Hockenbury Discovering Psychology 5th txtbk

54 CHAPTER 2 Neuroscience and BehaviorFOCUS ON NEUROSCIENCEIs “Runner’s High” an Endorphin Rush?“Runner’s high” is the rush of euphoriathat many people experience after sustainedaerobic exercise, especially runningor cycling. It’s thought to be causedby endorphins, which are associated withpositive moods and reduced feelings ofpain.Although blood levels of one type ofendorphins, called beta-endorphins, doincrease during intense exercise, it seemsunlikely that beta-endorphins cause runner’shigh. Why? Because the beta-endorphinscirculating in the blood do notaffect the brain. When you add the factthat it’s impossible to directly measureendorphin levels in the intact humanbrain, the “endorphin hypothesis” ofrunner’s high has lacked solid evidence tosupport it (Dietrich & McDaniel, 2004).But an ingenious experiment by Germanneuroscientist Henning Boecker andhis colleagues (2008) found a way to directlytest the endorphin hypothesis.Boecker recruited 10 highly conditioned male runners whotrained an average of eight or more hours per week. The runnerscompleted mood questionnaires and underwent a baseline PETscan after 24 hours with no physical exertion.To indirectly measure endogenous opioid (endorphin) activityin the brain, the runners were injected with a radioactivelytagged chemical that binds to opioid receptors. A PET scan detectsthe radioactively tagged chemical. The reasoning was thatif endorphins were released in the brain during exercise, the radioactivesubstance would be unable to bind to the opioidreceptors because the receptors wouldalready be occupied by the brain’s ownnatural opioids—the endorphins. Thus,the number of opioid receptors that didnot take up the radio active chemicalwould provide an accurate measure ofendorphin activity caused by the longdistancerunning.On the day of the second PET scan,each participant returned to the lab aftera two-hour run. After a 30-minute cooldown, each athlete again completed themood questionnaire, followed by a secondPET scan.The post-run PET scans showed highlevels of endorphins binding to opioid receptorsin several brain regions, especiallyfrontal regions known to be involved inpositive emotions (see figure). The analysisalso showed that the greater the subjectivefeelings of euphoria experiencedby each individual runner, the higher thebrain level of endorphin activity.After decades of speculation, Boecker and his colleagues(2008) have provided the first human evidence that runner’s highis at least partially due to the release of endorphins in the brain.The involvement of the brain’s opioid system in runner’s highsuggests one possible explanation for why some people canbecome addicted to excessive exercise, continuing to train despiteillness or injuries. But endorphins do not provide a completeexplanation. As the researchers point out, other neurotransmitters,such as dopamine, are probably also involved inrunner’s high.Endorphins and Runner’s High After two hours of endurancerunning, highly conditioned athletes underwent PET scans.Yellow and orange highlight regions in which opioid receptorswere blocked by the athlete’s naturally produced endorphins.Endorphin activity was highest in regions known to beinvolved in positive emotion and mood, including the frontalcortex. The scans also showed that endorphin activity waspositively correlated with subjective experience: The moreintense the euphoria experienced by the individual runner, thehigher the level of endorphin activity in his brain.How Drugs Affect Synaptic TransmissionMuch of what is known about different neurotransmitters has been learned fromobserving the effects of drugs and other substances. Many drugs, especially thosethat affect moods or behavior, work by interfering with the normal functioning ofneurotransmitters in the synapse (Self, 2005; Volkow & others, 2007).As Figure 2.6 illustrates, some drugs increase or decrease the amount of neurotransmitterreleased by neurons. For example, the venom of a black widow spider bitecauses acetylcholine to be released continuously by motor neurons, causing severe