3.3.3 Principle second messenger systems

Knowledge of the Principle Second Messenger Systems Related to Neurotransmitters

The second messenger systems are essentially small ions and molecules that rely upon the signal gathered from receptors present on the cell surface to the protein effector. The main function of the second messenger system is to receive the signal from the first messenger. The second messengers are intracellular and the first messengers are extracellular. Second messengers trigger physiological changes at a cellular level such as apoptosis, depolarization, differentiating, migration and proliferation. They are one of the triggers of intracellular signal transduction cascades.

Second messenger systems have the ability to pass along a chemical message from the first messenger (neurotransmitter) from the cell membrane toward the internal cell machinery. The activation process of the second messenger system includes a chemical energy source from the cell which is adenosine triphosphate (ATP). 

When noradrenaline is attached to the relevant receptors on the neuronal surface the activated receptors will attach a G-protein on the inside surface of the membrane. Activated G-protein acts in a way that causes the adenylyl cyclase enzyme with ATP to convert into cyclic adenosine monophosphate (cyclic AMP) the second messenger. Instead of working between neurons, in the synaptic cleft cAMP exerts changes to the ion channels in order to make changes in gene expression.

GABA receptors amplify the second messenger pathway providing neuronal transmission with an inhibitory signal.

Second messengers are involved in the development and growth processes. The second messenger’s direct effect on genetic messenger may lead to a cell alteration of its cellular functioning.

Neurotransmitters Related to Basic Neuronal Homeostasis and Plasticity

Neurotransmitters are small molecules that play a crucial role in transmitting information between neurons. They are synthesized and stored in the nerve terminals and are released into the synaptic cleft in response to an action potential. The neurotransmitter then binds to specific receptors on the postsynaptic neuron, triggering a series of events that lead to changes in the membrane potential and the transmission of an electrical signal.

Neuronal homeostasis:

Neurotransmitters play an important role in maintaining basic neuronal homeostasis, which refers to the stable and consistent functioning of the nervous system. Some neurotransmitters, such as GABA and glycine, have an inhibitory effect on the neurons, helping to regulate the excitability of the nervous system and maintain basic homeostasis.

Homeostatic changes in glutamate and GABA receptors have been found to occur on excitatory cortical neurons during insomnia. The role of homeostasis in serotonin is associated with behavioural activities, clinical disorders, and biological systems. The serotonergic neurons are found to be involved in these processes. Homeostasis of dopamine is also thought to play a role in neuronal health. Dysfunctions in dopamine homeostasis have been linked to a range of neurological disorders, including depression, addiction, and neurodegeneration (del Cid-Pellitero, 2017).

Plasticity:

Neurotransmitters also play a role in neuronal plasticity, which refers to the ability of the nervous system to change and adapt in response to new experiences and information. For example, the neurotransmitter dopamine is involved in reward-driven learning and is involved in the regulation of motivation, pleasure, and reward. Other neurotransmitters, such as glutamate and acetylcholine, are involved in synaptic plasticity, the ability of the synapses to change and strengthen over time in response to new experiences.

Long-term synaptic plasticity involves both dopamine and glutamate. Dopamine is a key neuromodulator that plays a significant role in reward-driven learning and motivation. It adjusts the synaptic dynamics by modifying the integration mechanism and synaptic transmission, which affects intrinsic properties of neural systems such as the probability of neurotransmitter release, membrane excitability, protein trafficking, gene transcription, and receptor response to neurotransmitters. However, dysfunction in dopamine can result in cognitive disorders (Kuhar, M.J., 1975).

In summary, neurotransmitters play a crucial role in both basic neuronal homeostasis and plasticity. They help to regulate the excitability of the nervous system, maintain stability, and support the ability of the nervous system to change and adapt in response to new experiences and information.

References:

(1) del Cid-Pellitero, E., Plavski, A., Mainville, L. and Jones, B.E., 2017. Homeostatic changes in GABA and glutamate receptors on excitatory cortical neurons during sleep deprivation and recovery. Frontiers in systems neuroscience, 11, p.17.

(2) Kuhar, M.J., 1975. Neurotransmitter uptake: a tool in identifying neurotransmitter-specific pathways. Minireviews of the Neurosciences from Life Sciences, pp.263-274.

(3) Pollard TD, Earnshaw WC, Lippincott-Schwartz J, Johnson G, eds. (2017-01-01). “Second Messengers”. Cell Biology (3rd ed.). Elsevier Inc. pp. 443–462.