Electrically induced release of endogenous noradrenaline and dopamine from brain slices: pseudo-one-pulse stimulation utilized to study presynaptic autoinhibition.

Slices of rat hypothalamus (noradrenaline experiments) or rabbit caudate nucleus (dopamine experiments) were prepared, superfused, and field-stimulated using series of monophasic rectangular pulses. Noradrenaline, dopamine and the main dopamine metabolite, dihydroxyphenylacetic acetic acid (DOPAC), were determined using HPLC with electrochemical detection. Electrical stimulation was performed using the following protocols: 1) 4 pulses delivered at 100 Hz; this type of stimulation is referred to as pseudo-one-pulse stimulation (POP); its short duration of only 32 ms does not allow the development of autoinhibition; 2) 2 bursts of 4 pulses at 100 Hz, delivered 1 s apart (2-POP-stimulation); 3) 8 pulses at 1 Hz (dopamine experiments only); 4) 36 pulses at 3 Hz. Noradrenaline experiments. The alpha 2-adrenoceptor antagonist yohimbine (1 mumol/l) did not enhance noradrenaline overflow following POP stimulation, but enhanced the overflow following 2-POP-stimulation by about 50% and that following 36-pulse-stimulation by almost 100%. Dopamine experiments. The D2-dopamine receptor antagonist sulpiride (3 mumol/l) facilitated the overflow of dopamine elicited with 2-POP-stimulation (66%), 8 pulses/1 Hz (92%), and 36 pulses/3 Hz (140%). It did not significantly facilitate the overflow of dopamine following POP-stimulation (19%). The overflow of DOPAC was not, or only slightly, increased by electrical stimulation, and its spontaneous outflow was more than three times higher than that of dopamine. Furthermore, the electrically induced overflow of dopamine did not exceed the outflow of DOPAC at any of the stimulation conditions employed. The results of the present study bear out important claims of the autoreceptor theory and confirm the data obtained in previous experiments using labelled transmitters.