The Age of Reason needed outposts where irrationality could be studied and perhaps admired: the insane asylum was one, the opera house another. Eighteenth-century London had Bedlam in the City of London (designed by Robert Hooke), where socialites came to giggle at the gesticulating loonies. Back in the fashionable centre of town, the theatres for which George Frideric Handel composed his operas displayed the same crazily capricious behaviour. Prima donnas of both sexes strutted and bickered, transfixed in attitudes as inflexible as straitjackets by the repetition of their da capo arias. Their tantrums were cosmic. The scheming politician Altomaro in Fernando, re di Castiglia - newly recorded by the conductor Alan Curtis and his Italian ensemble, Il Complesso Barocco - rants about Jove tossing thunderbolts, and promptly sings up a turbulent storm; in Floridante, also available in another new recording by Curtis, the tormented hero lets loose a blitz of what he calls "ardenti fulmini" - bolts of hot, angry lightning, made audible in the flurries of his vocal line.
These characters do a lot of fulminating, and the dramatic situations Handel contrives, like the crises of Racinian tragedy, serve to provoke their outbursts of rage, defiance or sexual torment. In Fernando, a Freudian family romance turns nasty, exacerbated by a territorial dispute between Portugal and Castille. The Portuguese king Dionisio, doubly befuddled like Lear and Gloucester combined, disinherits his rebellious heir Alfonso, and leaves the way open for his bastard son Sancio to inherit the throne. The dispute about succession is resolved in a duel between Dionisio and Alfonso: this is the Oedipus complex dramatised as a swordfight. In Floridante, the Persian tyrant Oronte makes love to his daughter. Elmira, sung by the brilliant Joyce DiDonato, spits contempt like a fizzing volcano. Only then does Oronte (Vito Priante) reveal that she is a foundling, so they are not related. He might have said so before, but then, of course, we would not have enjoyed the deliciously transgressive possibility of incest.
The characters of Fernando are besieged in the citadel at Coimbra, where they accumulate an energy that they can release only in their musical tirades. The Portuguese queen is a religious fanatic, goaded to hysteria: the pianissimi of Marianna Pizzolato sound eerie and ghostly. A succession of solo arias confirms the solitary confinement of these people, and the only duets occur between characters cruelly tugged apart. In Floridante, a barrage of military percussion menaces characters who worry more about their erotic tribulations than they do about the contentious affairs of state. DiDonato gives a feverish edge to Elmira's yearnings, and Sharon Rostorf-Zamir, as her sister Rossane, vents her own excitement in an aria that ends with a shriek of anticipated pleasure.
It is hardly surprising that the hero of Orlando - sung in Covent Garden's revival by the wired and freakily intense counter-tenor Bejun Mehta - should, like a Bedlamite, be driven to derangement by the convolutions of his chivalric quest. Orlando's mad scene is an exhibition of the mind's skittish free-associating powers, and of the equally unhinged virtuosity of the voice. He erupts in fury, then slumps into apathetic despair. Martial vigour alternates with mad clowning, and he celebrates his imagined arrival in the underworld by dancing a gavotte. At last he falls asleep, and is healed by the harmonious intercession of a violin. Awaking, he claims to have slain the marauding monsters that existed only in his imagination. This was the therapeutic, rationalising mission of the Enlightenment: Voltaire praised Descartes for rounding up and killing off all superstitious phantoms, persuading us to trust in our powers of reasoning.
The monsters, however, lived on in opera, defined by Dr Johnson as the most scandalously irrational of entertainments. A menagerie of un socialised ogres prowls and scavenges in David McVicar's production of Agrippina, imported from Brussels by English National Opera. Imperial Rome here is a lewd and murderous urban jungle. On John Macfarlane's bleeding drop cloth, a she-wolf suckles the founders of the city. At the end of the opera, with Agrippina's psychopathic son Nero installed as emperor on a throne that he licks with his drooling tongue, the chorus blithely celebrates the new regime, and Juno is supposed to float down from the sky to add her blessing. McVicar, however, dispenses with the ameliorating goddess; as the drop cloth descends, the vulpine foster-mother displays her fangs in a carnivorous smile that matches the mood of this ruthless comedy.
Handel reduces Roman politics to a squabble between rampant women of the Agrippina and Poppea sort, who usurp a power that ought to be exercised soberly by their menfolk. For McVicar, the empire falls apart in an orgy of sex, drugs and rock'n'roll. Agrippina inspects her soldiers' private parts, and attaches medals for feats of priapic valour. Nero, in a gloriously sullen and squalid performance by Christine Rice, snorts coke to fuel the flourishes of a particularly demanding aria, and muffles the bombast of a political ceremony by plugging in an MP3 player and jigging to a private beat. Poppea, betrayed by one of her many lovers, delivers an aria as a sozzled cabaret turn, with a cadenza improvised on air guitar. And Nero revises the genteel diction of the lib retto by emitting a row of expletives: somehow the words "Fuck fuckety fuck fuck" exactly match Handel's vocal line. Never has opera been grubbier, or grosser, or more scathingly truthful.
"Orlando" is at the Royal Opera House, London WC2, until 13 March. "Agrippina" is at English National Opera, London WC2, until 3 March. "Fernando, re di Castiglia" is on Virgin Classics and "Floridante" on Deutsche Grammophon