Hypothalamic neurone activity: hormone release and behaviour in freely moving rats

A technique for taking long-term recordings from single hypothalamic neurones in freely moving rats is described. The neurones were classed as magnocellular on the basis of antidromic activation from a chronically implanted neurohypophyseal stimulating electrode. Activity of magnocellular neurones in suckling revealed that 73% of the spontaneously discharging cells (n = 8) showed bursts of high-frequency excitation time-locked to reflex milk ejection. These cells were classed as putative oxytocinergic neurones. Non-responsive magnocellular neurones (n = 3) were tentatively identified as vasopressinergic. In many ways, these periods of activation were similar to those described in anaesthetized animals. The differences between these experimental situations are discussed. The discharge patterns of nine magnocellular neurones was studied in natural labour and correlated to the observed events of parturition. The activity of these cells was then followed in the post-partum suckling period and differentiated into activity recorded from putative oxytocinergic and vasopressinergic cells on the basis of their response before reflex milk ejection. All magnocellular neurone showed bursts of increased discharge after forceful abdominal straining movements but only the putative oxytocinergic neurones (n = 4) showed activation before delivery.