Postnatal development of preproenkephalin mRNA containing neurons in the rat lower brainstem

Postnatal developmental changes of preproenkephalin (PPE) gene expression in rat brainstem neurons were studied by in situ hybridization histochemistry. On the basis of PPE mRNA expression, brainstem neurons were categorized into three types: 1) type I neurons were characterized by constant or increasing expression of PPE mRNA during postnatal development; 2) type II neurons started to express PPE mRNA several days after birth and continued to do so thereafter; and 3) type III neurons showed transient expression of PPE mRNA or stopped expressing the mRNA during early postnatal development. Type I PPE neurons were observed in diverse brainstem structures including the mesencephalic and pontine central gray matter, various reticular and raphe nuclei, the ventral tegmental area of Tsai, the interpeduncular nucleus, the nucleus of the brachium of the inferior colliculus, the ventral and dorsal tegmental nuclei of Gudden, the sphenoid nucleus, the laterodorsal tegmental nucleus, Barrington's nucleus, the parabrachial region, the lateral lemniscus and its related nuclei, the trapezoid nucleus, the rostral and ventromedial periolivary nuclei, the mesencephalic trigeminal and principal sensory trigeminal nuclei, the locus coeruleus, the subcoeruleus nucleus, the medial and spinal vestibular nuclei, the dorsal and ventral cochlear nuclei, the medial and lateral cerebellar nuclei, the Roller nucleus, and the intermedius nucleus of the medulla. Type II PPE neurons were found in the superior colliculus, the inferior colliculus, the central part of the dorsal tegmental nucleus, and as Golgi neurons in the granular layer of the cerebellum. Type III PPE neurons were located in the substantia nigra, the red nucleus, the superior olive, the motor trigeminal nucleus, the facial nucleus, the inferior olive, the dorsal motor nucleus of the vagus, and the hypoglossal nucleus. Such region-specific expression of the PPE gene during postnatal ontogeny suggests that rat brainstem PPE neurons may be involved in a variety of developmental events, such as cell proliferation, differentiation, and migration.