Dorsal raphe nucleus stimulation reduces centrally-elicited fearlike behavior

Neuropeptide S (NPS) is a neuropeptide with cellular actions in arousal and anxiety-related nuclei: Functional implications for effects of NPS on wakefulness and mood

Neuropeptide S (NPS) is a peptide recently recognized to be present in the CNS, and believed to play a role in vigilance and mood control, as behavioral studies have shown it promotes arousal and has an anxiolytic effect. Although NPS precursor is found in very few neurons, NPS positive fibers are present throughout the brain stem. Given the behavioral actions of this peptide and the wide innervation pattern, we examined the cellular effects of NPS within two brain stem nuclei known to play a critical role in anxiety and arousal: the dorsal raphe (DR) and laterodorsal tegmentum (LDT). In mouse brain slices, NPS increased cytoplasmic levels of calcium in DR and LDT cells. Calcium rises were independent of action potential generation, reduced by low extracellular levels of calcium, attenuated by IP₃ - and ryanodine (RyR)-dependent intracellular calcium store depletion, and eliminated by the receptor (NPSR) selective antagonist, SHA 68. NPS also exerted an effect on the membrane of DR and LDT cells inducing inward and outward currents, which were driven by an increase in conductance, and eliminated by SHA 68. Membrane actions of NPS were found to be dependent on store-mediated calcium as depletion of IP₃ and RyR stores eliminated NPS-induced currents. Finally, NPS also had actions on synaptic events, suggesting facilitation of glutamatergic and GABAergic presynaptic transmission. When taken together, actions of NPS influenced the excitability of DR and LDT neurons, which could play a role in the anxiolytic and arousal-promoting effects of this peptide.

Translational approach to studying panic disorder in rats: hits and misses

Panic disorder (PD) patients are specifically sensitive to 5–7% carbon dioxide. Another startling feature of clinical panic is the counterintuitive lack of increments in ‘stress hormones’. PD is also more frequent in women and highly comorbid with childhood separation anxiety (CSA). On the other hand, increasing evidence suggests that panic is mediated at dorsal periaqueductal grey matter (DPAG). In line with prior studies showing that DPAG-evoked panic-like behaviours are attenuated by clinically-effective treatments with panicolytics, we show here that (i) the DPAG harbors a hypoxia-sensitive alarm system, which is activated by hypoxia and potentiated by hypercapnia, (ii) the DPAG suffocation alarm system is inhibited by clinically-effective treatments with panicolytics, (iii) DPAG stimulations do not increase stress hormones in the absence of physical exertion, (iv) DPAG-evoked panic-like behaviours are facilitated in neonatally-isolated adult rats, a model of CSA, and (v) DPAG-evoked responses are enhanced in the late diestrus of female rats. Data are consistent with the DPAG mediation of both respiratory and non-respiratory types of panic attacks.

Chronic immobilization stress occludes in vivo cortical activation in an animal model of panic induced by carbon dioxide inhalation

Breathing high concentrations of carbon dioxide (CO2) can trigger panic and anxiety in humans. CO2 inhalation has been hypothesized to activate neural systems similar to those underlying fear learning, especially those involving the amygdala. Amygdala activity is also upregulated by stress. Recently, however, a separate pathway has been proposed for interoceptive panic and anxiety signals, as patients exhibited CO2-inhalation induced panic responses despite bilateral lesions of the amygdala. This paradoxical observation has raised the possibility that cortical circuits may underlie these responses. We sought to examine these divergent models by comparing in vivo brain activation in unstressed and chronically-stressed rats breathing CO2. Regional cerebral blood flow measurements using functional Magnetic Resonance Imaging (fMRI) in lightly-anaesthetized rats showed especially strong activation of the somatosensory cortex by CO2 inhalation in the unstressed group. Strikingly, prior exposure to chronic stress occluded this effect on cortical activity. This lends support to recent clinical observations and highlights the importance of looking beyond the traditional focus on limbic structures, such as the hippocampus and amygdala, to investigate a role for cortical areas in panic and anxiety in humans.

Further evidence for involvement of the dorsal hippocampus serotonergic and γ-aminobutyric acid (GABA)ergic pathways in the expression of contextual fear conditioning in rats

Intra-dorsal hippocampus (DH) injections of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a serotonin-1A (5-hydroxytryptamine (5-HT)-1A) receptor agonist, were previously shown to inhibit the expression of contextual fear when administered six hours after conditioning. However, further understanding of the consolidation and expression of aversive memories requires investigations of these and other mechanisms at distinct time points and the regions of the brain to which they are transferred. Thus, the purpose of the present study was to investigate the role of DH serotonergic and γ-aminobutyric acid (GABA)ergic mechanisms in the expression of contextual fear 24 h after conditioning, reflected by fear-potentiated startle (FPS) and freezing behavior. The recruitment of the amygdala and medial prefrontal cortex (mPFC) in these processes was also evaluated by measuring Fos protein immunoreactivity. Although intra-DH injections of 8-OH-DPAT did not produce behavioral changes, muscimol reduced both FPS and the freezing response. Fos protein immunoreactivity revealed that contextual fear promoted wide activation of the mPFC, which was significantly reduced after intra-DH infusions of muscimol. The present findings, together with previous data, indicate that in contrast to 5-HT, which appears to play a role during the early phases of contextual aversive memory consolidation, longer-lasting GABA-mediated mechanisms are recruited during the expression of contextual fear memories.

Functional topography of serotonergic systems supports the Deakin/Graeff hypothesis of anxiety and affective disorders

Over 20 years ago, Deakin and Graeff hypothesized about the role of different serotonergic pathways in controlling the behavioral and physiologic responses to aversive stimuli, and how compromise of these pathways could lead to specific symptoms of anxiety and affective disorders. A growing body of evidence suggests these serotonergic pathways arise from topographically organized subpopulations of serotonergic neurons located in the dorsal and median raphe nuclei. We argue that serotonergic neurons in the dorsal/caudal parts of the dorsal raphe nucleus project to forebrain limbic regions involved in stress/conflict anxiety-related processes, which may be relevant for anxiety and affective disorders. Serotonergic neurons in the "lateral wings" of the dorsal raphe nucleus provide inhibitory control over structures controlling fight-or-flight responses. Dysfunction of this pathway could be relevant for panic disorder. Finally, serotonergic neurons in the median raphe nucleus, and the developmentally and functionally-related interfascicular part of the dorsal raphe nucleus, give rise to forebrain limbic projections that are involved in tolerance and coping with aversive stimuli, which could be important for affective disorders like depression. Elucidating the mechanisms through which stress activates these topographically and functionally distinct serotonergic pathways, and how dysfunction of these pathways leads to symptoms of neuropsychiatric disorders, may lead to the development of novel approaches to both the prevention and treatment of anxiety and affective disorders.

Involvement of serotonin-mediated neurotransmission in the dorsal periaqueductal gray matter on cannabidiol chronic effects in panic-like responses in rats

RATIONALE

Cannabidiol (CBD) is a non-psychotomimetic constituent of Cannabis sativa plant that promotes antianxiety and anti-panic effects in animal models after acute systemic or intra-dorsal periaqueductal gray (DPAG) administration. However, the effects of CBD repeated administration, and the possible mechanisms involved, in animal models of anxiety- and panic-related responses remain poorly understood.

OBJECTIVE

The present study evaluates the role of the serotonergic neurotransmission within the DPAG in the modulation of escape responses of rats chronically treated with CBD.

METHODS

Male Wistar rats received acute or repeated (5 mg/Kg/daily/21 days) administration of CBD and were submitted to the elevated T-maze (ETM). We also investigated if CBD effects on the ETM depend on facilitation of 5-HT1A-mediated neurotransmission in the DPAG. To this latter aim, we verified if these effects would be prevented by intra-DPAG injection of the 5-HT1A receptor antagonist WAY100635 (0.37 nmol/0.2 μL). Also, we verified, by in vivo microdialysis, if CBD chronic treatment increases serotonin (5-HT) release and, by quantitative polymerase chain reaction, if there are changes in 5HT-1A or 5HT-2C mRNA expression in DPAG.

RESULTS

The results showed that repeated but not acute peripheral administration of CBD decreases escape responses in the ETM, suggesting a panicolytic effect. This treatment did not change 5HT-1A or 5-HT-2C receptor mRNA expression nor modify serotonin extracellular concentrations in the DPAG. CBD effects were prevented by DPAG injection of the 5-HT1A receptor antagonist.

CONCLUSIONS

Together, these findings suggest that repeated treatment with CBD induces anti-panic effects by acting on 5-HT1A receptors in DPAG.

Orexin-A (hypocretin-1) is possibly involved in generation of anxiety-like behavior

Orexins (hypocretins) are neuropeptides expressed specifically in neurons in the lateral hypothalamic area and are known to be involved in the regulation of vigilance and feeding behavior. However, the relationship between orexin and emotional behaviors like anxiety is still poorly understood. Therefore, in this report we evaluated the effect of intracerebroventricular injection of orexin-A in two major anxiety tests, the light-dark exploration test (mouse) and the elevated plus-maze test (mouse, rat). Orexin increased time spent in the dark compartment in the light-dark test and time spent on the closed arms in the elevated plus-maze test. These results were not caused by a hypothetical sedative or activity-inducing effect of orexin-A because spontaneous locomotor activity did not alter upon orexin-A application under novel conditions. We therefore suggest an anxiogenic effect of orexin-A. To our knowledge, this is the first report about a relationship between orexin-A and anxiety.

Serotonin, the periaqueductal gray and panic

This article reviews experimental evidence and theoretical constructs that implicate serotonin (5-HT) modulation of defensive behavior within the midbrain periaqueductal gray in panic disorder (PD). Evidence with conflict tests in experimental animals indicates that 5-HT enhances anxiety, whereas results with aversive stimulation of the dorsal periaqueductal gray point to an anxiolytic role of 5-HT. To solve this contradiction, it has been suggested that the emotional states determined by the two types of animal model are different. Conflict tests would generate conditioned anxiety, whereas periaqueductal gray stimulation would produce unconditioned fear, as evoked by proximal threat. Clinically, the former would be related to generalized anxiety while the latter to PD. Thus, 5-HT is supposed to facilitate anxiety, but to inhibit panic. This hypothesis has been tested in the animal model of anxiety and panic named the elevated T-maze, in two procedures of human experimental anxiety applied to healthy volunteers or panic patients, and in CO2-induced panic attacks. Overall, the obtained results have shown that drugs that enhance 5-HT function increase different indexes of anxiety, but decrease indexes of panic. Drugs that impair 5-HT function have the opposite effects. Thus, so far the predictions derived from the above hypothesis have been fulfilled.

The dorsal raphe nucleus exerts opposed control on generalized anxiety and panic-related defensive responses in rats

It has been proposed that the ascending dorsal raphe (DR)-serotonergic (5-HT) pathway facilitates conditioned avoidance responses to potential or distal threat, while the DR-periventricular 5-HT pathway inhibits unconditioned flight reactions to proximal danger. Dysfunction on these pathways would be, respectively, related to generalized anxiety (GAD) and panic disorder (PD). To investigate this hypothesis, we microinjected into the rat DR the benzodiazepine inverse receptor agonist FG 7142, the 5-HT(1A) receptor agonist 8-OH-DPAT or the GABA(A) receptor agonist muscimol. Animals were evaluated in the elevated T-maze (ETM) and light/dark transition test. These models generate defensive responses that have been related to GAD and PD. Experiments were also conducted in the ETM 14 days after the selective lesion of DR serotonergic neurons by 5,7-dihydroxytriptamine (DHT). In all cases, rats were pre-exposed to one of the open arms of the ETM 1 day before testing. The results showed that FG 7142 facilitated inhibitory avoidance, an anxiogenic effect, while impairing one-way escape, an anxiolytic effect. 8-OH-DPAT, muscimol, and 5,7-DHT-induced lesions acted in the opposite direction, impairing inhibitory avoidance while facilitating one-way escape from the open arm. In the light/dark transition, 8-OH-DPAT and muscimol increased the time spent in the lighted compartment, an anxiolytic effect. The data supports the view that distinct DR-5-HT pathways regulate neural mechanisms underlying GAD and PD.

Neurochemical mechanisms of the defensive behavior in the dorsal midbrain

Some regions in the mesencephalon, such as dorsal periaqueductal gray, inferior colliculus and deep layers of superior colliculus have been grouped together as a continuous strip of midbrain structures involved in the integration of the different components of aversive states in the brain. In fact, escape behavior and defensive, or fear-like behavior often result when these sites are electrically or chemically stimulated. Moreover, the behavioral responses induced by stimulation of these structures are, in general, accompanied by increases in mean arterial blood pressure, heart rate and respiration, and by analgesia. Both the behavioral and autonomic consequences of electrical stimulation of the mesencephalic tectum was shown to be attenuated by minor tranquilizers, probably through enhancement of GABAergic neurotransmission. Besides GABAergic interneurons which exert a tonic inhibitory control on neural circuits responsible for the behavioral correlates of the aversion in the above-mentioned structures, several other mechanisms such as opioid, neuropeptides, serotonergic and excitatory amino acids have also been implicated in the regulation of these processes. As to the analgesia that accompanies these aversive states it is mediated by non-opioid mechanisms, particularly by serotonergic ones through 5-HT2 receptors. Now, efforts have been made to characterize the mode of action of these neurotransmitters on their multiple receptors and how they interact with each other to produce or regulate the neural substrates of aversion in the midbrain.

Projections from dorsal raphe nucleus to the periaqueductal grey matter: studies in slices of rat midbrain maintained in vitro

In coronal slices of rat midbrain localised injections of FluoroGold or DiO into the dorsolateral periaqueductal grey matter (PAG) labelled cells retrogradely in the dorsal half of the dorsal raphe nucleus (DRN) and the ipsilateral ventrolateral PAG. Low intensity electrical stimulation in the dorsolateral PAG (22.9 +/- 1.9 microA) evoked antidromic responses in neurones recorded intracellularly in the dorsal subnucleus of the DRN. Antidromic responses could also be evoked in neurones in the ventral DRN and its wings but only at much higher currents (40.9 +/- 2.5 microA) which likely spread to activate axons in the ventrolateral PAG that originated from perikarya in the ventral DRN. The findings are discussed in relation to modulation of the excitability of the aversive system in the dorsolateral PAG by the DRN.

5-Hydroxytryptamine pathways in anxiety and its treatment

The effects of manipulating 5-hydroxytryptamine (5HT) neuronal function in humans and in animals are reviewed. 5HT pathways do not have a unitary function in modulating anxiety. It is proposed that, rather than acting as input or output channels for brain aversive systems, these pathways provide information concerning waking/motor status, which is crucial to the organisation of appropriate responses to threat. Each terminal region can make use of this information in different ways. Globally, the influence of 5HT neurones on higher centres appears predominantly to facilitate information processing relevant to threat, while their major influence on brainstem centres may be a restraining one.

Inhibitory and excitatory projections from the dorsal raphe nucleus to neurons in the dorsolateral periaqueductal gray matter in slices of midbrain maintained in vitro

Coronal slices of midbrain from adult rats, maintained in an interface chamber at 33-35 degrees C, were used to investigate projections from the dorsal raphe nucleus to the dorsolateral periaqueductal gray matter. Extracellular recordings were made from neurons in the dorsolateral periaqueductal gray matter. The majority of cells were silent but activity could readily be induced by iontophoretic application of D, L-homocysteic acid from the recording pipette. Neuronal perikarya in the dorsal raphe nucleus were activated either by topical application of 15-100 nl droplets of 1 or 10 mM D,L-homocysteic acid or by iontophoresis into the dorsal raphe nucleus of 0.1 M D,L-homocysteic acid from a five-barrelled glass micropipette. Both forms of stimulation evoked increases or decreases in firing in neurons in the dorsolateral periaqueductal gray matter. The excitatory responses were evoked only from stimulation sites in the dorsal subnucleus of the dorsal raphe nucleus, whilst inhibitory responses could be evoked by stimulating throughout the nucleus. 5-Hydroxytryptamine was applied by iontophoresis to 55 neurons in the dorsolateral periaqueductal gray matter. Of these, 89% were inhibited whilst the remaining 11% were excited. No unresponsive cells were found. In a second series of experiments anterograde transport of biocytin was used to study projections from the dorsal raphe nucleus to the periaqueductal gray matter in vitro. Following injections of pellets of biocytin into the dorsal subnucleus of the dorsal raphe nucleus, two types of labelled axons could be distinguished which had either smooth or beaded profiles. Both types of axon could be followed into the region of the dorsolateral periaqueductal gray matter, from which recordings were made in the electrophysiological experiments. In contrast, following deposits of biocytin into the ventral half of the periaqueductal gray matter, very few labelled axons were found in the dorsolateral periaqueductal gray matter, although the ventrolateral periaqueductal gray matter was heavily labelled. We conclude that both excitatory and inhibitory projections from the dorsal raphe nucleus to the dorsolateral periaqueductal gray matter are present in the coronal midbrain slice preparation. Excitatory projections originate from the dorsal subnucleus of the dorsal raphe nucleus and may be mediated by a direct pathway. The inhibitory effects evoked from the ventral half of the dorsal raphe nucleus are probably mediated indirectly, perhaps by activation of inhibitory interneurons in the ventrolateral periaqueductal gray matter.

Neural substrate of defensive behavior in the midbrain tectum

It has been shown that the gradual increase in the intensity of electrical stimulation of the dorsal periaqueductal gray (DPAG), deep layers of the superior colliculus (DLSC) and inferior colliculus of rats induces, in a progressive manner, characteristic aversive responses such as arousal, freezing, and escape behavior. The DPAG-DLSC together with the periventricular gray substance of the diencephalon, amygdala and the inferior colliculus, constitute the neural substrate of aversion in the brain. In general, the behavioral responses induced by midbrain tectum stimulation are accompanied by increases in the mean arterial blood pressure, heart rate, and respiration. Both the behavioral and autonomic consequences of electrical stimulation of the mesencephalic tectum have been shown to be attenuated by minor tranquilizers, probably through enhancement of GABAergic neurotransmission. Besides GABAergic mechanisms several lines of evidence have clearly implicated opioid, serotonergic, and excitatory amino acids-mediated mechanisms in the control of the neural substrates commanding defensive behavior in the brain aversive system.

Influence of the dorsal and median raphe nuclei on neurons in the periaqueductal gray matter: role of 5-hydroxytryptamine

In rats anaesthetized with urethane, selective activation of neuronal perikarya in the dorsal raphe nucleus evoked inhibitory (n = 17) and excitatory (n = 10) responses in single neurons recorded in the dorsolateral and lateral sectors of the periaqueductal gray matter and in the adjacent tegmental area. A further 11 cells showed biphasic inhibitory/excitatory responses. Ongoing activity of > 85% of the cells was inhibited by iontophoretic application of 5-hydroxytryptamine (1-70 nA). The duration of the inhibitory response evoked from the dorsal raphe nucleus was increased by 66-75% during iontophoretic application (3-10 nA) of the 5-hydroxytryptamine (serotonin) reuptake blocker paroxetine (five of six cells). In contrast, excitatory responses evoked from the dorsal raphe nucleus were either reduced (n = 3) or replaced by inhibitory responses (n = 2) in the presence of paroxetine. Paroxetine also produced a reduction in baseline firing and potentiated the responses of neurons to iontophoretically applied 5-hydroxytryptamine. Stimulation in the median raphe nucleus did not produce any significant changes in the activity of five neurons tested in the periaqueductal gray matter. It is suggested that the inhibitory influence of the dorsal raphe nucleus on cells in the dorsal half of the periaqueductal gray matter is mediated by 5-hydroxytryptamine. This projection may be involved in modulating the level of excitability of neurons in the midbrain aversive system which integrate defence behaviour. In addition, there appears to be a non-serotonergic excitatory projection from the dorsal raphe nucleus to the periaqueductal gray matter. The functional role of this projection remains obscure.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of defense in the inferior colliculus

The inferior colliculus (IC) is a well known relay station for auditory pathways in the brainstem. In the present review we are suggesting that aversive states are also generated and elaborated in the inferior colliculus and that this structure may be part of a brain system commanding defensive behavior. The evidences presented in this review have been obtained from experiments carried out with the combined use of intracerebral microinjections and of electrical stimulation of the inferior colliculus. This electrical stimulation caused a behavioral activation together with autonomic reactions usually observed as part of the defense reaction. NMDA--an excitatory amino acid--, or bicuculline--a GABAA antagonist--injected into the IC mimicked the effects of its electrical stimulation. The IC electrical stimulation showed clear aversive properties as rats submitted to a switch-off paradigm quickly learned to interrupt it. Systemic administration as well as IC microinjections of the anxiolytic compound midazolam caused dose-dependent increases in the latency and reductions in the frequency of switch-off responses to the inferior colliculus electrical stimulation. Similar results were obtained following microinjections into this brainstem structure of the GABAA agonist muscimol. These results suggest that neural substrates responsible for defensive behavior in the inferior colliculus may be depressed by benzodiazepines as part of the anxiolytic action of these compounds. This anti-aversive action may be produced by the enhancement of GABAA mechanisms. Serotonergic mechanisms seem also to be involved in the modulation of these aversive states as IC microinjections of zimelidine, a 5-HT uptake blocker, caused a significant inhibition of the switch-off responses in the shuttle-box.(ABSTRACT TRUNCATED AT 250 WORDS)

Dye coupling between dorsal raphe neurones

Neurones in the dorsal raphe nucleus (DRN) were impaled and filled with biocytin in coronal slices of midbrain taken from young adult rats. The electrophysiological properties and gross morphology of the cells were similar to those reported previously for serotonergic neurones in the DRN. Of 27 cases in which filled neurones were recovered in histological material, almost half (48%) showed labelling of two or three cells, although only one cell had been recorded from. Coupled cells were identified as close or distantly coupled, depending on the distance from the soma of the presumed impaled cell (23.5 +/- 15 microns, n = 7 and 150 +/- 26.5 microns, n = 10 respectively). Whereas close-coupled cells may have been artefactually "coupled" by the penetrating electrode, coupling between distant cells is most likely to be a result of transfer of biocytin through gap junctions. Camera lucida reconstructions of pairs of labelled cells revealed extensive overlap of dendritic fields and numerous crossings between dendrites. When examined at high magnification under a light microscope, many of the crossing dendrites were found to travel in different focal planes. Nevertheless, for each pair of cells, at least one point of close apposition was observed between dendrites or between the axon and a dendrite of the presumed impaled and coupled cell. The incidence of dye coupling between neurones in the DRN may reflect a relatively high level of electronic coupling between the neurones. This form of coupling may be important in determining the synchronous nature of firing of neurones in the DRN.

Morphine applied to the ventral tegmentum differentially affects centrally and peripherally induced aversive effects

In the rat, a microinjection of 15 nmoles of morphine into the ventral tegmentum (VT) was found to suppress escape responding induced by electrical stimulation applied to the dorsal part of the mesencephalic central gray. This suppressant effect could be reversed by a systemic injection of naloxone and was unlikely to be due to gross motor impairment since morphine injected into the VT provoked a behavioral activation. A similar microinjection of morphine into the VT did not induce analgesic effects since it did not affect the reaction thresholds to a nociceptive stimulus. The mechanisms underlying these differential effects of morphine applied to the VT are discussed.

Differential control of hypothalamically elicited flight behavior by the midbrain periaqueductal gray in the cat

The purpose of the present study was to determine the role of the midbrain periaqueductal gray (PAG) and thalamic centrum medianum-parafascicular complex (CM-Pf) in the regulation of hypothalamically elicited flight behavior in the cat. The experimental paradigm involved a comparison of the differences in response latencies between single stimulation of the hypothalamus and concurrent stimulation of the hypothalamus and sites in the PAG or the CM-Pf. Dual stimulation of the ventral and dorsal aspects of the PAG resulted in differential modulation of flight behavior. Stimulation of the dorsal PAG suppressed hypothalamically elicited flight behavior while stimulation of the ventral aspects of the PAG facilitated flight behavior. Facilitation of flight behavior was also found from stimulation of ventral portions of the CM-Pf.

Fear is not critical to classically conditioned analgesia: the effects of periaqueductal gray lesions and administration of chlordiazepoxide

A high correlation between fear and analgesia classically conditioned to footshock in rats has been reported in the literature. However, it has never been directly tested whether or not fear is in fact causal to the production of conditioned analgesia. We therefore tested whether conditioned analgesia could be elicited in the absence of fear by employing two independent methods of fear suppression. First, areas of the periaqueductal gray (PAG) previously implicated in fear were selectively lesioned. Lesions of the dorsolateral PAG significantly attenuated conditioned analgesia and markedly decreased fear responses. Second, fear was attenuated via administration of chlordiazepoxide (CDP). Rats which had been conditioned while in the presence of CDP showed no reduction in conditioned pain inhibition. These results demonstrate that: (1) fear is not causal to classically conditioned analgesia and (2) the anatomical substrates for fear and conditioned analgesia are distinct but partially overlapping. The fact that fear is not a critical antecedent for classically conditioned analgesia suggests that classical conditioning techniques may be applied clinically to increase the effectiveness of some analgesic manipulations.

Afferent projections to the hypothalamic area controlling emotional responses (HACER)

The Mesulam technique for horseradish peroxidase was used to study the subcortical afferent projections to a location in the hypothalamus that has been shown to control the complete cardiovascular (CV) response accompanying a specific emotional behavior. Major projections common to all baboons injected included the lateral septal nucleus; medial, cortical and basal amygdala; the anteroventral third ventricle area; the preoptic areas; the subiculum; the paraventricular nucleus of the thalamus; periventricular gray and the central gray of the midbrain; the midbrain tegmentum; locus ceruleus, parabrachial and raphe cells in the pons; and in the medulla, raphe nuclei, the nucleus of the solitary tract, in and around the dorsal motor nucleus of the vagus, and in the region of the nucleus ambiguus. Other projections in some but not all baboons included the subfornical organ and the midline and dorsomedial nuclei of the thalamus. The nucleus of the diagonal band of Broca was labeled to some degree with all injections but was most heavily labeled with the injection extending more laterally in the hypothalamus. These results fit well with physiological and behavioral studies dealing with neural control of emotional and CV responses and support the concept of an integrative area in the hypothalamus concerned specifically with the control of CV response accompanying emotion.

Enhancing GABAergic transmission reverses the aversive state in rats induced by electrical stimulation of the periaqueductal grey region

In a proposed rat model for anxiety (electrical stimulation of the periaqueductal grey region), progabide (a GABA agonist) and diazepam both increased the latency to escape to a safe compartment and also the current needed to induce the escape response (escape threshold). Furthermore, the effects of progabide and diazepam were greater than additive in their actions on the escape response as when given together in normally subliminal doses, the combination exerted a marked anti-aversive effect. These actions of the drugs alone or in combination could not be explained by non-specific motor effects. Blockade of GABA receptors by bicuculline greatly reduced or abolished the action of progabide and diazepam (single administration). Sodium valproate, which indirectly augments GABAergic transmission, also increased the escape latency and escape threshold whereas, in contrast, diphenylhydantoin accentuated the aversive effects of stimulation of the periaqueductal grey. Haloperidol increased the escape latency and threshold but not other signs of distress following central stimulation (vocalization, jumping) which were effectively blocked by progabide and diazepam. The action of haloperidol was completely explicable by an interference with motor mechanisms. These results are interpreted as an indication that GABA agonists have an anti-aversive action in this proposed rat model for anxiety and, furthermore, that GABA receptors at least partially mediate the actions of benzodiazepines in this model.

A simple method for obtaining chronic cerebrospinal fluid samples from awake rats

A simple technique is described for taking repeated cerebrospinal fluid (CSF) samples from the cisterna magna of unanesthetized rats. The technique has the advantages that inexpensive, readily available materials are used, the cannulas remain patent for extended periods of time, and large volumes of CSF can be obtained.