9.3 Characteristics of Drugs to Treat Nervous System Disorders

Neurotransmitters and Nervous System Pharmacology

Neurotransmitters are chemical messengers that carry signals between neurons and their target cells. They drive everything from muscle contraction to mood regulation, and nearly every drug used to treat nervous system disorders works by altering neurotransmitter activity in some way. Some drugs mimic neurotransmitters, some block them, and others change how quickly they're released or cleared from the synapse. Understanding these mechanisms is how you'll connect a drug's action to its clinical use.

Neurotransmitters and Their Functions

Each major neurotransmitter has a distinct role, and knowing these roles helps you predict both the therapeutic effects and side effects of the drugs that target them.

• Acetylcholine (ACh)
  • Active in both the CNS and peripheral nervous system (PNS)
  • In the CNS, it supports memory formation and learning. In the PNS, it triggers skeletal muscle contraction and regulates autonomic functions like heart rate, digestion, and glandular secretion.
• Norepinephrine (NE)
  • The primary neurotransmitter of the sympathetic nervous system
  • Increases heart rate, blood pressure, and blood glucose during stress or physical activity. It's central to the "fight or flight" response, preparing the body to act quickly in emergencies.
• Dopamine (DA)
  • Functions in the CNS, especially in brain pathways controlling reward, motivation, and motor control
  • Regulates movement, emotional responses, and feelings of pleasure. Too little dopamine activity in motor pathways leads to Parkinson's disease. Too much dopamine activity in other pathways is associated with schizophrenia.
• Serotonin (5-HT)
  • Active in the CNS and the gastrointestinal tract
  • Regulates mood, sleep, appetite, and pain perception. Dysregulation of serotonin is linked to depression and anxiety disorders, which is why so many antidepressants target this neurotransmitter.
• Gamma-aminobutyric acid (GABA)
  • The primary inhibitory neurotransmitter in the CNS
  • Reduces neuronal excitability by hyperpolarizing the postsynaptic membrane, which makes neurons less likely to fire. GABA plays a major role in controlling anxiety and promoting relaxation. Drugs like benzodiazepines enhance GABA's effects.
• Glutamate
  • The primary excitatory neurotransmitter in the CNS
  • Involved in learning, memory, and synaptic plasticity (the ability of synapses to strengthen or weaken over time). Excessive glutamate activity can cause excitotoxicity, where neurons are damaged or killed by overstimulation. This process is linked to neurodegenerative disorders.

Drug Interactions with Autonomic Receptors

Drugs that target the autonomic nervous system work by binding to specific receptor subtypes. Knowing which receptor a drug targets tells you what it does clinically.

Cholinergic receptors respond to acetylcholine and come in two main types:

• Muscarinic receptors (M1–M5)
  • Found on smooth muscle, cardiac muscle, and glands
  • Drugs like atropine and scopolamine block these receptors (muscarinic antagonists). They're used to treat motion sickness, overactive bladder, and COPD by reducing smooth muscle contraction and secretion.
• Nicotinic receptors (NM, NN)
  • Found at the neuromuscular junction (NM) and in autonomic ganglia (NN)
  • Drugs like nicotine and varenicline stimulate or partially activate these receptors. Varenicline is used for smoking cessation; cholinesterase inhibitors (which increase ACh at nicotinic and muscarinic receptors) are used in Alzheimer's disease to support cognitive function.

Adrenergic receptors respond to norepinephrine and epinephrine. There are four key subtypes:

• Alpha-1 receptors ($\alpha_1$)
  • Stimulation causes vasoconstriction and smooth muscle contraction
  • Blockers like prazosin and tamsulosin relax smooth muscle. They're used to treat hypertension and benign prostatic hyperplasia (BPH).
• Alpha-2 receptors ($\alpha_2$)
  • Located on presynaptic neurons; stimulation decreases further NE release
  • Agonists like clonidine and guanfacine reduce sympathetic outflow. They're used to treat hypertension and ADHD.
• Beta-1 receptors ($\beta_1$)
  • Found primarily in the heart; stimulation increases heart rate and contractility
  • Blockers like atenolol and metoprolol reduce cardiac workload. They're used to treat hypertension, angina, and heart failure.
• Beta-2 receptors ($\beta_2$)
  • Found in bronchial smooth muscle; stimulation causes bronchodilation
  • Agonists like albuterol and salmeterol open the airways. They're used to treat asthma and COPD. Note that beta-2 receptors are primarily activated by epinephrine rather than NE.

Sympathetic vs. Parasympathetic Nervous System Agents

A helpful way to organize these drugs: think about whether they turn up or turn down each branch of the autonomic nervous system.

• SNS Stimulants (Sympathomimetics)
  • Mimic or enhance the effects of NE and epinephrine
  • Examples: epinephrine, norepinephrine, amphetamines
  • Clinical uses: anaphylaxis (epinephrine), cardiac arrest, and narcolepsy. These drugs increase alertness, heart rate, and blood pressure.
• SNS Blockers (Sympatholytics)
  • Block NE at adrenergic receptors, reducing sympathetic activity
  • Examples: alpha blockers (prazosin), beta blockers (propranolol)
  • Clinical uses: hypertension, angina, and migraine prevention. They work by lowering peripheral resistance and/or heart rate.
• PNS Stimulants (Cholinergic Agonists / Parasympathomimetics)
  • Mimic or enhance the effects of ACh
  • Examples: pilocarpine, bethanechol
  • Clinical uses: glaucoma (pilocarpine constricts the pupil to improve fluid drainage) and urinary retention (bethanechol contracts the bladder smooth muscle).
• PNS Blockers (Anticholinergics / Parasympatholytics)
  • Block ACh at muscarinic receptors
  • Examples: atropine, scopolamine, glycopyrrolate
  • Clinical uses: bradycardia (atropine increases heart rate by removing vagal tone), overactive bladder, and COPD. These drugs reduce smooth muscle contraction and secretion.

Drug Mechanisms and Nervous System Interactions

Beyond receptor binding, several broader concepts shape how nervous system drugs work.

Blood-Brain Barrier (BBB) The BBB is a selective permeability barrier formed by tightly joined endothelial cells in brain capillaries. It prevents most large or water-soluble molecules from entering the brain. For a CNS drug to be effective, it must be able to cross this barrier. This is why drug formulation matters: lipid-soluble drugs cross more easily, while many antibiotics and chemotherapy agents cannot reach the brain without special modifications.

Neurotransmitter Reuptake After a neurotransmitter is released into the synaptic cleft, transporter proteins pull it back into the presynaptic neuron. This process, called reuptake, terminates the signal. Many psychiatric drugs work by blocking reuptake, which keeps the neurotransmitter active in the synapse longer. SSRIs (selective serotonin reuptake inhibitors) like fluoxetine are a classic example: they block serotonin reuptake to increase serotonin signaling.

Synaptic Transmission This is the step-by-step process of signal transfer at a synapse:

1. An action potential arrives at the presynaptic terminal.
2. Voltage-gated calcium channels open, and calcium flows in.
3. Calcium triggers vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft.
4. The neurotransmitter binds to receptors on the postsynaptic cell, producing a response.
5. The signal is terminated by reuptake, enzymatic breakdown, or diffusion.

Drugs can intervene at any of these steps. For example, botulinum toxin blocks step 3 (vesicle release of ACh), while benzodiazepines enhance the postsynaptic response at step 4 (by increasing GABA receptor activity).

Receptor Agonists and Antagonists

• Agonists bind to a receptor and activate it, mimicking the natural neurotransmitter. Example: morphine is an agonist at opioid receptors.
• Antagonists bind to a receptor but don't activate it, blocking the natural neurotransmitter from binding. Example: naloxone is an antagonist at opioid receptors and reverses opioid overdose.

Neuroplasticity The brain's ability to form new neural connections and reorganize existing ones in response to experience, learning, or injury. Certain medications and rehabilitation therapies can promote neuroplasticity, which is relevant in stroke recovery and management of chronic neurological conditions.