1.Role in Evolution
2.Modulation of Pain: Pain Pathway
3.Endorphins and Hormones
4.Endorphins and Stress
5.Endorphins and Live Birth
6.Endorphins and Behavior
7.Endorphins and Emotions
8 Endorphins and Runner's High
9.Endorphins and Schizophrenia


It has been argued that endorphins have provided the foundation for change in brain structure. The change has occurred throughout the brain's long evolutionary journey and has provided changes of behavior that modulates pain reception and emotion so that organisms can respond in an adaptive way to eliminate or cope with the causes of pain. These feelings have made possible our gradual mastery of the environment (7).

The feeling of analgesia has proved to be extremely advantageous toward survival. Endorphins ensure that survival comes first, and recuperation comes later (7). Pain would ordinarily produce behaviors that would hurt your chances of survival. For instance, if an animal is attacked and stops to lick its wounds instead of fleeing away from its attacker, the animal's life is put in danger. But, the emotion of fear triggers an endorphin system that inhibits the processing of pain. Therefore, it is an evolutionary advantage for species who have developed a degree of pain control in times of stress. A phylogenetic study has shown that endorphins exist in the brain in all vertebrates from hagfish to humans (14). Humans can view themselves as part of a biological heritage. This heritage produced an endorphin system for the control of pain also known as analgesia. (7).


The main function of endorphins is the control of pain. The arrival of a pain stimulus comes from pain receptors in the skin. The pain receptors in the skin generate nerve impulses that travel a pathway up the spinal cord to the thalamus and then to the sensory and motor cortices (4). The pain receptor's impulses signal the body of pain by releasing excitatory neurons containing a transmitter called substance P. Substance P is a neuropeptide found in neurons on each side of the dorsal horns of the spinal cord and functions as a transmitter of pain. Substance P provokes other neurons in the spinal cord to fire. These transmitter neurons, containing substance P, diffuse across the fluid-filled cleft between neurons and bind to specific receptor sites on the postsynaptic membrane of the dorsal horns on either side of the spinal cord. The neurons sensitive to substance P then proceed to send the pain message to the brain .

The dorsal horns also house endorphin-containing neurons. The endorphin-containing neurons release enkephalin. Enkephalin is the smaller five amino acid chain of the endorphin family. Enkephalins released from the endorphin-containing neurons inhibit the release of substance P by synapsing between the terminal end of one neuron and the receiving surface of another pain transmitting neuron. This causes the receiving neuron in the spinal cord to receive less excitatory stimulation and hence sends fewer pain-related impulses to the brain (5).


Effects of narcotic analgesics on pituitary hormone release are observed with the endorphins. Endorphins can be injected either intraventricularly or parenterally . Like morphine, they stimulate the release of growth hormone, prolactin, ACTH, and anti diuretic hormone and inhibit the release of luteinizing hormone, follicle stimulating hormone, and thyrotropin. All of these effects are reversible by naloxone. The mechanism of opioid effects on pituitary hormone secretion is not understood. However, the evidence points to an action at the level of the hypothalamus, rather than effects directly on the pituitary gland (14). The hypothalamus is important in mediating some of the output of the limbic system as well as it being essential to the normal functioning of the pituitary gland, which lies directly beneath it and whose importance lies in the fact that many hormones are stored and released here. The hypothalamus also has a role in the control of feeding, drinking and the expression of emotional behavior (1).


There is a behavioral linkage between stress and endorphin release. Beta endorphin and adrenocorticotropin, the classic stress hormones were found to originate from the same precursor molecule that has been located in a range of places in the body such as the hypothalamus and other areas of the brain, as well as several peripheral tissues including the placenta and gastrointestinal tract. The pro-opiomelanocortin gene is expressed in the pituitary, and its peptide products are released into the blood stream in response to stress (7).


Endorphins counter stress of live birth with their analgesic powers. During the gestation period, the placenta provides necessary nourishment for the development of the fetus. The placenta also contains the crucial precursor molecule pro-opiomelanocortin from which beta endorphin, met-enkephalin and adrenocorticotropin are all derived (11). In the human placenta, beta-endorphin and met-enkephalin are present in the placental tissue and placental blood at higher levels than usual during pregnancy and labor.


The limbic system contains prominent structures that include the amygdala, septum pellucidum, hippocampus and cingulate cortex (7). All of these structures act with the hypothalamus as the integrator of emotional responses. The limbic system contains some of the highest concentrations of opiate receptors and endorphins in the brain. The evidence for linkage between brain endorphins and the concept of social behavior began to emerge in 1978 from experiments that administered morphine to young puppies and guinea pigs. They became less inclined to cry when they were separated from their mothers. The symptoms of separation distress were reduced. The antagonist naloxone, on the other hand increased the incidence of separation cries. This implicated the role of endorphins in this critical behavior of social bonding.


Neuroscientists have agreed that emotions are mediated by the limbic system of the brain. The amygdala and the hypothalamus are both classically considered the main components of the limbic system. Experiments were performed showing the connection between emotions and the limbic system. Neurologists found that when they used electrodes to stimulate the cortex over the amygdala they could evoke a whole array of emotional displays--- powerful reactions of grief, pain, pleasure associated with profound memories and also the total somatic accompaniment of emotional states. A map locating the opiate receptors in the brain, by a method involving radioactive molecules, found that the limbic system was highly enriched with opiate receptors, forty-fold higher than in other areas in the brain. These hot spots correspond to very specific nuclei or cellular groups that physiological psychologists have identified as mediating such processes as sexual behavior, appetite, and water balance in the body (8).


Euphoria and exhilaration are reported by many runners while in the course of their run (7). It has been tempting to relate these feelings ("the runners high") to an increase in brain endorphins. There is circumstantial evidence that a connection might exist. It has been found that physically untrained volunteers going through a two-month exercise program produced significantly higher levels of adrenocorticotropin and beta-endorphin in their blood system. In another study, for trained runners, beta-endorphin levels in blood plasma increased after an eight-mile race to levels three and a half times higher than levels taken immediately beforehand. It is possible however, that the endorphin increases could have been secondary to the adrenocorticotropin increases and that we might be seeing a general response to the stress of physical exertion (7).


Schizophrenia is a mental disorder consisting of agitation, disorientation, delusions and frequent hallucinations. Possible connections between schizophrenia and endorphins have been drawn by several experimental observations. It has been suggested that levels of endorphins are related to schizophrenia. This suggestion is based on the appearance of a catatonic-like state in injected animals. Other studies showed a high endorphin level in the cerebrospinal fluid of schizophrenics (7).