The Neuroscience of Springtime Bliss and Wintertime Doldrums

Seasonal rhythms may affect our mood via mu-opioid receptor (MOR) availability.

Posted Feb 25, 2021 | Reviewed by Matt Huston

Photobank gallery/Shutterstock
Source: Photobank gallery/Shutterstock

"We were shining our light into the days of blooming wonder. On and on and on, we kept singing our song. It's easy to describe leaves in the autumn. And it's oh so easy in the spring. But down through January and February, it's a very different thing. On and on and on, through the winter of our discontent. When the wind blows up your collar and the ears are frostbitten, too."
—from "A Sense of Wonder" by Van Morrison

Seasonal Affective Disorder's acronym, SAD, sums up how many of us in the Northern Hemisphere feel during this time of year—when the days tend to be shorter and colder. February is one of Americans' least favorite months, Gallup Polls have found.

Long before SAD was included in the DSM-IV in 1994, William Shakespeare summed up the seasonal pattern of recurrent depressive symptoms that usually begin in late autumn and continue through early spring in the opening line of William III: "Now is the winter of our discontent."

In 1985, this age-old phrase was repurposed by Van Morrison in "Sense of Wonder" to juxtapose how the song's protagonist feels in January and February compared to the spring and summer months. The Winter of Our Discontent is also the title of John Steinbeck's final novel, which has been described as a "tale of spiritual crisis."

When it comes to seasonal variations of mood, humans since de temps immémorial seem to grow increasingly happy and contented as the days get warmer and longer, which happens in opposite months for those living in Northern vs. Southern Hemispheres. (December to February is summer in Australia, for example.) Literally and figuratively, the transition from spring to summer is generally considered a hopeful and regenerative time of growth or rebirth.

Shakespeare begins one of his best-known love poems, Sonnet 18: "Shall I compare thee to a summer day?" but also notes: "Rough winds do shake the darling buds of May and summer's lease hath all too short a date." Songwriters from Gordon Lightfoot ("Summer Side of Life") to George Gershwin ("Summertime") represent this season as a time when "livin' is easy" and our "love is ripe."

Why do so many of us experience "winter doldrums" but tend to feel more joyful and gregarious in the spring and summertime? Historically, SAD has been associated with circadian rhythm-driven changes in melatonin and vitamin D levels that respond to seasonal changes in daylight (Melrose, 2015).

The latest neuroimaging research also identifies a potential correlation between SAD and daylength-affected fluctuations of the brain's mu-opioid receptor density. Just like flowers bloom in the spring, our brain's opioid receptors may also blossom then.

 University of Turku
Brain opioid receptors measured with positron emission tomography (A) and regions, where opioid receptor density varied seasonally.
Source: University of Turku

Seasonal Rhythms May Affect Our Moods and Sociability via Mu-Opioid Receptors

A new brain imaging study from Finland's Turku PET Centre suggests that fluctuations in the length of daylight hours may affect the availability of opioid receptors that regulate both mood and sociability in the brain. These findings (Sun et al., 2021) were published on February 10 in the Journal of Neuroscience.

"Seasonal rhythms influence emotion and sociability. The central mu-opioid receptor (MOR) system modulates numerous seasonally varying socioemotional functions, but its seasonal variation remains elusive," the authors explain their paper's Significance Statement. "Here, we used positron emission tomography to show that MOR levels in both human and rat brains show daylength-dependent seasonal variation."

For this study, the Finnish researchers compared how the length of daylight hours affected the opioid receptors in a cohort of 204 humans and nine lab rats. In both humans and rats, the researchers found that cerebral mu-opioid receptor availability shows significant seasonal variation based on daylength cycles.

Sun et al. found that "seasonally varying daylength had an inverted U-shaped functional relationship with brain mu-opioid receptor availability in humans. Brain regions sensitive to daylength spanned the socioemotional brain circuits, where MOR availability peaked during spring." In rats, they observed a similar phenomenon.

As the authors explain: "While rats underwent daylength cycle simulating seasonal changes, repeated [PET imaging] was conducted. Our data suggest that there is seasonal variation in brain mu-opioid receptor availability in both humans and rats, with MOR availability peaking in seasons with intermediate daylengths."

"In the study, we observed that the number of opioid receptors was dependent on the time of the year the brain was imaged. The changes were most prominent in the brain regions that control emotions and sociability," first author Lihua Sun of the University of Turku's PET Centre said in a news release. "The changes in the opioid receptors caused by the variation in the amount of daylight could be an important factor in seasonal affective disorder."

"Given the intimate links between mu-opioid receptor signaling and socioemotional behavior," the authors conclude, "these results suggest that the MOR system might underlie the seasonal variation in human mood and social behavior and imply that MOR might be a feasible target for treating seasonal affective disorders."

University of Turku image via EurekAlert, no usage restrictions with proper credit

References

Lihua Sun, Jing Tang, Heidi Liljenbäck, Aake Honkaniemi, Jenni Virta, Janne Isojärvi, Tomi Karjalainen, Tatu Kantonen, Pirjo Nuutila, Jarmo Hietala, Valtteri Kaasinen, Kari Kalliokoski, Jussi Hirvonen, Harry Scheinin, Semi Helin, Kim Eerola, Eriika Savontaus, Emrah Yatkin, Juha O. Rinne, Anne Roivainen and Lauri Nummenmaa. "Seasonal Variation in the Brain μ-Opioid Receptor Availability." The Journal of Neuroscience (First published: February 10, 2021) DOI: 10.1523/JNEUROSCI.2380-20.2020

Sherri Melrose. "Seasonal Affective Disorder: An Overview of Assessment and Treatment Approaches." Depression Research and Treatment (First published: November 25, 2015) DOI: 10.1155/2015/178564