Some in vivo electrophysiological properties of locus coeruleus neurones in fetal rats.

The electrical activity of locus coeruleus (LC) neurones was recorded extracellularly in fetal rats still in contact with their dams by an intact umbilical cord. Pregnant rats, at gestation days 18 to 22, were anesthetized with urethane. The head of a fetal rat was exposed from the uterus and fixed to a conventional stereotaxic apparatus by means of a simple device. The location of the LC in the fetal rats was determined by the appearance of field responses evoked by stimulation of the dorsal noradrenergic bundle. Antidromic spikes of single LC neurones were evoked superimposed upon the field responses. The mean conduction velocity of LC axons was calculated to be 0.25 m/s. Some fetal LC units were activated antidromically by stimulation of the frontal cortex (FC) with latencies ranging from 21 to 67 ms, values nearly the same as those obtained in neonates and adults. Although the majority of fetal LC neurones recorded were not spontaneously active, a small number of them revealed epochs of sporadic firing, which appeared to occur synchronously in many or all of the LC neurones. Sensory stimuli (e.g., air puffs to the skin) were effective in activating LC neurones. These results indicate that LC neurones have already developed projections to the FC, and are functionally active in prenatal periods.