Effect of Centrally Administered Oxytocin on the Association Between Cortical Electroencephalogram and Milk Ejection in the Rat

Abstract

The milk-ejection reflex of the rat is closely associated with synchronized activity of the cortical electroencephalogram (EEG), and the frequency of milk ejections has been shown to be greatly facilitated by central oxytocin. The following experiments were undertaken to examine the changes in the EEG during facilitation of the reflex by central oxytocin. lntracerebroventricular injection of 1 mU (2.2 ng) oxytocin during suckling caused a rapid increase in the frequency of milk ejections but no change in the predominantly synchronized pattern of the EEG. However, after a delay (11.3+0.8 min, meankSE) there appeared to be an increasing proportion of desynchronization, which correlated with cessation of the facilitated milk-ejection responses. Hence, the observed EEG desynchronization may signal activation of mechanisms inhibitory to the milk-ejection reflex.In the absence of the suckling stimulus oxytocin also caused a change to desynchronization. However, this effect was more pronounced and commenced after a much shorter latency (1.7k0.4 min, mean kSE; P<O.OOl), suggesting that the desynchronizing effect of oxytocin on the EEG can be attenuated by the suckling stimulus. These results demonstrate two phases in the action of central oxytocin in the suckled rat. During the initial phase, the milkejection reflex is facilitated and although there may be a concomitant desynchronizing influence on the EEG this is prevented by the influence of the suckling stimulus. In the later phase, this desynchronizing influence predominates and is accompanied by cessation of milk-ejection responses.Although milk ejections were generally restricted to periods of a synchronized EEG as previously reported, during the oxytocininduced change to desynchronization a number of milk ejections were observed to occur in the absence of a synchronized EEG. These results provide further evidence that the association between milk ejection and the EEG state is not an absolute causal relationship.The milk-ejection reflex o f the suckled rat i s characterized by a pulsatile pattern o f oxytocin release, which persists under urethane-anaesthesia ( 1 ). Intrdcerebroventricular (icv) administration o f oxytocin causes a striking facilitation o f this reflex, with an increase in both the frequency and amplitude o f the oxytocin pulses, lasting some 10 to 20 min (2). However, the mechanism of this effect, and the factors governing the time-course o f the facilitation, are poorly understood. The action o f oxytocin may be restricted to neural pathways directly involved in the milk-ejection reflex, or the peptide could exert more widespread effects within the brain. One important factor which correlates with occurrence o f milk ejection in the rat i s the state o f the cortical electroencephalogram (EEG) and, in the rat, milk ejection i s exclusively restricted to periods o f synchronized activity (3, 4). FreundMercier and Richard (2) reported that the facilitation o f milk ejections by oxytocin was not accompanied by any changes in the cortical EE...