Neonatal administration of thimerosal causes persistent changes in mu opioid receptors in the rat brain

Abstract

Thimerosal added to some pediatric vaccines is suspected in pathogenesis of several neurodevelopmental disorders. Our previous study showed that thimerosal administered to suckling rats causes persistent, endogenous opioid-mediated hypoalgesia. Here we examined, using immunohistochemical staining technique, the density of μ-opioid receptors (MORs) in the brains of rats, which in the second postnatal week received four i.m. injections of thimerosal at doses 12, 240, 1,440 or 3,000 μg Hg/kg. The periaqueductal gray, caudate putamen and hippocampus were examined. Thimerosal administration caused dose-dependent statistically significant increase in MOR densities in the periaqueductal gray and caudate putamen, but decrease in the dentate gyrus, where it was accompanied by the presence of degenerating neurons and loss of synaptic vesicle marker (synaptophysin). These data document that exposure to thimerosal during early postnatal life produces lasting alterations in the densities of brain opioid receptors along with other neuropathological changes, which may disturb brain development.

Fig. 1

Changes in MOR density in different brain structures of neonatally THIM-treated rats. Images are representative photographs of MOR reaction, visualized as brown/dark rings around cells (pointed with arrows). THIM administration led to increase of MOR density in the PAG (a–d) and the CPU (g–h) (increase pointed with black arrows), but to decrease of MOR density in the DG (e–f) (decrease pointed with white arrows). a DMPAG, control, 8-week, ×400; b DMPAG, 240 μg Hg/kg, 8-week, ×400; c DMPAG, control, 20-week, ×400; d DMPAG, 3,000 μg Hg/kg, 20-week, ×400; e DG, control, 20-week, ×100; f DG, 3,000 μg Hg/kg, 20-week, ×100; g CPU, Control, 20-week, ×400; h CPU, 3,000 μg Hg/kg, 20-week, ×400. For interpretation of the references to color in this figure legend, the reader is referred to the online version of this article

Fig. 2

Neuropathological changes in the hippocampi of neonatally THIM-treated rats, 8-week old. Images a–c show dark neurons and ischemic-like degeneration (marked with black arrows) of the hippocampal neurons in the dentate gyrus of THIM-treated rats. a Control group; b THIM dose 12 μg Hg/kg; c THIM dose 240 μg Hg/kg; magnification ×400. Images d–e show diminished synaptophysin reaction (loss of synapses or synaptic marker protein) in the hippocampi of THIM-treated rats (marked with white arrows). d Control group; e THIM dose 240 μg Hg/kg; magnification ×200