It has been difficult, so far, to obtain melatonin analogs possessing high
selectivity for the respective melatonin receptors, mt, and MT2. In the pre
sent work, we report the synthesis and pharmacological characterization of
a new compound N-{2-[5-(2-hydroxyethoxy)-1H-indol-3-yl)] ethyl} acetamide o
r 5-hydroxyethoxy-N-acetyltryptamine (5-HEAT). To assess the activity of th
e compound, the following tests were performed: affinity determination for
the high- and low-affinity receptor states (2-[I-125]iodomelatonin binding)
, potency and intrinsic activity in inducing G protein activation ([S-35]GT
P gammaS binding assay). 5-HEAT showed little selectivity for the mt(1) rec
eptor, with pK(i) values of 7.77 for mt(1) and 7.12 for the MT2 receptors,
respectively. 5-HEAT was able to differentiate between the high- and the lo
w-affinity receptor states in the mt(1) but not in the MT2 receptor. 5-HEAT
induced a high level of G protein activation when acting through the mt(1)
receptor, with a relative intrinsic activity of 0.92. On the contrary, it
elicited only minimal MT2 receptor-mediated G protein activation, with a re
lative intrinsic activity of 0.16, and was also able to inhibit the melaton
in-induced MT2 receptor-mediated G protein activation, with a pK(B) value o
f 7.4. In conclusion, it appears that 5-HEAT possesses very different effic
acies at the two melatonin receptors, behaving as a full melatonin receptor
agonist at the mt(1) and as an antagonist/weak partial agonist at the MT2
receptor. Therefore, it is a promising ligand for use in functional studies
aimed at distinguishing between the effects mediated by the different mela
tonin receptors in the human.