Chlorpromazine - Pharmacology - Pharmacodynamics and Central Effects

Pharmacodynamics and Central Effects

Chlorpromazine is a very effective antagonist of D₂ dopamine receptors and similar receptors, such as D₃ and D₅. Unlike most other drugs of this genre, it also has a high affinity for D₁ receptors. Blocking these receptors causes diminished neurotransmitter binding in the forebrain, resulting in many different effects. Dopamine, unable to bind with a receptor, causes a feedback loop that causes dopaminergic neurons to release more dopamine. Therefore, upon first taking the drug, patients will experience an increase in activity of dopaminergic neural activity. Eventually, dopamine production of the neurons will drop substantially and dopamine will be removed from the synaptic cleft. At this point, neural activity decreases greatly; the continual blockade of receptors only compounds this effect.

Chlorpromazine acts as an antagonist (blocking agent) on different postsynaptic receptors:

• Dopamine receptors (subtypes D₁, D₂, D₃ and D₄), which account for its different antipsychotic properties on productive and unproductive symptoms;in the mesolimbic dopamine system accounts for the antipsychotic effect whereas the blockade in the nigrostriatal system produces the extrapyramidal effects

• Serotonin receptors (5-HT₁ and 5-HT₂), with anxiolytic, and antiaggressive properties as well as an attenuation of extrapyramidal side effects, but also leading to weight gain, fall in blood pressure, sedation and ejaculation difficulties.

• Histamine receptors (H₁ receptors, accounting for sedation, antiemetic effect, vertigo, fall in blood pressure and weight gain)

• α₁- and α₂-adrenergic receptors (accounting for sympatholytic properties, lowering of blood pressure, reflex tachycardia, vertigo, sedation, hypersalivation and incontinence as well as sexual dysfunction, but may also attenuate pseudoparkinsonism—controversial. Also associated with weight gain as a result of blockage of the adrenergic alpha 1 receptor)

• M₁ and M₂ muscarinic acetylcholine receptors (causing anticholinergic symptoms such as dry mouth, blurred vision, constipation, difficulty or inability to urinate, sinus tachycardia, electrocardiographic changes and loss of memory, but the anticholinergic action may attenuate extrapyramidal side effects).

The presumed effectiveness of the antipsychotic drugs relied on their ability to block dopamine receptors. This assumption arose from the dopamine hypothesis that maintains that both schizophrenia and bipolar disorder are a result of excessive dopamine activity. Furthermore, psychomotor stimulants like cocaine that increase dopamine levels can cause psychotic symptoms if taken in excess.

Chlorpromazine and other typical antipsychotics are primarily blockers of D2 receptors. In fact an almost perfect correlation exists between the therapeutic dose of a typical antipsychotic and the drug's affinity for the D2 receptor. Therefore, a larger dose is required if the drug’s affinity for the D2 receptor is relatively weak. A correlation exists between average clinical potency and affinity of the antipsychotics for dopamine receptors. Chlorpromazine tends to have greater effect at serotonin receptors than at D2 receptors, which is notably the opposite effect of the other typical antipsychotics. Therefore, chlorpromazine with respect to its effects on dopamine and serotonin receptors is similar to the atypical antipsychotics than the typical antipsychotics.

Chlorpromazine and other antipsychotics with sedative properties such as promazine and thioridazine are among the most potent agents at α-adrenergic receptors. Furthermore, they are also among the most potent antipsychotics at histamine H1 receptors. This finding is in agreement with the pharmaceutical development of chlorpromazine and other antipsychotics as anti-histamine agents. Furthermore, the brain has a higher density of histamine H1 receptors than any body organ examined which may account for why chlorpromazine and other phenothiazine antipsychotics are as potent at these sites as the most potent classical antihistamines.

In addition to influencing the neurotransmitters dopamine, serotonin, epinephrine, norepinephrine, and acetylcholine it has been reported that antipsychotic drugs could achieve glutamatergic effects. This mechanism involves direct effects on antipsychotic drugs on glutamate receptors. By using the technique of functional neurochemical assay chlorpromazine and phenothiazine derivatives have been shown to have inhibitory effects on NMDA receptors that appeared to be mediated by action at the Zn site. It was found that there is an increase of NMDA activity at low concentrations and suppression at high concentrations of the drug. No significant difference in glutamate and glycine activity from the effects of chlorpromazine were reported. Further work will be necessary to determine if the influence in NMDA receptors by antipsychotic drugs contributes to their effectiveness.

Chlorpromazine does also act as FIASMA (functional inhibitor of acid sphingomyelinase).