The antihypertensive drug L-a-methyl-3,4-dihydroxyphenylalanine (a-methyl-DOPA, g-MD) affects the metabolism of noradrenaline (NA), dopamine (DA) and 5-hydrox~ryptamine (5-HT) in severa] ways. DOPA/5-hydroxytryptophan decarboxylase (SouRxEs, 1954), tyrosine hydroxylase (NAGATSU et al., 1964), tryptophan hydroxylase (Bu~KA~D et al., 1963; Ross and I-IALJASMAA, 1964; Roos and WE~D~IUS, 1963) are all inhibited to the same extent by a-MD. The levels of NA, DA and 5-HT in brain and of NA in peripheral sympathetically innervated tissues are decreased (for reviews, see Sou~K]~s, 1965 and MUSCHOLL, 1966). An important factor in the depletion of the catecholamines (CA) by ~-MD is the replacement of these amines by their corresponding ~-methylated analogues--"displacement" (CA~LssoN and LrSDQWST, 1962). These authors also suggested that the a-methylated amines, ~-methyl-NA and g-methyl-DA, might act as "false transmitters" in the brain, taking over the function of ~qA and DA, respectively. This idea was later extended to the peripheral adrenergie nerves (DAY and RAND, 1963 a). Interference with the function of the sympathetic nervous system is demonstrated through inhibition of hemodynamic reflexes which involve the central nervous system (MASON and B~AU~WALD, 1964; S~APmO and K~IFCHER, 1964), whereas the response to sympathetic nerve stimulation appears unaffected (GoLDBE~G et al., 1960 ; STONE et al., 1961 ; review by STONE and PO~TE~, 1966). Further evidence for a central effect of g-MD is seen in its inhibition of various conditioned behaviour patterns. RAY and B~vE~s (1965a and b) showed a suppression of learned behaviour in rats after treatment with ~-MD.
This study was performed with the aim of analyzing further the different central effects of ~-MD by using an active, negatively reinforced, conditioned avoidance response (CAR) in cats.
Methods
Twenty-two mature cats of either sex weighing 2.3--4.1 kg were used in the experiments and no animal was used more than once. The cats