Ergot alkaloids and cyclic nucleotides in the CNS

Ergotamine and dihydroergotamine show a higher blocking effect towards the dopamine-induced stimulation of adenylate cyclase activity than to the apomorphine-induced stimulation. This could reflect a possible specificity of the compound to dopaminergic receptor. Bromocriptine blocks in a more competitive way the activation of adenylate cyclase induced in vitro by dopamine, but increases the levels of cAMP in rat striatum in vivo. This effect is blocked by haloperidol and reserpine when given together with bromocriptine. On the base of the biochemical observation and various behavioral data the drug has been used in the treatment of parkinsonism with contrasting results and of Huntington's chorea. Moreover, bromocriptine induces a marked decrease of cGMP levels in cerebellum such as haloperidol and chloropromazine while apomorphine and other dopaminergic drugs increase the levels of cGMP. At the dose of 1 mg/kg, bromocriptine as well as DH-ergotoxine markedly reduces the levels of striatal DOPAC.