Expression of brain-derived neurotrophic factor in cortical neurons is regulated by striatal target area

Abstract

Changes in BDNF expression after different types of brain insults are related to neuroprotection, stimulation of sprouting, and synaptic reorganization. In the cerebral cortex, an autocrine-paracrine mechanism for BDNF has been proposed because the distribution patterns of BDNF and TrkB expression are almost identical. Moreover, cortical BDNF is anterogradely transported to the striatum, suggesting a role of BDNF in the functional interaction between the two brain regions. Here we have examined the expression of this neurotrophin in the cerebral cortex after various striatal lesions. Intrastriatal injection of quinolinate, kainate, 3-nitropropionic acid, or colchicine increased BDNF mRNA levels in cerebral cortex. In contrast, stimulation of neuronal activity in the striatum did not change cortical BDNF expression. Both excitatory amino acids increased BDNF expression in neurons of cortical layers II/III, V, and VI that project to the striatum. Moreover, grafting a BDNF-secreting cell line prevented both the loss of striatal neurons and the cortical upregulation of BDNF induced by excitotoxins. Because retrograde transport in the corticostriatal pathway was intact after striatal lesions, our results suggest that striatal damage upregulates endogenous BDNF in corticostriatal neurons by a transneuronal mechanism, which may constitute a protective mechanism for striatal and/or cortical cells.

Fig. 1.

Nissl-stained sections showing the striatal lesion (area inside dotted lines) induced by intrastriatal injection: A, C, QUIN (68 and 34 nmol, respectively); B, D, KA (6 and 2 nmol, respectively).

Fig. 2.

A, BDNF mRNA is upregulated in the cortex ipsilateral to excitotoxic lesion in the striatum. Intrastriatal QUIN injection (circles) induced a peak of expression between 4 and 6 hr. BDNF mRNA levels were also increased between 4 and 16 hr after KA-induced striatal injury (squares).Triangles represent sham-injected striata. Values are the mean ± SEM (n = 4–7; *p < 0.05; **p < 0.005). Autoradiograms show two representative experiments. B, Injection of QUIN or KA in the striatum through the contralateral cortex upregulated BDNF mRNA in the ipsilateral cortex. Values are mean ± SEM (n = 4; *p < 0.05).

Fig. 3.

In situ hybridization shows specific BDNF mRNA increased levels in the ipsilateral cortex after striatal EAA lesions. A, Autoradiograms showing a 20 d exposure of horizontal sections through brains receiving intrastriatal injection of PBS or high doses of EAAs (QUIN, 68 nmol; or KA, 6 nmol). B, C, Autoradiograms showing a 20 d exposure of intrastriatal injected brains with lower QUIN or KA doses (34 and 2 nmol, respectively). D, Bright-field photomicrographs of frontal, motor, and sensory cortical areas of QUIN-lesioned brains. E, Photomicrographs showing BDNF upregulation in the same cortical areas of KA intrastriatally injected animals. Scale bars, 150 μm.

Fig. 4.

A, Photomicrographs showing triple-labeling of cortical layer II/III. NeuN-positive neurons are labeled in red, GFAP-positive astrocytes are labeled ingreen, and blue corresponds to the BDNF hybridization signal as assessed in radioactive in situhybridization. B, High magnification ofA. Note that GFAP-positive cells are negative for BDNF hybridization (open arrows), whereas NeuN-positive neurons are positive for BDNF mRNA signal (filled arrows). Some NeuN-positive neurons are negative for BDNF (arrowheads). Scale bars: A, 30 μm;B, 15 μm.

Fig. 5.

BDNF immunohistochemistry in the cerebral cortex reveals a pattern of expression similar to that observed for BDNF mRNA after striatal excitotoxic lesions. A, B,C, Immunolocalization of BDNF protein in the prefrontal cortex ipsilateral to PBS-, QUIN-, or KA-injected striata, respectively. D, E, F, Detailed photomicrographs of prefrontal layer V after injection of PBS, QUIN, or KA in the striatum, respectively. Scale bars:A–C, 200 μm; D–F, 100 μm.

Fig. 6.

BDNF mRNA is transneuronally upregulated in the cortex by striatal cell death and not by synaptic activity per se.A–D, Nissl-stained sections in which unilateral lesions were performed. Note the areas of reduced staining (area insidedotted lines) in striata injected with the following:A, 3-NPA (500 nmol); B, 3-NPA (500 nmol) and kynurenic acid (Kyn; 60 nmol); C, 3-NPA (500 nmol) and an NMDA receptor antagonist (MK801; 30 nmol); andD, 3-NPA (500 nmol) and a non-NMDA receptor antagonist (CNQX; 1 nmol). E, Histogram represents BDNF mRNA levels from cerebral cortex 6 hr after intrastriatal injection of PBS, KCl, or 3-NPA alone or in combination with different glutamate receptor antagonists [kynurenic acid (Kyn), MK801, and CNQX]. Values, from three to four animals per condition, were normalized to those obtained in PBS-injected animals and are represented as mean ± SEM (*p < 0.05).

Fig. 7.

Intrastriatal grafting of a BDNF-secreting cell line (F3N-BDNF) prevents the upregulation of BDNF mRNA in the cerebral cortex of EAA-lesioned animals. A, Values obtained from intrastriatal PBS injection in the F3A-MT-grafted striata were taken as 100%, and all other values were normalized to these data. Values are the mean ± SEM of four animals (***p < 0.001). B, Autoradiograms showing a 10 d exposure of brains hybridized with a BDNF ³²P riboprobe in situ. Horizontal sections were obtained from animals intrastriatally grafted with the F3N-BDNF cell line and injected with PBS, QUIN, or KA. Although BDNF mRNA was detected in the cell line, no upregulation of cortical BDNF mRNA was observed.

Fig. 8.

Excitotoxic striatal lesions do not affect the corticostriatal transport. Fluorogold-labeled cortical cells were counted in animals receiving bilateral injection of this retrograde tracer and unilateral injection of PBS, QUIN, or KA in the striatum. No differences between the three groups were observed. Number of labeled cells in the ipsilateral cortex to the lesioned striatum (I) were normalized to the number of Fluorogold-positive neurons from the contralateral hemisphere (nonlesioned; C). The values are represented as mean ± SEM of three animals per group.

Fig. 9.

BDNF mRNA is upregulated in the cerebral cortex 6 hr after disruption of corticostriatal transport. A,B, Photomicrographs comparing the retrograde transport of Fluorogold to the cortical layer V after injection of KA or colchicine, respectively. No labeling in the cerebral cortex was observed in colchicine-injected animals. Scale bars, 100 μm.C, Histogram represents cortical BDNF mRNA levels after intrastriatal injection of colchicine alone or in combination with QUIN or KA. Values are the mean ± SEM of four different animals for each condition, normalized to those obtained from PBS-injected animals (*p < 0.05).