The developing mesencephalic reticular formation: changes in responsiveness during ontogeny of the rat

Multiunit activity (MUA) of the mesencephalic reticular formation (MRF) was recorded on freely moving 1 to 21-days-old young as well as adult rats. Consistent state-dependent changes in the frequency of amplitude discriminated cell discharges were observed in association with wakefulness and sleep cycle from the first day of life, and MRF neuron populations were found to gain adult-like functional characteristics of spontaneous activity by the 9th-11th postnatal day. MRF units were capable of responding to somatosensory, visual and acoustic stimulation by a significant activation from Day 1. As development proceeded, rate of responsiveness appeared to increase, however, MUA responses to different sensory modalities exhibited different developmental time-courses. High rate of responsiveness to somatosensory stimulation was characteristic of the newborn rat. Magnitude of somatosensory responses increased remarkably during the first three postnatal days reaching adult values around this age and only moderate alteration of response patterns could be detected afterwards. The MRF MUA responses to visual and acoustic stimulation increased during the first week of life and, following a striking decline of responsiveness occurring at the time of opening of eyes and external auditory meati (Day 13), visual and acoustic responses gained the adult properties by the 17th-19th postnatal day.