Release of acetylcholine and GABA, and activity of their synthesizing enzymes in the rat pontine reticular formation

Sleep and Anesthesia Interactions: A Pharmacological Appraisal

Anesthetics have been used in clinical practice for over a hundred years, yet their mechanisms of action remain poorly understood. One tempting hypothesis to explain their hypnotic properties posits that anesthetics exert a component of their effects by "hijacking" the endogenous arousal circuitry of the brain. Modulation of activity within sleep- and wake-related neuroanatomic systems could thus explain some of the varied effects produced by anesthetics. There has been a recent explosion of research into the neuroanatomic substrates affected by various anesthetics. In this review, we will highlight the relevant sleep architecture and systems and focus on studies over the past few years that implicate these sleep-related structures as targets of anesthetics. These studies highlight a promising area of investigation regarding the mechanisms of action of anesthetics and provide an important model for future study.

Pontine reticular formation (PnO) administration of hypocretin-1 increases PnO GABA levels and wakefulness

STUDY OBJECTIVES

GABAergic transmission in the oral part of the pontine reticular formation (PnO) increases wakefulness. The hypothalamic peptide hypocretin-1 (orexin A) promotes wakefulness, and the PnO receives hypocretinergic input. The present study tested the hypothesis that PnO administration of hypocretin-1 increases PnO GABA levels and increases wakefulness. This study also tested the hypothesis that wakefulness is either increased or decreased, respectively, by PnO administration of drugs known to selectively increase or decrease GABA levels.

DESIGN

Awithin-subjects design was used for microdialysis and microinjection experiments.

SETTING

University of Michigan.

PATIENTS OR PARTICIPANTS

Experiments were performed using adult male Crl:CD (SD)IGS BR (Sprague-Dawley) rats (n=46).

INTERVENTIONS

PnO administration of hypocretin-1, nipecotic acid (a GABA uptake inhibitor that increases extracellular GABA levels), 3-mercaptopropionic acid (a GABA synthesis inhibitor that decreases extracellular GABA levels; 3-MPA), and Ringer solution (vehicle control).

MEASUREMENTS AND RESULTS

Dialysis administration of hypocretin-1 to the PnO caused a statistically significant, concentration-dependent increase in PnO GABA levels. PnO microinjection of hypocretin-1 or nipecotic acid caused a significant increase in wakefulness and a significant decrease in non-rapid eye movement (NREM) sleep and REM sleep. Microinjecting 3-MPA into the PnO caused a significant increase in NREM sleep and REM sleep and a significant decrease in wakefulness.

CONCLUSIONS

An increase or a decrease in PnO GABA levels causes an increase or decrease, respectively, in wakefulness. Hypocretin-1 may promote wakefulness, at least in part, by increasing GABAergic transmission in the PnO.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Possible involvement of an endogenous benzodiazepine receptor ligand of the inverse agonist type in the regulation of rapid-eye movement (REM) sleep: an hypothesis

1. Rapid-eye movement (REM) sleep is a complex behavioral state characterized by desynchronized electroencephalographic (EEG) activity, postural atonia, rapid, saccadic movements of the eyes, and vivid dreaming. 2. A recently developed class of drugs, the inverse agonist beta-carboline-3-carboxylates, elicits a number of effects similar to the properties of REM sleep, such as desynchronized cortical EEG and penile erections. 3. The hypothesis is put forth that an endogenous beta-carboline-3-carboxylate exists which may initiate many aspects of REM sleep. 4. Clinical relevance of this hypothesis is discussed with regard to REM anxiety dreams, night terrors, narcolepsy, and depression.

Substance P and other putative transmitters modulate the activity of reticular pontine neurons: an electrophysiological and immunohistochemical study

In this study we investigated the effects of possible modulatory transmitters on acoustically responsive neurons of the caudal pontine reticular nucleus (PnC). From previous work in our laboratory it has been suggested that the acoustically responsive giant neurons of this nucleus are the sensorimotor interface mediating the acoustic startle response. Furthermore they are the site of some of the modulatory influence impinging on this response. Besides a possibly glutamatergic excitation from the amygdala a cholinergic input from the midbrain has been described which may use substance P as cotransmitter. Therefore we used electrophysiological and histochemical methods to study this possible modulatory influence in the caudal pontine reticular nucleus. In the first part of this study we recorded extracellularly from single units in the PnC in vivo and studied the effects of iontophoretically applied transmitters. Substance P elicited a long lasting excitation. This excitatory effect of SP was potentiated by acetyl-beta-methylcholine (AMCh, an acetylcholine agonist), whereas single application of AMCh showed no uniform response. Glutamate elicited a potent brief excitation, while application of GABA showed a potent brief inhibition of PnC neurons. In the second part of this study we employed immunoperoxidase staining for substance P, which revealed a fairly dense network of substance P-immunoreactive (SP-ir) fibers in the lateral and ventral aspects of the PnC. Combining retrograde tracing and immunocytochemistry for substance P, we demonstrated that the SP-ir axons in the PnC originate mainly in the laterodorsal tegmental nucleus. We therefore conclude that activation of the laterodorsal tegmental nucleus may facilitate the acoustic startle response by a long lasting excitation of neurons in the caudal pontine reticular nucleus.

Levels, uptake, and release of glycine and glutamate in the rat pontine reticular formation

In this work we have determined the levels of glycine, glutamate, and other amino acids in the rat pontine reticular formation (PRF), in addition to some properties of the uptake and release of labeled glycine and glutamate in slices of this region. Glutamate was the most concentrated amino acid in the PRF, although its content was about half that of the striatum. Surprisingly, glycine levels in the PRF were 3.2-fold higher than in the striatum, whereas GABA content was similar in both regions. The uptake of both glycine and glutamate by PRF slices was strictly Na(+)-dependent. Their release was stimulated by K(+)-depolarization, but only the release of glycine was Ca(2+)-dependent. These findings suggest that glycine is a strong candidate for a neurotransmitter role in the PRF and that glutamate might also play such a role in this region.

Microinjections of muscimol and bicuculline into the pontine reticular formation modify the sleep-waking cycle in the rat

The role of gamma-aminobutyric acid (GABA) in the sleep-waking cycle was evaluated by means of microinjections of muscimol and bicuculline into the rat pontine reticular formation (PRF). Muscimol (20 ng) produced a marked increase in wakefulness (70%), a decrease in slow-wave sleep (SWS) (35%) and a remarkable delay in the onset of both SWS and paradoxical sleep, without modifying the percentage of the latter. Bicuculline (4 ng) shortened SWS latency by about 70%. These results suggest that GABAergic transmission in the PRF is involved in the regulation of sleep-waking cycle in the rat.