Alterations of nitric oxide-producing neuronal system in the brain of a murine model for Down syndrome

Down's syndrome (DS) is the phenotypic expression of the partial or total trisomy of the chromosome 21. DS patients display visual deficits, impaired odour identification, inappropriate sexual and aggressive behaviour. Moreover, they are affected by mental retardation, and, in the adulthood, may develop anatomo-pathological alterations similar to those observed in the Alzheimer's disease. Recently, partial trisomic chromosome 16 mice (Ts65Dn) have been developed that contain most of the genes from human chromosome 21. In contrast to previous murine models for DS, these mice were born alive and survived. Ts65Dn mice show severe deficits in learning and they are more aggressive than wild type (WT) mice. Several behavioural studies have been already performed on this animal model, confirming a very dose parallelism with the DS behavioural deficits, on the contrary, only a few studies have investigated the neuroanatomical architecture of the Ts65Dn mouse brain. Experimental or genetic inhibition of neuronal NO synthase (nNOS) can induce aggression in male mice. Due to impaired social behaviour and to the presence, on the partial trisomic chromosome 16, of an extracopy of the SOD1-gene, an enzyme involved in the metabolism of NO, we studied the alteration of the nNOS system in Ts65Dn mice. In our experiment, 3 month-old mutants were significantly more aggressive than WT (p<0.05). Adjacent coronal brain sections were treated for nNOS immunoreactivity (nNOS-ir), and for NADPH-diaphorase (ND) histochemistry. Comparing nNOS- and ND-positivity on adjacent sections it resulted a strong and significant difference: in WT mice no disparity was observed between nNOS- and ND- positive elements while, in the trisomic mice the number of ND positive neurons was significantly reduced (p*0.05-0.01). This difference was not observed comparing nNOS elements, nor in other telencephalic regions as the caudato-putamen complex. Present results suggest hence that the neurones in the diagonal band, medial septum and magnocellular hypothalamic nuclei are still able to produce the enzyme, but its activity is extremely reduced. Recent studies demonstrated a decline in the number of cholinergic elements in Ts65Dn mice starting from 6 months of age. Our results, obtained in younger animals, when the cholinergic system is apparently not altered, suggest that the reduction in ND activity could represent a precocious signal of modification of the cholinergic system. Finally, due to the involvement of the paraventricular nucleus in the control of the autonomic nervous system, the decrease of ND activity in this nucleus may affect a large number of functions. Thus, precocious variations of nNOS/ND system in Ts65Dn mice could be one of the major causes of behavioural (aggressive behaviour, learning) and physiological alterations found in DS.