Changes in messenger RNAs coding for neurotransmitter receptors and voltage-operated channels in the developing rat cerebral cortex

Bilateral enucleation at birth modifies calcium spike amplitude, but not frequency, in neurons of the somatosensory thalamus and cortex: Implications for developmental cross-modal plasticity

Bilateral eye enucleation at birth (BE) leads to an expansion of the primary somatosensory cortex (S1) in rat pups. Although increased growth of the somatosensory thalamo-cortical afferents (STCAs) in part explains S1 expansion, timing mechanisms governing S1 formation are also involved. In this work, we begin the search of a developmental clock by intending to document the existence of putative clock neurons in the somatosensory thalamus (VPM) and S1 based upon changes of spontaneous spike amplitude; a biophysical property sensitive to circadian regulation; the latter known to be shifted by enucleation. In addition, we also evaluated whether STCAs growth rate and segregation timing were modified, as parameters the clock might time. We found that spontaneous spike amplitude transiently, but significantly, increased or decreased in VPM and S1 neurons of BE rat pups, respectively, as compared to their control counterparts. The growth rate and segregation timing of STCAs was, however, unaffected by BE. These results support the existence of a developmental clock that ticks differently in the VPM and S1 after BE. This observation, together with the fact that STCAs growth rate and segregation timing is unchanged, suggests that S1 expansion in BE rats may in part be controlled at the cortical level.

Ontogenetic distribution of 5-HT2C, 5-HT5A, and 5-HT7 receptors in the rat hippocampus

It is known that serotonin exerts its different nociceptive and motor functions by interacting with distinct receptors subtypes, which could be either G-protein coupled or ionotropic. Previous reports demonstrated the early activation of serotonin receptor transcripts during rat development, suggesting a potential role of the serotoninergic system during ontogeny. In this study we have compared the cellular distribution of three serotonin receptor subtypes: 5-HT2C, 5-HT5A, and 5-HT7. Immunocytochemical methods were used in slices of rat hippocampus obtained during the postnatal development. 5-HT2c immunoreactivity was strong at all developmental stages in the CA1 region, whereas differences were observed between P0 and P5 in the CA3 region. The 5-HT5A receptor immunosignal in CA1 and CA3 was strong at P0, decreased at P11, and then increased in the adult. The immunoreactivity to 5-HT7 receptors was high in all regions at P0 and then decreased progressively during postnatal development; the signal was stronger for 5-HT2c than for 5-HT5A and 5-HT7 receptors. Changes in the expression level of each receptor may result in differences in functional and pharmacological properties of the cells expressing them as well as in the hippocampal neuronal network. The distribution of the three serotonin receptor subtypes studied varied during the ontogeny, which supports their potential role during development and will help to understand their mechanisms.

Developmental expression of 5-HT 5A receptor mRNA in the rat brain

In the central nervous system, serotonin (5-HT) may function as a mitogen as well as a neurotransmitter; and its early appearance suggests a potential role in development. The present experiments were done to determine the localization of the mRNA coding for the 5-HT 5A receptor during development of the rat brain. 5-HT 5A gene transcription was assessed by in situ hybridization, from E18 and during postnatal (PN) development. An intense signal of 5-HT 5A mRNA was found in the cerebral cortex and olfactory nucleus at E18, PN0 and PN5. A sharp decrease at PN11 was followed by an increase until reaching the adult level in the cerebral cortex; whereas in the olfactory nucleus, transcription remained weak. In contrast, in the hippocampal formation the signal was weak in the CA1, CA2 and CA3 regions at E18 and PO; increased at P5 and then decreased at P11 before attaining the adult level. We conclude that the gene coding for the 5-HT 5A receptor is already active in the embryonic rat brain and is differentially expressed during development.

Developmental changes in the inducibility of fos-like immunoreactivity in primary embryonic spinal cord cultures

The immediate early gene (IEG) transcription factor c-fos coordinates changes in the pattern of long term gene expression and, therefore, it may be involved in mediating epigenetic control during neurodevelopment. We used pharmacological treatments mimicking various environmental and intracellular signals and assessed the inducibility of fos-like immunoreactivity (LIR) at various stages of neurodifferentiation in a primary embryonic spinal cord culture system by immunohistochemistry. Constitutive fos LIR exclusively found in neurons, was driven by the onset and extent of spontaneous electrical activity, as it was blockable by tetrodotoxin (TTX) at all developmental stages. Phorbol myristate 13 acetate (PMA) increased the number of fos-LIR cells equally effectively at all stages, but the predominant cellular localization of fos-LIR changed through ontogeny. The effect of veratridine, kainate and serum-derived factors in significantly inducing fos-LIR was restricted to the earliest developmental stage (4 days in vitro; DIV) investigated; whereas forskolin, the GABAA antagonist picrotoxin and NMDA failed to induce fos-LIR at this stage, but increased the number of fos-LIR neurons at later stages. Dihydropyridine agonists of the voltage-sensitive calcium channels (VSCC) raised the number of fos-LIR neurons and also prevented TTX-mediated down-regulation; whereas antagonists markedly reduced fos-LIR at all ages. Either type of NMDA antagonists (AP5 and MK801) and the GABAA agonist muscimol significantly reduced fos-LIR at all ages. These findings demonstrate that the inducibility of fos-LIR is substantially different in embryonic neurons than in adult ones and that inducibility by various first and second messengers is dependent on the development stage.

Developmental changes in intracellular calcium regulation in rat cerebral cortex during hypoxia

During the first weeks of life, injury to the central nervous system caused by brief periods of oxygen deprivation greatly increases. To investigate possible causes for this change, the effects of hypoxia or application of the excitatory neurotransmitter glutamate on intracellular calcium ([Ca2+]i) and ATP were studied in rat cerebrocortical brain slices. [Ca2+]i was measured fluorometrically with the indicator Fura-2. Hypoxia (95% N2/5% CO2) or 100 microM sodium cyanide produced gradual elevations in [Ca2+]i and ATP depletion in slices from rats < 2 weeks old, but rapid changes in older rats. After 20 min, [Ca2+]i in adult slices exposed to cyanide was 1,980 +/- 310 nM; in day 1-14 animals, it was 796 +/- 181 nM (p < 0.05). Combination of cyanide and a glycolytic inhibitor (iodoacetate) rapidly elevated [Ca2+]i and depleted ATP in all age groups. Energy utilization during anoxia, assessed by measuring ATP fall in cyanide/iodoacetate-treated brain slices, increased with age. Elevations in [Ca2+]i caused by application of 500 microM glutamate increased 240% from days 1-2 to day 28, but ATP loss caused by glutamate did not change with age. The N-methyl-D-aspartate antagonist MK-801 delayed calcium entry during the initial 5-7 min of hypoxia or cyanide in rats < 2 weeks old. We conclude that anaerobic ATP production, conservation of energy by reduced ATP consumption, and reduced sensitivity to glutamate contribute to delaying elevation in [Ca2+]i in neonatal rat brain during hypoxia.

Expression of neurotransmitter receptors by mRNAs from neurons developing in vitro: a Xenopus oocyte expression study

Poly(A)+ mRNA was extracted from cultures of neurons isolated from mouse embryonic day 14 cerebral cortex and injected into Xenopus oocytes. This led to the expression of receptors for gamma-aminobutyric acid (GABA), glycine, acetylcholine, serotonin, glutamate, kainate, N-methyl-D-aspartate, and quisqualate. Northern blot analysis of poly(A)+ mRNA from the cultured neurons with a GluR1 cDNA probe revealed the presence of three hybridization bands with estimated mRNA sizes of 5.1, 4.0, and 3.1 kb, respectively. The development of mRNAs coding for neurotransmitter receptors was investigated by isolating mRNA from neurons cultured for 2, 8, and 14 days in vitro and injecting it into Xenopus oocytes. The amplitude of membrane currents elicited by the transmitters gave a measure of the relative amounts of the different mRNAs. The size of the responses to kainate, aspartate (together with glycine), glutamate, acetylcholine, GABA, serotonin, and glycine increased with the time of culture in vitro. However, in contrast to all other agonist-induced currents, the current induced by glycine failed to increase further from 8 to 14 days in culture. It is concluded that the time course of receptor development in cortical neurons in vitro is similar to the development in vivo.

Stereoselective effects of AMOA on non-NMDA receptors expressed in Xenopus oocytes

Pharmacological characterization of the action of the novel non-N-methyl-D-aspartate (non-NMDA) antagonist AMOA (2-amino-3-[3-(carboxymethoxy)-5-methylisoxazol-4-yl]propionate) on glutamate receptors was investigated in Xenopus oocytes injected with mouse brain mRNA. AMOA (150 microM) produced a nearly parallel shift to the right of the dose-response curve for kainate-induced currents. AMOA was found to have two different effects on AMPA receptors: 1) currents elicited by low concentrations of AMPA (6 microM) were inhibited by AMOA with an IC50 value of 160 +/- 19 microM and 2) currents elicited by high concentrations of AMPA (100 microM) were potentiated with an IC50 value of 88 +/- 22 microM. The maximal potentiating effect of AMOA on AMPA currents was around 170%. Furthermore, the two opposing effects of AMOA on AMPA responses are specific for the L-configuration of AMOA. This unusual antagonistic/agonistic property of AMOA may explain its unusual properties with regard to antagonism of non-NMDA receptor-mediated events previously described.

Messenger RNAs coding for receptors and channels in the cerebral cortex of adult and aged rats

Poly(A)+ mRNAs from the cerebral cortex of aged (24 months) and young adult (3 months) rats were isolated and injected into Xenopus oocytes to express functional neurotransmitter receptors and voltage-operated channels. Electrophysiological recordings of induced membrane currents were used as a measure of the relative amounts of mRNA encoding different receptors and channels, and to study their functional properties. There were no large differences apparent between mRNAs from aged and adult rats, in marked contrast to the dramatic (1000-fold) changes in mRNA expression that occur during embryonic and postnatal development. The membrane currents induced by glutamate or acetylcholine (ACh) application were roughly one third smaller in oocytes injected with mRNA from aged cerebral cortex than in oocytes injected with mRNA from adult cerebral cortex, whereas currents induced by gamma-aminobutyric acid (GABA), kainate or serotonin (5-HT) application, and by activation of voltage-operated Na+ and Ca2+ channels were not significantly different. We did not observe any age-related differences in the properties of the receptors and channels studied.

Development of intracellular calcium responses to depolarization and to kainate and N-methyl-D-aspartate in cultured mouse hippocampal neurons

We have investigated the initial appearance of voltage-gated Ca channels and kainate- and NMDA-type glutamate receptors in cultured embryonic mouse hippocampal neurons. The Ca-dependent fluorescence change of the dye fura-2 was used as a sensitive assay for the presence of functional channels and receptors. Expression of functional NMDA receptors was observed on some hippocampal neurons as early as E14. By the equivalent of E15-16, 40-50% of cells responded to Ko-depolarization (50 mM), indicating the presence of functional voltage-gated Ca channels, approximately 20% of cells responded to kainate (50 microM), and just under 20% responded to NMDA (50 microM; in the presence of glycine and strychnine). By the equivalent of the end of the embryonic period 70-80% of cells responded to all 3 stimuli. As approximately 20% of cells in these cultures are glia, these data indicate that by the time of birth close to 100% of neurons express functioning kainate and NMDA receptors, and voltage-gated Ca channels. Increases in [Ca2+]i in embryonic neurons after application of NMDA were sensitive to APV and to external Mg, as are responses in mature neurons. The IC50 for block by external Mg of the [Ca2+]i increase induced by NMDA was 130 microM, and there was a slight positive correlation between the amplitude of the response to NMDA and sensitivity to external Mg.

Glutamate-operated channels: developmentally early and mature forms arise by alternative splicing

The expression of two alternative splice variants, Flip and Flop, in mRNAs encoding the four AMPA-selective glutamate receptors (GluR-A, -B, -C, and -D) was studied in the developing brain by in situ hybridization. These receptors are expressed prominently before birth, and patterns of distribution for Flip versions remain largely invariant during postnatal brain development. In contrast, the Flop versions are expressed at low levels prior to postnatal day 8. Around this time, the expression of Flop mRNAs increases throughout the brain, reaching adult levels by postnatal day 14. Thus, receptors carrying the Flop module appear to participate in mature receptor forms.