Organisation of the catecholaminergic system in the vagal motor nuclei of pigs: A retrograde fluorogold tract tracing study combined with immunohistochemistry of catecholaminergic synthesizing enzymes

The vagal motor system is involved in the regulation of cardiorespiratory and gastrointestinal functions. Vagal motor neurons are localized near or adjacent to catecholaminergic neurons, but their co-localisation seems species dependent, present in the cat but absent in the rabbit. In pig, a species commonly used as an experimental model in humans brain disorders (sudden infant death syndrome, hypoxia), the relationship is poorly understood. We aimed at describing the distribution of vagal motor neurons and tyrosine hydroxylase-immunoreactive (-ir) neurons by using a double staining method in combination with retrograde tracing of vagal efferent neurons. After fluorogold impregnation of the central part of the sectioned left cervical vagal trunk, two main vagal motor neuronal populations were located in the dorsal motor nucleus of the vagus nerve (DMX) and in the area of the nucleus ambiguus (Amb). Like in the human, the DMX was composed of different subpopulations of neurons with the same morphological characteristics. Immunohistochemistry of catecholaminergic synthesizing enzymes differentiated two main sites containing vagal motor populations: the dorsomedial and the ventrolateral medulla. TH-ir was rarely seen in vagal motor neurons of the DMX, but TH-ir neurons were present around the two main vagal motor neuronal populations that contained TH-ir fibres. The anatomical organisation of the vagal motor and the catecholaminergic neuronal systems are similar to those described in humans and suggest that the involvement of the catecholamines in the control of the vagal motor system may be similar in pigs and in humans.