PUPS REMOVAL ENHANCES THYROTROPIN-RELEASING-HORMONE MESSENGER-RNA IN THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS

Previous studies have shown that lactation and suckling alter thyrotro pin-releasing hormone (TRH) biosynthesis in hypothalamic paraventricul ar neurons. The amounts of paraventricular TRH mRNA and mediobasal hyp othalamus (MBH) TRH were determined following removal of the pups to e xamine whether paraventricular TRH neuron activity is altered during t he transition from lactation to estrous cycle. Paraventricular TRH mRN A and MBH TRH levels were determined by Northern blot analysis and rad ioimmunoassay, respectively, We had shown previously that after an 8-h withdrawal of the pups at mid-lactation the MBH TRH and paraventricul ar TRH mRNA levels are not modified, This condition was compared to on e where pups were removed for 56h, finding a significant decrease (46% , p < 0.005) of MBH TRH and a significant increase (156%, p < 0.02) of paraventricular TRH mRNA. The effect observed in the paraventricular TRH mRNA was correlated negatively with the serum corticosterone level s, a potential negative regulator of paraventricular TRH mRNA, The res ults were similar if a l-h suckling period was introduced 8h after wit hdrawal of the pups to induce a transient increase of corticosterone l evels. The pattern of TRW mRNA was specific to the paraventricular nuc leus because there was no enhancement in the preoptic. area-anterior h ypothalamus. In summary, our data suggest that TRH biosynthesis in par aventricular neurons is slowly adjusted after withdrawal of the pups, possibly to prepare TRH neurons to the new secretary demands of the es trous cycle.