Characteristics of Opioid Receptors

mu₁

Agonists : morphine phenylpiperidines

Actions:  analgesia bradycardia sedation

mu₂

Agonists : morphine phenylpiperidines

Actions:  respiratory depression euphoria physical dependence  

delta

Actions:  analgesia-weak,  respiratory depression

kappa

Agonists: ketocyclazocine dynorphin nalbuphine butorphanol

Actions:  analgesia-weak respiratory depression sedation

Sigma

Agonists: pentazocine

Action: dysphoria -delerium hallucinations tachycardia hypertension

epsilon:

Agonists: endorphin

Actions: stress response acupuncture

Organic Nitrates 
Relax smooth muscle in blood vessel
Produces vasodilatation
– Decreases venous pressure and venous return to the heart  Which decreases the cardiac work load and oxygen demand. 
– May have little effect on the coronary arteries CAD causes stiffening and lack of 
–    responsiveness in the coronary arteries 
– Dilate arterioles, lowering peripheral vascular resistance  Reducing the cardiac workload

Main effect related to drop in blood pressure by
– Vasodilation- pools blood in veins and capillaries, decreasing the volume of blood that the heart has to pump around (the preload)
– relaxation of the vessels which decreases the resistance the heart has to pump against (the afterload) 

Indications
- Myocardial ischemia 
– Prevention
– Treatment 

Nitroglycerin (Nitro-Bid)
• Used
– To relive acute angina pectoris 
– Prevent exercise induced angina 
– Decrease frequency and severity of acute anginal episodes

Type 
• Oral - rapidly metabolized in the liver only small amount reaches circulation 
• Sublingual – Transmucosal tablets and sprays 
• Transdermal  – Ointment s 
– Adhesive discs applied to the skin
• IV preparations 

Sublingual Nitroglycerine 
•  Absorbed directly into the systemic circulation,  Acts within 1-3 minutes , Lasts 30-60 min 

Topical Nitroglycerine 
• Absorbed directly into systemic circulation,   Absorption at a slower rate. ,  Longer duration of action 
Ointment - effective for 4-8 hours 
Transdermal disc - effective for 18-24 hours 

Isosorbide dinitrate 
• Reduces frequency and severity of acute anginal episodes
• Sublingual or chewable acts in 2 min. effects last 2-3 hours
• Orally, systemic effects in about 30 minutes and last about 4 hours after oral administration
    
Tolerance to Long-Acting Nitrates 
• Long-acting dosage forms of nitrates may develop tolerance
– Result in episodes of chest pain
– Short acting nitrates less effective 

Prevention of Tolerance 
• Use long-acting forms for approximately 12-16 hours daily during active periods and omit them during inactive periods or sleep 
• Oral or topical should be given every 6 hours X 3 doses allowing a rest period of 6 hours

Isosorbide dinitrate (Isordil, Sorbitrate) is used to reduce the frequency and severity of acute anginal episodes.
When given sublingually or in chewable tablets, it acts in about 2 minutes, and its effects last 2 to 3 hours. When higher doses are given orally, more drug escapes metabolism in the liver and produces systemic effects in approximately 30 minutes. Therapeutic effects last about 4 hours after oral administration

Isosorbide mononitrate (Ismo, Imdur) is the metabolite and active component of isosorbide dinitrate. It is well absorbed after oral administration and almost 100% bioavailable. Unlike other oral nitrates, this drug is not subject to first-pass hepatic metabolism. Onset of action occurs within 1 hour, peak effects occur between 1 and 4 hours, and the elimination half-life is approximately 5 hours. It is used only for prophylaxis of angina; it does not act rapidly enough to relieve acute attacks.

GLP-1 analogs

Exenatide

Mechanism

GLP-1 is an incretin released from the small intestine that aids glucose-dependent insulin secretion
basis for drug mechanism is the observation that more insulin secreted with oral glucose load compared to IV 

Exenatide is a GLP-1 agonist

↑ insulin
↓ glucagon release
the class of dipeptidyl peptidase inhibitors ↓ degradation of endogenous GLP-1
e.g.) sitagliptin, -gliptins 
 

Clinical use
type II DM
 

Toxicity
nausea, vomiting
pancreatitis
hypoglycemia
if given with sulfonylureas

TRICYCLIC ANTIDEPRESSANTS

e.g. amitriptyline, imipramine, nortriptyline

Belong to first generation antidepressants

ACTION:

Inhibit 5-HT(5-hydroxytryptamine) and norepinephrine reuptake

slow clearance of norepinephrine & 5-HT from the synapse 

enhance norepinephrine & 5-HT neuro-transmission

MODE OF ACTIONMODE OF ACTION

TCAs also block
– muscarinic acetylcholine receptors
– histamine receptors 
– 5-HT receptors
– α1 adrenoceptors

Onset of antidepressant activity takes 2-3 weeks

PHARMACOKINETICS

-  Readily absorbed from the gastro-intestinal tract 
- Bind strongly to plasma albumin
- Has a large volume of distribution(as a result of binding to extravascular tissues)
- Undergo liver CYP metabolism into biologically active metabolites
- These metabolites are inactivated via glucuronidation and excreted in urine

ADVERSE DRUG REACTIONS

Antimuscarinic - dry mouth, blurred vision, constipation and urinary retention
Antihistamine – drowsiness
adrenoceptor blockage(+/- central effect) postural hypotension
Reduce seizure threshold
Testicular enlargement, gynaecomastia, galactorrhoea
AV-conduction blocks and cardiac arrhythmias

TOXICITY

- Fatal in toxicity

- Most important toxic effect is, slowing of depolarisation of the cardiac action potential by blocking fast sodium channels ("quinidine-like" effect) 

- delays propagation of depolarisation through both myocardium and conducting tissue

- prolongation of the QRS complex and the PR/QT intervals

- predisposition to cardiac arrhythmias

DRUG INTERACTIONS

Pharmacodynamic:
– ↑ sedation with antihistamines, alcohol
– ↑ antimuscarinic effects with anticholinergics– ↑ antimuscarinic effects with anticholinergics
– Hypertension and arrhythmias with MAOIs- should be given at least 14 days apart

Pharmacokinetic (via altering CYP metabolism)
– ↓ plasma concentration of TCA by- carbamazepine, rifampicin
– ↑ plasma concentration of TCA by- cimetidine, calcium channel blockers,fluoxetine

OTHER CLINICAL USES OF AMITRIPTYLINE

- Treatment of nocturnal enuresis in children
- Treatment of neuropathic pain
- Migraine prophylaxis

Drugs used to induce vomiting

In case of poisoning with noncorrosive agents, and assuming incomplete absorption of the poison has taken place, induction of vomiting can be carried out. One of the drugs used for this purpose is emetine which causes irritation of the upper gut and, on absorption, it also acts on  CTZ.  

Chemotherapeutic agents (or their metabolites) can directly activate the medullary chemoreceptor trigger zone or vomiting center; several neuroreceptors, including dopamine receptor Type 2 and serotonin Type 3 (5-HT3) from cell damage(GIT and pharynx) play roles in vomiting.

Dextromethorphan  
O-methylated dextrorphan,  Excellent oral antitussive,  No analgesic effect,  No GI effects,  No respiratory depression