Central Nervous System
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The central nervous system (CNS) is the control center of the body, regulating everything from movement and sensation to thought and emotion. CNS pharmacology focuses on drugs that act on the brain and spinal cord to treat a variety of conditions, including mental health disorders, anxiety, pain, and neurodegenerative diseases. Unlike drugs that act on the peripheral nervous system, CNS drugs target receptors and pathways within the brain and spinal cord, often influencing neurotransmitter activity.
ANTIPSYCHOTICS
First-generation antipsychotics, also known as typical antipsychotics, are primarily used to treat schizophrenia and other psychotic disorders. These drugs work by blocking dopamine D2 receptors, which reduces the overactivity of dopamine in the brain – a hallmark of psychosis. Dopamine receptor antagonism in the mesolimbic pathway reduces positive symptoms of schizophrenia, such as hallucinations and delusions. Examples of first generation antipsychotics include haloperidol and chlorpromazine.
These drugs often cause extrapyramidal side effects (EPS), such as:
- tardive dyskinesia (involuntary, jerky movements)
- akathisia (restlessness)
- dystonia (muscle spasms).
Long-term use can lead to irreversible tardive dyskinesia.
Second-generation antipsychotics, also known as atypical antipsychotics, target both dopamine D2 receptors and serotonin 5-HT2A receptors. This dual mechanism improves efficacy and reduces the risk of extrapyramidal side effects, and make them effective in treating both positive and negative symptoms of schizophrenia (e.g., social withdrawal, lack of motivation). Examples include risperidone, olanzapine, clozapine.
Potential adverse effects include
- weight gain
- metabolic syndrome
- Sedation
Clozapine, in particular, carries a risk of agranulocytosis (a severe drop in white blood cells), requiring regular blood monitoring.
ANTIDEPRESSANTS
Antidepressants are used to treat depression, anxiety disorders, and other mood disorders. They work by increasing the levels of neurotransmitters such as serotonin, norepinephrine, and dopamine in the brain.
Selective Serotonin Reuptake Inhibitors (SSRIs) are the most commonly prescribed antidepressants due to their efficacy and relatively mild side effect profile. They block the reuptake of serotonin into presynaptic neurons, increasing serotonin levels in the synaptic cleft and enhancing mood. Examples include fluoxetine, sertraline, and citalopram.
Potential adverse effects include:
- Nausea
- Insomnia
- Sexual dysfunction
- Serotonin syndrome (a potentially life-threatening condition caused by excessive serotonin activity).
Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs) block the reuptake of both serotonin and norepinephrine, making them effective for depression and chronic pain conditions. Examples include venlafaxine and duloxetine.
Potential adverse effects are similar to SSRIs, with additional risks of increased blood pressure and sweating.
Monoamine Oxidase Inhibitors (MAOIs) are older antidepressants that inhibit the enzyme monoamine oxidase, which breaks down serotonin, norepinephrine, and dopamine. MAOIs are typically reserved for treatment-resistant depression due to their side effect profile. Examples include phenelzine and tranylcypromine.
The main potential adverse effects is hypertensive crisis when taken with tyramine-rich foods (e.g., cheese, wine).
Lithium is the gold standard for treating bipolar disorder, particularly manic episodes. Lithium stabilizes mood by modulating neurotransmitter release and second messenger systems. Regular blood monitoring is required to avoid toxicity.
Potential adverse effects include
- Tremors
- Hypothyroidism
- Nephrotoxicity
ANXIOLYTICS AND SEDATIVES
Benzodiazepines are widely used for anxiety, insomnia, and muscle relaxation. Benzodiazepines enhance the activity of GABA, the brain’s primary inhibitory neurotransmitter, by binding to the GABA-A receptor. This increases chloride ion influx, hyperpolarizing neurons and reducing CNS excitability.
Examples include diazepam (Valium), lorazepam, and alprazolam (Xanax).
Potential adverse effects include:
- Sedation
- Dependence
- Respiratory depression at high doses
Barbiturates are older sedatives that have largely been replaced by benzodiazepines due to their narrow therapeutic index and higher risk of overdose. Similar to benzodiazepines, barbiturates enhance GABA activity but also directly open chloride channels, making them more potent. Examples include phenobarbital and thiopental.
Potential adverse effects include:
- Severe respiratory depression
- High addiction potential
- Risk of overdose
PARKINSON’S DISEASE
Parkinson’s disease is caused by a deficiency of dopamine in the brain, leading to motor symptoms such as tremors, rigidity, and bradykinesia (slowness of movement). Levodopa is a dopamine precursor that crosses the blood-brain barrier and is converted to dopamine in the brain. Levodopa increases dopamine levels in the CNS, alleviating motor symptoms. Carbidopa is co-administered to inhibit peripheral conversion of levodopa to dopamine, ensuring more levodopa reaches the brain.
Potential adverse effects include:
- Nausea
- Dyskinesia (involuntary movements)
- Orthostatic hypotension.
Other Parkinson’s Drugs include:
- Dopamine Agonists: Pramipexole, ropinirole.
- MAO-B Inhibitors: Selegiline, which prevents dopamine breakdown.
- Anticholinergics: Benztropine, used to reduce tremors.
