Researchers trying to understand age-related memory loss in people have turned to large, slimy, sea slugs who bear two bunny-ear-like tentacles. When these sea hares reach senior citizen age, they aren’t able to learn a reflex. The findings were published in PLOS ONE this week.
As our brains age, we experience declining neurophysiological processes and increasing memory impairments. To better understand exactly how this happens, University of Miami’s Lynne Fieber and colleagues raised California sea hares (Aplysia californica) from egg masses of wild-caught animals. These sea hares have roughly a one-year lifespan, and they’re used pretty often as a model for studying the nervous system: Their simplicity allows researchers to examine the system quite directly. “You can count the number of nerve cells that are relevant to a reflex,” Fieber tells Washington Post.
It takes 7 to 8 months for the animals to mature, and by 12 to 13 months, the sea hares are considered “advanced age” animals. In their tail reflex experiments, the team “trained” slugs of these two age groups to forcefully contract their tail muscle when an electric shock was administered and also when the tail was simply touched. The process is called sensitization. The animal is exposed to a repeated stimulus of varying intensities, and its reaction is gauged. The more intense stimulus, of course, is the electric shock, and the result should be a forceful tail contraction in response to even just a touch.
By measuring their tail response learning, the team was able to determine patterns of aging in two kinds of neurons: those that sense the touch (sensory neurons) and those that signal for the tail muscle to contract (motor neurons).
“In a previous study focused on identification of the first cells in the nervous system to fail during aging of this reflex, we found that sensory neuron aging drives aging of the circuit,” Fieber says in a statement. “Here we examined the performance of sensory and motor neurons in short-term memory, and found that both neuron types had impaired performance in old Aplysia.”
Short-term sensitization to tail touch was gone in the aged sea hares. Their sensory and motor neurons were affected by aging in a way that resulted in their inability to learn sensitization. The team thinks that this is because the aged neurons aren’t responding to chemical messengers in the brain.