Wanhong Zuo

Dexmedetomidine Attenuates Neurotoxicity Induced by Prenatal Propofol Exposure.

Background: Anesthetic agents (eg, isoflurane, propofol) may cause neurodegeneration in the developing brains and impair animals’ learning ability. Dexmedetomidine (DEX), a selective alpha 2-adrenoreceptor agonist, has antiapoptotic properties in several brain injury models. Here, we tested whether DEX can protect the brain from neurodegeneration in rats exposed to propofol in utero. Materials and Methods: Fetal rats of embryonic day 20 were exposed in utero for 1 hour to propofol anesthesia with DEX or saline, or no anesthesia (control). The fetal brains were harvested 6 hours later. Cleaved caspase-3 levels and the relative number of ionized calcium-binding adaptor molecule 1 (IBA1)-positive cells were assessed by Western blot and immunohistochemistry. Learning and memory functions of the offspring in a separate cohort were assessed at postnatal day 35 by using an 8-arm radial maze. Results: Propofol anesthesia in pregnant rats augmented caspase-3 activation by 217% in the brain tissues of fetal rats and increased the number of IBA1-positive cells in the cortex by 40% and in the thalamus by 270%. Juvenile rats exposed prenatally to propofol were not different than controls on spontaneous locomotor activity, but made more errors of omission and took longer to complete visiting all 8 arms on days 1, 2, and 3 across a 5-day test in the radial arm maze. This neurocognitive deficit was prevented by administration of DEX (5.0 µg/kg, IP), which also significantly inhibited propofol-induced caspase-3 activation and microglial response in the fetal brains. Conclusions: DEX attenuates neuronal injury induced by maternal propofol anesthesia in the fetal brains, providing neurocognitive protection in the offspring rats.

Ethanol drives aversive conditioning through dopamine 1 receptor and glutamate receptor-mediated activation of lateral habenula neurons.

There has been increasing interest in the lateral habenula (LHb) given its potent regulatory role in many aversion‐related behaviors. Interestingly, ethanol can be rewarding as well as aversive; we therefore investigated whether ethanol exposure alters pacemaker firing or glutamate receptor signaling in LHb neurons in vitro and also whether LHb activity in vivo might contribute to the acquisition of conditioned place aversion to ethanol. Surprisingly, in epithalamic slices, low doses of ethanol (1.4 mM) strongly accelerated LHb neuron firing (by ~60%), and ethanols effects were much reduced by blocking glutamate receptors. Ethanol increased presynaptic glutamate release, and about half of this effect was mediated by dopamine subtype 1 receptors (D1Rs) and cyclic adenosine monophosphate (cAMP)‐dependent signaling pathways. In agreement with these findings, c‐Fos immunoreactivity in LHb regions was enhanced after a single administration of a low dose of ethanol (0.25 g/kg i.p.). Importantly, the same dose of ethanol in vivo also produced strong conditioned place aversion, and this was prevented by inhibiting D1Rs or neuronal activity within the LHb. By contrast, a higher dose (2 g/kg) led to ethanol conditioned place preference, which was enhanced by inhibiting neuronal activity or D1Rs within the LHb and suppressed by infusing aminomethylphosphonic acid or the D1R agonist SKF38393 within the LHb. Our in vitro and in vivo observations show, for the first time, that ethanol increases LHb excitation, mediated by D1R and glutamate receptors, and may underlie a LHb aversive signal that contributes to ethanol‐related aversion.

Salsolinol stimulates dopamine neurons in slices of posterior ventral tegmental area indirectly by activating μ-opioid receptors.

Previous studies in vivo have shown that salsolinol, the condensation product of acetaldehyde and dopamine, has properties that may contribute to alcohol abuse. Although opioid receptors, especially the μ-opioid receptors (MORs), may be involved, the cellular mechanisms mediating the effects of salsolinol have not been fully explored. In the current study, we used whole-cell patch-clamp recordings to examine the effects of salsolinol on dopamine neurons of the ventral tegmental area (VTA) in acute brain slices from Sprague-Dawley rats. Salsolinol (0.01–1 μM) dose-dependently and reversibly increased the ongoing firing of dopamine neurons; this effect was blocked by naltrexone, an antagonist of MORs, and gabazine, an antagonist of GABAA receptors. We further showed that salsolinol reduced the frequency without altering the amplitude of spontaneous GABAA receptor-mediated inhibitory postsynaptic currents in dopamine neurons. The salsolinol-induced reduction was blocked by both naltrexone and [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin, an agonist of MORs. Thus, salsolinol excites VTA-dopamine neurons indirectly by activating MORs, which inhibit GABA neurons in the VTA. This form of disinhibition seems to be a novel mechanism underlying the effects of salsolinol.

GABAergic actions mediate opposite ethanol effects on dopaminergic neurons in the anterior and posterior ventral tegmental area

It is known that the posterior ventral tegmental area (p-VTA) differs from the anterior VTA (a-VTA) in that rats learn to self-administer ethanol into the p-VTA, but not into the a-VTA. Because activation of VTA dopaminergic neurons by ethanol is a cellular mechanism underlying the reinforcement of ethanol consumption, we hypothesized that ethanol may exert different effects on dopaminergic neurons in the p-VTA and a-VTA. In patch-clamp recordings in midbrain slices from young rats (postnatal days 22-32), we detected no significant difference in electrophysiological properties between p-VTA and a-VTA dopaminergic neurons. However, acute exposure to ethanol (21–86 mM) stimulated p-VTA dopaminergic neurons but suppressed a-VTA dopaminergic neurons. Conversely, ethanol (>21 mM) dose-dependently reduced the frequency of the GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) generated by inhibitory neuronal firing but not miniature inhibitory postsynaptic currents (mIPSCs) in p-VTA dopaminergic neurons. By contrast, ethanol increased the frequency and amplitude of both sIPSCs and mIPSCs in a-VTA dopaminergic neurons. All of these effects of ethanol were abolished by a GABAA receptor antagonist. There was a strong negative correlation between ethanol-evoked modulation of sIPSCs and neuronal firing in VTA dopaminergic neurons. These results indicate that GABAergic inputs play an important role in ethanols actions in the VTA. The differential effects of ethanol on sIPSCs and neuronal firing in the p-VTA and a-VTA could be the basis for ethanol reinforcement via the p-VTA.

Salsolinol stimulates dopamine neurons in slices of posterior VTA indirectly by activating μ-opioid receptors

Previous studies in vivo have shown that salsolinol, the condensation product of acetaldehyde and dopamine, has properties that may contribute to alcohol abuse. Although opioid receptors, especially the μ-opioid receptors (MORs), may be involved, the cellular mechanisms mediating the effects of salsolinol have not been fully explored. In the current study, we used whole-cell patch-clamp recordings to examine the effects of salsolinol on dopamine neurons of the ventral tegmental area (VTA) in acute brain slices from Sprague-Dawley rats. Salsolinol (0.01–1 μM) dose-dependently and reversibly increased the ongoing firing of dopamine neurons; this effect was blocked by naltrexone, an antagonist of MORs, and gabazine, an antagonist of GABAA receptors. We further showed that salsolinol reduced the frequency without altering the amplitude of spontaneous GABAA receptor-mediated inhibitory postsynaptic currents in dopamine neurons. The salsolinol-induced reduction was blocked by both naltrexone and [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin, an agonist of MORs. Thus, salsolinol excites VTA-dopamine neurons indirectly by activating MORs, which inhibit GABA neurons in the VTA. This form of disinhibition seems to be a novel mechanism underlying the effects of salsolinol.

Serotonin modulates glutamatergic transmission to neurons in the lateral habenula

The lateral habenula (LHb) is bilaterally connected with serotoninergic raphe nuclei, and expresses high density of serotonin receptors. However, actions of serotonin on the excitatory synaptic transmission to LHb neurons have not been thoroughly investigated. The LHb contains two anatomically and functionally distinct regions: lateral (LHbl) and medial (LHbm) divisions. We compared serotonin’s effects on glutamatergic transmission across the LHb in rat brains. Serotonin bi-directionally and differentially modulated glutamatergic transmission. Serotonin inhibited glutamatergic transmission in higher percentage of LHbl neurons but potentiated in higher percentage of LHbm neurons. Magnitude of potentiation was greater in LHbm than in LHbl. Type 2 and 3 serotonin receptor antagonists attenuated serotonin’s potentiation. The serotonin reuptake blocker, and the type 2 and 3 receptor agonists facilitated glutamatergic transmission in both LHbl and LHbm neurons. Thus, serotonin via activating its type 2, 3 receptors, increased glutamate release at nerve terminals in some LHb neurons. Our data demonstrated that serotonin affects both LHbm and LHbl. Serotonin might play an important role in processing information between the LHb and its downstream-targeted structures during decision-making. It may also contribute to a homeostatic balance underlying the neural circuitry between the LHb and raphe nuclei.

Serotonin stimulates lateral habenula via activation of the post-synaptic serotonin 2/3 receptors and transient receptor potential channels.

There is growing interest on the role of the lateral habenula (LHb) in depression, because it closely and bilaterally connects with the serotoninergic raphe nuclei. The LHb sends glutamate efferents to the raphe nuclei, while it receives serotoninergic afferents, and expresses a high density of serotonin (5-HT) receptors. Recent studies suggest that 5-HT receptors exist both in the presynaptic and postsynaptic sites of LHb neurons, and activation of these receptors may have different effects on the activity of LHb neurons. The current study focused on the effect of 5-HT on the postsynaptic membrane. We found that 5-HT initiated a depolarizing inward current (I((5-HTi))) and accelerated spontaneous firing in ∼80% of LHb neurons in rat brain slices. I((5-HTi)) was also induced by the 5-HT uptake blocker citalopram, indicating activity of endogenous 5-HT. I((5-HTi)) was diminished by 5-HT(2/3) receptor antagonists (ritanserin, SB-200646 or ondansetron), and activated by the selective 5-HT(2/3) agonists 1-(3-Chlorophenyl) piperazine hydrochloride or 1-(3-Chlorophenyl) biguanide hydrochloride. Furthermore, I((5-HTi)) was attenuated by 2-Aminoethyl diphenylborinate, a blocker of transient receptor potential channels, and an IP3 receptor inhibitor, indicating the involvement of transient receptor potential channels. These results demonstrate that the reciprocal connection between the LHb and the 5-HT system highlights a key role for 5-HT stimulation of LHb neurons that may be important in the pathogenesis of depression.

Ethanol Withdrawal Drives Anxiety-Related Behaviors by Reducing M-type Potassium Channel Activity in the Lateral Habenula

Alcohol use disorders (AUDs) and anxiety disorders (ADs) are often seen concurrently, but their underlying cellular basis is unclear. For unclear reasons, the lateral habenula (LHb), a key brain region involved in the pathophysiology of ADs, becomes hyperactive after ethanol withdrawal. M-type K+ channels (M-channels), important regulators of neuronal activity, are abundant in the LHb, yet little is known about their role in AUDs and associated ADs. We report here that in rats at 24 h withdrawal from systemic ethanol administration (either by intraperitoneal injection, 2 g/kg, twice/day, for 7 days; or intermittent drinking 20% ethanol in a two-bottle free choice protocol for 8 weeks), the basal firing rate and the excitability of LHb neurons in brain slices was higher, whereas the amplitude of medium afterhyperpolarization and M-type K+ currents were smaller, when compared to ethanol naive rats. Concordantly, M-channel blocker (XE991)-induced increase in the spontaneous firing rate in LHb neurons was smaller. The protein expression of M-channel subunits, KCNQ2/3 in the LHb was also smaller. Moreover, anxiety levels (tested in open field, marble burying, and elevated plus maze) were higher, which were alleviated by LHb inhibition either chemogenetically or by local infusion of the M-channel opener, retigabine. Intra-LHb infusion of retigabine also reduced ethanol consumption and preference. These findings reveal an important role of LHb M-channels in the expression of AUDs and ADs, and suggest that the M-channels could be a potential therapeutic target for alcoholics.

High Frequency Electrical Stimulation of Lateral Habenula Reduces Voluntary Ethanol Consumption in Rats.

Background: Development of new strategies that can effectively prevent and/or treat alcohol use disorders is of paramount importance, because the currently available treatments are inadequate. Increasing evidence indicates that the lateral habenula (LHb) plays an important role in aversion, drug abuse, and depression. In light of the success of high-frequency stimulation (HFS) of the LHb in improving helplessness behavior in rodents, we assessed the effects of LHb HFS on ethanol-drinking behavior in rats. Methods: We trained rats to drink ethanol under an intermittent access two-bottle choice procedure. We used c-Fos immunohistochemistry and electrophysiological approaches to examine LHb activity. We applied a HFS protocol that has proven effective for reducing helplessness behavior in rats via a bipolar electrode implanted into the LHb. Results: c-Fos protein expression and the frequency of both spontaneous action potential firings and spontaneous excitatory postsynaptic currents were higher in LHb neurons of ethanol-withdrawn rats compared to their ethanol-naïve counterparts. HFS to the LHb produced long-term reduction of intake and preference for ethanol, without altering locomotor activity. Conversely, low-frequency electrical stimulation to the LHb or HFS applied to the nearby nucleus did not affect drinking behavior. Conclusions: Our results suggest that withdrawal from chronic ethanol exposure increases glutamate release and the activity of LHb neurons, and that functional inhibition of the LHb via HFS reduces ethanol consumption. Thus, LHb HFS could be a potential new therapeutic option for alcoholics.

Pharmacological Manipulation of the Rostromedial Tegmental Nucleus Changes Voluntary and Operant Ethanol Self-Administration in Rats

BACKGROUND The aversive properties of ethanol (EtOH) that limit its intake are poorly understood. There is an increasing interest in the role of the rostromedial tegmental nucleus (RMTg), because it encodes aversion signals and inhibits motivated behaviors. It is also a major source of inhibitory GABAergic inputs to the midbrain dopamine neurons. Up to this time, the role of the RMTg in EtOH-drinking behaviors has not been well explored. METHODS Male Long-Evans rats were trained either to drink EtOH under the intermittent 2-bottle-choice protocol or to self-administer EtOH in operant chambers under fixed-ratio-3 schedules. Changes in drinking behaviors induced by the bilateral infusion into the RMTg of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), an agonist of AMPA-type glutamate receptors, or muscimol, an agonist of GABAA receptors, were measured. RESULTS Consumption and preference for EtOH, numbers of active lever pressing, and head entrance to the EtOH port were all significantly decreased upon activation of the RMTg by the infusion of AMPA, but were increased upon inhibition of the RMTg by the infusion of muscimol. By contrast, intra-RMTg infusion of these agents did not change sucrose consumption. CONCLUSIONS These data show for the first time that EtOH-drinking and EtOH-seeking behaviors of rats changed inversely with RMTg function, supporting the idea that the RMTg plays a crucial role in EtOH-drinking behaviors.