Close appositions between tyrosine hydroxylase immunoreactive boutons and respiratory neurons in the rat ventrolateral medulla
The extent of the adrenergic input to respiratory neurons in the ventrolateral medulla oblongata of rats was assessed by using a combination of intracellular recording, dye filling, and immunohistochemistry. Twenty-two neurons that displayed a pronounced respiration-related modulation of their membrane potential, and could not be antidromically activated by electrical stimulation of the superior laryngeal, vagus, or facial nerves, were labelled by intracellular injection of biocytin. Three types of respiration-related neurons were labelled: small neurons located in the Bötzinger complex between 0.5 and 1.0 mm caudal to the facial nucleus; medium-sized neurons located in the ventral respiratory group 1.0 to 2.0 mm caudal to the facial nucleus; and large motoneurons located within the nucleus ambiguus 0.5 to 2.0 mm caudal to the facial nucleus. Small Bötzinger neurons [length = 22 ± 5 μm, width = 13 ± 3 μm, area = 222 ± 79 μm2; (mean ± SD, n = 5)] had membrane potentials of − 15 to − 27 mV during the recording period. Four of five of these cells had profuse axonal terminations between 50 μm caudal and 450 μm rostral to their somata, suggesting that they may form part of local networks responsible for generating respiratory activity. Medium-sized ventral respiratory group neurons (length = 26 ± 5 μm, width = 18 ± 4 μm, area = 377 ± 141 μm2; n = 5) were found in the vicinity of the nucleus ambiguus dorsal to the lateral reticular nucleus. Three of five of these neurons had an axon that crossed the midline and travelled caudally. One axon had a collateral with varicosities close to its soma. The somata of motoneurons (length = 29 ± 6 μm, width = 21 ± 4 μm, area = 485 ± 142 μm2; n = 12) were located within the nucleus ambiguus, and had axons that could be traced to exit points from the medulla. Tyrosine hydroxylase immunoreactive cells and their terminal fibres within the medulla were localised by immunocytochemistry. Small Bötzinger neurons received the largest number of close appositions from tyrosine hydroxylase immunoreactive boutons (13 ± 2 appositions/neuron; n = 5). Medium-sized ventral respiratory group neurons received fewer appositions (8 ± 4 appositions/neuron; n = 5). Most motoneurons (n = 10) received few appositions from tyrosine hydroxylase immunoreactive boutons, while two received none. The average number was 3 ± 3 appositions/neuron (n = 12). On all neurons, appositions occurred most commonly on small dendrites rather than large proximal dendrites. Axosomatic appositions were never seen. These results suggest that catecholamine-containing nerve fibres may provide a direct synaptic input to small respiration-related neurons that are thought to be involved in the generation of central respiratory activity. A catecholamine input to these neurons could provide a basis for the previously observed effects of noradrenaline on respiratory frequency.