Agrp neurons and neuronal plasticity

Throughout development, neurons are rewired for new functions. Certain behaviors can be useful early in life, but they can become unnecessary as an organism undergoes maturation. In response, neurons can change from their original, neonatal function into their new, adult function. The ability to remodel neurons also allows organisms to gain adult-specific behaviors, such as reproduction. Neuronal plasticity is key for the proper development of many organisms. To better understand this phenomenon, researchers in the Dietrich lab at Yale University investigated a group of neurons in mice that alter their function during maturation.

In mice, there are a specific group of neurons in the hypothalamus called Agouti-related peptide neurons, or Agrp neurons. In adult mice, the Agrp neurons have been shown to regulate normal food ingestion. When Agrp neurons have been removed, adult mice will not eat or swallow, a condition known as aphagia. The Dietrich lab asked if Agrp neurons in neonatal mouse pups also regulate ingestion.

Researchers used Fos as a marker of Agrp activation. When they isolated 10-day old mouse pups for 90 minutes, they observed that Agrp neurons were ~30% Fos-positive. When mouse pups remained in the nest, there were almost no Fos-positive Agrp neurons.

In their first experiment, the Dietrich lab tested if mouse pups would drink more milk after Agrp activation. If the mouse pups did drink more milk, then one could assume the Agrp neurons regulate ingestion in both neonates and adults. Researchers isolated mouse pups for 90 minutes, thereby activating the Agrp neurons. After isolation, they reunited the pups with their mothers and measured the weight of the mouse pups’ stomachs. Surprisingly, the mouse pups did not ingest more milk after Agrp activation.

From this result, the Dietrich lab wondered if feeding could be enough to modulate Agrp activity. The researchers gave milk to pups during the 90-minute isolation period, but their Agrp activity did not change compared to controls. To further test this, instead of isolating pups, researchers paired pups with foster mothers for 90 minutes and measured the percentage of Fos-positive cells in Agrp neurons. Because maternal care is linked to suckling and lactation, the researchers divided the foster mothers into lactating and non-lactating cohorts. Regardless of the foster mother's ability to produce milk, mouse pups had significantly less Agrp activation when paired with a foster mother. These results suggest that the Agrp neurons in mouse pups do not regulate feeding as they do in adults. Instead, the presence of maternal care modulates Agrp activity.

Additionally, warmth was capable of decreasing Agrp activity in mouse pups. When mouse pups were isolated and placed into cages at 35°C, their Agrp activity significantly decreased compared to controls. Warmth is another important aspect of maternal care. This supports the previous result that maternal care dictates Agrp activity.

To determine the dynamics of Agrp activation, researchers used Cre-dependent GCaMP signaling in mouse pups. Using fiber photometry, the researchers isolated mouse pups and measured Calcium levels from the Agrp neurons. High Calcium levels demonstrate neuronal activation. As soon as the pups were isolated, Calcium levels highly increased. They maintained at high levels throughout isolation. When pups were returned to their mothers, the Calcium levels of their Agrp neurons dramatically decreased to normal levels.

Combined, this data shows the functional properties of the Agrp neurons in neonatal mice. Isolation rapidly activates the Agrp neurons. Only aspects of maternal care, such as suckling and warmth, can decrease Agrp activity. Feeding does not modulate the Agrp neurons in neonatal mice. When do the Agrp neurons change their function to regulate ingestion? The Dietrich lab used chemogenetic activation to constantly activate the Agrp neurons in 15-day old mice. These mouse pups drank more milk when Agrp neurons were active. At 18 and 21 days old, mouse pups with activated Agrp neurons also ate more solid food than control mice. The Agrp neurons change their function as mouse pups begin to wean at 15-days old. Initially, the Agrp neurons have a specific neonatal function and are regulated by maternal care.  Instead of disappearing, these neurons are modified to regulate an adult-specific behavior, eating solid foods. This example shows the nervous system’s capability to functionally rewire during development. With this capacity, the nervous system can gain greater complexity and utility, without destroying pre-existing neurons.