Science

Inside the Neuron

Neurotransmitters are about outside the cell. Let's open up the hood.

Until very recently, it was thought that the brain could not grow new cells. From a mood disorders point of view, this was particularly distressing, as brain scans and post mortem studies have found reduction in the volume of the prefrontal cortex of depressed and bipolar patients, as well as cell atrophy and loss. In one classic study, Wayne Drevets MD, Chief of Neuroimaging of Mood and Anxiety Disorders at the NIMH, found that the subgenual prefrontal cortex of the brain was 38 percent smaller in bipolar patients and 48 percent smaller for those with chronic unipolar depression In experiments by Robert Sapolsky PhD of Stanford, animals subjected to stress resulted in dead or atrophied neurons in the hippocampus, as well as endangered neurons that were more likely to die when subjected to another stressful event.

One of these casualties is brain derived neurotrophic factor (BDNF). BDNF is a neuropeptide that is crucial to the survival and growth of neurons.

Stress elevates cortisol, which in turn ups the excitatory neurotransmitter glutamate, which increases calcium influx into the neuron and activates certain calcium-dependent “death” enzymes. Cortisol may also reduce the neuron’s capacity to take energy-sustaining glucose into the cell so it doesn’t have the strength to deal with a subsequent crisis. Another casualty may be the glia, the “other” brain cell, once thought of as mere mind-glue but now recognized as an active partner of the neuron. One of its functions is thought to be clearing glutamate from the synapse.

Basically, the cells can’t handle the load, Husseini Manji MD, Director of the Mood and Anxiety Disorders Program at the NIMH, explained in a grand rounds lecture delivered at UCLA in Feb 2003 and webcast the same day. Their atrophy and death tends to isolate neurons, affecting their ability to connect to and communicate with other neurons.

Then, in the late nineties, came the startling discovery that under the right conditions, animal brains can grow new cells and shrunken brain cells could grow to normal sizes and make new connections, a process called neurogenesis that takes place mainly in the hippocampus. In 2000, Fred Gage PhD of the Salk Institute discovered neurogenesis also takes place in humans - assuming stress and depression do not sabotage the brain's best efforts.

In 2001 came the finding that antidepressants can cause new cells to grow in the hippocampus. The next year came the discovery that exercise also causes brain cells to grow. Ron Duman PhD and his team at Yale first found that repeated antidepressant treatment "up-regulates" a process known as the cAMP-CREB cascade. cAMP is a signaling molecule inside the cell that is upstream of the protein CREB, which controls the expression for certain genes, among them BDNF. Significantly, CREB and BDNF play critical roles in neuroplasticity, that is, of the brain's capacity to constantly remap itself by learning and forming memories.

The cAMP-CREB cascade also figures in neurogenesis. Dr Duman and his team exposed lab rats to repeated footshocks to induce behavioral helplessness, resulting in a long-lasting down-regulation of neurogenesis. But when the animals were treated with different classes of antidepressants, the behavior was reversed.

Approximately 9,000 new cells a day grow in the hippocampus of an adult rodent. Of these, about 75 to 80 percent become neurons, and half survive after four weeks. It is estimated that in humans the rate of new cell growth is only 10 to 20 percent of rodents, but this may still be a number that is sufficient to influence the function of the hippocampus.

Meanwhile, a study led by Dr Manji found that lithium "significantly increases total gray matter volume in the human brain of people with manic-depressive illness."

Using a gene chip micro-array (a process that allows researchers to record the interactions among thousands of genes simultaneously), Dr Manji and his colleagues started experimenting with lithium and Depakote on brain cell tissue, and found to their surprise these two completely different medications indirectly affected some of the same cell pathways associated with cell survival and death. One protective protein that utilizes these pathways is Bcl-2, which in one experiment was doubled by lithium and Depakote administration. Subsequent experiments on rats found lithium mitigated the effects of lab-induced stroke and led to the growth of new neurons in the hippocampus. When Dr Manji asked Dr Drevets to revisit his study, it was found that those patients on lithium or Depakote did not show brain atrophy. More recently, a study on human patients with bipolar found lithium increased overall brain grey matter.

But producing new brain cells is only part of the picture, and probably not the main part of the picture. What may be even more important is the ability to protect and rescue damaged brain cells and helping them to re-establish connections, according to Dr Manji. To appreciate lithium’s possibilities we need to realize that both depression and bipolar disorder are more than mere mood disorders. The impairments to function and cognition may last far beyond the course of an actual episode, and although not "classic" neurodegenerative diseases such as Parkinson’s and Alzheimer’s, they are clearly illnesses associated with brain cell loss and shrinkage.

Tellingly, Bcl-2 protects against free radicals that can damage brain cells, as well as Parkinson’s and possibly the ravages of mood disorders, Dr Manji informed a session at the National Alliance for the Mentally Ill annual conference held in Cincinnati in July 2002. Dr Manji also drew attention to the related work of Ron Duman PhD and his team at Yale, who found antidepressants turned on the expression of BDNF. Significantly, Dr Duman and his team have recently found that an infusion of BDNF may produce an antidepressant effect in lab rats.

Dr Manji explained how for the last three decades, neurotransmitters have been the focus of mental health research. But recently, he went on to say, we have been learning that mental illness is much more complicated than that. Nerve cells communicate with each other through neurotransmitters, but do not actually go inside the nerve cell. Rather, they are merely the keys that unlock what is going on inside the neuron, "where all the action is."

“You can mess all you want with serotonin and dopamine, etc,” Dr Manji told his audience at UCLA, “but if you don’t have the appropriate [cell] circuitry in place it’s not going to have any effect.”

According to Dr Manji, there are some 10 different potential targets within the nerve cell that we did not even suspect 10 years ago. Eight of these targets are being actively investigated.

A potential target inside the nerve cell includes protein kinase C (PKC), a signaling pathway that is implicated in nerve cellular excitability. Dr Manji’s team discovered that lithium and Depakote have very similar effects on the PKC system, taking days or weeks to act. A PKC inhibitor, however, may be more direct. The drug Novladex (tamoxifen), used to treat breast cancer, inhibits PKC and has been found to significantly reduce mania scores in one small study. Larger placebo-controlled studies are now underway at the NIMH and at Harvard. If these studies work, he said, we can develop a better PKC inhibitor.

And the ALS drug Rilutek (riluzole), a glutamate inhibitor, has “a remarkable antidepressant effect.”

Forget the neurotransmitters. Clearly "beneath the cell membrane" is the future.

A review article by Dr Manji and his colleagues at the NIMH in the May 2003 Biological Psychiatry goes into far greater detail:

To start, neurotransmitters, particularly the three we are familiar with, are only a small part of the picture in mood disorders, with the new and old generation antidepressants that target them representing at best a partial remedy for many patients. Other places to look involve a number of processes vital to cellular function rather than just mood, separate but interrelated, including:

* Neurotrophins - proteins that mediate neuron survival, inhibit cell death, and modulate synaptic neurotransmitter activity, which include nerve growth factor and brain-derived nerve growth factor (BDNF). It is believed these are secreted from the neuron and bind to specific tyrosine kinase (Trk) postsynaptic receptors. BDNF also acts as both an excitatory and inhibitory molecule through different mechanisms.
* Bcl-2 is a neuroprotective protein whose numbers are multiplied by a cellular signaling cascade that begins with nerve growth factor acting through several intermediate steps on the chemical pathway, MAP kinase. Increased bcl-2 protects neurons from ischemia, free radicals, excessive glutamate, and other catastrophes. The protein may also promote nerve cell growth and regeneration.
* The cAMP-CREB cascade is a signaling pathway that acts on BDNF and plays a role in neurogenesis - new cell growth - in the hippocampus. According to Manji et al, "it is quite plausible that alterations in hippocampal neurogenesis are fundamental to the clinical syndrome of depression."

Dr Manji and other scientists such as Ron Duman PhD at Yale have discovered that antidepressants or lithium indirectly enhanced one or more of the above processes in rats, leading researchers to speculate that much of the benefit of our medications may lie well downstream of the three neurotransmitters we know so well. This raises the possibility of formulating compounds with a direct action on novel targets, as well as taking a second look at what we already have in the stockpile.

One such target could be PDE, the enzyme responsible for breaking down cAMP. One "PDE inhibitor," rolipram, already exists, with rapid onset of action but with burdensome side effects. Second generation drugs are in development. Also under investigation is BDNF, experimentally administered to patients with ALS. A gleam in the eyes of researchers are “bcl-2 enhancers.” Pramipexole, a Parkinson’s drug acting mainly on dopamine, has also been found to increase bcl-2 in several brain areas, and shows antidepressant effects, according to preliminary studies. Larger studies at the NIMH are underway.

The trouble with current antidepressant treatment, the authors conclude, lies in the faulty assumption that our cell circuitry is intact and will faithfully relay meds-enhanced neurotransmitter activity to their intended targets. In fact, we are discovering just the opposite, that some of our brain cells take the kind of physical beating that necessitates “both trophic and neurochemical support” to restore neuronal connectivity and molecular signaling.

One day, cAMP, bcl-2, BDNF, and the rest may be as familiar to us as serotonin is today, not as academic curiosities, but as the targets of new drugs that promise to radically improve our lives. Bring on the CREBzac.

Updated June 11, 2003, reviewed Feb 11, 2008