GABAA and GABAB receptors and the ionic mechanisms mediating their effects on locus coeruleus neurons

Norepinephrine system at the interface of attention and reward

Reward learning is key to survival for individuals. Attention plays an important role in the rapid recognition of reward cues and establishment of reward memories. Reward history reciprocally guides attention to reward stimuli. However, the neurological processes of the interplay between reward and attention remain largely elusive, due to the diversity of the neural substrates that participate in these two processes. In this review, we delineate the complex and differentiated locus coeruleus norepinephrine (LC-NE) system in relation to different behavioral and cognitive substrates of reward and attention. The LC receives reward related sensory, perceptual, and visceral inputs, releases NE, glutamate, dopamine and various neuropeptides, forms reward memories, drives attentional bias and selects behavioral strategies for reward. Preclinical and clinical studies have found that abnormalities in the LC-NE system are involved in a variety of psychiatric conditions marked by disturbed functions in reward and attention. Therefore, we propose that the LC-NE system is an important hub in the interplay between reward and attention as well as a critical therapeutic target for psychiatric disorders characterized by compromised functions in reward and attention.

Altered expression of vesicular glutamate transporter-2 and cleaved caspase-3 in the locus coeruleus of nerve-injured rats

Neuropathic pain is a debilitating chronic condition provoked by a lesion in the nervous system and it induces functional alterations to the noradrenergic locus coeruleus (LC), affecting distinct dimensions of pain, like sensorial hypersensitivity, pain-induced depression, and anxiety. However, the neurobiological changes induced by nerve damage in the LC remain unclear. Here, we analyzed excitatory and inhibitory inputs to the LC, as well as the possible damage that noradrenergic neurons suffer after the induction of neuropathic pain through chronic constriction injury (CCI). Neuropathic pain was induced in male Sprague-Dawley rats, and the expression of the vesicular glutamate transporter 1 or 2 (VGLUT1 or VGLUT2), vesicular GABA transporter (VGAT), and cleaved caspase-3 (CC3) was analyzed by immunofluorescence 7 (CCI7d) or 28 days after the original lesion (CCI28d). While no significant differences in the density of VGLUT1 puncta were evident, CCI7d induced a significant increase in the perisomatic VGLUT2/VGAT ratio relative to Sham-operated and CCI28d animals. By contrast, when the entire region of LC is evaluated, there was a significant reduction in the density of VGLUT2 puncta in CCI28d animals, without changes in VGLUT2/VGAT ratio relative to the CCI7d animals. Additionally, changes in the noradrenergic soma size, and a lower density of mitochondria and lysosomes were evident in CCI28d animals. Interestingly, enhanced expression of the apoptotic marker CC3 was also evident in the CCI28d rats, mainly co-localizing with glial fibrillary acidic protein but not with any neuronal or noradrenergic marker. Overall, short-term pain appears to lead to an increase of markers of excitatory synapses in the perisomatic region of noradrenergic cells in the LC, an effect that is lost after long-term pain, which appears to activate apoptosis.

Calcium, Magnesium, Potassium, and Sodium Oxybates Oral Solution: A Lower-Sodium Alternative for Cataplexy or Excessive Daytime Sleepiness Associated with Narcolepsy

Lower-sodium oxybate (LXB) is an oxybate medication approved to treat cataplexy or excessive daytime sleepiness (EDS) in patients with narcolepsy 7 years of age and older in the United States. LXB was developed as an alternative to sodium oxybate (SXB), because the incidence of cardiovascular comorbidities is higher in patients with narcolepsy and there is an elevated cardiovascular risk associated with high sodium consumption. LXB has a unique formulation of calcium, magnesium, potassium, and sodium ions, containing 92% less sodium than SXB. Whereas the active oxybate moiety is the same for LXB and SXB, their pharmacokinetic profiles are not bioequivalent; therefore, a phase 3 trial in participants with narcolepsy was conducted for LXB. This review summarizes the background on oxybate as a therapeutic agent and its potential mechanism of action on the gamma-aminobutyric acid type B (GABA_B) receptor at noradrenergic and dopaminergic neurons, as well as at thalamocortical neurons. The rationale leading to the development of LXB as a lower-sodium alternative to SXB and the key efficacy and safety data supporting its approval for both adult and pediatric patients with narcolepsy are also discussed. LXB was approved in August 2021 in the United States for the treatment of idiopathic hypersomnia in adults. Potential future developments in the field of oxybate medications may include novel formulations and expanded indications for other diseases.

Secretin Regulates Excitatory GABAergic Neurotransmission to GnRH Neurons via Retrograde NO Signaling Pathway in Mice

In mammals, reproduction is regulated by a wide range of metabolic hormones that maintain the proper energy balance. In addition to regulating feeding and energy expenditure, these metabolic messengers also modulate the functional performance of the hypothalamic-pituitary-gonadal (HPG) axis. Secretin, a member of the secretin-glucagon-vasoactive intestinal peptide hormone family, has been shown to alter reproduction centrally, although the underlying mechanisms have not been explored yet. In order to elucidate its central action in the neuroendocrine regulation of reproduction, in vitro electrophysiological slice experiments were carried out on GnRH-GFP neurons in male mice. Bath application of secretin (100 nM) significantly increased the frequency of the spontaneous postsynaptic currents (sPSCs) to 118.0 ± 2.64% compared to the control, and that of the GABAergic miniature postsynaptic currents (mPSCs) to 147.6 ± 19.19%. Resting membrane potential became depolarized by 12.74 ± 4.539 mV after secretin treatment. Frequency of evoked action potentials (APs) also increased to 144.3 ± 10.8%. The secretin-triggered elevation of the frequency of mPSCs was prevented by using either a secretin receptor antagonist (3 μM) or intracellularly applied G-protein-coupled receptor blocker (GDP-β-S; 2 mM) supporting the involvement of secretin receptor in the process. Regarding the actions downstream to secretin receptor, intracellular blockade of protein kinase A (PKA) with KT-5720 (2 μM) or intracellular inhibition of the neuronal nitric oxide synthase (nNOS) by NPLA (1 μM) abolished the stimulatory effect of secretin on mPSCs. These data suggest that secretin acts on GnRH neurons via secretin receptors whose activation triggers the cAMP/PKA/nNOS signaling pathway resulting in nitric oxide release and in the presynaptic terminals this retrograde NO machinery regulates the GABAergic input to GnRH neurons.

Blockade of α2-adrenergic or metabotropic glutamate receptors induces glutamate release in the locus coeruleus to activate descending inhibition in rats with chronic neuropathic hypersensitivity

Locus coeruleus (LC)-spinal noradrenergic projections are important to endogenous analgesic mechanisms and can be activated by local glutamate signaling in the LC. The current study examined the local glutamatergic, GABAergic, and noradrenergic influences on glutamate release in the LC and noradrenergic descending inhibition in rats 6 weeks after spinal nerve ligation (SNL). Intra-LC injection of the α2 adrenoceptor antagonist idazoxan or the group 2 metabotropic glutamate receptor (mGluR) antagonist (RS)-α-Methyl-4-tetrazolylphenylglycine (MTPG) increased withdrawal thresholds in SNL animals and this was reversed by the blockade of α-amino-3-hydroxy-5-methyl- 4-isoxazolepropionic acid (AMPA) receptors in the LC or α2-adrenoceptors in the spinal cord, but not in normal animals. Neither blockade of GABA-A nor GABA-B receptors in the LC affected withdrawal thresholds in normal and SNL animals. Intra-LC perfusion of idazoxan increased extracellular glutamate in the LC in SNL animals but not in normal animals. Intra-LC perfusion of MTPG increased extracellular glutamate in the LC in both normal and SNL animals. These results suggest that local noradrenaline and glutamate tonically inhibit glutamate release in the LC after peripheral nerve injury and this may contribute to reduced descending inhibition in response to noxious input during chronic neuropathic pain.

gamma-Hydroxybutyrate/sodium oxybate: neurobiology, and impact on sleep and wakefulness

gamma-Hydroxybutyrate (GHB) is an endogenous short chain fatty acid and a, mostly oral, pharmacological compound that has been utilised in a variety of ways. Endogenously, GHB is synthesised locally within the CNS, mostly from its parent compound GABA. Sodium oxybate is the sodium salt of GHB and is used for the exogenous oral administration of GHB. It is likely that supraphysiological concentrations of GHB from exogenous administration produce qualitatively different neuronal actions than those produced by endogenous GHB concentrations. Evidence suggests a role for GHB as a neuromodulator/neurotransmitter. Under endogenous conditions and concentrations, and depending on the cell group affected, GHB may increase or decrease neuronal activity by inhibiting the release of neurotransmitters that are co-localised with GHB. After exogenous administration, most of the observed behavioural effects appear to be mediated via the activity of GHB at GABA(B) receptors, as long as the concentration is sufficient to elicit binding, which does not happen at endogenous concentrations. Endogenous and exogenous GHB is rapidly and completely converted into CO(2) and H(2)O through the tricarboxylic acid cycle (Krebs cycle). Sodium oxybate has been observed to modulate sleep in nonclinical study participants, and sleep and wakefulness in clinical populations, including groups with insomnia, fibromyalgia and narcolepsy. In narcolepsy, sodium oxybate has shown dose-related effects on various properties of sleep, including increases in slow-wave sleep duration and delta power, and a reduced number of night-time awakenings. Furthermore, multiple measures of daytime sleepiness and cataplexy demonstrated consistent short- and long-term improvement in response to night-time sodium oxybate therapy. The most common reported adverse events include dose-related headache, nausea, dizziness and somnolence.

From waking to sleeping: neuronal and chemical substrates

Multiple arousal systems maintain waking through the actions of chemical neurotransmitters that are released from broadly distributed nerve terminals when the neurons fire. Among these, noradrenaline-, histamine- and orexin-containing neurons fire during waking with behavioral arousal, decrease firing during slow-wave sleep (SWS) and cease firing during paradoxical sleep (PS), which is also known as rapid-eye-movement sleep. By contrast, acetylcholine (ACh)-containing neurons discharge during waking, decrease firing during SWS and fire at high rates during PS in association with fast cortical activity. Neurons that do not contain ACh, including GABA-containing neurons in the basal forebrain and preoptic area, are active in a reciprocal manner to the neurons of the arousal systems: one group discharges with slow cortical activity during SWS, and another discharges with behavioral quiescence and loss of postural muscle tone during SWS and PS. The reciprocal activities and interactions of these wake-active and sleep-active cell groups determine the alternation between waking and sleeping. Selective enhancement and attenuation of their discharge, transmitter release and postsynaptic actions comprise the substrates for the major stimulant and hypnotic drugs.

Numerous GABAergic afferents to locus ceruleus in the pericerulear dendritic zone: possible interneuronal pool

Most nuclei in the CNS are composed of principal neurons that project to other areas and interneurons that serve to integrate information among afferents. The noradrenergic brain nucleus locus ceruleus (LC) has appeared to be an exception to this general rule, because the LC is composed almost entirely of noradrenergic principal neurons. Here, we report that numerous small neurons in the peri-LC region become retrogradely labeled after focal injections of wheat germ agglutinin-apo (inactivated) horseradish peroxidase conjugated to colloidal gold, or pseudorabies virus (PRV), into the nuclear core of the rat LC. A substantial number of these neurons were routinely found within the dendritic field of the LC, in the area surrounding the compact cell-dense region classically defined as LC. Double labeling revealed that a large percentage of these cells stained for GABA. Ultrastructural analyses revealed axodendritic and axosomatic contacts between PRV-labeled afferents and LC neurons labeled with tyrosine hydroxylase immunohistochemistry. In addition, PRV-labeled neurons or axons were immunopositive for GABA in ultrastructural localizations. Analysis of the synaptology of immunopositive profiles demonstrated that these LC afferents in the peri-LC region receive several non-LC synaptic inputs. These results indicate that a population of small GABAergic neurons in the peri-LC dendritic zone may provide interneuronal integration for LC noradrenergic neurons.

Effects of sustained gamma-hydroxybutyrate treatments on spontaneous and evoked firing activity of locus coeruleus norepinephrine neurons

BACKGROUND

Gamma-hydroxybutyrate is currently used to promote nighttime sleep in the treatment of narcolepsy; however, it is also a drug of abuse ("Liquid Ecstasy") associated with a withdrawal syndrome with anxiety features. Of interest, the activity of locus coeruleus neurons is a reflective index of these above mentioned behavioral states.

METHODS

Using in vivo extracellular unitary recordings, sustained administration of gamma-hydroxybutyrate (40 mg/kg/day via minipump implanted subcutaneously) on the spontaneous and sensory-evoked burst firing of locus coeruleus norepinephrine neurons was assessed in rats.

RESULTS

A 2-day and 10-day gamma-hydroxybutyrate administration decreased the spontaneous firing activity of locus coeruleus neurons by 52% and 54%, respectively, when compared with controls. A similar degree of attenuation on evoked burst firing of norepinephrine neurons also occurred in these rats (2-day gamma-hydroxybutyrate: 47% and 10-day gamma-hydroxybutyrate: 58%), when compared with controls. In contrast, rats treated with gamma-hydroxybutyrate for 10 days followed by removal of the minipump for 36 hours resulted in a 33% augmentation in spontaneous locus coeruleus activity as compared with controls. Furthermore, a robust 79% increase in burst firing in response to paw-pinch was exhibited in theses rats.

CONCLUSIONS

Chronic gamma-hydroxybutyrate treatment inhibits the spontaneous and sensory-evoked burst firing of locus coeruleus norepinephrine neurons, whereas these indices are enhanced during drug withdrawal. The alteration in norepinephrine activity during chronic gamma-hydroxybutyrate administration may contribute to the ability of this agent to induce sleep and regulate narcoleptic episodes. Enhanced norepinephrine activity during withdrawal may be related to symptoms of anxiety on rapid termination of this drug in abusers.

Activation of μ-opioid receptors excites a population of locus coeruleus-spinal neurons through presynaptic disinhibition

The nucleus locus coeruleus (LC) plays an important role in analgesia produced by opioids and by modulation of the descending noradrenergic pathway. The functional role of micro-opioid receptors (muOR) in regulation of the excitability of spinally projecting LC neurons has not been investigated. In the present study, we tested the hypothesis that activation of presynaptic mu-opioid receptors excites a population of spinally projecting LC neurons through attenuation of gamma-aminobutyric acid (GABA)-ergic synaptic inputs. Spinally projecting LC neurons were retrogradely labeled by a fluorescent dye injected into the spinal dorsal horn of rats. Whole-cell current- and voltage-clamp recordings were performed on labeled LC neurons in brain slices. All labeled LC noradrenergic neurons were demonstrated by dopamine-beta-hydroxylase (DbetaH) immunofluorescence. In 37 labeled LC neurons, (D-Ala(2),N-Me-Phe(4),Gly-ol(5))-enkephalin (DAMGO) significantly increased the discharge activity of 17 (45.9%) neurons, but significantly inhibited the firing activity of another 15 (40.5%) cells. The excitatory effect of DAMGO on seven labeled LC neurons was diminished in the presence of bicuculline. DAMGO significantly decreased the frequency of GABA-mediated miniature inhibitory postsynaptic currents (mIPSCs) in all nine labeled LC neurons. However, DAMGO had no effect on glutamate-mediated miniature excitatory postsynaptic currents (mEPSCs) in 12 of 15 neurons. Furthermore, DAMGO significantly inhibited the peak amplitude of evoked inhibitory postsynaptic currents (eIPSCs) in all 11 labeled neurons, but had no significant effect on the evoked excitatory postsynaptic currents (eEPSCs) in 10 of these 11 neurons. Thus, data from this study suggest that activation of micro-opioid receptors excites a population of spinally projecting LC neurons by preferential inhibition of GABAergic synaptic inputs. These findings provide important new information about the descending noradrenergic modulation and analgesic mechanisms of opioids.

Shared mechanisms of antidepressant and antiepileptic treatments: drugs and devices

Many treatments for the epilepsies and affective disorder share the properties of seizure suppression and mood stabilization. Moreover, affective disorders and the epilepsies appear to share partially similar pathogenic mechanisms. A component of the shared predisposition appears to arise from noradrenergic and serotonergic deficits. Increasing evidence supports the hypothesis that noradrenergic and/or serotonergic elevation is a mechanism of therapeutic benefit shared by most antidepressants and many antiepileptic medications. Medication induced alterations in GABAergic, glutamatergic, and CRH (corticotropin releasing hormone) containing neurons may also contribute to the shared therapeutic properties of antidepressant and antiepileptic medications.

GABAB receptor subunits, R1 and R2, in brainstem catecholamine and serotonin neurons

GABA(B) receptors have been implicated in the GABAergic modulation of catecholaminergic and serotonergic pathways in the central nervous system. The GABA(B) receptor may require two subunits, GABA(B)R1 and GABA(B)R2, for functional activity. Using dual immunofluorescent labelling on adjacent cryostat sections, we investigated the presence of immunoreactivity for the GABA(B)R1 and GABA(B)R2 subunits in brainstem catecholamine (tyrosine hydroxylase-immunoreactive) and serotonin (tryptophan hydroxylase-immunoreactive) neurons. All neurons (>98%) examined in catecholamine groups A1, A2, A5, A6, C1, and serotonin groups B1-3 and B6-8 were immunoreactive for the GABA(B)R1 subunit. All A5 and A6 neurons (>97%) and at least 86% of A1, A2, C1, B2, B3, B7 and B8 neurons examined were GABA(B)R2-immunoreactive. The proportion of neurons with immunoreactivity for the GABA(B)R2 subunit varied between 0% and 99% for B1 neurons, and between 35% and 93% for B6 neurons. Statistical analysis showed that similar proportions of sampled neurons were immunoreactive for GABA(B)R1 and GABA(B)R2 in the A1, A5, A6, C1, B2 and B7 cell groups, whereas a smaller proportion of A2, B1, B3, B6 and B8 neurons were GABA(B)R2-immunoreactive than GABA(B)R1-immunoreactive. In general, our results suggest that GABA(B)R1 and GABA(B)R2 co-exist in the great majority of brainstem catecholamine and serotonin neurons. In the neurons that lack GABA(B)R2, the GABA(B)R1 subunit may act alone or with another protein.

Activation of delta-opioid receptors excites spinally projecting locus coeruleus neurons through inhibition of GABAergic inputs

Stimulation of the noradrenergic nucleus locus coeruleus (LC) releases norepinephrine in the spinal cord, which inhibits dorsal horn neurons and produces analgesia. Activation of this descending noradrenergic pathway also contributes to the analgesic action produced by systemic opioids. The delta-opioid receptors are present presynaptically in the LC. However, their functional role in the control of the activity of spinally projecting LC neurons remains uncertain. In this study, we tested the hypothesis that activation of presynaptic delta-opioid receptors excites spinally projecting LC neurons through inhibition of GABA release. Spinally projecting LC neurons were retrogradely labeled by a fluorescent dye, DiI, injected into the spinal dorsal horn of rats. Whole cell voltage- and current-clamp recordings were performed on DiI-labeled LC neurons in brain slices in vitro. Retrogradely labeled LC noradrenergic neurons were demonstrated by dopamine-beta-hydroxylase immunofluorescence. [D-Pen(2), D-Pen(5)]-enkephalin (DPDPE, 1 microM) significantly decreased the frequency of GABA-mediated miniature inhibitory postsynaptic currents (IPSCs) of nine DiI-labeled LC neurons from 2.1 +/- 0.5 to 0.7 +/- 0.2 Hz without altering their amplitude and the kinetics. On the other hand, the miniature excitatory postsynaptic currents (EPSC) of nine DiI-labeled LC neurons were not significantly altered by DPDPE. Furthermore, DPDPE significantly inhibited the amplitude of evoked IPSC but not EPSC in eight DiI-labeled LC neurons. Under the current-clamp condition, the firing activity in 9 of 11 DiI-labeled LC neurons was significantly increased by 1 microM DPDPE from 4.6 +/- 0.7 to 6.2 +/- 1.0 Hz. Bicuculline (20 microM) also significantly increased the firing frequency in 13 of 20 neurons from 1.8 +/- 0.5 to 2.8 +/- 0.6 Hz. Additionally, the excitatory effect of DPDPE on LC neurons was diminished in the presence of bicuculline. Collectively, these data strongly suggest that activation of presynaptic delta-opioid receptors by DPDPE excites a population of spinally projecting LC neurons by preferential inhibition of GABA release. Thus presynaptic delta-opioid receptors likely play an important role in the regulation of the excitability of spinally projecting LC neurons and the descending noradrenergic inhibitory system.

Role of locus coeruleus in foot shock-evoked Fos expression in rat brain

The robust activation of locus coeruleus neurons in response to a variety of stressors, in conjunction with the widespread outputs of the locus coeruleus, suggest that the locus coeruleus may be important in mediating responses to stress. Previous studies in rats have demonstrated that exposure to foot shock elicits Fos expression, a marker of neuronal activation, in the locus coeruleus and other brain sites. In order to evaluate the involvement of the locus coeruleus in foot shock-induced activation of other brain sites, shock-induced Fos expression was examined in the locus coeruleus and other brain areas known to be activated by foot shock, following direct inhibition of the locus coeruleus by local infusion of muscimol, a GABA agonist, prior to foot shock. Control rats received infusions of artificial cerebrospinal fluid into the locus coeruleus or muscimol into areas outside of locus coeruleus. Rats infused with artificial cerebrospinal fluid and then exposed to foot shock had significant increases in Fos expression in several brain areas, including locus coeruleus, nucleus O, several subdivisions of the hypothalamus, subnuclei of amygdala, bed nucleus of the stria terminalis and cingulate cortex. Inhibition of the locus coeruleus prior to foot shock significantly inhibited Fos expression in the locus coeruleus, nucleus O, some subdivisions of the hypothalamus including the magnocellular and medial parvicellular paraventricular hypothalamic nucleus, subnuclei of amygdala, and cingulate cortex. In contrast, inhibition of the locus coeruleus did not affect shock-induced Fos expression in other areas, including certain subdivisions of the hypothalamus and bed nucleus of the stria terminalis. We suggest that foot shock may activate multiple pathways, with activation of certain discrete nuclei requiring input from the locus coeruleus and activation of others occurring independently of locus coeruleus input.

Activation of pontine and medullary motor inhibitory regions reduces discharge in neurons located in the locus coeruleus and the anatomical equivalent of the midbrain locomotor region

Activation of the pontine inhibitory area (PIA) including the middle portion of the pontine reticular nucleus, oral part (PnO), or the gigantocellular reticular nucleus (Gi) suppresses muscle tone in decerebrate animals. The locus coeruleus (LC) and midbrain locomotor region (MLR) have been implicated in the facilitation of muscle tone. In the current study we investigated whether PIA and Gi stimulation causes changes in activity in these brainstem motor facilitatory systems. PIA stimulation evoked bilateral muscle tone suppression and inhibited 26 of 28 LC units and 33 of 36 tonically active units located in the anatomical equivalent of the MLR (caudal half of the cuneiform nucleus and the pedunculopontine tegmental nucleus). Gi stimulation evoked bilateral suppression of hindlimb muscle tone and inhibited 20 of 35 LC units and 24 of 24 neurons located in the MLR as well as facilitated 11 of 35 LC units. GABA and glycine release in the vicinity of LC was increased by 20-40% during ipsilateral PnO stimulation inducing hindlimb muscle tone suppression on the same side of the body. We conclude that activation of pontine and medullary inhibitory regions produces a coordinated reduction in the activity of the LC units and neurons located in the MLR related to muscle tone facilitation. The linkage between activation of brainstem motor inhibitory systems and inactivation of brainstem facilitatory systems may underlie the reduction in muscle tone in sleep as well as the modulation of muscle tone in the isolated brainstem.

Double-blind randomized controlled trial of baclofen vs. clonidine in the treatment of opiates withdrawal

BACKGROUND

A variety of detoxification methods have been utilized for the treatment of opiate withdrawal syndrome, of which alpha-adrenergic agonists have attracted considerable attention over the last two decades. However, accumulating evidence in rats shows the efficacy of the GABAB receptor agonist, baclofen, in reducing alcohol intake and self-administration of cocaine.

OBJECTIVE

To examine the ability of baclofen, in the management of opiate withdrawal.

METHOD

A total of 62 opiate addicts randomly assigned to treatment with baclofen or clonidine during a 14-day, double-blind clinical trial. All patients met the DSM IV criteria for opioid dependence. Maximum daily doses were 40 mg for baclofen and 0.8 mg for clonidine given three times a day in divided doses. The severity of the opiate withdrawal syndrome was measured on days 0, 1, 2, 3, 4, 7 and 14 using the Short Opiate Withdrawal Scale (SOWS).

RESULTS

Baclofen and clonidine were equally effective in treating the physical symptoms of withdrawal syndromes. However, baclofen showed a significant superiority over clonidine in the management of mental symptoms.

CONCLUSION

These results suggest that baclofen might be a novel therapeutic agent for opiate withdrawal syndrome. However, a larger study to confirm our results is warranted.

Activation of rat locus coeruleus neuron GABA(A) receptors by propofol and its potentiation by pentobarbital or alphaxalone

The action of propofol on the rat locus coeruleus was examined using intracellular recording from in vitro brain slice preparations. Concentrations of propofol between 3 and 300 microM were tested. At 100 microM, propofol completely inhibited the firing of all neurons tested (n=34); this was associated with a 5.7-mV hyperpolarization (range 0-16 mV, n=33) and a 35.6% reduction in input resistance (range 7.3-66.1%, n=33). The propofol-induced responses were not affected by 2-hydroxysaclofen (50 microM) or BaCl(2) (300 microM), but were completely blocked by bicuculline methiodide (100 microM) or picrotoxin (100 microM), indicating that propofol acts on GABA(A) receptors. As assessed by inhibition of the spontaneous firing rate, propofol was 5.6-fold more potent than GABA (gamma-aminobutyric acid). Potentiation of the propofol effect by other general anesthetics or other drugs was also investigated. When pentobarbital (100 microM) was tested alone on locus coeruleus cells, no change in membrane potential or input resistance was seen and there was only a 20.3+/-7.2% (n=8) inhibition of firing rate; however, in combination with 30 microM propofol, it caused a 6.1-fold greater increase in membrane hyperpolarization and a 9.7-fold greater reduction in input resistance than 30 microM propofol alone. A relatively low concentration of alphaxalone (10 microM), when tested alone, had little effect on the membrane potential or input resistance and only produced a 46.0+/-8.9% (n=8) inhibition of firing rate; however, in combination with 30 microM propofol, it caused a 9.3-fold greater hyperpolarization and an 8.6-fold greater reduction in input resistance compared with 30 microM propofol alone. In contrast, diazepam caused no potentiation of either propofol- or GABA-induced responses. Our data also indicate that locus coeruleus neuron GABA(A) receptors possess distinctive pharmacologic characteristics, such as blocking of the propofol effects by zinc and insensitivity to diazepam and the direct action of pentobarbital. On the basis of these pharmacologic properties, we suggest that locus coeruleus neuron GABA(A) receptors do not contain the gamma subunit.

Slow oscillations as a probe of the dynamics of the locus coeruleus-frontal cortex interaction in anesthetized rats

Multiunit or single unit activity recorded simultaneously from frontal cortex (FC) and locus coeruleus (LC) under ketamine anesthesia revealed that both regions show slow oscillatory activity, together or separately. If, however, both regions are engaged in this oscillatory activity, there is a systematic relationship between their phases with peak LC firing always following FC firing by 200-400 ms. This was confirmed by cross-correlational analyses, which indicated that the two structures temporarily form a resonant system. The FC-LC resonant state is, however, loose enough to remain open to other intrinsic or extrinsic influences, keeping the measured frequencies of oscillations at each site slightly different, as demonstrated by a detailed analysis of the autocorrelograms. An injection of lidocaine at the frontal cortex site, while sharply reducing the prefrontal activity to essentially zero, leads to an increase of the LC activity and to a modification of the shape of the LC autocorrelogram, but does not change appreciably the phase relationship between the activity in the two structures during the diminishing activity in FC.

Mesencephalic trigeminal neuron responses to gamma-aminobutyric acid

Mesencephalic trigeminal neurons are primary sensory neurons which have cell somata located within the brain stem. In spite of the presence of synaptic terminals on and around the cell somata, applications of a variety of neurotransmitter substances in earlier studies have failed to demonstrate responses. Using intracellular recording in a brain slice preparation, we have observed prominent depolarizations and decreases in input resistance in response to applications of gamma-aminobutyric acid (GABA) in most recorded mesencephalic trigeminal neurons. Those cells failing to respond were located deeply within the slice, and the low responsiveness was shown to be related to uptake of GABA in the slice. The responses were direct, since they remained during perfusion with a low calcium, high magnesium solution that blocks synaptic transmission. The responses were mimicked by the GABA(A) receptor agonist isoguvacine, and blocked by GABA(A) receptor antagonists. The GABA(B) receptor agonist baclofen evoked no changes in membrane potential or input resistance in neurons exhibiting depolarizations with GABA application. Tests of neuronal excitability during GABA applications indicated that the excitatory effects of the depolarization prevail over the depressant effects of the increase in membrane conductance. In situ hybridization histochemistry indicated that the GABA(A) receptors in Me5 cells are comprised of alpha2, beta2 and gamma2 subunits.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.

Radioautographic evidence that the GABAA receptor antagonist SR 95531 is a substrate inhibitor of MAO-A in the rat and human locus coeruleus

The locus coeruleus (LC), a major noradrenergic nucleus in the brain, probably has a functional role in the regulation of anxiety states as well as vigilance, attention, learning and memory. LC neurons are under the inhibitory control of gamma-aminobutyric acid (GABA) via ionotropic GABAA receptors. However, to date, little is known of the receptor binding characteristics of these neurons. In the present investigation we therefore examined by receptor radioautography the localization of the binding sites for different components of the GABAA receptor complex in the rat and human LC. Both rat and human LC neurons have a high density of binding sites for the pyridazinyl-GABA derivative [3H]SR 95531 (gabazine, a GABAA receptor antagonist for low affinity GABA recognition sites). However, at the concentrations used, no binding sites in the LC were detectable for the benzodiazepine receptor antagonist [3H]flumazenil, the GABAA receptor agonist (for high affinity sites) [3H]muscimol or the ionophore ligand [35S]t-butyl bicyclophosphorothionate (TBPS). Unexpectedly, the pharmacological specificity of [3H]SR 95531 binding to the LC differed markedly from that to most brain regions (IC50 values for GABA and RU 5135 respectively in the LC were > 10(-2) and 10(-3) M; and, for example, in the dentate gyrus the most labelled structure after the LC, 8 x 10(-7) and 1.8 x 10(-9) M). These differences prompted the further characterization of [3H]SR 95531 binding in the LC, revealing a significant affinity for monoamine oxidase type A (MAO-A), which is highly concentrated in this nucleus. In a competition binding study, a reduction of up to 25% of the [3H]SR 95531 binding was observed with MAO-A but not MAO-B inhibitors, at concentrations which produce maximum but selective enzyme inhibition. Correspondingly, 2 h after the oral administration of supramaximal doses of the MAO-A inhibitors moclobemide and Ro 41-1049 (but not the MAO-B inhibitor lazabemide) the in vitro binding of [3H]SR 95531 was markedly reduced (by 77 and 82% of controls respectively). Moreover, enzyme radioautography with [3H]Ro 41-1049 revealed that SR 95531 has a significant affinity for MAO-A (IC50 values were 10(-5) and 4 x 10(-6) M in the LC and dentate gyrus respectively) but not for MAO-B ([3H]lazabemide binding). Altogether, these findings suggest that the high-affinity binding of [3H]SR 95531 to the LC mainly reflects its affinity for MAO-A, which questions its utility as a selective ligand for low-affinity GABA recognition sites in the CNS.(ABSTRACT TRUNCATED AT 400 WORDS)

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)