In the classic Aesop's fable, the tortoise triumphs over the smug hare by patiently making its way along the race track. While reptiles such as turtles and tortoises are famously ambling voyagers, how they learn their routes is less well known. One chemical that may be required is acetylcholine. Acetylcholine is a brain chemical that is broadly important for cognition and appears to be particularly relevant for spatial navigation. It's needed by rats for negotiating mazes, by birds when caching seeds, and homing pigeons flying home. Recently, Timothy Roth II (Franklin and Marshall College, USA) and Aaron Krochmal (Washington College, USA) found that acetylcholine is also important for painted turtles (Chrysemys picta) that are journeying to new water sources. However, acetylcholine acts on a variety of receptors in the brain; therefore, it was unclear which one might be responsible for regulating spatial learning.
In a follow-up study recently published in Proceedings of the Royal Society B, Roth and Krochmal set out to determine if M1, a specific type of acetylcholine receptor, facilitates turtle migration. To do so, the researchers first radio-tagged adult turtles residing at a man-made pond that is regularly drained. Once the scheduled drain happened, Roth and Krochmal tracked the turtles’ routes to nearby ponds as they ventured in search of alternative watering holes. After mapping these routes, the team returned the following year to intercept the turtles mid-way to their new watering holes after their home had been drained. They then administered a drug that would block the M1 receptor and prevent acetylcholine from contributing to the turtles’ ability to recall the route that they had learned the previous year. While the turtles had been trotting along the predicted route just fine beforehand, blocking the M1 receptor immediately sent the turtles off course. Therefore, M1 receptors are needed for turtles to recall directions; however, it was unclear whether M1 was also necessary to memorize the route.
To test whether turtles rely on M1 to memorize the directions, Roth and Krochmal next turned off the M1 receptor while juvenile turtles were in the midst of learning their route. Knowing that juvenile turtles must memorize watering hole routes within the first three years of life, the researchers caught radio-tagged three-year-old juvenile turtles that were still learning their way as they set sail for a new pond, and temporarily blocked the M1 receptors with a drug. The juvenile turtles were unfazed by the disruption and kept on trucking right along to the pond. However, when the researchers tracked the turtles during the following annual water drain, the presently sober turtles – which had been drugged a year earlier while memorizing their route – struggled to find their way to the pond. Thus, while juvenile turtles are unperturbed when the M1 receptor is blocked during real-time navigation, the receptor is essential for them to create spatial memories early in life.
Taken together, Roth and Krochmal's findings suggest that painted turtles require acetylcholine acting specifically on M1 receptors for both memorizing and recalling navigation routes.