Compounds of formula (I) have activity in alleviating the effects of impaired dopaminergic signaling, for example in the treatment of Parkinsons Disease: ##STR00001## wherein: R.sub.1 is a carboxyl, carboxyl ester, or carboxamide group; R.sub.2 and R.sub.3 are independently hydrogen, or a group --C(.dbd.O)R.sub.6 or --C(.dbd.O)OR.sub.6 wherein Re is C.sub.1-C.sub.6 alkyl, or a group --CH.sub.2Q wherein Q is an optionally substituted monocyclic carbocyclic or heterocyclyl ring of 3 to 6 ring atoms; R.sub.4 and R.sub.5 are independently (a) the side chain of a natural amino acid, or (b) optionally substituted C.sub.1-C.sub.4 alkyl, C.sub.2-C.sub.4 alkenyl, or C.sub.2-C.sub.4 alkynyl, or (c) --CH.sub.2XCH.sub.3, --CH.sub.2CH.sub.2XCH.sub.3, or --CH.sub.2XCH.sub.2CH.sub.3, wherein X is --O--, S, or --NR.sub.7 wherein R.sub.7 is hydrogen, methyl or ethyl; or (d) --CH.sub.2Q or CH.sub.2OQ wherein Q is as defined in relation to R.sub.6; or R.sub.4 and R.sub.5 taken together with the carbon atom to which they are attached form an optionally substituted cycloalkyl or heterocyclic ring of 3 to 8 ring atoms, optionally fused to a second, optionally substituted, carbocyclic or heterocyclic ring