Question

Diagram the activation of the 5-HT Serotonin Receptor (G-Protein Coupled Receptor) leading to transcriptional activation in a neuron. Include signaling pathways you have learned about in lecture or in your reading that involve G-Protein Coupled Receptors and lead to gene activation. Demonstrate how Lithium acts to disrupt one of these signaling pathways.

Answer 1

Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter that influences multiple processes, including autonomic function, motor activity, hormone secretion, cognition, and complex processes associated with affection, emotion, and reward 1.

In the terminal axon of the serotonergic neuron, free tryptophan (TRP) is converted to 5-HT 2. 5-HT synthesis is a two-step process catalyzed by tryptophan hydroxylase (TPH) and aromatic decarboxylase (DDC). TPH is the rate-limiting enzyme and exists in two isoforms TPH1 and TPH2. The TPH2 isoform is the predominant form in neuronal tissue 3, 4. The 5-HT uptake into presynaptic storage vesicles is mediated by the vesicular monoamine transporter (SLC18A2). The transporter accumulates serotonin into synaptic vesicles using a proton gradient across the vesicular membrane 5. 5-HT that is not stored in vesicles is degraded by monoamine oxidase A (MAOA) to 5-hydroxyindoleacetic acid (5-HIAA).

An action potential stimulates a calcium-dependent exocytotic release of serotonin from presynaptic vesicles into the synaptic cleft, where it interacts with both post- and presynaptic receptors. At the presynaptic side, 5-HT activates 5-hydroxytryptamine (serotonin) receptor 1A (HTR1A), B (HTR1B), and D (HTR1D), which results in an attenuation of the 5-HT exocytosis 2. This feedback loop regulates the 5-HT concentration in the synaptic cleft and therefore, the extent of stimulation of various HTR receptor subclasses at the postsynaptic membrane 6. Prolonged administration of selective serotonin reuptake inhibitors (SSRI) desensitizes these feedback loops. Thus, their regulatory effects on the serotonergic neurotransmission are weakened 7. Postsynaptic HTR1 receptors (HTR1A, HTR1B, HTR1D, HTR1E, HTR1F) work together with HTR2 receptor subtypes (HTR2A, HTR2C) in mediating effector signals via activation of second messenger cascades 2. Postsynaptic, the main signaling pathway for HTR1 receptor subtypes is via coupling of Gi/o protein alpha subunit (GNAI). This interaction decreases cyclic AMP formation by inhibiting adenylate cyclases (ADCY) 8. After interaction with 5-HT, the main signaling linkage for the HTR2 receptor subpopulation is to activate phospholipase C (PLCB) through coupling of Gq/11 protein alpha (GNAQ) 8. PLCB catalyzes the formation of myoinositol- 1, 4, 5-trisphosphate (IP3) and diacylglycerol (DAG) 9.The postsynaptic, ionotropic HTR3 receptor is a cation-specific ligand-gated ion channel, which does not activate a second messenger system 10. The binding of 5-HT to this receptor depolarizes the postsynaptic membrane by sodium influx and potassium efflux, which is assumed to influence the activation of HTR2 receptors. HTR4, HTR6, and HTR7 primarily couple Gs protein alpha (GNAS), which results in an activation of adenylate cyclase, and consequently in an increase of cyclic AMP levels 9. Further operational diversity is supported by the existence of a great number of splice and editing variants for several HTRs, their possible modulation by accessory proteins and chaperones, as well as their potential to form homo or heteromers both at the GPCR and at the ligand-gated channel level 11.

The amplification of all those second messenger signals in further downstream reactions leads to the mediation of neurotransmitter release from central serotonergic, noradrenergic, and dopaminergic neurons in the brain by regulating potassium channels, several protein kinases, and other calcium dependent signals.

Chronic administration of antidepressant treatments have been reported to commonly increase the expression of brain-derived neurotrophic factor (BDNF), an activity-dependent secreted protein that is critical to organization of neuronal networks and synaptic plasticity 12, 13.

The solute carrier family 6 (neurotransmitter transporter, serotonin), member 4 (SLC6A4) is responsible for terminating the action of 5-HT in the synaptic cleft. Released serotonin is transported back into the presynaptic terminals via this integral membrane protein. SLC6A4 is a member of the Na+/Cl--dependent transporter family 14.