A role for serotonin in the circadian system revealed by the distribution of serotonin transporter and light-induced Fos immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet

Components of the circadian system, the suprachiasmatic nucleus and the intergeniculate leaflet receive serotonin input from the raphe nuclei. Manipulations of serotonin neurotransmission disrupt cellular, electrophysiological, and behavioural responses of the circadian system to light, suggesting that serotonin plays a modulatory role in photic regulation of circadian rhythms. To study the relation between serotonin afferents and light-activated cells in the suprachiasmatic nucleus and intergeniculate leaflet, we used immunostaining for the serotonin transporter and for the transcription factor, Fos. Serotonin transporter, a plasma membrane protein located on serotonin neurons, regulates the amount of serotonin available for neurotransmission by re-accumulating released serotonin into presynaptic neurons; expression of Fos in the suprachiasmatic nucleus identifies light-activated cells involved in photic resetting of circadian clock phase. In the suprachiasmatic nucleus, immunostaining for serotonin transporter revealed a dense plexus of fibres concentrated primarily in the ventrolateral region. In the intergeniculate leaflet, serotonin transporter immunostaining identified vertically-oriented columns of fibres. Serotonin transporter immunostaining was abolished by pretreatment with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Exposure to light for 30 min during the dark phase of the light cycle induced Fos expression in the ventrolateral suprachiasmatic nucleus and intergeniculate leaflet regions. In both structures the Fos-expressing cells were encircled by serotonin transporter-immunoreactive fibres often in close apposition to these cells. These results support the idea that serotonin activity plays a modulatory role in processing of photic information within the circadian system.

Induction of Fos expression in the circadian system by unsignaled light is attenuated as a result of previous experience with signaled light: a role for Pavlovian conditioning

A circadian clock responsive to light is located in the hypothalamic suprachiasmatic nucleus. In rodents, light induces the expression of the transcription factor Fos in cells of the suprachiasmatic nucleus and this effect is associated with light-induced resetting of the circadian clock. Until recently, it was thought that the induction of Fos in the suprachiasmatic nucleus was mediated by a mechanism uniquely sensitive to photic cues. We have shown, however, using Pavlovian conditioning procedures, that a nonphotic stimulus that has been repeatedly paired with light can, in the absence of light, induce Fos expression in the suprachiasmatic nucleus. In the present study we asked whether, as a result of conditioning, the ability of light alone to induce Fos expression in the suprachiasmatic nucleus might be altered. We suspected that once the mechanism mediating Fos expression in the suprachiasmatic nucleus had become tuned to receiving light signaled by a conditioned stimulus, the response to presentation of light alone would be diminished. To study this possibility we investigated whether induction of Fos expression in the suprachiasmatic nucleus by unsignaled light would be altered as a result of previous experience with signaled light. Consistent with our hypothesis, we found that a series of conditioning trials not only confers upon a nonphotic stimulus the ability to activate the mechanism mediating Fos expression in the suprachiasmatic nucleus, but also reduces the efficacy of light itself to activate this mechanism.

Blockers of nitric oxide synthase inhibit stress activation of c-fos expression in neurons of the hypothalamic paraventricular nucleus in the rat

The cellular transcription factor Fos, product of the immediate early gene c-fos, is induced in secretory neurons of the hypothalamic paraventricular nucleus in response to stress. Recent evidence indicates that the paracrine-acting messenger molecule nitric oxide may be involved in the activation or regulation of c-fos expression in neurons, and many neurons in the paraventricular nucleus contain the enzyme nitric oxide synthase. Furthermore, nitric oxide has been implicated in the regulation of corticotropin-releasing factor and arginine vasopressin release from neurons of the paraventricular nucleus. To study whether nitric oxide is involved in stress activation of c-fos expression in the paraventricular nucleus, we assessed the effect of treatment with competitive nitric oxide synthase blockers on expression of Fos protein in neurons of the paraventricular nucleus of rats subjected to immobilization stress. We found that such treatment blocks stress-induced Fos expression in the paraventricular nucleus. Furthermore, using double staining for Fos and the nitric oxide synthase histochemical marker, nicotinamide adenine dinucleotide phosphatediaphorase, we found that many neurons in the paraventricular nucleus that express Fos in response to immobilization stress also contain nitric oxide synthase. These results indicate that nitric oxide is involved in the regulation of Fos expression in stress-activated cells of the paraventricular nucleus.

Lactation reduces Fos induction in the paraventricular and supraoptic nuclei of the hypothalamus after urethane administration in rats

Both neuroendocrine and behavioral responses to stressors are reduced in lactating animals. In these studies we determined whether Fos induction following treatment with urethane would differ between nonlactating and lactating rats. Urethane treatment produced robust Fos expression in the central nucleus of the amygdala and the paraventricular and supraoptic nuclei of the hypothalamus 1 h after treatment. Fos expression in both the paraventricular and supraoptic nuclei was attenuated in lactating rats compared to that seen in nonlactating animals but Fos expression in the central nucleus of the amygdala did not differ between these two groups. Removing litters from lactating rats 48 h prior to urethane administration restored the Fos response in the parvocellular division of the paraventricular nucleus (PVN) to the level seen in nonlactating rats.

A blocker of nitric oxide synthase, NG-nitro-L-arginine methyl ester, attenuates light-induced Fos protein expression in rat suprachiasmatic nucleus

Nitric oxide (NO) serves as a messenger molecule in some of the neuronal systems that use glutamate as a transmitter. Because glutamate mediates the transmission of photic signals from retinal ganglion cell axons to the suprachiasmatic nucleus (SCN) circadian pacemaker, and because pharmacological treatments which block NO production by NO synthase (NOS) inhibit light-induced pacemaker phase-resetting, it has been proposed that NO is involved in circadian light signaling in the SCN. In the present study we investigated this hypothesis by assessing in rats the effect of treatment with the NOS blocker, NG-nitro-L-arginine methyl ester (L-NAME), on light-induced expression of the transcription factor Fos, a cellular marker of light signaling in the SCN. We found that systemic administration of L-NAME (100 mg/kg) but not of the inactive analog, D-NAME, significantly attenuates light-induced expression of Fos immunoreactivity in the SCN.

Reproductive state changes NADPH-diaphorase staining in the paraventricular and supraoptic nuclei of female rats

It has been demonstrated in guinea pigs that nitric oxide synthase (NOS) activity is increased in late pregnancy in some peripheral tissues and in the cerebellum. To determine whether similar changes would be observed in areas of the brain known to play a role in parturition, staining for NADPH-diaphorase, a histochemical marker of NOS synthase, in the paraventricular (PVN) and supraoptic nuclei (SON) was compared among ovariectomized, virgin and late pregnant rats. The number of cells showing dense staining for NADPH-diaphorase increased in both the SON and PVN in late pregnancy compared to that observed in virgin and ovariectomized females. Thus, changes in reproductive state are associated with changes in NADPH-diaphorase staining in areas of the brain that are intimately involved in the control of reproductive function.

Constant light induces persistent Fos expression in rat intergeniculate leaflet

Fos protein expression in retinorecipient suprachiasmatic nucleus (SCN) neurons is a marker of photic entrainment of circadian rhythms. Light-induced Fos in neurons of the intergeniculate leaflet (IGL) is not well-characterized. We compared Fos immunoreactivity (Fos-IR) in SCN and IGL neurons of rats housed under various lighting conditions and sacrificed at different phases of the circadian period. IGL neurons of rats that received 1 h-3 weeks of light exposure prior to sacrifice displayed Fos-IR, whereas the IGL of animals exposed only to darkness displayed little if any staining. In contrast with light-induced Fos in SCN neurons, Fos-IR was observed in the IGL regardless of circadian time. This work supports the idea that the IGL is involved in transmission of photic information to the SCN in rats.

Fos expression in rat visual cortex induced by ocular input of ultraviolet light

We used immunostaining for the cellular transcription factor Fos to assess patterns of neuronal activation in rat visual cortex during exposure to ultraviolet light. Exposure to monochromatic ultraviolet light (lambda max 360 nm: half-bandwidth 8.8 nm, 10 microW/cm2 at eye level) induced strong expression of Fos immunoreactivity in the primary visual cortex and associated cortical visual areas of dark-adapted rats. The stimulatory effect of ultraviolet light on Fos expression was related to exposure duration, was independent of stimulus novelty or phase of the circadian cycle in which exposure occurred, and it was mediated by a mechanism located in the eye. These results demonstrate that ocular input of ultraviolet light is capable of altering neuronal activity in cortical structures involved in visual processing and are consistent with the hypothesis that rodents may use ultraviolet light for vision.

Resetting of the circadian clock by a conditioned stimulus

Environmental light is the dominant temporal cue for the entrainment of circadian rhythms. In mammals, light entrains circadian rhythms by daily resetting a pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). Although it is widely held that phase resetting by light involves cellular elements within the SCN that are uniquely responsive to photic cues, we now report that non-photic cues that reliably precede the onset of light can, through associative learning, come to activate these elements. In rats, a neutral non-photic stimulus paired with light in pavlovian conditioning trials was capable of eliciting cellular and behavioural effects characteristic of phase-dependent resetting of the pacemaker by light, the expression of the transcription factor Fos in SCN cells, and phase shifts in free-running activity and temperature rhythms. Thus an associative learning process, pavlovian conditioning, provides a means whereby environmental cues that predict light onset can come to mimic the effect of light on the SCN pacemaker and thereby bring about entrainment of circadian rhythms.

Ultraviolet light entrains rodent suprachiasmatic nucleus pacemaker

It has long been assumed that, in contrast to other vertebrates, mammals are ultraviolet blind. Recent evidence indicates, however, that the spectral sensitivity of the retina in rodents extends into the ultraviolet range. This finding, combined with reports that ultraviolet light can suppress nocturnal melatonin release and reverse the effect of short photoperiod on the gonads, invites speculation about the role of ultraviolet light in photoperiodic control of physiological and behavioral functions. One idea is that ultraviolet light participates in retinally mediated processes underlying photic entrainment of a pacemaker located in the hypothalamic suprachasmatic nucleus that generates circadian rhythms. Consistent with such a function, we now show that ultraviolet light is capable of inducing phase shifts in circadian rhythms in the rat and of inducing in the hypothalamic suprachiasmatic nucleus expression of the transcription factor Fos, a known cellular correlate of light-induced phase shifts of the hypothalamic suprachiasmatic nucleus pacemaker.

Distribution of NADPH-diaphorase staining and light-induced Fos expression in the rat suprachiasmatic nucleus region supports a role for nitric oxide in the circadian system

Nitric oxide serves as a messenger molecule in some neuronal systems that use glutamate as a transmitter and it has been shown that glutamate mediates the transmission of photic signals by retinal ganglion cell axons terminating in the hypothalamic suprachiasmatic nucleus, site of the circadian pacemaker in rodents. Recent experiments have demonstrated that pharmacological treatments which block nitric oxide synthesis by nitric oxide synthase prevent glutamate-induced phase shifts of the cell firing rhythm in suprachiasmatic nucleus slice preparation in vitro; similar treatments were found to inhibit light transmission to the suprachiasmatic nucleus as well as light-induced phase shifts in activity rhythms in vivo, implicating nitric oxide in circadian light signalling in vivo. There is limited information, however, about the presence and function of nitric oxide synthase-containing neurons within retinorecipient regions of the rodent suprachiasmatic nucleus. In the present study we used NADPH-diaphorase histochemistry and immunostaining for the nuclear phosphoprotein Fos to assess the co-distribution of nitric oxide synthase-containing neurons and light-responsive cells in the rat suprachiasmatic nucleus region. A strong convergence between NADPH-diaphorase-stained cell bodies and fibres and cells that expressed Fos in response to photic stimulation was noted in the anterior periventricular nucleus, suprachiasmatic preoptic nucleus, retrochiasmatic area, the inter-suprachiasmatic nucleus region, and the dorsal aspect of the optic chiasm, below the suprachiasmatic nucleus. A similar convergence between NADPH-diaphorase-stained fibres and Fos-immunoreactive cells was noted inside the suprachiasmatic nucleus, but the number of NADPH-diaphorase-stained elements found in this region was substantially low compared with that found in retinorecipient regions bordering the nucleus. In many cases both inside and outside the suprachiasmatic nucleus, the Fos-immunoreactive cells appeared to make direct contact with NADPH-diaphorase-stained cells or fibres, but no co-localization of Fos immunoreactivity and NADPH-diaphorase histochemical activity within individual cells was detected. Extensive co-distribution of NADPH-diaphorase-stained cells and fibres and cells that express Fos in response to photic stimulation in the suprachiasmatic nucleus region is in line with the hypothesis that nitric oxide participates in the mechanism mediating circadian light signalling in the suprachiasmatic nucleus. However, lack of co-localization of the two markers to individual cells rules out the possibility that retinorecipient cells in the suprachiasmatic region synthesize and release nitric oxide when photically-activated. Instead, the results support the possibility that photic stimulation triggers nitric oxide synthesis in nitric oxide synthase-containing neurons located near the photically-activated cells.

Non-photic manipulations induce expression of Fos protein in the suprachiasmatic nucleus and intergeniculate leaflet in the rat

Expression of Fos protein in the suprachiasmatic nucleus (SCN) and intergeniculate leaflet (IGL) is considered a cellular correlate of light-induced phase-shift of circadian rhythms in rodents. Non-photic stimuli also induce phase shifts, but their effects on Fos expression have not been established. We examined induction of Fos protein in SCN and IGL regions, in response to cage change, intraperitoneal saline injection, and restraint stress. Fos immunoreactivity was observed in SCN and IGL regions, with greater expression observed in IGL during the light phase of the light-dark cycle. Results suggest that cells in SCN and IGL respond to several types of non-photic manipulations and that expression of Fos in these regions is not light-specific.

Induction of Fos protein in a model of closed head injury in rats

The induction of Fos immunoreactivity in the pyriform cortex and the hippocampal formation after closed head injury was determined and compared to that seen following cortical needle injury in pentobarbital anaesthetized male rats. Robust Fos expression was observed in the ipsilateral pyriform cortex following both types of injury but was observed in the ipsilateral dentate gyrus only following closed head injury. Pretreatment with the NMDA receptor blocker MK-801 eliminated closed head injury induced Fos expression in the pyriform cortex and attenuated that seen in the hippocampus. Similar amounts of Fos expression were observed in urethane anaesthetized lactating and nonlactating rats following closed head injury. No gross behavioural impairments as reflected in body weight gain and locomotor activity were seen in animals subjected to closed head injury. These results demonstrate that as with other forms of brain damage, closed head injury at levels that produce no overt brain lesion nor gross behavioural impairment induce Fos expression in the pyriform cortex and the dentate gyrus which is dependent on the activation of NMDA receptors. Further, this response to brain injury is not modulated by lactation.

Neonatal monosodium glutamate treatment prevents effects of constant light on circadian temperature rhythms of adult rats

Housing rats under continuous illumination (LL) disrupts circadian rhythms controlled by a pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). The neural mechanisms underlying this effect are not well understood. The present study examined the effects of LL on circadian rhythms and on light-induced expression of Fos protein in the SCN, intergeniculate leaflet (IGL), and ventrolateral geniculate nucleus (vLGN) in adult rats treated neonatally with monosodium glutamate (MSG). Such treatment is known to lead to acute degeneration of retinal ganglion cells. Despite degeneration of the optic nerve, neonatal MSG treatment (2 mg/g SC on postnatal days 1,3,5,7, and 9) had no effect on daily temperature rhythms in the adult animal under a light-dark cycle. However, the disintegration of circadian rhythms under LL conditions observed in adult rats treated neonatally with 10% saline was prevented in MSG-treated rats. Furthermore, neonatal MSG treatment attenuated light-induced expression of Fos protein in the IGL and vLGN, but not in the SCN. These data suggest that neonatal MSG treatment alters the response of the circadian system to LL and that cells within the IGL/vLGN region may mediate this response.

The nitric oxide synthase inhibitor NW-nitro-L-arginine methylester attenuates brain catalase activity in vitro

Nitric oxide has been implicated in mediating the neurotoxic effects of ischemia in the brain. However, studies of the effects of nitric oxide inhibition with nitric oxide synthase inhibitors have provided controversial results. One of the reasons for the controversy may be related to the specificity of the nitric oxide synthase inhibitors, such as Nw-nitro-L-arginine methylester (L-NAME), which has recently been questioned. The present work investigated the possible interaction of L-NAME with the enzyme catalase in vitro. Catalase is an iron containing enzyme which could potentially interact with the iron-binding groups of L-NAME. Since the normal function of catalase in the brain is to remove excess hydrogen peroxide, the inhibition of this process could have potentially toxic effects. L-NAME was found to attenuate the catalase inhibiting effects of the known catalase inhibitor cyanamide in vitro, suggesting a competition between cyanamide and L-NAME for catalase. In addition, L-NAME by itself attenuated catalase activity in vitro. These results indicate that in addition to inhibiting nitric oxide synthase, L-NAME may have effects on catalase activity.

Induction of Fos protein in the piriform cortex after brain injury in pentobarbital-anaesthetized rats: lack of effect of lactation

The ability of lactation and progesterone administration to inhibit the induction of Fos protein in the piriform cortex after brain injury in pentobarbital-anaesthetized rats was assessed in three studies. Consistent with previous reports we found that brain injury-induced Fos expression in the piriform cortex could be eliminated by the administration of the non-competitive NMDA receptor antagonist MK 801 (4 mg/kg i.p.). Fos induction was not reduced, however, in lactating dams (days 7-9) post-partum or in progesterone-treated males. These results are not consistent with the hypothesis that suckling stimulation and progesterone have a direct effect at the NMDA receptor complex.

Thermogenesis in brown adipose tissue is activated by electrical stimulation of the rat dorsal raphe nucleus

Brainstem mechanisms involved in the central control of thermogenesis in brown adipose tissue (BAT) were investigated, in urethane-anaesthetized rats, by observing changes in the temperature of interscapular BAT following electrical stimulation of sites in the dorsal raphe nucleus (DRN). Large increases in temperature could be produced by single 30 s stimulation trains. The magnitude of the temperature increase grew as a function of both current and frequency. Non-linear temporal summation was observed when near-threshold trains were delivered in close succession. These results are consistent with the view that ascending projections from the DRN relay thermal information to the basal forebrain and hypothalamus.

N-Methyl-D-aspartate receptor-mediated signaling in the supraoptic nucleus involves activation of a nitric oxide-dependent pathway

N-Methyl-D-aspartate (NMDA) microinjection (1 mM, 0.2 microliter) into the hypothalamic supraoptic nucleus (SON) stimulated heart rate in urethane-anaesthetized rats. This effect was inhibited by coinjection of a competitive blocker of NMDA receptors, CPP (20 nmol) or by pretreatment with a sympathetic ganglionic blocker, chlorisondamine chloride (5 mg/kg i.p.), but not by prior hypophysectomy. Furthermore, the cardioexcitatory effect of intra-SON NMDA was inhibited by prior intra-SON injection of a competitive blocker of nitric oxide (NO) synthesis, NG-nitro-L-arginine methyl ester (40 nmol) or a blocker of the soluble guanylate cyclase, Methylene blue (20 nmol), and was mimicked by intra-SON injection of a calcium ionophore, A23187 (10 nmol), which stimulates NO production by raising intracellular free calcium levels. Finally, intra-SON microinjection of a membrane-permeating cGMP analog, 8-bromo-cGMP (20 nmol) stimulated heart rate in urethane-anaesthetized rats. The results point to a functional link between a sympathetically mediated cardiophysiological effect of NMDA receptor stimulation in the SON and activation of the NO/cGMP signal transduction pathway.

The young diabetic subjects in the Israel Army

Between 1978 to 1986, 145 of the Type 1 diabetic patients under our care reached the age of 18 (military service recruitment age). Of the 77 (45 men, 32 women) who decided to volunteer for service, 60 (35 men, 25 women) responded to a questionnaire relating to the conditions of their military service following its completion-these comprised Group A. Of the 68 patients who did not volunteer for service (20 men, 48 women), 44 patients comprised Group B (10 men, 34 women). Group A and Group B were compared in regard to their diabetes history and diabetes management and control during two periods, 17-18 years and 18-20 years. Group A was also evaluated regarding military employment, working and accommodation conditions and diabetes management during army service. The patients in Group A came from a higher socioeconomic level (p < 0.009) and had a significantly higher educational level (p < 0.008). The men in Group A achieved significantly better diabetes control, as evaluated by HbA1, in both periods compared to all the others in both groups. During army service the patients in Group A underwent fewer hospitalizations than the others and only two of them developed complications, whereas among those in Group B 5 patients developed complications. Our data demonstrate that youngsters with Type 1 diabetes can maintain satisfactory diabetes management and control and can function not only adequately, but often far beyond, in the stressful situations with which they are confronted within the rigid authoritative system of military service. Our data also indicated that diabetic patients wishing to volunteer for such service constitute a selected group with a higher socioeconomic background and a higher motivation to prove themselves.

Inhibition of nitric oxide synthase does not block the development of sensitization to the behavioral activating effects of amphetamine

Pretreatment with the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), a competitive blocker of NO production, did not interfere with the development of sensitization to the behavioral activating effects of amphetamine (AMPH). On five pre-exposure sessions, at 3-day intervals, rats were given two i.p. injections, either 50 mg/kg L-NAME 30 min prior to 1.5 mg/kg D-AMPH sulfate, saline and AMPH, L-NAME and saline, or saline only. L-NAME reduced the levels of activity recorded during the pre-exposure session but had no effect on the degree of sensitization shown to a challenge injection of 0.5 mg/kg AMPH given 10 days later. A separate study using in vivo microdialysis showed that pretreatment with L-NAME did not alter AMPH-stimulated dopamine release in nucleus accumbens.

The effects of prostaglandin E2 injected into the paraventricular nucleus of the hypothalamus on brown adipose tissue thermogenesis in spontaneously hypertensive rats

We examined brown adipose tissue (BAT) thermogenic responses to prostaglandin E2 (PGE2) injected into the paraventricular nucleus of the hypothalamus in the spontaneously hypertensive rat (SHR). 250 ng of PGE2 produced smaller increases in BAT and core temperatures in SHRs compared to their normotensive controls, the WKYs. The results of the present study suggest that the ability of SHRs to mount a febrile response may be compromised.

Blocking NMDA receptors or nitric oxide production disrupts light transmission to the suprachiasmatic nucleus

Retinal stimulation with a brief pulse of light (200 lx, 3 min) stimulated heart rate in dark-adapted urethane-anaesthetized rats. This effect was inhibited by prior infusion of a competitive blocker of N-methyl-D-aspartate (NMDA) receptors, (+-)-3-(2-carboxypiperazin-4-yl)-propyl-L-phosphonic acid (CPP, 20 nmol) into the hypothalamic suprachiasmatic nucleus (SCN) region. Furthermore, this inhibition of the stimulatory effect of light on heart rate was mimicked by prior infusion in the SCN region of a competitive blocker of nitric oxide (NO) production from L-arginine, NG-nitro-L-arginine methyl ester (40 nmol), or a blocker of the soluble guanylate cyclase. Methylene blue (20 nmol). None of these effects was seen when infusions were made in a region located 2 mm dorsal to the SCN or when a non-visual stimulus (tail pinch) was used to stimulate heart rate. These results point to a functional link between activation of an NMDA receptor coupled NO/cGMP signalling pathway and light transmission to the SCN.

Activation of brown adipose tissue thermogenesis by chemical stimulation of the hypothalamic supraoptic nucleus

Glutamate microinjection (1 M, 250 nl) into the hypothalamic supraoptic nucleus (SON) stimulated heat production in brown adipose tissue (BAT) and caused a rapid and sustained increase in interscapular BAT and core temperatures in urethane-anaesthetized rats. This effect was blocked by intraperitoneal pretreatment with a sympathetic ganglionic blocker, chlorisondamine chloride (2.5 mg/kg), or a beta-adrenergic receptor blocker, propranolol (2.5 mg/kg), but not by prior hypophysectomy or intracerebroventricular pretreatment with specific receptor blockers to vasopressin (d(CH2)5[Tyr(Me)2]AVP, 5 micrograms) or oxytocin (d(CH2(5)[Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT, 5 micrograms). The results demonstrate that stimulation of SON cells with glutamate elicits a non-vasopressinergic/non-oxytocinergic neural signal that can bring about a sympathetically-mediated increase in BAT thermogenesis. Heat production in BAT is an important mechanism of thermal protection during cold stimulation, and there is evidence that osmotic stimulation can influence thermoregulation. SON neurons play a major role in osmoregulation via release of the peptide hormones vasopressin and oxytocin. The present results suggest the possibility that apart from releasing peptide hormones for osmoregulation, SON neurons might be involved in mediating the effect of osmotic stimulation on thermoregulatory responses involved in thermal adaptation.

An inhibitor of nitric oxide production, NG-nitro-L-arginine-methyl ester, improves survival in anaphylactic shock

Induction of anaphylactic shock in mice by i.v. antigen challenge (bovine serum albumin, 100 micrograms) or i.v. treatment with the mast cell degranulator compound 48/80 resulted in 80 and 90% mortality rate, respectively. Inhibition of nitric oxide (NO) synthesis from L-arginine by co-injection of the L-arginine analog NG-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg) reduced the mortality rate by 40 and 20% in the antigen- and compound 48/80-induced shock models. Treatment with 60 mg/kg L-NAME reduced the mortality rate by 60% in these shock models. This beneficial effect was reversed by addition of L-arginine (120 mg/kg) but not D-arginine (120 mg/kg). These results suggest NO production as a possible mechanism involved in the pathophysiology of anaphylactic shock.