Extracellular histamine levels in the feline preoptic/anterior hypothalamic area during natural sleep-wakefulness and prolonged wakefulness: an in vivo microdialysis study.

Increased activity of the histaminergic neurons of the posterior hypothalamus has been implicated in the facilitation of behavioral wakefulness. Recent evidence of reciprocal projections between the sleep-active neurons of the preoptic/anterior hypothalamus and the histaminergic neurons of the tuberomammillary nucleus suggests that histaminergic innervation of the preoptic/anterior hypothalamic area may be of particular importance in the wakefulness-promoting properties of histamine. To test this possibility, we used microdialysis sample collection in the preoptic/anterior hypothalamic area of cats during natural sleep-wakefulness cycles, 6 h of sleep deprivation induced by gentle handling/playing, and recovery sleep. Samples were analyzed by a sensitive radioenzymatic assay. Mean basal levels of histamine in microdialysate during periods of wakefulness (1.155+/-0.225 pg/microl) did not vary during the 6 h of sleep deprivation. However, during the different sleep states, dramatic changes were observed in the extracellular histamine levels of preoptic/anterior hypothalamic area: wakefulness>non-rapid eye movement sleep>rapid eye movement sleep. Levels of histamine during rapid eye movement sleep were lowest (0.245+/-0.032 pg/microl), being significantly lower than levels during non-rapid eye movement sleep (0.395+/-0.081 pg/microl) and being only 21% of wakefulness levels. This pattern of preoptic/anterior hypothalamic area extracellular histamine levels across the sleep-wakefulness cycle closely resembles the reported single unit activity of histaminergic neurons. However, the invariance of histamine levels during sleep deprivation suggests that changes in histamine level do not relay information about sleep drive to the sleep-promoting neurons of the preoptic/anterior hypothalamic area.

Adenosinergic modulation of basal forebrain and preoptic/anterior hypothalamic neuronal activity in the control of behavioral state.

This review describes a series of animal experiments that investigate the role of endogenous adenosine (AD) in sleep. We propose that AD is a modulator of the sleepiness associated with prolonged wakefulness. More specifically, we suggest that, during prolonged wakefulness, extracellular AD accumulates selectively in the basal forebrain (BF) and cortex and promotes the transition from wakefulness to slow wave sleep (SWS) by inhibiting cholinergic and non-cholinergic wakefulness-promoting BF neurons at the AD A1 receptor. New in vitro data are also compatible with the hypothesis that, via presynaptic inhibition of GABAergic inhibitory input, AD may disinhibit neurons in the preoptic/anterior hypothalamus (POAH) that have SWS-selective activity and Fos expression. Our in vitro recordings initially showed that endogenous AD suppressed the discharge activity of neurons in the BF cholinergic zone via the AD A1 receptor. Moreover, in identified mesopontine cholinergic neurons, AD was shown to act post-synaptically by hyperpolarizng the membrane via an inwardly rectifying potassium current and inhibition of the hyperpolarization-activated current, I(h). In vivo microdialysis in the cat has shown that AD in the BF cholinergic zone accumulates during prolonged wakefulness, and declines slowly during subsequent sleep, findings confirmed in the rat. Moreover, increasing BF AD concentrations to approximately the level as during sleep deprivation by a nucleoside transport blocker mimicked the effect of sleep deprivation on both the EEG power spectrum and behavioral state distribution: wakefulness was decreased, and there were increases in SWS and REM sleep. As predicted, microdialyis application of the specific A1 receptor antagonist cyclopentyltheophylline (CPT) in the BF produced the opposite effects on behavioral state, increasing wakefulness and decreasing SWS and REM. Combined unit recording and microdialysis studies have shown neurons selectively active in wakefulness, compared with SWS, have discharge activity suppressed by both AD and the A1-specific agonist cyclohexyladenosine (CHA), while discharge activity is increased by the A1 receptor antagonist, CPT. We next addressed the question of whether AD exerts its effects locally or globally. Adenosine accumulation during prolonged wakefulness occurred in the BF and neocortex, although, unlike in the BF, cortical AD levels declined in the 6th h of sleep deprivation and declined further during subsequent recovery sleep. Somewhat to our surprise, AD concentrations did not increase during prolonged wakefulness (6 h) even in regions important in behavioral state control, such as the POAH, dorsal raphe nucleus, and pedunculopontine tegmental nucleus, nor did it increase in the ventrolateral/ventroanterior thalamic nucleii. These data suggest the presence of brain region-specific differences in AD transporters and/or degradation that become evident with prolonged wakefulness, even though AD concentrations are higher in all brain sites sampled during the naturally occurring (and shorter duration) episodes of wakefulness as compared to sleep episodes in the freely moving and behaving cat. Might AD also produce modulation of activity of neurons that have sleep selective transcriptional (Fos) and discharge activity in the preoptic/anterior hypothalamus zone? Whole cell patch clamp recordings in the in vitro horizontal slice showed fast and likely GABAergic inhibitory post-synaptic potentials and currents that were greatly decreased by bath application of AD. Adenosine may thus disinhibit and promote expression of sleep-related neuronal activity in the POAH. In summary, a growing body of evidence supports the role of AD as a mediator of the sleepiness following prolonged wakefulness, a role in which its inhibitory actions on the BF wakefulness-promoting neurons may be especially important.

Behavioral state-related changes of extracellular serotonin concentration in the pedunculopontine tegmental nucleus: a microdialysis study in freely moving animals.

Neurons of the cholinergic mesopontine tegmentum preferentially discharge during REM sleep and are thought to promote this state. It has been hypothesized they are inhibited during wakefulness by serotonergic input. The present study used the microdialysis sampling procedure coupled to microbore HPLC to measure extracellular serotonin levels in the pedunculopontine tegmental nucleus (PPT) in naturally sleeping cats. Extracellular serotonin levels were found to be highest during periods of wakefulness, lower during slow wave sleep, and lowest during periods of REM sleep. During wakefulness serotonin levels (mean A+/-SEM) measured in 10 A microliter samples were 1.14 A+/- 0.13 fmol/sample, whereas during slow wave sleep levels declined significantly to 72% of the wakefulness baseline (0.85 A +/- 0.11 fmol/sample), and dropped further to 45% of the wakefulness baseline in REM samples (0.52 A +/- 0.10 fmol/sample; all p's<0.003). The decrease in PPT serotonin levels during sleep may be an important determinant in the timing of REM sleep cyclicity. The data support the hypothesis that, during slow wave sleep and REM sleep, the declining levels of serotonin release the PPT REM-promoting neurons from serotonergic inhibition, which, in turn, leads to increases in acetylcholine release in terminal areas, facilitating the emergence of REM sleep.