pain threshold will change and result in a stronger pain response. Researchers believe that there are parallels that can be drawn between these animal trials and persistent pain in people. For example, after a back surgery that removed a herniated disc from causing a pinched nerve, the patient may still continue to "feel" pain. Also, newborns who are circumcised without anesthesia have shown tendencies to react more greatly to future injections, vaccinations, and other similar procedures. The responses of these children are an increase in crying and a greater hemodynamic response (tachycardia and tachypnea). Drug sensitization occurs in drug addiction, and is defined as an increased effect of drug following repeated doses (the opposite of drug tolerance). Such sensitization involves changes in brain mesolimbic dopamine transmission, as well as a protein inside mesolimbic neurons called delta FosB. An associative process may contribute to addiction, for environmental stimuli associated with drug taking may increase craving. This process may increase the risk for relapse in addicts attempting to quit. Cross-sensitization Cross-sensitization is a phenomenon in which sensitization to a stimulus is generalized to a related stimulus, resulting in the amplification of a particular response to both the original stimulus and the related stimulus. For example, cross-sensitization to the neural and behavioral effects of addictive drugs are well characterized, such as sensitization to the locomotor response of a stimulant resulting in cross-sensitization to the motor-activating effects of other stimulants. Similarly, reward sensitization to a particular addictive drug often results in reward cross-sensitization, which entails sensitization to the rewarding property of other addictive drugs in the same drug class or even certain natural rewards. In animals, cross-sensitization has been established between the consumption of many different types of drugs of abuse – in line with the gateway drug theory – and also between sugar consumption and the self-administration of drugs of abuse. As a causal factor in pathology Sensitization has been implied as a causal or maintaining mechanism in a wide range of apparently unrelated pathologies including addiction, allergies, asthma, overactive bladder and some medically unexplained syndromes such as fibromyalgia and multiple chemical sensitivity. Sensitization may also contribute to psychological disorders such as post-traumatic stress disorder, panic anxiety and mood disorders. See also Long-term potentiation Multiple chemical sensitivity Neuroplasticity Synaptic plasticity References Behaviorism Learning