CAROTID-SINUS NERVE-TERMINALS WHICH ARE TYROSINE-HYDROXYLASE IMMUNOREACTIVE ARE FOUND IN THE COMMISSURAL NUCLEUS OF THE TRACTUS-SOLITARIUS

Tyrosine hydroxylase immunoreactive sensory neurons in the petrosal ga nglion selectively innervate the carotid body via the carotid sinus ne rve. Central projections of the carotid sinus nerve were traced with h orseradish peroxidase. The commissural nucleus of the tractus solitari us was examined by dual labelling light and electron microscopy. Dense bilateral labelling with horseradish peroxidase was found in the trac tus solitarius and commissural nucleus of the tractus solitarius. Hors eradish peroxidase was found in unmyelinated axons, myelinated axons, and nerve terminals. About 88% of horseradish peroxidase-labelled caro tid sinus nerve axons were unmyelinated. Tyrosine hydroxylase immunore activity was identified in unmyelinated axons, myelinated axons, dendr ites, perikarya, and nerve terminals. Most tyrosine hydroxylase immuno reactive axons (93%) in the commissural nucleus of the tractus solitar ius were unmyelinated. Tyrosine hydroxylase immunoreactivity was simul taneously identified in carotid sinus nerve unmyelinated axons, myelin ated axons, and nerve terminals. These double-labelled terminals compr ised 28% of the number of tyrosine hydroxylase immunoreactive terminal s in the commissural nucleus of the tractus solitarius, and 55% of tra nsganglionically-labelled terminals. Therefore, there are both central and peripheral sources of tyrosine hydroxylase immunoreactive nerve t erminals in the commissural nucleus of the tractus solitarius. These d ata support the hypothesis that peripheral tyrosine hydroxylase immuno reactive neurons are involved in the origination of the chemoreceptor reflex. Axo-axonic synapses between peripheral carotid sinus nerve aff erent terminals and central terminals containing tyrosine hydroxylase immunoreactivity were observed in 22% of the axo-axonic synapses obser ved. Thus, central tyrosine hydroxylase immunoreactivity neurons are i nvolved in the modulation of the chemo- and/or baroreceptor reflexes. Synaptic contacts were not observed between carotid sinus nerve affere nts and tyrosine hydroxylase immunoreactive perikarya of dendrites. Ca techolaminergic neurons are thus unlikely to be the second order neuro ns of either the chemo- or baroreceptor reflex in the commissural nucl eus of the tractus solitarius.