Showing papers by "Dan Lindholm published in 1995"

GABAergic stimulation switches from enhancing to repressing BDNF expression in rat hippocampal neurons during maturation in vitro

Abstract: gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the adult mammalian central nervous system. However, GABA depolarizes immature rat hippocampal neurons and increases intracellular Ca2+ ([Ca2+]i). Here we show, that GABA and the GABAA receptor agonist muscimol induce c-Fos immunoreactivity and increase BDNF mRNA expression in embryonic hippocampal neurons cultured for 5 days. In contrast, after 3 weeks in culture, GABA and muscimol failed to induce c-fos and BDNF expression. Fura-2 fluorescence microscopy revealed that muscimol produces a dihydropyridine-sensitive transient increase in [Ca2+]i, comparable to the effect of the non-NMDA receptor agonist kainic acid in neurons cultured for 5 days, but not in 3-week-old cultures. The increase in c-Fos immunoreactivity and BDNF mRNA levels by GABA were dependent upon the activation of voltage-gated Ca2+ channels, as shown using the L-type specific Ca2+ channel blocker nifedipine. The differential regulation of c-fos and BDNF expression by GABA and muscimol in developing and mature hippocampal neurons is due to a switch in the ability of GABAA receptors to activate voltage-gated Ca2+ channels. These observations support the hypothesis that GABA might have neurotrophic effects on embryonic or perinatal hippocampal neurons, which are mediated by BDNF.

Fibroblast Growth Factors Regulate Calcitonin Gene‐related Peptide mRNA Expression in Rat Motoneurons after Lesion and in Culture

Abstract: In this study, we have investigated the effect of fibroblast growth factors (bFGF and FGF-5) and brain derived neurotrophic factor (BDNF) on the expression of calcitonin gene-related peptide (CGRP) in rat motoneurons in vivo and in vitro. Following sciatic nerve transection in adult rats, the levels of alpha-CGRP and beta-CGRP mRNA were up- and down-regulated respectively in axotomized motoneurons, revealed by in situ hybridization histochemistry. Local administration of 1 microgram bFGF was able to entirely abolish the up-regulation of alpha-CGRP mRNA, and to further down-regulate beta-CGRP. These effects, albeit less pronounced, were still evident with 0.2 micrograms bFGF. In contrast, bFGF did not attenuate the lesion-induced decrease of choline acetyltransferase (ChAT) mRNA. Administration of BDNF did not significantly alter the expression of CGRP or ChAT mRNA in axotomized motoneurons. Both alpha- and beta-CGRP mRNAs could be detected by PCR in enriched motoneuron cultures prepared from rat embryos at embryonic day 14-15. Comparing the amplification of alpha- and beta-CGRP mRNAs with that of mRNA encoding glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in parallel samples, we found that cultures treated with FGF-5 had a lower ratio of alpha- and beta-CGRP mRNA to GAPDH mRNA, than did control or BDNF-treated cultures. BDNF, on the other hand increased alpha-CGRP and decreased beta-CGRP mRNA levels, though these effects were moderate compared with the effects of FGF-5. The results obtained in this study suggest that members of the FGF family of growth factors influence the expression of CGRP in rat motoneurons, and that the increase of this neuropeptide induced by axotomy may, at least in part, be due to deprivation of these target-derived factors.

TrkA Tyrosine Residues Involved in NGF-induced Neurite Outgrowth of PC12 Cells

Abstract: The proto-oncogene product gp140prototrk (TrkA) is the receptor tyrosine kinase that mediates nerve growth factor-induced neuronal survival and differentiation. In receptor tyrosine kinases, specific intracellular tyrosine residues become phosphorylated after ligand binding and the phosphorylated tyrosines induce the cascade of signal transduction. Here we have identified intracellular tyrosine residues of TrkA involved in nerve growth factor-induced neurite outgrowth of PC12 cells, using site-directed mutagenesis and a PC12 cell line expressing very low levels of endogenous TrkA (PC12nnr5 cells). We analysed eight conserved intracellular tyrosine residues of TrkA while the three putative autophosphorylation sites conferring tyrosine kinase activity were left intact. Five tyrosine residues, Y499, Y643, Y704, Y760 and Y794, in rat TrkA were involved in nerve growth factor-induced neurite outgrowth. None of these tyrosines mediated the full activity of wild-type TrkA, and a pair of these tyrosines, Y760 and Y794, promoted neurite outgrowth in an additive manner. These data indicate that no single tyrosine is sufficient to induce complete neurite outgrowth but the five tyrosine residues Y499, Y643, Y704, Y760 and Y794 cooperate to exhibit the full activity of wild-type TrkA.

Neurotrophins as Mediators of Neuronal Plasticity

Abstract: Publisher Summary Neurotrophic molecules — in particular, the members of the nerve growth factor (NGF) gene family — have so far predominantly been considered under the aspect of their function in regulating neuronal survival and differentiation of specific populations of neurons during embryonic development and in maintaining characteristic structural and functional properties of these neurons in adulthood. Apart from the trophic support of specific populations of neurons during embryonic development and in adulthood, there is increasing evidence that neurotrophins also play a role in synaptic plasticity. This can be deduced from the following observations — (1) the regionally differential rapid regulation of synthesis of NGF and brain-derived neurotrophic factor (BDNF) by neuronal activity in response to subtle physiological stimuli; (2) the activity-dependent release of neurotrophins from all neuronal processes demonstrated so far only for NGF. This, together with the enhancement of transmitter release by neurotrophins from nerve terminals, expressing the corresponding Trk receptors, is compatible with the hypothesis that the neurotrophins may stabilize and/or rearrange specific synapses in an activity-dependent manner. This interpretation is also supported by recent observations that long-term potentiation is strongly impaired in hippocampal slices of homo- and heterozygote BDNF knock-out mice.

Role of neurotrophic factors in cerebellar development

Abstract: Publisher Summary Development of the rodent cerebellum mainly occurs in the postnatal stage and that provides an interesting model to study external factors influencing neurogenesis and differentiation of this brain region. The precursor cells of the cerebellar granule neurons proliferate in the external granule layer (EGL) and subsequently migrate inward to reach the internal granule layer (IGL). In the molecular layer (ML), the granule neurons establish synaptic contacts with the dendrites of developing Purkinje cells that are the only efferent neurons in the cerebellum. The afferent input to the cerebellum is conveyed by the mossy fibers to the granule neurons and the climbing fibers that make synaptic contacts with the Purkinje cells. There exist mutant mice that exhibit disturbed cerebellar development and these animals have been studied to understand a normal cerebellar development. However, the genetic defects of these mice are unknown as is the possible involvement of various neurotrophic factors therein.