The hypothalamic suprachiasmatic nuclei (SCN) comprise the main site in the 
brain involved in the control of the homeostatic mechanism which respond to 
environmental daily light changes. The sympathetic nervous system and 
hypothalamic releasing or inhibiting factors mediate the SCN control of a number 
of peripheral organs and tissues. In this work we analyzed the involvement of 
two environmental light conditions, constant light (LL) and constant dark (DD) 
for 20 days, on the expression of mRNAs for catecholamines biosynthetic enzymes 
and neuropeptide Y (NPY) genes in rat superior cervical ganglia (SCG) and 
adrenal gland. The results of Northern blot analysis show that LL exposure 
reduces mRNA levels for tyrosine hydroxylase (TH) the rate limiting 
catecholamine biosynthetic enzyme and also of dopamine beta-hydroxylase (DBH) as 
well as for NPY in SCG to about half the levels in control animals. In contrast, 
exposure of the rats to DD did not elicit any change in the SCG. In the adrenal 
gland, both, LL and DD conditions increased the TH, DBH as well as 
phenylethanolamine N-methyltransferase (PNMT) mRNA levels. Under the same 
conditions, adrenal NPY mRNA levels were decreased by either LL or DD. The 
results show, for the first time, that prolonged changes in environmental light 
can alter the gene expression of catecholamine biosynthetic enzymes and of NPY. 
There was differential response in SCG and adrenal gland.