EFFECT OF THYROID-HORMONES ON G-PROTEINS IN SYNAPTOSOMES OF CHICK-EMBRYO

We have described a thyroid hormone receptor in synaptosomes or the ch ick embryo brain. To understand how the hormones exert their actions a t this level, we performed a series of studies to demonstrate that thi s receptor could be linked to G proteins. Guanosine 5'-[gamma-thio]tri phosphate (GTP gamma S) (100 mu M) lowered the binding capacity or the receptor high affinity site nom 8.9 +/- 1.3 to 3.4 +/- 1.3 ng T-3/mg protein, a finding consistent with the coupling of receptor to G prote ins. Furthermore, ADP ribosylation with pertussis toxin showed that th yroid hormones induced a dose-dependent increase in the inactive alpha (0)-subunit of the G(0) protein. This effect was detected at 10 pM, wi th a maximal increase (mean +/- SEM, 50 +/- 3.6%) at 100 nM, and T-4 w as as effective as T-3. Both hormones also decreased the intrinsic gua nine triphosphatase activity of G proteins by lowering the binding of GTP to the alpha-subunit and their rate of hydrolysis. This inhibition was greater with T-4 (25 +/- 5%) than with T-3 (14 +/- 2%), suggestin g that the former could be the more active hormone at the synaptosomal level. The effect on guanine triphosphatase activity confirms that th e synaptosomal thyroid hormone receptor is coupled to a G(0) protein. These results demonstrate that thyroid hormones increase or favor the ADP ribosylation of G alpha(0) by pertussis toxin. Thus, they enhance the alpha(0)-GDP form of the G(0) protein namely its inactive conforma tion. By decreasing the activity of this protein, these hormones may m odulate- the formation of second messengers in synaptosomes and interv ene in the regulation of neuronal proliferation and differentiation in duced by several factors. Therefore, thyroid hormones may exert their action on brain maturation at least in part by modulating G alpha(0) t hrough their synaptosomal receptor.