Agonistas Colinérgicos (parte 1 - receptores) | Aula 9 | Farmacologia rápida e fácil | Flavonoide

Introduction to Autonomic Nervous System Pharmacology

Overview of the Lesson

• The video introduces a lesson on pharmacology related to the autonomic nervous system, emphasizing the importance of understanding physiology before diving into pharmacological concepts.

• The session is structured into two parts: first discussing receptors and then focusing on drugs for clarity.

Structure of the Autonomic Nervous System

• The autonomic nervous system (ANS) is part of the peripheral nervous system and is divided into sympathetic and parasympathetic systems.

• Sympathetic responses are associated with "fight or flight," while parasympathetic responses relate to "rest and digest."

Cholinergic Neurons

• Cholinergic neurons produce, store, and release acetylcholine (ACh). Preganglionic neurons in both sympathetic and parasympathetic systems are cholinergic.

• Postganglionic neurons in the parasympathetic system also use ACh, which binds to muscarinic receptors; however, there are exceptions like sweat glands that utilize cholinergic postganglionic fibers.

Mechanism of Acetylcholine Release

Steps in Acetylcholine Synthesis and Release

• The process begins with choline entering the neuron via sodium-dependent transport.

• Inside the neuron, choline combines with acetyl coenzyme A through choline acetyltransferase to form ACh.

• ACh is stored in synaptic vesicles until an action potential causes depolarization, leading to calcium channel opening and vesicle fusion with the membrane for release into the synaptic cleft.

Feedback Mechanism

• ACh can bind to presynaptic receptors as a feedback mechanism indicating sufficient levels present in the synapse, preventing further release.

Degradation of Acetylcholine

Breakdown Process

• Acetylcholinesterase breaks down excess ACh into choline and acetate in the synaptic cleft, halting signal transmission.

• The resulting choline is reabsorbed by presynaptic neurons for reuse.

Types of Cholinergic Receptors

Muscarinic vs. Nicotinic Receptors

• There are two main types of receptors for ACh: muscarinic (G-protein coupled receptors found mainly in glands and smooth muscles), and nicotinic (ion channels located at neuromuscular junction).

Muscarinic Receptor Subtypes

• Muscarinic receptors include M1 (found in autonomic glands), M2 (located on cardiac cells), M3 (present in smooth muscle), while M4 & M5 are primarily central nervous system receptors.

G Protein Coupling Effects

Nicotinic and Muscarinic Receptors Overview

Types of Nicotinic Receptors

• There are two types of nicotinic receptors: N1 (or NM) found at neuromuscular junctions, and N2 (or NN) located in autonomic ganglia and the central nervous system.

Effects of Agonists and Antagonists on Muscarinic Receptors

• The M1, M4, and M5 receptors are linked to cognition; agonists can enhance learning and communication abilities, while antagonists may impair cognitive functions.

Specific Receptor Functions

• The M2 receptor is associated with heart rate regulation; activation leads to bradycardia (decreased heart rate), while antagonism results in tachycardia (increased heart rate).

• The M3 receptor affects smooth muscle contraction. For example:

• In the urinary tract, agonists cause bladder contraction for urination.

• In the gastrointestinal tract, they increase peristalsis; excessive stimulation can lead to diarrhea.

Glandular and Ocular Effects

• Activation of M3 receptors in exocrine glands increases secretion (e.g., saliva), while antagonism decreases it, leading to dry mouth.

• In the eyes, agonists cause miosis (pupil constriction), whereas antagonists induce mydriasis (pupil dilation).

Respiratory System Response

• In pulmonary smooth muscle, agonist activation leads to bronchoconstriction; antagonist use promotes bronchodilation.

Summary of Agonist Actions

Understanding the Autonomic Nervous System: Sympathetic vs. Parasympathetic Actions

Key Concepts of Sympathetic and Parasympathetic Systems

• The speaker discusses the intuitive nature of remembering sympathetic actions compared to parasympathetic ones, suggesting that sympathetic responses are easier to recall.

• Agonist cholinergic drugs are mentioned as having effects opposite to those of the sympathetic system, indicating a clear distinction in their actions.

• Cholinergic drugs are defined as those that exert effects similar to the parasympathetic nervous system, highlighting their role in physiological processes.

• Anticholinergic drugs (antagonists) produce anti-parasympathetic effects, which can be understood as mimicking or enhancing sympathetic responses.