RTN TRH causes prolonged respiratory stimulation.

Cream, Carlos L., Aihua Li, and Eugene E. Nattie. RTN TRH causes prolonged respiratory stimulation. J. Appl. Physiol. 83(3): 792–799, 1997.—We injected thyrotropin-releasing hormone (TRH; 10 nl; 0.25, 0.5, 1.0, or 10 mM), its inactive free acid form (TRHOH; 1 mM), or a metabolite with low TRH-receptor binding affinity, histidine-proline diketopiperazine (cHP; 1 mM), into the retrotrapezoid nucleus of anesthetized rats. Injection location was verified by anatomic analysis. Lower doses (0.25–0.5 mM) significantly increased both the product of integrated phrenic amplitude and frequency ($\int$ Phr ⋅ f) and f for 20–30 min compared with artificial cerebrospinal fluid control injections. Higher doses (1.0–10 mM) produced greater and long-lasting stimulation of $\int$ Phr ⋅ f,$\int$ Phr, and f and of blood pressure. This stimulation reached values 150% of baseline and durations of 270 min after a single injection. TRHOH (1 mM ) or cHP (1 mM) had no effect on$\int$ Phr but increased f, as did 1 mM TRH. We conclude that TRH has a very powerful stimulatory effect in the retrotrapezoid nucleus region on $\int$ Phr ⋅ f, with the $\int$ Phr response seemingly specific for TRH receptors. Similar responses of f to TRHOH and cHP suggest it may be nonspecific.