Autoreceptors and alpha 2-adrenoceptors at the serotonergic axons of rabbit brain cortex

Slices of the rabbit occipito-parietal cortex were preincubated with 3H-serotonin and then superfused and stimulated electrically (2 min at 3 Hz). In the absence of drugs, the stimulation-evoked overflow of tritium was approximately 3% of the tritium content of the tissue. Unlabelled serotonin and 5-carboxamido-tryptamine, when administered in the presence of 6-nitroquipazine, reduced the evoked overflow of tritium. Their effects were antagonized by metitepin (apparent pA2 value 8.1) and (+/-)-cyanopindolol (apparent pA2 value 6.4). Metitepin, but not cyanopindolol, increased evoked tritium overflow; the effect of metitepin was greater in the presence than in the absence of nitroquipazine. The evoked overflow of tritium was also depressed by clonidine, an effect antagonized by idazoxan (apparent pA2 value 7.0) but not by prazosin. Phenylephrine caused a decrease only at high concentrations that simultaneously accelerated basal tritium efflux. Prazosin and idazoxan did not change evoked tritium overflow, and phentolamine increased it significantly only when administered in the presence of (+)-oxaprotiline. Rauwolscine produced an inhibition that was prevented by metitepin. It is concluded that the serotonergic axons of the rabbit occipitoparietal cortex possess presynaptic, release-inhibiting serotonin autoreceptors and alpha 2-adrenoceptors. The receptors appear to receive an input of endogenous serotonin and, to a lesser extent, noradrenaline, under the conditions of these in vitro experiments.