Presynaptic inhibition and negative feedback
The regulation of neurotransmitter release into the synaptic cleft occurs via a negative feedback mechanism. This process unfolds on presynaptic membranes and directly depends on the state of ion currents.
The primary physiological effect of this mechanism is a significant reduction in mediator release from nerve endings. This primarily concerns noradrenaline, though similar processes also occur at cholinergic nerve endings where α2-receptors are present. Their stimulation leads to the inhibition of acetylcholine release.
The molecular cascade of presynaptic inhibition includes the following steps:
- Activation of specific Gi/0 proteins occurs.
- This activation entails a marked decrease in the activity of the enzyme protein kinase A.
- The drop in protein kinase A activity triggers the opening of potassium channels.
- Potassium ions (K+) begin to rapidly flow out of the cell into the extracellular space.
- The loss of positively charged ions leads to hyperpolarization of the cell membrane.
Against the background of the developed hyperpolarization, the normal functioning of voltage-gated calcium channels is disrupted. As a result, the influx of Ca2+ ions into the nerve ending is reduced, which is critical for synaptic transmission since calcium is precisely what initiates the reduction of vesicle exocytosis with the neurotransmitter into the synaptic cleft.
Calcium channel blockade in smooth muscle
In addition to participating in synaptic transmission, calcium currents play a key role in maintaining vascular wall tone. A general group of calcium channel blockers, specifically dihydropyridine derivatives, is used for therapeutic intervention in this process.
Their mechanism of action is based on the selective blockade of L-type voltage-gated calcium channels localized in the smooth muscle of blood vessels.
The pharmacodynamic effect of these compounds consists of several sequential reactions:
- The entry of Ca2+ ions into the smooth muscle cell is disrupted.
- A predictable decrease in blood vessel tone occurs.
- Total peripheral vascular resistance (TPVR) decreases.
- Consequently, arterial blood pressure (BP) drops.
In clinical practice, there is a clear priority when choosing drugs from this group. Preference is always given to extended-release medications. They are precisely the ones capable of providing a prolonged and stable therapeutic effect, as well as minimizing the risk of adverse hemodynamic side effects that often arise from a sudden change in vascular tone.
Pharmacological modulation of ion currents
The influence on the electrical potential of cell membranes is actively used in clinical pharmacology, specifically within the ATC classification N03A (antiepileptic drugs). The effect is exerted via two main targets: calcium and sodium channels.
Group 1: Calcium channel modulators The mechanism of action of these drugs consists in modulating the electrical potential of calcium channels, which makes it possible to control excessive neuronal excitability. The main representatives include:
- Pregabalin: used in a daily dosage from 150 to 600 mg.
- Gabapentin: prescribed in a wide dosage range, from 300 to 3600 mg daily.
Group 2: Sodium channel blockers An alternative path to reducing excitability is blocking the electrical potential of sodium channels. This group includes:
- Carbamazepine: standard dosage varies within 200–600 mg/day.
- Oxcarbazepine: baseline daily dose is 600–1200 mg. It is important to note the specific indications for this drug: it is used only for acute sensory (atypical) diabetic polyneuropathy (DPN) and in cases of strict inefficacy of prior therapy. In such severe conditions, the daily dose of oxcarbazepine can be increased up to 2400 mg.
To memorize the presynaptic inhibition cascade: "G-protein lowers kinase, potassium leaves, calcium gets stuck" (activation of Gi/0 protein → decrease in protein kinase A → K+ efflux → hyperpolarization → block of Ca2+ influx).
Class and exam questions
How does the activation of Gi/0 proteins affect the membrane potential?
Activation of Gi/0 proteins decreases protein kinase A activity, leading to the opening of potassium channels. The efflux of potassium ions from the cell causes membrane hyperpolarization.
Why are extended-release calcium channel blockers preferred when reducing blood pressure?
Extended-release drugs provide a prolonged therapeutic effect. In addition, they minimize hemodynamic side effects that can occur with a sharp change in vascular tone.
Which neurotransmitters stop being released during presynaptic inhibition?
As a result of impaired voltage-gated calcium channel function, vesicle exocytosis decreases. This leads to reduced release of noradrenaline from nerve endings, as well as acetylcholine from cholinergic nerves.
What else to study on this topic
- Differences between voltage-gated and ligand-gated calcium channels.
- The role of T-type calcium channels in the physiology of excitable tissues.
- Mechanisms of development of acute sensory (atypical) diabetic polyneuropathy.
- Comparative characteristics of short-acting and extended-release dihydropyridine drugs.
- The impact of total peripheral vascular resistance on blood pressure indicators.
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