Effects of preoptic and hypothalamic thermal stimulation on electrical activity of neurosecretory cells in the supraoptic nucleus

Synaptic control of rat magnocellular neurosecretory cells by warm-sensing neurons in the organum vasculosum lamina terminalis

Increases in core body temperature cause secretion of vasopressin (vasopressin, antidiuretic hormone) to promote water reabsorption and blunt water losses incurred through homeostatic evaporative cooling. Subtypes of transient receptor potential vanilloid (Trpv) channels have been shown to contribute to the intrinsic regulation of vasopressin-releasing magnocellular neurosecretory cells (MNCs) in the supraoptic nucleus (SON) and paraventricular nucleus (PVN). However, MNCs in vivo can also be excited by local heating of the adjacent preoptic area, indicating they receive thermosensory information from other areas. Here, we investigated whether neurons in the organum vasculosum lamina terminalis (OVLT) contribute to this process using in vitro electrophysiological approaches in male rats. We found that the majority of OVLT neurons are thermosensitive in the physiological range (36-39°C) and that this property is retained under conditions blocking synaptic transmission. A subset of these neurons could be antidromically activated by electrical stimulation in the SON. Whole cell recordings from SON MNCs revealed that heating significantly increases the rate of spontaneous excitatory postsynaptic currents (sEPCSs), and that this response is abolished by lesions targeting the OVLT, but not by bilateral lesions placed in the adjacent preoptic area. Finally, local heating of the OVLT caused a significant excitation of MNCs in the absence of temperature changes in the SON, and this effect was blocked by inhibitors of ionotropic glutamate receptors. These findings indicate that the OVLT serves as an important thermosensory nucleus and contributes to the activation of MNCs during physiological heating.

Differences in hypothalamic Fos expressions between two heat stress conditions in conscious mice

Hyperthermia and dehydration were important physiological phenomena in heat stress. But, the degrees of these phenomena were changed by heat stress conditions, and the distinction between both phenomena is necessary for investigation of response for individual phenomenon. Heat stress at 34 degrees C for 60 min increased rectal temperature, and heat stress at 38.5 degrees C for 60 min further increased rectal temperature and increased osmolality in mice. We investigated the activated region in hypothalamus, which played a role in thermoregulation, fluid regulation and so on, using immunostaining for Fos protein under these conditions in conscious mice. At 34 degrees C, Fos-positive neurons increased in the median preoptic nucleus, lateral preoptic area and anterior hypothalamic area, which were known to be the thermoregulatory center, and the dorsomedial hypothalamic nucleus, which was known to control eating. At 38.5 degrees C, Fos-positive neurons further increased in the regions mentioned above and appeared in the lateral septal nucleus, medial preoptic area, lateral hypothalamic area and zona incerta, which were thought to be involved in thermoregulation and/or fluid regulation, and the paraventricular hypothalamic nucleus, supraoptic nucleus and supraoptic nucleus in the retrochiasmatic part, which were known to be involved in neuroendocrine effector systems. These results support that the activated regions in hypothalamus differed with heat stress conditions, which induced only hyperthermia and both hyperthermia and dehydration.

Increases in amino-cupric-silver staining of the supraoptic nucleus after sleep deprivation

Sleep deprived rats undergo a predictable sequence of physiological changes, including changes in skin condition, increased energy expenditure, and altered thermoregulation. Amino-cupric-silver staining was used to identify sleep deprivation related changes in the brain. A significant increase in staining was observed in the supraoptic nucleus (SON) of the hypothalamus of rats with high sleep loss (>45 h) vs. their yoked controls. Follow-up experiments showed that staining was not significantly different in rats sleep deprived for less than 45 h, suggesting that injurious sleep deprivation-related processes occur above a threshold quantity of sleep loss. These anatomical changes suggest that the effects of sleep deprivation may be related to protein metabolism in certain brain regions.

Peptidergic excitation of supraoptic nucleus neurons: involvement of stretch-inactivated cation channels

Although the primary stimulus regulating vasopressin (VP) release is a change in systemic osmolality, other physiological parameters are known to affect VP secretion or modulate the osmotic control over its release. Neuropeptides feature prominently in afferents underlying the central regulation of the VP-releasing magnocellular neurosecretory cells (MNCs). Although little is yet known of the circumstances under which peptides are released onto MNCs, previous studies have shown that a common response profile to exogenous peptide application is a slow excitation that seems to result from the activation of a nonselective cation conductance. In this paper we review the basis for the excitatory effects of angiotensin II, cholecystokinin, and neurotensin in MNCs acutely isolated from the supraoptic nucleus of adult rats. Saturating concentrations of these three peptides evoked nonadditive increases in macroscopic cation conductance. During single-channel recordings Ang II, CCK, and NT caused kinetically identical increases in the probability of opening of 35-pS nonselective cation channels. Patches containing only one channel further revealed that the activity of single channels could be regulated by separate applications of all three peptides. Peptide-stimulated channels were also found to be inactivated by increases in membrane stretch and to be blocked by low concentrations of gadolinium (Gd(3+)). It is concluded that many excitatory peptides depolarize MNCs by stimulating the stretch-inactivated cation channels underlying osmoreception. Convergent regulation of these channels provides a potentially powerful mechanism for integrating signals derived from the various afferents involved in the regulation of MNCs.

Thermally induced changes in neural and hormonal control of osmoregulation in a bird with salt glands (Anas platyrhynchos)

In conscious Pekin ducks adapted to hypertonic saline (1.9%) as drinking water, steady state secretion of the salt glands was established by continuous intravenous salt loading and the effects of hypothalamic thermal stimulation on salt gland activity and on the plasma concentrations of arginine vasotocin (AVT) and angiotensin II (AII) were observed. Hypothalamic cooling depressed salt gland secretion and the plasma level of AVT. Hypothalamic warming caused transient activation and subsequent inhibition of salt gland secretion without consistent changes of the plasma levels of AVT and AII. Whole body cooling by heat extraction with a colonic thermode produced moderate inhibition of salt gland activity, without changes in plasma AVT and AII, which may be explained by peripheral vasoconstriction. The results are consistent with the view that hypothalamic osmoregulation is under an influence of local temperature by combined osmo/thermo-responsiveness of hypothalamic neurons and temperature dependence of signal transmission in hypothalamic neural integration of osmoregulation.

Responsiveness of monkey preoptic thermosensitive neurons to non-thermal emotional stimuli

Responsiveness of 143 preoptic neurons to changes in hypothalamic temperature and to non-thermal emotional stimuli were investigated while rewarding (foods) and aversive objects (hypertonic saline, a toy snake, an air puffer) were given. About 71% of thermosensitive neurons and 32% of thermally insensitive neurons changed the activity when emotional stimuli were shown to and/or tasted by the monkey. Such responses were modulated by satiety/hunger state and were dependent on the degree of perturbation of emotional state. About half of the neurons tested responded when the monkey opened the mouth and protruded the tongue or moved fingers in trying to obtain foods with strong motivation, but did not when the animal made such movements less readily or reluctantly with the progress of satiation. This response was most frequently found among warm-units. The results raise a possibility that preoptic thermosensitive neurons, besides their postulated thermoregulatory functions, might be involved in the response of coordination with thermal and non-thermal emotional behaviors controlled in the hypothalamus.