Neurohumoral transmission in ans

Learn the nor adrenergic transmission in ANS. Synthesis, storage ,release, uptake,metabolism of nor-adrenaline. Types of adrenoceptors. Agonist and antagonist of adrenoceptors.

1. NORADRENERGIC TRANSMISSION NEUROHUMORAL TRANSMISSION IN ANS - By Lalita shahgond B.pharm SSR COLLEGE OF PHARMACY , SILVASSA

2. The transmission of impulse through synapse and neuro-effector junction by the release of humoral (chemical) substances… What is neurohumoral transmission

3. CATECHOLAMINES Catecholamine's are compounds containing a catechol  moiety (a benzene ring with two adjacent hydroxyl  groups) and an amine side chain  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 nor adrenaline  and adrenaline, also a transmitter/neuromodulator  in the central nervous system • isoprenaline (isoproterenol), a synthetic derivative  of nor adrenaline, not present in the body.

4. PHYSIOLOGY OF NORADRENERGIC TRANSMISSION The noradrenergic neuron: Noradrenergic neurons in the periphery are postganglionic sympathetic neurons whose cell bodies are situated in sympathetic ganglia.

5. - they generally have long axons that end in the series of varicosities strung along the branching terminal network   These varicosities contain numerous synaptic vesicles, which are the sites of synthesis and  release  of  nor adrenaline  and  of  co-released  mediators  such as ATP and neuropeptide Y  which are  stored in vesicles and released by exocytosis.

6. NORADRENALINE SYNTHESIS  The metabolic precursor for noradrenaline is L-tyrosine, an aromatic  amino acid that is present in the body fluids and is taken  up by adrenergic neurons. Tyrosine hydroxylase, a cytosolic  enzyme that catalyses the conversion of tyrosine to dihydroxyphenylalanine (dopa). This above  hydroxylation step is the main control point for  noradrenaline synthesis (rate limiting step).  Tyrosine hydroxylase is inhibited  by  the  end  product  of  the  biosynthetic  pathway,  noradrenaline.

7. The biosynthetic pathway for nor adrenaline synthesis

8. The  tyrosine  analogue  α- methyltyrosine  strongly  inhibits tyrosine hydroxylase and has been used experimentally to block nor adrenaline synthesis . The next step, conversion of dopa to dopamine, is catalyzed by dopa decarboxylase, a cytosolic enzyme that is by  no means confined to catecholamine-synthesising cells.    Dopa decarboxylase activity is not rate-limiting for  noradrenaline synthesis  It  is a relatively non-specific enzyme, and catalyses the decarboxylation of various other L-aromatic amino acids, such  as L-histidine and L-tryptophan, which are precursors in the  synthesis of histamine.

9. Dopamine-β-hydroxylase (DBH) is also a relatively nonspecific  enzyme, that catalyses the conversion of   dopamine to noradrenaline. Many drugs inhibit DBH, including copper-chelating  agents and disulfiram (a drug used mainly for its effect  on ethanol metabolism. Phenylethanolamine N-methyl transferase (PNMT) catalyses the N-methylation of nor adrenaline to adrenaline. The  main location of this enzyme is in the adrenal medulla.

10. NORADRENALINE STORAGE Most of the nor adrenaline is stored in nerve terminals or chromaffin cells is contained in synaptic vesicles; only a little is free in the  cytoplasm under normal circumstances.  The concentration in the vesicles is very high (0.3–1.0 mol/l) and is  maintained by the vesicular monoamine transporter (VMAT),  which is similar to the amine transporter responsible for  nor adrenaline uptake into the nerve terminal. The  vesicles  contain  two  major  constituents  besides  nor adrenaline, namely ATP (about four molecules per  molecule of nor adrenaline).   This  substance  is  released  along  with  nor adrenaline.

11. 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 nor adrenaline from vesicles.   Noradrenaline  escapes via the NET transporter (reverse transport). • Transmitter action is terminated mainly by reuptake of  nor adrenaline into nerve terminals via the NET transporter.  Nor adrenaline with ATP are released by exocytosis.

12. NET is blocked by tricyclic antidepressant drugs and cocaine. • Noradrenaline release is controlled by autoinhibitory  feedback mediated by α 2  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.  Noradrenaline, by acting on presynaptic  β 2  receptors, can regulate its own release, and also that of  co-released ATP.

13. Feedback control of noradrenaline (NA) release.   The presynaptic  α 2   receptor inhibits Ca +2  influx in response to  membrane depolarisation via an action of the  βγ  subunits of the  associated G protein on the voltage-dependent Ca +2  channels 

14. The depolarization of the nerve terminal causes activation of the calcium channel. The calcium channel opens and ca +2 ion enters into the cell, the calcium ions influx causes depolarization inside the cell. Depolarization leads to exocytosic release of nor adrenaline with ATP from the vesicle. Released NA bind to the postsynaptic receptors and produce respective effects. NA bind to the α 2 adrenoceptor which is auto receptor it inhibits the NA release.

15. G i / G o that are the α subunit of GPCR they are responsible for 3 activities :- - G i regulates the opening of potassium channel and the inhibition of adenyl cyclase. - G o regulates the opening of the calcium channel. -The potassium channel opens and the K + ions move outside the cell this causes repolarisation of the cell. This inhibits exocytosis of NA - The inhibition of the adenyl cyclase inhibits the conversion of ATP to C AMP this inhibits the opening of calcium channel. This inhibits exocytosis of NA.

16. UPTAKE AND DEGRADATION OF CATECHOLAMINES (nor adrenaline) The action of released nor adrenaline is terminated mainly  by  reuptake  of  the  transmitter  into  noradrenergic    nerve terminals. Circulating adrenaline and noradrenalin  are degraded enzymically, but much more slowly than  acetylcholine. 

17. UPTAKE OF CATECHOLAMINES About 75% of the nor adrenaline released by sympathetic  neurons is recaptured and repackaged into vesicles.   This  serves to cut short the action of the released nor adrenaline, as well as recycling it.  The remaining 25% is captured  by non-neuronal cells in the vicinity, limiting its local  spread.  These two uptake mechanisms depend on distinct  transporter molecules.   Neuronal uptake is performed by  the plasma membrane nor adrenaline transporter (generally known as NET, the nor epinephrine transporter), which  belongs to the family of neurotransmitter transporter proteins(NET,DAT, SERT, etc)  

18. THE UPTAKE OF NOR ADRENALINE IS OF 3 TYPES :- 1. AXONAL UPTAKE 2. VESICULAR UPTAKE 3. EXTRANEURONAL UPTAKE

19. AXONAL UPTAKE An active amine pump (NET) is present at the neuronal membrane which transports NA by a Na+ coupled mechanism. It takes up NA at a higher rate than Adrenaline and had been labelled uptake-1. The indirectly acting sympathomimetic amines like tyramine, but not isoprenaline, also utilize this pump for entering the neurone. This uptake is the most important mechanism for terminating the postjunctional action of NA. From 75% to 90% of released NA is retaken back into the neurone. This pump is inhibited by cocaine, desipramine and few other drugs.

20. VESICULAR UPTAKE The membrane of intracellular vesicles has another amine pump the ‘vesicular monoamine transporter’ (VMAT-2), which transports NA from the cytoplasm to the interior of the storage vesicle. The VMAT-2 transports monoamines by exchanging with H+ ions. The vesicular NA is constantly leaking out into the axoplasm and is recaptured by this mechanism. This carrier also takes up DA formed in the axoplasm for further synthesis to NA. Thus, it is very important in maintaining the NA content of the neurone. This uptake is inhibited by reserpine, resulting in depletion of CAs.

21. Extraneuronal uptake Extraneuronal uptake of CAs (uptake-2) is carried out by extraneuronal amine transporter (ENT or OCT3) and other organic cation transporters OCT1 and OCT2 into cells of other tissues. In contrast to NET this uptake transports Adrenaline at a higher rate than NA, is not Na+ dependent and is not inhibited by cocaine, but inhibited by corticosterone. It may capture circulating Adr, but is quantitatively minor and not of physiological or pharmacological importance.

22. METABOLIC DEGRADATION OF CATECHOLAMINES (NOR ADRENALINE) - Endogenous and exogenous catecholamines are metabolised  mainly  by  two  intracellular  enzymes:-   - Monoamine oxidase (MAO)    - Catechol-O-methyl transferase (COMT).  - MAO  (of  which  there  are  two  distinct  isoforms)    MAO-A and MAO-B  is bound to the  surface  membrane  of  mitochondria.  - It  is  abundant  in  noradrenergic nerve terminals but is also present in liver,  intestinal epithelium and other tissues. - MAO converts  catecholamines to their corresponding aldehydes, which in  the  periphery,  are  rapidly  metabolised  by  aldehyde dehydrogenase to the corresponding carboxylic acid.

23. NA Converted to NM by COMT MAO catalyses conversion of NM to NM aldehyde NM aldehyde is directly converted to VMA by ADH, VMA is the major metabolite which is excreted through urine. when MAO enzyme acts on the NA it gets converted to NA aldehyde AR converts the NM aldehyde directly into the MHPG, which is a minor metabolite of NA and it is excreted through urine. ADH converts NA aldehyde to DHMA further the DHMA is converted to VMA by COMT AR catalyses the conversion of NA aldehyde to DHPG which is catalysed to MHPG by COMT.

24. The catecholamine's should bind to the adrenoceptors to produce effects… ADRENOCEPTORS

25. α β α 1 α 2 β 1 β 2 β 3 А В Ϲ А В Ϲ α: nor adrenaline > adrenaline > isoprenaline β:  isoprenaline > adrenaline > nor adrenaline

26. 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, α 1  and α 2 , each  divided into three further subtypes (1-/2- A,B,C) – three β-adrenoceptor subtypes (β 1 , β 2 , β 3 ) – all belong to the super family of G protein-coupled  receptors .

27.   Second messengers: –  α 1   receptors activate phospholipase C, producing  inositol trisphosphate and diacylglycerol as second  messengers –  α 2   receptors inhibit adenylyl cyclase, decreasing  cAMP formation – all types of  β  receptor stimulate adenylyl cyclase  

28. • . •  The main effects of receptor activation are as follows: – α 1   receptors: vasoconstriction, relaxation of  gastrointestinal smooth muscle, salivary secretion  and hepatic glycogenolysis –  α 2   receptors: inhibition of: transmitter release  (including nor adrenaline and acetylcholine release  from autonomic nerves); platelet aggregation;  vascular smooth muscle contraction; insulin release –  β 1   receptors: increased cardiac rate and force –  β 2   receptors: bronchodilatation; vasodilatation;  relaxation of visceral smooth muscle; hepatic  glycogenolysis; muscle tremor –  β 3   receptors: lipolysis and thermogenesis; bladder  detrusor muscle relaxation.

29. ADRENOCEPTOR AGONISTS  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 nor adrenaline release  and partly by a central action.  • 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. • A selective  β3  agonist, mirabegron, is used to treat  overactive bladder;  β3  agonists promote lipolysis and  have potential in the treatment of obesity.