SNIP (through introduction.)
Endorphins: "The Morphine of the Mind"
Endorphins (meaning "morphine within") and enkephalins ("in the head") were only two of several morphine-like substances (opioids) discovered within our brains as recently as the '70s. Enkephalins are pentapeptides, and endorphins are polypeptides containing 30 amino acid units. Opioids are considered stress hormones -- alongside others including corticotrophin, cortisol, and catecholamines (adrenaline, noradrenaline). Stress hormones are released by activity in sympathetic fibers; following pain, excitement, anxiety, hypoglycemia, cold, or hemorrhage. While adrenaline's release is accompanied by renewed synthesis, other hormones require positive stress (eustress) for reproduction.
Diet, exercise, and general wellbeing control the production of endorphins, but stress and pain trigger their release. Examples: eating spicy foods (your mind believes your mouth is on fire and supplies some natural morphine), having charging sex (that sudden giddy feeling, or those painless pin pricks in your brain), or stubbing your toe (pain again). Why would a natural analgesic occur during something as pleasurable as sex? Endorphins (and elevated serotonin levels) cause "runner's high", a release which occurs due to necessary pain alleviation in stressed muscles. Invigorating sex is exertion, and therefore causes the same release. Intense pleasure also relates to pain; women often have the same extroverted, and chemical, reactions in child birth as they do during sex. You know those moments when you feel like it's just too much, but you're loving it and don't ever want to stop? Ever get the cranial pin pricks and euphoria right then? Some people release endorphins during orgasm, others do so from non-climactic over-stimulation.
Endorphins control emotions as well. The psychological model is "Glad, sad, and mad", with fear as a sidebar. The average person (harrumph) is typically in glad mode. If duress downshifts them to sad or mad, endorphins are released for re-elevating them to glad. If fear strikes, endorphins similarly allow coping by providing a feeling of calm euphoria. Such a nice feeling, perhaps too nice of one. Your brain (the primal portion) maintains a certain quota of endorphins to ensure survival under duress. If you don't have that quota, you've got problems. In the instance of pain you simply aren't supplied with an analgesic, but it becomes more complex with emotional reactions. A startle reflex can escalate into terror and paralysis, depression can downgrade into catatonia. Or your body can compensate by supplying adrenaline, which is intended for lifting you back up to glad mode but often overshoots you into an explosive rage. Why would your mind consider this better? It's simple. If your upset is inspired by a physical threat, then obviously paralysis and catatonia musn't be allowed. Rage may be draining, but it is more likely to get you through situations than lying there with your head in the sand is. Adrenaline is your body's response to the threat of atrophy and death.
SNIP (through opioid mechanisms and basic biochemical function.)
Heroin: Synthesized Opioids
During WWI morphine was the most common analgesic for wounded soldiers and, as a result, numbers in the range of 40,000+ became addicted to it. A cure was needed and, presumably, found. Faith in this new drug's ability for curing the addiction was so great that the Bayer Company even assigned the trade name Heroin(e) to it as a show of everyones' hopeful expectations. Heroin, though derived from morphine, could satisfy the addicts' cravings with far less quantity. Same effect, lesser amounts, should be good for weaning the soldiers down. It was, they lost their morphine addictions miraculously fast. Then heroin addiction surged. It may seem unnerving that such hazardous methods were used for treating addiction, but no one realized that heroin happened to be more addictive than morphine. These were our fine soldiers they were treating, not a bunch of street scum who picked up bad habits they couldn't self-manage.
If you follow modern research involving junkies at all, you'll notice that they tend to be present through virtue of their high drug tolerances alone. They are, effectively, lab rats for the wealthier man's analgesics. You can see the research heading into possibly helpful directions, then it simply never becomes applied. We all know why junkies would volunteer for drug trials: they get paid, they go score. It's a hazard, though, because their biochemistries are already far too scrambled and using any drug risks causing further damage. "Ah, who cares, they obviously didn't mind ruining their bodies with drugs that much to begin with." The surging trendiness of heroin amongst party and rave hopping rich teens might ultimately invoke an increase in applied research, but we can only wait for the scientific community to decide junkies are people too. In the mean time, let's take a look at why the biochemistry of an addict (or recovering addict) is so screwy.
Any drug works within your body's biochemistry. LSD and psilocybin target serotonin, preventing it from reaching specific neurons in the brain. This increases activity in the accessible brain cells (which, in turn, can produce hallucinations). Nicotine mimics acetylcholine, opening channels that allow sodium molecules to rush into your nerves. Speed and cocaine block the reuptake of norephedrine, hence it continues to contact membranes and over-stimulate them. Opiates (of opium poppy descent: opium, morphine, heroin, codeine) function by fitting into the sensory receptors which opioids bind to. They truly are synthetic opioids.
SNIP (through mechanisms of opiates, and standard withdrawal "medicines".)
And the Twain Addictions Shall Meet in Overwhelming Withdrawal
Picture yourself as an opioid connoisseur who has become terrified of sex, where you relied on getting your fix. The sudden aversion could be the result of anything from rape trauma to getting used and dumped by your first object of lust. Either way, sex abruptly becomes a thing to avoid. At this point, you can't achieve a natural fix but you do have $50 for a score. A sober person probably wouldn't think "Bummer, no endorphins, I should get on heroin", but incidental usage would trigger (on a subconscious level) recognition of the craved opioid binding -- albeit a synthetic version of it. The predisposed "dependency" to this particular rush would lend to the appeal of the much longer version provided, on demand, by heroin. The quick-rush morphine, replaced by a synthetic endorphin... Addiction transference is ever so simple.
Biochemical instability (such as from junk sickness) causes you to produce less endorphins, further aggravating the lack of binding. If you kick heroin, you suddenly find yourself far less capable of getting a natural fix than you were before you began using it. You'll be craving endorphins and heroin, and the only one you'll be able to get is the latter. Prior to using synthetics you were still capable of having normal opioid bindings, you simply needed to find non-sexual methods for stimulating them. Stubbing your toe or consuming incredibly spicy food would've worked. After heroin... if you stub that toe, you'll just have to deal with the pain. No rush, no analgesic, no satisfaction for overwhelming cravings, no nothing. Tough luck for now, you fucked up a perfectly good natural drug system. Your body is also spitting out all of the wrong hormones, trying to compensate for the lacking reactions, hence increasing illness. Quite a predicament, what can be done?
SNIP (through biochemical/immune stimulation, sedatives, anti- depressants, downscaling of addictions, etc.)