The direct cooling of the preoptic-hypothalamic area elicits the release of thyroid stimulating hormone during wakefulness but not during REM sleep

Abstract

Thermoregulatory responses to temperature changes are not operant during REM sleep (REMS), but fully operant in non-REM sleep and wakefulness. The specificity of the relationship between REMS and the impairment of thermoregulation was tested by eliciting the reflex release of Thyrotropin Releasing Hormone (TRH), which is integrated at hypothalamic level. By inducing the sequential secretion of Thyroid Stimulating Hormone (TSH) and Thyroid Hormone, TRH intervenes in the regulation of obligatory and non-shivering thermogenesis. Experiments were performed on male albino rats implanted with epidural electrodes for EEG recording and 2 silver-copper wire thermodes, bilaterally placed in the preoptic-hypothalamic area (POA) and connected to small thermoelectric heat pumps driven by a low-voltage high current DC power supply. In preliminary experiments, a thermistor was added in order to measure hypothalamic temperature. The activation of TRH hypophysiotropic neurons by the thermode cooling of POA was indirectly assessed, in conditions in which thermoregulation was either fully operant (wakefulness) or not operant (REMS), by a radioimmunoassay determination of plasmatic levels of TSH. Different POA cooling were performed for 120 s or 40 s at current intensities of 80 mA and 125 mA, respectively. At both current intensities, POA cooling elicited, with respect to control values (no cooling current), a significant increase in plasmatic TSH levels in wakefulness, but not during REMS. These results confirm the inactivation of POA thermal sensitivity during REMS and show, for the first time, that this inactivation concerns also the fundamental endocrine control of non-shivering thermogenesis.

Figure 1. Temperature changes in the preoptic-hypothalamic area elicited by local cooling

. Values (°C; means ± S.E.M. from 4 independent observations for each current intensity) are differences from the average baseline levels in the 60 s preceding the onset of cooling. Heat pumps were driven by current intensities of 80 mA (top) and 125 mA (bottom). Asterisks indicate a significant difference from the baseline value at time 0.

Figure 2. Thyroid Stimulating Hormone concentration in plasma following preoptic-hypothalamic area cooling in behavioral states.

Values (ng/mL; means ± S.E.M) are shown for wakefulness (white bars, Wake) and REM sleep (black bars, REMS). Suffixes are: 0, control (no cooling current); 80, 125, cooling current of 80 mA and 125 mA, respectively. # Significant difference between behavioral states. * Significant difference from control.