by quantitative in situ hybridization histochemistry. In the weaver mutant most granule neurons die early in postnatal development, thus eliminating the major source of excitatory input to Purkinje cells. Despite this loss, the three subunit mRNAs were expressed in all Purkinje neurons. However, the levels of expression were generally lower in the mutants than in the littermate controls. These results suggest that the onset of GABA(A) receptor gene expression in cerebellar Purkinje neurons occurs in the absence of extensive synapse formation by mechanisms which may be intrinsic to the neurons. In contrast, the absolute level of transcript expression attained appears to be modulated by cell-cell interactions or by other extrinsic cues present in the cerebellar environment.

GABAA receptor subunit polypeptides increase in parallel but exhibit distinct distributions in the developing rat cerebellum

The GABAA receptor, a multisubunit ligand-gated ion channel, plays a central role in cell-cell communication in the developing and adult nervous system. Although the developmental expression of mRNAs encoding many subunit isoforms has been extensively characterized throughout the central nervous system, little is known concerning the relationship between subunit mRNA and polypeptide expression. To address this issue, we examined the developmental expression of the alpha 1, beta 2/3, and gamma 2 subunit polypeptides, subunits that are thought to coassemble in many brain regions. Western blot analysis using subunit-specific antibodies revealed that the levels of these polypeptides in both the cerebral cortex and cerebellum increased severalfold during the second postnatal week. Whereas polypeptide expression in the cerebellum paralleled that of the corresponding subunit mRNAs, increases in beta 2/3 and gamma 2 polypeptide expression in the cerebral cortex occurred in the absence of detectable changes in the mRNA levels. To determine whether the increases in subunit polypeptide expression in the cerebellum were accompanied by changes in distribution, immunohistochemistry was performed. These studies demonstrated that the subunits exhibited different but partially overlapping distributions that remained constant throughout postnatal development. Our findings suggest that although GABAA receptor subunit polypeptide expression may be regulated primarily at the level of the mRNA, additional regulatory mechanisms may play a role. Furthermore, the observation that subunit distribution remains constant in the cell bodies of cerebellar Purkinje neurons, which express the alpha 1, beta 2, beta 3, and gamma 2 subunit mRNAs exclusively, suggests that GABAA receptor subunit composition in this cell population does not change during postnatal maturation.

Chronic NMDA receptor blockade or muscimol inhibition of cerebellar cortical neuronal activity alters the development of spinocerebellar afferent topography

The requirement for cerebellar cortical neuronal activity in the development of spinocerebellar afferent topography was investigated in neonatal rats. In adult rats lower thoracic-upper lumbar spinocerebellar projections are localized to sharply circumscribed patches in the granule cell layer of the cerebellar anterior lobe. In transverse sections these patches appear as sagittally oriented stripes. This pattern develops postnatally as many spinal axons which initially project between the incipient stripes are eliminated thereby sharpening the stripe boundaries. We attempted to alter cerebellar cortical neuronal activity in neonatal animals to study the effects of these changes on the development of spinocerebellar stripes. In some experiments glutaminergic excitatory synaptic transmission was chronically blocked with the N-methyl-D-aspartate (NMDA) receptor antagonist 2-aminophosphovaleric acid (APV). In other experiments postsynaptic activity was directly inhibited by the gamma-aminobutyric acid agonist muscimol. Chronic exposure to APV or to muscimol did not affect the initial development of spinocerebellar projections; many spinal axons were present in the anterior lobe and arranged in incipient stripes. Both the APV and the muscimol appeared to prevent the elimination of interstripe projections; consequently the boundaries of the stripes remained poorly defined. These findings suggest that cerebellar cortical neuronal activity is a necessary requirement for the refinement of spinal afferent topography in the anterior lobe.

GABA alters GABAA receptor mRNAs and increases ligand binding

In addition to its role as an inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) influences the cytodifferentiation of developing neurons both in culture and in vivo. Here, we report some of the targets of GABA action and the mechanism through which GABA acts. In primary cultures of cerebellar granule cells, GABA specifically stimulates an increase in the levels of mRNAs for alpha 1 and beta 2 GABAA receptor subunits. The GABAA agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) mimics this effect, and the GABAA antagonist bicuculline prevents it. In addition, GABA and THIP trigger an increase in the number of GABA binding sites. This increase parallels that seen in vivo, where the total number of GABAA receptor sites increases during postnatal cerebellar development. It is interesting that the period of the greatest increase in the number of receptor sites coincides with the development of the granule cells. Taken together, our data suggest that GABA may play an important role during maturation of cerebellar granule cells by influencing the number and composition of its own receptors.

Differential expression of GABAA/benzodiazepine receptor beta 1, beta 2, and beta 3 subunit mRNAs in the developing mouse cerebellum

Gamma aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian cerebellum. Cerebellar granule, Purkinje, and deep nuclear neurons are known to receive GABAergic afferents. Since GABA exerts its inhibitory effects via GABA receptors, it is of interest to determine the temporal relationship between the formation of GABAergic synapses and the expression of genes coding for the GABA receptor. In a previous study, we have examined the developmental expression of binding sites for [3H]muscimol, which binds with high affinity to the beta subunits of the GABAA/benzodiazepine (GABAA/BZ) receptor. In the present study, [35S]cRNA probes were used to examine the appearance and distribution of GABAA/BZ beta 1, beta 2, and beta 3 subunit mRNAs in the developing C57BL/6 mouse cerebellum by in situ hybridization. In the adult cerebellum, the distribution of the three subunit mRNAs was clearly different, despite considerable overlap, and their temporal expression differed throughout postnatal development. The beta 1 hybridization signal appeared within the cerebellar cortex during the second postnatal week as a discrete band at the interface of the molecular and granule cell layers. Grains were distributed diffusely over small densely staining cells surrounding the Purkinje cells; relatively few grains were visible over Purkinje cell bodies themselves. This distribution may reflect an association with Bergmann glia or basket cells. The beta 2 and beta 3 hybridization signals were present considerably earlier than that of the beta 1 mRNA. The beta 2 signal was present at birth in the molecular/Purkinje cell layer; as development progressed, the signal became increasingly intense over both granule and Purkinje cells. At birth, the beta 3 subunit mRNA was present in the external germinal and molecular layers, later becoming largely localized within the granule cell layer. Dense beta 2 and beta 3 cRNA probe labeling was present over the adult granule cell layer. Moderate levels of beta 2 signal were seen over Purkinje cell bodies; considerably less labeling was observed with the beta 3 probe. The adult distribution of beta 2 and beta 3 cRNA probes showed good spatial correspondence with the known GABAA receptor beta subunit markers, [3H]-muscimol and the mAb 62-3G1 antibody, each being present within the granule cell layer. Our results indicate that the temporal expression of GABAA/BZ receptor beta subunit messages within a given cell type may be independently regulated, and that acquisition of the beta 2 and beta 3 mRNAs occurs before these cells become integrated into mature synaptic circuits.

Ontogeny of GABAA/benzodiazepine receptor subunit mRNAs in the murine inferior olive: transient appearance of beta 3 subunit mRNA and [3H]muscimol binding sites

The GABAA/benzodiazepine receptor consists of at least four subunits, alpha, beta, gamma and delta, each comprised of several variants. The developmental expression of the alpha 1, beta 1-3, gamma 2 and delta subunits was studied in the murine inferior olivary nucleus by in situ hybridization with antisense cRNA probes. The postnatal appearance and distribution of [3H]flunitrazepam and [3H]muscimol binding sites, alpha and beta subunit-specific ligands respectively, were also studied autoradiographically. The beta 3 subunit was transiently expressed in each of the subnuclei of the inferior olive: The signal was strong at birth, increased throughout postnatal week 1 and rapidly declined thereafter to low adult levels. A similar pattern of labeling was observed with [3H]muscimol. Detectable levels of alpha 1 subunit mRNA hybridization signal and [3H]flunitrazepam binding sites were also present in the inferior olive at birth, decreasing thereafter. Low to moderate levels of beta 1, beta 2, and gamma 2 subunit mRNAs were present in olivary neurons throughout postnatal development, while delta mRNAs were largely absent. It has been reported previously that, during the 2nd postnatal week, the ratio of climbing fiber terminals to Purkinje cells is reduced from 3:1, as observed in neonates, to the 1:1 relationship observed in the adult cerebellar cortex. Our results raise the possibility that the subunit composition of the GABAA/benzodiazepine receptor in inferior olivary neurons undergoes changes during development, and that this process may be related to the elimination of multiple climbing fiber innervation of cerebellar Purkinje cells.

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).

Muscimol and flunitrazepam binding sites in a subcellular fraction enriched in rat cerebellar glomeruli

In the internal granular layer of the cerebellar cortex the polysynaptic complexes called glomeruli consist mainly of homogeneous populations of glutamatergic and GABAergic synapses, both located on granule cell dendrites. A subcellular fraction enriched in glomeruli was prepared from rat cerebellum, and the distribution of GABAA and of benzodiazepine binding sites between membranes derived from this fraction (fraction G) and from a total cerebellar homogenate (fraction T) was studied. The benzodiazepine and GABA binding sites were measured by the binding of agonists [3H]flunitrazepam and [3H]muscimol, respectively. The results indicate that both binding sites are present, but only slightly enriched, in the glomerular synapses. We found a muscimol/flunitrazepam binding site ratio of two, which is consistent with the enrichment of muscimol binding sites in the granular layer shown by both autoradiographic with radioactive glutamatergic ligands and in situ hybridization experiments respectively.

[3H]muscimol and [3H]flunitrazepam binding sites in the developing cerebellum of mice treated with methylazoxymethanol at different postnatal ages

Two models of perturbed cerebellar ontogenesis were obtained by a single administration of methylazoxymethanol (MAM), a potent antimitotic agent, to mouse pups either on the day of birth (MAM0 mice) or at postnatal day 5 (MAM5 mice). The alterations of the cerebellar GABAergic system were studied by measuring glutamic acid decarboxylase activity, [3H]muscimol binding sites, which are known to be concentrated in the GABAA receptors in the internal granular layer, and [3H]flunitrazepam binding sites, which are more abundant in the molecular layer. The primary target of the antimitotic agent are the precursors of the glutamatergic and GABAceptive granule cells. In both models GABAergic structures, as revealed by GAD activity measurements, appear to be relatively spared, and recovery of granule cell numbers occurs during development in MAM5 mice. In MAM treated mice the number of [3H]muscimol binding sites (on a per cerebellum basis) decrease as the number of granule cells decrease, although some recovery occurred in MAM5 mice, but not in MAM0 mice. In MAM5 mice, [3H]flunitrazepam binding sites (on a per cerebellum basis) were relatively unaffected, while they were decreased significantly, but to a lesser extent than [3H]muscimol binding sites, in MAM0 animals. The more significant reduction of granule cell numbers and the cytoarchitectural disruption resultant from the more precocious application of the antimitotic appear responsible for the significant alteration and lack of recovery in MAM0 mice.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Stability of GABAA/benzodiazepine receptor alpha 1 subunit mRNA expression in reeler mouse cerebellar Purkinje cells during postnatal development

A [35S]cRNA probe was used for the visualization of GABAA/benzodiazepine (GABAA/BZ) receptor alpha 1 subunit mRNA in developing reeler mutant mouse cerebellum. A clear hybridization signal was observed throughout the malformed reeler cerebellum from birth. Labeling was associated with Purkinje cell bodies located in three subcortical masses. Additional labeled Purkinje cells were observed within the granule cell layer and at their normal position at the interface between the molecular and granule cell layers. All reeler Purkinje cells had comparable levels of grain density, regardless of their location within the cerebellar cortex. These results indicate that Purkinje cell malpositioning, and the resulting absence of a major complement of afferents throughout development, does not impair the expression of mRNA coding for the alpha 1 subunit of the GABAA/BZ receptor.

Calcium binding protein (calbindin-D28k) gene expression in the developing and aging mouse cerebellum

Calbindin-D28k (CaBP28k) protein and gene expression were examined in the mouse cerebellum during development and aging utilizing slot and Northern blot hybridization analyses for mRNA levels, Western blot analysis and radioimmunoassay (RIA) for protein levels, and by in situ studies using immunocytochemistry and hybridization cytochemistry on prepared tissue sections. Samples were obtained and analyzed from C57BL/6J mice aged day of birth and postnatal weeks 1, 2, 4, 8, and 120. A specific cDNA and antibody for CaBP28k were utilized in these studies. Analysis of mRNA levels showed a steady rise in CaBP28k mRNA from birth to a peak at postnatal week (3.4-fold increase) and then a decline to steady-state levels at postnatal weeks 4 and 8 (47% reduction of peak level) followed by a reduction of CaBP28k mRNA to birth levels at postnatal week 120. The specificity of the changes observed was tested by reprobing blots with beta-actin cDNA. Analysis of CaBP28k protein levels by both Western blot and RIA showed a similar pattern. In situ analysis of CaBP28k mRNA levels, based on hybridization signal (silver grains per cell), demonstrated a rise in cellular CaBP28k mRNA levels which peaked at postnatal week 2 (416.9 +/- 52.1) and then declined to steady-state levels by postnatal weeks 4 and 8 (267.4 +/- 35.8). Cellular CaBP28k mRNA levels exhibited a dramatic reduction in the aged cerebellum (postnatal week 120; 78.3 +/- 16.0). The levels of cellular CaBP28k mRNA corresponded to the intensity of immunoreactive CaBP28k localized by immunocytochemistry. The results are consistent with the hypothesis that CaBP28k may play a critical role in Purkinje cell maturation and maintenance.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of endogenous taurine and gamma-aminobutyric acid from brain slices from the adult and developing mouse

The spontaneous and potassium-stimulated release of endogenous taurine and gamma-aminobutyric acid (GABA) from cerebral cortex and cerebellum slices from adult and developing mice was studied in a superfusion system. The spontaneous release of GABA was of the same magnitude in slices from adult and developing mice, but the spontaneous release of taurine was considerably greater in the adults. The potassium-stimulated release of GABA from cerebral cortex slices was about five times greater in adult than in 3-day-old mice, but the potassium-stimulated release of taurine was more than six times greater in 3-day-old than in adult mice. In cerebellar slices from 7-day-old mice, potassium stimulation also evoked a massive release of taurine, whereas the evoked release from slices from adult mice was rather negligible. Also in cerebellar slices the potassium-stimulated release of GABA exhibited the opposite quantitative pattern. The stimulated release of both GABA and taurine was partially calcium dependent. The results suggest that taurine may be an important regulator of excitability in the developing brain.

Release of taurine and GABA from cerebellar slices from developing and adult mice

The properties of the release of exogenous radioactive taurine and GABA from cerebellar slices from developing and adult mice were investigated using a superfusion system. Potassium stimulation (50 mM K+) caused, approximately, a 1.4-fold enhancement in the release of preloaded taurine from slices from adult mice, while the response to potassium in 7-day-old mice was about 6-fold. The potassium stimulation caused, approximately, a 3-fold increase in the release of preloaded GABA from cerebellar slices from 7-day-old mice, whereas the enhancement was about 10-fold in the adult. The actual molar amount of taurine released from the immature cerebellum was strikingly large, about 16 times larger than the amount of GABA released upon the same stimulus. Spontaneous taurine efflux was potentiated by taurine and GABA, the responses being more pronounced in the 7-day-old cerebellum, suggesting that the immature cerebellum is more prone to stimulation by homo- and heteroexchange than the mature cerebellum. Potassium-stimulated taurine release was inhibited by GABAergic substances in the adult but not in the developing cerebellum. Potassium-stimulated GABA release from the adult cerebellum was greatly increased by GABA and also moderately by muscimol and bicuculline, the effect of the latter being antagonized by taurine and hypotaurine. Taurine was thus able to modulate GABA release through bicuculline-sensitive receptors, but this modulation was not evident in cerebellar slices from 7-day-old mice. An exposure of the slices to sodium-free media greatly enhanced taurine and GABA release in both age groups. The stimulated release of GABA generally exhibited a similar calcium dependency in the adult and 7-day-old cerebella but in 7-day-old mice the stimulated release of taurine was not strictly calcium-dependent.