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Antipsychotic drugs (AKA Neuroleptics, antischizophrenics and major tranquillisers) Main categories – Typical/classical antipsychotics – e.g. chlorpromazine, haloperidol. - Atypical antipsychotics e.g. risperidone, clozapine. - Difference is that classicals (developed pre-1980) have pretty similar properties, whereas the atypical drugs are more diverse, have less tendency to cause unwanted motor side effects and/or have an effect on the negative symptoms as well as the positives. Mechanism of action o All are antagonists at dopamine D2 receptors, but most also block other monoamine receptors, especially 5HT2. Antipsychotic potency generally runs parallel to activity on D2- receptors, but other activities may determine side-effect profile. o Anti-psychotics take days to weeks to work, suggesting that secondary effects, such as increased D2 receptor numbers in the limbic system, may be more important that the direct effect of D2 receptor block. Behavioral effects o Produces a state of apathy and reduced initiative. o The patients normally show fewer emotions, respond slowly to external stimuli and tend to become drowsy. o The patients are easily aroused and have no loss of intellectual function. o Aggressive tendencies are strongly inhibited. o Many antipsychotics also show antiemetic activity, reflecting antagonisms at dopamine receptors. Unwanted effects Antipsychotic-induced motor disturbances o Two main kinds seen, which are collectively termed the extrapyramidal side- effects, which result directly or indirectly from D2-receptor blockade. Atypical antipsychotics show less tendency to cause these effects. o Acute dystonia – involuntary movements (muscle spasm, protruding tongue, torticollis etc) and often a Parkinson-type syndrome. Occur in first few weeks, often declining in time, and are reversible upon Rx cessation. Effect consistent with blocking dopaminergic nigrostriatal pathway, and thus is less often seen with atypical antipsychotics that target the mesolimbic/mesocortical pathway. o Tardive dyskinesia – involuntary movements of face, tongue, trunk and limbs, appearing after months to years of Rx in 20-40% of Pts on classical antipsychotics. Side-effect is disabling and often irreversible, and often gets worse when Rx stops. Associated with proliferation of dopamine receptors (possibly presynaptic) in the corpus striatum. Endocrine effects o Increases prolactin release leading to breast swelling, pain and lactation in both sexes. Other side-effects o Dry mouth, blurred vision, hypotension etc result from blockade of other receptors, such as α-adrenoceptors and muscarinic ACh receptors. o Sedation and weight gain are also common. o Leucopenia and agranulocytosis are rare but potentially fatal and occur in the first few weeks. o Antipsychotic malignant syndrome – rare but serious, and is similar to malignant hyperthermia syndrome seen with certain anaesthetics. Muscle rigidity is accompanied by rapid rise in body temp and mental confusion. Usually reversible, but is fatal in 10-20% of cases. Clinical efficacy o Effective in controlling symptoms of acute schizophrenia, when large doses may be needed. o Long term Rx is often effective in preventing recurrence of attacks, and enables most Pts to live normal lives o Generally not effective in controlling the negative symptoms, though newer drugs are claimed to relieve these symptoms o Approximately 40% of chronic schizophrenic Pts are poorly controlled by antipsychotic drugs. o There are negligible differences in efficacy between different antipsychotic drugs, though side effects differ significantly.
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