Can Altered Brain Signals Turn a Mother’s Love On and Off?

Source: Piyapong Saydaung/Pixabay

Most mothers are hardwired to love and nurture their children, and these moms are willing to take extraordinary risks to protect their kids. The simple act of holding an umbrella out to keep a child dry, while Mom gets sopping wet, exemplifies typical parental behavior and maternal instincts.

But not every mother has these instincts. Nor does every mother develop secure attachment to her infants. Whereas most moms would walk through fire to protect their offspring, some others neglect their babies and are unwilling to make sacrifices to protect them.

Sons and daughters who grow up feeling unloved and neglected by a biological mother who seems disinterested in them may wonder, "Why doesn't my mom have the maternal instincts of other mothers?"

Maternal neglect seems to go against evolutionary design. Doesn't the survival of each mammalian species rely on a mother taking care of her offspring before they can fend for themselves? What might explain such differences in maternal instincts and nurturing motivation? Can neuroscience or evolutionary psychology help to explain these parental behaviors?

Calcitonin Receptor Signaling in the MPOA Can Turn Nurturing On and Off

Previous research (Geher et al., 2020) from the New Paltz Evolutionary Psychology Lab has inked suboptimal parenting styles to a Dark Triad of personality traits. (See "Dark Parenting Styles.") A new neuroscience-based study in mice offers some fresh clues about neural mechanisms in the mammalian brain that appear to drive maternal nurturing behaviors—or the lack thereof. These findings (Yoshihara et al., 2021) were published on June 1 in Cell Reports.

This research was conducted by a team of scientists from the Laboratory for Affiliative Social Behavior at the RIKEN Center for Brain Science in Japan, led by Kumi Kuroda.

Previous RIKEN research (Kuroda & Numan, 2014) suggested that the medial preoptic area (MPOA) subregion of the hypothalamus might be a hub for nurturing behaviors in mothers and non-parent mice. The MPOA contains at least seven different neuron types; this study's objective was to pinpoint which neurons are most vital for nurturing behaviors.

"Here, we report that the calcitonin receptor (Calcr) and its ligand amylin are expressed in distinct neuronal populations in the medial preoptic area (MPOA) and are upregulated in mothers," the authors explain in the abstract of their recent (2021) paper. "Calcr+ MPOA neurons activated by parental care project to somatomotor and monoaminergic brainstem nuclei."

In mice, nurturing behaviors of young pups involve building a nest, hovering over infants while they're in the nest, and something called "pup retrieval," which involves picking up stray pups and bringing them back to the nest.

The elevated pup-retrieval test was used to assess the willingness of mice to care for infants in risky/dangerous situations.

Source: RIKEN, no usage restrictions with credit.

When These Neurons Are Silenced, Nurturing Stops

To establish whether calcitonin receptor signaling in the MPOA is key to maternal nurturing, the researchers silenced these neurons in some mice before conducting a pup-retrieval experiment. Risky or "scary" pup-retrieval tests are often used to measure the degree of nurturing motivation in mice.

Interestingly, mice with active Calcr+ MPOA neurons were willing to take dangerous risks to retrieve their pups in an elevated pup-retrieval maze. Conversely, when the Calcr+ MPOA neurons were silenced, mice displayed little motivation for nurturing behavior and were unwilling to take risks to retrieve pups in the elevated maze. As the authors sum up:

"These data indicate that Calcr+ MPOA neurons are required for both maternal and allomaternal nurturing behaviors and that upregulation of amylin-Calcr signaling in the MPOA at least partially mediates risk-taking maternal care, possibly via modified connectomics of Calcr+ neurons postpartum."

Of course, mice mothers aren't human moms. Nonetheless, mouse research is a first step for clarifying the underlying neuromolecular mechanisms that enhance a maternal mammals' motivation to nurture and protect her offspring. "The next step is to examine calcitonin receptor-expressing cMPOA neuron's role in the nurturing behavior of non-human primates, which should be more similar to what happens in humans," senior author Kuroda said in a June 1 news release.

"Obviously, there is also much more to be studied to fully elucidate the mode of action of amylin-Calcr signaling in the MPOA to mediate complex mammalian nurturing behaviors," the authors conclude.

RIKEN pup-retrieval test photo via EurekAlert