Nitrogen substitution modifies the activity of cytisine on neuronal nicotinic receptor subtypes.

Cytisine very potently binds and activates the α3β4 and α7 nicotinic subtypes, but only partially agonises the α4β2 subtype. Although with a lower affinity than cytisine, new cytisine derivatives with different substituents on the basic nitrogen (CC1-CC8) bind to both the heteromeric and homomeric subtypes, with higher affinity for brain [3H]epibatidine receptors. The cytisine derivatives were tested on the Ca2+ flux of native or transfected cell lines expressing the rat α7, or human α3β4 or α4β2 subtypes using Ca2+ dynamics in conjunction with a fluorescent image plate reader. None elicited any response at doses of up to 30-100 μM, but all inhibited agonist-induced responses. Compounds CC5 and CC7 were also electrophysiologically tested on oocyte-expressed rat α4β2, α3β4 and α7 subtypes. CC5 competitively antagonised the α4β2 and α3β4 subtypes with similar potency, whereas CC7 only partially agonised them with maximum responses of respectively 3% and 11% of those of 1 mM acetylcholine. Neither compound induced any current in the oocyte-expressed α7 subtype, and both weakly inhibited acetylcholine-induced currents. Adding chemical groups of a different class or size to the basic nitrogen of cytisine leads to compounds that lose full agonist activity on the α3β4 and α7 subtypes.