Degranulation, density, and distribution of mast cells in the rat thalamus: a light and electron microscopic study in basal conditions and after intracerebroventricular administration of nerve growth factor.

In the adult rat brain mast cells reside selectively in the thalamus. We investigated thalamic mast cells stained by acidic toluidine blue or pinacyanol, and with histamine immunocytochemistry, focusing on their state of activity revealed by degranulation. Mast cells exhibited perivascular prevalence and high quantitative variability, between cases and in different sections, with no asymmetry or topographical selectivity in thalamic nuclei. Pinacyanol, alone or with erythrosine, stained mast cells with higher sensitivity than toluidine blue. However, toluidine blue was highly predictive of pinacyanol staining and provided the best resolution of mast cell cytoplasmic features. Histamine immunocytochemistry labeled 61% of pinacyanol-stained mast cells. Intensely toluidine blue-stained granulated cells, as well as cells exhibiting different degrees of degranulation that paralleled lighter staining, were observed. The response of thalamic mast cells to intracerebroventricular administration of nerve growth factor (NGF) and control cytochrome-c injections was evaluated after 2, 24, and 72 hours. No obvious changes in mast cell number or distribution were found after treatment, but massive degranulation was frequently observed after NGF administration. Significant decrease of staining intensity of mast cells, supporting enhanced degranulation, was documented in NGF-treated animals by quantitative image analysis. Ultrastructural features of mast cell degranulation, with granule coalescence and matrix dissolution, were detected in untreated and NGF-treated cases. The findings point out that mast cells are active in the thalamus in basal conditions and that NGF has the potential to elicit long-lasting degranulation of thalamic mast cells in vivo, exerting a direct effect and/or priming these cells to react to endogenous stimuli. Copyright 2000 Wiley-Liss, Inc.