Phenothiazines receptor blocking actions

The answer is a. (Katzung, p 4850 Agents with lb receptor blocking actions are effective in reducing itching. Hj receptor blockade is typical of phenothiazines with short side chains. 

Phenothiazines with shorter side chains have considerable H -receptor-blocking action and have been used for relief of pruritus or, in the case of promethazine, as preoperative sedatives. The butyrophenone droperidol is used in combination with an opioid, fentanyl, in neuroleptanesthesia. The use of these drugs in anesthesia practice is described in Chapter 25. 

Fluphenazine (Prolixin, Permitil) [Antipsychotic/ Phenothiazine] Uses Schizophrenia Action Phenothiazine antipsychotic blocks postsynaptic mesolimbic dopamin gic brain receptors Dose 0.5-10 mg/d in % doses PO q6-8h, av age maint 5 mg/d or 1.25 mg IM, then... 

Mechanism of Action A phenothiazine that blocks dopamine neurotransmission at postsynaptic dopamine receptor sites. Possesses strong anticholinergic, sedative, and antiemetic effects moderate extrapyramidal effects and slight antihistamine action. Therapeutic Effect Relieves nausea and vomiting improves psychotic conditions controls intractable hiccups and porphyria. 

Alpha-receptor blocking effects can be demonstrated for many Hi antagonists, especially those in the phenothiazine subgroup, eg, promethazine. This action may cause orthostatic hypotension in susceptible individuals. Beta-receptor blockade is not observed. Serotonin-Blocking Action... 

Cyproheptadine resembles the phenothiazine antihistaminic agents in chemical structure and has potent H receptor-blocking as well as 5-HT2-blocking actions. The actions of cyproheptadine are predictable from its histamine and 5-HT receptor affinities. It prevents the smooth muscle effects of both amines but has no effect on the gastric secretion stimulated by histamine. It also has significant antimuscarinic effects and causes sedation. 

Emesis. BA receptor blocking agents suppress nausea and vomiting. Phenothiazines, such as chlorpromazine and prochlorperazine, are usually employed. Their action is probably mediated via blockade of BA receptors in the chemoreceptor trigger zone of the medulla oblongata. 

Sedation Sedation is more marked with phenothiazines than with other antipsychotics this effect is usually perceived as unpleasant by nonpsychotic individuals. With the exception of sertindole, the atypical dmgs all block histamine receptors, an action that contributes to sedation. 

The answer is a. (Katzung, p 482.) Phenothiazines as a class and, in particular, the aliphatic phenothiazines are most likely to produce marked sedation. The mechanism of action for this effect is associated with its ability to block histamine and acetylcholine receptors. 

Hf and H Receptors. Histamine exerts its actions by binding to receptors on cell membranes. Two types of histamine receptors, the Hi and H2 receptors, are known specific agonists and antagonists exist for each of these receptors. Black et al. (55) differentiated H and H2 receptors with the compounds, 2-methylhistamine and 4 methylhistamine. 2-Methylhistamine is active on tissues with H receptors 4-methylhistamine is active on tissues with H2 receptors. Classical antihistaminic drugs were developed in the 1930 s these compounds block H but not H2 receptors. Among the clinically used H -blockers are derivatives of ethanolamine, ethylenediamine, alkylamine, piperazine and phenothiazine (32). These agents are valuable in the treatment of... 

It also seems plausible that antipsychotic drugs competitively bind with dopamine receptors and block the action of dopamine on corresponding receptor sites, thus lowering psychotic activity. Central dopamine receptors are subdivided into Dj, D2, and according to some sources, Dj receptors. These receptors have a high affinity for dopamine, but they differ in sensitivity to neuroleptics of various chemical classes. For example, drugs of the phenothiazine series are nonselective competitive Dj and D2 antagonists. Unlike phenoth-iazines, antipsychotics of the butyrophenone series such as haloperidol display selective action only on D2 receptors. 

Phenothiazine derivatives are nonselective, competitive Dj and D2 antagonists that block dopamine activity on corresponding receptor sites. In addition, their action is... 

A number of different compounds of the piperidine and piperazine series with p-fluorobuty-rophenone group substitutions at the nitrogen atom display significant neuroleptic activity (haloperidol, trifluperidol, droperidol, methorin). There is a considerable interest in butyrophenone derivatives as antipsychotic agents as well as in anesthesiology. They exhibit pharmacological effects and a mechanism of action very similar to that of phenothiazines and thioxanthenes in that they block dopaminergic receptors. However, they are more selective with respect to D2 receptors. 

Another form of parkinsonism is drug-induced, that is, iatrogenic parkinsonism, which often is a comphca-tion of antipsychotic therapy, especially following the use of the butyrophenone and phenothiazine drug classes (see Chapter 34). Unlike idiopathic parkinsonism, striatal content of dopamine is not reduced by administration of these drugs. In contrast, they produce a functional decrease in dopamine activity by blocking the action of dopamine on postsynaptic dopamine receptors. 

Mechanism of Action A phenothiazine that antagonizes dopamine neurotransmission at synapses by blocking postsynaptic dopaminergic receptors in the brain. Therapeutic Effect Decreases psychotic behavior. Also produces weak anticholinergic, sedative, and antiemetic effects and strong extrapyramidal effects. Pharmacokinetics Erratic absorption. Protein binding greater than 90%. Metabolized in liver. Excreted in urine. Half-life 33 hr. 

Mechanism of Action A piperazine phenothiazine that acts centrally to block dopamine receptors in chemoreceptor trigger zone in central nervous system (CNS). Tiicr-apeutic Effect Relieves nausea and vomiting. 

Mechanism of Action A phenothiazine derivative that blocks dopamine at postsynap-tic receptor sites. Possesses strong extrapyramidal and antiemetic effects and weak anticholinergic and sedative effects. Therapeutic Effect Suppresses behavioral response in psychosis reduces locomotor activity and aggressiveness. Pharmacokinetics Readily absorbed following PO administration. Protein binding 90%-99%. Metabolized in liver. Excreted in urine. Half-life 24 hr. 

The first phenothiazine antipsychotic drugs, with chlorpromazine as the prototype, proved to have a wide variety of central nervous system, autonomic, and endocrine effects. Although efficacy of these drugs is primarily driven by D2-receptor blockade, their adverse actions were traced to blocking effects at a wide range of receptors including a adrenoceptors and muscarinic, Hi histaminic, and 5-HT2 receptors. 

The classical antipsychotic (see p. 380) drugs block dopamine receptors and their antipsychotic activity relates closely to this action, which notably involves the Dj-receptor, the principal target in Parkinson s disease. It comes as no surprise, therefore, that these drugs can induce a state whose clinical features are very similar to those of idiopathic Parkinson s disease. The piperazine phenothiazines, e.g. trifluoperazine, and the butyrophenones, e.g. haloperidol, are most commonly involved. In one series of 95 new cases of parkinsonism referred to a department of geriatric medicine, 51% were associated with prescribed drugs and half of these required hospital admission. After withdrawal of the offending drug most cases resolved completely in 7 weeks. But... 

A number of the antihistamines, particularly the phenothi-azines and aminoalkyl ethers, have antiemetic actions and thus may be u.seful in the treatment of nausea, vomiting, and motion sickness. Also, those agents that produce pronounced. sedation have application as nonpiescription sleeping aids." - "Several of the phenothiazines have limited u.se in Parkinson-like syndromes as a result of their ability to block central muscarinic receptors." -And. a number of antihistamines, including promethazine, pyrilamine. tri-pelennamine and diphenhydramine, display local anesthetic activity that may be therapeutically useful. ... 

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