Scientists often use fear conditioning to study rodents’ learning and behavior. If a particular stimulus (such as a light, a sound, or an odor) is presented paired with the delivery of a mild shock, the animal begins to associate the stimulus with the shock and will freeze when it is presented and avoid the stimulus.
New research shows that if a pregnant rat (known as a dam) goes through fear conditioning that pairs an odor with a shock, the rat’s offspring will also avoid that odor into adolescence. Even if the pups are raised by a different mother who never went through the fear conditioning, they still avoid the odor into adolescence, showing that they do not learn the behavior through watching their mother.
The conditioning is specific to the particular odor, such that a different odor not used in the fear conditioning does not evoke a heightened reaction from the pups. It appears that the pup learns the fear through chemical signals, such as alarm pheromones that can pass through the placenta.
Rats who are taught to associate a light with an electric shock learn to avoid the light. This process is known as conditioned fear. New research shows that if one rat watches another rat go through fear conditioning, the observing rat will also show the effects of fear conditioning. It will also avoid the light, but only if it had previous experience with fear conditioning. It appears that rats have the ability to learn from other rats’ painful experiences.
Colleen Hanlon, a researcher at the Medical University of South Carolina, has found that biofeedback can be used to decrease cocaine craving in people with substance abuse problems. In her research, patients were given real time feedback from functional magnetic resonance imaging (fMRI) and learned to decrease the activation of a part of the brain called the anterior cingulate when exposed to cocaine cues (reminders of their desire for cocaine). They were able to decrease drug craving as well as heart rate and skin conduction, which often accompany it.
Brain-derived neurotrophic factor (BDNF) is a protein in the brain that protects neurons and is necessary for long-term memory and learning. Different people have different genetic variations in BDNF depending on which amino acid the gene that codes for it inserts into the protein, valine or methionine. There are three possible combinations that vary in their efficiency. The Val66Val allele of BDNF is the most efficient for secreting and transporting BDNF within the cell body to synapses on dendrites, and is also a risk factor for early onset of bipolar disorder and rapid cycling. Twenty-five percent of the population has a Met variant (either Val66Met or Met66Met), which functions less efficiently. These people have mild decrements in some cognitive processing.
Increases in BDNF are necessary to the antidepressant effects of intravenous ketamine. In animals, ketamine also rapidly changes returns dendritic spines that had atrophied back to their healthy mushroom shape in association with its antidepressant effects. According to research published by Gonzalo Laje and colleagues in the journal Biological Psychiatry in 2012, depressed patients with the better functioning Val66Val allele of BDNF respond best to ketamine, while those with the intermediate functioning Val66Met allele respond less well.
Researcher Ronald S. Duman of Yale University recently found that increases in BDNF in the medial prefrontal cortex are necessary to the antidepressant effects of ketamine. If antibodies to BDNF (which block its effects) are administered to the prefrontal cortex, antidepressant response to ketamine is not observed.
Duman also found that calcium influx through voltage sensitive L-type calcium channels is necessary for ketamine’s antidepressant effects. A genetic variation in CACNA1C, a gene that codes for a subunit of the dihydropiridine L-type calcium channel, is a well-replicated risk factor for bipolar disorder. One might predict that those patients with the CACNA1C risk allele, which allows more calcium influx into cells, would respond well to ketamine.
Most children recover from an episode of bipolar disorder after a considerable period of time, but the majority eventually relapse. At the 2013 meeting of the American Academy of Child and Adolescent Psychiatry (AACAP), Boris Birmaher of the University of Pittsburgh presented new data on the long-term prospective course of bipolar disorder in 255 children with bipolar I, 30 children with bipolar II, and 153 children with bipolar NOS (not otherwise specified), who together had an average age of onset of 9.3 +/- 3.9 years. The children participated in the study for an average of 8 years. Most of the children (81.5%) recovered from their episode, but only after an average of 2.5 years of follow up treatment. Yet 62.5% of those who recovered experience a recurrence after an average of 1.5 years.
Editor’s Note: It takes a long, long time to achieve recovery, and longer for bipolar NOS (more than 2 years on average) than for either Bipolar I or II (about 1.8 years). However, the high rate of relapse within 1 to 2 years is equally disturbing. These data are similar to those in many other prospective follow up studies of children, and suggest that it is important for parents to be aware that this illness is difficult to treat, and good results within weeks are not likely to be the norm. At the same time, 43% of the children with a bipolar diagnosis eventually achieved euthymia (wellness) in the longer term, so there is cause for some optimism.
Four Trajectories in Children with Bipolar Illness
Birmaher described four different long-term,trajectories observed over an average of 8 years of follow up with 438 children with bipolar disorder.
- Predominately euthymic (24%)
- Ill early then much improved (19%)
- Mild to moderately ill—euthymic only 47% of the time (34.6%)
- Predominantly ill—euthymic 11.5% of the time (20.3%)
The predominantly well group (1) was associated in a univariate analysis with a later onset of illness, higher socio-economic status, less conflict, fewer stressors, less sexual abuse, fewer anxiety and ADHD comorbidities, and less medication (including stimulant use). In a multivariate analysis, this group was independently associated with less severe depression/mania, less suicidal ideation, less substance use, less sexual abuse, and less family history of mania and substance abuse.
This group had the best functioning, almost to 80 on the Children’s Global Assessment Scale (C-GAS). In comparison, despite considerable time euthymic for groups 2 and 3, these children still had considerable functional impairment, in the realm of 65 on the C-GAS scale. Even in Group 1, about half of the children had low C-GAS scores.
Birmaher suggested the importance of trying to find ways to delay the onset of the illness (to graduate more children into the good prognosis group) and allowing them time to develop socially and educationally and graduate from high school. Potential preventive strategies could include omega-3 fatty acids, more time spent exercising, good sleep hygiene, family focused therapy (FFT), dialectic behavior therapy, treating subsyndromal depression, and even treating parents with mood disorders to complete remission (which has been shown to improve behavioral health in offspring).
Editor’s Note: As this editor Post, Chang, and Frye wrote in the Journal of Clinical Psychiatry in 2013, beginning to study the effectiveness of these kinds of early primary and secondary prevention strategies in children who can now be readily identified clinically as at risk for a mood disorder, should be given the highest priority.
Children who have at least one parent with a bipolar or unipolar disorder, some further environmental risk factors (such as adversity in early childhood), and early symptoms of depression, anxiety, or prodromal bipolar disorder are at very high risk for bipolar disorder, and there is an urgent need for randomized studies (even open ones) of safe potential preventive strategies for these children.
Omega-3 fatty acids in particular have a strong record of safety, compelling rationale for use in bipolar disorder, and have already been shown to have significant preventive effects in decreasing the transition from early prodromal psychosis to full-blown schizophrenia.
People with bipolar disorder often show signs of inflammation. These could eventually help clarify diagnosis, illness activity, and treatment response, and predict illness progression. Previous studies have shown increases in c-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha) in adults with mood disorder. These high levels tend to improve with medications, are related to illness severity, and are also related to manic and mixed states.
At the 2013 meeting of the American Academy of Child and Adolescent Psychiatry (AACAP), Ben Goldstein reported on a study that examined levels of TNF alpha, IL-6, and high sensitivity CRP (hsCRP) in 123 adolescents with an average age of 20.4 years, who had been ill for an average of 12.7 years.
CRP levels in adolescents with bipolar illness were equivalent to those with rheumatoid arthritis, and much higher than healthy controls. In children with bipolar disorder, higher levels of CRP were related to more time symptomatic. High hsCRP was related to lower socio-economic status and to substance abuse disorders.
Increases in IL-6 were linked to a longer time to achieve remission and more weeks depressed. High IL-6 was related to duration of illness, positive family history of substance use, and family conflict.
High TNF alpha was related to low socioeconomic status (SES), self-injury, suicidal ideation, and positive life events.
Goldstein said studies of these markers could eventually lead to therapeutic advances, but the process would be long and would require several steps: proof of concept studies, prospective validation studies in independent samples, and demonstration of clinical gains over standard predictive markers, culminating in enhanced patient care and outcome through better, faster prediction of response.
Editor’s Note: Ideally clinicians could jump ahead by immediately attempting to determine whether adding a medication with direct anti-inflammatory effects could enhance therapeutic effects in children with elevated inflammatory markers. Treating inflammation could also theoretically help prevent cognitive deterioration and decrease the considerable risk for cardiovascular dysfunction in patients with bipolar disorder.
Many children with bipolar disorder also present with other comorbid Axis I psychiatric illnesses. Now it seems that the worsening of these comorbidities, such as attention-deficit hyperactivity disorder (ADHD) or an anxiety disorder, can signal a more difficult course of bipolar illness itself. At a symposium on the course of bipolar disorder in children at the 2013 meeting of the American Academy of Child and Adolescent Psychiatry (AACAP), Shirley Yen from Brown University discussed findings on comorbidities of childhood onset bipolar disorder from COBY, the Collaborative Child Bipolar Network. Upon study entry, 60% of children with bipolar disorder also had ADHD, 40% had oppositional defiant disorder (ODD), 39% had an anxiety disorder, 12.5% had both oppositional defiant disorder and a conduct disorder, and 9% had a substance abuse disorder.
The prevalence of most of these comorbid illnesses increased over time (e.g. anxiety disorder rates increased from 39% to 62%). The illnesses were also related to the time it took participants to achieve recovery (eight consecutive weeks well), and the time until a recurrence of a depressive or manic episode.
Increases in anxiety were linked to longer time to achieve recovery and a shorter time to a recurrence. Increases in ADHD were linked to a more rapid onset of a depressive recurrence. Increases in oppositional defiant disorder and conduct disorder had no relationship with either remission or recurrence. Increases in substance abuse disorders were linked to a longer time to recover from a manic episode. Thus, worsening of the comorbid conditions had definite consequences for both recovery and recurrence.
Research on early-onset bipolar disorder has often hinged on identifying the key characteristics of the disorder. At a symposium on the course of bipolar disorder in children at the 2013 meeting of the American Academy of Child and Adolescent Psychiatry (AACAP), Jeff Hunt of Brown University discussed findings from COBY, the Collaborative Child Bipolar Network. He described the course of illness in 446 children with bipolar disorder, including 10% who had irritability at baseline, 15% who had elated mood at baseline, and a majority (75%) who had both irritability and elation at baseline.
Most factors such as positive family history of bipolar illness and comorbidities including attention deficit hyperactivity disorder (ADHD) did not differ across the three groups. The three subtypes (irritable, elated, or mixed) did not remain stable, and most of the children eventually converted to the combined irritable and elated subtype. These data contrast with those of Ellen Liebenluft et al., who found that those with severe mood dysregulation or chronic irritability (but not other key characteristics of bipolar disorder) did not go on go on to receive a bipolar diagnosis and tended not to have a family history of bipolar disorder.
No one gene explains the risk of developing bipolar disorder. Many genes are involved, each with a small effect. However, the effects of one particular gene have been validated in multiple different ways. The gene is called CACNA1C, and it codes for one subunit of the dihydropyridine L-type calcium channel. Calcium channels are structures on the membranes of neurons that allow calcium to enter cells and alter their excitability.
Different people can have different variants of the CACNA1C gene, depending on which nucleotides appear there: valine (Val) or methionine (Met). One particular variant (known as the Met/Met single nucleotide polymorphism, rs1006737) has been associated with executive function deficits compared to the Val/Val variant in multiple tests in patients with bipolar disorder. Executive function refers to abilities like planning, organizing, and retaining information. This was reported by Soeiro-de-Souza et al. in the journal Acta Psychiatrica Scandinavica in 2013.
Importantly, CACNA1C has also been linked to risk of bipolar disorder, a finding that was replicated in several large genome-wide association studies (GWAS). Autopsy studies of people who had been diagnosed with bipolar disorder show more calcium channels in their brains. The Met/Met variant of the CACNA1C gene also lets more calcium ions into cells. Those who have the gene variant also show differences in some brain structures known to be involved in the modulation of emotions compared to those without the variant.
In addition to these findings, more than a dozen studies report increased intracellular calcium in the white blood cells of people with bipolar disorder compared to controls. To the extent that these increases in intracellular calcium reflect changes in neurons, this would be consistent with the findings about CACNA1C. High levels of calcium influx and the associated intracellular calcium may increase cellular excitability and potentially dysregulate normal neuronal functioning.
The final piece of evidence linking altered calcium channel regulation to bipolar disorder is a direct therapeutic test of a drug that blocks calcium influx through the dihydropyridine L-type calcium channel. There is evidence that nimodipine, which selectively blocks dihydropyridine L-type calcium channels, has therapeutic effects in bipolar disorder.
Researchers have identified neurons responsible for remembering conditioned fear in the amygdala of rodents, and can turn them on and off. At the 2013 meeting of the Society of Biological Psychiatry, Sheena A. Josselyn gave a breath-taking presentation on this process.
When animals hear a tone they have learned to associate with the imminent delivery of a shock in a given environment, they learn to avoid that environment, and they reveal their learning of the tone-shock association by freezing in place. Josselyn was able to observe that 20% of the neurons in the lateral nucleus of the amygdala were involved in this memory trace. They were revealed by their ability to increase the transcription factor CREB, which is a marker of cell activation. Using cutting-edge molecular genetic techniques, the researchers could selectively eliminate only these CREB-expressing neurons (using a new technology in which a diphtheria toxin is attached to designer receptors exclusively activated by designer drugs, or DREADDs) and consequently erase the fear memory.
The researchers could also temporarily inhibit the memory, by de-activating the memory trace cells, or induce the memory, so that the animal would freeze in a new context. Josselyn and colleagues were able to identify the memory trace for two different tones in two different populations of amygdala neurons.
The same molecular tricks with memory also worked with cocaine cues, using what is known as a conditioned place preference test. A rodent will show a preference for an environment where it received cocaine. Knocking out the selected neurons would remove the memory of the cocaine experience, erasing the place preference.
The memory for cocaine involved a subset of amygdala neurons that were also involved in the conditioned fear memory trace. Incidentally, Josselyn and her group were eventually able to show that amygdala neurons were in competition with each other as to whether they would be involved in the memory trace for conditioned fear or for the conditioned cocaine place preference.