Function and Role:

Enkephalin is an endogenous opiate.  They behave in a similar way to opium in that they have an analgesic effect thus the similar name.  There are opiate receptors throughout the body, in the brain and peripheral tissues.  Enkephalin is categorized as an endorphin (endogenous morphine).
Some of the different types of endorphins include:
1.  Beta-endorphins
2.  enkephalins
3.  dynorphin
 Although it is a short peptide (five amino acids) there are numerous functions:

                            1)There is evidence that it enhances natural killer cells in an immune response.
                            2)Reduces growth and metastases of tumor cells.
                            3)Enhances immune functions in patients with cancer and AIDS.

Receptor Sites:

There are three receptor sites that interact with endorphin peptides and all three influence each other->allowing for higher complexity, more options for neural signals.
Patterns of distribution and quantity of receptor sites vary between species and anatomical regions.

1) mu-receptor: sensitive to morphine and similar drugs.  Beta-endorphin also interacts here slightly although it prefers the delta-receptor.  The mu-receptor is thought to deal primarily with analgesia.  There are two subtypes
        a)one deals primarily with the analgesic effects
        b)another deals with inhibition of breathing->principal reason of death by overdose on heroin and morphine.
2) kappa-receptor: dynorphin interacts here.  Stimulation at the kappa-receptor site antagonizes the effect of mu-receptor-mediated analgesia.  Calcium channels of the neuron are indirectly affected here and at the delta-receptor site.
 
mu stimulation produces euphoria kappa stimulation results in dysphoria and aversion
mu deals with rewarding, increases dopamine release kappa blocks morphine-induced rewarding, lowers dopamine.
 
3) delta-receptor: this receptor is more sensitive to enkephalins ([met]-enkephalin, [leu]-enkephalin).  Stimulation here affects mood (ie. the euphoria of heroin, morphine).  Calcium channels are indirectly affected here like the kappa-receptor.  60%-70% of mu and delta receptors are the same; they differ in the extracellular N-terminus and the intracellular C-terminus.
Why are there three receptors?



Opiates and stress- evolution of enkephalins:
B-endorphin and ACTH (adrenaline) are released at the same time from the anterior pituitary in response to certain stimuli.
Role that endorphins played early in evolution relate to survival
1)  temporary analgesia: in the presence of a predator->ignore pain, stay still until danger passes.
2)  flight: flee fast without feeling pain from an injury
3)  fight:  don't feel pain in battle.

The presence of endorphins/enkephalins insured that survival comes first,  recuperation later.
Areas of the brain that release endorphins are primitive suggesting an early beginning, estimated at 200 mya.
a)lower hindbrain
b)mid-brain
c)entry at spinothalamic pathway
Role of endorphins in humans now-> no longer linked to a hostile environment, now focused on social behaviors and relationships.
 



Neurons and the brain:
There are 10-100 billion neurons in the brain which consume 25% of the oxygen and 15% of the blood (glucose) in the body, yet accounting for only 2% of the body's weight-->neurons use a lot of energy.
 In formative years (first four years of life) the brain uses 50% of the bodies oxygen-->neurons are the most active during this time.
 The hypothalamus secretes enkephalins most.

Signal transmission:
A neurotransmitter is released from the presynaptic neuron, aided by Ca++ transport.  Receptor sites on the dendrite of the recieving neuron match the molecular structure of the neurotransmitter allowing a fit.  Successful binding changes the membrane which either excites or inhibits the post-synaptic neuron.
Opioid receptors are coupled with K+ conductors.  Na + and K+ are held on opposite sides of the neural membrane of the post-synaptic neuron, with the inside being slightly more negative than the outside creating an action potential.  When enkephalin binds to the delta receptor, the permeability of the membrane is increased for K+ only, it leaks out-->increasing the action potential of the synapse.  Normally when the neuron accepts a transmission Na+ flows into the cell creating a slightly positive charge.  This charge change lasts about a tenth of a second as Na+ starts flowing back out to create the action potential again.   The signal proceeds to travel down the neuron in the same fashion until it reaches the next synaptic cleft where it either excites or inhibits the next neuron.  When K+ leaks out due to enkephalin binding the charge difference increases as the inside of the cell body becomes more negative.  The action potential is decreased as it takes greater energy for the neuron to reach the firing threshold.

1)  Current causes the presynaptic neuron to become Calcium++ permeable from extracellular fluid.
2)  C++ causes exocytosis of synaptic vesicles (contain neurotransmitters).
3)  A neurotransmitter travels the synaptic cleft->binds to a complementary receptor.
4)  The result is either:
                a)EPSP (excitatory post synaptic potential) ->depolarization
                                                    or
                b)IPSP (inhibitory post synaptic potential)->hyperpolarization (i.e. making the membrane permeable to K+).
Both IPSP and EPSP can occur together as many neurotransmitters act on the neuron at once-->integration of the signal.

Two neurotransmitters that are thought to transmit nociceptive information are somatostatin and substance P (SP).  The latter will be discussed here.

Release of substance P can be inhibited by enkephalin through presynaptic inhibition.

G-protein receptors:
The delta-receptor which enkephalin binds to is a type of receptor called a G-protein receptor.
The G-protein is a hetero-trimer with subunits:
       1)alpha (39-40 Kda)
       2)beta (31 Kda)
       3)gamma (8 Kda)
  The receptor binds to an enkephalin, which causes the intracellular shape of the protein to change, allowing the G-protein to bind.  Once the G-protein is bound, the neurotransmitter (enkephalin in this case) is done, it can leave ->receptor relaxes.  The activated GTP-alpha complex migrates to effector region to relax the K+ pump and allow potassium to leak out.  Many G-proteins can be activated by one receptor.  Activation is rapid.  The alpha subunit remains active until the GTP is hydrolyzed to GDP.


a) Hetero-trimer and inactivated receptor
b) Agonist binding at the receptor causes conformational change-->the G-protein binds.
c) The beta-gamma dimer dissociates
d) The activated GTP-alpha complex then travels to the effector enzyme which makes the membrane K+ soluble.
e) The cycle can be repeated for as long as the agonist binds to the receptor.
 



Experimental Support- Mice without enkephalin:
Mice without the proenkephalin gene, responsible for the formation of enkephalins, were bred to test enkephalin activity in mammals.

(-)mice exhibited some similar characteristics with (+)mice and some not:

abnormal behavior in (-) pups and adolescents included: Both genotypes (+) and (-) exhibited similar levels of analgesia when stimulated with spinal-mediated pain in tail flick latency tests (see graph a).  Enkephalins don't act in the spine to mediate pain.

 

Hot-plate assay was used to measure pain responses by supraspinal mechanisms.  (-)mice immediately shook their foot when put on the plate while (+) explored the area, then shook their foot (analgesia was greater).  (-)mice showed a jump-latency as almost half that of the (+)mice (see graph b)-->supraspinal threshold of pain of (-)mice is lower than that of (+)mice.

A swim test was conducted to examine the effects of stress on analgesia related to enkephalins.  The mice swam in a 4 degree celsius water for 90 seconds.  The degree of analgesia was the same in (-) and (+) mice.  Therefore this test seems to suggest that enkephalins aren't involved in stress-induced analgesia, contrary to previous research.  The conclusion of this test might not be wholly accurate since a presumption is being made that swimming in cold water accurately represents a stressful environment.

(-)mice were less inquisitive about a new environment than (+)mice.  They tended to stay near a wall, didn't enter the open field much.  Less inquisitive, curious-->perhaps lower intelligence.


  Back to table of contents 



References:

Lazarus, L. M.,  Bryant, S. D.,  Salvadori, S.,  Attila, M.,  Jones, L. S. (1996).  Opioid infidelity: novel opioid peptides
    with dual high affinity for delta and mu-receptors.  Trends in Neuroscience, 19, 31-35.

Pasternak, Gavril W. (1988).  Multiple Morphine and Enkephalin Receptors and the Relief of Pain.  Journal of the
    American Medical Association, 259, 1362-1366.

Plotnikoff, N. P.,  Faith, R. E.,  Murgo, A. J.,  Heberman, R. B.,  Good, R. A. (1997).  Methionine Enkephalin: A New Cytokine-Human Studies.  Clinical Immunology and Immunopathology, 82, 93-94.

Heagy, W.,  Teng, E.,  Lopez, P.,  Finberg, R. W. (1998).  Enkephalin Receptors and Receptor-Mediated Signal
    Transduction in Cultured Human Lymphocytes.  Cellular Immunology, 191, 34.

Konig, M.,  Zimmer, A. M.,  Steiner, H.,  Holmes, P. V.,  Crawley, J. N.,  Brownstein, M. J.,  Zimmer, A. (1996).
    Pain responses, anxiety and aggression in mice deficient in pre-proenkephalin.  Nature, 383, 535-537.

Plotnikoff, N. P.,  Faith, R. E., Murgo, A. J.,  Good, R. A. (1986).  Enkephalins and Endorphins: Stress and the Immune   System.  New York, Plenum Press.

Almeida, O. F. X.,  Shippenberg, T. S. (1991).  Neurobiology of Opioids.  Germany, Springer-Verlag Berlin Heidelberg.

Rapaka, Rao S.,  Dhawan, Bhola N. (1988).  Opioid Peptides: An Update.  NIDA Research Monograph.

Levinthal, Charles F. (1988).  Messengers of Paradise: Opiates and the Brain.  New York, Anchor Press.

Orinthal, P. E.  (1985).  Neuromodulation.  New York, Doubleday Press.