DYNAMICAL ENTROPY IS CONSERVED DURING COCAINE-INDUCED CHANGES IN FETAL-RAT MOTOR PATTERNS

Our previous studies demonstrated that the intra-cisternal (IC) admini stration of cocaine to fetal rats increased motor activity and decreas ed responsiveness to perioral stimulation. One explanation for these o bservations comes from the behavioral pharmacology of stimulant drugs: increased motor activity is often associated with a decrease in its v ariety. Previous power spectral transformation of this data suggests a n alternative explanation: cocaine-induced hyperactivity fixates a new behavioral pattern with complexity equal to that of saline controls. We explore these possibilities using statistical techniques derived fr om studies of nonlinear dynamical systems, examining patterns of the t otal motor activity of the individual fetus as counts per 5 s interval on either gestational day E20 or E21 for 20 min following IC injectio ns of saline, 2.5 or 10 mg/kg of cocaine. The results are consistent w ith a state in which increased spontaneous activity is associated with the emergence of a new dynamical pattern which conserves entropy and provides experimental support for a fundamental conservation-variation al relation, h(T) approximate to L(1) x D-R, that has been proven for abstract models of chaotic dynamical systems. A multivariate analysis of variance (MANOVA) followed by appropriate analyses of variance (ANO VAs) and pairwise comparisons revealed that, whereas cocaine induced i ncreases in the total amount of motor activity, the rate of increase i n the variety of new sequences in activity counts over time did not ch ange with treatment and age conditions. This invariant is quantified b y an absence of change in topological entropy, Delta h(T)=0. The analy ses also showed that, in order to maintain h(T) values, compensatory c hanges took place in the leading Lyapounov characteristic exponent, L( 1) (the distance between sequential values 'stretched' along the incre asing amplitudes of the variations) such that Delta L(1) > 0, and the correlation dimension, DR (the hierarchical range of possible values, 'complicated clustering') was reduced, so that Delta D-R < 0. Our find ings are consistent with the idea that the association between cocaine -induced increases in activity and decreases in adaptive response are not due to the dynamical entropy loss of decreased behavioral variety, but are rather the result of competitive interference by a drug-induc ed, equally complex, new pattern of spontaneous behavior. Copyright (C ) 1996 Elsevier Science Ltd