The Cholinergic Hypothesis of Depression?

Come the year 2012 there could be a new antidepressant with a novel mechanism of action on the market in these United States (1). As the drug is still in development, it is known as "TC-5214."

According the the press release, TC-5214 is a "nicotinic channel blocker that is thought to treat depression by acting on neuronal nicotinic receptors, or NNRs, according to Targacept. Targacept says NNRs are found on nerve cells throughout the nervous system and regulate nervous system activity."

This is the first I've heard the term "neuronal nicotinic receptors." In all my texts they are referred to as nicotinic cholinergic receptors (nAChRs). In common parlance, they are simply referred to as "nicotinic receptors." My bias leads me to believe that this is the term that polled best in a focus group as being both "sciency" sounding and "catchy." But I digress...

In this abstract (2), this rationale is put forward, "based on the notion that the depressive states involve hypercholinergic tone, we have examined the potential palliative role of NNR antagonism in these disorders, using TC-5214" (my emphasis).

I have never heard this "cholinergic" hypothese of depression before.

They add, "TC-5214 demonstrated positive effects in a number of animal models of depression and anxiety... forced swim test, a classical depression model....behavioral despair test ... the social interaction paradigm, a model of generalized anxiety disorder...the light/dark chamber paradigm , a model of GAD and phobia."

Take this for what it is worth. I hold almost no faith in animal studies (3) since the animal models simply cannot mimic the complexities of human mental illness and that the majority of drugs that pass animal trials fail to generate positive result in humans.

And just like many other theories of depression, a complex mental illness can be boiled down to a single receptor, "the antidepressant and anxiolytic effects seen in these studies are likely attributable to antagonist effects at the α4β2."

Since so much has been written about serotonin, norepinephrine, and dopamine, I've decided to dedicate the rest of post to acetylcholilne, the proposed "cause" of depression that this new drug will treat. (NERD ALERT: If you find this kind of stuff boring, stop reading now!)

Acetylcholine (ACh) is synthesized in one step (as opposed to the multiple steps required for the catecholamines). As you can see in the above image, there are two precursors: choline and acetyl coenyzme A (acetyl CoA). Choline is primarily derived from the fat in our daily diets. Acetyl CoA is produced within the cell by way of fat and sugar metabolism. The synthesis of ACh is catalyzed by the enzyme choline acetyltransferase (ChAT), which does as the name implies, it transfers the acetyl from CoA to choline to form ACh. ChAT is present in the cytoplasm of neurons that use ACh as their neurotransmitter.

Acetylcholinesterase (pictured above) breaks down ACh into choline and acetic acid (the acid that gives vinegar its smell and taste, 4). AChE is found within the presynaptic cell to metabolize excess ACh, on the membrane of the postsynaptic cell to break down ACh released into the synaptic cleft. It's also found in the neuromuscular junction (where PNS nerves stimulate muscles).

Cholinergic neurons play important roles in both the central and peripheral nervous systems (CNS & PNS). ACh neurons modulate both the sympathetic branch and the parasympathetic branches of the PNS. In the brain, the main neuclei that produce ACh are clustered in only a few areas. The first major pathway originates in the dorsal tegmental areas and projects to the thalamus. This pathway is part of the "reticular activating system," which govern the arousal level and alertness. Next is the septal nucleus that projects to the fornix and terminates in the hippocampus (ACh plays a prominent role in long-term memory formation). Next is the "ACh forebrain complex," which includes three "bands," the largest being the nucleus basalis of Meynert. These projections go all through the cortex and amygdala, the olfactory bulbs and the vestibular-cochlear nerve (important in balance).

There are two acetylcholine receptor subtypes, muscarinic and nicotinic.

Nicotinic receptors are highly concentrated on muscle cells, ganglionic neurons in the PNS, and on certain brain neurons. They are ionotropic (made up of an ion channel). When ACh binds to this receptor, sodium (Na+) and calcium (Ca2+) rush into the cell, causing depolorization, and increasing the cell's excitability. These receptors also enhance the release of other neurotransmitters.

The nicotinic receptor (above) is comprised of five proteins that form a channel. The subunits are label with Greek letters: beta, gamma, sigma, and two alpha subunits (ACh needs to bind to both of these to open the channel up). The structure, however, of the brain nicotinic neurons and muscles neurons are different, leading to different pharmacological difference between the two (see below).

The drug TC-5214 reportedly binds to neuron type three at the alpha4, beta2 subunit. (Contrary to what Wikipedia says, 5, this drug has NOT been tested in MDD patients).

Muscarinic receptors, conversely, are metabotropic (similar to monoamine NTs). So far, 5 muscarinic receptors have been discovered (M1 to M5), with some being excitatory and others inhibitory. Receptors are widely distributed throughout the brain including the neocortex, hippocampus, thalamus, striatum, and the basal forebrain. Outside of the brain muscarinic receptors are found mainly in cardiac muscle and smooth muscle (such as those found in the bladder). This is the receptor system associated with "anticholinergic syndrome" (6).

ACh's exact role in mood and cognition is still not known. It's associated with long-term memory formation (e.g., Alzheimer's disease) and attention and arousal (e.g., focused attention). Will TC-5214 actually treat depression? I doubt it. It seems to me, as a nicotinic receptor antagonist, it is better suited for smoking cessation (Chantix loses patent protection in 2012). Either way, it will be interesting to see how this one develops.