1. Noradrenergic transmission

4. Noradrenergic Transmission
The peripheral noradrenergic neuron and the structures that it innervates are important targets for drug action, both as objects for investigation in their own right and as points of attack for many clinically useful drugs

5. Noradrenergic Transmission… Learning Objectives:
Physiology and function of noradrenergic neurons
The properties of adrenoceptors (the receptors on which noradrenaline and adrenaline act)
Classes of drugs affecting Noradrenergic transmission

6. CATECHOLAMINES
Catecholamines are compounds containing a catechol moiety (a benzene ring with two adjacent hydroxyl groups) and an amine side chain

7. CATECHOLAMINES… Pharmacologically, the most important ones are:
Noradrenaline (norepinephrine), a transmitter released by sympathetic nerve terminals.
Adrenaline (epinephrine), a hormone secreted by the adrenal medulla.
Dopamine, the metabolic precursor of noradrenaline and adrenaline, also a transmitter/neuromodulator in the central nervous system.
Isoprenaline (also known as isoproterenol), a synthetic derivative of noradrenaline, not present in the body.

8. PHYSIOLOGY OF NORADRENERGIC TRANSMISSION

9. The Noradrenergic Neuron
The noradrenergic neurons in the periphery are postganglionic sympathetic neurons whose cell bodies lie in the sympathetic ganglia.
They generally have long axons that end in a series of of varicosities strung along the branchial terminal network.
These varicosities contain numerous synaptic vesicles, which are the sites of synthesisi and release of noradrenaline and of co-released mediatots such as ATP and neuropeptide Y,

10. The Noradrenergic Neuron…
which are stored in vesicles and released by exocytosis
In most peripheral tissues, the tissue content of noradrenaline closely parallels the density of the sympathetic innervation
With exception of the adrenal medulla, sympathetic nerve terminals account all NA content of peripheral tissues.
Organs such as heart, spleen, vas deferens and some blood vessels are particularly rich in NA and have been widely used for studies of noradrenergic transmission

11. Classification of adrenoceptors
Main pharmacological classification into α and β subtypes, based originally on order of potency among agonists, later on selective antagonists.
Adrenoceptor subtypes:
two main α-adrenoceptor subtypes, α1 and α2, each divided into three further subtypes
three β-adrenoceptor subtypes (β1, β2, β3)
all belong to the superfamily of G-protein-coupled receptors.

12. Classification of adrenoceptors…
Second messengers:
α1 receptors activate phospholipase C, producing inositol trisphosphate and diacylglycerol as second messengers
α2 receptors inhibit adenylyl cyclase, decreasing cAMP formation
all types of β receptor stimulate adenylyl cyclase.

13. Classification of adrenoceptors…
The main effects of receptor activation are as follows:
α1 receptors: vasoconstriction, relaxation of gastrointestinal smooth muscle, salivary secretion and hepatic glycogenolysis
α2 receptors: inhibition of transmitter release (including noradrenaline and acetylcholine release from autonomic nerves), platelet aggregation, contraction of vascular smooth muscle, inhibition of insulin release

14. Classification of adrenoceptors…
β1 receptors: increased cardiac rate and force, delayed cardiac hypertrophy
β2 receptors: bronchodilatation, vasodilatation, relaxation of visceral smooth muscle, hepatic glycogenolysis and muscle tremor
β3 receptors: lipolysis.

15. Assignment!
Describe the characteristics of adrenoceptors

16. Assignment!
Describe the synthesis, storage, release and degradation of Noradrenaline

17. Noradrenaline Synthesis
Transmitter synthesis involves the following:
L-tyrosine is converted to dihydroxyphenylalanine (dopa) by tyrosine hydroxylase (rate-limiting step). Tyrosine hydroxylase occurs only in catecholaminergic neurons.
Dopa is converted to dopamine by dopa decarboxylase.
Dopamine is converted to noradrenaline by dopamine β-hydroxylase (DBH), located in synaptic vesicles.
In the adrenal medulla, noradrenaline is converted to adrenaline by phenylethanolamine N- methyltransferase.

18. NORADRENALINE SYNTHESIS

19. Noradrenaline Storage
Noradrenaline is stored at high concentration in synaptic vesicles, together with ATP, chromogranin and DBH (dopamine β-hydroxylase), all of which are released by exocytosis.
Transport of noradrenaline into vesicles occurs by a reserpine-sensitive transporter (VMAT-vesicular monoamine transporter), which is similar the amine transporter responsible for noradrenaline uptake into the nerve terminal, but uses the transvesicular proton gradient as its driving force.

20. Noradrenaline Storage…
Noradrenaline content of cytosol is normally low due to monoamine oxidase in nerve terminals.

21. Noradrenaline Release
Transmitter release occurs normally by Ca2+- mediated exocytosis from varicosities on the terminal network.
Non-exocytotic release occurs in response to indirectly acting sympathomimetic drugs (e.g. amphetamine), which displace noradrenaline from vesicles.
Noradrenaline escapes via the NET transporter (reverse transport).
(NET=Norepinephrine transporter)

22. Transmitter action is terminated mainly by reuptake of noradrenaline into nerve terminals via the NET transporter.
NET is blocked by tricyclic antidepressant drugs and cocaine.
Noradrenaline release is controlled by autoinhibitory feedback mediated by α2 receptors.
Co-transmission occurs at many noradrenergic nerve terminals, ATP and neuropeptide Y being frequently co-released with NA.
ATP mediates the early phase of smooth muscle contraction in response to sympathetic nerve activity.

23. UPTAKE AND DEGRADATION OF CATECHOLAMINES
The action of released noradrenaline is terminated mainly by reuptake of the transmitter into noradrenergic nerve terminals.
Circulating adrenaline and noradrenaline are degraded enzymically.
The two main catecholamine-metabolising enzymes are located intracellularly, so uptake into cells necessarily precedes metabolic degradation.

24. UPTAKE OF CATECHOLAMINES
About 75% of the noradrenaline released by sympathetic neurons is recaptured and repackaged into vesicles.
This serves to cut short the action of the released noradrenaline, as well as recycling it.
The remaining 25% is captured by non- neuronal cells in the vicinity, limiting its local spread
Neuronal uptake is performed by the plasma membrane noradrenaline transporter (generally known as NET, the norepinephrine transporter),

25. UPTAKE OF CATECHOLAMINES (2)
Packaging into vesicles occurs through the vesicular monoamine transporter (VMAT), driven by the proton gradient between the cytosol and the vesicle contents.
Extraneuronal uptake is performed by the extraneuronal monoamine transporter (EMT), which belongs to a large and widely distributed family of organic cation transporters.
NET is relatively selective for noradrenaline, with high affinity and a low maximum rate of uptake, and it is important in maintaining releasable stores of noradrenaline.
EMT has lower affinity and higher transport capacity than NET, and transports adrenaline and isoprenaline as well as noradrenaline

26. UPTAKE OF CATECHOLAMINES (3)
The effects of several important drugs that act on noradrenergic neurons depend on their ability either to inhibit NET or to enter the nerve terminal with its help.
Table 14.2 summarises the properties of neuronal and extraneuronal uptake.
Read on the characteristics of noradrenaline transport system

27. METABOLIC DEGRADATION OF CATECHOLAMINES
Endogenous and exogenous catecholamines are metabolised mainly by two enzymes:
monoamine oxidase (MAO) and
catechol-O-methyl transferase (COMT).
MAO (of which there are two distinct isoforms, MAO- A and MAO-B; occurs within cells, bound to the surface membrane of mitochondria. It is abundant in noradrenergic nerve terminals but is also present in many other places, such as liver and intestinal epithelium.
MAO converts catecholamines to their corresponding aldehydes, which, in the periphery, are rapidly metabolised by aldehyde dehydrogenase to the corresponding carboxylic acid (3,4- dihydroxyphenylglycol being formed from noradrenaline;

28. METABOLIC DEGRADATION OF CATECHOLAMINES (2)
MAO can also oxidise other monoamines, including dopamine and 5-HT.
It is inhibited by various drugs, which are used mainly for their effects on the central nervous system, where these three amines all have transmitter functions
These drugs have important side effects that are related to disturbances of peripheral noradrenergic transmission.
Within sympathetic neurons, MAO controls the content of dopamine and noradrenaline, and the releasable store of noradrenaline increases if the enzyme is inhibited.

29. METABOLIC DEGRADATION OF CATECHOLAMINES (3)
The second major pathway for catecholamine metabolism involves methylation of one of the catechol hydroxyl groups by COMT to give a methoxy derivative.
COMT is absent from noradrenergic neurons but present in the adrenal medulla and many other cells and tissues.
The final product formed by the sequential action of MAO and COMT is 3-methoxy-4-hydroxyphenylglycol (MHPG).
This is partly conjugated to sulfate or glucuronide derivatives, which are excreted in the urine, but most of it is converted to vanillylmandelic acid (VMA) and excreted in the urine in this form.

30. Regulation of noradrenaline release
Feedback control of noradrenaline release. The presynaptic α2 receptor inhibits adenylyl cyclase, thereby reducing intracellular cAMP. cAMP acts to promote Ca2+ influx in response to membrane depolarisation, and hence to promote the release of noradrenaline and ATP.

34. DRUGS ACTING ON NORADRENERGIC TRANSMISSION

35. DRUGS ACTING ON ADRENOCEPTORS
The overall activity of these drugs is governed by their affinity, efficacy and selectivity with respect to different types of adrenoceptor
Broadly speaking,
β-adrenoceptor agonists are useful as smooth muscle relaxants (especially in the airways), while
β-adrenoceptor antagonists (often called β-blockers) are used mainly for their cardiodepressant effects.
α-Adrenoceptor antagonists are used mainly for their vasodilator effects in cardiovascular indications and also for the treatment of prostatic hyperplasia.
α-Adrenoceptor agonists have few clinical uses.

37. ADRENOCEPTOR AGONISTS
Adrenoceptor agonists Noradrenaline and adrenaline show relatively little receptor selectivity.
Selective α1 agonists include phenylephrine and oxymetazoline.
Selective α2 agonists include clonidine and α- methylnoradrenaline.
They cause a fall in blood pressure, partly by inhibition of noradrenaline release and partly by a central action.
Methylnoradrenaline is formed as a false transmitter from methyldopa, developed as a hypotensive drug (now largely obsolete).

38. ADRENOCEPTOR AGONISTS…
Selective β1 agonists include dobutamine.
Increased cardiac contractility may be useful clinically, but all β1 agonists can cause cardiac dysrhythmias.
Selective β2 agonists include salbutamol, terbutaline and salmeterol, used mainly for their bronchodilator action in asthma.
Selective β3 agonists may be developed for the treatment of obesity.

39. Clinical Uses of Adrenoceptor Agonists
Clinical uses of adrenoceptor agonists Cardiovascular system:
cardiac arrest: adrenaline
cardiogenic shock : dobutamine .
Anaphylaxis (acute hypersensitivity,: adrenaline.
Respiratory system:
asthma: selective β2-receptor agonists (salbutamol, terbutaline, salmeterol, formoterol)
nasal decongestion: drops containing xylometazoline or ephedrine for short- term use.

40. Clinical Uses of Adrenoceptor Agonists…
Miscellaneous indications:
adrenaline: with local anaesthetics to prolong their action
premature labour (salbutamol;
α2 agonists (e.g. clonidine): to lower blood pressure and intraocular pressure; as an adjunct during drug withdrawal in addicts; to reduce menopausal flushing; and to reduce frequency of migraine attacks

41. Clinical uses and unwanted effects of α-adrenoceptor antagonists
Drugs that block α1- and α2 adrenoceptors (e.g. phenoxybenzamine and phentolamine) were once used to produce vasodilatation in the treatment of peripheral vascular disease, but this use is now largely obsolete.
Selective α1 antagonists (e.g. prazosin, doxazosin, terazosin) are used in treating hypertension.
Postural hypotension and impotence are unwanted effects.
Yohimbine is a selective α2 antagonist.
It is not used clinically.
Tamsulosin is α1A selective and acts mainly on the urogenital tract.

42. Clinical uses of α-adrenoceptor antagonists
Severe hypertension: α1-selective antagonists (e.g. doxazosin) in combination with other drugs.
Benign prostatic hypertrophy (e.g. tamsulosin, a selective α1A-receptor antagonist).
Phaeochromocytoma: phenoxybenzamine (irreversible antagonist) in preparation for surgery.

43. β-Adrenoceptor antagonists
Non-selective between β1- and β2- adrenoceptors:
propranolol, alprenolol, oxprenolol.
β1 selective: atenolol, nebivolol.
Alprenolol and oxprenolol have partial agonist activity.
Many clinical uses
Important hazards are bronchoconstriction, and bradycardia and cardiac failure (possibly less with partial agonists).
Side effects include cold extremities, insomnia, depression, fatigue.
Some show rapid first-pass metabolism, hence poor bioavailability.
Some drugs (e.g. labetalol, carvedilol) block both α and β-adrenoceptors.

44. Clinical uses of β-adrenoceptor antagonists
Cardiovascular :
- angina pectoris
- myocardial infarction
- dysrhythmias
- heart failure
- hypertension.
Other uses:
- glaucoma (e.g. timolol eye drops)
- thyrotoxicosis, as adjunct to definitive treatment (e.g. preoperatively)
- anxiety (Ch. 43), to control somatic symptoms (e.g. palpitations, tremor)
- migraine prophylaxis
- benign essential tremor (a familial disorder).

45. Unwanted effects Bronchoconstriction Cardiac depression Bradycardia
Hypoglycaemia
Fatigue
Cold extremities

46. Drugs acting on noradrenergic nerve terminals
Drugs that inhibit noradrenaline synthesis include:
- α-methyltyrosine: blocks tyrosine hydroxylase; not used clinically
- carbidopa: blocks dopa decarboxylase and is used in treatment of parkinsonism; not much effect on noradrenaline synthesis.
Methyldopa gives rise to false transmitter (methylnoradrenaline), which is a potent α2 agonist, thus causing powerful presynaptic inhibitory feedback (also central actions). Rarely used as antihypertensive agent.
Reserpine blocks noradrenaline accumulation in vesicles by VMAT, thus depleting noradrenaline stores and blocking transmission.
Effective in hypertension but may cause severe depression. Clinically obsolete.

47. Drugs acting on noradrenergic nerve terminals (2)
Noradrenergic neuron-blocking drugs (e.g. guanethidine, bethanidine) are selectively concentrated in terminals and in vesicles (by NET and VMAT respectively), and block transmitter release, partly by local anaesthetic action.
Effective in hypertension but cause severe side effects (postural hypotension, diarrhoea, nasal congestion, etc.), so now little used.
6-Hydroxydopamine is selectively neurotoxic for noradrenergic neurons, because it is taken up and converted to a toxic metabolite.
Used experimentally to eliminate noradrenergic neurons, not clinically.

48. Drugs acting on noradrenergic nerve terminals (3)
Indirectly acting sympathomimetic amines (e.g. amphetamine, ephedrine, tyramine) are accumulated by NET and displace noradrenaline from vesicles, allowing it to escape.
Effect is much enhanced by monoamine oxidase (MAO) inhibition, which can lead to severe hypertension following ingestion of tyramine-rich foods by patients treated with MAO inhibitors.
Indirectly acting sympathomimetic agents are central nervous system stimulants.
Methylphenidate and atomoxetine are used to treat attention deficit-hyperactivity disorder.
Drugs that inhibit NET include cocaine and tricyclic antidepressant drugs.
Sympathetic effects are enhanced by such drugs

49. INHIBITORS OF NORADRENALINE UPTAKE
The main class of drugs whose primary action is inhibition of NET are the tricyclic antidepressants , for example imipramine.
These drugs have their major effect on the central nervous system but also cause tachycardia and cardiac dysrhythmias, reflecting their peripheral effect on sympathetic transmission.
Cocaine, known mainly for its abuse liability and local anaesthetic activity enhances sympathetic transmission, causing tachycardia and increased arterial pressure.
Its central effects of euphoria and excitement are probably a manifestation of the same mechanism acting in the brain.

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