Regulated nocturnal hypothermia induced in pigeons by food deprivation

The daily body temperature (Tb) cycle of pigeons is altered by food deprivation in that Tb falls to lower and lower levels on consecutive nights after the onset of deprivation, whereas the Tb levels during corresponding days remain nearly unchanged. Manipulations of spinal cord temperature, a major feedback parameter in the avian thermoregulatory system, reveal that episodes of nocturnal hypothermia are regulated. The spinal cord threshold temperature for inducing increases in metabolic heat production falls to progressively lower levels each night and returns to normal euthermic levels during the day.

Spinal Cord Injury in the Monkey

We have investigated the effectiveness of surface cord cooling in reducing spinal cord temperature with respect to the rate of cooling and the temperature gradient within the spinal cord parenchyma. Another question of some practical importance if spinal cooling is to be used clinically is that of the temperature within the spinal cord as a function of the dura being intact or open. In five monkeys, a laminectomy was carried out at T-10 and an impact injury of 350 g-cm force was applied to the spinal cord with the dura intact. Hypothermic perfusion with Elliott's B solution (artificial cerebrospinal fluid) was started, and we measured temperatures in the spinal cord with a thermistor probe mounted on a stereotactic drive. A series of measurements were made with the dura intact and the measurements were then repeated with the dura open. For comparison, we also recorded temperatures in one animal that had not been cord-injured. The rate of cord cooling was rapid during the first 3 minutes of hypothermic perfusion, after which there was a slight further reduction in cord temperature; a low level plateau of 6.7° C was reached within 21 minutes after the start of cooling. The temperature gradient at varying depths of the spinal cord was approximately 6° C (3.8° C at the posterior surface to 10.1° C in the deepest portion of the spinal cord), representing a temperature gradient of 1.3° C/mm of cord tissue. Transmission of the cooling effect from perfusate to the spinal cord was not appreciably affected by the dura being intact or open.

Spinal cord thermosensitivity and sorting of neural signals in cold-exposed rats

In the present study, data relevant to the presence or absence of sorting of neural signals were obtained by evaluating the thermal responses to spinal warming in the chronically prepared rat. Specifically, shivering activity and the rate of oxygen consumption (VO2) were measured in unanesthetized rats during cold exposure (10–16 degrees C). Warming the spinal cord at the level of T2 resulted in a significant decrease in shivering (P less than 0.001), without a significant change in VO2. The shivering response was reversed upon cessation of heating. These results are interpreted as indicating a direct influence of spinal cord temperature on shivering but not nonshivering thermogenesis in the rat. Similarly, in previous work with the rat, we have obtained data supporting hypothalamic receptor control of nonshivering but not shivering heat production. These findings are thus consistent with the suggestion that in the rat there occurs a sorting of neural signals. That is, impulses from the three thermoreceptor locations are not integrated in an identical manner for the control of shivering and nonshivering thermogenesis.

Midbrain neuronal responses to local and spinal cord temperatures

Water-perfused thermodes were implanted over the lumbothoracic spinal cord and unilaterally in the midbrain of urethan-anesthetized rabbits. Single-unit activities were recorded with steel microelectrodes from the thermosensitive neurons in the midbrain reticular formation (MRF), and the effects of heating and cooling of the spinal cord were studied. Of 38 cold-sensitive MRF neurons studied, 7 units decreased their firing rate upon elevation of spinal cord temperature (Tsc) and 3 units showed the opposite type of response to Tsc. The remaining 28 cold units were not affected by the changes in Tsc between 30 and 43 degrees C. Of 17 warm units, 3 units increased and one unit decreased the firing rate during spinal cord heating. These results suggest that the temperature signal arising from thermosensitive structures in the spinal cord may be transmitted to some of the locally thermosensitive neurons in the MRF.