Some general anesthetics reduce serotonergic neocortical activation and enhance the action of serotonergic antagonists

A Comparison of Brain-State Dynamics across Common Anesthetic Agents in Male Sprague-Dawley Rats

Anesthesia is a powerful tool in neuroscientific research, especially in sleep research where it has the experimental advantage of allowing surgical interventions that are ethically problematic in natural sleep. Yet, while it is well documented that different anesthetic agents produce a variety of brain states, and consequently have differential effects on a multitude of neurophysiological factors, these outcomes vary based on dosages, the animal species used, and the pharmacological mechanisms specific to each anesthetic agent. Thus, our aim was to conduct a controlled comparison of spontaneous electrophysiological dynamics at a surgical plane of anesthesia under six common research anesthetics using a ubiquitous animal model, the Sprague-Dawley rat. From this direct comparison, we also evaluated which anesthetic agents may serve as pharmacological proxies for the electrophysiological features and dynamics of unconscious states such as sleep and coma. We found that at a surgical plane, pentobarbital, isoflurane and propofol all produced a continuous pattern of burst-suppression activity, which is a neurophysiological state characteristically observed during coma. In contrast, ketamine-xylazine produced synchronized, slow-oscillatory activity, similar to that observed during slow-wave sleep. Notably, both urethane and chloral hydrate produced the spontaneous, cyclical alternations between forebrain activation (REM-like) and deactivation (non-REM-like) that are similar to those observed during natural sleep. Thus, choice of anesthesia, in conjunction with continuous brain state monitoring, are critical considerations in order to avoid brain-state confounds when conducting neurophysiological experiments.

Brain areas that influence general anesthesia

This document reviews the literature on local brain manipulation of general anesthesia in animals, focusing on behavioral and electrographic effects related to hypnosis or loss of consciousness. Local inactivation or lesion of wake-active areas, such as locus coeruleus, dorsal raphe, pedunculopontine tegmental nucleus, perifornical area, tuberomammillary nucleus, ventral tegmental area and basal forebrain, enhanced general anesthesia. Anesthesia enhancement was shown as a delayed emergence (recovery of righting reflex) from anesthesia or a decrease in the minimal alveolar concentration that induced loss of righting. Local activation of various wake-active areas, including pontis oralis and centromedial thalamus, promoted behavioral or electrographic arousal during maintained anesthesia and facilitated emergence. Lesion of the sleep-active ventrolateral preoptic area resulted in increased wakefulness and decreased isoflurane sensitivity, but only for 6 days after lesion. Inactivation of any structure within limbic circuits involving the medial septum, hippocampus, nucleus accumbens, ventral pallidum, and ventral tegmental area, amygdala, entorhinal and piriform cortex delayed emergence from anesthesia, and often reduced anesthetic-induced behavioral excitation. In summary, the concept that anesthesia works on the sleep-wake system has received strong support from studies that inactivated/lesioned or activated wake-active areas, and weak support from studies that lesioned sleep-active areas. In addition to the conventional wake-sleep areas, limbic structures such as the medial septum, hippocampus and prefrontal cortex are also involved in the behavioral response to general anesthesia. We suggest that hypnosis during general anesthesia may result from disrupting the wake-active neuronal activities in multiple areas and suppressing an atropine-resistant cortical activation associated with movements.

Cellular Actions of Urethane on Rat Visual Cortical Neurons In Vitro

Urethane is widely used in neurophysiological experiments to anesthetize animals, yet little is known about its actions at the cellular and synaptic levels. This limits our ability to model systems-level cortical function using results from urethane-anesthetized preparations. The present study found that action potential discharge of cortical neurons in vitro, in response to depolarizing current, was strongly depressed by urethane and this was accompanied by a significant decrease in membrane resistance. Voltage-clamp experiments suggest that the mechanism of this depression involves selective activation of a Ba2+-sensitive K+ leak conductance. Urethane did not alter excitatory glutamate-mediated or inhibitory (GABAA- or GABAB-mediated) synaptic transmission. Neither the amplitude nor decay time constant of GABAA- or GABAB-mediated monosynaptic inhibitory postsynaptic currents (IPSCs) were altered by urethane, nor was the frequency of spontaneous IPSCs. These results are consistent with observations seen in vivo during urethane anesthesia where urethane produced minimal disruption of signal transmission in the neocortex.

Generalized cortex activation by the auditory midbrain: Mediation by acetylcholine and subcortical relays

The inferior colliculus (IC) is a critical component of the ascending projection system carrying auditory information from the brainstem to the forebrain. Recent evidence indicates that, in addition to its role in auditory processing, the IC can exert a generalized, modulatory effect on the forebrain by activating the neocortical electrocorticogram (ECoG). Given the sparse direct projections from the IC to the cortex, it appears that the effect of the IC to produce ECoG activation is indirect, mediated by one or several neuromodulatory systems that have diffuse access to the entire cortical mantle. However, the anatomical relays that permit the IC to influence cortical activity have not been elucidated. In the present experiments, electrical stimulation of the IC suppressed slow, large amplitude oscillations in the ECoG of urethane anesthetized rats, replacing them with higher-frequency cortical activation. This effect was blocked by the muscarinic receptor antagonist scopolamine (0.5-1.0 mg/kg, i.p.), suggestive of a critical role of acetylcholine (ACh) release. Consistent with this hypothesis, localized lidocaine infusions (2%, 1 microl) into the cholinergic basal forebrain complex strongly reduced ECoG activation elicited by IC stimulation. To identify additional relays between the IC and basal forebrain, the effects of lidocaine infusions into the superior colliculus, medial prefrontal cortex, midline thalamus, and dorsal raphe were also studied. Inactivation of the superior colliculus and dorsal raphe reduced IC-induced activation, while prefrontal cortex and thalamic infusions were ineffective. Concurrent basal forebrain and raphe inactivation produced effects similar to that of inactivation of the basal forebrain alone, suggesting that these two areas are arranged in series, rather than acting as independent, parallel pathways. These results suggest that the ability of the IC to induce ECoG activation is mediated, in large parts, by the basal forebrain cholinergic system. Consistent with anatomical evidence, the superior colliculus and dorsal raphe appear to provide important links to functionally connect the IC to the basal forebrain, allowing the IC to indirectly access the entire cortical mantle and enhance processing in neocortical networks.

Methodological considerations in rat brain BOLD contrast pharmacological MRI

RATIONALE AND OBJECTIVES

Blood oxygen level dependent (BOLD) contrast pharmacological magnetic resonance imaging (phMRI) is an increasingly popular technique that allows the non-invasive investigation of spatial and temporal changes in rat brain function in response to pharmacological stimulation in vivo. Rat brain BOLD contrast phMRI is, at present, established in few neuropharmacological laboratories, and various issues associated with the technique require attention. The present review is primarily aimed at psychopharmacologists with no previous experience of phMRI, who are interested in the practical aspects that phMRI studies entail.

RESULTS AND DISCUSSION

Experimental and analytical considerations, including anaesthesia, physiological monitoring, drug dose and delivery, scanning protocols, statistical approaches and the interpretation of phMRI data, are discussed.

Locus coeruleus activation shortens synaptic drive while decreasing spike latency and jitter in sensorimotor cortex. Implications for neuronal integration

Chronic recording of locus coeruleus (LC) neurons in rat and monkey have pointed out that brief, phasic LC discharges, but not sustained activity, are specifically related to salient stimuli and attention. However, the sensory consequences of phasic activation of the noradrenergic system by a brief conditioning stimulation of the LC have not been fully investigated. This study examined the effect of LC activation on synaptic and neuronal responses to a tactile stimulus in the sensorimotor cortex of the anaesthetized rat, by analysing the fine temporal structure of sensory discharges and current source-density profiles recorded from the same electrodes. LC stimulation, with minimal EEG effects, consistently reduced the synaptic input in layers IV and V-VI, by decreasing the amplitude and duration of short-latency current sinks, but not the slope of their early rising phase. Simultaneously, most multiple and single unit excitatory responses were shortened by the suppression of their late component after 25-30 ms, whereas robust temporal facilitation of the early discharge was found for spike latency mean and variance, spike timing and synchronization to the stimulus, but leaving the number of spikes unaffected. These two apparently opposite effects on the synaptic drive and neuronal response are reminiscent of the noradrenergic depression of afferent synaptic potentials observed with an increased neuronal excitability in vitro. They are interpreted as a noradrenergic sharpening of thalamocortical processing consistent with a presumed role of synchronous discharges in perception that would depend on activated states, particularly when LC activity is correlated with vigilance or attention.

Neocortical activation by electrical and chemical stimulation of the rat inferior colliculus: intra-collicular mapping and neuropharmacological characterization

Classic experiments suggested that the midbrain reticular formation plays an important role in the induction and maintenance of high-frequency, low-amplitude activation of the electrocorticogram (ECoG). However, recent studies have shown that generalized activating systems are not restricted to the reticular formation in that non-reticular brain systems (e.g., basal forebrain, amygdala, superior colliculus) can effectively produce ECoG activation. Here, we investigated the role of the inferior colliculus (IC) in regulating ECoG activation in rats. Urethane-anesthetized rats displayed continuous large amplitude ECoG activity with peak power in the delta frequency range (0.5-3.9 Hz). Electrical 100-Hz stimulation (0.1-0.5 mA) of 40/88 (46%) stimulation sites in the IC suppressed low frequency oscillations and induced ECoG activation (>/=50% suppression of peak delta power). Systematic mapping of different IC territories (central nucleus, external and dorsal cortex) revealed that stimulation of all IC parts was equally effective in producing activation. Chemical stimulation of the IC with intra-collicular glutamate infusions (50 mM, 0.5 micro l) produces similar, but more consistent effects, with ECoG activation elicited in eight of nine rats. Pharmacological experiments were carried out in order to identify transmitters that mediate cortical activation in response to IC stimulation. The muscarinic receptor antagonist scopolamine (1 mg/kg, i.p.) reduced, but did not abolish, activation, as did the serotonergic receptor antagonist methiothepin (1 mg/kg, i.p.). A combination of the two drugs produced a complete block of IC-induced ECoG activation. These experiments demonstrate that the IC contains a distributed network, spanning all IC territories, which can participate in regulating the generalized activation state of the rat neocortex. Rather than by some direct cortical projections, IC neurons appear to induce ECoG activation by acting through both cholinergic and serotonergic systems, thought to provide the final effector mechanisms for cortical activation.

Histaminergic facilitation of electrocorticographic activation: role of basal forebrain, thalamus, and neocortex

The neuromodulator histamine plays an important role in the regulation of behavioural state and the neocortical electrocorticogram (ECoG). With the present experiments, we characterized the anatomical targets that mediate the cortical-activating effects of histamine. Urethane-anaesthetized rats displayed continuous large-amplitude, low-frequency oscillations with a maximal spectral power in the delta (0.5-3.9 Hz) frequency band. Electrical (100 Hz) stimulation of the pontine-tegmentum suppressed slow, large-amplitude oscillations and induced ECoG activation. Application of histamine (1 mm) into the basal forebrain cholinergic complex by reverse microdialysis enhanced ECoG activation elicited by tegmental stimulation without changing resting ECoG activity. Ventrolateral or central thalamic application of histamine had no effect on resting ECoG activity, and ventrolateral thalamic application produced only a slight enhancement of brainstem-induced activation. Neocortical application of histamine in close proximity (< 500 micro m) to the recording electrode reduced low-frequency delta power in the resting ECoG without affecting stimulation-induced ECoG activation. These data suggest that, under the present experimental conditions, histamine facilitates ECoG activation primarily by potentiating the excitatory influence of brainstem fibers at the level of the basal forebrain. Histamine release in some parts of the thalamus results in a minor enhancement of ECoG activation, and cortical histamine release produces a small but consistent suppression of slow delta oscillations in the resting ECoG. These concurrent subcortical and cortical actions probably permit histamine to effectively modulate cortical activation and excitability across different behavioural states.

Superior colliculus stimulation enhances neocortical serotonin release and electrocorticographic activation in the urethane-anesthetized rat

Recent evidence indicates that the superior colliculus (SC), in addition to its functions in sensory detection, also participates in controlling the generalized activation state of the forebrain, as measured by the electroencephalogram (EEG) or electrocorticogram (ECoG). The mechanisms by which the SC modulates forebrain activation are not well understood. By using in vivo microdialysis, we examined the role of serotonin release as a mechanism by which the SC can control neocortical activity in the urethane-anesthetized rat. Electrical 100 Hz stimulation of the SC increased frontal cortex serotonin output to 116, 118, and 140% of baseline levels for stimulation intensities of 0.5, 0.75, and 1.0 mA, respectively. Further, 75% of extracellularly recorded single (putative serotonergic) dorsal raphe neurons increased their discharge rate in response to 100 Hz stimulation of the SC. Stimulation of the SC also suppressed frontal cortex low frequency (1-6 Hz) synchronized ECoG activity, replacing it with high-frequency desynchronization. This activation response was resistant to cholinergic-muscarinic receptor antagonists (atropine, 50 mg/kg; scopolamine, 2 mg/kg), but was reduced or abolished by systemic treatment with the serotonergic receptor antagonists ketanserin (10 mg/kg) or methiothepin (5 mg/kg). These data suggest that efferents from the SC, possibly by an excitatory action on serotonergic dorsal raphe cells, produce an enhanced release of serotonin and ECoG activation in the neocortex. The stimulation of cortical serotonin output may constitute a mechanism by which the SC acts on the forebrain to increase cortical excitability in response to sensory stimuli processed by SC neurons.

Serotonergic receptor antagonists alter responses to general anaesthetics in rats

Serotonergic neurotransmission is involved in controlling arousal levels in humans and other animals. Here, the effects of serotonergic receptor antagonists on the induction and depth of anaesthesia produced by three different general anaesthetics were investigated. Rats were pretreated (i.p.) with either methiothepin (1.5 mg kg(-1)), mianserin (5 mg kg(-1)), ketanserin (7 mg kg(-1)) or saline. Subsequently, successive, cumulative doses (i.p.) of either ketamine (final, cumulative dose of 350 mg kg(-1)), sodium pentobarbital (final dose 77 mg kg(-1)), or chloral hydrate (final dose 600 mg kg(-1)) were administered. The response to the anaesthetics was measured using a behavioural test battery assessing nocifensive reflexes and hypnotic state. Pre-treatment with methiothepin enhanced responses to all three anaesthetics; mianserin enhanced responses to chloral hydrate. These results show that some serotonergic receptor antagonists change anaesthetic requirements, resulting in enhanced anaesthesia to hypnotics with different mechanisms of action.

Anesthetics eliminate somatosensory-evoked discharges of neurons in the somatotopically organized sensorimotor striatum of the rat

The somatotopic organization of the lateral striatum has been demonstrated by anatomical studies of corticostriatal projections from somatosensory and motor cortices and by single-cell recordings in awake animals. The functional organization in the rat, characterized thus far in the freely moving rat preparation, could be mapped more precisely if a stereotaxic, and possibly an anesthetized, preparation could be used. Because striatal discharges evoked by innocuous somatosensory stimulation are used in mapping, this study tested whether such discharges can be observed during anesthesia, encouraged by responsiveness during anesthesia in somatosensory cortical layers projecting to the striatum. Electrode tracks through lateral striatum of anesthetized rats (pentobarbital or ketamine) revealed spontaneously discharging neurons but no discharges evoked by somatosensory examination (passive manipulation and cutaneous stimulation of 14 body parts). Similar tracks in chronically implanted rats showed evoked firing at numerous sites during wakefulness but not during anesthesia (pentobarbital or urethane). Comparisons of the activity of individual neurons between wakefulness and anesthesia showed that pentobarbital, ketamine, chloral hydrate, urethane, or metofane eliminated evoked firing and suppressed spontaneous firing. Recovery time was greater for neural than for behavioral measures. Thus, mapping as proposed is ruled out, and more importantly, the data show that somatotopically organized lateral striatal neurons stop discharging in response to natural stimulation during anesthesia. Available data indicate they do not reach threshold in response to depolarizations produced by glutamatergic corticostriatal synaptic transmission projected from the somatosensory cortex. These data and demonstrations of anesthetic-induced imbalances in most striatal neurotransmitters emphasize that many results regarding striatal physiology and pharmacology during anesthesia cannot be extrapolated to behavioral conditions, thus indicating the need for more empirical testing in conscious animals.

Regional differences in the effect of the combined treatment of WAY 100635 and fluoxetine: an in vivo microdialysis study

We studied the changes in extracellular serotonin (5-HT) levels in the frontal cortex (FC) and ventral hippocampus (vHi) in conscious rats, induced by the combined administration of a highly selective 5-HT1A receptor antagonist, WAY 100635 (0.1 mg/kg, i.v.), and fluoxetine (1 mg/kg, i.p.), a selective 5-HT reuptake inhibitor (SSRI). In the two brain areas studied, no change in extracellular 5-HT concentrations was observed following fluoxetine administration over the 210 min post-injection period. However, in animals co-administered with [WAY 100635 + fluoxetine], the maximal increase in 5-HT levels in the FC was to 215% of the respective basal value (100%), while no significant change in 5-HT was observed in dialysates from the vHi. Furthermore, the [norfluoxetine]-to-[fluoxetine] ratio in the FC was significantly higher than in the hippocampus as measured in homogenates of animals treated with either fluoxetine alone or a prior administration of WAY 100635. Thus, WAY 100635 made the fluoxetine short-lasting effect apparent in the FC, but not by interfering with pharmacokinetic parameters of fluoxetine. Taken together, our data suggest the possibility, that either 5-HT1A autoreceptor sensitivity or uptake carrier density or higher [metabolite]-to-[parent drug] ratios in the FC than in the hippocampus may be involved in regional specific responses to SSRIs.

5-Hydroxytryptamine (5-HT) agonists: effects on neocortical slow wave activity after combined muscarinic and serotonergic blockade

In freely-moving rats treated with a combination of reserpine (10 mg/kg, i.p.) and scopolamine (5 mg/kg, i.p.), neocortical low voltage fast activity (LVFA) associated with continuous multiunit activity (MUA) was abolished and replaced by 2-6 Hz large irregular slow activity (LISA) above 1.5 mV associated with a burst-suppression pattern of MUA. Administration of the monoamine oxidase inhibitor pargyline (50-100 mg/kg, i.p.) completely suppressed 2-6 Hz LISA and restored normal-appearing LVFA and continuous MUA. The 5-hydroxytryptamine (5-HT) receptor agonists quipazine (0.5-20 mg/kg, i.p.), (+/-)-DOI (0.1-5 mg/kg, s.c.), and buspirone (0.1-10 mg/kg, i.p.), but not 8-hydroxy-2-(di-n-propylamine) tetraline (8-OH-DPAT, 0.05-0.8 mg/kg, s.c.) and RU 24969 (1-30 mg/kg, i.p.), produced a partial suppression of 2-6 Hz LISA and restored some lower voltage activity (< 1 mV) above 6 Hz associated with continuous MUA. However, as opposed to pargyline, no receptor agonist tested restored continuous, normal-appearing LVFA. Even though agonists at 5-HT receptors can produce some activation of neocortical slow wave activity after combined cholinergic and serotonergic blockade, this effect is not equivalent to that observed after restoration of endogenous 5-HT transmission.

Anti-serotonergic effects of urethane and chloral hydrate may not be mediated by a blockade of 5-HT2 receptors. Short communication

The general anesthetics urethane and chloral hydrate have profound anti-serotonergic effects both in the rat cortex in vivo and the rat aortic ring in vitro. The suggestion that these effects may be due to an action on 5-HT2 receptors was tested using ex vivo and in vitro [3H]ketanserin binding assays with membrane-enriched fractions from rat brain. Urethane did not alter [3H]ketanserin binding in the ex vivo assay. In the in vitro assay, urethane, chloral hydrate, and its active metabolite 2,2,2-trichloroethanol produced slight reductions (of 16%, 9%, and 18%, respectively) of [3H]ketanserin binding. These studies suggest that anti-serotonergic effects of urethane and chloral hydrate may not be mediated by a blockade of 5-HT2 receptors.

Urethane reduces contraction to 5-hydroxytryptamine (5-HT) and enhances the action of the 5-HT antagonist ketanserin on the rat thoracic aortic ring

The general anesthetic urethane (ethyl carbamate) is widely used in electrophysiological in vivo experiments. However, its pharmacological effects are poorly understood. Here, the effects of urethane on in vitro contractile responses of the rat thoracic aortic ring preparation were investigated. Bath application of 5-HT produced a concentration-dependent contractile response (EC50 = 4.3 x 10(-6) M). Urethane (11.2 mM = 1 mg/ml) shifted the concentration-response curve (CRC) for 5-HT to the right (EC50 = 1.7 x 10(-5) M) and decreased the maximal contraction by 30.8%. The CRC for NA (EC50 = 7.2 X 10(-9)M) was also shifted to the right by urethane (EC50 = 1.4 X 10(-8)M), but the shift of the 5-HT-CRC was twice that of the NA-CRC (3.95 vs. 1.95). The CRC to KCl was shifted rightwards only slightly by urethane (ratio 1.27) and the maximal contraction to KCl was not affected. The CRC to replacement of CaCl2 (0.1-10 mM) to KCl-depolarized vessels in a Ca(2+)-free Krebs solution was unaffected by urethane. Ketanserin (10(-9)M) antagonized the contraction to 5-HT, and a combination of ketanserin and urethane was markedly more effective than either drug alone, decreasing the maximal contraction by 58%. Antagonism of NA contraction by prazosin (5 X 10(-8)M) was not increased by addition of urethane. The urethane dose used here approximates blood and brain concentrations required to produce anesthetic effects in mammals. It is possible that reductions in 5-HT transmission and, to a lesser extent, in NA transmission, but not blockade of Ca2+ or K+ channels, may contribute to the anesthetic effect of urethane. In addition, the action of the selective 5-HT2 antagonist ketanserin is clearly altered by urethane. These findings are important to consider when urethane is used for in vivo neurophysiological investigations, particularly when 5-HT mechanisms are involved.