Carotid sinus nerve terminals which are tyrosine hydroxylase immunoreactive are found in the commissural nucleus of the tractus solitarius

Abstract

Tyrosine hydroxylase immunoreactive sensory neurons in the petrosal ganglion selectively innervate the carotid body via the carotid sinus nerve. Central projections of the carotid sinus nerve were traced with horseradish peroxidase. The commissural nucleus of the tractus solitarius was examined by dual labelling light and electron microscopy. Dense bilateral labelling with horseradish peroxidase was found in the tractus solitarius and commissural nucleus of the tractus solitarius. Horseradish peroxidase was found in unmyelinated axons, myelinated axons, and nerve terminals. About 88% of horseradish peroxidase-labelled carotid sinus nerve axons were unmyelinated. Tyrosine hydroxylase immunoreactivity was identified in unmyelinated axons, myelinated axons, dendrites, perikarya, and nerve terminals. Most tyrosine hydroxylase immunoreactive axons (93%) in the commissural nucleus of the tractus solitarius were unmyelinated. Tyrosine hydroxylase immunoreactivity was simultaneously identified in carotid sinus nerve unmyelinated axons, myelinated axons, and nerve terminals. These double-labelled terminals comprised 28% of the number of tyrosine hydroxylase immunoreactive terminals in the commissural nucleus of the tractus solitarius, and 55% of transganglionically-labelled terminals. Therefore, there are both central and peripheral sources of tyrosine hydroxylase immunoreactive nerve terminals in the commissural nucleus of the tractus solitarius. These data support the hypothesis that peripheral tyrosine hydroxylase immunoreactive neurons are involved in the origination of the chemoreceptor reflex. Axo-axonic synapses between peripheral carotid sinus nerve afferent terminals and central terminals containing tyrosine hydroxylase immunoreactivity were observed in 22% of the axo-axonic synapses observed. Thus, central tyrosine hydroxylase immunoreactivity neurons are involved in the modulation of the chemo-and/or baroreceptor reflexes. Synaptic contacts were not observed between carotid sinus nerve afferents and tyrosine hydroxylase immunoreactive perikarya of dendrites. Catecholaminergic neurons are thus unlikely to be the second order neurons of either the chemo-or baroreceptor reflex in the commissural nucleus of the tractus solitarius.

Sudden Death and Myocardial Lesions after Damage to Catecholamine Neurons of the Nucleus Tractus Solitarii in Rat

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“…Blood O2 tension is measured by chemoreceptors in the carotid body, which are innervated by the carotid sinus branch of the glossopharyngeal nerve (Gonzalez et al, 2010;Izumizaki et al, 2004). Afferents from the carotid sinus nerve terminate in the lateral and commissural parts of the NTS (Abbott et al, 2013;Finley and Katz, 1992;Gonzalez et al, 2003;Massari et al, 1996), and hypoxia activates neurons in the ventrolateral medulla, including the C1 neurons, as well as in the locus coeruleus and the orexin neurons in the LH, in addition to the PBel, lateral crescent, and Kolliker-Fuse nucleus (Song et al, 2010;Teppema et al, 1997). Rising CO2 is in part recognized by the carotid body chemoreceptors, but also by central chemoreceptors.…”

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“…The central terminations of these afferent fibers lie in the commissural or medial subnuclei of the nucleus tractus solitarius (68, 157, 185, 276, 392, 554, 668), a subcomponent of the dorsal respiratory group respiratory neurons in the brainstem and convey sensory information regarding cardiorespiratory homeostasis (421) in the form of graded action potential frequencies in fibers of the carotid sinus branch of the IXth cranial (glossopharyngeal) nerve.…”