Effects of Experience and Environment on the Developing and Mature Brain: Implications for Laboratory Animal Housing

Ann C. Benefiel, B.S., is Director of the Biological Resources Facility at the Beckman Institute, Urbana, Illinois. William T. Greenough, Ph.D., is Swanlund Professor and Center for Advanced Study Professor at the Beckman Institute and in the Departments of Psychology, Psychiatry, and Cell and Structural Biology, University of Illinois at Urbana-Champaign. Ann C. Benefiel, B.S., is Director of the Biological Resources Facility at the Beckman Institute, Urbana, Illinois. William T. Greenough, Ph.D., is Swanlund Professor and Center for Advanced Study Professor at the Beckman Institute and in the Departments of Psychology, Psychiatry, and Cell and Structural Biology, University of Illinois at Urbana-Champaign. In normal animal development, experience molds the brain and behavior. The potential for experience to alter brain organization and behavioral capabilities and tendencies continues through most of, if not all, an animal's (or human's) life. A key component of this view is that a “normal” nervous system is one that has been exposed to some minimal amount of experience, allowing it to develop normally. However, laboratory animals such as rats, housed in minimally stimulating surroundings through their lives, may have abnormal, underdeveloped brains and may, under certain conditions, react in ways that are different from animals exposed to a broader array of experience. In the context of regulations regarding current, standard animal housing and alternative housing that may be more optimal for the welfare of laboratory animals (particularly rodents), we first address special sensitivity of the developing nervous system to experience, using the term “experience-expectant information storage” ( Black and Greenough 1986 ; Greenough and Black 1992 ). We then discuss lifelong information storage, termed “experience-dependent information storage” ( Black and Greenough 1986 ; Greenough and Black 1992 ). The development of many, if not all, mammals involves certain consistencies in the information available in the typical rearing environment. For example, in visual development, it is very likely that all members of a species have been exposed to a common set of visual stimuli that, at least for diurnal animals, includes a diverse array of patterned input via the retina. Deprivation studies have indicated that the visual systems of cats and monkeys require or “expect” this stimulation for normal development. If normal pattern vision is prevented, either through occlusion ( Hubel and Wiesel 1970 ) or through presenting a limited range of visual stimuli such as only vertically oriented lines ( Hirsch and Spinelli 1970 ), the performance of the visual system will be relatively insensitive to stimuli at any orientation, in the case of occlusion, or at the nonexposed orientations, in the second case. Similarly, if 1 eye gets normal experience and the other is deprived, the experienced eye develops normal vision and the deprived eye becomes effectively blind ( LeVay and others 1980 ). In both cases, there is a critical or sensitive period when normal visual experience must occur if normal vision is to develop. The changes that occur do so within the visual cortex of the brain, whereas the eyes themselves show little if any effect of these manipulations. Once this experience has occurred, the system becomes nearly invulnerable to deprivation. The nervous system appears to have evolved a special mechanism for encoding “expected” information. During development, the visual cortex overproduces the synaptic connections through which nerve cells communicate, and the numbers peak within the sensitive period for experiential organization. Subsequently, as experience “tunes” a functional system, a subset of these connections is strengthened while the remainder is pruned away ( Boothe and others 1979 ; Bourgeois and others 1994 ; Cragg 1975 ; LeVay and others 1980 ). This process is similar to that of marble sculpture, in which the functional form is achieved by removing the marble that does not belong. Hence, loss of synapses is essential to the development of a functional visual system; and survival and loss of synapses--as well as the operational quality of the system--depend on experience. Similar overproduction and loss processes have been reported for the human visual system and have been used to characterize human language and cognition development ( Huttenlocher 1994 ). Beyond early development, the brain continues to acquire information from experience. Much of this information is unique to the individual and hence may involve different mechanisms from those used for species-typical information storage. The nature of the environment and the animal's interaction with it determines much of the character of the brain--in fact, most of what we would use to describe ourselves to others reflects this sort of information storage. Considerable data indicate that 1 mechanism underlying this lifelong information storage is the formation of new synapses in response to experience. The best-known laboratory example of this, and one very germane to our interest in laboratory animal welfare, is a series of experiments involving rearing and adult housing environments that differ in physical and social complexity. A typical experiment involves laboratory animals such as rats housed individually in standard laboratory cages (“individual cage” [IC 1 ]), animals housed in pairs in similar cages (“social condition” [SC 1 ]), and animals housed in a group of approximately 12 in a large cage filled with an arrangement of play and exploration objects that is changed daily (“environmental complexity” [EC 1 ] or “enriched condition“). It has been known since Hebb (1949 ) initiated this research that animals from EC housing outperform the other 2 groups on complex, appetitively-motivated tasks such as mazes. Although this may in part reflect greater familiarity with large, open, spatially-complex environments, there is also evidence that EC animals use different strategies and have more well-developed concepts of the properties of objects ( Juraska and others 1984 ; Thinus-Blanc 1981 ). Some time ago, it was found that the nerve cells of animals reared in, or as adults housed in, the EC environment have larger dendritic fields (the “input side” of the neuron) than those of the other 2 groups ( Volkmar and Greenough 1972 ). As this finding might predict, EC rats also have more synapses per neuron ( Turner and Greenough 1985 ). These results are compatible with the hypothesis that information arising from experience in the complex environment is encoded, at least in part, in new circuitry enabled by the added synapses. It should be noted that in addition to these brain effects, other differences include lowered body weights and slowed growth of certain organs and of bones in EC rats ( Black and others 1989b ). Further discussion of whether learning or some other aspect of complex environmental exposure drives and is encoded by brain changes appears below (see “Are Experience-dependent Effects Due to Learning?”). The support systems of the brain--the vascular and glial systems--exhibit a form of plasticity similar to experience-dependent synaptic plasticity, although it appears to be more age-dependent and probably of shorter duration. In rats housed from weaning in EC, the capillary volume per nerve cell is about 80% greater than for IC rats ( Black and others 1987 ). The size of the effect diminishes in rats first housed in EC as adults ( Black and others 1991 ) and is further reduced with aging ( Black and others 1989a ). We compared the surface density (Sv) of astrocytic processes, identified as immunopositive for their characteristic glial fibrillary acidic protein, and found that the total surface area of astrocytes per neuron was similarly greater in EC rats reared in the complex environment from weaning than in IC or SC littermates ( Sirevaag and Greenough 1991 ). There are indications that the astrocytic effects are largely driven by the needs of the new synapses, as glial volume correlates with synaptic numbers ( Anderson and others 1994 ) and the glia appear to envelop synapses more thoroughly in the EC rats ( Jones and Greenough 1996 ). Other work has suggested that oligodendroglia and the myelin with which they envelop axons are similarly sensitive to these experience manipulations ( Juraska and Kopcik 1988 ; Szeligo and LeBlond 1977 ). With the synapse data, these results present a very dynamic view of the brain. Essentially all cellular components appear to adjust to the demands placed on the brain by the organism's interactions with its environment. It is clear that the anatomy and physiology of the brain vary with the housing environment. Thus, it would not be surprising for the results of some experimental designs to differ in important ways from others, depending on the housing conditions of the animal subjects. One example of a housing-based physiological difference that is interpretable from the anatomical data comes from an experiment (T. L. Ivanco, Beckman Institute, Urbana, Illinois, personal communication, 1997) in which we recorded from the visual cortex of rats reared in EC and IC conditions while stimulating the subcortical white matter afferents to the visual cortex. In our recordings, we could identify the electrical potential from visual cortex neurons termed the “population spike,” the sum of the individual firings of visual cortex nerve cells in response to the stimulus. As we expected, since the neurons in the visual cortex of the EC rats had more synapses, the summed firing reflected in the population spike was greater in this group. This result supports our anatomical work and indicates that the physiology of the brain can differ depending on the kind of housing in which the animal has been reared or maintained. While these experimental environments are extremes, relative to the laboratory norm, the results show that housing conditions can affect outcomes of experiments in ways that could make replicating work from another laboratory very difficult if the housing conditions are not also replicated. Two basic interpretations exist for the effects of complex environmental (EC) housing on the brain. According to the first interpretation, as described above, the synaptic changes encode information from experience. In short, the synaptic changes mediate learning and memory, which occurs to a much greater extent in animals in a complex environment. In the second, contrasting interpretation, the brain is functionally much like a muscle, and its nerve cells hypertrophy when they are exercised. According to the second view, the combination of neuronal hypertrophy and neovascularization is very similar to the condition of muscles with exercise. As a test of these alternative interpretations, we devised an experiment in which 1 group of rats underwent a great deal of learning with very little exercise and associated neural activity in the brain (“acrobat condition” [AC 1 ]), 2 groups had considerable exercise and little learning (“voluntary exercise” [VX 1 ] in a running wheel attached to the home cage, and “forced exercise” [FX 1 ] on a treadmill), and another group of “cage potatoes” (“lack of exercise” [LX 1 ]) received neither exercise nor learning but were handled each day to control for the effects of handling. We first examined the paramedian lobule of cerebellar cortex ( Black and others 1990 ), and we have since obtained comparable data for motor cerebral cortex ( Kleim and others 1996 ). Results indicated that the AC rats formed new synapses compared with all 3 other groups in which synapse numbers did not differ, indicating that learning rather than neuronal activity was the important variable with regard to synapses. By contrast, the 2 exercise groups formed new blood vessels compared with the AC and LX groups, whose blood vessel measures did not differ. Thus, the formation of new synapses and capillaries is independently governed and, at least in this case, is differentially activated by aspects of particular behavioral experiences. In separate work, we found that astrocyte volume was correlated with synaptic number change both within and across groups ( Anderson and others 1994 ). However, when training was suspended for 4 weeks, astrocyte effects decreased to nonsignificant levels, whereas synapse numbers and behavioral performance remained at presuspension levels ( Kleim and others 1997 ). Thus, astrocytes appear to be jointly governed by the formation of new synapses and by their behaviorally maintained activity. These findings reinforce the important roles that housing conditions can play in physiologic characteristics of laboratory animals. As we consider modifications of housing for the improved welfare of laboratory animals, we must take the potential physiologic effects of these changes into account. The foregoing findings were from studies using rats. Similar findings have been reported in mice, cats, and monkeys (for example, Beaulieu and Colonnier 1987; Floeter and Greenough 1979 ). However, recent studies using pigs remind us of the limits of generalization. In 2 studies (T. Grandin, Colorado State University, Fort Collins, Colorado, personal communication, 1997; Morrow-Tesch 1997 ), swine were reared in conditions intended to simulate the complex environmental conditions described above--basically either group housing with access to objects with which the animals could play and explore or an outside enclosure allowing social interaction and exploration. Comparison groups were housed in conditions reflecting more standard agricultural methods that provided some social contact but limited additional stimulation. Both studies found no effect on the visual cortex, which was heavily affected in the studies described above. Surprisingly, Grandin and others found somatosensory cortical dendritic branching to be greater in the animals housed under the standard agricultural conditions, in this case an indoor pen without bedding containing 2 pigs each. Morrow-Tesch, however, found no differences in somatosensory or visual cortex between preweanling pigs housed in indoor farrowing and those housed in with bedding and with Morrow-Tesch did numbers of on neurons in cortex of pigs housed in the environment from weaning to weeks, to the differences in the environment in this It is that the more deprived animals levels of somatosensory in the as Grandin and others it may be that the to a is a is a is a reflects the to a basic known to those and animal species to environmental in different In this we have not the data that these environmental effects are to in the (for example, 1980 ), to that rats placed in EC, and environments do not differ in across provided that the about of does that early individual cage which should be The to additional information is to Black and others ). 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