SUCKLING AND COLD STRESS RAPIDLY AND TRANSIENTLY INCREASE TRH MESSENGER-RNA IN THE PARAVENTRICULAR NUCLEUS

Thyrotropin releasing hormone (TRH) is released from the median eminen ce in response to neural stimuli evoked by different physiologic condi tions (i.e. cold stress or suckling). The paraventricular nucleus (PVN ) synthesizes pro-TRH and responds to negative thyroid hormone feedbac k. With the aim of determining if TRH biosynthesis is regulated in coo rdination with its release, we quantified TRH mRNA levels in PVN and i n preoptic area-anterior hypothalamus (POA-AH) of rats sacrificed at d ifferent times during cold (0.5, 1, 2 or 6 h) or suckling (15, 30 and 60 min) stimulus; TRH-like immunoreactivity (TRH-LI) in medial basal h ypothalamus (MBH) and in POA-AH as well as corticosterone, triiodothyr onine and prolactin levels in serum were also measured. Increases of s erum hormones were observed in both paradigms as has been reported. MB H TRH-LI content decreased during suckling by 33% (p < 0.0 1) after 1 h, but did not change after cold stimulation. At short stimulation tim es, PVN TRH mRNA levels were 85% (30 min of suckling) and 97% (1 h in the cold) higher than their respective controls, decreasing to normal after 1-2 h. In the POA-AH, another TRH synthesizing region not involv ed in TRH hypophysiotropic function, a similar transient enhancement o f TRH mRNA (146%) was observed only in cold stimulated animals after 3 0 min, consistent with its suggested role in thermogenesis. These resu lts show a fast and transient response of TRH mRNA in PVN evoked by a neural stimulus.