Pharmacology of antidepressants

The earliest and probably most popular scientific theory of antidepressant action is the monoamine hypothesis (which can be traced back to the 1950s), which states that depression is due to an imbalance (most often a deficiency) of the monoamine neurotransmitters (namely serotonin, norepinephrine and dopamine). It was originally proposed based on the observation that certain hydrazine anti-tuberculosis agents produce antidepressant effects, which was later linked to their inhibitory effects on monoamine oxidase, the enzyme that catalyses the breakdown of the monoamine neurotransmitters. All antidepressants that have entered the market before 2011 had the monoamine hypothesis as their initial theoretical basis, with the possible exception of agomelatine, which acts on a dual melatonergic-serotonergic pathway.

Despite the success of the monoamine hypothesis, it has a number of limitations: for one, all monoaminergic antidepressants have a delayed onset of action of at least a week; and secondly, there are a sizeable portion (>40%) of depressed patients that do not adequately respond to monoaminergic antidepressants. Further evidence to the contrary of the monoamine hypothesis are the recent findings that a single intravenous infusion with ketamine, an antagonist of the NMDA receptor — a type of glutamate receptor — produces rapid (within 2 hours), robust and sustained (lasting for up to 2 weeks) antidepressant effects. Monoamine precursor depletion also fails to alter mood. To overcome these flaws with the monoamine hypothesis a number of alternative hypotheses have been proposed, including the glutamate, neurogenic, epigenetic, cortisol hypersecretion, and inflammatory hypotheses. Another hypothesis which could explain the delay suggests monoamines don't directly influence mood, but influence emotional perception biases.

Monoamine hypothesis

In 1965, Joseph Schildkraut published a review article stating that several researchers had found an association between depression and deficiency of the catecholamine family of monoamine neurotransmitters, which they had begun calling the "catecholamine hypothesis", also known as the monoamine hypothesis.

By 1985, the monoamine hypothesis was mostly dismissed until it was revived with the introduction of selective serotonin reuptake inhibitors (SSRIs) through the successful direct-to-consumer advertising, often revolving around the claim that SSRIs correct a chemical imbalance caused by a lack of serotonin within the brain.

Serotonin levels in the human brain are measured indirectly by sampling cerebrospinal fluid for its main metabolite, 5-hydroxyindole-acetic acid, or by measuring the serotonin precursor, tryptophan. In one placebo controlled study funded by the National Institutes of Health, tryptophan depletion was achieved, but they did not observe the anticipated depressive response. Similar studies aimed at increasing serotonin levels did not relieve symptoms of depression. At this time, decreased serotonin levels in the brain and symptoms of depression have not been linked

Although there is evidence that antidepressants inhibit the reuptake of serotonin, norepinephrine, and to a lesser extent dopamine, the significance of this phenomenon in the amelioration of psychiatric symptoms is not known. Given the low overall response rates of antidepressants, and the poorly understood causes of depression, it is premature to assume a putative mechanism of action of antidepressants.

While monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants (TCAs), and SSRIs increase serotonin levels, others prevent serotonin from binding to 5-HT_2Areceptors, suggesting it is too simplistic to say serotonin is a "happy neurotransmitter". In fact, when the former antidepressants build up in the bloodstream and the serotonin level is increased, it is common for the patient to feel worse for the first weeks of treatment. One explanation is that 5-HT_2A receptors evolved as a saturation signal (people who use 5-HT_2A antagonists often gain weight), telling the animal to stop searching for food, a mate, etc., and to start looking for predators. In a threatening situation, it is beneficial for the animal not to feel hungry even if it needs to eat. Stimulation of 5-HT_2A receptors will achieve that. But if the threat is long lasting, the animal needs to start eating and mating again - the fact that it survived shows that the threat was not as dangerous as perceived. So the number of 5-HT_2A receptors decreases through a process known as downregulation and the animal goes back to its normal behavior. This suggests that there are two ways to relieve anxiety in humans with serotonergic drugs: by blocking stimulation of 5-HT_2A receptors or by overstimulating them until they decrease via tolerance.

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Hypothalamic-pituitary-adrenal axis

One manifestation of depression is an altered hypothalamic-pituitary-adrenal axis (HPA axis) that resembles the neuro-endocrine (cortisol) response to stress, that of increased cortisol production and a subsequent impaired negative feedback mechanism. It is not known whether this HPA axis dysregulation is reactive or causative for depression. A 2003 briefing suggests that the mode of action of antidepressants may be in regulating HPA axis function.

A 2011 study combines aspects of the HPA axis theory and the neurogenic theory (see below). The researchers showed that mice under unpredictable chronic mild stress (a well-known animal model of depression) have impaired hippocampal neurogenesis and greatly reduced ability of the hippocampus to regulate the HPA axis, causing anhedonia as measured by the Cookie Test. Administration of fluoxetine (an SSRI) without removing the stressor causes increased hippocampal neurogenesis, normalization of the HPA axis, and improvement of anhedonia. If X-ray irradiation is used on the hippocampus before drug treatment to prevent neurogenesis, no improvement of anhedonia occurs. However, if an irradiated mouse is given a corticotropin-releasing factor 1 antagonist – a drug that directly targets the HPA axis – anhedonia is improved. Combined with the fact that irradiation without stressing does not impair hippocampal control of the HPA axis, the authors conclude that fluoxetine works by improving hippocampal neurogenesis, which then helps restore the HPA axis, in turn leading to improvements in depression symptoms such as anhedonia.

Neurogenic adaptations

The neurogenic hypothesis states that molecular and cellular mechanisms underlying the regulation of adult neurogenesis are required for remission from depression and that neurogenesis is mediated by the action of antidepressants. A broader view is that antidepressants help by increasing neuroplasticity in general.

Chronic use of SSRI antidepressant increased neurogenesis in the hippocampus of rats and mice. Other antidepressant treatments also appear associated with hippocampal neurogenesis and/or neuroplasticity: electroconvulsive therapy, which is known to be highly effective for depression, is associated with higher brain-derived neurotrophic factor (BDNF) expression in the hippocampus as well as global rewiring; lithium and valporate, two mood stabilizers occasionally used as add-on treatment, are associated with increased survival and proliferation of neurons. Ketamine (see also esketamine), a new fast-acting antidepressant, can increase the number of dendritic spines and restore aspects of functional connectivity after a single infusion.

Other animal research suggests that long term drug-induced antidepressants effects modulate the expression of genes mediated by clock genes, possibly by regulating the expression of a second set of genes (i.e. clock-controlled genes).

The delayed onset of clinical effects from antidepressants indicates involvement of adaptive changes in antidepressant effects. Rodent studies have consistently shown upregulation of the 3, 5-cyclic adenosine monophosphate (cAMP) system induced by different types of chronic but not acute antidepressant treatment, including serotonin and norepinephrine uptake inhibitors, MAOs, TCAs, lithium, and electroconvulsions. cAMP is synthesized from adenosine 5-triphosphate (ATP) by adenylyl cyclase and metabolized by cyclic nucleotide phosphodiesterases (PDEs).

Studies on human patients have used imaging approaches to measure the changes in density and volume of specific brain areas. The grey matter volume of parts of the brain are differently increased or decreased by SSRI use. It appears possible to use brain imaging to predict which patients are likely to respond to SSRI antidepressants.

Anti-inflammatory and immunomodulation

Recent studies show pro-inflammatory cytokine processes take place during clinical depression, mania and bipolar disorder, and it is possible that symptoms of these conditions are reduced by the pharmacological effect of antidepressants on the immune system.

Studies also show that the chronic secretion of stress hormones as a result of disease, including somatic infections or autoimmune syndromes, may reduce the effect of neurotransmitters or other receptors in the brain by cell-mediated pro-inflammatory pathways, thereby leading to the dysregulation of neurohormones. SSRIs, serotonin-norepinephrine reuptake inhibitors (SNRIs) and TCAs acting on serotonin, norepinephrine and dopamine receptors have been shown to be immunomodulatory and anti-inflammatory against pro-inflammatory cytokine processes, specifically on the regulation of interferon-gamma (IFN-gamma) and interleukin-10 (IL-10), as well as TNF-alpha and interleukin-6 (IL-6). Antidepressants have also been shown to suppress TH1 upregulation.

Antidepressants, specifically TCAs and SNRIs (or SSRI-NRI combinations), have also shown analgesic properties. Antidepressants are now standard of care in chronic neuropathic pain. Antidepressants also work in mouse models of both neuropathic and inflammatory pain. In addition, tramadol is an opioid with SNRI-like activity; it retains some analgesic potency when administered with the opioid antagonist naloxone, indicating a role of its non-opioid activities.

These studies warrant investigation for antidepressants for use in both psychiatric and non-psychiatric illness and that a psycho-neuroimmunological approach may be required for optimal pharmacotherapy. Future antidepressants may be made to specifically target the immune system by either blocking the actions of pro-inflammatory cytokines or increasing the production of anti-inflammatory cytokines.

Pharmacological data

Receptor affinity

A variety of monoaminergic antidepressants have been compared below:

The values above are expressed as equilibrium dissociation constants in nanomoles/liter. A smaller dissociation constant indicates more affinity. SERT, NET, and DAT correspond to the abilities of the compounds to inhibit the reuptake of serotonin, norepinephrine, and dopamine, respectively. The other values correspond to their affinity for various receptors.

Pharmacokinetics

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