Medchem Module 5

Autoreceptors

• Autoreceptors are typically located on the presynapse, exerting effects opposite to those of postsynaptic receptors.
• Functions include modulating neurotransmitter release and synthesis.

Receptor Regulation

• Receptor regulation involves changes in receptor function due to under- or overstimulation.
• Repeated drug exposure can alter receptor mechanisms, prompting signaling pathways to adjust and restore normal function.
• Drug tolerance and sensitization can arise from under- and overstimulation.

Review of Neurotransmission

• Essential components include the synapse, neurotransmitter release, storage, and synthesis in the presynapse.
• Also includes receptor binding and second messenger signaling in the postsynapse.
• Understanding specifics of dopamine, serotonin, and norepinephrine neurotransmission is crucial.
• Tyrosine hydroxylase is the rate-limiting enzyme for DOPA synthesis, a key factor in dopamine production.
• Neurotransmission involves vesicle fusion, neurotransmitter release, and calcium level changes.

Case Study: Dopamine Synapse

Presynaptic Events

• Action potentials trigger the opening of voltage-sensitive calcium channels.
• Increased calcium influx into the presynaptic terminal.
• Increased calcium leads to an increase in the formation of dopamine vesicles.
• Dopamine is released into the synapse
• Glutamate activates presynaptic NMDA receptors, further increasing calcium influx.
• Activation of Gs-coupled adrenergic receptors increases adenylate cyclase activity, leading to increased cyclic AMP and protein kinase A activation.
• Protein kinase A activates vesicle-moving proteins.
• Activation of Gi-coupled D2 receptors inhibits adenylate cyclase, decreasing cyclic AMP and reducing vesicle movement.
• Dopamine reuptake and binding to D2 autoreceptors activates Gi coupling, inhibiting cyclic AMP production and dopamine vesicle formation.
• D2 autoreceptors decrease cyclic AMP, affecting tyrosine hydroxylase and reducing the conversion of tyrosine to DOPA, decreasing dopamine synthesis.
• D2 receptors interact with potassium channels, increasing potassium outflow and causing an inhibitory postsynaptic potential.

Postsynaptic Events

• Dopamine binds to postsynaptic D1 receptors.
• D1 receptor activation increases adenylate cyclase activity, leading to protein kinase A activation.
• A signaling cascade is initiated, and protein kinase A phosphorylates GABA receptors, sensitizing or inhibiting them.
• Dopamine can be metabolized, halting the signal

Regulatory Mechanisms & Feedback Loops

• D2 receptors act as homologous autoreceptors, modulating dopamine release.
• Autoreceptors are generally inhibitory, reducing further dopamine release when dopamine levels are high.
• Heterologous receptors for other neurotransmitters (e.g., glutamate, GABA, norepinephrine) can modulate dopamine release.
• Synaptic plasticity ensures homeostasis is maintained.

Autoreceptor Details

• A homologous receptor, or autoreceptor, influences its own release.
• Autoreceptors are typically inhibitory.
• Autoreceptors exist for nearly all neurotransmitters like norepinephrine, GABA-B, and adrenergic receptors.
• Glutaminergic terminals from the NMDA receptor and adrenergic receptors can stimulate dopamine release through calcium and adenylate cyclase.
• Postsynaptically, dopamine binding to dopamine receptors activates adenylate cyclase and cyclic AMP, activating protein kinase A, which can phosphorylate GABA receptors.

Location of Autoreceptors

• Presynaptic D2 receptors decrease cyclic AMP through Gi coupling and decrease protein kinase A, reducing dopamine vesicle formation.
• Other D2 autoreceptors decrease cyclic AMP, affecting the tyrosine hydroxylase enzymatic rate, affecting dopamine synthesis.
• Autoreceptors on the cell body interact with Gi to inhibit adenylate cyclase, interact with calcium channels to decrease calcium signaling, and interact with potassium channels. Activation of these autoreceptors results in more potassium, leading to an inhibitory postsynaptic potential and decreasing action potential probability.

Autoreceptor Effects

• Autoreceptors affect cyclic AMP, calcium, and potassium pathways.
• The downstream target can be vesicle movement, action potential, or a synthetic pathway.
• Most autoreceptors are inhibitory, but some serotonin autoreceptors can stimulate the neuron.

Receptor Regulation

• Receptors undergo a balance between internalization and replenishment.
• Neurotransmission binding to receptors can change their affinity state or number via internalization.
• In an understimulated terminal, the system upregulates recycling to the membrane, increasing Bmax.
• In an overstimulated case, the system internalizes receptors and shuts down recycling to downregulate the system, decreasing the Bmax.
• Understimulation increases Bmax, while overstimulation decreases Bmax.
• The system normalizes the signal in both understimulated and overstimulated scenarios.

Antagonists and Agonists

• In an understimulated synapse, an antagonist blocks receptors, shutting down stimulation, and the system upregulates receptors to overcome the block, increasing Bmax.
• In an overstimulated case, an agonist stimulates receptors, and the system internalizes receptors and does not recycle them, lowering Bmax.

Drug Tolerance

• Drug tolerance can occur with both understimulation (antagonist) and overstimulation (agonist).
• The system changes to counter the antagonist or combat the agonist.

Synaptic Plasticity

• The system adapts to normalize the signal by changing synthesis, storage, and release pathways.
• Postsynaptically, it considers second messengers, receptor binding, second messenger signaling, and metabolism.
• Synaptic plasticity ensures homeostasis and maintains balance.

Mechanism of Drug Tolerance

• The system has plasticity to respond to varying neurotransmitter stimulation via synthesis, storage, and release changes presynaptically.
• Postsynaptically, receptor binding, second messenger amplification, and metabolism are adjusted.

Downregulation of a Signal

• In an overstimulated system, mechanisms to downregulate an overstimulated system include decreasing receptor binding (Bmax and Kd).
• Also includes decreasing second messenger activity, increasing metabolism, and pre-synaptic changes like decreasing neurotransmitter synthesis and release.
• Another aspect includes increasing neurotransmitter metabolism, activating autoreceptors, and increasing re-uptake.
• These changes normalize the signal in response to overstimulation and contribute to drug tolerance.

Upregulation of a Signal

• When understimulated, the system upregulates by increasing receptor binding (Bmax, affinity), increasing second messenger activity.
• Also includes decreasing metabolism, increasing synthesis and release, decreasing autoreceptor effects, and decreasing re-uptake.
• The outcome is that these changes normalize the signal in response to understimulation and contribute to drug tolerance.

Receptor Regulation: Intrinsic Regulation

• Receptors regulate themselves through various events and are subject to homeostatic controls.
• Repeated drug use leads to pharmacodynamic changes normalizing the signal but causing drug tolerance.
• Disease states alter receptor number and function, complicating intrinsic regulation.

Drug Tolerance

• Tolerance occurs when larger doses of a drug are needed to produce the same effect.
• Tolerance can be pharmacodynamic (receptor-related) or pharmacokinetic (clearance-related).
• Physiological and behavioral adaptations can also contribute.

Response Curve and Affinity Curve

• In a one-to-one molecule interaction, the response curve and affinity curve overlap, and EC50 = Kd.
• Response can differ from affinity due to intrinsic efficacy and spare receptors.
• Receptor regulation (changes in Bmax) can shift the affinity curve.

Ligand Concentration and Receptor Occupancy

• Drug duration of action is influenced by the off rate (Koff).
• Kd = Koff/Kon.
• Required receptor occupancy for a maximal response determines whether changes in off rate are beneficial.

Adverse Drug Reactions

• An ADR is a non-preventable adverse event occurring with the intended use of a drug.
• Side effects are well-known ADRs with varying harm.
• MEDWATCH is the FDA's gateway for safety information on medical products.
• FAERS Database is a reporting system where healthcare providers can report ADRs.
• FAERS is populated by healthcare providers, consumers, advocates, and drug manufacturers.
• SIDER 4.1 is a European version of FAERS.
• Caution is advised due to reporting bias and non-specific information in the FAERS database.

Survey of ADRs Using the FAERS Database

COX-2 Inhibitors (Vioxx)

• Vioxx, a COX-2 inhibitor, was associated with an increased risk of myocardial infarction (10% of cases).
• COX-2 selective drugs were initially thought to avoid the side effects of COX-1 inhibition.
• Post-approval use showed an increased risk of clots, stroke, and heart attack.
• COX-2 is beneficial in endothelial cells because it generates prostacyclin, promoting vasodilation and decreasing clotting.
• Vioxx blocks the beneficial effect, increasing the risk of clots and heart attacks.
• The FDA discontinued Vioxx due to the risk outweighing the benefit.

Serotonin Reuptake Inhibitors (SSRIs)

• SSRIs can cause QT prolongation, which is the delay in repolarization of heart cells.
• SSRIs can block hERG potassium channels, disrupting the contraction-relaxation cycle.
• This can lead to irregular heartbeat rhythm, potentially leading to tachycardia, heart attack, and death.
• Serotonin syndrome: SSRIs block serotonin reuptake channels, leading to excess serotonin and hyperstimulation of serotonin receptors.
• Approximately 11,000 out of 15,000 cases in the FAERS database for serotonin syndrome are reported for SSRIs.
• Sertraline and fluoxetine are specific branded drugs with considerable risk.
• Varies from mild to severe, affecting neuromuscular action and mental state, with severe cases including hyperthermia and respiratory failure.

Convulsive Ergotism

• Convulsive ergotism is similar to serotonin syndrome.
• In the medieval world, fungal infections in grain led to consumption of excess serotonin, resulting in symptoms similar to serotonin syndrome.

Zantac (Ranitidine) and Breast Cancer

• Zantac, an over-the-counter antacid, has been linked to an increased incidence of breast cancer.
• The high risk of cancer is due to process impurities in manufacturing and the generation of nitrosamines (carcinogens) under acidic conditions.

Opioids

• Nearly 300,000 Americans died from opioid overdoses between 1999 and 2021.
• Physical dependence is a major ADR associated with opiates.
• Oxycodone is the most abused drug that causes dependence.
• Opioid receptors can directly lead to increased insulin synthesis and secretion from beta cells of the pancreas.
• Fentanyl is at the top of the list in terms of the number of cases of respiratory depression.
• Respiratory depression and cardiovascular depression, hallucinations, sedation, and addiction/dependence.
• Opioids activate opioid receptors, leading to downregulation or inhibition of cyclic AMP levels, which mutes the excitation of neurons.
• Opioid receptor-dependent and independent mechanisms lead to respiratory depression.

Reporting ADRs and Research

• Reporting ADRs is important because it leads to researchers addressing why and how it is tied to a specific drug.
• Investigators are looking at the signaling downstream of the opioid receptor, including G proteins and beta-arrestin signaling.
• The goal is to identify specific pathways responsible for respiratory depression to selectively prevent it from happening.
• ADRs drive new research into attenuating those effects and making drugs better.

Future Directions: Machine Learning and AI

• Machine learning can help predict ADRs before a drug is administered to many patients.
• Novartis used the FAERS and SIDER databases to look at adverse drug reactions reported.
• They connected the off-targets to the adverse drug reaction.
• PDE3 is very well represented in ADRs as one of the major ADR-causing targets.
• hERG channel is a known connect to the ADR, which is QT prolongation and heart attack.