ESTROUS HORMONES ENHANCE COUPLED, RHYTHMIC OLIVARY DISCHARGE IN CORRELATION WITH FACILITATED LIMB STEPPING

Synchronized oscillatory behaviour of neurons within the dorsal access ory olive of the rat were monitored across spontaneous changes in rapi d, co-ordinated limb movements associated with the estrous (hormone) a nd circadian cycles as well as following local administration of estra diol. Facilitation of rapid alterations in limb movement is observed f ollowing increases in circulating estradiol and progesterone on the ni ght of behavioural estrus, as assessed by the ability of a rat to main tain a consistent position on a treadmill in response to variable chan ges in acceleration. Synchronized olivary activity was determined usin g chronically implanted bundles of microwires (50 mu m diameter) to re cord from as many as 23 individual olivary neurons simultaneously acro ss several four to five day estrous cycles, and in some cases after in jection of the estrous hormones, estradiol and/or progesterone, either by systemic or local administration. Rats were tested during non-move ment, constant speed locomotion or using a variable acceleration parad igm. Olivary oscillations were most commonly observed during treadmill locomotion with variable changes in acceleration. Local administratio n of estradiol, in combination with systemic progesterone administrati on, enhanced rhythmic olivary oscillations during this paradigm. At th is time, at least a six-fold increase in the size of the coupled clust er of rhythmically discharging olivary neurons was observed compared w ith values obtained on diestrus, a low hormone state. Similar facilita ting effects on olivary oscillations were observed on estrus, or follo wing systemic injection of both estrous hormones. Administration of ei ther steroid alone was not as effective. During the dark phase of the light:dark cycle, the oscillatory behaviour of these neurons was enhan ced more than five-fold than during the light phase, and coupling diam eter was maximal on the night of behavioural estrus. The synchronized oscillatory discharge of neurons within the olive is a putative timing mechanism which may underlie hormone-associated facilitation of rapid limb movements. The results from the present study provide evidence t hat both hormonal and circadian factors can enhance olivary rhythmicit y in association with behavioural rhythmicity. (C) 1997 IBRO. Publishe d by Elsevier Science Ltd.