VANILLOID RECEPTORS

Aakrati Gupta

(2011H108048H)

16-04-2012

CONTENTS

Introduction

TRPV1 Structure

TRPV1 Expression

Implications in various disease states

Agonists

Antagonists

TRANSIENT RECEPTOR POTENTIAL

RECEPTORS(TRP)

TRP superfamily

TRPC TRPM TRPV TRPP TRPML TRPA TRPN

6 types(mammals 2 types(non-

mammals)

TRPV1

TRPV2

TRPV6

Osm-9

NanchungTRPV3

TRPV4

TRPV5High Ca2+ selectivity & low temperature

sensitivity

Heat activated receptors & non

selective for cations(also Ca2+)In drosophila

Activated by

vanilloids(capsaicin)

INTRODUCTION

TRP family

TRP channels were initially discovered in trp mutant strain of the fruit fly Drosophila.

It includes > 30 cation channels, the majority of which are permeable for

divalent and monovalent cations, including Ca2+, Na+, and Mg2+.

They have broad tissue distribution and may participate in divergent functions such as:

visual and auditory functions, speech, pain signal transduction,

regulation of blood circulation, gut motility, mineral absorption and fluid balance, airway

and bladder hypersensitivities, cell survival, growth and death.

TRPV1

It is a non-selective cation channel.

in humans, is encoded by the TRPV1 gene

It has been considered as a ‘pathological’ receptor, it has a

significant role in the pain transduction pathway and pro-

inflammatory role in a variety of disease and injury states.

It is activated by capsaicin, the main pungent principle of hot chilli

peppers.

It is a polymodal TRP channel that can be activated by noxious

heat, pH changes, fatty acid amides, and endogenous lipid ligands.

First cloned from rat dorsal root ganglia(DRG).

STRUCTURE OF TRPV1

It is a single polypeptide, 838 amino acid, 95kD.

It has a large intracellular amino-(N-) and carboxy-(C-) terminal and 6

transmembrane segment.

An additional short hydrophobic stretch between TM5 and TM6.

TRPV-1 subunits assemble as tetramers.

N- terminus (432 a.a) has 3 ankyrin repeats(essential for channel

function).

C- terminus(154 a.a) has a TRP domain which serves as a molecular

determinant.

C- terminus has amino acid residues for PIP2 binding

To the N-terminus PKA and PKC binds.

TRPV1 EXPRESSION

Neuronal cells:

small to medium diameter primary afferent fibres.

sensory neurons

Dorsal root ganglia(DRG)

Trigeminal neurons

o Non –neuronal cells:

Keratinocytes, bladder urothelium, smooth muscle, liver, polymorphonuclear granulocytes, pancreatic B cells, endothelial cells, lymphocytes, macrophages

o Brain :

Dopaminergic neurons of substantia nigra, hippocampal pyramidal neurons, hypothalamus.

A-δ myelinated fibres

unmyelinated C fibres

IN NEURONAL CELLS

IN NON NEURONAL CELLS

Location/Cell type Function

Keratinocytes Release of pro-inflammatory

mediator, sensor for noxious

cutaneous stimulation

Bladder urothelial cells Regulation of bladder reflex

contractions

Smooth muscle Vasoconstriction

Cerebromicrovascular

endothelial cells

Contribution to the regulation

of cerebral blood flow and

BBB permeability

polymorphonuclear

granulocytes,

Lymphocytes, macrophages

Possible pro-inflammatory

role, yet the role of TRPV1 on

cells of the

immune system is currently

elusive

Preadipocytes and adipose

tissue

Regulation of adipogensis

PARADOXICAL EFFECTS OF TRPV1

Nociception and pro-inflammatory effects

Diabetic painful

neuropathy

Cancer pain

Peripheral neuropathic

pain

Chronic persistent

cough

osteoarthritis

Rheumatoid arthritis

Faecalincontinence

Oesophagealreflux

disease

cystitis

DIABETIC NEUROPATHY:

Study done in:

STZ induced diabetic rats.

Observations:

TRPV1 protein levels were down-regulated, while the function of

TRPV1 was increased in the DRG neurones isolated from early

diabetic rats.

Also DRG neurons from diabetic rats exhibit increased levels of

oxidative stress in vitro, an effect that is reduced by incubation of

cells with the TRPV1 antagonist, capsazepine.

Conclusion:

early diabetic neuropathy is associated with enhanced function of

TRPV1 in DRG neurones , which may result in compensatory down-

regulation of TRPV1 receptor expression.

PERIPHERAL NEUROPATHIC PAIN

Study done:

total or partial sciatic nerve transection, or spinal nerve ligation.

Observations:

TRPV1 mRNA levels were downregulated in the somata of damaged

sensory neurones.

TRPV1-immunoreactivity was significantly reduced in the somata of

damaged DRG neuronal profiles, compared to controls.

In case of partial transection or spinal nerve ligation, TRPV1

expression was increased in the undamaged DRG somata compared

to controls.

Conclusion:

The persistence of TRPV1 expression in sites close to nerve

injury,although down-regulated in injured nerves, is possibly due to

depletion of growth factors such as NGF in injured nerves.

CANCER PAIN

chronic bone pain(due to bone cancer)or metastases of non-bone

primary tumours as breast, prostate and lung.

osteoclast-induced bone remodelling is accompanied by the robust

production of extracellular protons, which are known to be potent

activators of primary afferent neurones.

acidic microenvironment produced by osteoclasts contributes to bone

cancer-associated pain via activation of acid-sensitive nociceptors

which innervate the marrow and mineralised bone.

Study done in :

osteolytic sarcoma cell-induced-bone cancer model of mice

1. TRPV1 knockout mice,

2. wild-type control mice,

3. with the administration of a selective TRPV1 antagonist.

Int J Cancer 2004;109:182–8.

Observations:

TRPV1 knockout mice and TRPV1 antagonist-treated mice

demonstrated :

-ve ongoing and movement-evoked pain-related behaviour

-ve bone cancer severity

No effect on tumour growth

Conclusions:

3 mechanisms contribute simultaneously to activation and

sensitisation of TRPV1 receptors expressed by sensory fibres

innervating the tumour-bearing joint .

inflammation, tumour-released products and tumour-induced injury to

primary afferent neurones.

ACUTE AND CHRONIC ARTHRITIS:

neuropeptide-containing nerve fibres are present in the knee joint

synovium and adjacent bone.

levels of neuropeptides are significantly increased in samples of

synovial fluid from patients with rheumatoid arthritis.

1.Study done :

CFA-induced hind paw inflammation

Observation:

Myelinated axons were not affected during inflammation.

Conclusion:

an increase in the number of unmyelinated sensory axons

expressing TRPV1 is one of the mechanisms underlying peripheral

sensitisation in inflammation.

2. Study done :

wild-type mice and TRPV1 knockout mice after intra-articular

injection of CFA.

Observation:

knee swelling and vascular hyperpermeability were significantly

higher in the CFA-treated joints of wild-type mice in comparison to

TRPV1 null mice.

No effect on leukocyte accumulation and cytokine production.

thermal hyperalgesia and joint swelling were decreased in TRPV1

knockout mice compared with wild-type controls after intraarticular

injection of mouse recombinant (TNF-α)

Conclusion:

(TNF-α) requires the presence of TRPV1 receptors to function

AIRWAY HYPERSENSITIVITY: number of neuropeptide CGRP containing nerves in the airway

submucosa of patients with chronic persistent cough was shown to be increased.

Also as the cough reflex is mediated by the activation of A-δ fibres, in addition to C-fibres.(TPRV1 present in these)

Study done in:

capsaicin-induced cough in guinea-pigs

Observations:

TRPV1 antagonist, capsazepine, inhibited the capsaicin-induced cough and the endogenous TRPV1 ligand, anandamide, induced coughing.

Conclusion:

The activation of A-δ fibres and C-fibres in the airways of guinea-pigs induced by decreasing pH involves TRPV1 as protons increase the TRPV1 channel function .An increase in the content of protons in exhaled breath condensate in chronic cough

FAECAL INCONTINENCE:

Faecal urgency is a distressing, debilitating disorder in rectal cancer

and IBD.

Rectal sensation is conveyed by the polymodal C-fibres and A-δ

fibres.

in patients suffering from rectal hypersensitivity, the number of

TRPV1-expressing nerve fibres was increased in muscle,

submucosal and mucosal layers of rectal biopsy samples.

OESOPHAGEAL REFLUX DISEASE

some patients perceive oesophageal acid exposure as painful,

suggesting that visceral hyperalgesia may contribute to their

symptoms.

Study:

capsaicin injection into the oesophageal wall resulted in severe

chest pain and heartburn.

Observations:

TRPV1 immunoreactive fibres were increased in patients with

erosive oesophagitis.

increased acid exposure is associated with an increase in the

density of nerve fibres expressing TRPV1.

Protective effects

Ischaemia and reperfusion

injury

Hypertension

Inflammatory bowel disease

Sepsis

Obesity

TRPV1 in CVS and GIT: TRPV1 rich nerves are densely distributed in the CV and GI system.

activation of TRPV1 either by endogenous ligands or by exogenous agonists exert hypotensive /protective effects against these systems injury through stimulating the synthesis and release of NT such as CGRP and substance P.

1.CVS:

Study done in:

isolated rat heart

Observations:

cardioprotection provided by CGRP- or capsaicin-induced preconditioning was abolished by CGRP-(8–37), the selective CGRP receptor antagonist.

Also preconditioning-induced cardiac protection against ischemia–reperfusion

injury was significantly impaired in TRPV1 knockout hearts.

European Journal of Pharmacology 627(2010) 1–7

Conclusion:

Activation of TRPV1 either by endogenous or exogenous ligand

exert beneficial effects against cardiac injury.

TRPV1 gene deletion results in excessive inflammation,

disproportional left ventricular remodeling, and deteriorated cardiac

function after myocardial ischemia, indicating that TRPV1 may

prevent infarct expansion and cardiac injury by inhibiting

inflammation and abnormal tissue remodelling.

2.GIT:

It is rich in TRPV1 nerves, play pivotal role in the maintenance of

gastrointestinal mucosa integrity against injurious interventions.

also found in non-nervous tissue such as gastric epithelial cells,

gastrin cells etc.

Study done in:

TRPV1 knockout mice, isolated human antral glands.

o Observations:

luminal acidification did not activate mechanosensory neurons in

TRPV1 knockout mice.

activation of TRPV1 by capsaicin was able to stimulate the secretion

of gastrin from antral glands.

Conclusion:

TRPV1 exerts multiple physiological functions as acid secretion ,

intestinal motility and visceral sensation to gastrin secretion.

Mechanism of protection: release of NT (CGRP ) and the activation

of cyclooxygenase-1 enzymes(inducing the production of

prostaglandin E2).

NO

DRUG

IN ANXIETY:

TRPV1 is expressed also in the brain, where it seems to be

involved in antinociception, locomotor control, and regulation

of affective behavior.

Model: rats , EPM

Observations:

ANANDAMIDE

CB1 TRPV1

Decrease in intracellular Ca2+ and attenuation of

synaptic transmission Elevated

calcium levels and potentiated synaptic transmission

Endogenous agonist

of CB1 receptor

Neu

roscie

nce 2

04

(20

12

) 18

6–

192

This suggests a tripartite regulatory system with antagonistic effects

of CB1 and TRPV1, which are tied together by the same endogenous

ligand. Such a system may have important implication for the

modulation of behavioural responses

TRPV1 is also present in glutamatergic synapses, but its role in fear

and anxiety is less well explored than that of CB1.

Study :

systemic injection of the TRPV1 antagonist capsazepine in rats in the

EPM test reduced anxiety-like behavior.

Also behavioral analyses of TRPV1 knockout mice(i.e.decreased

anxiety in the EPM , impaired fear expression in response to a tone)

strengthened this notion that TRPV1 affects anxiety diametrically

opposite to CB1.

Conclusion:

Opposite roles of CB1 receptors and TRPV1 channels in synaptic

transmission, fear, and anxiety: whereas activation of CB1 receptors

promotes acute fear adaptation and decreases anxiety, TRPV1

channels exert anxiogenic actions.

TRPV1 AND PARKINSON’S DISORDER

Problem with L-dopa : dyskinesias and psychiatric complications.

Endocannabinoid/endovanilloid system - an important modulator of

basal ganglia functions and its pharmacologic manipulation can be a

therapy to alleviate L-dopa induced dyskinesias.

Study done in:

Male Wistar rats

DA-denervating lesions were performed by unilateral injection of 6-

OHDA.

Two weeks after the lesion, the rats were screened for apomorphine-

induced contralateral rotation to assess the efficacy of the lesion.

Net contralateral turns were calculated by

number of ipsilateral - contralateral rotations.

Only rats displaying more than 300 rotations per 30 min

(corresponding to about 90% depletion of tyrosine hydroxylase

(TH) positive neurons were included in the study.

Exp Neurol. 2007 November ; 208(1): 110–119

daily injection of levodopa + carbidopa for up to 12 days, led to a

gradual development of increasingly severe axial, limb, locomotor

and oro-facial abnormal involuntary movements (AIMs).

Observations:(Catalepsy and AIM score)

Administration of the CB1 agonist WIN 55, attenuated levodopa-

induced axial, limb and oral AIMs dose-dependently via a CB1–

mediated mechanism, it had no effect on locomotive AIMs.

URB597, a potent FAAH inhibitor alone , did not affect AIMs scoring

despite its ability to increase anandamide concentration throughout

the basal ganglia.

As known that anandamide can also bind and activate TRPV1

receptors.

URB597+ TRPV1 antagonist capsazepine- significantly decreased all

AIMs subtypes .

Conclusions :

blockade of TRPV1 receptors unmasks the anti-dyskinetic effects of

FAAH inhibitors.

CB1 and TRPV1 receptors play opposite roles in levodopa-induced

dyskinesias.

TRPV1 AND ALZIEMER’S DISEASE

To investigate both the basal and beta-amyloid peptide impaired cognitive performance of D3R((-/-)) mice, and the role therein of endocannabinoids/endovanilloids.

Study done in :

D3R knock out mice D3R((-/-)) and wild type(WT) mice were divided into 3 groups:

1. Untreated

2. Vehicle

3. beta-amyloid peptide 1-42(BAP).

(TRPV1) antagonist SB366791, were injected intraperitoneally for 11 /7 days.

The retention test was performed 1, 7 and 14 days after the learning trial.

Observations:

D3R((-/-)) mice exhibited a better basal cognitive performance as compared to WT mice , which was reversed by TRPV1 antagonism.

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(TRPV1) antagonist SB366791 did not affect the cognitive

performance of healthy mice, but fully counteracted BAP 1-42-

induced amnesic effects in both D3R((-/-)) and WT mice -when

administered for 11 days

when administered for 7 days, only transiently affected WT mice and

caused more prolonged cognitive ameliorations in D3R((-/-)) mice.

Conclusion :

there is involvement of D3R and TRPV1 in cognitive processes and

the A beta peptides inhibit memory retention in mice through the

involvement of endovanilloids.

Physiological functions

thermoregulation

neurogenesis

Urinary bladder

function

AGONISTS

ANTAGONISTS

EXPRESSION IN RETINA

TRPV1 was detected in astrocytes, blood vessels, microglial cells

and in neurons, indicating that this receptor could be involved in

both visual coding and generating signals to provide retinal

homeostasis.

Int. J. Devl Neuroscience 27 (2009) 709–718