Role of lateral preoptic area alpha-1 and alpha-2 adrenoceptors in sleep-wakefulness and body temperature regulation

Sleep loss disrupts decision-making ability and neuronal cytomorphology in zebrafish and the effects are mediated by noradrenaline acting on α1-adrenoceptor

Sleep is an instinct behavior, and its significance and functions are still an enigma. It is expressed throughout one's life and its loss affects psycho-somatic and physiological processes. We had proposed that it might maintain a fundamental property of the neurons and the brain. In that context, it was shown that sleep, rapid eye movement sleep (REMS) in particular, by regulating noradrenaline (NA), maintains the brain excitability. It was also reported that sleep-loss affected memory, reaction time and decision-making ability among others. However, as there was lack of clarity on the cause-and-effect relationship as to how the sleep-loss could affect these basic behaviors, their association was questioned and it was difficult to propose a cure or at least ways and means to ameliorate the symptoms. Also, we wanted to conduct the studies in a simpler model system so that conducting future molecular studies might be easier. Hence, using zebrafish as a model we evaluated if sleep-loss affected the basic decision-making ability, a cognitive process and if the effect was induced by NA. Indeed, our findings confirmed that upon sleep-deprivation, the cognitive decision-making ability of the prey zebrafish was compromised to protect itself by running away from the reach of the exposed predator Tiger Oscar (TO) fish. Also, we observed that upon sleep-loss the axonal arborization of the prey zebrafish brain was reduced. Interestingly, the effects were prevented by prazosin (PRZ), an α1-adrenoceptor (AR) antagonist and when the zebrafish recovered from the lost sleep.

Melatonin MT1 and MT2 Receptors Exhibit Distinct Effects in the Modulation of Body Temperature across the Light/Dark Cycle

Melatonin (MLT) is a neurohormone that regulates many physiological functions including sleep, pain, thermoregulation, and circadian rhythms. MLT acts mainly through two G-protein-coupled receptors named MT₁ and MT₂, but also through an MLT type-3 receptor (MT₃). However, the role of MLT receptor subtypes in thermoregulation is still unknown. We have thus investigated the effects of selective and non-selective MLT receptor agonists/antagonists on body temperature (T_b) in rats across the 12/12-h light-dark cycle. Rectal temperature was measured every 15 min from 4:00 a.m. to 9:30 a.m. and from 4:00 p.m. to 9:30 p.m., following subcutaneous injection of each compound at either 5:00 a.m. or 5:00 p.m. MLT (40 mg/kg) had no effect when injected at 5 a.m., whereas it decreased T_b during the light phase only when injected at 5:00 p.m. This effect was blocked by the selective MT₂ receptor antagonist 4P-PDOT and the non-selective MT₁/MT₂ receptor antagonist, luzindole, but not by the α₁/MT₃ receptors antagonist prazosin. However, unlike MLT, neither the selective MT₁ receptor partial agonist UCM871 (14 mg/kg) nor the selective MT₂ partial agonist UCM924 (40 mg/kg) altered T_b during the light phase. In contrast, UCM871 injected at 5:00 p.m. increased T_b at the beginning of the dark phase, whereas UCM924 injected at 5:00 a.m. decreased T_b at the end of the dark phase. These effects were blocked by luzindole and 4P-PDOT, respectively. The MT₃ receptor agonist GR135531 (10 mg/kg) did not affect T_b. These data suggest that the simultaneous activation of both MT₁ and MT₂ receptors is necessary to regulate T_b during the light phase, whereas in a complex but yet unknown manner, they regulate T_b differently during the dark phase. Overall, MT₁ and MT₂ receptors display complementary but also distinct roles in modulating circadian fluctuations of T_b.

Sleep and Sedative States Induced by Targeting the Histamine and Noradrenergic Systems

Sedatives target just a handful of receptors and ion channels. But we have no satisfying explanation for how activating these receptors produces sedation. In particular, do sedatives act at restricted brain locations and circuitries or more widely? Two prominent sedative drugs in clinical use are zolpidem, a GABA_A receptor positive allosteric modulator, and dexmedetomidine (DEX), a selective α2 adrenergic receptor agonist. By targeting hypothalamic neuromodulatory systems both drugs induce a sleep-like state, but in different ways: zolpidem primarily reduces the latency to NREM sleep, and is a controlled substance taken by many people to help them sleep; DEX produces prominent slow wave activity in the electroencephalogram (EEG) resembling stage 2 NREM sleep, but with complications of hypothermia and lowered blood pressure—it is used for long term sedation in hospital intensive care units—under DEX-induced sedation patients are arousable and responsive, and this drug reduces the risk of delirium. DEX, and another α2 adrenergic agonist xylazine, are also widely used in veterinary clinics to sedate animals. Here we review how these two different classes of sedatives, zolpidem and dexmedetomideine, can selectively interact with some nodal points of the circuitry that promote wakefulness allowing the transition to NREM sleep. Zolpidem enhances GABAergic transmission onto histamine neurons in the hypothalamic tuberomammillary nucleus (TMN) to hasten the transition to NREM sleep, and DEX interacts with neurons in the preoptic hypothalamic area that induce sleep and body cooling. This knowledge may aid the design of more precise acting sedatives, and at the same time, reveal more about the natural sleep-wake circuitry.

Involvement of the α1-adrenoceptor in sleep-waking and sleep loss-induced anxiety behavior in zebrafish

Sleep is a universal phenomenon in vertebrates, and its loss affects various behaviors. Independent studies have reported that sleep loss increases anxiety; however, the detailed mechanism is unknown. Because sleep deprivation increases noradrenalin (NA), which modulates many behaviors and induces patho-physiological changes, this study utilized zebrafish as a model to investigate whether sleep loss-induced increased anxiety is modulated by NA. Continuous behavioral quiescence for at least 6s was considered to represent sleep in zebrafish; although some authors termed it as a sleep-like state, in this study we have termed it as sleep. The activity of fish that signified sleep-waking was recorded in light-dark, during continuous dark and light; the latter induced sleep loss in fish. The latency, number of entries, time spent and distance travelled in the light chamber were assessed in a light-dark box test to estimate the anxiety behavior of normal, sleep-deprived and prazosin (PRZ)-treated fish. Zebrafish showed increased waking during light and complete loss of sleep upon continuous exposure to light for 24h. PRZ significantly increased sleep in normal fish. Sleep-deprived fish showed an increased preference for dark (expression of increased anxiety), and this effect was prevented by PRZ, which increased sleep as well. Our findings suggest that sleep loss-induced anxiety-like behavior in zebrafish is likely to be mediated by NA's action on the α1-adrenoceptor.

Sleep duration varies as a function of glutamate and GABA in rat pontine reticular formation

The oral part of the pontine reticular formation (PnO) is a component of the ascending reticular activating system and plays a role in the regulation of sleep and wakefulness. The PnO receives glutamatergic and GABAergic projections from many brain regions that regulate behavioral state. Indirect, pharmacological evidence has suggested that glutamatergic and GABAergic signaling within the PnO alters traits that characterize wakefulness and sleep. No previous studies have simultaneously measured endogenous glutamate and GABA from rat PnO in relation to sleep and wakefulness. The present study utilized in vivo microdialysis coupled on-line to capillary electrophoresis with laser-induced fluorescence to test the hypothesis that concentrations of glutamate and GABA in the PnO vary across the sleep/wake cycle. Concentrations of glutamate and GABA were significantly higher during wakefulness than during non-rapid eye movement sleep and rapid eye movement sleep. Regression analysis revealed that decreases in glutamate and GABA accounted for a significant portion of the variance in the duration of non-rapid eye movement sleep and rapid eye movement sleep episodes. These data provide novel support for the hypothesis that endogenous glutamate and GABA in the PnO contribute to the regulation of sleep duration.

Influence of alpha-adrenoceptor agonists and antagonists on imipramine-induced hypothermia in mice

Effects of adrenoceptor agonists and antagonists on imipramine-induced hypothermia in mice were studied. Intraperitoneal injection of imipramine (10-40 mg x kg(-1)), alpha2-adrenoceptor agonist clonidine (0.05-0.1 mg x kg(-1)), alpha1-adrenoceptor agonist phenylephrine (6 mg x kg(-1)) and alpha1-adrenoceptor antagonist prazosin (1-4 mg x kg(-1)) but not alpha2-adrenoceptor antagonist yohimbine (1-4 mg x kg(-1)) induced significant hypothermia. The hypothermic response induced by imipramine (10-30 mg x kg(-1)) was not altered by clonidine (0.05-0.1 mg x kg(-1)) or phenylephrine (2-6 mg x kg(-1)). The response of imipramine (10-30 mg x kg(-1)) was reduced significantly by yohimbine (2 mg x kg(-1)) and was potentiated by prazosin (1 mg x kg(-1)). The hypothermic effect of clonidine (0.1 mg x kg(-1)) and imipramine (20 mg x kg(-1)) were also decreased significantly by different doses of yohimbine (1-4 mg x kg(-1)). The hypothermia induced by different doses of prazosin (1-4 mg x kg(-1)) was not altered by yohimbine (2 mg x kg(-1)) or by low dose of imipramine (10 mg x kg(-1)). It is concluded that alpha2-adrenoceptor mechanism may be involved in the hypothermic effect of imipramine.

GABA exerts opposite influence on warm and cold sensitive neurons in medial preoptic area in rats

The preoptic area regulates body temperature. GABA-ergic terminals and receptors are present in this area. Local microinjection studies have shown that GABA, its agonist, and its antagonist in this area may modulate body temperature. However, there are warm and cold sensitive neurons, and they are known to be affected by local and peripheral temperatures. In order to understand the mechanism of action of GABA in temperature regulation at the cellular level it was necessary to study the effect of GABA on individual thermosensitive neurons in in vivo preparations. Hence, in this study the responses of preoptic area thermosensitive and insensitive neurons to microiontophoretic application of picrotoxin, a GABA-A antagonist, were studied in anaesthetized rats. It was observed that a majority of both the thermosensitive and insensitive neurons were affected by microiontophoretic application of picrotoxin. Although almost an equal number of cold and warm sensitive neurons were affected, a majority of the cold sensitive neurons were excited, while a majority of the warm sensitive neurons were inhibited by picrotoxin. The results suggested that in normal conditions GABA acts through GABA-A receptor in modulating the spontaneous activity of thermosensitive neurons in the preoptic area. Furthermore, the results of the present study taken together with other reports suggest that normally GABA exerts a direct inhibitory action on the cold sensitive neurons, while it acts on presynaptic heteroreceptors, possibly on norepinephrinergic afferent input terminals on the warm sensitive neurons, for mediating its action.

Baclofen-impairment of memory retention in rats: possible interaction with adrenoceptor mechanism(s)

This study concerned the influence of adrenoceptor agonists and antagonists on baclofen-induced impairment of memory retention. Intracerebroventricular injection of baclofen (0.25--2 microg/rat) reduced memory retention in rats. The combination of different doses of baclofen with a low dose of clonidine (0.5 microg/rat) elicited a greater decrease in memory retention. Yohimbine (1 microg/rat) potentiated the response to a low dose, but decreased the response to higher doses of baclofen. Single administration of clonidine (0.5--2 microg/rat) but not yohimbine (1--4 microg/rat) itself decreased memory retention. The combination of clonidine with yohimbine did not show any interaction. The low dose of phenylephrine (0.5 microg/rat) or prazosin (0.5 microg/rat) also potentiated the inhibition of memory retention by baclofen. Phenylephrine (0.5--3 microg/rat) increased, while prazosin (0.5--2 microg/rat) decreased memory retention. The combination of the two drugs showed an interaction. It may be concluded that an adrenoceptor mechanism may interact with the memory retention impairment induced by baclofen.

Heterogeneous effects of rapid eye movement sleep deprivation on binding to alpha- and beta-adrenergic receptor subtypes in rat brain

Quantitative receptor autoradiography was used to map alterations in binding to alpha1-, alpha2-, beta1- and beta2-adrenergic receptors throughout the brain of rats deprived of rapid eye movement sleep for 96 h. Binding of [3H]prazosin to alpha1 sites, while not significantly different in any of 46 brain regions examined, showed a clear overall tendency towards decreased values after sleep deprivation. [3H]UK-14,314-labeled alpha2 binding sites were not significantly affected by sleep deprivation in any of 91 brain regions analysed, despite a trend towards increased values. In contrast, beta-adrenergic binding was significantly reduced throughout the brain. Binding to beta1 sites labeled by [125I]iodopindolol in the presence of ICI-11855 was significantly reduced in 13 of 69 brain areas examined; binding to beta2 sites labeled by [125I]iodopindolol in the presence of CGP-20712A was likewise reduced throughout the brain and significantly so in 25 of the 72 brain areas analysed. Rank ordering of the binding changes indicated that reductions in beta1 vs beta2 binding were maximal in different brain areas. This pattern of results may reflect a particular configuration of effects specifically associated with sleep loss stress. The results are consistent with evidence of persisting noradrenergic cell activity during sleep deprivation. The observed heterogeneity of effects suggests that not all norepinephrine receptors are equally affected by rapid eye movement sleep deprivation.

Distribution of alpha 1a-, alpha 1b- and alpha 1d-adrenergic receptor mRNA in the rat brain and spinal cord

The technique of in situ hybridization with specific ribonucleotide probes was used to determine the distribution patterns of mRNA encoding the alpha 1a-, alpha 1b- and alpha 1d-adrenoceptor (AR) subtypes in rat brain and spinal cord. The expression pattern of alpha 1a-AR mRNA has not been reported previously, and was found to be widespread throughout the rat central nervous system. High levels were found in regions of the olfactory system, several hypothalamic nuclei, and regions of the brainstem and spinal cord, particularly in areas related to motor function. Regions expressing moderate levels of mRNA for this receptor were the septum, bed nucleus of the stria terminalis, cerebral cortex, amygdala, cerebellum and pineal gland. Low expression levels were detected in the hippocampal formation. Most nuclei in the basal ganglia and thalamus expressed extremely low or undetectable levels of alpha 1a-AR mRNA. The expression patterns of the alpha 1b- and alpha 1d-AR mRNAs were similar to those described using oligonucleotide probes in earlier studies. High expression of alpha 1b-AR mRNA was noted in the pineal gland, most thalamic nuclei, lateral nucleus of the amygdala and dorsal and median raphe nuclei. Moderate expression levels were noted throughout the cerebral cortex, and in some olfactory, septal, and brainstem regions. The distribution of alpha 1d-AR mRNA was the most discrete of the three receptors examined. Expression was strong in the olfactory bulb, cerebral cortex, hippocampus, reticular thalamic nucleus, regions of the amygdala, motor nuclei of the brainstem, inferior olivary complex and spinal cord. Comparison of the distributions of the alpha 1a-, alpha 1b- and alpha 1d-AR mRNA suggests unique functional roles for each of these receptors.