Brain tissue transplanted to the anterior chamber of the eye. 4. Drug-modulated transmitter release in central monoamine nerve terminals lacking normal postsynaptic receptors.

Small pieces of fetal rat brain selected to contain a high number of noradrenaline (NA), dopamine (DA), or 5-hydroxytryptamine (5-HT) neuroblasts were transplanted to the anterior chamber of the eye of adult rats. The sympathetic ground plexus of the host iris was removed by superior cervical ganglionectomy so that transmitter mechanisms of the different central monoamine fibers innervating the iris could be selectively studied after intraocular maturation. Such irides, containing NA, DA, or 5-HT nerve terminals were incubated with radiolabelled transmitters and then stimulated by an electrical field while superfused, to investigate the spontaneous and stimulation-induced release of amine, both in drug-free buffer and buffer containing drugs acting on monoamine receptors. The central monoamine neurons of all three types were able to take up exogenous amines and release them upon stimulation by an electrical field, in much the same way as corresponding nerves in situ in slices of cerebral cortex (NA, 5-HT) or olfactory tubercle (DA). The alpha-adrenergic receptor blocking agent phentolamine increased the stimulation-induced release of 3H-NA from central NA fibers on the iris significantly. The dopamine receptor stimulating agent apomorphine decreased the stimulation-induced release of 3H-DA from central DA fibers on the iris. Pimozide, a DA receptor blocking drug tended to increase the 3H-DA release. The 5-HT receptor stimulating agent ergocornine tended to reduce the stimulation-induced release of 3H-5-HT from central 5-HT fibers on the iris. It was concluded that all three types of central monoamine nerve fibers develop essentially normal transmitter storage and release mechanisms also in an environment completely devoid of normal postsynaptic receptors. The drug experiments add strong support to the view that there are presynaptic monoamine receptors ("autoreceptors") able to modulate transmitter release present on the monoamine nerve terminals.