The variability of basal action potential firing is positively correlated with bursting in hypothalamic oxytocin neurones

Magnocellular oxytocin neurones are proposed as a suitable system for study ing the mechanisms involved in the regulation of neuronal bursting activity . They display high frequency (50 sp./s) bursts of spikes (approximately ev ery 300 s), in response to specific stimuli, which are superimposed on a va riable level of basal activity and are tightly co-ordinated as a result of network interactions. The relationship between the strength of the bursting activity (as quantified by burst amplitude and interburst interval) and th e characteristics of the interburst basal activity were assessed. During co ntrol conditions, mean basal activity and variability of firing increased j ust before bursts. During experimental conditions leading to burst facilita tion, burst amplitude increased and interburst interval decreased while a s ustained increase in mean firing rate occurred. Variability of firing (meas ured by both the standard deviation of firing rate, and the index of disper sion which corrected this standard deviation for differences in mean firing rate), increased demonstrating an increase in spike clustering greater tha n expected as a result of increased basal activity. When bursting was restr ained (i.e. interburst interval increased), mean basal activity increased s ubstantially, but index of dispersion decreased. A narrowing of the intersp ike interval distribution occurred, indicating increased regularity of firi ng. The aspect of basal activity most strongly correlated with bursting was variability of firing rate. The strongest correlate of burst amplitude was the standard deviation of mean firing rate, whereas the strongest and most consistent correlate of interburst interval was the index of dispersion. I n conclusion, bursting behaviour is most strongly related to the irregulari ty rather than the level of basal activity.