The invention provides indole derivatives of general formula (I)
wherein R¹ is lower alkyl, R² and R³ are each hydrogen or are joined together so as to form, with the carbon atoms to which they are attached, a benzyl ring which may optionally be substituted by 4-halogen, 2,4-dihalogen, 3-trifluoromethyl or 4-methoxy and R⁴ and R⁵ are hydrogen, alkyl containing 1 to 6 carbon atoms or halogen, and the pharmaceutically acceptable salts thereof.