live, love, hurt

A long-experienced phenomenon, tolerance is the need to increase the dosage of medication to achieve the same effect. This is most frequently seen in illicit drug abuse. Heroin users quickly have a need to take increasing doses in order to achieve the same “high.” The same goes for other illicit drugs like amphetamines and cocaine. With most drugs tolerance is a complicated process and is not fully understood. In most cases we assume the neurotransmitters (or their receptors) affected by the specific drug type is down-regulated in some way. That is to say, if you take a lot of opioid analgesics your body reduces the amount of naturally-produced (endogenous) opioids, and also decreases the amount of opioid receptors in the body.

Some time ago, it was noticed that NMDA receptor antagonists (dissociative anesthetics) like ketamine, phencyclidine, and dextromethorphan have the side-effect of reducing the amount of tolerance formed to opioid analgesics. This has far-reaching implications because if you can mediate opioid tolerance, you can control the amount of opioid needed for pain relief.

The NMDA receptor both induces and maintains persistent enhancements of the excitability of neurons to prolonged stimulation, or “wind-up.” Wind-up is a key spinal mechanism requiring activation of the NMDA receptor that both amplifies and prolongs certain types of pain. As a result, wind-up may be one of the events underlying prolonged or chronic pain. Evidence from animal studies indicates that this mechanism is involved in the induction and maintenance of certain types of pain, most notably inflammatory and neuropathic.

Neuropathic pains are at least partly mediated by the NMDA receptor, which may relate to changes in opioid sensitivity. All opioids reduce, or with high doses block the input that causes certain types of pain, probably via activation of the presynaptic opioid receptors to prevent the release of primary afferent transmitters and so prevent pain input from actually activating the neurons that make you feel pain. However, if the pain continues, wind-up overcomes the inhibitions of input and the neurons commence firing, causing pain. As wind-up increases the activity of neurons, a higher dose of opioid will be required to block the increased excitability. Thus, at moderate doses, opioids delay wind-up without inhibiting the process itself. In contrast, NMDA antagonists abolish wind-up. Thus, threshold doses of morphine combined with low doses of NMDA antagonists are able to elicit dramatic inhibitory effects, a synergism that suggests low probability of side effects. Importantly, in a model of neuropathic pain where morphine is inoperative, the co-application of an NMDA antagonist restored the ability of morphine to inhibit the response.

All that medical speak translates to this: the pain input that’s prolonged and intensified by NMDA receptors can be delayed by opioids, but not inhibited. However, NMDA antagonists (mentioned above) completely turn off the prolongation and intensification, allowing opioids to take away that pain. Basically stated, it amounts to the aforementioned. Adding a mild NMDA receptor antagonist (in extremely sub-anesthetic doses) to an opioid enhances the effects of the opioid, allowing smaller amounts of opioid, and thus fewer side-effects.

At least there’s one good use for dextromethorphan.

Recently my friend and I have been going back and forth on a campaign called ResearchSaves.org, which is a campaign that promotes medical animal testing. My friend is an avid vegetarian and PETA-type guy, and I’m a medical type guy who also enjoys steak. I don’t advocate for the needless suffering of animals, but I frequently leave comments on his blog defending animal research in certain circumstances, such as the following.

The spinal action of opioids is an excellent example of how basic research in animals can lead to improvements in the clinical relief of pain. The knowledge gained from basic animal studies showing an opioid inhibition of nociceptive spinal neurons and the direct analgesia following epidural and intrathecal opioids was soon applied to humans. Importantly, the use of various different models of clinical pain states has led to animal studies addressing the extent and mechanisms of plasticity in opioid spinal function, since pathological and physiological and pharmacological events can alter the degree of opioid antinociception. It is noteworthy that opioid receptors originally cloned from rats and mice allowed much in vivo research, and ultimately it was discovered that the animal opioid receptors are identical biochemically, and pharmacologically, to human opioid receptors.

These animals were used to develop pain and suffering relief in humans. If a couple of massively overpopulated rodent had to die for me to be pain free, I have to tell you that I’m not that upset by it.