CLOSE APPOSITIONS BETWEEN TYROSINE-HYDROXYLASE IMMUNOREACTIVE BOUTONSAND RESPIRATORY NEURONS IN THE RAT VENTROLATERAL MEDULLA

The extent of the adrenergic input to respiratory neurons in the ventr olateral medulla oblongata of rats was assessed by using a combination of intracellular recording, dye filling, and immunohistoehemistry. Tw enty-two neurons that displayed a pronounced respiration-related modul ation of their membrane potential, and could not be antidromically act ivated by electrical stimulation of the superior laryngeal, vagus, or facial nerves, were labelled by intracellular injection of biocytin. T hree types of respiration-related neurons were labelled: small neurons located in the Botzinger complex between 0.5 and 1.0 mm caudal to the facial nucleus; medium-sized neurons located in the ventral respirato ry group 1.0 to 2.0 mm caudal to the facial nucleus; and large motoneu rons located within the nucleus ambiguus 0.5 to 2.0 mm caudal to the f acial nucleus. Small Botzinger neurons [length = 22 +/- 5 mu m, width = 13 +/- 3 mu m, area = 222 +/- 79 mu m(2); (mean+/-SD, n = 5)] had me mbrane potentials of -15 to -27 mV during the recording period. Four o f five of these cells had profuse axonal terminations between 50 mu m caudal and 450 mu m rostral to their somata, suggesting that they may form part of local networks responsible for generating respiratory act ivity. Medium-sized ventral respiratory group neurons (length = 26 +/- 5 mu m, width = 18 +/- 4 mu m, area = 377 +/- 141 mu m(2); n = 5) wer e found in the vicinity of the nucleus ambiguus dorsal to the lateral reticular nucleus. Three of five of these neurons had an axon that cro ssed the midline and travelled caudally. One axon had a collateral wit h varicosities close to its soma. The somata of motoneurons (length = 29 +/- 6 mu m, width = 21 +/- 4 mu m, area = 485 +/- 142 mu m(2); n = 12) were located within the nucleus ambiguus, and had axons that could be traced to exit points from the medulla. Tyrosine hydroxylase immun oreactive cells and their terminal fibres within the medulla were loca lised by immunocytochemistry. Small Botzinger neurons received the lar gest number of close appositions from tyrosine hydroxylase immunoreact ive boutons (13 +/- 2 appositions/neuron; n = 5). Medium-sized ventral respiratory group neurons received fewer appositions (8 +/- 4 apposit ions/neuron; n = 5). Most motoneurons (n = 10) received few apposition s 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 rat her than large proximal dendrites. Axosomatic appositions were never s een. These results suggest that catecholamine-containing nerve fibres may provide a direct synaptic input to small respiration-related neuro ns that are thought to be involved in the generation of central respir atory activity. A catecholamine input to these neurons could provide a basis for the previously observed effects of noradrenaline on respira tory frequency. (C) 1994 Wiley-Liss, Inc.