General anaesthetics do not impair developmental expression of the KCC2 potassium-chloride cotransporter in neonatal rats during the brain growth spurt

Disruption of Rapid Eye Movement Sleep Homeostasis in Adolescent Rats after Neonatal Anesthesia

BACKGROUND

Previous studies suggest that rapid eye movement sleep rebound and disruption of rapid eye movement sleep architecture occur during the first 24 h after general anesthesia with volatile anesthetics in adult rats. However, it is unknown whether rapid eye movement sleep alterations persist beyond the anesthetic recovery phase in neonatal rats. This study tested the hypothesis that rapid eye movement sleep disturbances would be present in adolescent rats treated with anesthesia on postnatal day 7.

METHODS

Forty-four neonatal rats were randomly allocated to treatment with anesthesia consisting of midazolam, nitrous oxide, and isoflurane or control conditions for 2 h or 6 h. Electroencephalographic and electromyographic electrodes were implanted and recordings obtained between postnatal days 26 and 34. The primary outcome was time spent in rapid eye movement sleep. Data were analyzed using two-tailed unpaired t tests and two-way repeated measures analysis of variance.

RESULTS

Rats treated with midazolam, nitrous oxide, and isoflurane exhibited a significant increase in rapid eye movement sleep three weeks later when compared with control rats, regardless of whether they were treated for 2 h (174.0 ± 7.2 min in anesthetized, 108.6 ± 5.3 in controls, P < 0.0001) or 6 h (151.6 ± 9.9 min in anesthetized, 108.8 ± 7.1 in controls, P = 0.002).

CONCLUSIONS

Treatment with midazolam, nitrous oxide, and isoflurane on postnatal day 7 increases rapid eye movement sleep three weeks later in rats.

Postnatal Changes in K+/Cl- Cotransporter-2 Expression in the Forebrain of Mice Bearing a Mutant Nicotinic Subunit Linked to Sleep-Related Epilepsy

The Na+/K+/Cl- cotransporter-1 (NKCC1) and the K+/Cl- cotransporter-2 (KCC2) set the transmembrane Cl- gradient in the brain, and are implicated in epileptogenesis. We studied the postnatal distribution of NKCC1 and KCC2 in wild-type (WT) mice, and in a mouse model of sleep-related epilepsy, carrying the mutant β2-V287L subunit of the nicotinic acetylcholine receptor (nAChR). In WT neocortex, immunohistochemistry showed a wide distribution of NKCC1 in neurons and astrocytes. At birth, KCC2 was localized in neuronal somata, whereas at subsequent stages it was mainly found in the somatodendritic compartment. The cotransporters' expression was quantified by densitometry in the transgenic strain. KCC2 expression increased during the first postnatal weeks, while the NKCC1 amount remained stable, after birth. In mice expressing β2-V287L, the KCC2 amount in layer V of prefrontal cortex (PFC) was lower than in the control littermates at postnatal day 8 (P8), with no concomitant change in NKCC1. Consistently, the GABAergic excitatory to inhibitory switch was delayed in PFC layer V of mice carrying β2-V287L. At P60, the amount of KCC2 was instead higher in mice bearing the transgene. Irrespective of genotype, NKCC1 and KCC2 were abundantly expressed in the neuropil of most thalamic nuclei since birth. However, KCC2 expression decreased by P60 in the reticular nucleus, and more so in mice expressing β2-V287L. Therefore, a complex regulatory interplay occurs between heteromeric nAChRs and KCC2 in postnatal forebrain. The pathogenetic effect of β2-V287L may depend on altered KCC2 amounts in PFC during synaptogenesis, as well as in mature thalamocortical circuits.

K-Cl Cotransporter 2-mediated Cl- Extrusion Determines Developmental Stage-dependent Impact of Propofol Anesthesia on Dendritic Spines

BACKGROUND

General anesthetics potentiating γ-aminobutyric acid (GABA)-mediated signaling are known to induce a persistent decrement in excitatory synapse number in the cerebral cortex when applied during early postnatal development, while an opposite action is produced at later stages. Here, the authors test the hypothesis that the effect of general anesthetics on synaptogenesis depends upon the efficacy of GABA receptor type A (GABAA)-mediated inhibition controlled by the developmental up-regulation of the potassium-chloride (K-Cl) cotransporter 2 (KCC2).

METHODS

In utero electroporation of KCC2 was used to prematurely increase the efficacy of (GABAA)-mediated inhibition in layer 2/3 pyramidal neurons in the immature rat somatosensory cortex. Parallel experiments with expression of the inward-rectifier potassium channel Kir2.1 were done to reduce intrinsic neuronal excitability. The effects of these genetic manipulations (n = 3 to 4 animals per experimental group) were evaluated using iontophoretic injection of Lucifer Yellow (n = 8 to 12 cells per animal). The total number of spines analyzed per group ranged between 907 and 3,371.

RESULTS

The authors found a robust effect of the developmental up-regulation of KCC2-mediated Cl transport on the age-dependent action of propofol on dendritic spines. Premature expression of KCC2, unlike expression of a transport-inactive KCC2 variant, prevented a propofol-induced decrease in spine density. In line with a reduction in neuronal excitability, the above result was qualitatively replicated by overexpression of Kir2.1.

CONCLUSIONS

The KCC2-dependent developmental increase in the efficacy of GABAA-mediated inhibition is a major determinant of the age-dependent actions of propofol on dendritic spinogenesis.

Acute and Long-Term Effects of Brief Sevoflurane Anesthesia During the Early Postnatal Period in Rats

The possibility that exposure to general anesthetics during early life results in long-term impairment of neural function attracted considerable interest over the past decade. Extensive laboratory data suggest that administration of these drugs during critical stages of central nervous system development can lead to cell death, impaired neurogenesis, and synaptic growth as well as cognitive deficits. These observations are corroborated by several recent human epidemiological studies arguing that such cognitive impairment might also occur in humans. Despite the potential public health importance of this issue, several important questions remain open. Amongst them, how the duration of anesthesia exposure impact on outcome is as yet not fully elucidated. To gain insight into this question, here we focused on the short- and long-term impact of a 30-min-long exposure to clinically relevant concentrations of sevoflurane in rat pups at 2 functionally distinct stages of the brain growth spurt. We show that this treatment paradigm induced developmental stage-dependent and brain region-specific acute but not lasting changes in dendritic spine densities. Electrophysiological recordings in hippocampal brain slices from adult animals exposed to anesthesia in the early postnatal period revealed larger paired-pulse facilitation but no changes in the long-term potentiation paradigm when compared with nonanesthetized controls. 5-bromo-2-deoxyuridine pulse and pulse-chase experiments demonstrated that neither proliferation nor differentiation and survival of hippocampal progenitors were affected by sevoflurane exposure. In addition, behavioral testing of short- and long-term memory showed no differences between control and sevoflurane-exposed animals. Overall, these results suggest that brief sevoflurane exposure during critical periods of early postnatal development, although it does not seem to exert major long-term effects on brain circuitry development, can induce subtle changes in synaptic plasticity and spine density of which the physiological significance remains to be determined.

GABAA receptor drugs and neuronal plasticity in reward and aversion: focus on the ventral tegmental area

GABA_A receptors are the main fast inhibitory neurotransmitter receptors in the mammalian brain, and targets for many clinically important drugs widely used in the treatment of anxiety disorders, insomnia and in anesthesia. Nonetheless, there are significant risks associated with the long-term use of these drugs particularly related to development of tolerance and addiction. Addictive mechanisms of GABA_A receptor drugs are poorly known, but recent findings suggest that those drugs may induce aberrant neuroadaptations in the brain reward circuitry. Recently, benzodiazepines, acting on synaptic GABA_A receptors, and modulators of extrasynaptic GABA_A receptors (THIP and neurosteroids) have been found to induce plasticity in the ventral tegmental area (VTA) dopamine neurons and their main target projections. Furthermore, depending whether synaptic or extrasynaptic GABA_A receptor populations are activated, the behavioral outcome of repeated administration seems to correlate with rewarding or aversive behavioral responses, respectively. The VTA dopamine neurons project to forebrain centers such as the nucleus accumbens and medial prefrontal cortex, and receive afferent projections from these brain regions and especially from the extended amygdala and lateral habenula, forming the major part of the reward and aversion circuitry. Both synaptic and extrasynaptic GABA_A drugs inhibit the VTA GABAergic interneurons, thus activating the VTA DA neurons by disinhibition and this way inducing glutamatergic synaptic plasticity. However, the GABA_A drugs failed to alter synaptic spine numbers as studied from Golgi-Cox-stained VTA dendrites. Since the GABAergic drugs are known to depress the brain metabolism and gene expression, their likely way of inducing neuroplasticity in mature neurons is by disinhibiting the principal neurons, which remains to be rigorously tested for a number of clinically important anxiolytics, sedatives and anesthetics in different parts of the circuitry.

Cation-chloride cotransporters in neuronal development, plasticity and disease

Electrical activity in neurons requires a seamless functional coupling between plasmalemmal ion channels and ion transporters. Although ion channels have been studied intensively for several decades, research on ion transporters is in its infancy. In recent years, it has become evident that one family of ion transporters, cation-chloride cotransporters (CCCs), and in particular K(+)-Cl(-) cotransporter 2 (KCC2), have seminal roles in shaping GABAergic signalling and neuronal connectivity. Studying the functions of these transporters may lead to major paradigm shifts in our understanding of the mechanisms underlying brain development and plasticity in health and disease.