The Effects of Differential Maternal Environments Prior to Pregnancy on Future Offspring in Hooded Lister Rats

Central to any understanding of the nature of an organism is the examination of the relative contributions of heredity and environment to its development. Set within this framework are the literatures studying the environmental forces which interact with biological predispositions to produce the mature individual (Shaffer 1985). One such area of research which has provided evidence that the environment has a beneficial impact on an animal's neurochemistry, neuroanatomy and behaviour is the environmental enrichment literature (Rosenzweig 1984; Renner and Rosenzweig 1987; Rose 1988) where animals reared in socially and perceptually stimulating environments are compared with their litter mates raised in impoverished environments. Within this literature are a handful of studies which suggest that these beneficial effects are not only confined to those animals directly exposed to an enriching environment but also can be passed across generations. It is this intergenerational research which provides the focus of this thesis in which the effects of exposing female rats to differential environments prior to pregnancy on successive generations were investigated in Hooded Lister rats. Chapter one, the introduction to this present work, provides a historical background to the investigation of early experience, enrichment and its effect on the brain and reviews those few studies which have investigated the results of maternal enrichment on the offspring generation. Enrichment as an environmental manipulation has been extensively researched and those studies investigating the behavioural consequences of exposing animals directly to Enriched (EC) and Impoverished (IC) conditions are reviewed in chapter two, to provide a profile against which to compare the offspring generations investigated in this thesis. The impact of inter generational effects has of course been explored using manipulations other than enrichment. Indeed, it is now well established that various kinds of stressors imposed upon females of different species can affect both the physiology and behaviour of their offspring (Joffe 1969b; 1978; 1982; Thompson and Grusec 1970; Archer and Blackman 1971). Chapter three of this thesis provides an overview of the literature investigating the effects of manipulation of the maternal generation either prior to pregnancy, during pregnancy or postnatally on offspring and grandoffspring generations emphasising the diversity of manipulations other than enrichment that have been employed. Following chapter four, which describes the general methodology employed in this thesis, with details of the breeding programmes, the environmental manipulations and behavioural test apparatus used, are the four experimental studies designed to investigate the effects of enrichment on successive generations. In particular, chapter five (study one) provided a profile of animals exposed directly to environmental enrichment, impoverishment and standard housing (SC) against which to compare future generations’ behavioural patterns. Futhermore, this chapter also tested the efficacy of the enriched environment employed in this thesis, best described as a Superenriched environment (SEC) in male, female and postpartum female rats. The inclusion of the latter group was to ensure that the commonly found enrichment effects would continue postpartum despite undergoing pregnancy and litter-rearing. Moving on to successive generations, chapter six (study two) explored the effects of differential maternal environments prior to pregnancy on offspring and grandoffspring behaviour. Animals were put through a battery of tests to investigate their activity, perceptual and learning performances. From this work qualitatively different behavioural profiles were observed in both the offspring and grandoffspring of the three maternal conditions. Possible causes for the observed performance differences were discussed and it was suggested that they might reflect amongst other things, different learning capacities between the groups or differential arousal and/or stress levels. The last two studies of this thesis were designed to investigate these postulated causes further. Chapter seven (study three) analysed the effects of differential maternal environments prior to pregnancy on offspring performance in the Hebb-Williams maze and in an operant conditioning task, whilst chapter eight (study four) considered the hypothesis that offspring of SEC, SC and IC dams are differentially aroused, by artificially manipulating arousal levels with d-amphetamine sulphate. In the final chapter of this thesis (chapter nine) the main findings of the four studies are summarised and possible causes of the intergenerational transfer of effects discussed. In addition, the individual experiments are critically assessed and avenues for future research suggested