Absence of long-term changes in biochemical markers of monoaminergic systems afferent to the excitotoxically neuron-depleted somatosensory thalamus

Abstract

In a previous study, it was shown that, one month after kainic acid (KA) injection into the thalamus, afferents deprived of postsynaptic target neurons exhibit structural alteration, including the loss of synaptic vesicles. The present study was undertaken to determine whether these long-term morphological changes were associated with changes in biochemical markers of monoaminergic pathways. In situ injection of KA was performed into the right ventrobasal complex of the rat thalamus (VB). Protein content and total amount of norepinephrine, dopamine, 5-hydroxytryptamine and 5-hydroxyindolacetic acid were analyzed in the lesioned area one, two, three and four months after injection using high-performance liquid chromatography and electrochemical detection. The results were compared to those obtained in an equal volume of tissue dissected out from the opposite (intact) VB. Protein content per unit volume decreased progressively to 50% of control in the neuron-depleted area. In contrast, whatever the amine considered, its total amount remained unaltered up to 4 months after the excitotoxic lesion. 5-hydroxyindolacetic acid was also unchanged 4 months after lesion. This study suggests that (i) the quantity of monoamines in afferents to the rat VB does not depend upon the presence of postsynaptic target neurons, (ii) a non-vesicular storage compartment may compensate the loss of synaptic vesicles in afferent fibers to the lesioned area.