4 April 2007. The ups and downs of bipolar disorder are related to a somewhat different kind of oscillation—circadian rhythm. However, the exact relationship between the two, especially at the molecular level, is uncertain. But in last week’s PNAS online, Colleen McClung and colleagues at University of Texas Southwestern Medical Center in Dallas reported that mutation of the CLOCK protein, a key cog of the mammalian circadian oscillator, causes manic symptoms in mice akin to those found in people with bipolar disorder. The finding adds to a growing literature tying neuropsychiatric disorders, particularly mood disorders, to the mammalian circadian clock and sleep/wake cycles, and suggests that a molecular explanation for the correlations may be within reach. There may also be benefits in terms of understanding schizophrenia—disturbances in sleep patterns have long been recorded in schizophrenia patients, for example (see Keshavan et al., 1998).
In humans, the central circadian oscillator resides in the suprachiasmatic nucleus (SCN), a tiny area of the brain that sits above the optic chiasm. There, in a small number of cells, a molecular feedback loop, involving CLOCK and many other proteins, operates on a precise, almost 24-hour cycle. This endogenous clock acts as the body’s central pacemaker, setting the rhythm for a wide range of biological processes, including sleep/wake cycles, body temperature, and hormone levels. Though it has been 30 years since scientists reported that the cyclical episodes of mania and depression that characterize bipolar disorder oscillate in phase with the circadian cycle (see Sitaram et al., 1978), it is only recently that scientists have been able to investigate that relationship at the molecular level.
“We wanted to look at some of the specific genes involved in circadian rhythm to see if they would have any effect on mood and reward or other measures that might be associated with psychiatric disorders,” McClung said in an interview with SRF. First author Kole Roybal and colleagues chose to examine mice originally described by coauthor Joe Takahashi and colleagues at Northwestern University, Illinois (see King et al., 1997). In these animals, a point mutation causes exon skipping and a loss of 51 amino acids in the CLOCK protein. The mutation renders CLOCK transcriptionally inactive and perturbs the transcriptional feedback loop that serves as the central circadian oscillator.
“In every way that we could test them they looked like bipolar patients in the manic state,” said McClung. Roybal and colleagues found that the CLOCK mutants have heightened preference for rewarding stimuli, such as self-administered stimulation of the medial forebrain bundle in the brain, which leads to a type of hedonic state, preference for sucrose, and the effects of cocaine. “This is similar to behavior seen in the manic state when patients with bipolar disease often succumb to shopping or gambling sprees,” suggested McClung.
The CLOCK mutant mice were also less likely to become depressed, as when put through stressful tests such as a forced swim. In these tasks normal mice gave up much earlier than CLOCK mutant mice. The latter were also more likely to remain exposed in an open field test, suggesting that they are generally less anxious than their wild-type counterparts. In addition, the researchers found that lithium, which has been used for years as a mood stabilizing drug, calms the animals and restores behavior to nearly wild-type levels.
CLOCK Around the Brain
It is unclear whether these behavioral changes are related to perturbation in the circadian oscillator itself or some other area of the brain. Some SCN proteins that are either part of the circadian oscillator or governed by it are also found in other areas of the brain where they may have different functions. CLOCK, for example, is highly expressed in the hippocampus, as is SCN oscillatory protein (SCOP), which was recently shown to modulate signal transduction pathways involved in long-term memory (see Alzheimer Research Forum related news story). “It could be that CLOCK has an independent function that is unrelated to its role in the SCN, but it could also be that CLOCK is having an effect in dopamine cells,” said McClung. “One thing that we and others have found is that basically all the components of the dopamine system are circadian. Dopamine levels, receptors, the enzymes involved in dopamine synthesis all have a circadian rhythm, so maybe its normal function is to control the rhythm in dopamine firing and dopamine transmission.”
The researchers tested this idea by introducing functional CLOCK into the ventral tegmental area (VTA) of the brain, a region that plays a key role in the brain’s reward pathways and which is laden with dopaminergic cells—previous work showed that VTA dopamine transmission in CLOCK mutants is elevated. McClung and colleagues used viral vectors to transfect the VTA of CLOCK mutant mice with functional CLOCK genes, a technique that coauthor William Carlezon at McLean Hospital, Belmont, Massachusetts, has successfully employed to transfect cells in specific areas of the brain. Though only 35 percent of VTA cells were infected with the viral vectors, the treatment was sufficient to normalize behavior. “This gives us a focal point to target for future studies,” said McClung.
More generally, these CLOCK mice may represent a useful model of manic symptoms. “These mice are a complete and well-characterized model of human mania and that is something that has been lacking in the field. We haven’t really had a good mouse model to test why mania develops and also to test how mood stabilizers function. It is still somewhat of a mystery as to how they have their effects in the brain. So this mouse gives us an opportunity to study both the development of bipolar disorder and the treatment of bipolar disorder,” said McClung.
In an accompanying PNAS commentary, Joseph Coyle, also from McLean Hospital, noted that “an important limitation of the mutant CLOCK model is that it recreates only the behavioral homologues of mania but not the mood oscillations characteristic of bipolar disorder.” McClung said that she is uncertain why the mice do not have the full-blown spectrum. “They seem to only show manic phase. As far as we looked they don’t seem to go into depression spontaneously, but we are planning to test them in some different paradigms to see if activity or stress may make them go into depression, but we haven’t tested that yet,” she said. She did note that bipolar disorder is a very complicated disease and people can go through long periods where they seem perfectly normal.—Tom Fagan.
Roybal K, Theobold D, Graham A, DiNeri JA, Russo SJ, Krishnan V, Chakravarty S, Peevey J, Oehrlein N, Birnbaum S, Vitaterna MH, Orsulak P, Takahashi JS, Nestler EJ, Carlezon Jr. WA, McClung CA. Mania-like behavior induced by disruption of CLOCK. PNAS. 2007, March 26, early edition. AbstractCoyle JT. What can a clock mutation in mice tell us about bipolar disorder? PNAS. 2007, March 26 early edition. Abstract