BEHAVIORAL AND NEURAL BIOLOGY 29, 522-526 (1980)
BRIEF REPORT Suppression of Adult Polydipsia by Early Stimulation L. D. DEVENPORT AND J. A . DEVENPORT 1 Department of Psychiatry and Behavioral Sciences, University of Oklahoma Health Sciences Center, P.O. Box 26901, Oklahoma City, Oklahoma 73190 Sprague-Dawley rat pups received stimulation during two preweaning phases (postpartum Days 5-8 or 25-28). They were tested as adults for onset and magnitude of schedule-induced polydipsia. Stimulation during either phase significantly retarded the onset of polydipsia as compared to nonstimulated controls. As the stimulation procedure was applied during hippocampal development, and in view of the modulatory action of this structure on schedule-induced polydipsia, the results suggest that the enduring effects of early stimulation were mediated by the hippocampus.
The effects of infant stimulation or "handling" on later behavior are of unusual interest for their apparent permanence. Moreover, this area of investigation holds the promise of explaining broad differences in adult behavior that are not directly attributable to genetics or conventional learning. The literature in this field has come to focus upon the hypothalamicpituitary-adrenocortical axis as the mechanism through which the effects of handling are mediated. Early stimulation acutely activates (Denenberg, Brumaghim, Haltmeyer, & Zarrow, 1967) and permanently modifies (Levine & Mullins, 1966) the responsiveness of this system; and those behaviors for which adrenocorticotrophic hormone (ACTH) and adrenal steroids have been implicated (diGiusto, Cairncross, & King, 1971) are the ones in which positive handling effects are seen (Denenberg, 1964). The construction of the handling model has generally followed that established for sexual differentiation. That is, early release of steroids exerts an organizing action on parts of the CNS such that the behaviors controlled by these areas and their interaction with the hormone are permanently altered (Levine & Mullins, 1966).
i The work was carried out at Southern Oregon State College, Ashland, and was supported, in part, by a grant from the SOSC Institutional Research Committee. 522 0163-1047/80/080522-5502.00/0 Copyright© 1980by AcademicPress, Inc. All rights of reproductionin any formreserved.
EARLY STIMULATION AND SIP
To date, the effects of handling in rodents have been most strongly associated with fear-motivated behavior (shock avoidance and open-field activity). The present experiment relates handling to another class of behavior, adjunctive responses. These responses, of which a wide variety have been identified (Falk, 1971), are schedule-induced behaviors which arise from intermittent reinforcement situations (Falk, 1971). Although the function served by adjunctive responses is uncertain, they have important physiological consequences (e.g., overhydration; Falk, t971); and as a class they interact with conditioned behavior to produce some of the characteristic features of operant performance (e.g., behavioral contrast, Hinson & Staddon, 1978). Recent findings raise the possibility of a link between the prototypical adjunctive response, schedule-induced polydipsia (SIP), and early stimulation. SIP is under the modulatory influence of the hippocampus (Devenport, 1978). This is significant in the context of handling because the hippocampus is a target for the rat's major glucocorticoid, corticosterone (reviewed in Devenport, 1978). Changes in the secretion of corticosterone as a consequence of handling could influence the expression of SIP, as fluctuations in corticosterone are known to alter the emergence of the polydipsic pattern in adult rats (Devenport, 1978), presumably as a result of changes in hippocampal activity. Alternatively, changes in infant adrenal output, as induced by handling, might have lasting CNS effects. The rat hippocampus does not cease to develop until about 30 days after birth (Altman & Das, 1965) and handling during this period affects cell mitosis in the hippocampus (Altman, Das, & Anderson, 1968) and hippocampaldependent behaviors (Douglas, 1975). This suggests that the developing hippocampus might be especially vulnerable to acute handling-induced adrenocortical secretions. In this way, handling might bring about enduring changes in adjunctive responsiveness. We tested this possibility by stimulating rat pups for 4 days during an early or late part of the reported phase of hippocampal maturation. SIP performance was then compared with that of unstimulated controls. Breeding animals were Sprague-Dawley rats obtained commercially (Simonsen Laboratories, Gilroy, Calif.) and housed in pairs by like sex in suspended wire-mesh cages. They were maintained on a 14/10 light-dark cycle and had free access to food and water. Twelve nulliparous females (approximately 70 days old) were allowed to cycle for several days after which they were paired individually with males. Following pregnancy, as confirmed by vaginal smears, each female was housed in a ventilated glass maternity cage (60 x 41 × 31 cm). Wood shavings and nesting materials were provided along with food and water. At that time prospective litters were randomly assigned to either an early-stimulation (ES), latestimulation (LS), or nonstimulation (NS) group (n -- 4 litters per condition). Animals were checked each day between 0900 and 1100 hr. Within
DEVENPORT AND DEVENPORT
12 hr of birth, litters were sexed and reduced to eight pups, retaining four males where possible. This necessitated separation of the mother from her litter for approximately 10 min. Thereafter, litters were not disturbed except to replenish food and water, with care taken to cause minimal disruption. As it has been reported that hippocampal maturation spans about the first 30 days postpartum, we chose to stimulate animals at an early and late stage of this process. The early treatment (ES), administered daily across Days 5-8, consisted of removing the mother from the litter and gathering all pups into a small box with wood shavings in the bottom. Each pup was transferred to an individual cup with shavings, picked up, held a few seconds, and a 25-gauge needle was inserted subcutaneously. This form of stimulation was employed to make the experiment comparable to a hormone study in progress.. The entire procedure was held constant at 10 min, after which pups were gathered into the box and returned to the maternity cage, followed by the mother. This procedure was applied to each litter of the LS group on Days 25-28. Litters assigned to the NS group were not disturbed. At 30 days of age, each pup was weighed and housed with four littermates of like sex. When 40 days old, animals were housed in pairs for 10 days and singly thereafter. Operant chambers (Scientific Prototype) equipped with food dispensers were programmed to deliver standard 45-rag Noyes pellets on a FixedTime 100-sec schedule of reinforcement. Each test cage held a graduated tube fitted with a stainless-steel spout which extended into the chamber and served as the SIP target. Consistent with an earlier study (Devenport, 1978) and one in progress, the tubes contained isotonic saline. Following food deprivation to 85% body wt, the 70-day-old subjects were placed in the chambers with a pellet available. When this and two more had been consumed by each animal, assuring consistent eating, pellet delivery came under the control of programming equipment. The sessions of 50 pellets each were scored by both authors. Any drink that was taken during the interpellet interval was scored as occurring with the preceding pellet delivery. Multiple drinks were not counted. An animal was considered polydipsic when it had drunk following any 9 out of I0 consecutive pellet deliveries. Its score was the pellet delivery number on which this sequence began. Amount drunk per session was recorded as well. Subjects were run daily at the same hour in separate squads with equal group representation until criterion was met. Animals not displaying polydipsia were terminated after the 10th session and assigned a score of 500. As seen in Fig. 1, infant stimulation suppressed the onset of SIP. Four stimulated subjects never became polydipsic while 66% of the NS subjects met criterion within the first session. A nonparametric analysis of variance yielded a significant difference among groups on number of
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I I ES
FIG. 1. Median pellets to criterion across stimulation conditions. Abbreviations: ES, early stimulation (n = 11); LS, late stimulation (n = 10); NS, nonstimulation (n = 15). pellets to criterion, H(2) = 9.34, p < .01. Individual two-tailed tests isolated differences between the ES and NS groups, U = 23, p < .05, as well as between the LS and NS groups, U = 18.5, p < .01, but not between the two stimulated groups, U = 48, p > .05. Analysis of the data from the criterion session revealed that there were no significant differences in mean drink size among the groups, F(2, 33) = .0057, p > .05. This finding was expected. When other factors are held constant, the increased intake that characterizes SIP is primarily attributable to the development of more frequent but not larger drinks (e.g., Devenport, 1978). In a sense, the unit of analysis in our study is the litter, not individual animals. Accordingly, the mean pellets to criterion were calculated for each litter. F o r this analysis, the two stimulation groups were combined (n = 8 litters) and compared with NS litters (n = 4). The median pellets to criterion for stimulated litters (212.6) was significantly greater than that for NS litters (53.3), U = 2, p < .05 (two-tailed). Stimulating rats before 29 days of age retards the development of SIP as adults. Although all subjects received some form of stimulation at 30 days (weaning), 40 days (paired housing), 50 days (individual housing), 70 days and thereafter (testing), only the early treatments applied during hippocampal development were effective in differentiating SIP behavior. The results of this study, in light of the acute corticosterone secretion as a result of handling, the hippocampal target of this hormone, the anatomical changes of the hippocampus resulting from handling, and the relation between the hippocampus and SIP support an interpretation in terms of stimulation-induced alteration of hippocampal function. As for whether early stimulation attenuates broadly the class of be-
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haviors of which SIP is a member, and whether this attenuation involves steroid-induced changes in hippocampal "tuning" remains to be seen. Although supportive of our guiding hypothesis, the findings of this experiment leave other possibilities open. Handling may permanently alter regulation of ACTH, corticosterone, or both, the permanency of handling being mediated hormonally, not neurally. This explanation is rendered less likely by the finding that handling effects remain strong after adrenalectomy (Devenport & Devenport, 1978). In any case, early handling plainly affects adult SIP. In some handled rats, polydipsia was never observed. But in others the pattern eventually appeared, and when it did, its form (postpellet) and magnitude were indistinguishable from those of nonhandled controls. Taken together, these findings suggest that early stimulation adjusts the threshold for the release of SIP, but it does not interfere with mechanisms responsible for its organization. REFERENCES Altman, J., & Das, G. D. (1965). Autoradiographic and histological evidence of postnatal hippocampal neurogenesis in rats. Journal of Comparative Neurology, 124, 319-336. Altman, J., Das, G. D., & and Anderson, W. J. (1968). Effects of infantile handling on morphological development of the rat brain: An exploratory study. Developmental Psychobiology, 1, 10-20. Denenberg, V. H. (1964). Critical periods, stimulus input, and emotional reactivity: A theory of infantile stimulation. Psychological Review, 71, 335-351. Denenberg, V. H., Brumaghim, J. T., Haltmeyer, G. C., & Zarrow, M. X. (1967). Increased adrenocortical activity in the neonatal rat following handling. Endocrinology, 81, 1047-1052. Devenport, L. D. (1978). Schedule-induced polydipsia in rats: Adrenocortical and hippocampal modulation. Journal of Comparative and Physiological Psychology, 92, 651-660. Devenport, J. A., & Devenport, L. D. (1978). Adjunctive Behavior in Rats: Modification by Early Handling. Paper presented at the annual meeting of the Western Psychological Association, San Francisco, Calif., April, 1978. diGiusto, E. L., Cairncross, K., & King, M. G. (1971). Hormonal influences on fearmotivated responses. Psychological Bulletin, 75, 432-444. Douglas, R. J. (1975). The development of the hippocampal function: Implications for theory and for therapy. In R. L. Isaacon & K. H. Pribram (Eds.), The Hippocampus, Vol. 2, pp. 327-361. New York: Plenum. Falk, J. L. (1971). The nature and determinants of adjunctive behavior. Physiology and Behavior, 6, 577-588. Hinson, J. M., & Staddon, J. E. R. (1978). Behavioral competition: A mechanism for schedule interactions. Science, 202, 432-434. Levine, S., & Mullins, R. F. (1966). Hormonal influences on brain organization in infant rats. Science, 152, 1585-1592.