Continuous vs. pulsatile administration of thyrotropin-releasing hormone (TRH) in the model of the chronically cannulated rat: long-term effects on thyroid function

Thyroid-stimulating hormone (TSH) belongs to a family of glycoprotein hormones secreted by the anterior lobe of the pituitary gland in a pulsatile fashion. Although this pulsatile pattern of release has been characterized by several groups, its exact physiological relevance remains to be elucidated. We established a model of a chronically cannulated rat allowing us to apply quantified pulses of thyrotropin-releasing hormone intravenously and to monitor the biological response on the anterior pituitary as well as on the thyroid level. Serum rTSH, T3 and T4 levels were measured as functional parameters. During the five day infusion period, TSH increased under both continuous and pulsatile TRH stimulation, but this increase was only maintained following pulsatile TRH but not after continuous application. No differences between the continuous and the pulsatile infusion regimen could be observed in terms of TT3 release, whereas TT4 decreased following an initial stimulation under continuous application, but remained stimulated during pulsatile application. Our data indicate that the model of the chronically cannulated rat appears to be a valuable tool to study the impact of the temporal pattern of TRH/TSH on thyroid physiology.