A. When a neurotransmitter has a stimulating effect on receptors, this leads to agonistic actions. Drugs which stimulate receptors similar to these neurotransmitters are therefore called agonists. Drugs which block the action of the neurotransmitter on the receptors are antagonists. They act only in the presence of the neurotransmitter and do not have any intrinsic activity on their own. Inverse agonists have the opposite action of an agonist. For example, instead of opening an agonist operated channel, they close the channel after binding to the receptor. Inverse agonists thus have intrinsic activity (can act even in the absence of neurotransmitter molecules in the vicinity) and so can also be blocked by the antagonist. Partial agonists act as net agonists in the absence of the neurotransmitter, but act as a net antagonist in the presence of the neurotransmitter. Aripiprazole is a partial agonist at the dopamine (D2) receptor.
Lofexidine is an:
A. Lofexidine is an alpha 2 agonist which acts at the autoreceptor. When the presynaptic autoreceptors are stimulated, they generally stop the release of the particular neurotransmitter. Thus they act as a brake. Lofexidine stimulates the alpha 2 autoreceptors, thus stopping the release of noradrenaline into the synapse. The antagonists of autoreceptors increase the release of the neurotransmitter. In the case of serotonin, 5HT1B are autoreceptors. Some alpha 2 receptors are found on serotonergic neurones and control the release of 5HT; these alpha 2 receptors are called heteroreceptors.
Which of the following is NOT a reuptake inhibitor?
E. Buspirone is a partial agonist at 5-HT1A autoreceptors and can infl uence the fi ring of serotonergic (5-HT) neurones. Neurotransmitters in the synaptic cleft are actively transported back into the presynaptic neurone by a reuptake pump. Most antidepressants block this pump, thus increasing the availability of the monoamine neurotransmitters. Cocaine is a dopamine reuptake inhibitor. Bupropion inhibits reuptake of both dopamine and noradrenaline.
Which of the following combinations is NOT paired correctly?
E. Phenytoin is an anticonvulsant. Alprazolam is a benzodiazepine and it is not used as a mood stabilizer; valproate, lithium, lamotrigine, and carbamazepine can be considered as mood stabilizers. Haloperidol is a butyrophenone and not a phenothiazine. Acamprosate is used to treat alcohol dependence. It is not an opiate. It has a structure similar to GABA and it is thought to act via glutamate receptor mechanism. Zopiclone is not a benzodiazepine. It is a hypnotic classed with other ‘Z drugs’; Z drugs are non-benzodiazepines which act on the GABA complex but at a slightly different site than benzodiazepines.
Paralytic ileus is most likely to be associated with which one of the following antipsychotics?
C. Clozapine causes anticholinergic effects by blocking muscarinic (M3) receptors. This can cause side-effects such as dry mouth, constipation, loss of accommodation, and urinary retention. Constipation is a side-effect in around 15% of patients taking clozapine, and, in most, simple advice about diet and fluid intake is sufficient. More significant ileus is an uncommon but potentially more serious side-effect. It may particularly be associated with bowel surgery. It could potentially be fatal. Though isolated reports of similar problems with olanzapine and risperidone have been recorded, these are not well-established associations.
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