Axonal projections of pulmonary slowly adapting receptor relay neurons in the rat

We elucidated efferent projections of second-order relay neurons (P-cells) activated by afferents originating from slowly adapting pulmonary receptors (SARs) to determine the central pathway of the SAR-evoked reflexes. Special attention was paid to visualizing the P-cell projections within the nucleus tractus solitarii (NTS), which may correspond to the inhibitory pathway from P-cells to second-order relay neurons (RAR-cells) of rapidly adapting pulmonary receptors. P-cells were recorded from the NTS in Nembutal-anesthetized, paralyzed, and artificially ventilated rats. First, we used electrophysiological methods of antidromic mapping and showed that the majority of the P-cells examined projected their axons to the caudal NTS and to the dorsolateral pons corresponding to the parabrachial complex. Second, a mixture of HRP and Neurobiotin was injected intracellularly or juxtramembranously into P-cells. (1) Stained P-cells (n = 7) were located laterally to the solitary tract and had dendrites extending characteristically along the lateral border of the solitary tract. (2) All P-cells had stem axons projecting to the ipsilateral medulla. Of these, the axons from five P-cells projected to the nucleus ambiguus and its vicinity with distributing boutons. Some of these axons further ascended in the ventrolateral medulla, and distributed boutons in the areas ventral or ventrolateral to the nucleus ambiguus. (3) All the P-cells had axonal branches with boutons in the NTS area. In particular, axons from three P-cells projected bilaterally to the medial NTS caudal to the obex, i.e., to the area of RAR-cells. These results show anatomic substrates for the connections implicated in the P-cell inhibition of RAR-cells as well as the SAR-induced respiratory reflexes.