While stress is a well-validated causal factor in anxiety disorders and depression, understanding the mechanisms that confer susceptibility to stress in some people and resilience in others is an active focus of research efforts. Such factors may be key in developing the next generation of therapies to treat anxiety, depression, and other mood disorders.

In a new paper appearing in Nature Neuroscience, a team led by 2016 BBRF Young Investigator Caroline Menard, Ph.D., of Université Laval, Quebec, Canada, report results of experiments that identify new mechanisms involving the endocannabinoid system in brain cells that appear to contribute to stress resilience.

In past research, Dr. Menard and colleagues have shown that levels of the key protein called Claudin-5 (Cldn5) were low in mice susceptible to becoming depressed after exposure to chronic social stress. Cldn5 is one of the proteins found in cells lining the inside of blood vessels in the brain which are responsible for integrity of the blood-brain barrier (BBB). This barrier normally protects the brain from toxins, viruses and pro-inflammatory molecules circulating in the bloodstream. In stress-vulnerable mice, the team documented blockage of the regulatory mechanism that causes the gene for Cldn5 to become active, a factor perhaps involved in loss of BBB integrity.

In their new paper, Dr. Menard and team focus on a particular involvement of the endocannabinoid system in stress resilience. Endocannabinoids are naturally occurring co-regulators of the stress response throughout the body (among various other functions). The two main receptors for endocannabinoids are called CB1 and CB2, with the former being more plentiful and important in the brain. Dysfunction of the endocannabinoid system has been linked to depressive behaviors in both animals and people.

The researchers knew from past studies that chronic severe social stress not only can cause mice to manifest depression-like behaviors; they also knew that such stress leads to disruptions in the BBB, specifically a leakage in its tight junctions (forged by proteins like Cldn5) that normally prevent pro-inflammatory molecules circulating in the blood from reaching brain tissue.

The brain has its own unique immune cells, called astrocytes. Astrocytes play a mediating role between the neurovascular system and brain cells. Threadlike projections from astrocytes connect them with neurons and other brain cells including glial “helper” cells. At the same time, they also send robust projections to blood vessels in the brain. These astrocytic projections culminate in oblong surfaces that attach to the exterior wall of brain blood vessels. They are called “astrocytic endfeet,” and in the words of the researchers, “they are perfectly positioned" to modulate properties of the BBB during stress exposure.

The team used super-resolution microscopy and gene-expression technology to look closely at astrocytes in two brain areas in mice subjected to chronic social stress: the prefrontal cortex (PFC) and the nucleus accumbens (NAc). The PFC is involved in social behaviors, executive function, and decision-making. The NAc has key roles in reward and mood regulation.

They found that in a portion of the NAc called the nucleus accumbens shell, CB1 receptors for endocannabinoids were highly expressed in male animals that were resilient in the face of chronic social stress. This elevated expression of the CB1 receptor was particularly evident in the endfeet of astrocytes in the NAc shell—the portion of the astrocyte that comes in direct contact with brain vasculature.

Separate experiments showed that overexpression of the gene (called Cnr1) that encodes the CB1 receptor in astrocytes within the NAc shell dampened anxiety- and depression-like behaviors in male mice. Analysis indicated that this overexpression of the Cnr1 gene promoted the expression of genes in the astrocytes involved in vascular regulation.

The team reported two other related experiments of note. In one, both physical exercise and antidepressant treatment increased the expression of astrocytic Cnr1 around blood vessels in the NAc of male mice. In the other, performed in postmortem tissue from people who had been diagnosed with major depressive disorder, the team confirmed a loss of the human gene that encodes the CB1 receptor in astrocytes within the NAc.

These results are consistent with the finding in this study of increased expression of endocannabinoid receptors in astrocytes in the NAc of stress-resilient animals. More broadly, the study suggests one specific way in which the endocannabinoid system may be involved in moderating stress, not only in mice but possibly also in people. Increasing numbers of CB1 receptors in NAc astrocytes appears to promote resilience to stress by dampening alterations of the BBB normally induced by chronic stress exposure.

Endocannabinoid activity in astrocytes appears “to promote vascular remodeling and attenuates inflammation contributing to biological adaptation underlying stress resilience,” the team wrote. Identifying such beneficial adaptations related to endocannabinoid-associated changes within the BBB “can represent a promising approach to development of innovative therapies,” they said.

Future research should investigate how stress appears to affect brain vasculature in a sex-specific manner, the team suggested. Different responses of blood vessels and the BBB to stress in males and females may contribute to sex differences in major depressive disorder prevalence and treatment response, they said.

The team also included Gustavo Turecki, M.D., Ph.D., a 2016 BBRF Distinguished Investigator, 2008 BBRF Independent Investigator, and 2000 BBRF Young Investigator.