State-dependent changes of brain endogenous opioids in mammalian hibernation

Peptidomic Analysis Reveals Seasonal Neuropeptide and Peptide Hormone Changes in the Hypothalamus and Pituitary of a Hibernating Mammal

During the winter, hibernating mammals undergo extreme changes in physiology, which allow them to survive several months without access to food. These animals enter a state of torpor, which is characterized by decreased metabolism, near-freezing body temperatures, and a dramatically reduced heart rate. The neurochemical basis of this regulation is largely unknown. Based on prior evidence suggesting that the peptide-rich hypothalamus plays critical roles in hibernation, we hypothesized that changes in specific cell-cell signaling peptides (neuropeptides and peptide hormones) underlie physiological changes during torpor/arousal cycles. To test this hypothesis, we used a mass spectrometry-based peptidomics approach to examine seasonal changes of endogenous peptides that occur in the hypothalamus and pituitary of a model hibernating mammal, the thirteen-lined ground squirrel (Ictidomys tridecemlineatus). In the pituitary, we observed changes in several distinct peptide hormones as animals prepared for torpor in October, exited torpor in March, and progressed from spring (March) to fall (August). In the hypothalamus, we observed an overall increase in neuropeptides in October (pre-torpor), a decrease as the animal entered torpor, and an increase in a subset of neuropeptides during normothermic interbout arousals. Notable changes were observed for feeding regulatory peptides, opioid peptides, and several peptides without well-established functions. Overall, our study provides critical insight into changes in endogenous peptides in the hypothalamus and pituitary during mammalian hibernation that were not available from transcriptomic measurements. Understanding the molecular basis of the hibernation phenotype may pave the way for future efforts to employ hibernation-like strategies for organ preservation, combating obesity, and treatment of stroke.

Towards understanding the neural origins of hibernation

Hibernators thrive under harsh environmental conditions instead of initiating canonical behavioral and physiological responses to promote survival. Although the physiological changes that occur during hibernation have been comprehensively researched, the role of the nervous system in this process remains relatively underexplored. In this Review, we adopt the perspective that the nervous system plays an active, essential role in facilitating and supporting hibernation. Accumulating evidence strongly suggests that the hypothalamus enters a quiescent state in which powerful drives to thermoregulate, eat and drink are suppressed. Similarly, cardiovascular and pulmonary reflexes originating in the brainstem are altered to permit the profoundly slow heart and breathing rates observed during torpor. The mechanisms underlying these changes to the hypothalamus and brainstem are not currently known, but several neuromodulatory systems have been implicated in the induction and maintenance of hibernation. The intersection of these findings with modern neuroscience approaches, such as optogenetics and in vivo calcium imaging, has opened several exciting avenues for hibernation research.

Delta Opioids: Neuroprotective Roles in Preclinical Studies

Since ancient times, opioids have been used clinically and abused recreationally. In the early stages (about 1,000 AD) of opium history, an Arab physician, Avicenna, administered opioids to control diarrhea and eye diseases. ¹ Opioids have very strong pain relieving properties and they also regulate numerous cellular responses. Opioid receptors are expressed throughout the body, including the nervous system, heart, lungs, liver, gastrointestinal tract, and retina. ^2-6 Delta opioid receptors (DORs) are a very attractive target from the perspective of both receptor function and their therapeutic potential. Due to a rapid progress in mouse mutagenesis and development of small molecules as DOR agonist, novel functions and roles of DORs have emerged in recent years. This review article focuses on the recent advances in the neuroprotective roles of DOR agonists in general and retina neuroprotection in particular. Rather than being exhaustive, this review highlights the selected studies of DOR function in neuroprotection. We also highlight our preclinical studies using rodent models to demonstrate the potentials of DOR agonists for retinal neuroprotection. Based on existing literature and our recently published data on the eye, DOR agonists possess therapeutic abilities that protect the retina and optic nerve injury against glaucoma and perhaps other retinopathies as well. This review also highlights the signaling events associated with DOR for neuroprotection in the eye. There is a need for translational research on DORs to recognize their potential for clinical application such as in glaucoma.

Current research on opioid receptor function

The use of opioid analgesics has a long history in clinical settings, although the comprehensive action of opioid receptors is still less understood. Nonetheless, recent studies have generated fresh insights into opioid receptor-mediated functions and their underlying mechanisms. Three major opioid receptors (μ-opioid receptor, MOR; δ-opioid receptor, DOR; and κ-opioid receptor, KOR) have been cloned in many species. Each opioid receptor is functionally sub-classified into several pharmacological subtypes, although, specific gene corresponding each of these receptor subtypes is still unidentified as only a single gene has been isolated for each opioid receptor. In addition to pain modulation and addiction, opioid receptors are widely involved in various physiological and pathophysiological activities, including the regulation of membrane ionic homeostasis, cell proliferation, emotional response, epileptic seizures, immune function, feeding, obesity, respiratory and cardiovascular control as well as some neurodegenerative disorders. In some species, they play an essential role in hibernation. One of the most exciting findings of the past decade is the opioid-receptor, especially DOR, mediated neuroprotection and cardioprotection. The upregulation of DOR expression and DOR activation increase the neuronal tolerance to hypoxic/ischemic stress. The DOR signal triggers (depending on stress duration and severity) different mechanisms at multiple levels to preserve neuronal survival, including the stabilization of homeostasis and increased pro-survival signaling (e.g., PKC-ERK-Bcl 2) and antioxidative capacity. In the heart, PKC and KATP channels are involved in the opioid receptor-mediated cardioprotection. The DOR-mediated neuroprotection and cardioprotection have the potential to significantly alter the clinical pharmacology in terms of prevention and treatment of life-threatening conditions like stroke and myocardial infarction. The main purpose of this article is to review the recent work done on opioids and their receptor functions. It shall provide an informative reference for better understanding the opioid system and further elucidation of the opioid receptor function from a physiological and pharmacological point of view.

Mass spectrometric analysis of activity-dependent changes of neuropeptide profile in the snail, Helix pomatia

Terrestrial snails are able to transform themselves into inactivity ceasing their behavioral activity under unfavorable environmental conditions. In the present study, we report on the activity-dependent changes of the peptide and/or polypeptide profile in the brain and hemolymph of the snail, Helix pomatia, using MALDI TOF and quadrupole mass spectrometry. The present data indicate that the snails respond to low temperature by increasing or decreasing the output of selected peptides. Average mass spectra of the brain and hemolymph revealed numerous peaks predominantly present during the active state (19 and 10 peptides/polypeptides, respectively), while others were observed only during hibernation (11 and 13). However, there were peptides and/or polypeptides or their fragments present irrespective of the activity states (49 and 18). The intensity of fourteen peaks that correspond to previously identified neuropeptides varied in the brain of active snails compared to those of hibernating animals. Among those the intensity of eight peptides increased significantly in active animals while in hibernated animals the intensity of another six peptides increased significantly. A new peptide or peptide fragment at m/z 1110.7 was identified in a brain of the snail with the following suggested amino acid sequence: GSGASGSMPATTS. This peptide was found to be more abundant in active animals because the intensity of the peptide was significantly higher compared to hibernating animals. In summary, our results revealed substantial differences in the peptide/polypeptide profile of the brain and hemolymph of active and hibernating snails suggesting a possible contribution of peptides in the process of hibernation.

Identification of qRT-PCR reference genes for analysis of opioid gene expression in a hibernator

Previous work has suggested that central and peripheral opioid signaling are involved in regulating torpor behavior and tissue protection associated with the hibernation phenotype. We used quantitative real-time PCR (qRT-PCR) to measure mRNA levels of opioid peptide precursors and receptors in the brain and heart of summer ground squirrels (Ictidomys tridecemlineatus) and winter hibernating squirrels in the torpid or interbout arousal states. The use of appropriate reference genes for normalization of qRT-PCR gene expression data can have profound effects on the analysis and interpretation of results. This may be particularly important when experimental subjects, such as hibernating animals, undergo significant morphological and/or functional changes during the study. Therefore, an additional goal of this study was to identify stable reference genes for use in qRT-PCR studies of the 13-lined ground squirrel. Expression levels of 10 potential reference genes were measured in the small intestine, liver, brain, and heart, and the optimal combinations of the most stable reference genes were identified by the GeNorm Excel applet. Based on this analysis, we provide recommendations for reference genes to use in each tissue that would be suitable for comparative studies among different activity states. When appropriate normalization of mRNA levels was used, there were no changes in opioid-related genes in heart among the three activity states; in brain, DOR expression was highest during torpor, lowest in interbout arousal and intermediate in summer. The results support the idea that changes in DOR expression may regulate the level of neuronal activity in brain during the annual hibernation cycle and may contribute to hibernation-associated tissue protection.

Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature

Mammalian hibernators undergo a remarkable phenotypic switch that involves profound changes in physiology, morphology, and behavior in response to periods of unfavorable environmental conditions. The ability to hibernate is found throughout the class Mammalia and appears to involve differential expression of genes common to all mammals, rather than the induction of novel gene products unique to the hibernating state. The hibernation season is characterized by extended bouts of torpor, during which minimal body temperature (Tb) can fall as low as -2.9 degrees C and metabolism can be reduced to 1% of euthermic rates. Many global biochemical and physiological processes exploit low temperatures to lower reaction rates but retain the ability to resume full activity upon rewarming. Other critical functions must continue at physiologically relevant levels during torpor and be precisely regulated even at Tb values near 0 degrees C. Research using new tools of molecular and cellular biology is beginning to reveal how hibernators survive repeated cycles of torpor and arousal during the hibernation season. Comprehensive approaches that exploit advances in genomic and proteomic technologies are needed to further define the differentially expressed genes that distinguish the summer euthermic from winter hibernating states. Detailed understanding of hibernation from the molecular to organismal levels should enable the translation of this information to the development of a variety of hypothermic and hypometabolic strategies to improve outcomes for human and animal health.

Structure and function of mitochondria in hepatopancreas cells from metabolically depressed snails

Mitochondria in cells isolated from the hepatopancreas of aestivating land snails (Helix aspersa) consume oxygen at 30% of the active control rate. The aim of this study was to investigate whether the lower respiration rate is caused by a decrease in the density of mitochondria or by intrinsic changes in the mitochondria. Mitochondria occupied 2% of cellular volume, and the mitochondrial inner membrane surface density was 17 microm(-1), in cells from active snails. These values were not different in cells from aestivating snails. The mitochondrial protein and mitochondrial phospholipid contents of cells were also similar. There was little difference in the phospholipid fatty acyl composition of mitochondria isolated from metabolically depressed or active snails, except for arachidonic acid, which was 18% higher in mitochondria from aestivating snails. However, the activities of citrate synthase and cytochrome c oxidase in mitochondria isolated from aestivating snails were 68% and 63% of control, respectively. Thus the lower mitochondrial respiration rate in hepatopancreas cells from aestivating snails was not caused by differences in mitochondrial volume or surface density but was associated with intrinsic changes in the mitochondria.

Neuronal activity in the mediobasal hypothalamus of hibernating jerboas (Jaculus orientalis)

The expression of the c-fos protein in the mediobasal hypothalamus (MBH) of the jerboa was examined both during hibernation and on arousal from hibernation. Expression was examined by c-fos immunohistochemistry using a polyclonal antibody raised against c-fos protein. In jerboas hibernating for 2 days, a significant number of c-fos immunopositive neurons were found in the median eminence and ventrolateral arcuate nucleus. Such an immunoreaction was not observed in non hibernating control animals. In animals hibernating for 10 days, c-fos immunoreactivity was localized in the lateral arcuate nucleus and ventromedial hypothalamus (VMH), the median eminence displayed no immunoreaction. After arousal from hibernation, the fos immunoexpression within the MBH was exclusively limited to the VMH nucleus. Thus, the present study shows different patterns of c-fos protein expression during hibernation, notably in the neuroendocrine systems of MBH. Consequently, the mediobasal hypothalamus seems to be implicated in a physiological regulation during hibernation. Moreover, the present data suggest that the c-fos gene is either implicated in mediating or is a consequence of physiological activation of specific neuron systems of the desert jerboa.

Diurnal variations of opioid peptides and synenkephalin in vitro release in the amygdala of kindled rats

Pentylenetetrazol (PTZ) kindling was induced in male Wistar rats (250-300 g) by daily intraperitoneal injections of 35 mg/kg of the convulsant agent. Immunoreactive (IR)-Met-enkephalin (IR-ME), IR-Leu-enkephalin (IR-LE), IR-heptapeptide (IR-HE), IR-octapeptide (IR-OC) and IR-synenkephalin (IR-Syn) in vitro release was measured from amygdala slices 24 h after the last stimulus, in groups of eight rats, every 4 h beginning at 08:00 h. Opioid peptides in vitro release displayed diurnal variations. IR-ME and IR-Syn showed maximal levels before the onset of darkness (16:00 h). IR-LE and IR-OC release was enhanced 4 h later (20:00 h), no changes were detected for IR-HE. These results show that endogenous opioid system (EOS) release displays diurnal variations. The peak for the analysed peptides was reached before and during the dark phase. It is suggested that EOS release enhancement in PTZ-kindled rats, seems to be due to a compensatory mechanism against the excitation induced by the blockade of the GABAergic transmission.

Hibernation in ground squirrels induces state and species-specific tolerance to hypoxia and aglycemia: an in vitro study in hippocampal slices

Hibernation in mammals is associated with a regulated depression of global cellular functions accompanied by reductions of cerebral blood flow that would render the brain profoundly ischemic under normal conditions. Homeostatic control is preserved, however, and brain damage does not occur. We investigated the possibility that hibernation not only confers tolerance to profound hypothermia, but also to hypoxia and aglycemia independent of temperature. Hippocampal slices from ground squirrels Citellus tridecemlineatus in both the active and hibernating states and from rats were subjected to in vitro hypoxia and aglycemia at incubation temperatures of 36 degrees C, 20 degrees C, and 7 degrees C and evaluated histologically. A binary bioassay was used to determine the duration of hypoxia/aglycemia tolerated in each group. At all temperatures, slices from hibernating animals were most tolerant compared with both active squirrels and rats. Slices from active ground squirrels were more tolerant than rat at 20 degrees C and 7 degrees C but not at 36 degrees C indicating a species-specific difference that becomes manifest at lower temperatures. These results indicate that hibernation is associated not only with tolerance to profound hypothermia but also to deprivation of oxygen and glucose. Because tolerance was already demonstrable at the shortest duration of hibernation studied, rapid therapeutic induction of a similar state may be possible. Therefore, identification of the regulatory mechanisms underlying this tolerance may lead to novel neuroprotective strategies.

Hibernation triggers and cryogens: do they play a role in hibernation?

A survey of the literary evidence on cryogens and hibernation induction triggers is given and the results of experiments on the effect of hypothalamic or i.v. injections of opioids and plasma from hibernating European hamsters on body temperature control of rabbits are presented. Pharmacological doses of a delta opioid--DADLE (25 or 50 micrograms), when injected into the anterior hypothalamus, induce a small and short-lasting hypothermic effect in cold exposed rabbits, due to the downward shift of the temperature threshold for shivering. Lower doses (5 micrograms) are without effect, similarly as i.v. administrations (500 micrograms/kg) of this substance. Intrahypothalamic injections of met-enkephalin (0.1-1 microgram) induce a slight hyperthermia due to the shift of all thermoregulatory effectors to higher body temperatures. Intrahypothalamic injections of plasma from hibernating European hamsters do not influence the body temperature control in rabbits.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.