Summary
Highlights
The video begins by defining depression as unusually long periods of sadness, guilt, and decreased interest in activities, sometimes occurring alone or as part of bipolar disorder. It introduces antidepressant drugs, which work by elevating neurotransmitter levels, leading to the monoamine hypothesis of depression involving deficiencies in serotonin, norepinephrine, and dopamine. Other hypotheses include postsynaptic receptor upregulation and abnormal gene functioning.
Antidepressants are categorized into five classes: Selective Serotonin Reuptake Inhibitors (SSRIs), Serotonin Norepinephrine Reuptake Inhibitors (SNRIs), Tricyclic Antidepressants (TCAs), Monoamine Oxidase Inhibitors (MAOIs), and Atypical Antidepressants. The video illustrates the mechanisms of neurotransmitter release, reuptake (via SERT and NET), and breakdown by monoamine oxidase (MAO) in presynaptic serotonergic and noradrenergic neurons.
SSRIs inhibit serotonin reuptake by blocking the serotonin transporter (SERT), increasing serotonin levels in the synapse. Examples include Citalopram, Fluoxetine, and Sertraline. SSRIs are used for depression, anxiety, PTSD, and OCD. The delay in effect is explained by the movement of G-proteins out of lipid rafts, allowing better signaling. Side effects include insomnia, anxiety, sexual dysfunction, nausea, and withdrawal symptoms.
SNRIs inhibit the reuptake of both serotonin and norepinephrine by blocking SERT and the norepinephrine transporter (NET). Examples include Venlafaxine and Duloxetine. They are used for depression, anxiety, panic disorders, and pain associated with fibromyalgia and neuropathy. Side effects are similar to SSRIs but can also include increased blood pressure and heart rate due to noradrenergic activity.
TCAs, named for their three-ring structure, inhibit the reuptake of serotonin and norepinephrine, though with varying selectivity. They also block alpha, histamine, and muscarinic receptors, which contribute to their side effects rather than their antidepressant activity. Examples include Amitriptyline and Imipramine. TCAs are used for depression, migraine prevention, neuropathic pain, and insomnia. Side effects include orthostatic hypotension, sedation, anticholinergic effects, and cardiac conduction abnormalities.
MAOIs inhibit monoamine oxidase enzymes (MAO-A and MAO-B), preventing the breakdown of monoamine neurotransmitters and increasing their availability. MAO-A inhibition is responsible for antidepressant effects. Examples include Isocarboxazid and Phenelzine. Selegiline, a selective MAO-B inhibitor, is used for Parkinson's disease. MAOIs are a last-line treatment due to significant drug-drug and drug-food interactions, particularly with tyramine-rich foods, which can lead to hypertensive crisis.
This class includes agents with diverse mechanisms. Bupropion inhibits norepinephrine and dopamine reuptake and is also used for nicotine cravings. Mirtazapine is an alpha-2 receptor antagonist, increasing noradrenergic and serotonergic neurotransmission, and has antihistaminic effects. Trazodone and Nefazodone inhibit serotonin reuptake and block postsynaptic serotonin 2a receptors, with sedative effects due to antagonism of histaminic H1 and adrenergic alpha-1 receptors. Vilazodone is a serotonin partial agonist reuptake inhibitor. Vortioxetine's mechanism involves serotonin reuptake inhibition and modulation of various serotonin receptors.
Lithium, a mood-stabilizing drug, is primarily used for bipolar disorder despite its narrow therapeutic index and risk of toxicity. Its exact mechanism is unknown but proposed mechanisms include inhibiting the recycling of neuronal membrane inositol lipids, thereby decreasing cellular responses to neurotransmitters linked to this second messenger system. Lithium also inhibits glycogen-synthase-kinase-3 (GSK3), mimicking the Wnt protein signaling pathway, which is relevant as abnormal GSK3 activation is linked to bipolar disorder. Lithium affects all major neurotransmitter systems.