ULTRASTRUCTURAL FEATURES OF TYROSINE-HYDROXYLASE-IMMUNOREACTIVE AFFERENTS AND THEIR TARGETS IN THE RAT AMYGDALA

Interrelations of tyrosine-hydroxylase-immunoreactive afferent fibres with neuronal elements were studied in central, basal and intercalated nuclei of the rat amygdaloid complex. Comparison with dopamine-beta-h ydroxylase-immunoreacted and enylethanolamine-N-methyltransferase-immu noreacted parallel sections indicated that the tyrosine-hydroxylase im munoreaction labelled preferentially dopaminergic axons. At the electr on-microscopic level, the majority of tyrosine-hydroxylase-immunoreact ive axons possessed small boutons containing small clear vesicles and contacting dendrites, spines or somata of amygdala neurons, forming mo stly symmetric synapses. They were often directly apposed to or in the vicinity of unlabelled terminals synapsing on the same structure. Syn aptic density was highest in the central lateral part of the central n ucleus. In the central and basal nuclei labelled axons synapsed prefer entially on small dendrites and dendritic spines, and on somata of a f ew neurons. A detailed study of the neuronal ultrastructure showed tha t innervated somata possessed the differential characteristics display ed by the predominant neuron types in the medial and central lateral c entral nucleus and resembled the typical projection neurons in the bas al nuclei. In the paracapsular intercalated cell groups the majority o f neurons possessed intense perisomatic innervation by immunoreactive terminals. The results suggest that tyrosine-hydroxylase-immunoreactiv e, predominantly dopaminergic amygdaloid afferent fibres preferentiall y modulate the effect of extrinsic inputs into neurons of the central and basal nuclei, while a nonselective regulation is exerted upon the output of paracapsular intercalated neurons. It is suggested that this innervation pattern may be important for the coordinated integration of extrinsic and intraamygdaloid connections and thus for balanced out put of the structure.