SOMATOSTATIN AND LEU-ENKEPHALIN IN THE RAT AUDITORY BRAIN-STEM DURINGFETAL AND POSTNATAL-DEVELOPMENT

A transient expression of the neuropeptide somatostatin has been descr ibed in several brain areas during early ontogeny and several opioid p eptides, such as leu-enkephalin, have also been found in the brain at this stage in development. It is therefore believed that somatostatin and leu-enkephalin may play a role in neural maturation. The aim of th e present study was to describe the spatiotemporal pattern of somatost ain and leu-enkephalin immunoreactivity in the auditory brainstem nucl ei of the developing rat and to correlate it with other developmental events. In order to achieve this goal, we applied peroxidase-antiperox idase immunocytochemistry to rat brains between embryonic day (E) 17 a nd adulthood. Somatostatin immunoreactivity (SIR) was found in all nuc lei of the auditory brainstem, yet it was temporally restricted in mos t nuclei. SIR appeared prenatally and reached maximum levels around po stnatal day (P) 7, when great numbers of immunoreactive neurons were p resent in the ventral cochlear nucleus (VCN) and in the lateral lemnis cus. At that time relatively low numbers of cells were labeled in the dorsal cochlear nucleus, the lateral superior olive (LSO), and the inf erior colliculus (IC). During the same period, when somata in the VCN were somatostatin-immunoreactive (SIR), a dense network of labeled fib ers was also present in the LSO, the medial superior olive (MSG), and the medial nucleus of the trapezoid body (MNTB). As these nuclei recei ve direct input from VCN neurons, and as the distribution and morpholo gy of the somatostatinergic fibers in the superior olivary complex (SO C) was like that of axons from VCN neurons, these findings suggest a t ransient somatostatinergic connection within the auditory system. Asid e from the LSO, MSG, and MNTB, labeled fibers were found to a smaller extent in all other auditory brainstem nuclei. After P7, the SIR decre ased and only a few immunoreactive elements were found in the adult au ditory brainstem nuclei, indicating that somatostatin is transiently e xpressed in the rat auditory brainstem. Leu-enkephalin immunoreactivit y showed a lower number and weaker intensity of labeled structures as compared to SIR, with E18 being the earliest day at which labeled fibe rs appeared in the SOC. At birth, immunoreactive fibers were also pres ent in the cochlear nuclear complex and in the IC. Leu-enkephalin immu noreactive somata were found only after P12 in the CN and after P16 in the IC. Leu-enkephalin immunoreactivity was not transient, but increa sed progressively with age until about P21, when the adult levels were reached. Our results demonstrate somatostatinergic and leu-enkephalin ergic inputs onto auditory brainstem neurons during perinatal life, i. e., during a period when the processes of synapse maturation occur. It is thus likely that both neuropeptides may influence the development of synaptic connections in the auditory brainstem.