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Neuro-pharmacology > Opioid Receptors and Systems > Flashcards

Opioid Receptors and Systems Flashcards

1
Q

A great variety of opioids exist

A

natural and synthetic, owing to
their unsurpassed potency as
analgesics.

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2
Q

Natural narcotics

A

Opium
morphine, codeine, thebaine

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3
Q

semisynthetic narcotics

A

Heroin, Hydromorphone, oxycodone, etorphine

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4
Q

totally synthetic narcotics

A

Pentazocine, meperidine, fentanyl, methadone, LAAM, Propoxyphene

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5
Q

endogenous opioids

A

endomorphines, enkaphalins, endorphins, dynorphins, Nociceptin

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6
Q

Opioid Potency

A

Analgesic effects are difficult to
directly measure in lab based assays
* In animal and human trials ethics limit
the types of experimental pain that
can be applied
* Human trials can be highly
confounded by subjectivity of pain
measures

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7
Q

We know opioids are potent
modulators of

A

GI mobility

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8
Q

Investigators developed a GI based

A

assay to measure the potency of
opioids

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9
Q

Ex vivo preparation of the

A

guinea pig
ileum

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10
Q

Application of hydraulic pressure
stimulates the

A

ileum peristaltic reflex

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11
Q

Morphine reversibly

A

Inhibits the
ileum peristaltic reflex,

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12
Q

opioid antagonist naloxone
rapidly restores

A

inhibition of Ileum peristaltic reflex by morphine

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13
Q

Opioid receptor discovery

A

Candace Pert and Soloman Snyder
finally identified the receptor using
radiolabelled naloxone (opioid
antagonist) in 1973

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14
Q

Opioid binding was demonstrated to
be

A

reversible, saturable, and of high
affinity.

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15
Q

Opioid receptor
binding and
potency

A

Opioid receptor binding
by the radioligand assay
was shown to correlate
with the potency of
opioids in the guinea pig
ileum bioassay

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16
Q

Opioid receptor distribution

A
  • High binding observed in the striatum,
    locus coeruleus, thalamus, raphe
    nuclei, and periaqueductal gray
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17
Q

Receptor subtypes Four main subtypes exist

A

δ (delta)
κ (kappa)
μ (mu)
Nociceptin

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18
Q

Opioid receptors are

A

e G-protein coupled (to
Gi)

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19
Q

Putative receptors include

A

ε (EOR) and ζ
(ZOR) and previously σ

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20
Q

δ (delta) –

A

DOR / OP1

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21
Q

κ (kappa)

A

– KOR / OP2

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22
Q

μ (mu)

A

MOR / OP3

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23
Q

Nociceptin

A

NOP / OP4

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24
Q

Varied expression of opioid receptors in the rat brain suggest

A

subtype specific roles.

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25
Q

where are Mu receptors located

A

Striatum, thalamus, olfactory areas, cortex, Raphe Nuclei, Locus Coeruleus

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26
Q

WHere are delta receptors located

A

striatum, olfactory areas, cortex

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27
Q

where are Kappa receptors located

A

Striatum, Cortex, Thalamus, Raphe nuclei, Locus coeruleus

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28
Q

μ-opioid receptor (MOR) High affinity for

A

morphine

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29
Q

μ-opioid receptor (MOR) High expression in

A

thalamus, periaqueductal gray,
median raphe suggests roles in analgesia

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30
Q

μ-opioid receptor (MOR) Expression in nucleus accumbens suggests role in

A

reinforcement

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31
Q

μ-opioid receptor (MOR) Expression in brainstem suggests roles in

A

n respiratory
depression, cough suppression, and vomit reflex

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32
Q

δ-opioid receptor (DOR) Similar expression to

A

μ but more restricted

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33
Q

δ-opioid receptor (DOR) Not sensitive to

A

morphine

34
Q

δ-opioid receptor (DOR) Roles in

A

olfaction, motor integration, reinforcement,
and analgesia

35
Q

κ-opioid receptor (KOR) Distinct

A

expression pattern

36
Q

κ-opioid receptor (KOR) High affinity for

A

ketocyclazocine

37
Q

κ-opioid receptor (KOR) Expressed in

A

striatum and amygdala, also
hypothalamus and pituitary

38
Q

κ-opioid receptor (KOR)Regulation of

A

pain perception, gut motility, and
dysphoria

39
Q

κ-opioid receptor (KOR)Additional roles in

A

water balance, feeding,
temperature control, neuroendocrine function

40
Q

ketocyclazocine

A

Synthetic opioid that is hallucinogenic and induces
dysphoria

41
Q

Nociceptin receptor Expressed in

A

amygdala, hippocampus, hypothalamus,
and spinal cord

42
Q

Nociceptin receptor Roles in

A

anxiety, depression, appetite, and
development of tolerance to μ-opioid agonists

43
Q

Enkephalins

A

‘in brain’
Selective for δ-receptor
* Two subtypes

44
Q
  • Dynorphins
A

– from Greek dynamis, meaning
power
* Selective for the κ-receptor
* Four subtypes

45
Q

Endorphins

A

contraction from endogenous
morphine
Selective for the μ-receptor
* Five subtypes

46
Q

Endomorphins

A

also a contraction from
endogenous morphine
Selective for the μ-receptor
* Extremely high affinity
* At least two subtypes
* Gene or prepeptide not yet identified

47
Q

Nociceptin

A

Selective for the nociceptin receptor
* Anti-analgesic
* Single species

48
Q

Despite dramatic size differences, endogenous peptides show

A

structural similarity to opiates but are considerably more potent

49
Q

Endogenous peptide genes and synthesis

A

Endorphins, enkephalins, and
dynorphins are synthesized
from pre-propeptide genes

50
Q
  • Endorphins are expressed from
A

POMC, which also gives rise to
melanocyte stimulating
hormones and
adrenocorticotropic hormone

51
Q

β-endorphin release POMC is highly expressed in the

A

pituitary
–
peptides for both adrenocorticotropic
hormone (ACTH) and
β-endorphin

52
Q

ACTH is released in response to

A

hypothalamic
corticotropin releasing hormone (CRH) and
acts on the adrenal cortex to release
glucocorticoid hormones

53
Q

Co-release of
β-endorphin from the pituitary
provides a

A

a physiological link between stresses
and pain signaling

54
Q

Endogenous opioid signalling is

A

inhibitory
– this can affect
neurotransmitter release through
a number of different ways.

55
Q

Postsynaptic inhibition is a result
of

A

Gi signalling to adenylate cyclase and Gβγ signalling to
hyperpolarizing
K + -channels
(GIRK).

56
Q

Axoaxonal inhibition can be
elicited through

A

G
i and cAMP
signalling to inhibit voltage gated
Ca2+
-channels

57
Q

Endogenous opioid signalling is
inhibitory
– this can affect
neurotransmitter release through
a number of different ways Presynaptic autoreceptors to

A

inhibit neurotransmitter release.

58
Q

Pain is unique among the senses as it can be induced by a range of factors

A

mechanical,
chemical, electrical, thermal, and inflammatory stimuli all affect nociceptive neurons.

59
Q

Opioids are involved in modulating

A

pain pathways at both the spinal level and at supraspinal sites.

60
Q

Pain perception has two components

A

Early pain
late pain

61
Q

Early pain

A

– immediate sensory component signalling stimulus location to cause withdrawal or escape from
stimulus

62
Q

Late pain

A

signals a strong emotional component, the unpleasantness of pain sensation – prolongs sensation
of pain to focus behaviours to limit further damage and aid recovery

63
Q

arly and late pain are signalled through

A

distinct neuronal pathways.

64
Q

Ascending pain
pathways

A

Sensory neurons in the dorsal root ganglia
transmit signals in the dorsal horn to
ascending pathways.

65
Q

Early pain is signalled through

A

A
δ fibers
(large, myelinated axons
– fast transmission).

66
Q

A
δ fibers project to the

A

thalamus and
somatosensory cortex to provide location
information on pain.

67
Q

Late pain is signalled through

A

C fibers (small,
unmyelinated axons
– slower transmission).

68
Q

C fibers project to the

A

thalamus but also
innervate the limbic system
(hypothalamus,
amygdala, and anterior cingulate cortex).

69
Q

early pain (pain
recognition) responses correlate
with

A

somatosensory activation

70
Q

Late pain (identification of
unpleasantness of pain) correlates

A

with ACC activation

71
Q

Both components of pain

A

bilaterally activate the secondary
somatosensory complex.

72
Q

Sites of opioid analgesia

A

Spinal sites
Supraspinal sites

73
Q

Opioid receptors are expressed at multiple
steps of pain Spinal sites

A

Opioidergic neurons are involved in
descending modulatory pathways (either
acting directly on projection neurons or on
excitatory interneurons)

74
Q

Opioidergic interneurons release

A

endorphins to
inhibit ascending projection neurons

75
Q

Supraspinal sites

A

Opioids function in the limbic system, thalamus,
and sensory areas to modulate emotional
components of pain

76
Q

Descending pain
modulation
pathways The most important descending pathways
originate in the

A

periaqueductal gray (PAG) in
the midbrain.

77
Q

periaqueductal gray (PAG) neurons project to the

A

raphe nuclei
where seratonergic projections descend to
provide inhibitory input to pain afferents

78
Q

Further projections from the PAG terminate in

A

the locus ceruleus
– noradrenergic cells
increase firing in response to pain and are inhibited by
μ-receptor agonists

79
Q

Sustained pain results in extensive
activation of

A

endogenous opioid signalling
in limbic structures.

80
Q

PET scan measuring displacement of a
radiolabelled ligand for the μ
-receptor
([11C]carfentanil) by endogenous opioids.
Since the endogenous and exogenous
ligand compete for

A

r the same site
decreased signal from the PET ligand is
proportional to increased release of
endogenous opiates.

81
Q

Opioid peptide effects on pain sensation In PET displacement studies, sensory pain scores correlated
negatively with

A

opioid release in the nucleus accumbens,
amygdala, and thalamus

82
Q

Affective pain scores correlated negatively with

A

opioid release in
the anterior cingulate cortex, thalamus, and nucleus accumbens

Neuro-pharmacology (26 decks)

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