Monkeys can slowly increase or decrease the amplitude of the purely spinal, largely monosynaptic portion of the response to sudden muscle stretch, the spinal stretch reflex (SSR), when confronted by a task requiring such change (Wolpaw, J.R., V.A. Kieffer, R.F. Seegal, D.J. Braitman, and M.G. Sanders (1983) Brain Res. 267: 196-200; Wolpaw, J.R., D.J. Braitman, and R.F. Seegal (1983) J. Neurophysiol. 50: 1296-1311). Change occurs without alteration in initial muscle length or in background activity of agonist, antagonist, or synergist muscles. This study uses composite curves to describe in detail the development of SSR amplitude change. It reveals important, previously unexpected features of this development. SSR increase or decrease appears to occur in two distinct phases. Phase I, a nearly immediate 8% change, occurs within the first 6 hr. Phase II, a 2%/day change, continues for at least 2 months. Although phase II is much slower than phase I, its final magnitude is far greater. Phase I indicates a nearly immediate change in suprasegmental influence of the segmental arc of the SSR. Because stretch onset time is unpredictable and the SSR occurs before any other possible response, this change in descending activity must be tonic; it must be present continually, day after day, for the 5 to 7 hr/day the animal spends at the task. Phase I produces a rapid and significant increase in reward probability. Thus, it may be readily interpreted as an example of operant conditioning, provoked by the reward contingency.(ABSTRACT TRUNCATED AT 250 WORDS)
Oxytocin (OT) and vasopressin (AVP) gene expression are enhanced in the rat hypothalamus in late gestation and during the second and third weeks of lactation. We report that during the first 3 postpartum days, OT and AVP cytoplasmic mRNAs in the supraoptic and paraventricular nuclei of lactating rats decreased dramatically, reaching less than one fifth of peak gestational levels by day 2 postpartum. Differences in the temporal pattern of OT and AVP expression were observed in the supraoptic and paraventricular nuclei from days 4-10 of lactation. We also compared OT and AVP cytoplasmic mRNAs isolated from the hypothalamus of day 3 lactating rats to cohorts that had litters removed at the time of parturition. Lactating rats had significantly lower OT and AVP cytoplasmic mRNA levels than their nonlactating cohorts. We further compared OT and AVP cytoplasmic mRNAs in the hypothalamus of day 12 lactating rats that had been ovariectomized or sham ovariectomized on day 3 of lactation. Ovariectomized day 12 lactating animals had significantly lower OT and AVP cytoplasmic mRNA levels than their intact cohorts. These data refute the hypothesis that lactation is characterized by persistently elevated hypothalamic cytoplasmic OT and AVP mRNAs produced as a result of continuous stimulation by suckling and suggest that ovarian steroids may exert a modulatory effect on hypothalamic OT and AVP expression during early lactation.
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