A discrete neuronal circuit induces a hibernation-like state in rodents.

Hibernating mammals actively lower their body temperature to reduce energy expenditure when facing food scarcity1. This ability to induce a hypometabolic state has evoked great interest owing to its potential medical benefits2,3. Here we show that a hypothalamic neuronal circuit in rodents induces a long-lasting hypothermic and hypometabolic state similar to hibernation. In this state, although body temperature and levels of oxygen consumption are kept very low, the ability to regulate metabolism still remains functional, as in hibernation4. There was no obvious damage to tissues and organs or abnormalities in behaviour after recovery from this state. Our findings could enable the development of a method to induce a hibernation-like state, which would have potential applications in non-hibernating mammalian species including humans.

Involvement of the dorsomedial hypothalamus and the nucleus tractus solitarii in chronic cardiovascular changes associated with anxiety in rats.

Anxiety disorders in humans reduce both the heart rate variability (HRV) and the sensitivity of the cardiac baroreflex (BRS). Both may contribute to sudden death. To elucidate the mechanisms underlying these alterations, male rats were subjected to social defeat sessions on four consecutive days. Five days later, the rats were found to be in an anxiety-like state. At this time point, we analysed HRV and BRS in the defeated rats, with or without treatment with the anxiolytic chlordiazepoxide (CDZ). HRV was reduced after social defeat, due to changes in the autonomic balance favouring the sympathetic over the parasympathetic component. Spontaneous and pharmacological baroreflex gains were also reduced. CDZ abolished anxiety-like symptoms as well as HRV and BRS alterations. Inhibition of the dorsomedial hypothalamus (DMH) with muscimol reversed all cardiovascular alterations, whereas blockade of the nucleus tractus solitarii (NTS) 5-HT3 receptor by the local or systemic administration of granisetron restored only baroreflex gains and the parasympathetic component of HRV. In conclusion, repeated social defeat in the rat lead to an anxiety-like state that was associated with lasting reduction in HRV and baroreflex gains. The DMH and the NTS were responsible for these chronic cardiovascular alterations. These regions may therefore constitute new therapeutic targets for reducing cardiac dysfunction and fibrillation in anxiety disorders.

The role of N-methyl-D-aspartate receptor subunits in the rat thalamic mediodorsal nucleus during central sensitization.

We have identified tooth pulp-driven neurons (TPDNs) in the thalamic mediodorsal nucleus (MD) in rats and showed that the TPDNs' responsiveness in the MD is increased by chemical conditioning stimulation of allyl-isothiocyanate (mustard oil) to the molar tooth pulp. The aim of the present study was to address the role of N-methyl-d-aspartate receptors (NMDA receptors) in the sensitized central nervous system following the mustard oil application to the rat tooth pulp. Microinjection of MK-801, a noncompetitive NMDA receptor antagonist, to the thalamic MD nucleus reduced the TPDNs' responsiveness in the thalamic MD nucleus. Gene expression analysis showed that expression levels of NMDA receptor subunits NR2A and NR2D mRNAs in the thalamus were increased by the mustard oil application and that the increases were reduced by MK-801. When naloxone, an opioid receptor antagonist, was given systemically following the MK801 microinjection, the TPDNs' responsiveness was rekindled and expression levels of NR2D and NR2A mRNAs were increased. Moreover, lidocaine pretreatment abolished the mustard oil-induced upregulation of NR2D and NR2A mRNAs. These results suggest that, during central sensitization, interaction of NMDA receptors and endogeneous opioid-related inhibitory mechanisms plays critical role in the alteration of the TPDNs' responsiveness in the thalamic MD nucleus.

Electrical activities of neurones in the dorsomedial hypothalamic nucleus projecting to the supraoptic nucleus during milk-ejection reflex in the rat.

Using urethane-anesthetized lactating rats, extracellular action potentials were recorded from single cells in the dorsomedial hypothalamus (DMH), which were identified as projecting to the supraoptic nucleus (SON). Sixty-two DMH cells were identified as projecting to the SON. Of these 53, 4 and 5 cells had ipsilateral, contralateral and bilateral projections, respectively. Two cells (1 ipsilaterally and 1 contralaterally projecting cell) showed bursting activities preceding milk ejection that were similar to those of oxytocin (OT) cells in the SON or paraventricular nucleus. Two ipsilaterally and 2 bilaterally projecting cells reduced their firing rates preceding milk ejection. The results suggest that some of the projections from the DMH to the SON are contralateral or bilateral and that these projections may contribute to synchronized activation of OT cells bilaterally distributed in the hypothalamus during milk-ejection reflex.

Cardiovascular and thermal responses evoked from the periaqueductal grey require neuronal activity in the hypothalamus.

Stimulation of neurons in the lateral/dorsolateral periaqueductal grey (l/dlPAG) produces increases in heart rate (HR) and mean arterial pressure (MAP) that are, according to traditional views, mediated through projections to medullary autonomic centres and independent of forebrain mechanisms. Recent studies in rats suggest that neurons in the l/dlPAG are downstream effectors responsible for responses evoked from the dorsomedial hypothalamus (DMH) from which similar cardiovascular changes and increase in core body temperature (T(co)) can be elicited. We hypothesized that, instead, autonomic effects evoked from the l/dlPAG depend on neuronal activity in the DMH. Thus, we examined the effect of microinjection of the neuronal inhibitor muscimol into the DMH on increases in HR, MAP and T(co) produced by microinjection of N-methyl-D-aspartate (NMDA) into the l/dlPAG in conscious rats. Microinjection of muscimol alone modestly decreased baseline HR and MAP but failed to alter T(co). Microinjection of NMDA into the l/dlPAG caused marked increases in all three variables, and these were virtually abolished by prior injection of muscimol into the DMH. Similar microinjection of glutamate receptor antagonists into the DMH also suppressed increases in HR and abolished increases in T(co) evoked from the PAG. In contrast, microinjection of muscimol into the hypothalamic paraventricular nucleus failed to reduce changes evoked from the PAG and actually enhanced the increase in T(co). Thus, our data suggest that increases in HR, MAP and T(co) evoked from the l/dlPAG require neuronal activity in the DMH, challenging traditional views of the place of the PAG in central autonomic neural circuitry.

Investigation of the hypothalamic defensive system in the mouse.

The hypothalamus plays especially important roles in various endocrine, autonomic, and behavioral responses that guarantee the survival of both the individual and the species. In the rat, a distinct hypothalamic defensive circuit has been defined as critical for integrating predatory threats, raising an important question as to whether this concept could be applied to other prey species. To start addressing this matter, in the present study, we investigated, in another prey species (the mouse), the pattern of hypothalamic Fos immunoreactivity in response to exposure to a predator (a rat, using the Rat Exposure Test). During rat exposure, mice remained concealed in the home chamber for a longer period of time and increased freezing and risk assessment activity. We were able to show that the mouse and the rat present a similar pattern of hypothalamic activation in response to a predator. Of particular note, similar to what has been described for the rat, we observed in the mouse that predator exposure induces a striking activation in the elements of the medial hypothalamic defensive system, namely, the anterior hypothalamic nucleus, the dorsomedial part of the ventromedial hypothalamic nucleus and the dorsal premammillary nucleus. Moreover, as described for the rat, predator-exposed mice also presented increased Fos levels in the autonomic and parvicellular parts of the paraventricular hypothalamic nucleus, lateral preoptic area and subfornical region of the lateral hypothalamic area. In conclusion, the present data give further support to the concept that a specific hypothalamic defensive circuit should be preserved across different prey species.

The dorsomedial hypothalamic nucleus as a putative food-entrainable circadian pacemaker.

Temporal restriction of feeding can phase-shift behavioral and physiological circadian rhythms in mammals. These changes in biological rhythms are postulated to be brought about by a food-entrainable oscillator (FEO) that is independent of the suprachiasmatic nucleus. However, the neural substrates of FEO have remained elusive. Here, we carried out an unbiased search for mouse brain region(s) that exhibit a rhythmic expression of the Period genes in a feeding-entrainable manner. We found that the compact part of the dorsomedial hypothalamic nucleus (DMH) demonstrates a robust oscillation of mPer expression only under restricted feeding. The oscillation persisted for at least 2 days even when mice were given no food during the expected feeding period after the establishment of food-entrained behavioral rhythms. Moreover, refeeding after fasting rapidly induced a transient mPer expression in the same area of DMH. Taken in conjunction with recent findings (i) that behavioral expression of food-entrainable circadian rhythms is blocked by cell-specific lesions of DMH in rats and (ii) that DMH neurons directly project to orexin neurons in the lateral hypothalamus, which are essential for proper expression of food-entrained behavioral rhythms, the present study suggests that DMH plays a key role as a central FEO in the feeding-mediated regulation of circadian behaviors.

Disinhibition of maternal behavior following neurotoxic lesions of the hypothalamus in primigravid rats.

Virgin female rats do not respond maternally to foster pups due to an endogenous neural circuit that actively inhibits the display of maternal behavior. Once pregnant, primigravid rats will continue to avoid foster pups until just prior to or at parturition. Anosmia or lesions of the olfactory tract, medial amygdala, and areas of the hypothalamus will stimulate virgin females to display maternal behavior rapidly, but little is known of the effect of these lesions in primigravid rats. The objective of the present study was to determine if neurotoxic lesions of the dorsomedial (DMH) and ventromedial nuclei (VMH) of the hypothalamus will advance the onset of maternal behavior in primigravid rats. Nulliparous Sprague-Dawley female rats were mated and then on day 8 of gestation bilaterally infused with N-methyl-d-aspartic acid (NMDA; 8 microg/0.2 microl/side) or vehicle directed toward either the DMH or VMH. Beginning on day 15 of gestation until parturition, females were tested daily for maternal responsiveness. DMH and VMH lesions significantly advanced the onset of maternal behavior (5-6 days vs. 0-1 day before parturition) in first-time pregnant rats. These results indicate that the DMH and VMH are involved in the regulation of maternal behavior and may be part of an endogenous neural circuit that inhibits maternal behavior during pregnancy.

Involvement of hypothalamic histamine H1 receptor in the regulation of feeding rhythm and obesity.

Histamine H(1) receptors (H(1)-Rs) are found in peripheral tissues and in regions of the hypothalamus that are concerned with regulating body composition. In the present study, we investigated the detailed mechanisms of histamine H(1)-Rs in the development of obesity. Histamine H(1)-R knockout (H1KO) mice gradually developed mature-onset obesity, which was accompanied by hyperphagia and decreased expression of uncoupling protein-1 (UCP-1) mRNA. Both younger nonobese (12-week-old) and older obese (48-week-old) H1KO mice exhibited impairment of the responsiveness to the leptin. In addition, disruption of the diurnal rhythm of feeding occurred before the onset of obesity in H1KO mice. Correction of these abnormal feeding rhythms by means of scheduled feeding caused a reduction in obesity and associated metabolic disorders in H1KO mice. Furthermore, central administration of a histamine H(1)-R agonist affected feeding behavior, body weight, and c-fos-like immunoreactivity in the hypothalamus. Taken together, these findings suggest that histamine H(1)-Rs are crucial for the regulation of feeding rhythm and in mediating the effects of leptin. Early disruption of H(1)-R-mediated functions in H1KO mice may lead to hyperphagia and decreased expression of UCP-1 mRNA, which may contribute to the development of obesity in these animals. In addition, centrally acting histamine H(1)-R may be a novel therapeutic target for the treatment of obesity and related metabolic disorders.

Trophic action of leptin on hypothalamic neurons that regulate feeding.

In adult mammals, the adipocyte-derived hormone leptin acts on the brain to reduce food intake by regulating the activity of neurons in the arcuate nucleus of the hypothalamus (ARH). Here, we report that neural projection pathways from the ARH are permanently disrupted in leptin-deficient (Lepob/Lepob) mice and leptin treatment in adulthood does not reverse these neuroanatomical defects. However, treatment of Lepob/Lepob neonates with exogenous leptin rescues the development of ARH projections, and leptin promotes neurite outgrowth from ARH neurons in vitro. These results suggest that leptin plays a neurotrophic role during the development of the hypothalamus and that this activity is restricted to a neonatal critical period that precedes leptin's acute regulation of food intake in adults.

Multiple hypothalamic sites control the frequency of hippocampal theta rhythm.

Stimulation of a neural pathway originating in the brainstem reticular formation, with synapses in the medial hypothalamus, activates the hippocampal theta rhythm. The frequency of reticular-elicited theta is determined in the medial supramammillary nucleus (mSuM) completely in anaesthetised rats, but only partially when the animal is awake. We tested other medial hypothalamic sites for their capacity to control theta frequency in awake rats. Blockade of sodium channels (1 microl fast infusion of the local anaesthetic procaine, experiment 1) or increased inhibition by GABA (Chlordiazepoxide [CDP], experiment 2) was found to reduce or increase the frequency of reticular-elicited theta, depending on the precise site of injection, in the region of the dorsomedial hypothalamic nucleus (DMH) and the posterior hypothalamic nucleus (PH). A band of null sites for CDP was located in the region of the ventral border of PH and dorsal border of mSuM. Using 0.5 and 1 microl CDP, and slow infusions (experiment 3), it was found that effective PH sites were also separate from mSuM in the rostrocaudal direction. In experiment 4, the DMH/PH region was mapped with unilateral and bilateral slow infusions of 0.5 microl CDP. CDP significantly reduced frequency in medial (periventricular) and dorsal PH, but not DMH. Bilateral injections appeared to generally sum the usual effects of unilateral injection, producing effects of intermediate size. However, the absolute frequency change in any given site, or with any pair of sites, did not exceed 1 Hz, which is similar to what is seen with single injections in mSuM. Overall, it appears that at, any one time, theta frequency may be determined by a complex interplay between distinct but interacting modulatory regions in the medial hypothalamus.

Chemical coding of GABA(B) receptor-immunoreactive neurones in hypothalamic regions regulating body weight.

Gamma-aminobutyric acid (GABA) interacts with hypothalamic neuronal pathways regulating feeding behaviour. GABA has been reported to stimulate feeding via both ionotropic GABA(A) and metabotropic GABA(B) receptors. The functional form of the GABA(B) receptor is a heterodimer consisting of GABA(B) receptor-1 (GABA(B)R1) and GABA(B) receptor-2 (GABA(B)R2) proteins. Within the heterodimer, the GABA-binding site is localized to GABA(B)R1. In the present study, we used an antiserum to the GABA(B)R1 protein in order to investigate the cellular localization of GABA(B)R1-immunoreactive neurones in discrete hypothalamic regions implicated in the control of body weight. The colocalization of GABA(B)R1 immunoreactivity with different chemical messengers that regulate food intake was analysed. GABA(B)R1-immunoreactive cell bodies were found in the periventricular, paraventricular (PVN), supraoptic, arcuate, ventromedial hypothalamic, dorsomedial hypothalamic, tuberomammillary nuclei and lateral hypothalamic area (LHA). Direct double-labelling showed that glutamic acid decarboxylase (GAD)-positive terminals were in close contact with GABA(B)R1-containing cell bodies located in all these regions. In the ventromedial part of the arcuate nucleus, GABA(B)R1-immunoreactive cell bodies were found to contain neuropeptide Y, agouti-related peptide (AGRP) and GAD. In the ventrolateral part of the arcuate nucleus, GABA(B)R1-immunoreactive cell bodies were shown to contain pro-opiomelanocortin and cocaine- and amphetamine-regulated transcript. In the LHA, GABA(B)R1 immunoreactivity was present in both melanin-concentrating hormone- and orexin-containing cell populations. In the tuberomammillary nucleus, GABA(B)R1-immunoreactive cell bodies expressed histidine decarboxylase, a marker for histamine-containing neurones. In addition, GAD and AGRP were found to be colocalized in some nerve terminals surrounding GABA(B)R1-immunoreactive cell bodies in the parvocellular part of the PVN. The results may provide a morphological basis for the understanding of how GABA regulates the hypothalamic control of food intake and body weight via GABA(B) receptors.

Smelling of odorous sex hormone-like compounds causes sex-differentiated hypothalamic activations in humans.

The anatomical pathways for processing of odorous stimuli include the olfactory nerve projection to the olfactory bulb, the trigeminal nerve projection to somatosensory and insular cortex, and the projection from the accessory olfactory bulb to the hypothalamus. In the majority of tetrapods, the sex-specific effects of pheromones on reproductive behavior is mediated via the hypothalamic projection. However, the existence of this projection in humans has been regarded as improbable because humans lack a discernable accessory olfactory bulb. Here, we show that women smelling an androgen-like compound activate the hypothalamus, with the center of gravity in the preoptic and ventromedial nuclei. Men, in contrast, activate the hypothalamus (center of gravity in paraventricular and dorsomedial nuclei) when smelling an estrogen-like substance. This sex-dissociated hypothalamic activation suggests a potential physiological substrate for a sex-differentiated behavioral response in humans.

Lesions of the dorsomedial hypothalamic nucleus do not influence the daily profile of pineal metabolism in rats.

The present study attempted to characterize the effects of electrolytic lesions of the hypothalamic dorsomedial nucleus on the daily profile of pineal metabolism as well as on the inhibition of pineal melatonin synthesis induced by acute light exposure during the night. Adult male Wistar rats (n = 107, 12:12 h light-dark cycle) were left intact (n = 47) or lesioned (n = 60). Lesioned rats and their respective controls were killed at six time points distributed throughout the light-dark cycle. At ZT (zeitgeber time) 18 the animals were killed either in the dark or after 15 min of light stimulation. Pineal glands were assayed using high-performance liquid chromatography with electrochemical detection (HPLC-ED). There was no difference in the amounts of pineal indoles between lesioned and control rats under any of the experimental situations tested. These results suggest that in rats, the hypothalamic dorsomedial nucleus does not participate in either the neural control of daily pineal metabolism or the nocturnal light-induced inhibition of the pineal metabolism.

Cardiovascular responses to NMDA injected into nuclei of hypothalamus or amygdala in conscious rats.

The nuclei of hypothalamus and amygdala have been shown to be involved in the central cardiovascular homeostasis. Recent studies suggest that glutamate-containing neurons have an important role in the regulation of the central cardiovascular function. In this study, we demonstrate the roles of the central nucleus of the amygdala and the paraventricular nucleus of the amygdala and the paraventricular nucleus or the dorsomedial nucleus of the hypothalamus in N-methyl-D-aspartate (NMDA) induced blood pressure and heart rate changes in conscious Sprague-Dawley rats. Intracerebroventricular or parenchymal injections of NMDA evoke increases in arterial pressure. The NMDA-induced elevations in blood pressure are more prominent when NMDA is administered into the dorsomedial nucleus of the hypothalamus. Microinjections of NMDA into the dorsomedial hypothalamus exert significant heart rate increases, whereas NMDA when administered into the paraventricular nucleus of the hypothalamus or into the central nucleus of the amygdala has no significant effect on the heart rate. The dorsomedial nucleus of the hypothalamus is found to be the most effective site in this respect. The present study provides strong evidence for the tonic glutamatergic influence on blood pressure and heart rate via NMDA receptors located within the dorsomedial nucleus and to a lesser extent via those located within the paraventricular nucleus of the hypothalamus.

[Change in lipid peroxidation depending on hormonal reactions during lengthy electric stimulation of the rabbit hypothalamic dorsomedial nucleus]. / Izmenenie perekisnogo okisleniia lipidov v zavisimosti ot gormonal'nykh reaktsii pri dlitel'noi élektrostimuliatsii dorsomedial'nykh iader gipotalamusa krolikov.

A model of chronic emotional stress (ES) induced by electrostimulation of the hypothalamic dorsomedial nucleus in 69 rabbits was used to examine the relationship of blood hormonal changes and lipid peroxidation (LPO) activity in blood and myocardial cells. The elevated concentrations of stress hormones at the initial stages of chronic ES (the first 2 series) caused LPO with high activities of antioxidative enzymes (AOEs). The subsequent stages of chronic ES (from 60 to 120 days) were associated with the decreased role of major stress hormones (adrenocorticotropic hormone, cortisol, catecholamines) and the increased significance of parathyroid hormone and active renin (angiotensin I). A significant direct correlation was found between the blood level of these hormones and the rate of LPO in the myocardium and blood. At the same time the activity of AOEs progressively decreased and all rabbits exhibited myocardial cell necrotic foci.

[Effect of electroacupuncture on the discharges of pain-sensitive neurons in the hypothalamic dorsomedial nucleus of rats].

The discharges of pain-sensitive neurons of the hypothalamic dorsomedial nucleus (DMH) in Sprague-Dawley rats were recorded with glass microelectrodes, After electroacupuncture acupoints "Zusanli" and "Sanyinjiao", the rate of spontaneous discharges and the rate and duration of the pain-evoked discharges of pain-excitatory units of DMH were profoundly decreased, and the spontaneous firing rate of pain-inhibitory units was increased while their inhibitory response to nociceptive stimulation was released by the electroacupuncture. The results mentioned above suggest that DMH participates in the activity of acupuncture analgesia.

Cardiovascular effects and changes in midbrain periaqueductal gray neuronal activity induced by electrical stimulation of the hypothalamus in the rat.

The effects of low-intensity electrical stimulation of sites in the hypothalamus and zona incerta (ZI) on mean blood pressure (MBP), heart rate (HR), and neuronal activity in the midbrain periaqueductal gray (PAG) were investigated in rats. Long-lasting depressor responses were elicited from 67 sites in the hypothalamus and ZI. Effects on HR were variable, except for the ZI where bradycardiac responses were evoked. The amplitude of the depressor responses was significantly correlated with baseline MBP on stimulation of the dorsomedial hypothalamic nucleus (DMH) or the dorsal hypothalamic area. Extracellular single-unit recordings were made from 94 PAG neurons. Most units were located in the ventral half of the PAG (62/94), where spontaneous firing rates were significantly higher than in the dorsal half: 12.5 +/- 1.4 spikes/s as compared to 6.0 +/- 0.9 spikes/s. Changes in PAG neuronal activity to both ipsi- and contralateral hypothalamic stimulation were observed. Most neurons were inhibited or unresponsive. There was no site specificity: responsive and unresponsive neurons were scattered throughout the PAG. Inhibition was most effectively evoked by stimulation of the DMH (25 out of 39 neurons) and the ZI (9 out of 15 neurons). In most neurons, the inhibition strictly followed the time course of hypothalamic stimulation. The results suggest that PAG as well as nonPAG pathways participate in the hypothalamic control of cardiovascular function.

Alterations in behavioral responses to stressors following excitotoxin lesions of dorsomedial hypothalamic regions.

The dorsomedial hypothalamus is important for regulation of cardiovascular responses associated with emotional arousal. This region has also been identified as a component of neural circuitry involved in fear/anxiety, yet clear evidence as to the effects of lesioning on stress-related behaviors is missing. In this study, we lesioned the dorsomedial hypothalamic region with the neurotoxin, ibotenic acid (IBO; 2.0 micrograms in 0.2 microliter), and studied the impact on spontaneous and unlearned behavioral responses to stressors. In the open field test, we observed non-generalized increases in motility parameters in the IBO rats with the differences occurring in the latter two-thirds of the test. In the elevated plus-maze, the IBO rats displayed a classic anxiolytic response with a greater proportion of entries into (and greater time spent in) the open arms of the maze. In the environment-specific social interaction (SI) test, the IBO rats showed a normal familiar/unfamiliar environment discrimination with respect to Total SI; however, the composition of the behaviors ('curiosity' vs. physical contact) by the IBO rats was markedly altered, with there being a 2-fold increase in non-violent physical interactions. Additionally, the differences in these traditional indices of anxiety were associated with lesioned animals exhibiting greater acoustic startle responsiveness than controls as a function of prepulse intensity. Overall, the results following IBO lesions indicate an altered responsiveness to sudden stressors, particularly as relates to novelty or exploration-oriented behaviors. The hypothalamic lesion may, therefore, have resulted in a disinhibition of normally suppressed responding to innate fear or challenging stimuli. This study contributes to those that have begun to define neural interactions that are essential for integrated stress responses.