Dopamine 1-like receptor in the brain holds key to zombie cockroach transformation

Abstract

It's a grizzly scene; a female jewel wasp (Ampulex compressa) wrestles an American cockroach (Periplaneta americana) to the ground. But what follows is a masterclass in neurosurgery. Following a precise sting aimed at the first bundle of nerves in the body, immobilising the cockroach's front legs, the wasp then deals its coup de grâce through the neck, aiming a second sting directly into a region of the brain known as the central complex, transforming the cockroach into a zombie-incubator for her egg. ‘Biochemical analysis of the venom shows the presence of [the neurotransmitter] dopamine’, says Frederic Libersat from Ben Gurion University of the Negev, Israel, adding that the neurotransmitter is entirely responsible for the initial stage of the transformation, when the cockroach begins to groom obsessively. However, it wasn't clear which of the two possible dopamine receptors (D1-like receptor or D2-like receptor) in the cockroach brain could be mediating the neurotransmitter's gruesome effect. So, Stefania Nordio (Ben Gurion University of the Negev) and Libersat set about bamboozling the cockroach's nervous system with drugs to identify which receptors are responsible for the deadly transformation that robs the insects of free will.‘These experiments require some technical skill and lots of practice’, says Libersat, who chose a series of drugs that selectively activate or disable the dopamine receptors in the cockroach's brain, to identify which receptors are targeted by the wasp's mind-bending venom. Injecting a drug that specifically activates the D1-like receptor into the central complex, Nordio and Libersat watched the insects as they ceased ambling around the arena and settled down to grooming, as if they'd been stung. However, the cockroaches that were injected with a drug that activates the D2-like receptor did not behave as if they had been stung and continued wandering around. The D1 receptor is the key receptor that controls the cockroaches’ urge to settle down and groom.The duo then injected cockroaches with a drug that blocks both dopamine receptors and waited a few hours before releasing jewel wasps in with the cockroaches to see if the drugs could prevent the wasps’ venom cocktail from disabling the cockroaches. Sure enough, the cockroaches were protected from the wasps’ stings, continuing to stroll around the arena even after being stung 6 h after the drug was applied. In addition, selectively blocking the D2 receptor with a drug, leaving the D1 receptor responsive, left the cockroaches vulnerable to the wasps’ stings, while the cockroaches treated to prevent the D1 receptor from responding to the toxic venom continued moving around the arena unaffected, providing more evidence that the D1 receptor is essential for initiating the transformation.Finally, the team tested the protective powers of the dopamine receptor blocking drugs to find out whether the dopamine receptors are targeted by wasp venom later in the cockroach's zombie transformation. Treating the cockroaches with a drug before letting the wasps loose, the team kept an eye on the stung insects for 2 days to find out whether they lost the ability to flee. Initially the cockroaches were unable to make a run for it. However, a day after being stung, when most cockroaches can no longer walk independently, the cockroaches that had been pre-treated with the drug had regained their free will and could make a dash for it; the wasp venom was targeting both dopamine receptors to rob the zombies of the ability to walk independently.So, jewel wasp venom specifically targets the D1 receptors in the cockroach brain to trigger the first stage of the zombie transformation, while both receptors rob the cockroach of the ability to walk unaided as the mind-altering venom takes hold.