Stress in the horse and its modification with drugs

The aim of this study was to estimate the effects of stress on horses by measuring alterations in adrenocortical sympathetic nervous activity. The effects On the adrenocortical system were determined directed by measuring circulating levels of plasma 11-hydroxycorticosteroids. The effects of stress on the sympathetic nervous system were assessed indirectly by recording charges in heart rate and circulating packed cell volume and in a few caeer by direct measurement of circulating plasma catecholamine levels. Plasma 11-hydroxycorticosteroid levels were measured using a modified fluorometric technique. Standard reliability criteria were used to validate the method. The range of corticosteroid levels in horses and ponies under various natural and experimentally induced conditions was established. Changes in adrenocortical and sympathetic nervous activity were measured during physical stimulation (either exercise or surgical tran.ma) and following administration of certain drugs. The significance as a means of quantifying stress of alterations in plasma 11-hydroxycorticosteroid levels and in sympathetic nervous activity was assessed. A number of tranquillising drugs were compared both for their central nervous depressant and peripheral actions and the effects) of these drugs on stress-induced changes in adrenocortical and sympathetic nervous activity were studied. Alterations in packed cell volume caused by tranquillising drugs was investigated. Finally, changes in adrenocortical and sympathetic nervous activity produced by the commonly used neuroleptanalgesic mixture of storphine and acepromazine were studied and attempts were made modify these changes by incorporating various tranquillising and adrenoreceptor blocking drugs in the etorphine/acepromazine combination. It was concluded from this study that alterations in adrenocortical and sympathetic nervous activity can be used as means of assessing stress in the horse. As the tranquillising drugs used however modified adrenocortical and sympathetic activity, changes In these systems could not be employed in assessing alleviation of stress by these compounds. It was also concluded from these experiments that the marked effects of the etorphine/acepromazins mixture on sympathetic nervous activity could be modified by the central and peripheral actions of the various tranquillising drugs employed in this study