The preoptic area in the hypothalamus is the source of the additional respiratory drive at raised body temperature in anaesthetised rats.

In mammals that use the ventilatory system as the principal means of increasing heat loss, raising body temperature causes the adoption of a specialised breathing pattern known as panting and this is mediated by the thermoregulatory system in the preoptic area of the hypothalamus. In these species an additional respiratory drive is also present at raised body temperature, since breathing can reappear at low Pa,CO2 levels, when stimulation of chemoreceptors is minimal. It is not known whether the preoptic area is also the source of this additional drive. Rats do not pant but do possess this additional respiratory drive at raised body temperatures. We have therefore tested whether the preoptic area of the hypothalamus is the source of this additional respiratory drive in rats. Urethane anaesthesia and hyperoxia were used in eleven rats to minimise behavioural and chemical drives to breathe. The presence of the additional respiratory drive was indicated if rhythmic diaphragmatic EMG activity reappeared during hypocapnia (a mean Pa,CO2 level of 21+/-2 mm Hg, n = 11), induced by mechanical ventilation. The additional respiratory drive was absent at normal body temperature (37¿C). When the temperature of the whole body was raised using an external source of radiant heat, the additional respiratory drive appeared at 40.6+/-0.5 degrees C (n = 3). In two further rats this drive was induced at normal body temperature by localised warming in the preoptic area of the intact hypothalamus. The additional respiratory drive appeared at similar temperatures to those in control rats in three rats following isolation of the hypothalamus from more rostral areas of the brain. In contrast, the additional respiratory drive failed to appear at these temperatures in three rats after isolating the hypothalamus from the caudal brainstem, by sectioning pathways medial to the medial forebrain bundle. Since the preoptic area is known to contain thermoreceptors and to receive afferents from peripheral thermoreceptors, the results show that this area is also the source of the additional respiratory drive at raised body temperature in anaesthetised rats.

Behavioral changes in fetal sheep caused by vibroacoustic stimulation: the effects of cochlear ablation.

We measured the effects of 2 minutes of vibroacoustic stimulation on the activity of unanesthetized fetal sheep in utero. We were unable to detect any changes in activity after stimulation with a model 5C electrolarynx. With more powerful stimulation using a mechanical oscillator, fetal electrocortical activity desynchronized. Fetal breathing and eye movements changed with electrocortical activity and there was an increase in nuchal muscle activity. Stimulation caused a small decrease in fetal heart rate. Responses to stimulation were not seen in four of the five fetuses with bilateral cochlear ablation. These results indicate that in fetal sheep the auditory apparatus is necessary for the detection of vibroacoustic stimuli. The difference in responsiveness of the sheep and human fetus appears to be because of a difference in central processing after detection of the stimulus.

Vibroacoustic stimulation is not associated with sudden fetal catecholamine release.

The safety of vibroacoustic stimulation (VAS), which produces marked changes in fetal heart rate, movements and behavioural state, remains unclear. In order to determine whether VAS is associated with catecholamine release, we measured plasma noradrenaline and adrenaline in 13 appropriately grown normoxaemic fetuses between 28 and 40 weeks gestation immediately before and 60 and 75 s after VAS. Over this time interval, VAS is known to increase fetal heart rate. There was no significant change in either noradrenaline (median change = +0.06 ng/ml, P = 0.26) or adrenaline levels (median change = +0.03 ng/ml, P = 0.4). This study suggests that sympathoadrenal activation is not part of the fetal response to VAS. These findings do not support the recent suggestion that VAS may be deleterious to the fetus by provoking sudden release of catecholamines.