Chloral hydrate anesthesia alters the responsiveness of central serotonergic neurons in the cat

Ventral hippocampus is more sensitive to fluoxetine-induced changes in extracellular 5-HT concentration, membrane 5-HT transporter level and immobility times

Hippocampal responses to selective 5-HT reuptake inhibitor (SSRI) have long been studied. However, its sub-regional involvements in mediating SSRI's pharmacological effects have not been fully addressed. The current study sought to investigate neurochemical, neurobiological and neurobehavioral changes in response to direct fluoxetine perfusion into the ventral and dorsal sub-regions of the hippocampus in C57BL/6 mice. Following fluoxetine perfusion, time courses of dialysate 5-HT, 5-HT transporter (5-HTT) protein (total, membrane and cytoplasmic fractions), locomotion, and immobility times in the forced swim test (FST) and tail suspension test (TST) were determined. At baseline, 5-HT uptake efficiency assessed by the no-net-flux microdialysis, and 5-HTT protein were measured as well. Results show that fluoxetine dose-dependently increased dialysate 5-HT, lowered membrane 5-HTT protein and increased cytoplasmic fraction without changing the total level, decreased immobility times in both the FST and TST, with greater responses all detected in the ventral sub-region compared to the dorsal sub-region. Fluoxetine didn't affect locomotor activity, ruling out the possibility that fluoxetine's effects on immobility maybe due to alteration in locomotion. Besides, lower 5-HT uptake efficiency and lower membrane 5-HTT protein level were found in the ventral sub-region at baseline. Together, the sub-regional differences at baseline and in responses to fluoxetine added powerful evidence to support the existence of two distinct 5-HT sub-systems in the hippocampus, with greater changes to fluoxetine detected in the ventral sub-system.

5-HT1A Receptor-Mediated Autoinhibition and the Control of Serotonergic Cell Firing

The idea that serotonergic synaptic transmission plays an essential role in the control of mood and the pharmacotherapy of anxiety and depression is one of the cornerstones of modern biological psychiatry. As a result, there is intense interest in understanding the mechanisms controlling the activity of serotonin-synthesizing (serotonergic) neurons. One of the oldest and most durable ideas emerging from this work is that serotonergic neurons are capable of autonomously regulating their own basal firing rate. Serotonergic neurons express on their surface 5-HT1A receptors (autoreceptors) that, when activated, induce the opening of potassium channels that hyperpolarize and thereby inhibit cell firing. Activity-dependent release of serotonin within serotonergic nuclei is thought to activate these autoreceptors, thus completing an autoinhibitory feedback loop. This concept, which was originally proposed in the 1970s, has proven to be enormously fruitful and has guided the interpretation of a broad range of clinical and preclinical work. Yet, remarkably, electrophysiological studies seeking to directly demonstrate this phenomenon, especially in in vitro brain slices, have produced mixed results. Here, we critically review this work with a focus on electrophysiological studies, which directly assess neuronal activity. We also highlight recent work suggesting that 5-HT1A receptor-mediated autoinhibition may play other roles in the control of firing besides acting as a feedback regulator for the pacemaker-like firing rate of serotonergic neurons.

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.

Arterial chemoreceptor activation reduces the activity of parapyramidal serotonergic neurons in rats

The parapyramidal (ppy) region targets primarily the intermediolateral cell column and is probably involved in breathing and thermoregulation. In the present study, we tested whether ppy serotonergic neurons respond to activation of central and peripheral chemoreceptors. Bulbospinal ppy neurons (n=30) were recorded extracellularly along with the phrenic nerve activity in urethane/α-chloralose-anesthetized, paralyzed, intact (n=7) or carotid body denervated (n=6) male Wistar rats. In intact animals, most of the ppy neurons were inhibited by hypoxia (n=14 of 19) (8% O2, 30s) (1.5 ± 0.03 vs. control: 2.4 ± 0.2 Hz) or hypercapnia (n=15 of 19) (10% CO2) (1.7 ± 0.1 vs. control: 2.2 ± 0.2 Hz), although some neurons were insensitive to hypoxia (n=3 of 19) or hypercapnia (n=4 of 19). Very few neurons (n=2 of 19) were activated after hypoxia, but not after hypercapnia. In carotid body denervated rats, all the 5HT-ppy neurons (n=11) were insensitive to hypercapnia (2.1 ± 0.1 vs. control: 2.3 ± 0.09 Hz). Biotinamide-labeled cells that were recovered after histochemistry were located in the ppy region. Most labeled cells (90%) showed strong tryptophan hydroxylase immunocytochemical reactivity, indicating that they were serotonergic. The present data reveal that peripheral chemoreceptors reduce the activity of the serotonergic premotor neurons located in the ppy region. It is plausible that the serotonergic neurons of the ppy region could conceivably regulate breathing automaticity and be involved in autonomic regulation.

Differential effects of cocaine on dopamine neuron firing in awake and anesthetized rats

Cocaine (benzoylmethylecgonine), a natural alkaloid, is a powerful psychostimulant and a highly addictive drug. Unfortunately, the relationships between its behavioral and electrophysiological effects are not clear. We investigated the effects of cocaine on the firing of midbrain dopaminergic (DA) neurons, both in anesthetized and awake rats, using pre-implanted multielectrode arrays and a recently developed telemetric recording system. In anesthetized animals, cocaine (10 mg/kg, intraperitoneally) produced a general decrease of the firing rate and bursting of DA neurons, sometimes preceded by a transient increase in both parameters, as previously reported by others. In awake rats, however, injection of cocaine led to a very different pattern of changes in firing. A decrease in firing rate and bursting was observed in only 14% of DA neurons. Most of the other DA neurons underwent increases in firing rate and bursting: these changes were correlated with locomotor activity in 52% of the neurons, but were uncorrelated in 29% of them. Drug concentration measurements indicated that the observed differences between the two conditions did not have a pharmacokinetic origin. Taken together, our results demonstrate that cocaine injection differentially affects the electrical activity of DA neurons in awake and anesthetized states. The observed increases in neuronal activity may in part reflect the cocaine-induced synaptic potentiation found ex vivo in these neurons. Our observations also show that electrophysiological recordings in awake animals can uncover drug effects, which are masked by general anesthesia.

A combined in vivo neurochemical and electrophysiological analysis of the effect of high-frequency stimulation of the subthalamic nucleus on 5-HT transmission

Movement disability in advanced Parkinson's disease (PD) can be treated by high frequency stimulation (HFS) of the subthalamic nucleus (STN) but some patients experience psychiatric side-effects including depression, which is strongly linked to decreases in 5-hydroxytryptamine (5-HT). The current study investigated the effect of bilateral STN HFS on extracellular 5-HT in brain regions of anesthetized and freely moving rats as measured with microdialysis. Parallel in vivo electrophysiological experiments allowed a correlation of changes in extracellular 5-HT with the firing of 5-HT neurons. Bilateral STN HFS decreased (by up to 25%) extracellular levels of 5-HT in both striatum and medial prefrontal cortex of anesthetized rats. STN HFS also decreased extracellular 5-HT in the medial prefrontal cortex of freely moving rats. This decrease in extracellular 5-HT persisted after turning off the stimulation, and was present in dopamine-denervated rats. As with changes in extracellular 5-HT, in anesthetized rats STN HFS evoked a decrease in the in vivo firing of midbrain raphe 5-HT neurons that also persisted after cessation of stimulation. These data provide neurochemical evidence for an inhibition of 5-HT neurotransmission by STN HFS, which may contribute to its psychiatric side effects and guide therapeutic options.

Serotonin in the inferior colliculus fluctuates with behavioral state and environmental stimuli

Neuromodulation by serotonin (5-HT) could link behavioral state and environmental events with sensory processing. Within the auditory system, the presence of 5-HT alters the activity of neurons in the inferior colliculus (IC), but the conditions that influence 5-HT neurotransmission in this region of the brain are unknown. We used in vivo voltammetry to measure extracellular 5-HT in the IC of behaving mice to address this issue. Extracellular 5-HT increased with the recovery from anesthesia, suggesting that the neuromodulation of auditory processing is correlated with the level of behavioral arousal. Awake mice were further exposed to auditory (broadband noise), visual (light) or olfactory (2,5-dihydro-2,4,5-trimethylthiazoline, TMT) stimuli, presented with food or confined in a small arena. Only the auditory stimulus or restricted movement increased the concentration of extracellular 5-HT in the IC. Changes occurred within minutes of stimulus onset, with the auditory stimulus increasing extracellular 5-HT by an average of 5% and restricted movement increasing it by an average of 14%. These findings suggest that the neuromodulation of auditory processing by 5-HT is a dynamic process that is dependent on internal state and behavioral conditions.

Evolving concepts of sleep cycle generation: From brain centers to neuronal populations

As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

The influence of sex and ovarian hormones on temporomandibular joint nociception in rats

The aim of this study was to investigate the influence of sex and ovarian hormones on formalin- and glutamate-induced temporomandibular joint (TMJ) nociception in rats. The influence of sex and ovarian hormones on the nociceptive behavior induced by formalin or glutamate was virtually the same. The nociceptive behavior of males was similar to that of females in the proestrus phase of the estrous cycle but was significantly lower than that in the diestrus phase. Since the serum level of estradiol but not of progesterone was significantly higher in the proestrus than in the diestrus phase, these data suggest that females with lower endogenous serum level of estradiol have an exacerbation of TMJ nociception. The nociceptive behavior of ovariectomized rats was similar to that of diestrus females and significantly greater than that of proestrus females. Although the administration of estradiol or progesterone in ovariectomized females significantly reduced TMJ nociception, the combination of both hormones did not increase the antinociceptive effect induced by each of them. These findings suggest that estradiol and progesterone decrease TMJ nociception in an independent way.

PERSPECTIVE

We report that ovarian hormones have an antinociceptive effect on the TMJ formalin and glutamate nociceptive behavior models. Therefore, the greater prevalence and severity of TMJ pain in women of reproductive age may be a consequence of hormonal fluctuation during the reproductive cycle, in that during low endogenous estradiol serum level TMJ pain sensitivity is increased, enhancing the risk of females experiencing TMJ pain.

Effects of 8-OH-DPAT on hippocampal NADH fluorescence in vivo in anaesthetized rats

Systemic administration of the 5-HT1A receptor agonist 8-OH-DPAT modifies 5-HT neuronal transmission via stimulation of presynaptic and postsynaptic receptors. Compared to the effects of presynaptic receptor stimulation, there are less data on the effects of postsynaptic 5-HT1A receptors and the net effects of a stimulation of pre- and postsynaptic 5-HT1A receptors available. We measured the neuronal activity in the rat hippocampus after systemic treatment with 8-OH-DPAT in doses (30-300 microg/kg) known to reduce 5-HT release and anxiety-like behavior in rodents. Neuronal activity was assessed by laser-induced fluorescence spectroscopy determining changes in nicotinamide adenine dinucleotide (NADH) fluorescence in the ventral hippocampus of anaesthetized rats in vivo. NADH, a co-substrate for energy transfer in the respiratory chain, mirrors mitochondrial activity. Increased NADH fluorescence signals lower consumption of NADH caused by neuronal inhibition. 8-OH-DPAT in a dose of 300 microg/kg, but not 100 microg/kg and 30 microg/kg, increased NADH fluorescence by maximal +27 +/- 3.5%, suggesting a decreased neuronal activity in the ventral hippocampus. The selective 5-HT1A antagonist WAY-100635 (3 mg/kg) prevented the increased NADH fluorescence after 8-OH-DPAT, but had no own effect. The results show that systemic administration of the 5-HT1A agonist 8-OH-DPAT dose-dependently affects neuronal activity in the ventral hippocampus. The dose of 300 microg/kg seemingly activates presynaptic and postsynaptic receptors with dominating inhibitory postsynaptic effects.

Moderate differences in circulating corticosterone alter receptor-mediated regulation of 5-hydroxytryptamine neuronal activity

Circulating glucocorticoid levels vary with stress and psychiatric illness and play a potentially important role in regulating transmitter systems that regulate mood. To determine whether chronic variation in corticosterone levels within the normal diurnal range altered the control of 5-hydroxytryptamine (5-HT) neuronal activity, male rats were adrenalectomized and implanted with either a 2% or 70% corticosterone/cholesterol pellet (100 mg). Two weeks later, the regulation of 5-HT neuronal activity in the dorsal raphe nucleus was studied by in vitro electrophysiology. At this time, serum corticosterone levels approximated the low-point (2%) and mid-point (70%) of the diurnal range. The excitatory response of 5-HT neurones to the alpha1-adrenoceptor agonist phenylephrine (1-11 microM) was significantly greater in the 2% group compared to the 70% group. By contrast, the inhibitory response to 5-HT (10-50 microM) was significantly lower in the 2% group compared to the 70% group. Thus, chronic variation in circulating corticosterone over a narrow part of the normal diurnal range causes a shift in the balance of positive and negative regulation of 5-HT neurones, with increased alpha 1-adrenoceptor-mediated excitation and reduced 5-HT-mediated autoinhibition at lower corticosterone levels. This shift would have a major impact on control of 5-HT neuronal activity.

Regulation of the release of serotonin in the dorsal raphe nucleus by alpha1 and alpha2 adrenoceptors

To investigate the modulation of serotonin release in the dorsal raphe nucleus (DRN) by alpha(1) and alpha(2) adrenoceptors, dual-probe microdialysis was performed in conscious rats. The specific alpha(1) and alpha(2) adrenoceptor agonists and antagonists were locally infused into the DRN via retrograde microdialysis. The release of serotonin was simultaneously sampled from the DRN and prefrontal cortex (PFC). Infusion of the alpha(1) adrenoceptor agonist cirazoline into the DRN (100 microM) produced an increase in the release of serotonin in the DRN to 200% of the basal levels, but no effect was seen in the PFC. After infusion of the alpha(1) adrenoceptor antagonist prazosin into the DRN (100 microM) the release of serotonin decreased in the DRN and PFC to about 40% and 65% of the basal levels, respectively. Infusion of the alpha(2) adrenoceptor agonist clonidine into the DRN (100 microM) slightly but significantly decreased the level of serotonin in the DRN as well as in the PFC to about 70% of the basal levels. Infusion of the alpha(2A) adrenoceptor antagonist BRL 44408 into the DRN (100 microM) caused an increase of serotonin release in the DRN to 270% of the basal levels, but at the same time no changes were seen in the extracellular levels of serotonin in the PFC. The present study demonstrates that alpha(1) as well as alpha(2) adrenoceptors in the DRN modulate the release of serotonin in the DRN, and that alpha(1) adrenoceptors in the DRN are maximally stimulated during resting conditions.

Highly H+-sensitive neurons in the caudal ventrolateral medulla of the rat

The ventral surface of the caudal ventrolateral medulla (cVLM) has been shown to generate intense respiratory responses after surface acid-base stimulation. With respect to their chemosensitive characteristics, cVLM neurons have been less studied than other rostral-most regions of the brainstem. The purpose of these experiments was to determine the bioelectric responses of cVLM neurons to acidic stimuli and to determine their chemosensitive properties. Using extracellular and microiontophoretic techniques, we recorded electrical activities from 117 neurons in an area close to the ventral surface of the cVLM in anaesthetised rats. All neurons were tested for their sensitivity to H+. The fluorescent probe BCECF was used to measure extracellular pH changes produced by the microiontophoretic injection of H+ in brainstem slices. This procedure provided an estimation of the local changes in pH produced by microiontophoretic H+ application in the anaesthetised rat. Neurons coupled to the respiratory cycle, R (n = 51), were not responsive to direct stimulation with H+. Sixty-six neurons that did respond to H+ stimulation were uncoupled from respiration, and identified as NR neurons. These neurons presented distinct ranges of H+ sensitivity. The neuronal sensitivity to H+ was mainly assessed by the slope of the stimulus-response curve, where the steeper the slope, the higher the H+ sensitivity. On this basis, NR neurons were classed as being either weakly or highly sensitive to H+. NR neurons with a high H+ sensitivity (n = 12) showed an average value of 34.17 +/- 7.44 spikes s-1 (100 nC)-1 (mean +/- S.D.) for maximal slope and an EC50 of 126.76 +/- 33 nC. Suprathreshold H+ stimulation of highly sensitive NR neurons elicited bursting pattern responses coupled to the respiratory cycle. The bursting responses, which were synchronised with the inspiratory phase and the early expiratory phase of the respiratory cycle, lasted for several seconds before returning to the steady state firing pattern characteristic of the pre-stimulus condition. These NR neurons, which possess the capacity to detect distinct H+ concentrations in the extracellular microenvironment, are excellent candidates to serve in a chemoreceptor capacity in the caudal medulla.

5-HT1A receptor agonist 8-OH-DPAT acts in the hindbrain to reverse the sympatholytic response to severe hemorrhage

Central administration of serotonergic 5-HT1A receptor agonists delays the reflex sympatholytic response to severe hemorrhage in conscious rats. To determine the region where 5-HT1A receptor agonists act to mediate this response, recovery of mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) was compared in hemorrhaged rats after injection of the selective 5-HT1A agonist, (+)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), in various regions of the cerebroventricular system or the systemic circulation. Three minutes after injection of 8-OH-DPAT (48 nmol/kg), MAP and RSNA were higher in hemorrhaged rats given drug in the fourth ventricle (94 +/- 5 mmHg, 82 +/- 18% of baseline) or the systemic circulation (90 +/- 4 mmHg, 113 +/- 15% of baseline) than in rats given drug in the Aqueduct of Sylvius (63 +/- 4 mmHg, 27 +/- 11% of baseline), the lateral ventricle (42 +/- 3 mmHg, -8 +/- 18% of baseline), or in rats given saline in various brain regions (47 +/- 5 mmHg, -42 +/- 10% of baseline). A lower-dose injection of 8-OH-DPAT (10 nmol/kg) also accelerated the recovery of MAP and RSNA in hemorrhaged rats when given in the fourth ventricle (94 +/- 26 mmHg, 72 +/- 33% of baseline 3 min after injection) but not the systemic circulation (46 +/- 4 mmHg, -25 +/- 30% of baseline). These data indicate that 8-OH-DPAT acts on receptors in the hindbrain to reverse the sympatholytic response to hemorrhage in conscious rats.

Firing modes of midbrain dopamine cells in the freely moving rat

There is a large body of data on the firing properties of dopamine cells in anaesthetised rats or rat brain slices. However, the extent to which these data relate to more natural conditions is uncertain, as there is little quantitative information available on the firing properties of these cells in freely moving rats. We examined this by recording from the midbrain dopamine cell fields using chronically implanted microwire electrodes. (1) In most cases, slowly firing cells with broad action potentials were profoundly inhibited by the dopamine agonist apomorphine, consistent with previously accepted criteria. However, a small group of cells was found that were difficult to classify because of ambiguous combinations of properties. (2) Presumed dopamine cells could be divided into low and high bursting (>40% of their spikes in bursts) groups, with the majority having low bursting rates. The distribution of burst incidence was similar to that previously reported with chloral hydrate anaesthesia, but the average intraburst frequency was higher in the conscious animal at rest and was higher again in bursts triggered by salient stimuli. (3) There was no evidence for spike frequency adaptation within bursts on average, consistent with the hypothesis that afterhyperpolarisation currents may be disabled during behaviourally induced bursting. (4) Presumed dopamine cells responded to reward-related stimuli with increased bursting rates and significantly higher intraburst frequencies compared to bursts emitted outside task context, indicating that modulation of afferent activity might not only trigger bursting, but may also regulate burst intensity. (5) In addition to the irregular single spike and bursting modes we found that extremely regular (clock-like) firing, previously only described for dopamine cells in reduced preparations, can also be expressed in the freely moving animal. (6) Cross-correlation analysis of activity recorded from simultaneously recorded neurones revealed coordinated activity in a quarter of dopamine cell pairs consistent with at least "functional" connectivity. On the other hand, most dopamine cell pairs showed no correlation, leaving open the possibility of functional sub-groupings within the dopamine cell fields. Taken together, the data suggest that the basic firing modes described for dopamine cells in reduced or anaesthetised preparations do reflect natural patterns of activity for these neurones, but also that the details of this activity are dependent upon modulation of afferent inputs by behavioural stimuli.

Effects of ethanol on the dorsal raphe nucleus and its projections to the caudate putamen

The objective of this study was to examine the effects of intraperitoneal injection of ethanol on the activity of the dorsal raphe nucleus (DRN) serotonin (5-hydroxytryptamine [5-HT]) system and its projections to the rostral caudate putamen (CPu) and determine whether rapid tolerance to the effects of ethanol develops in this system. Adult, male, Wistar rats were used in these experiments. In experiment 1, a microdialysis procedure was used to determine (a) the effects of acute intraperitoneal administration of ethanol (1.75 and 2.5 g/kg) on the extracellular levels of 5-HT in the rostral CPu and (b) whether rapid tolerance develops to these effects. In experiment 2, firing rates of 5-HT neurons were determined in the DRN after intraperitoneal administration of 2.5 g/kg of ethanol. The results of the microdialysis experiments indicated that the 2.5-g/kg dose significantly (P < .005) increased the extracellular levels of 5-HT to 150%-160% of baseline. Compared with findings for rats pretreated with saline 24 h earlier, prior treatment 24 h earlier with 2.5 g/kg of ethanol had no effect on the extracellular levels of 5-HT produced by a challenge dose of 2.5 g/kg of ethanol. Contrary to the effects in the CPu, intraperitoneal administration of 2.5 g/kg of ethanol significantly (P<.005) decreased the firing rates of 5-HT neurons in the DRN to approximately 50% of control. Overall, the results suggest to us that there is a dissociation between the effects of acute administration of ethanol on 5-HT cell body neuronal activity and 5-HT synaptic activity. The higher extracellular levels of 5-HT in the CPu may be due to increased release of 5-HT from a direct or an indirect action of ethanol, a result of inhibiting 5-HT reuptake, or related to both of these mechanisms. In addition, the findings suggest to us that rapid tolerance did not develop to the effects of ethanol on the 5-HT system within the CPu.

Serotonergic Raphe magnus cells that respond to noxious tail heat are not ON or OFF cells

Pharmacological studies have suggested that serotonergic cells in RM contribute to both the inhibition and facilitation of spinal nociceptive transmission. Physiological studies in the medullary nucleus raphe magnus (RM) and adjacent nucleus reticularis magnocellularis have identified putative nociceptive-inhibitory OFF cells and nociceptive-facilitatory neurons ON cells by their responses to noxious thermal stimulation. The present study was designed to determine 1) whether any serotonergic RM cells respond to noxious thermal stimulation and 2) whether noxious heat-responsive serotonergic cells should be classified as ON or OFF cells. Serotonergic cells (n = 150) were identified by physiological criteria in anesthetized rats; 30 of 32 cells tested contained serotonin immunoreactivity. Noxious tail heat elicited a neuronal response in less than a quarter of the serotonergic cells. Most serotonergic cells that responded to tail heat were excited (n = 25), while a small minority of the cells tested were inhibited (n = 8). The tail heat-evoked responses of serotonergic cells were small in magnitude, averaging five to eight spikes in 10 s. Excitatory responses rarely persisted for more than 10 s, while inhibitory responses rarely persisted for more than 20 s. The tail heat-evoked responses of serotonergic cells were compared to those of non-serotonergic cells (n = 186). Non-serotonergic cells that responded to noxious tail heat had significantly greater response magnitudes, averaging 75-95 spikes in 10 s, than heat-responsive serotonergic cells. In addition, most heat-responsive non-serotonergic cells responded for at least 30 s after stimulus onset. These results demonstrate that the tail heat-evoked responses of serotonergic RM cells are qualitatively and quantitatively distinct from those of non-serotonergic ON and OFF cells. It is therefore unlikely that serotonergic RM cells, even the subpopulation that responds to noxious tail heat, share a physiological function with ON and OFF cells.

Single-unit and polygraphic recordings associated with systemic or local pharmacology: a multi-purpose stereotaxic approach for the awake, anaesthetic-free, and head-restrained rat

In order to avoid any artifactual pharmacological interferences with anaesthetic agents, a procedure has been developed for working on the awake, anaesthetic-free rat in a head-restrained condition. It allows, on the same animal and over several consecutive days, single-unit recordings in combination with systemic or local pharmacology (microiontophoresis or micropressure ejections), as well as monitoring vigilance states via the electroencephalogram and the electromyogram. After the cementing of a special "U"-shaped device on its skull under general anaesthesia, the animal is progressively habituated to stay daily, for several hours, under a painless corresponding stereotaxic restraint. This system can be easily adapted to different stereotaxic frames and, because of its spatial flexibility for targetting the desired rostrocaudal or lateral positions, allows access to a large number of cerebral structures. Experiments performed on Globus Pallidus, Substantia Nigra, and Locus Coeruleus neurons, combining the different possibilities of this system, are reported. They demonstrate, on the awake anaesthetic-free head-restrained rat, and under suitable ethical conditions, the feasibility of single-unit recordings of identified neurons associated with the study of their pharmacological reactivity after systemic or local drug administrations without any other drug interferences, and in physiologically relevant conditions such as the spontaneous alternance of vigilance states.

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.

Physiological identification of pontomedullary serotonergic neurons in the rat

Spinal serotonin is derived entirely from bulbar sources and plays an important role in spinal modulatory processes, including pain modulation. Establishing the electrophysiological properties of SEROTONERGIC bulbospinal neurons in the pontomedullary raphe and reticular formation is critical to understanding the physiological role of serotonin in the spinal cord. Neurons were characterized by their responses to noxious stimulation and their background discharge pattern in the lightly anesthetized rat. Characterized cells were intracellularly labeled with Neurobiotin, which was visualized with a Texas Red fluorophore. Sections containing the labeled cells were processed for serotonin immunocytochemistry with the use of a Bodipy fluorophore. Forty-seven intracellularly labeled cells were tested for serotonin immunoreactivity. The labeled neurons were located in raphe magnus, the nucleus reticularis magnocellularis, and the adjacent reticular and raphe nuclei at levels from the inferior olivary complex to the superior olivary complex. SEROTONERGIC cells were located in the raphe nuclei, in nucleus reticularis magnocellularis pars alpha, and in nucleus reticularis magnocellularis pars beta or nucleus reticularis gigantocellularis. Thirteen intracellularly labeled cells contained serotonin immunoreactivity. The background discharge rate of SEROTONERGIC cells average 1.8 Hz (range: 0.5-3.1 Hz). Discharge was steady and without sustained pauses or bursts in firing. Most serotonin-immunoreactive cells were unaffected or slightly excited by pinch and were unaffected by noxious heat. Three SEROTONERGIC cells were weakly excited by both noxious pinch and heat, whereas two SEROTONERGIC cells were briefly inhibited by these stimuli. Cells that lacked serotonin immunoreactivity were heterogeneous and included ON, OFF, and NEUTRAL cells. Nonserotonergic cells differed from SEROTONERGIC cells in having an irregular discharge pattern and/or a high mean discharge rate. A linear discriminant function, employing background discharge characteristics as independent variables, was calculated that successfully classified 13 of 13 SEROTONERGIC and 32 of 33 nonserotonergic neurons. The probability of misclassification with the use of this discriminant function was estimated to be < 10%. Employing the discriminant function on a test group of cells whose immunochemical content was unknown revealed a population of SEROTONERGIC-LIKE cells that resembled the labeled SEROTONERGIC cells in background discharge pattern, response to noxious stimulation, and nuclear location. The discharge of pontomedullary SEROTONERGIC neurons is slow and steady, suggesting that these neurons may have a role in the tonic, rather than phasic, modulation of spinal processes.

Local infusion of brain-derived neurotrophic factor modifies the firing pattern of dorsal raphé serotonergic neurons

Previous studies have reported a neuromodulatory effect of brain-derived neurotrophic factor (BDNF) on serotonin neurons in the central nervous system. In the present study, we examined the effects of local infusion of BDNF on the electrophysiological activity of serotonergic neurons in the rat dorsal raphé nucleus with extracellular single unit recording in vivo. Compared with vehicle-infused rats, chronic administration of BDNF (10-14 days) caused serotonergic neurons to fire in a significantly less regular pattern, without altering the mean firing rate or other measures of electrical activity. These results suggest that the ability of similar infusions of BDNF to produce behavioral effects (i.e. analgesia and an antidepressant-like effect) associated with elevated serotonin turnover may be in part the result of more irregular firing patterns of dorsal raphé neurons.

Effects of isoflurane concentration on the activity of pontomedullary raphe and medial reticular neurons in the rat

Neurons in the pontomedullary raphe magnus (RM) and adjacent nucleus reticularis paragigantocellularis pars alpha (NRPG alpha) are thought to participate in the modulation of spinal nociceptive transmission. In order to determine whether these cells also contribute to the suppression of nocifensive reflexes produced by general anesthetics, the spontaneous activity of RM/NRPG alpha cells was recorded in rats anesthetized with isoflurane (IF) at several steady state concentrations, corresponding to depths which are below, equal to, or above the threshold for blocking the motor response to noxious stimuli (minimum alveolar concentration, MAC). Neurons were classified by their spontaneous activity patterns and their responses to noxious stimulation as OFF, ON, REGULAR or NEUTRAL cells. After cell classification, unit activity, arterial blood pressure, heart rate, and EEG activity were simultaneously recorded, in the absence of somatic stimulation, for 1 h at each of two or three concentrations of IF. The concentrations tested were low (1.05-1.25%), medium (1.30-1.45%) and high (1.70-1.90%). ON, OFF and some NEUTRAL cells exhibited alternating periods of inactivity and activity when recorded during periods of low and medium anesthetic concentrations. At high steady state anesthetic concentrations, the mean discharge of most OFF, ON and NEUTRAL cells decreased by greater than 25% from their mean discharge rate at the low concentration. REGULAR cells maintained a uniform firing rate at all steady state anesthetic concentrations studied. Since high concentrations of IF do not activate OFF cells, the putative inhibitory output neuron of the RM/NRPG alpha, it is unlikely that the activity of RM/NRPG alpha neurons contributes to the suppression of nocifensive movement by the general anesthetic, IF.

Single-unit recordings at dorsal raphe nucleus in the awake-anesthetized rat: spontaneous activity and responses to cutaneous innocuous and noxious stimulations

In this study, we recorded the single-unit activity of the dorsal raphe nucleus (DRN) in rats tested first awake and, a few days later, anesthetized with sodium pentobarbital and recorded again. This was achieved by means of a small chronically implanted device supporting a 25 micron platinum-iridium wire as the recording electrode. In both the awake and anesthetized conditions, and in agreement with most of the studies performed at the DRN level, we found that a vast majority of the units, displaying small amplitude and long-duration action potentials, possessed a low level of spontaneous activity (0.2-4 Hz). Among these units, found in greater number under pentobarbital, it was possible to establish that this activity was regular or irregular, in accordance with the literature reports. However, as opposed to these studies, we determined that the 'regularity' was relative, only noticeable in more or less prolonged phases of activity. In particular, we never recorded the so-called 'clock-like' activity, largely reported as an unambiguous criterion for selecting the serotoninergic neurons. In both the awake and anesthetized conditions, the responses of the DRN neurons to peripheral mechanical innocuous and noxious stimulations were observed in only one-half of the units recorded and were weak in comparison to other results that we obtained at the nucleus raphe magnus level in previous studies. When present, these responses were excitation or inhibition, occurring during or after the stimulus application. These results question the direct involvement of the DRN in acute nociception.

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.

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

In urethane-anesthetized rats, neocortical LVFA induced by 100 Hz electrical stimulation of the median raphe area or by tail pinching was completely eliminated by a combination of scopolamine (5 mg/kg, IP) and p-chlorophenylalanine (500 mg/kg/day x 3, IP), providing evidence that LVFA is dependent on cholinergic-muscarinic and serotonergic inputs to the neocortex in urethane-anesthetized as well as in freely moving rats. The serotonergic receptor antagonists ketanserin and methiothepin (1-10 mg/kg, IP) also produced a dose-dependent blockade of LVFA in urethane-anesthetized rats, and eliminated virtually all LVFA when combined with scopolamine. A combination of diethyl ether anesthesia and scopolamine completely eliminated all neocortical LVFA without additional antiserotonergic treatment, and a combination of chloral hydrate anesthesia and scopolamine similarly blocked all LVFA in about 50% of the rats tested. In the remaining chloral hydrate-anesthetized rats, the residual LVFA could be eliminated by the serotonergic antagonist ritanserin (10 mg/kg, IP). As shown previously, in nonanesthetized rats treated with scopolamine, LVFA can be maintained by a serotonergic input to the neocortex. The present data suggest that some general anesthetics reduce or completely abolish this serotonergic LVFA. Further, the serotonergic antagonists used here exert much stronger antiserotonergic effects in rats anesthetized with urethane or chloral hydrate than in freely moving rats. Therefore, studies of serotonergic transmission or antagonist action, especially in the neocortex, in anesthetized rats may not be applicable to the waking state.

Biochemical evidence for the regulation of central noradrenergic activity by 5-HT1A and 5-HT2 receptors: microdialysis studies in the awake and anaesthetized rat

Here we have studied the effect of various 5-HT1A and 5-HT2 receptor-selective drugs on noradrenaline release in the hippocampus on anaesthetized and awake rats using microdialysis. In the anaesthetized rat, administration of the 5-HT1A agonists buspirone, gepirone and ipsapirone increased noradrenaline levels in the microdialysates. However, the common metabolite of these compounds, 1-PP (an alpha-2 adrenoceptor antagonist with low affinity for 5-HT1A receptors), also increased noradrenaline efflux whilst the 5-HT1A receptor agonist 8-OH-DPAT and MDL 73005EF, which are not metabolized to 1-PP, did not. In the awake rat, buspirone but also 8-OH-DPAT increased noradrenaline efflux. A similar effect was observed in response to MDL 73005EF and the 5-HT1A ligand NAN-190. Since the latter two drugs have weak intrinsic activity at the post-versus presynaptic 5-HT1A receptor, a presynaptic mechanism (inhibition of 5-HT release) was implicated. The 5-HT2 receptor may be important to this mechanism as noradrenaline increased following administration of the 5-HT2 receptor antagonists, ritanserin and ICI 170,809. In conclusion, our data indicate that there are clear differences in the effects of 5-HT1A and 5-HT2 receptor-selective drugs on noradrenaline efflux in hippocampus of the anaesthetized versus awake rat. Our findings are reconcilable with the hypothesis that in the awake (but not anaesthetized) rat, release of noradrenaline in hippocampus is influenced by an inhibitory tone mediated via 5-HT2 receptors. If this inhibitory tone is removed, either by decreasing 5-HT release through activation 5-HT1A autoreceptors or by blocking postsynaptic 5-HT2 receptors, noradrenaline release increases.

Effect of RU 24969 on 5-HT metabolism in the medullary dorsal horn as studied by in vivo voltammetry

The effect of i.p. administration of the preferential 5-HT1B agonist 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole succinate (RU 24969) (10 mg/kg) has been investigated by in vivo 5-hydroxyindole electrochemical (peak 3) detection in the medullary dorsal horn (MDH) of acute anesthetized and unanesthetized freely moving rats. RU 24969 induced a significant decrease in peak 3 in the MDH of anesthetized rats. In freely moving animals, RU 24969 induced a biphasic effect. Thus, after the injection the curve remained above that of the saline group and returned to control levels up to 60 min. Subsequently the curve decayed to below the control values and rapidly plateaued for up to 180 min. The initial increase and the decrease thereafter were both statistically significant vs. saline. With reference to similar in vivo studies demonstrating the responsiveness of ascending serotonergic systems to RU 24969, it is concluded that the 5-HT metabolism in the serotonergic NMR-dorsal horn system is affected by this 5-HT1B agonist. However, the biphasic effect reported here in unanesthetized animals suggests that RU 24969 could act by two different ways on 5-HT metabolism and indicates that there could be a primary interaction of RU 24969 on the 5-HT uptake system (inhibition) which could, at first, prevail over the interaction with terminal autoreceptors.

Serotonin-dependent cerebral activation: effects of methiothepin and other serotonergic antagonists

In scopolamine-treated (5.0 mg/kg, s.c.) rats hippocampal rhythmical slow activity (RSA) and neocortical low voltage fast activity (LVFA) occur only in close correlation with head movements, spontaneous changes in posture, or locomotion (Type I behavior). Previous work indicates that such scopolamine-resistant RSA and LVFA are dependent on ascending serotonergic projections. A test of 9 serotonergic antagonists (methiothepin; ritanserin; ketanserin; pizotifen; mianserin; pirenperone; ICS-205-930; metoclopramide; methysergide) showed that methiothepin produces a partial reduction in RSA and LVFA in scopolamine-treated rats, while the other antagonists are completely inactive over a wide range of doses. It may be that serotonergic cerebral activation depends on both 5-HT1 and 5-HT2 receptors.

Effects of raphe stimulation on hippocampal and neocortical activity and behaviour

In chronically prepared rats, electrical stimulation (100 Hz, 0.1 ms pulses) of the dorsal raphe nucleus, some sites in the median raphe nucleus, and adjoining regions of the midbrain produced locomotion accompanied by hippocampal rhythmical slow activity (RSA) and neocortical low voltage fast activity (LVFA). Both the behaviour and the cerebral waveforms persisted after injection of scopolamine HBr (5 mg/kg, s.c.). Median raphe stimulation usually produced behavioural freezing or an unnatural forced movement accompanied by RSA and LVFA. The behavioural response and the LVFA were not affected by scopolamine but scopolamine eliminated the RSA, replacing it with a low amplitude irregular (suppressed) pattern. p-Chlorophenylalanine (PCPA, 500 mg/kg/day x 3, i.p.) reduced the RSA and LVFA normally present during walking after scopolamine but did not reduce the hippocampal suppression produced by median raphe stimulation in scopolamine-treated rats. Hippocampal suppression and LVFA in response to median raphe stimulation were also present in urethane (1.0-1.5 g/kg, i.p.) anesthetized rats, whether pretreated with PCPA or not. Stimulation at most other midbrain sites produced RSA and LVFA in the urethane condition. RSA was abolished in the urethane plus scopolamine condition. The data support the view that scopolamine-resistant RSA and LVFA are dependent on serotonergic projections. The hippocampal suppression produced by median raphe stimulation may be dependent on non-serotonergic neurotransmission.

Drastic changes of ventromedial medulla neuronal properties induced by barbiturate anesthesia. I. Comparison of the single-unit types in the same awake and pentobarbital-treated rats

By means of single-unit recordings, as we have already performed in other studies, we have found that in the awake, drug-free, freely moving rat, there is only one neuronal class potentially involved in nociception and its control at the ventromedial medulla level (VMM, a structure involved in the spinal descending control systems of nociception): the 'multireceptive multimodal' units. These neurons are always activated by very light mechanical (air puff, light touch) and mechanical (pinch, pin-prick) or thermal noxious stimuli, in addition to an auditory stimulus. During identical VMM penetrations, performed in the same animals tested first awake and then anesthetized a few days later with 30 mg/kg of i.p. pentobarbital, we once again found the 'multireceptive multimodal' units, but this time with physiological properties that were strongly modified: in particular, we noted a disappearance of the nociceptive responses consecutive to a strong noxious heat pulse application (36-51 degrees C), associated sometimes with a reduction of the responses due to innocuous stimulation. This is in agreement with the classical effects of barbiturates. In light of previous observations reported in the literature devoted to the VMM physiology in the anesthetized rat, the most important observation in our study was that, with pentobarbital anesthesia, we recorded 'new' neuronal classes as compared to the awake condition. In these classes, which appeared to be qualitatively similar to those already reported under anesthesia, we found the units exclusively driven by innocuous stimulation (excited for the majority), the units specifically driven by noxious stimulation (half excited, half inhibited) and a 'multireceptive multimodal' group inhibited or excited-inhibited by non-noxious and noxious stimuli (half of the multireceptive group). All these data demonstrate that barbiturate anesthesia strongly modifies the VMM physiology in relation to nociception. Furthermore, since our results, that were obtained in anesthetized rats, were qualitatively identical to those described in the literature under similar experimental conditions, they raise the question of the appropriateness of using a barbiturate anesthetic in order to study the cellular mechanisms related to nociception at this level. In addition, these findings indicate that the obtention of only one neuronal class in the awake, drug-free, freely moving rat (the excited 'multireceptive' neurons) is not due to an experimental bias, which strongly emphasizes the reliability of using awake animals. However, it remains to be determined by which mechanisms pentobarbital 'distorts' the VMM physiology as compared to the normal, standard physiological conditions of the awake animal.

In vivo electrochemical evidence that the tricyclic antidepressant femoxetine potentiates the morphine-induced increase in 5-HT metabolism in the medullary dorsal horn of freely moving rats

Acute administration of tricyclic antidepressants (TCAs) is known to potentiate morphine antinociception. At the medullary dorsal horn (MDH) level systemic morphine has been shown to increase serotonin (5-HT) metabolism as measured by in vivo electrochemistry in freely moving rats. Using similar electrochemical detection of 5-hydroxyindole (peak '3') within the MDH, the present study investigated the effect of the specific 5-HT uptake inhibitor femoxetine on peak 3 and the effects of this TCA on changes in 5-HT metabolism induced by morphine. Acutely administered femoxetine (40 mg/kg i.p.) (i) induced a small but significant increase in peak 3 and (ii) strongly potentiated the effect of morphine (10 mg/kg i.p.) on 5-HT metabolism, this potentiation being opiate specific since simultaneous injection of naloxone (1 mg/kg i.p.) abolished the effect of morphine. These findings provide an in vivo neurochemical basis for the potentiation of morphine antinociception by TCAs. They further emphasize the importance of 5-HT bulbospinal descending pathways in morphine antinociception.

The organization of serotonergic projections to cerebral cortex in primates: regional distribution of axon terminals

Serotonergic axons are widely distributed in the primate forebrain and represent the most abundant ascending projection from the reticular formation. Immunocytochemical methods have been utilized to examine the density, laminar distribution and morphology of serotonergic axons in both primary projection (motor, somatosensory) and association areas (dorsolateral prefrontal, area 5) as well as in the hippocampus and in cingulate cortex of rhesus and cynomolgus macaques. Serotonergic axons are present in all areas of cortex examined, and all cortical layers receive serotonergic afferents. However, the intracortical distribution of serotonergic axon terminals is not uniform; rather, different regions of cortex exhibit dissimilarities in both the density and laminar distribution of serotonergic axons. Thus, there are local patterns of serotonin innervation that are characteristic of each cortical area. Highly diverse patterns of serotonin innervation are found in heterotypical areas of cortex; more subtle variations are present among homotypical areas. Two morphologic types of serotonergic axon terminals, fine and beaded, are present in all cortical areas, and they typically exhibit different laminar distributions: in most areas of neocortex, beaded axons predominate in layer I while fine axons predominate in layers II-VI. However, exceptions to this pattern were observed in primary visual cortex and in the hippocampal formation. The distinctive local patterns of serotonin innervation observed in this study indicate that raphe-cortical projections are likely to have differential influences on particular cytoarchitectonic areas of cerebral cortex in the primate. Moreover, the discrete laminar distribution of serotonin axons suggests that serotonergic projections selectively innervate particular neuronal elements in cerebral cortex. The present findings suggest that the two classes of serotonergic axons, fine and beaded, which have different patterns of termination, affect different sets of cortical neurons. In addition, these two serotonergic projections may be associated with different sets of serotonergic receptors and thus produce selective effects on cortical function.

Chloral hydrate in intractable status epilepticus

Five adult patients were admitted to the neurological department in a state of status epilepticus. All were treated unsuccessfully with IV diazepam and diphenylhydantoin. Administration of sodium valporate or phenobarbital also was ineffective. However, after treatment with intrarectal chloral hydrate, all seizures ceased. The excellent effect of this drug was proved both clinically and electrodiagnostically. Discussed is the possibility of using chloral hydrate to treat patients with status epilepticus in whom conventional treatment has failed.

The involvement of 5-hydroxytryptaminergic and dopaminergic mechanisms in the eating induced by buspirone, gepirone and ipsapirone

1. The roles of 5-hydroxytryptamine (5-HT) and dopamine systems in mediating the increased feeding induced by buspirone, gepirone and ipsapirone were investigated. 2. All three compounds induced dose-dependent increases in food intake when administered subcutaneously to free feeding rats. Buspirone was effective over a narrower dose range than either gepirone or ipsapirone, and the maximal effect observed was smaller than the effects elicited by gepirone and ipsapirone. 3. Depletion of brain 5-HT with parachlorophenylalanine (PCPA) prevented the effects of equi-effective doses of gepirone (2.5 mg kg-1) and ipsapirone (2.5 mg kg-1), but failed to prevent buspirone (1 mg kg-1)-induced eating. Thus buspirone does not appear to interact with 5-HT systems to elicit feeding. 4. Gepirone (0.2 micrograms) and ipsapirone (0.04 and 0.2 micrograms) increased food intake when injected into the dorsal raphé nucleus (DRN), presumably by inhibiting the activity of DRN 5-hydroxytryptaminergic afferents. Buspirone (0.04-5 micrograms) was ineffective when injected into the DRN. 5. Pretreatment with haloperidol (0.1 mg kg-1, 30 min) significantly attenuated the effects of equi-effective doses of buspirone, gepirone and ipsapirone, indicating that these drugs interact with dopaminergic systems to increase feeding. 6. Previously it has been shown that each of these drugs increases striatal dopamine activity. Increased dopaminergic neurotransmission in the striatum induces a general behavioural activation, which under certain conditions facilitates feeding. It is possible that this mechanism underlies the behavioural effects of buspirone, gepirone and ipsapirone. The effects of gepirone and ipsapirone probably involve an indirect action to inhibit the activity of DRN 5-hydroxytryptaminergic afferents, whereas buspirone interacts directly with dopaminergic systems.

Cardiac arrhythmias induced in cats by stimulation of the anteromedial hypothalamus

Negative emotional behavior induced during stimulation of the anteromedial hypothalamus is frequently accompanied by cardiac arrhythmias just after cessation of the stimulation (the poststimulus arrhythmias: the PSAs). A mechanism related to the PSAs may be neurogenic in origin. Whether or not the PSAs are influenced by hypothalamically induced adrenal catecholamines has not been settled. We assessed the correlation between these influences on the PSAs induced by anteromedial hypothalamic stimulation in anesthetized cats. Thirty-five adult cats were given stimulation through an electrode inserted stereotaxically into the hypothalamus and arterial blood pressure, electrocardiogram and heart rate were recorded on a polygraph. PSAs were never observed after vagotomy and intravenous injections of methyl-atropine and phentolamine. Carteolol was without effect. When an intentional gradual reduction of stimulus intensity was applied instead of sudden cessation of hypothalamic stimulation, there was a gradual decline in the blood pressure and no PSAs. Adrenalectomy did not alter the threshold for the PSAs but did reduce the frequency of the PSAs. Intravenous injections of catecholamines produced cardiac arrhythmias similar to those seen in PSAs. After vagotomy, stimulation of the distal nerve of the right vagus during or just after hypothalamic stimulation produced the same arrhythmias as seen in PSAs, but vagus stimulation alone did not. These results suggest that the PSAs are due to 'sudden' change from sympathetic to parasympathetic influences on the heart and that adrenal catecholamines are involved in the prolongation of the train of the PSAs. These findings support the proposal that the PSAs are a stress-related symptom induced by stimulation of the anteromedial hypothalamus.

Single-unit responses of serotonergic dorsal raphe nucleus neurons to environmental heating and pyrogen administration in freely moving cats

Single-unit activity of serotonergic neurons in the dorsal raphe nucleus was examined in response to environmental heating and pyrogen-induced fever in freely moving cats. In the heating study, ambient temperature was rapidly raised from a baseline of 25 degrees to 43 +/- 1 degrees C and maintained at this level for 2 h. Cats displayed hyperthermia, intense panting, and signs of heat stress, however, the discharge rate of serotonergic neurons of the dorsal raphe nucleus was not significantly different from baseline at any time during heat exposure. Similarly, the activity of these neurons was not significantly altered at any time during an approximately 6-h long febrile response induced by the synthetic pyrogen muramyl dipeptide (50 micrograms/kg, i.v.). These results indicate that serotonergic unit activity in the dorsal raphe nucleus is not related to either the activation of behavioral or physiological mechanisms underlying heat defense, or to alterations in thermoregulatory mechanisms during the febrile response to exogenous pyrogen. Furthermore, because these neurons do not respond to an elevation in body temperature induced by either ambient heating or pyrogen, they do not appear to be directly temperature-sensitive. These results do not support a specific role for serotonergic neurons of the dorsal raphe nucleus in thermoregulation.

Effects of physiological manipulations on locus coeruleus neuronal activity in freely moving cats. I. Thermoregulatory challenge

This and the following two papers examine the activity of locus coeruleus noradrenergic (LC-NE) neurons in response to a variety of physiological manipulations in unanesthetized, unrestrained cats. Unit responses were studied during a constant behavioral state in order to avoid the potentially confounding effects of state changes upon LC-NE unit discharge. In the present study, LC-NE unit activity was recorded during two thermoregulatory challenges: ambient heating and pyrogen-induced fever. These two conditions are particularly interesting since the direction of body temperature change that they produce is the same, but the thermoregulatory responses elicited are opposite, i.e. heat loss and heat gain. LC-NE neurons were activated by both manipulations. In response to ambient heating, neuronal activity increased only with the occurrence of panting, and not during earlier portions of the heating session. Following pyrogen administration, LC-NE neurons responded only during the peak increase in body temperature. Both of these effects on unit activity occurred independent of changes in behavioral state. Since both manipulations similarly increased LC-NE unit activity, despite eliciting opposite thermoregulatory responses, it appears that these neurons do not play a specific role in thermoregulation, but may participate in the response to physiological challenges in general.

Effects of physiological manipulations on locus coeruleus neuronal activity in freely moving cats. II. Cardiovascular challenge

Several cardiovascular manipulations were examined for their effects on single-unit activity of locus coeruleus noradrenergic (LC-NE) neurons in unanesthetized, unrestrained cats: hydralazine (1 mg/kg, i.v.) was administered to present a tonic hypotensive stimulus, and to activate preferentially the neural component of the sympathoadrenal system; hemorrhage was used to decrease blood volume and to activate both the neural and hormonal components of the sympathoadrenal system; intravenous infusion of isotonic saline was used to increase blood volume. LC-NE neurons were activated by hydralazine, in parallel with the sympathetic response (indicated by elevated heart rate and plasma NE). LC-NE unit activity was decreased following a volume load. However, contrary to previous findings in anesthetized animals, hemorrhage had no effect on LC-NE unit activity, but did activate both components of the sympathetic response. It is concluded that: (1) cardiovascular stimuli can influence the activity of LC-NE neurons, though they show less sensitivity to such stimuli than do primary regulatory mechanisms; (2) the response of LC-NE neurons to physiological stimuli can occur independent of changes in behavioral state; (3) these neurons do not appear to play a specific role in cardiovascular regulation, but may respond to physiological challenges in general; (4) finally, in agreement with previous studies, our data show that LC-NE neurons are generally co-activated with the sympathetic nervous system, but also that the two can be dissociated (e.g. hemorrhage).

Effects of physiological manipulations on locus coeruleus neuronal activity in freely moving cats. III. Glucoregulatory challenge

Insulin-induced hypoglycemia and the subsequent administration of glucose were examined for their effects on single unit activity of locus coeruleus noradrenergic (LC-NE) neurons in unanesthetized, unrestrained cats. LC-NE neuronal activity showed an inverse relationship to blood glucose levels. The activity of most cells increased during sustained hypoglycemia, and then decreased following glucose administration. Some neurons were unaffected by hypoglycemia, but were inhibited following glucose. The activation of LC-NE neurons in response to insulin administration generally paralleled the increase in plasma epinephrine, although the adrenal response was more sensitive. These data, together with those reported in the preceding papers, suggest the following general conclusions: (1) physiological stimuli can influence the activity of LC-NE neurons in unanesthetized subjects (although they do so less strongly than environmental stimuli); (2) these effects of physiological stimuli upon LC-NE neurons can be exerted independent of changes in behavioral state; (3) LC-NE neurons do not appear to play a specific role in the regulation of any of the systems examined, but may instead play a more global role in the response to physiological challenges in general; (4) LC-NE neurons are generally co-activated with both the neural and hormonal components of the sympatho-adrenal system, although sympathetic activation can occur in the absence of increased LC-NE activity. A previously hypothesized role for LC-NE neurons in facilitating the behavioral response to environmental stressors may thus be extended to include the response to physiological challenges, and perhaps facilitation of the physiological as well as the behavioral components of the stress response.