BRAIN RESEARCH 471 DEPLETION OF CATECHOLAMINES IN VIVO INDUCED BY ELECTRICAL STIMULATION OF CENTRAL MONOAMINE PATHWAYS GORDON W. ARBUTHNOTT*, TIMOTHY J. CROW*, KJELL FUXE, LARS OLSON AND URBAN UNGERSTEDT Department of Histology, Karolinska Institute, Stockholm (Sweden) (Accepted June 29th, 1970) INTRODUCTION A large number of investigations have been concerned with the release of mono- amines in vitro from brain slices by means of electrical field stimulationS,L Some studies have been concerned with the release of monoamines in vivo by electrical stimulation in different parts of the brain10, 4s. Various techniques for measuring release of mono- amines in vivo are available but most have serious drawbacks. The push-pull cannula introduced by Gaddum 2a has been used by McLennan 31 and Stein and WisO 5. Chase and Kopin 12 have shown, however, that the damage produced by this type of cannula may lead to non-specific release of substances unrelated to transmitter activity. Although this does not happen ~3 with cups placed on the cortex to collect released transmitted 9, this technique has not been very successful and can be applied only to a limited number of sites in the nervous system. This limitation also holds for the method of collecting ventricular perfusatO a. Another technique involves the estimation of decreases in total amine levels or changes in the levels of their metab- olites in brain tissue1, 29. This technique has the disadvantage, however, that it requires relatively large samples of tissue and, in addition, it has been shown, in the periphery at least, that nerve activity may lead to an increase rather than a decrease in NA** levels zl. An alternative approach is now possible as a result of the detailed mapping of the central monoamine pathwaysS,6,22,za,a6, aT. By the use of synthesis inhibitors to prevent the regeneration of amine stores, it is possible to utilize a reduction in the specific fluorescence in aminergic terminals, as demonstrated by the histochemical technique of Falck et al. ~o, as an index of transmitter release following nerve activity both in the periphery 3z, and in the central nervous system 4. This technique has the * Present address: Physiology Department, Marischal College, University of Aberdeen, Aberdeen, Scotland, Great Britain. ** Abbreviations: CA, catecholamine(s); NA, noradrenaline; DA, dopamine; 5-HT, 5-hydroxy- tryptamine. Brain Research, 24 (1970) 471-483