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Excitotoxic striatal lesions induced by quinolinic acid, a model for Huntington's disease, were used to test for neuroprotective actions of nerve growth factor on striatal cholinergic and GABAergic neurons. Expressions of the trk A receptor for nerve growth factor, choline acetyltransferase and glutamate decarboxylase were analysed by messenger RNA in situ hybridization in adult rats following quinolinic acid lesion (150 nmol) and daily striatal administration of nerve growth factor (1 μg) or control protein (cytochrome C) for one week. One week after toxin administration, the numbers of cells expressing trk A or choline acetyltransferase messenger RNAs were decreased when compared with unlesioned animals. Moreover, the surviving cells showed a strong down-regulation of these messenger RNAs as deduced from grain count analysis of sections processed for emulsion autoradiography. Daily intrastriatal nerve growth factor administration for one week completely prevented the reduction in the number of cells expressing either of the two markers. Nerve growth factor treatment increased the cellular expression of choline acetyltransferase messenger RNA three times above control levels and restored the levels of trk A messenger RNA expression to control levels. In contrast to the protective effects on cholinergic cells, nerve growth factor treatment failed to attenuate the quinolinic acid-induced decrease in glutamate decarboxylase messenger RNA levels. Optical density measurements of the entire striatum on autoradiographs of brain sections from quinolinic acid-lesioned animals revealed a reduction of the glutamate decarboxylase messenger RNA-specific hybridization signal, which was unaltered by infusion of nerve growth factor or control protein. Our findings strongly suggest that in both the intact and the quinolinic acid-lesioned adult rat striatum, nerve growth factor action is confined to trk A-expressing cholinergic neurons. Striatal glutamate decarboxylase messenger RNA-expressing GABAergic neurons which degenerate in Huntington's disease are not responsive to nerve growth factor.

The expression of the specific trk receptors for nerve growth factor and brain-derived neurotrophic factor ( trk A and trk B) has been assayed by messenger RNA in situ hybridization in adult rats with partial fimbrial transections along with intraventricular treatment of nerve growth factor or brain-derived neurotrophic factor. In the forebrain, specific hybridization labeling for trk A messenger RNA showed an identical pattern to that of choline acetyltransferase messenger RNA, supporting the view that trk A expression is confined to the cholinergic population in the basal forebrain and the cholinergic interneurons in the striatum. After partial unilateral transections of the fimbria there was a progressive loss of choline acetyltransferase and trk A messenger RNA expression in the septal region ipsilateral to the lesion. Daily intraventricular administration of brain-derived neurotrophic factor or nerve growth factor partially prevented the lesion-induced decrease in the levels of both messengers, the latter being more effective than the former. Grain count analysis of individual cells was used to test whether the two factors upregulated choline acetyltransferase or trk A expression in individual cells surviving the lesion. Brain-derived neurotrophic factor treatment failed to induce any change in the levels of both messengers per neuron in the septal area. In contrast, daily intraventricular administration of nerve growth factor upregulated both choline acetyltransferase and trk A messenger RNA expression in individual neurons. This upregulation was evident on ipsilateral and contralateral sides, suggesting that nerve growth factor is able to upregulate these markers in intact and injured cholinergic cells in the basal forebrain. Similar to the situation in the septum, brain-derived neurotrophic factor did not upregulate choline acetyltransferase or trk A expression in the striatum. However, nerve growth factor administration strongly upregulated choline acetyltransferase messenger RNA expression by individual cholinergic neurons of the striatum. A medial to lateral gradient decrease in this upregulation was detected in the striatum ipsilateral to the side of administration, suggesting a limited diffusion of the nerve growth factor protein from the ventricle into brain parenchyma. In contrast to the strong effect on choline acetyltransferase expression, nerve growth factor treatment was ineffective in altering trk A messenger RNA in the striatum. The contrasting findings between septum and striatum suggest different regulatory mechanisms for trk A messenger RNA expression in the two cholinergic populations. Since nerve growth factor was found to upregulate the expression of its trk A receptor, we tested whether brain-derived neurotrophic factor administration had similar effects on the regulation of its trk B receptor. Neither brain-derived neurotrophic factor administration nor fimbrial transection induced any change in the expression of full-length or truncated trk B messenger RNAs. However, in situ hybridization for brain-derived neurotrophic factor messenger RNA detected lower levels in CA2-CA3 following the fimbrial transections in the ipsilateral side, which confirmed that septal cholinergic afferents are necessary for maintaining normal levels of brain-derived neurotrophic factor messenger RNA expression in the hippocampus.

Ribonuclease protection analysis and quantitative in situ hybridization histochemistry were used to investigate the coordination and regional expression of catalytic and non-catalytic trk B messenger RNAs in the adult rat hippocampus following systemic kainate-induced seizures. Changes in trk B expression were compared with the messenger RNA expression of its neurotrophic ligands, brain-derived neurotrophic factor and neurotrophin-3. Trk B messenger RNA expression was increased in the dentate granule cells at 1–4 h following the onset of seizures, and returned to control levels 16–24 h thereafter. In addition, seizures also induced expression of trk B messenger RNA in putative non-neuronal cells at four to seven days in the molecular layer of the dentate gyrus and the stratum lacunosum moleculare of the CA1 region. Hybridization with probes specific for the non-catalytic trk B receptor and the catalytic trk B receptor revealed that the increases at four and seven days in the molecular layers of the hippocampus reflected an up-regulation of only the non-catalytic form of the receptor. Furthermore, the neuronal increases observed 1–4 h were due to an up-regulation of both trk B TK− and trk B TK+ messenger RNAs. It was established that systemic administration of kainate increased brain-derived neurotrophic factor messenger RNA levels in the pyramidal and granule cell regions of the hippocampus 1–4 h following the onset of behaviorally manifested seizure activity. Early changes in neuronal expression of trk B TK− and trk B TK+ messenger RNA paralleled changes in brain-derived neurotrophic factor messenger RNA in the dentate granule cell and CA1 pyramidal cell layers, but not in the CA3 subregion. These data suggest that concomitant regulation of brain-derived neurotrophic factor and its cognate receptor may play a role in the selective vulnerability of hippocampal subregions to kainate-induced neuropathology. Furthermore, these data suggest a dual function for trk B receptor expression in the hippocampus following kainate-induced seizures, possibly related to both the plastic and degenerative consequences of seizure induction by kainate.

We previously reported the presence of trk A messenger RNA expressing non-cholinergic neurons in the paraventricular anterior and reuniens nuclei, which are located in the thalamic midline. In the present study, retrograde labeling with iodinated ( 125 I) nerve growth factor was used to identify the innervation target of these cells. Neurons in the paraventricular anterior and reuniens nuclei were labeled following injection of iodinated nerve growth factor into amygdala and hippocampus, but not into nucleus accumbens and entorhinal cortex, the two other main areas receiving strong innervation from the thalamic midline. Target ablation of hippocampus or amygdala failed to down-regulate trk A messenger RNA expression in the two thalamic nuclei, thus suggesting a role for nerve growth factor different from a critical survival factor. The thalamic paraventricular anterior and reuniens nuclei are part of the reticular formation which plays a role in general cortical activation, behavioral arousal and control of awareness. Retrograde transport of nerve growth factor by trk A messenger RNA expressing neurons in these nuclei suggests a physiological role of this trophic factor in the function of these cells.

The present study determined the effects of chronic intranigral injections of recombinant human brain-derived neurotrophic factor (1 μg) every second day for 19 days on the functional capacity of dopaminergic neurons of the nigrostriatal pathway of unlesioned adult rats. In animals chronically treated with brain-derived neurotrophic factor, we observed amphetamine (5 mg/kg)-induced circling behavior directed toward the neurotrophin-injected side (33 turns/5 min). The behavioral asymmetry was paralleled by reductions of striatal [ 3 H]dopamine uptake (27%), tyrosine hydroxylase activity (68%), dopamine content (36%) and [ 3 H]mazindol binding site density (35%) on the same side as brain-derived neurotrophic factor treatment. While chronic injections of brain-derived neurotrophic factor produced a modest decrease in the number of tyrosine hydroxylase-positive cell bodies in the vicinity of the injection site, a similar reduction in cell number was observed in animals injected with a control protein, cytochrome c. However, in contrast to the animals treated with brain-derived neurotrophic factor, rats treated with the control protein showed no amphetamine-induced circling behavior, and there were no significant reductions in neurochemical parameters of striatal dopaminergic function. Lastly, we found that in brain-derived neurotrophic factor-injected animals there was a 30% decrease of tyrosine hydroxylase messenger RNA levels in the ventral mesencephalon. We also determined the effects of brain-derived neurotrophic factor treatment on animals with transections of the medial forebrain bundle. Medial forebrain bundle-lesioned animals challenged with amphetamine circled (55 turns/5 min) ipsilateral to the lesioned side. The medial forebrain lesions decreased the following markers of striatal dopaminergic function: [ 3 H]dopamine uptake (65%), tyrosine hydroxylase activity (79%), dopamine content (80%) and [ 3 H]mazindol binding site density (52%), induced a pronounced loss of tyrosine hydroxylase-positive cell bodies within the substantia nigra and also reduced tyrosine hydroxylase messenger RNA levels. Chronic intranigral brain-derived neurotrophic factor treatment did not attenuate nor did it exacerbate the medial forebrain bundle lesion-induced decreases of dopaminergic parameters in either the substantia nigra or striatum. The results of the present study indicate that chronic intranigral administration of brain-derived neurotrophic factor to normal adult rats induces a dopaminergic hypofunction in the striatum which is manifested behaviorally by amphetamine-induced rotations. The brain-derived neurotrophic factorinduced striatal function is not the result of significant cell loss at the levels of the substantia nigra, but seems to be related to brain-derived neurotrophic factor-induced down-regulation of dopaminergic-specific proteins. In addition, we found that chronic brain-derived neurotrophic factor treatment does not attenuate the loss of striatal dopaminergic function following transection of the medial forebrain bundle.