Effect of ambient temperature on brain temperature and sleep-wakefulness in medial preoptic area lesioned rats

The changes in brain temperature and sleep-wakefulness were studied in rats during their exposure to different ambient temperatures of 18 degrees C, 24 degrees C and 30 degrees C, before and after N-methyl D-aspartic acid lesion of the medial preoptic area. The medial preoptic area lesion produced a decrease in sleep, and increase in brain temperature except at 30 degrees C. Increase and decrease in brain temperature with slow wave sleep and paradoxical sleep respectively, were observed both in normal and lesioned rats. Sleep-wakefulness and brain temperature cycle durations were increased and their frequencies decreased at higher ambient temperature in normal rats. After the medial preoptic area lesion, sleep-wakefulness cycle duration was decreased and frequency increased at 30 degrees C. There was no significant change in brain temperature cycles at higher ambient temperature in lesioned rats. The medial preoptic area, in normal rats, possibly interlinks the neuronal circuits involved in regulating brain temperature and sleep-wakefulness cycles. The medial preoptic area is essential for increasing the sleep-wakefulness cycle duration with higher ambient temperature. The possible contribution of the increased brain temperature variation in producing sleep-wakefulness changes cannot be ruled out. The results of the study show that this area may serve as a fine tuning mechanism which helps to interlink the sleep-wakefulness with the thermoregulation.     

Role of adenosine in sleep and temperature regulation in the preoptic area of rats.

We have examined the effects on sleep and brain temperature of bilateral microinjections of adenosine and adenosine analogs to the preoptic area (PO) of rats. Administration of adenosine (12.5 nmoles), a nonselective adenosine A1/A2 receptor agonist NECA (N-ethyl-carboxamido-adenosine, 1.0 nmole), and the selective adenosine A1 receptor agonist CPA (cyclopentyladenosine, 0.25, 0.5 nmoles) increased total sleep primarily through an enhancement in deep slow-wave sleep (SWS2), while adenosine also increased REM sleep. Administration of 12.5 nmoles adenosine and 0.25 nmoles CPA did not affect brain temperature, while 1.0 nmole NECA and 0.5 nmoles CPA caused a transient and prolonged hypothermia, respectively. Administration of the selective adenosine A2 receptor agonist CV-1808 (2-phenylaminoadenosine, 5, 10 nmoles) had no effect on sleep or brain temperature. The present results demonstrate a site for the central hypnotic action of adenosine, and a functional role for adenosine A1 receptors in the hypothalamus.     

Why the medial preoptic area is important for sleep regulation

The medial preoptic area (mPOA) is one of the many areas in the brain that control sleep. Apart from sleep, the mPOA is important for the regulation of body temperature, and other important body functions aimed at energy homeostasis. In sleep regulation, the major function of this area is to maintain sleep. Though the mPOA controls sleep and body temperature through independent neuronal circuits, it is essential for organising the sleep architecture, as per the thermoregulatory requirement. The functional integrity of the mPOA may be essential for the regulation of energy homeostasis, in response to alterations in the ambient temperature, heat producing physical activity and sleep-wakefulness. Thus, the mPOA forms part of the brain that integrates regulations aimed at preservation of self. The mPOA is important for maintaining the "set point" for not only body temperature, but it is also important for maintaining the "set point" for several physiological parameters including sleep-wakefulness.     

Enhancement of sleep by microinjection of triazolam into the medial preoptic area

A growing literature suggests that the basal forebrain may contain structures involved in the regulation of sleep. As part of a series of studies designed to locate the site(s) of hypnotic action of benzodiazepines, we have injected triazolam into the medial preoptic area (MPA) of the hypothalamus of rats. Total sleep time was increased, due primarily to an increase in non-rapid eye movement (non-REM) sleep and a trend toward a decrease in intermittent waking time. A previous study from this laboratory reported that injections into the raphe nucleus decreased sleep, whereas injections at adjacent sites were without effect. These studies indicate the selectivity with which different brain regions respond to triazolam in terms of actions on sleep.     

Serotonergic modulation of GABAergic and glutamatergic synaptic transmission in mechanically isolated rat medial preoptic area neurons.

The medial preoptic area (MPOA) of the hypothalamus is critically involved in the regulation of male sexual behavior and has been implicated in several homeostatic processes. Serotonin (5-hydroxytryptamine, 5-HT) inhibits sexual behavior via effects in the MPOA, where there are high densities of 5-HT(1A) and 5-HT(1B) receptor subtypes. We used whole-cell recordings under voltage-clamp conditions to investigate the serotonergic modulation of gamma-aminobutyric acid (GABA)ergic and glutamatergic synaptic transmission in mechanically dissociated rat MPOA neurons with native presynaptic nerve endings. Spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in the MPOA were completely blocked by bicuculline. Serotonin reversibly reduced the GABAergic mIPSC frequency without affecting the mean current amplitude. Serotonergic inhibition of mIPSC frequency was mimicked by (+/-)-8-hydroxy-2-dipropylaminotetralin hydrobromide, a specific 5-HT(1A) receptor agonist, and blocked by 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide, a specific 5-HT(1A) receptor antagonist. 6-Cyano-7-nitroquinoxaline-2,3-dione completely blocked spontaneous glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in the MPOA. Serotonin reversibly decreased the glutamatergic mEPSC frequency without affecting the mean current amplitude. Serotonergic inhibition of mEPSC frequency was mimicked by CGS 12066B, a specific 5-HT(1B) receptor agonist, and blocked by SB 216641, a specific 5-HT(1B) receptor antagonist. Stimulation of adenylyl cyclase with forskolin increased the frequencies of GABAergic mIPSCs and glutamatergic mEPSCs, and blocked the inhibitory effects of 5-HT. H-89, a selective protein kinase A (PKA) inhibitor, decreased the frequencies of GABAergic mIPSCs and glutamatergic mEPSCs, and blocked their reduction by 5-HT. These findings suggest that 5-HT reduces the frequency of GABAergic mIPSCs and glutamatergic mEPSCs through 5-HT(1A) and 5-HT(1B) receptor-mediated inhibition, respectively, of the PKA-dependent pathway in the presynaptic nerve terminals of MPOA neurons.     

Thermal changes produced by norepinephrine application in the preoptic area of monkeys

The effect of injection of norepinephrine in the anterior regions of hypothalamus on rectal temperature, skin temperature, heart rate and respiratory rate in rhesus monkeys was studied. The injection of 2 micrograms of norepinephrine in the preoptic area produced a fall in body temperature without any accompanying change in skin temperature, heart rate and respiratory rate. The findings suggest that the suppression of heat production may be responsible for the norepinephrine induced hypothermia in monkeys.     

Sleep changes during chronic cold exposure showed that the homeostatic requirement of sleep is reduced in the medial preoptic area lesioned rats

The effects of chronic exposure to a mildly cold ambient temperature (T(a)) of 18 degrees C on sleep wakefulness (S-W) and brain temperature (T(br)) were studied in the medial preoptic area (mPOA) lesioned male Wistar rats. Electroencephalogram (EEG), electrooculogram (EOG) and electromyogram (EMG) electrodes were chronically implanted to assess S-W, and a thermocouple above the dura to record the T(br). After three recordings (24 h each) of S-W and T(br) at 24 degrees C, N-methyl D-aspartic acid (NMDA) was intracerebrally injected to produce bilateral destruction of neurons in the mPOA. There was decreased sleep and increased T(br) even four weeks after the mPOA lesion. T(a) of the environmental chamber was then reduced to 18 degrees C, and the S-W and T(br) were again recorded for 24 h each on the 1st, 7th, 14th, 21st, and on 28th days of continuous exposure to the mild cold T(a). Exposure to the cold produced further decrease in sleep and increase in the T(br). However, sleep came back to the pre-exposure level by the 14th day. An increase in the duration of sleep episodes was responsible for the restoration of sleep during chronic cold exposure. The study showed that the requirement of sleep was reset at a lower level in the mPOA lesioned rats. The mPOA lesion affected the sleep maintenance and sleep initiation, though the latter became evident only during chronic cold exposure. The magnitude of the acute changes in T(br) and S-W were less in the lesioned rats, as compared to those observed in the normal rats exposed to similar cold T(a). On the basis of these observations, it could be proposed that the mPOA plays some role in cold induced changes in thermoregulation and sleep regulation. The T(br) remained elevated throughout the period of cold exposure. Resetting of the T(br), at a higher level may be part of the homeostatic readjustment to restore sleep.     

Cocaine and body temperature in the rat: effects of ambient temperature

The laboratory rat is being developed as a model to determine whether abuse of cocaine constitutes a risk factor in the pathogenesis of stress or exertion-induced heatstroke. Under thermoneutral conditions (Ta 20 degrees C) cocaine (10-40 mg/kg i.p.) caused a dose-dependent fall in core temperature ranging from 0.45 +/- 0.18 to 1.77 +/- 0.26 degrees C. When the ambient temperature (Ta) was increased to 35 degrees C, cocaine (10-40 mg/kg i.p.) led to a dose-dependent hyperthermia (0.3 +/- 0.08 to 1.43 +/- 0.43 degrees C). Repeated injection of cocaine (40 mg/kg at Ta 20 degrees C or 20 mg/kg at Ta 40 degrees C) on days 1, 3, 8, 15, and 23 did not alter the magnitude of the temperature change compared to that following the first injection, i.e., neither tolerance nor potentiation occurred.     

Presynaptic mu-opioid receptors regulate a late step of the secretory process in rat ventral tegmental area GABAergic neurons

γ-Aminobutyric acid (GABA)-containing interneurons of the ventral tegmental area (VTA) regulate the activity of dopaminergic neurons. These GABAergic interneurons are known to be innervated by synaptic terminals containing enkephalin, an endogenous ligand of μ-opioid receptors. Bath application of μ-opioid receptor agonists inhibits the activity of VTA GABAergic neurons but the mechanism whereby μ-opioid receptors regulate synaptic GABA release from these neurons has not been directly identified. Using cultured VTA neurons we have confirmed that μ-opioid receptor agonists inhibit synaptic GABA release. DAMGO, a selective μ-opioid receptor agonist, had four distinct effects on GABAergic IPSCs: (1) it inhibited the frequency and amplitude of spontaneous IPSCs (sIPSCs), (2) it reduced the amplitude of IPSCs evoked by single action potentials, (3) it inhibited the frequency, but not the amplitude of miniature IPSCs (mIPSCs), and (4) DAMGO inhibited mIPSCs evoked by ionomycin, a Ca²⁺ ionophore. The inhibition of action potential-evoked IPSCs and of spontaneous and ionomycin-evoked mIPSCs by DAMGO was prevented by the K⁺ channel blocker, 4-aminopyridine (4-AP). In conclusion, our work shows that one of the mechanisms through which μ-opioid receptors inhibit GABA release by VTA neurons is through inhibition of the secretory process at the nerve terminal level. In addition, considering that ionomycin stimulates exocytosis through a mechanism that should be insensitive to membrane polarization, our experiments with 4-AP suggest that K⁺ channels are implicated in the inhibition of the efficacy of the secretory process by μ-opioid receptors.     

Preliminary investigation of changes in the sexually dimorphic nucleus of the rat medial preoptic area following prenatal exposure to fenitrothion

In vitro, the organophosphate insecticide fenitrothion is a potent competitive androgen receptor antagonist, whereas in vivo it affects the development of the male rat reproductive system. The purpose of this pilot study was to determine whether prenatal exposure to fenitrothion affects development of the rat sexually dimorphic nucleus of the medial preoptic area (SDN-POA). Pregnant rats (n = 5-6 litters/group) were orally dosed with corn oil (vehicle) or fenitrothion (20 or 25 mg kg(-1) day(-1)) from gestation day (GD) 12-21. Offspring were euthanized after reaching sexual maturity (females 60-65 days old and males 96-105 days old) and the SDN-POA volumes determined for two rats/sex/litter. Tremors, increased lacrimation and decreased body weight gain were observed in dams from both fenitrothion exposure groups. Reproductive effects in male offspring, including reduced anogenital distance on postnatal day (PND) 1 and increased retention of areolae (PND 13) were observed following fenitrothion exposure at these dose levels. These effects did not persist into adulthood. There was a dose-related increase in the SDN-POA volume in males and a dose-related decrease in SDN-POA volume in females exposed to fenitrothion. These SDN-POA volume changes contrast with those seen with flutamide, another potent anti-androgen, and suggest that fenitrothion may have mixed endocrine effects on the developing brain.     

Effect of ambient temperature on thermoregulation in rats following preoptic/anterior hypothalamic injection of physostigmine

This experiment was designed to study the effect of ambient temperature (Ta) on the thermoregulatory response after the injection of the acetylcholinesterase blocking agent, physostigmine, into the preoptic/anterior hypothalamic area (POAH) of the rat. Three doses of physostigmine (3.0, 30.0 and 60.0 micrograms) were injected in a volume of 1.0 microliter in the preoptic/anterior hypothalamic area of unrestrained rats at three different ambient temperatures (15, 25 and 35 degrees C). Brain temperature (Tbr) and gross changes in behavior were monitored continuously throughout the duration of each experiment. Physostigmine induced hypothermia at ambient temperatures of 15 and 25 degrees C but not at 35 degrees C. Immediately prior to and during the hypothermic response the animals displayed behavioral reflexes such as fur licking and a sprawled posture which presumably enhanced heat loss. Generally, soon after the peak of the hypothermic response (approximately 30 min), the rats displayed heat-conserving behavior (huddled position, piloerection of the fur). These data indicate that the activity of cholinergic synapses within the preoptic/anterior hypothalamic area increases with decreasing ambient temperature. The behavioral observations suggest some role for the cholinergic system in the activation of heat-dissipating responses in the rat.     

5-HT1A receptor-mediated activation of a G-protein-coupled inwardly rectifying K+ current in rat medial preoptic area neurons

The medial preoptic area plays an important role in the regulation of sexual behavior, and serotonin (5-hydroxytryptamine, 5-HT) exerts an inhibitory effect on sexual behavior by acting on the medial preoptic area region. This study was designed to clarify the inhibitory effect of 5-HT on the medial preoptic area neurons and to elucidate the electrophysiological mechanisms involved in the action of 5-HT. Superfusion of 100 nM 5-HT hyperpolarized the membrane potential and inhibited the action potential firing. When the membrane potential was stepped to various potentials, the inward K+ currents were potentiated in the presence of 100 nM 5-HT. When the concentration of K+ in the external solution was increased from 5 mM to 30 mM, 5-HT markedly potentiated the inward K+ currents. In the steady-state current-voltage relationship, the 5-HT-activated inward current was carried by K+ ions and showed characteristics typical of an inwardly rectifying K+ current. The 5-HT-activated K+ current was mimicked by a 5-HT1A receptor agonist, (+/-)-8-hydroxy-2-dipropylaminotetralin hydrobromide, and was reversibly blocked by a 5-HT1A receptor antagonist, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide, but not by a 5-HT2 receptor antagonist, ketanserin. The 5-HT-activated K+ current was sensitively blocked by Ba2+, but not by 4-aminopyridine, and was completely suppressed by N-ethylmaleimide. These results indicate that 5-HT-induced hyperpolarization of the medial preoptic area neurons occurs as a result of activation of the G-protein-coupled inwardly rectifying K+ currents by 5-HT1A receptors.     

Bicuculline free base blocks voltage-activated K+ currents in rat medial preoptic neurons

The effects of the well-known GABA(A)-receptor blocker bicuculline on voltage-gated K(+) currents were studied in neurons from the medial preoptic nucleus (MPN) of rat. Whole-cell currents were recorded using the perforated-patch technique. Voltage steps from -54 to +6 mV resulted in tetraethylammonium-sensitive K(+) currents of delayed rectifier type. The total K(+) current (at 300 ms), including Ca(2+)-dependent and Ca(2+)-independent components, was reversibly reduced (17 +/- 4%) by 100 microM bicuculline methiodide and (37 +/- 5%) by 100 microM bicuculline as free base. The Ca(2+)-independent fraction (77 +/- 2%) of K(+) current evoked by a voltage step was, however, reduced (54 +/- 6%) only by bicuculline free base, but was not affected by bicuculline methiodide. The half-saturating concentration of bicuculline free base for blocking this purely voltage-gated K(+) current was 113 microM, whereas for blocking a steady Ca(2+)-dependent K(+) current it was 36 microM. The bicuculline-sensitive voltage-gated K(+) current was composed of 4-AP-sensitive and 4-AP-resistant components with different kinetic properties. No component of the purely voltage-gated K(+) current was affected neither by 100 nM alpha-dendrotoxin nor by 100 nM I-dendrotoxin. The possible K(+)-channel subtypes mediating the bicuculline-sensitive current in MPN neurons are discussed.     

腹外侧视前区5-羟色胺对大鼠睡眠-觉醒周期的作用

目的　通过腹外侧视前区 (VLPO)微量注射 5 羟色胺酸 (5 HTP)、非特异性 5 HT受体阻断剂麦角新碱 (MS)和5 HT再摄取抑制剂氟西汀观察 5 HT在VLPO对大鼠睡眠-觉醒周期的影响。方法　采用脑立体定位、核团微量注射和多导睡眠描记技术。结果　VLPO双侧分别微量注射小剂量 5 HTP(0 5μg ,0 1 μl)对大鼠睡眠 -觉醒周期无明显影响 ;而双侧注射大剂量 5 HTP(1 0 μg ,0 1 μl)使大鼠睡眠减少 ,觉醒增加 ;VLPO双侧注射 5 HT再摄取抑制剂氟西汀(6 0 μg ,0 1 μl)可产生与大剂量 5 HTP类似的作用 ;而VLPO双侧微量注射非特异性 5 HT受体阻断剂麦角新碱(MS ,1 0 μg ,0 1 μl)使大鼠睡眠增加 ,觉醒减少。大剂量 5 HTP和MS对觉醒 ,睡眠成分改变具有明显的时间相关性。结论　 5 HT在VLPO参与睡眠 -觉醒周期调节且有促觉醒作用 ,其促觉醒作用可能与突触后物质表达有关     

Effects of thalidomide and pentobarbital on neuronal activity in the preoptic area during sleep and wakefulness in the cat

To test the hypothesis that sleep produced by thalidomide, unlike that of pentobarbital, is associated with increased neuronal activity in the preoptic area (POA), the spontaneous activity of 96 POA neurons was recorded in chronically prepared cats during alert wakefulness (W), deep slow-wave sleep (SWS), and REM sleep in a drug-free preparation and after administration of thalidomide (4mg/kg) and pentobarbital (4 or 8 mg/kg). Thalidomide, unlike pentobarbital, at a dose that significantly increased the amount of SWS, failed to depress neuronal activity in the POA compared to drug-free controls. Mean discharge rates during thalidomide treatment were similar to drug-free rates. In contrast, rates during low-dose pentobarbital treatment were significantly less than those of drug-free and thalidomide-treated animals. Rates during high-dose pentobarbital treatment were significantly less than those in all other groups. Thalidomide, compared with the other groups, in addition to increasing the amount of SWS, significantly increased the total amount of REM sleep as well as REM sleep as a percent of total sleep, but did not produce ataxia or behavioral excitement. These results do not confirm the initial hypothesis, but suggest that hypnotic drugs that do not depress neuronal activity in the POA may be devoid of some of the unwanted side effects often associated with the more commonly prescribed hypnotic medications.     

Presynaptic μ-opioid receptors regulate a late step of the secretory process in rat ventral tegmental area GABAergic neurons

γ-Aminobutyric acid (GABA)-containing interneurons of the ventral tegmental area (VTA) regulate the activity of dopaminergic neurons. These GABAergic interneurons are known to be innervated by synaptic terminals containing enkephalin, an endogenous ligand of μ-opioid receptors. Bath application of μ-opioid receptor agonists inhibits the activity of VTA GABAergic neurons but the mechanism whereby μ-opioid receptors regulate synaptic GABA release from these neurons has not been directly identified. Using cultured VTA neurons we have confirmed that μ-opioid receptor agonists inhibit synaptic GABA release. DAMGO, a selective μ-opioid receptor agonist, had four distinct effects on GABAergic IPSCs: (1) it inhibited the frequency and amplitude of spontaneous IPSCs (sIPSCs), (2) it reduced the amplitude of IPSCs evoked by single action potentials, (3) it inhibited the frequency, but not the amplitude of miniature IPSCs (mIPSCs), and (4) DAMGO inhibited mIPSCs evoked by ionomycin, a Ca²⁺ ionophore. The inhibition of action potential-evoked IPSCs and of spontaneous and ionomycin-evoked mIPSCs by DAMGO was prevented by the K⁺ channel blocker, 4-aminopyridine (4-AP). In conclusion, our work shows that one of the mechanisms through which μ-opioid receptors inhibit GABA release by VTA neurons is through inhibition of the secretory process at the nerve terminal level. In addition, considering that ionomycin stimulates exocytosis through a mechanism that should be insensitive to membrane polarization, our experiments with 4-AP suggest that K⁺ channels are implicated in the inhibition of the efficacy of the secretory process by μ-opioid receptors.     

GABA-A receptors in mPOAH simultaneously regulate sleep and body temperature in freely moving rats

Sleep-wakefulness and body temperature are two circadian rhythmic biological phenomena. The role of GABAergic inputs in the medial preoptico-anterior hypothalamus (mPOAH) on simultaneous regulation of those phenomena was investigated in freely moving normally behaving rats. The GABA-A receptors were blocked by microinjecting picrotoxin, and the effects on electrophysiological parameters signifying sleep-wakefulness, rectal temperature and brain temperature were recorded simultaneously. The results suggest that, normally, GABA in the medial preoptic area acts through GABA-A receptor that induces sleep and prevents an excessive rise in body temperature. However, the results do not allow us to comment on the cause and effect relationship, if any, between changes in sleep-wakefulness and body temperature. The changes in brain and rectal temperatures showed a positive correlation, however, the former varied within a narrower range than that of the latter.     

Prominent burst firing of dopaminergic neurons in the ventral tegmental area during paradoxical sleep.

Dopamine is involved in motivation, memory, and reward processing. However, it is not clear whether the activity of dopamine neurons is related or not to vigilance states. Using unit recordings in unanesthetized head restrained rats we measured the firing pattern of dopamine neurons of the ventral tegmental area across the sleep-wake cycle. We found these cells were activated during paradoxical sleep (PS) via a clear switch to a prominent bursting pattern, which is known to induce large synaptic dopamine release. This activation during PS was similar to the activity measured during the consumption of palatable food. Thus, as it does during waking in response to novelty and reward, dopamine could modulate brain plasticity and thus participate in memory consolidation during PS. By challenging the traditional view that dopamine is the only aminergic group not involved in sleep physiology, this study provides an alternative perspective that may be crucial for understanding the physiological function of PS and dream mentation.     

The medial septum acts through the medial preoptic area for thermoregulation and works with it for sleep regulation

The chronic changes in sleep-wakefulness (S-W), body temperature (Tb), locomotor activity (LMA) and thermal preference were studied in male Wistar rats after the destruction of neurons in both the medial preoptic area (mPOA) and the medial septum (MS) by intracerebral injection of N-methyl-D-aspartic acid. An increase in the Tb, and a preference for higher ambient temperature (Tamb) of 30 degrees C were observed after the combined lesion of the mPOA and the MS. Similar changes were reported to occur after the lesion that was restricted to the mPOA. But these alterations were in contrast to the decrease in Tb and preference for lower Tamb, observed after the MS lesion. The thermostat of the brain would have been reset at a higher level after the combined lesion, as there was an increase in Tb, along with a preference for a higher Tamb, and an increase in LMA. There was a reduction in the frequency and the duration of the slow wave sleep (SWS) episodes, and a reduction in the frequency of the paradoxical sleep (PS) episodes after the combined lesion. The destruction of the MS neurons was probably responsible for the reduction in the frequency of SWS, whereas the loss of mPOA neurons was responsible for the decrease in the duration of SWS and frequency of PS. It can be suggested that the MS exerts its influence on thermoregulation through the mPOA. However, the MS and the mPOA seem to play independent, but complementary roles in sleep promotion.