Suckling and cold stress rapidly and transiently increase TRH mRNA in the paraventricular nucleus.

Thyrotropin releasing hormone (TRH) is released from the median eminence in response to neural stimuli evoked by different physiologic conditions (i.e. cold stress or suckling). The paraventricular nucleus (PVN) synthesizes pro-TRH and responds to negative thyroid hormone feedback. With the aim of determining if TRH biosynthesis is regulated in coordination with its release, we quantified TRH mRNA levels in PVN and in preoptic area-anterior hypothalamus (POA-AH) of rats sacrificed at different 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 hypothalamus (MBH) and in POA-AH as well as corticosterone, triiodothyronine and prolactin levels in serum were also measured. Increases of serum hormones were observed in both paradigms as has been reported. MBH TRH-LI content decreased during suckling by 33% (p < 0.01) after 1 h, but did not change after cold stimulation. At short stimulation times, 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 involved in TRH hypophysiotropic function, a similar transient enhancement of TRH mRNA (146%) was observed only in cold stimulated animals after 30 min, consistent with its suggested role in thermogenesis. These results show a fast and transient response of TRH mRNA in PVN evoked by a neural stimulus.