Researchers investigating how stressful experiences early in life promote later drug abuse have homed in on the glucocorticoid receptor (GR). In experiments with mice, augmenting GRs in the forebrain during the early postnatal period increased animals’ anxiety and sensitivity to cocaine as adults.
The GR plays a pivotal role in producing the physiological response to stress. The new NIDA-supported research suggests that GR levels during early brain development also affect the hard wiring of neural circuits that shape an individual’s basic emotional makeup. Increasing mouse GRs prior to the animals’ weaning was associated with alterations in the expression of more than 5,000 genes in the nucleus accumbens and hippocampus.
Mice Under Pressure
When a mouse or a person confronts a threat, the brain signals the adrenal gland to release glucocorticoid hormones. The hormones stimulate GRs on cells throughout the brain and body. The cells, in turn, alter their activity in ways that produce stress-related symptoms and behaviors.
Dr. Huda Akil and colleagues at the University of Michigan, Ann Arbor, have developed a mouse strain that enables them to study the impact of greatly amplified GR activity, such as occurs in traumatic stress. The mice have an extra copy of the GR gene that gives them a superabundance of GRs in the forebrain. In early experiments, the researchers showed that these GR-augmented mice exhibit behaviors suggestive of heightened anxiety and depression and hypersensitivity to cocaine.
To investigate whether early-life GR levels have lifetime consequences, the researchers attached an off-switch to the extra GR gene. They raised male mice to the age of weaning with the gene turned on, then turned it off by administering the antibiotic doxycycline. When the animals reached adulthood, the researchers tested their responses to stressful situations and cocaine.
Compared to normal animals, the mice that had early-life GR augmentation responded more fearfully to stress-inducing situations. They were more reluctant to venture out on a narrow and—to a mouse—lofty beam in the “elevated plus maze” test; and they hesitated longer before emerging from the dark to explore a brightly lit novel space in “light-dark box” and “defensive withdrawal” tests.
Cocaine sensitizes the mouse brain so that re-exposures to the drug produce more locomotor stimulation than initial doses. The Michigan researchers’ genetically manipulated mice exhibited this effect more markedly than normal mice. Given two injections of the drug 14 days apart, they covered 2.5 times as much distance after the second compared to after the first.
Further experiments indicated that early life is the critical period for GR activation to influence anxiety and drug sensitivity. In behavioral tests, compared with control animals with normal GR:
- Mice in which GR was augmented from birth to weaning exhibited increased anxiety and drug sensitivity responses—but continuing augmentation after weaning and into adulthood produced no additional rise.
- Mice in which GR was not augmented from birth to weaning did not exhibit increases in anxiety and drug sensitivity responses—even when the researchers turned on the extra GR gene after weaning.
“These findings demonstrate the critical nature of early development in resilience or liability to drug abuse,” says Dr. Akil.
Many Genes Are Affected
“Glucocorticoid receptors are all over the brain, and they have an impact on the regulation of many, many genes,” says Dr. Akil. “Through stress, you can influence molecules that have a very broad impact on the wiring and reactivity of the brain.”
Dr. Akil and colleagues analyzed brain tissue for differences in gene expression that might underlie the lasting effects of early-life stress. As adults, mice that had an active extra GR gene prior to weaning differed from normal mice in the expression of more than 5,000 genes in brain areas that influence reward and emotion. These included more than 1,000 genes in the nucleus accumbens and more than 4,000 genes in the dentate gyrus region of the hippocampus.
|Dentate Gyrus||Nucleus Accumbens|
|Early-Life Exposure Group||4,393||562|
|Lifetime Exposure Group||4,648||1,058|
|Differences Shared by Early-Life and Lifetime Exposure Groups||2,785 (62%)||162 (10%)|
Among the 5,000 changes in gene expression, the researchers identified about 3,000 that may result from heightened GR activity during early stressful experiences and persist throughout life. These changes were present both in mice with early-life and mice with lifetime heightened GR activity. Most were in the dentate gyrus, and might influence anxiety. Relatively few were in the nucleus accumbens, but these could influence responses to the rewarding properties of addictive drugs.
|Dentate Gyrus||Nucleus Accumbens|
“We’re very interested in following up on these findings, and looking at downstream molecules that might play a role in driving the increased sensitization to cocaine we observed,” says Dr. Akil. “The hope is to come up with strategies to understand who is at risk and the different types of risk. For prevention and treatment, understanding both the genetic background and the developmental trajectory of addiction is really going to be helpful. The consequences of addiction may be different for people according to their temperaments—for example, between those attracted to drugs of abuse by novelty seeking versus those looking to blunt anxiety—and that might mean that you need different treatments also.
“Studies that are done at early ages have turned out to be very critical in our thinking about how addiction occurs and which changes are most important,” says Dr. Nancy Pilotte, chief of NIDA’s Functional Neuroscience Research Branch. “They’ve forced us to recognize the importance of early developmental changes and how, because the brain is in such a plastic mode during this early period, these changes can critically set the path for life.”
This study was supported by NIDA grants; DA021633-02, L99MH60398, DA021633-01A2, and DA021633-02.
Wei, Q., et al. Early-life forebrain glucocorticoid receptor overexpression increases anxiety behavior and cocaine sensitization. BiolPsychiatry 2012;71:224–231. Full Text