Developmental study of cytochrome oxidase activity in the brain stem respiratory nuclei of postnatal rats

We utilized cytochrome oxidase (CO) as a marker of neuronal functional acti vity to examine metabolic changes in brain stem respiratory nuclei of rats from newborn to 21 day of age. The pre-Botzinger complex (PBC), upper airwa y motoneurons of nucleus ambiguus (NA(UAM)), ventrolateral nucleus of solit ary tract (NTSVL), and medial and lateral parabrachial nuclei (PBM and PBL, respectively) were examined at postnatal days (P) 0, 1, 2, 3, 4, 5, 7, 14, and 21. CO histochemistry was performed, and the intensity of CO reaction product was quantitatively analyzed by optical densitometry. In addition, C O histochemistry was combined with neurokinin-1 receptor (NK1R) immunogold- silver staining to doubly label neurons of PBC in P14 animals. The results showed that levels of CO activity generally increased with age in all of th e nuclei examined. However, a significant decrease was found in NA(UAM) at P3 (P < 0.01), and a distinct plateau of CO activity was noted at P3 in PBC and at P3 and P4 in NTSVL, PBM, and PBL. Of the neurons examined in PBC, 8 3% were doubly labeled with CO and NK1R. Of these, CO activity was high in 33.9%, moderate in 27.3%, and light in 38.8% of neurons, suggesting differe nt energy demands in these metabolic groups that may be related to their ph ysiological or synaptic properties. The transient decrease or plateau in CO activity at P3 and P4 implies a period of synaptic adjustment or reorganiz ation during development, when there may be decreased excitatory synaptic d rive or increased inhibitory synaptic drive, or both, in these brain stem r espiratory nuclei. The adjustment, in turn, may render the system less resp onsive to respiratory insults. This may bear some relevance to our understa nding of pathological events during postnatal development, such as occurs i n sudden infant death syndrome.