Functional development of the prenatal brain. I. Recording of extracellular action potentials from the magnocellular system of the 18-day-old chicken embryo.

For the full understanding of the ontogeny of the electrical activity in the brain it is essential to record single unit activity of the fetus. However, investigations of the functional development of neuronal properties in mammals are largely limited by the inaccessibility of the prenatal brain. Therefore, we have designed a new method to record extracellular single unit activities of identified magnocellular neurones in the Nucleus paraventricularis of the chicken embryo after 18 days of incubation. One hundred and four magnocellular neurones were identified by antidromic stimulation from the neural lobe. In a high percentage of the neurones an A-B inflexion of the action potential could be observed similar to that frequently encountered in mammalian magnocellular neurones. The mean duration of the action potential was 2.8 ms with a range between less than 1 ms and 7 ms. This large range is probably due to developmental processes of the cell membrane and subsequent changes in the extra- und intracellular ion concentration. Fourty-six percent of the neurones generated spontaneous action potentials with a slow irregular firing pattern. The mean discharge frequency was estimated as approx. 1 Hz. In further 13% of the cells orthodromic action potentials could be observed only after the occurrence of several antidromic spikes. The data presented are the first recordings of single unit activity in the magnocellular system in the prenatal brain. They demonstrate that the chicken embryo may offer a suitable model to study the ontogeny of neuroendocrine systems in the fetal brain in-vivo.