OUTLO()K Behavioral Toxicology Looks at
Research at.o?mpts to relate subtle chemical and physiological
Air Pollutants Health, as onl! definition has it, is the ability to function efficiently in a given envir0nment. Thus, while millions suffered no immediate ill effects from the London air pollution episode of 1952, thou~ands-many of them with respiratory problems-died from it. The word "immediate" adds another dimension to the problem of defining health. How is the individual affected hy long-term, low·level exposures to air pollution? Respiratory effects are the number one suspect. But a new school of thought asks: "Do we know what the effects really are. For example. how is the heart affected by air pollution exposures?" "And how about behavior?" asks Dr. Charles Xintaras, head of the National Air Pollution Control Administration's hehavioral toxicology unit. Xintaras and a small group of research workers in Cincinnati have been studying how information is processed in the brains of rats and monkeys and how certain pollutants change this processing. The research workers' efforts in behavioral toxicology are in three general areas: • Neurochemistry. This phase of the work is just getting underway; it will consider how pol:utants interfere with chemicals such as serotonin, acetylcholine. and noradrenaline, which are responsible for transmission of electrical signals in the brain. The chemical processes of the brain are probably the first to be changed by certain pollutants. A profile of the subcellular distributions of neurotransmitters is being developed in sucrose homogenates of rat brain fractionated by the techniques of differential and density gradient ultracentrifugation.
~hanges
induced in the brain to changes in behavior
• Neuro(Wbysiology--or, more precisely, electrophysiology. Chemical changes will probably be followed by changes in function. Xintaras and his group have concentrated on what happens to electrical activity in the visual cortex and in the superior colliculus, which controls the visual motor system. (The visual system was chosen for study because it is important in decision making processes.) • Behavior. "Starting from the idea that the brain-and eventually behavior-is affected by pollutants, we had to decide what to look for and how to measure it," Xintaras says. "We chose to look at timing sense, problem solving, and decision making. It has taken us considerable time to work out our methods of measure~ ment, and we are now ready to investigate how low-level pollutants affect some of the central factors in visual discrimination and perception." Within the year, Xintaras' associate, Dr. Barry L. Johnson, a bioengineer, plans to have a process control computer connected on-line with the experimental animals. "Then we'll be able to look at how pollutants affect the whole animal organism, not just an isolated part. w~·n · be recording a variety of physiological responsesbody temperature, EEG, EKG, and respiratory ga~es-while simultaneously controlling and evaluating how the animal copes with increasingly complex decisions and performance tasks." Three pollutants studied The Cincinnati group has concentrated its studies on three commonly occurring pollutants:
• Carbon monoxide. Its primary effect appears to be on the brain through an interference with the subject's awareness of his environment. • Ozone. Research at Wright-Patterson Air Force Base suggests that ozone may impair vision. In addition, the Cincinnati group has found that b0dy temperature is depressed. This finding could be a consequence of peripheral vasodilation or an impairment of the brain center that regulates body temperature. Further studies are in pr:ogress to try to account for this decrease in temperature. • Lead. The brain is protected against foreign materials that have entered the blood stream (blood-brain barrier), but heavy metals like lead are exceptions. The metals appear to interfere with brain function rather than to damage the cells themselves. Carbon monoxide and ozone response The results of the research to date indicate that the carbon monoxide, ozone, and lead pollutants selectively affect the electrical activity of the brain. Using the EEG and an on-line averaging cor:tputer to evaluate 'the responses, Xintaras compares how an animal responds to a flashing light both before and after exposure' to carbon monoxide or ozone. Each light flash evokes a complex pattern of electrical potentials in the areas of · the brain responsible for vision. Such potentials, called visual evoked potentials. can be used to characterize the stat~ of an animal's visual system. The responses induced by carbon monoxide are strikingly similar~ to those obtained when pentobar~ital is injected; in fact, the depressanfeffects Volume 2, Number 10, October l~8 731
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• the same experimental de~igh.'An animal's responses to aflashing light are determinedtwhen he is awake and alert, then relaxed, drowsy, .arid, rinal)y. asleep. These responses· are cbmpared with those attp~'he has been exposed to.· pollutants. ·. · · .· · · ;, ·. To ensure that the anif\Jal pa'ys attention to U·.e light and is always iri the same relative position, Xintaras uses this pro~edure:. the. animal· is. y~mditioned to depress a lever to get food. Afterhe ha~ fear11ed this response, the lightis.fla~hed whenever he presses the lever. When his rate of depreSSing the lever h'~.~ been stabilized, he is ready for the experiment . ... The lever~md light are remo~~d and the animal is ex· posed to a pollutant, generally f9r about two hours. The 13verand light'arethen restor~& and the responses mea· sur~d again. . . ·.· ... : lhe principal tt:mi.Xinta.ras .~~es to measure these re~ . Sponses 'is the electroeinceph~Iogram (EEG), which re· . cords the eiectrical poten,ti.al~'!/developed between two . . electrodes Contacting S3parale points of the head. In dinica] situations: ari exp~riknced observer analyzes the EEG data directly .as recorcle:,d by/an ink writer. · felt he needed rn?re quantitative, objective ofanalysis;,so he •iv~bt)o: use of an on· line digital . to a~erage .hurgr¥CI,s:'of responses. The ave rag· · increasesAHe. resolution of the responses evoked responses SUC· the. . .light by, \\ere noted immediately and grew progre~sively larger each day. When the tests ended, 48 hours were required to restore the animal's responses to the preexposure baseline. However. throughout the exposures the animals continued to depress a lever for food at the preexposure rate. "What appears to be happening," Xintaras thinks. "is that carbon monoxide changes what we call the 'sensory awarene~s· wave in the EEG. The result is that sensory information is no longer pw.::essed properly-just as it is no longer processed properly when an animal falls asieep. A shutter closes and information no longer gets through." The response' induced by ozone are quite diiTcrent from those induced by carbon monoxide and pentobarbital. This suggests to Xintaras that ozone acts at a different site in the brain. In addition. t1zone-induced changes do not appear until one hour after exposure, whereas carbon monoxide-induced changes appear promptly. The ozone experiments were carried out for 1 hour at 0.5-1.0 p.p.m. ozone. The threshold limit value for industrial establishments is 0.1 p.p.m. However. Xiutara'i feels the changes in response are " l marked that they would occur at con.::entrations less than 0.5 p.p.m. He .::oncludes that people who must make accurate judgments, correct decisiom. and rapid responses in performing their duties, and also are exposed w low levels of carbon monoxide and ozone. may lose efficiency in performing their tasks. It is possible that a similar situation may exist for other toxic substances. Lead· effects To study the effects of chronic lead absorption. Xintaras examined the spontaneous EEG sleep records of rats and related them to behavior changes. Sleep tends ro follow orderly patterns,
so changes are relatively easy to detect. Also, sluggishness is one of the early symptoms of lead poisoning; The research indicates that lead changes the period of deep, or dream, sleep, shortening the period and making it less stable. Treated animals showed more dream sleep in the early sleep period as compared to baseline data; in addition, the patterns of deep sleep became less regular. These changes in dream sleep may directly (or indirectly) reflect the impairment of a neural control system, according to Xintaras. The fragile quality of dream sleep may make it a sensitive indicator of "good sleep." Altered functions of the central nervous system may be evident before clinical signs and symptoms have been induced by exposure to toxic materials. In other words, sleep impairment will provide a stress which the animal will have to overcome to perform a given task efficiently. In particular, the changes in dream sleep may involve the mechanism responsible for controlling transitions between the various levels of consciousness. This may account for the sluggishness associated with exposure to lead and other toxic metal's. Bigger things to come With the approach to the electrophysiological part of the research established, Xintaras and his group are now ready to move on to behavior itself. Within the next year, he plans to have a process .control computer connected on-line with each of five rhesus monkey chambers. The computer will be able to control and evaluate behavioral tasks presented to the animals. The animals will carry implanted electrodes to gather data on electrical activity. A variety of physiological data will be gathered-heart and lung actions will be monitored, for example. While correlating all this information, the computer will present animals with two types of behavioral tests that involve the type of decision making humans are called on to do: o Time discrimination. The animal sees a one-second light flash; after a pause, he will see another flash. His task will be to determine if the time duration of the second light is equal to, greater than, or less than. the duration of the first light.
o Pattern recognition. The aniwa! .is presented with a 5" x 5" square containing 25 light panels, each of which can be separately illuminated. The computer will have stored in its memory various patterns which the animals will be called on to compare wi:th other patterns. Temporal factors in perception are of particular c.oncern for two reasons. First, with modern electronic equipment it is easy to generate and control time-limited stimuli and to record and measure the temporal aspects of the response. Second, temporal faCtors are more readily compared .in different species than almost any other stimulus or response parameter. Consequently, time-limited phenomena may be measured and compared in essentially parallel investigations. · Once the animal's base responses are established, he will be exposed to carbon monoxide. "The computer will be looking at how an animal system responds to the environment," Xintaras declares. It will correlate what is happening to the brain's electrical activity, what is happening to other important systems of the body, and how the animal behaves. 'The underlying importance of all our work," Xintaras declares, "is ·to discover what stressors in the environment may ma1.e it more difficult for people to perform efficiently at given tasks. These stressors might be heat, noise, or pollution, for example. Of course, we can't hope to assess every chemical in the air. We can hope to assess the more important ones 'in these terms: how they affect performance efficiency of a specific task. To do this we must also learn what that task demands of the brain. For example, typing involves largely small muscle coordination. Automobile driviqg, on the other hand, involves decision making. A pollutant may be tolerable in one situation, but not in another," he points out. "Another critical factor is redundancy. In most real life situations it isn't essential that the brain process all information properly. But it may be essential in the case of an air traffic controller or an astronaut," Xintaras goes on. "So we try to design our experiments tu get around redundancy arid to find out how information is laid down."
