Effects of morphine and midazolam on reactivity to peripheral noxious and central aversive stimuli

Decoding the language of fear: Unveiling objective and subjective indicators in rodent models through a systematic review and meta-analysis

While rodent models are vital for studying mental disorders, the underestimation of construct validity of fear indicators has led to limitations in translating to effective clinical treatments. Addressing this gap, we systematically reviewed 5054 articles from the 1960 s, understanding underlying theoretical advancement, and selected 68 articles with at least two fear indicators for a three-level meta-analysis. We hypothesized correlations between different indicators would elucidate similar functions, while magnitude differences could reveal distinct neural or behavioral mechanisms. Our findings reveal a shift towards using freezing behavior as the primary fear indicator in rodent models, and strong, moderate, and weak correlations between freezing and conditioned suppression ratios, 22-kHz ultrasonic vocalizations, and autonomic nervous system responses, respectively. Using freezing as a reference, moderator analysis shows treatment types and fear stages significantly influenced differences in magnitudes between two indicators. Our analysis supports a two-system model of fear in rodents, where objective and subjective fears could operate on a threshold-based mechanism.

The Effects of Amphetamine and Methamphetamine on the Release of Norepinephrine, Dopamine and Acetylcholine From the Brainstem Reticular Formation

Amphetamine (AMPH) and methamphetamine (METH) are widely abused psychostimulants, which produce a variety of psychomotor, autonomic and neurotoxic effects. The behavioral and neurotoxic effects of both compounds (from now on defined as AMPHs) stem from a fair molecular and anatomical specificity for catecholamine-containing neurons, which are placed in the brainstem reticular formation (RF). In fact, the structural cross-affinity joined with the presence of shared molecular targets between AMPHs and catecholamine provides the basis for a quite selective recruitment of brainstem catecholamine neurons following AMPHs administration. A great amount of investigations, commentary manuscripts and books reported a pivotal role of mesencephalic dopamine (DA)-containing neurons in producing behavioral and neurotoxic effects of AMPHs. Instead, the present review article focuses on catecholamine reticular neurons of the low brainstem. In fact, these nuclei add on DA mesencephalic cells to mediate the effects of AMPHs. Among these, we also include two pontine cholinergic nuclei. Finally, we discuss the conundrum of a mixed neuronal population, which extends from the pons to the periaqueductal gray (PAG). In this way, a number of reticular nuclei beyond classic DA mesencephalic cells are considered to extend the scenario underlying the neurobiology of AMPHs abuse. The mechanistic approach followed here to describe the action of AMPHs within the RF is rooted on the fine anatomy of this region of the brainstem. This is exemplified by a few medullary catecholamine neurons, which play a pivotal role compared with the bulk of peripheral sympathetic neurons in sustaining most of the cardiovascular effects induced by AMPHs.

Interaction between μ-opioid and 5-HT1A receptors in the regulation of panic-related defensive responses in the rat dorsal periaqueductal grey

A wealth of evidence indicates that the activation of 5-HT1A and 5-HT2A receptors in the dorsal periaqueductal grey matter (dPAG) inhibits escape, a panic-related defensive behaviour. Results that were previously obtained with the elevated T-maze test of anxiety/panic suggest that 5-HT1A and μ-opioid receptors in this midbrain area work together to regulate this response. To investigate the generality of this finding, we assessed whether the same cooperative mechanism is engaged when escape is evoked by a different aversive stimulus electrical stimulation of the dPAG. Administration of the μ-receptor blocker CTOP into the dPAG did not change the escape threshold, but microinjection of the μ-receptor agonist DAMGO (0.3 and 0.5 nmol) or the 5-HT1A receptor agonist 8-OHDPAT (1.6 nmol) increased this index, indicating a panicolytic-like effect. Pretreatment with CTOP antagonised the anti-escape effect of 8-OHDPAT. Additionally, combined administration of subeffective doses of DAMGO and 8-OHDPAT increased the escape threshold, indicating drug synergism. Therefore, regardless of the aversive nature of the stimulus, μ-opioid and 5-HT1A receptors cooperatively act to regulate escape behaviour. A better comprehension of this mechanism might allow for new therapeutic strategies for panic disorder.

Serotonergic neural links from the dorsal raphe nucleus modulate defensive behaviours organised by the dorsomedial hypothalamus and the elaboration of fear-induced antinociception via locus coeruleus pathways

Decrease of γ-aminobutyric acid (GABA)-mediated neurotransmission in the dorsomedial hypothalamus (DMH) evokes instinctive fear-like responses. The aim of the present study was to investigate the involvement of the serotonin (5-HT)- and norepinephrine-mediated pathways of the endogenous pain inhibitory system, including the dorsal raphe nucleus (DRN) and the locus coeruleus (LC), in the defensive responses and antinociceptive processes triggered by the blockade of GABAergic receptors in the DMH. The intra-hypothalamic microinjection of the GABA(A) receptor antagonist bicuculline (40 ng/200 nL) elicited elaborate defensive behaviours interspersed with exploratory responses. This escape behaviour was followed by significantly increased pain thresholds, a phenomenon known as fear-induced antinociception. Furthermore, at 5 and 14 days after DRN serotonin-containing neurons were damaged using the selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), the frequency and duration of alertness and escape behaviour evoked by the GABA(A) receptor blockade in the DMH decreased, as well as fear-induced antinociception. Pre-treatment with the non-selective 5-HT receptor antagonist methysergide, the 5-HT(2A/2C) receptor antagonist ketanserin and the 5-HT(2A) receptor selective antagonist R-96544 in the LC also decreased fear-induced antinociception, without significant changes in the expression of defensive behaviours. These data suggest that the serotonergic neurons of the DRN are directly involved in the organisation of defensive responses as well as in the elaboration of the innate fear-induced antinociception. However, serotonin-mediated inputs from the NDR to the LC modulate only fear-induced antinociception and not the defensive behaviours evoked by GABA(A) receptor blockade in the DMH.

The role of mu-opioid receptor signaling in the dorsolateral periaqueductal gray on conditional and unconditional responding to threatening and aversive stimuli

Here we examined how mu-opioid receptor signaling in the periaqueductal gray (PAG) mediates conditional and unconditional responses to aversive stimuli. The mu-opioid agonist morphine (MOR) and/or the partially mu-selective antagonist naltrexone (NAL) were infused into dorsolateral PAG (dlPAG) during a fear conditioning task, in which rats were trained to fear an auditory conditional stimulus (CS) by pairing it with a unilateral eyelid shock unconditional stimulus (US). During drug-free test sessions, the CS elicited movement suppression responses (indicative of freezing) from trained rats that had not recently encountered the US. In trained rats that had recently encountered the US, the CS elicited flight behavior characterized by turning in the direction away from the eyelid where US delivery was anticipated. Infusions of MOR (30 nmol/side) into dlPAG prior to the test session did not impair CS-evoked movement suppression, but did impair CS-evoked turning behaviors. MOR infusions also reduced baseline motor movement, but US-evoked reflex movements remained largely intact. NAL was infused at two dosages, denoted 1x (26 nmol/side) and 10x (260 nmol/side). Infusions of NAL into dlPAG did not affect CS- or US-evoked behavioral responses at the 1x dosage, but impaired CS-evoked movement suppression at the 10x dosage, both in the presence and absence of MOR. When rats were co-infused with MOR and NAL, MOR-induced effects were not reversed by either dosage of NAL, and some measures of MOR-induced movement suppression were enhanced by NAL at the 1x dosage. Based on these findings, we conclude that mu-opioid receptors in dlPAG may selectively regulate descending supraspinal motor pathways that drive active movement behaviors, and that interactions between MOR and NAL in dlPAG may be more complex than simple competition for binding at the mu receptor.

GABA(A) receptor blockade in dorsomedial and ventromedial nuclei of the hypothalamus evokes panic-like elaborated defensive behaviour followed by innate fear-induced antinociception

Dysfunction in the hypothalamic GABAergic system has been implicated in panic syndrome in humans. Furthermore, several studies have implicated the hypothalamus in the elaboration of pain modulation. Panic-prone states are able to be experimentally induced in laboratory animals to study this phenomenon. The aim of the present work was to investigate the involvement of medial hypothalamic nuclei in the organization of panic-like behaviour and the innate fear-induced oscillations of nociceptive thresholds. The blockade of GABA(A) receptors in the neuronal substrates of the ventromedial or dorsomedial hypothalamus was followed by elaborated defensive panic-like reactions. Moreover, innate fear-induced antinociception was consistently elicited after the escape behaviour. The escape responses organized by the dorsomedial and ventromedial hypothalamic nuclei were characteristically more elaborated, and a remarkable exploratory behaviour was recorded during GABA(A) receptor blockade in the medial hypothalamus. The motor characteristic of the elaborated defensive escape behaviour and the patterns of defensive alertness and defensive immobility induced by microinjection of the bicuculline either into the dorsomedial or into the ventromedial hypothalamus were very similar. This was followed by the same pattern of innate fear-induced antinociceptive response that lasted approximately 40 min after the elaborated defensive escape reaction in both cases. These findings suggest that dysfunction of the GABA-mediated neuronal system in the medial hypothalamus causes panic-like responses in laboratory animals, and that the elaborated escape behaviour organized in both dorsomedial and ventromedial hypothalamic nuclei are followed by significant innate-fear-induced antinociception. Our findings indicate that the GABA(A) receptor of dorsomedial and ventromedial hypothalamic nuclei are critically involved in the modulation of panic-like behaviour.

Evidence for mediation of nociception by injection of the NK-3 receptor agonist, senktide, into the dorsal periaqueductal gray of rats

RATIONALE

Ultrasound vocalizations (USVs) at approximately 22 kHz are usual components of the defensive response of rats. However, depending on the neural substrate that is activated, such as the dorsal periaqueductal gray (dPAG), USV emissions may be reduced. Activation of neurokinin-1 (NK-1)-mediated mechanisms of the dPAG causes analgesia, reduced 22 kHz USVs, and anxiogenic-like effects in rats exposed to the elevated plus maze (EPM). Involvement of other types of neurokinin receptors in this activation has not yet been evaluated.

OBJECTIVES

The present study examined whether local injections of the selective NK-3 agonist senktide (1-100 pmol/0.2 microL) into the dPAG can (1) cause anxiogenic effects in the EPM, (2) influence novelty-induced 22 kHz USVs, or (3) change nociceptive reactivity in the tail-flick test.

RESULTS

Senktide elicited a significant increase in exploratory behavior, an effect accompanied by hyperalgesia and an increase in the number of 22 kHz USVs. The nociceptive effects, increased locomotor activity, and USV emissions elicited by local injections of senktide (50 pmol/0.2 microL) were reduced by prior injections of the selective NK-3 receptor antagonist SB222200 (50 pmol/0.2 microL) into the dPAG.

CONCLUSIONS

These findings show that NK-3 receptors in the dPAG mediate nociceptive responses in this area, contrasting with the known fear-related processes mediated by NK-1 receptors in the dPAG. Both hyperalgesia and fear-related processes are accompanied by emissions of 22 kHz USVs.

Effects of ovine CRF injections into the dorsomedial, dorsolateral and lateral columns of the periaqueductal gray: a functional role for the dorsomedial column

Corticotropin-releasing factor (CRF) and its receptor subtypes have been implicated in the regulation of endocrine, behavioral and autonomic responses to stress, fear and anxiety. Ovine CRF (oCRF) is a nonspecific CRF receptor agonist that produces anxiogenic-like effects when injected locally into the dorsal aspects of the periaqueductal gray (PAG). This structure is subdivided into four distinct longitudinal columns but their exact functional role is not fully understood. The purpose of the present study was to characterize the effects of oCRF (0.25, 0.5 and 1 microg/0.2 microL) injections into the dorsomedial (dmPAG), dorsolateral (dlPAG) and lateral (lPAG) columns of the PAG using an analysis of the exploratory behavior of rats in the elevated plus-maze (EPM) test. The results showed that microinjections of oCRF intra-dmPAG reduced entries and time spent in the open arms and decreased end-arm exploration and head-dipping. In contrast, oCRF intra-dlPAG or lPAG did not affect the exploratory behavior of the animals in the EPM. These findings point to a columnar specificity for the oCRF effects in the PAG, that is, it increased spatial avoidance measures of the EPM test only in the dmPAG. The proaversive effects of oCRF in the dmPAG gain further relevance when combined with previous immunohistochemical studies showing that CRF-containing projections from the periventricular hypothalamic system arch dorsomedially to the PAG, which could function as an important relay station in the midbrain tectum for avoidance behaviors.

Anxiogenic effects of activation of NK-1 receptors of the dorsal periaqueductal gray as assessed by the elevated plus-maze, ultrasound vocalizations and tail-flick tests

Ultrasound vocalizations (USVs) known as 22kHz are usual components of the defensive responses of rats exposed to threatening conditions. The amount of emission of 22kHz USVs depends on the intensity of the aversive stimuli. While moderate fear causes an anxiolytic-sensitive enhancement of the defensive responses, high fear tended to reduce the defensive performance of the animals to aversive stimuli. The dorsal periaqueductal gray (dPAG) is an important vocal center and a crucial structure for the expression of defensive responses. Substance P (SP) is involved in the modulation of the defensive response at this midbrain level, but the type of neurokinin receptors involved in this action is not completely understood. In this study we examined whether local injections of the selective NK-1 agonist SAR-MET-SP (10-100 pmol/0.2microL) into the dPAG (i) cause anxiogenic effects in the elevated plus-maze (EPM) (Exp. I), (ii) influence the novelty-induced 22kHz USVs recorded within the frequency range of 20-26kHz (Exp. II) and (iii) change the nociceptive reactivity to heat applied to the rat's tail (Exp III). The data obtained showed that SAR-MET-SP elicited significant "anxiety-like" behaviors, as revealed by the decrease in the number of entries into and time spent onto the open arms of the EPM. These anxiogenic effects were accompanied with antinociception and disruption of the novelty-induced increase in the number and duration of 22kHz USVs. These findings are in agreement with the notion that NK-1 receptors of the dPAG may be an important neurochemical target for new selective drugs aimed at the control of pathological anxiety states.

Different patterns of freezing behavior organized in the periaqueductal gray of rats: association with different types of anxiety

Freezing defined as the complete absence of body movements is a normal response of animals to unavoidable fear stimuli. The present review presents a series of evidence relating different defensive patterns with specific anxiety disorders. There are at least four different kinds of freezing with specific neural substrates. The immobility induced by stimulation of the ventral column of the periaqueductal gray (vPAG) has been considered a quiescence characteristic of the recovery component of defense-recuperative processes. There is an isomorphism between freezing response to contextual stimuli paired with electrical shocks and generalized anxiety disorder. Besides, two types of freezing emerge with the electrical stimulation of the dorsal aspects of the periaqueductal gray (dPAG): the dPAG-evoked freezing and the dPAG post-stimulation freezing. Evidence is presented in support of the hypothesis that whereas dPAG-evoked freezing would serve as a model of panic attacks, the dPAG post-stimulation freezing appears to be a model of panic disorder. It is also proposed that conditioned freezing plus dPAG electrical stimulation might also mimic panic disorder with agoraphobia. A model of serotoninergic modulation through on- and off-cells of the defense reaction generated in the dPAG is also presented. The understanding of how the periaqueductal gray generates and elaborates different types of freezing is of relevance for our better knowledge of distinct types of anxiety such as panic disorder or generalized anxiety disorder.

CRF type 1 receptors in the dorsal periaqueductal gray modulate anxiety-induced defensive behaviors

The dorsal periaqueductal gray (dPAG) is involved in defensive coping reactions to threatening stimuli. Corticotropin releasing factor (CRF) is substantially implicated as a direct modulator of physiological, endocrine and behavioral responses to a stressor. Previous findings demonstrate a direct role of the central CRF system in dPAG-mediated defensive reactions toward a threatening stimulus. These include anxiogenic behaviors in the elevated plus maze (EPM) in rats and defensive reactions in both the mouse defense test battery (MDTB) and rat exposure test (RET) paradigms in mice. Furthermore, CRF was shown to directly and dose-dependently excite PAG neurons in vitro. The aim of the present series of experiments was to directly evaluate the role of the CRF1 receptor (CRF1) in dPAG-induced defensive behaviors in the MDTB and the RET paradigms. For this purpose, cortagine, a novel CRF1-selective agonist, was directly infused into the dPAG. In the RET the high dose of cortagine (100 ng) significantly affected spatial avoidance measures and robustly increased burying behavior, an established avoidance activity, while having no effects on behaviors in the MDTB. Collectively, these results implicate CRF1 in the dPAG as a mediator of temporally and spatially dependent avoidance in response to controllable and constant stimuli.

Innate defensive behaviour and panic-like reactions evoked by rodents during aggressive encounters with Brazilian constrictor snakes in a complex labyrinth: behavioural validation of a new model to study affective and agonistic reactions in a prey versus predator paradigm

Defensive behaviour has been extensively studied, and non-invasive methodologies may be interesting approaches to analyzing the limbic system function as a whole. Using experimental models of animals in the state of anxiety has been fundamental in the search for new anxiolytic and antipanic compounds. The aim of this present work is to examine a new model for the study of affective behaviour, using a complex labyrinth consisting of an arena and galleries forming a maze. Furthermore, it aims to compare the defensive behaviour of Wistar rats, Mongolian gerbils and golden hamsters in a complex labyrinth, as well as the defensive behaviour of Meriones unguiculatus in aggressive encounters with either Epicrates cenchria assisi or Boa constrictor amarali in this same model. Among species presently studied, the Mongolian gerbils showed better performance in the exploration of both arena and galleries of the labyrinth, also demonstrating less latency in finding exits of the galleries. This increases the possibility of survival, as well as optimizes the events of encounter with the predator. The duration of alertness and freezing increased during confrontation with living Epicrates, as well as the duration of exploratory behaviour in the labyrinth. There was an increase in the number of freezing and alertness behaviours, as well as in duration of alertness during confrontations involving E.c. assisi, compared with behavioural reactions elicited by jirds in presence of B.c. amarali. Interestingly, the aggressive behaviour of Mongolian gerbils was more prominent against B.c. amarali compared with the other Boidae snake. E.c. assisi elicited more offensive attacks and exhibited a greater time period of body movement than B.c. amarali, which spent more time in the arena and in defensive immobility than the E.c. assisi. Considering that jirds evoked more fear-like reaction in contact with E.c. assisi, a fixed E.c. assisi kept in a hermetically closed acrylic box was used as control. In these prey/predator encounter-based experiments, there was an increase in the number of alertness and freezing behaviours exhibited by gerbils, and a decrease in the number of crossing elicited by them, when comparing confrontations between the living E.c. assisi and the control. The experiments were performed at 7.0 p.m. In the labyrinth, the snakes showed in confrontation similar performance to that observed in nature (organizing hunting behaviour, offensive/defensive attack, constriction, prey inspection and feeding behaviour), which were essential to the validity of the experiments and gave behavioural validation within the complex labyrinth.

Interactions between opioid-peptides-containing pathways and GABAA-receptors-mediated systems modulate panic-like-induced behaviors elicited by electric and chemical stimulation of the inferior colliculus

Aiming to clarify the effect of interactive interconnections between the endogenous opioid peptides-neural links and GABAergic pathways on panic-like responses, in the present work, the effect of the peripheral and central administration of morphine or the non-specific opioid receptors antagonist naloxone was evaluated on the fear-induced responses (defensive attention, defensive immobility and escape behavior) elicited by electric and chemical stimulation of the inferior colliculus. Central microinjections of opioid drugs in the inferior colliculus were also performed followed by local administration of the GABA(A)-receptor antagonist bicuculline. The defensive behavior elicited by the blockade of GABAergic receptors in the inferior colliculus had been quantitatively analyzed, recording the number of crossing, jump, rotation and rearing, in each minute, during 30 min, in the open-field test. The opioid receptors stimulation with morphine decreased the defensive attention, the defensive immobility and escape behavior thresholds, and the non-specific opioid receptors blockade caused opposite effects, enhancing the defensive behavior thresholds. These effects were corroborated by either the stimulation or the inhibition of opioid receptors followed by the GABA(A) receptor blockade with bicuculline, microinjected into the inferior colliculus. There was a significant increase in the diverse fear-induced responses caused by bicuculline with the pretreatment of the inferior colliculus with morphine, and the opposite effect was recorded after the pretreatment of the inferior colliculus nuclei with naloxone followed by bicuculline local administration. These findings suggest an interaction between endogenous opioid-peptides-containing connections and GABA(A)-receptor-mediated system with direct influence on the organization of the panic-like or fear-induced responses elaborated in the inferior colliculus during critical emotional states.

Functional and ultrastructural neuroanatomy of interactive intratectal/tectonigral mesencephalic opioid inhibitory links and nigrotectal GABAergic pathways: Involvement of GABAA and μ1-opioid receptors in the modulation of panic-like reactions elicited by electrical stimulation of the dorsal midbrain

In the present study, the functional neuroanatomy of nigrotectal-tectonigral pathways as well as the effects of central administration of opioid antagonists on aversive stimuli-induced responses elicited by electrical stimulation of the midbrain tectum were determined. Central microinjections of naloxonazine, a selective mu(1)-opiod receptor antagonist, in the mesencephalic tectum (MT) caused a significant increase in the escape thresholds elicited by local electrical stimulation. Furthermore, either naltrexone or naloxonazine microinjected in the substantia nigra, pars reticulata (SNpr), caused a significant increase in the defensive thresholds elicited by electrical stimulation of the continuum comprised by dorsolateral aspects of the periaqueductal gray matter (dlPAG) and deep layers of the superior colliculus (dlSC), as compared with controls. These findings suggest an opioid modulation of GABAergic inhibitory inputs controlling the defensive behavior elicited by MT stimulation, in cranial aspects. In fact, iontophoretic microinjections of the neurotracer biodextran into the SNpr, a mesencephalic structure rich in GABA-containing neurons, show outputs to neural substrate of the dlSC/dlPAG involved with the generation and organization of fear- and panic-like reactions. Neurochemical lesion of the nigrotectal pathways increased the sensitivity of the MT to electrical (at alertness, freezing and escape thresholds) and chemical (blockade of GABA(A) receptors) stimulation, suggesting a tonic modulatory effect of the nigrotectal GABAergic outputs on the neural networks of the MT involved with the organization of the defensive behavior and panic-like reactions. Labeled neurons of the midbrain tectum send inputs with varicosities to ipsi and contralateral dlSC/dlPAG and ipsilateral substantia nigra, pars reticulata and compacta, in which the anterograde and retrograde tracing from a single injection indicates that the substantia nigra has reciprocal connections with the dlSC/dlPAG featuring close axo-somatic and axo-dendritic appositions in both locations. In addition, ultrastructural approaches show inhibitory axo-axonic synapses in MT and inhibitory axo-somatic/axo-axonic synapses in the SNpr. These findings, in addition to the psychopharmacological evidence for the interaction between opioid and GABAergic mechanisms in the cranial aspects of the MT as well as in the mesencephalic tegmentum, offer a neuroanatomical basis of a pre-synaptic opioid inhibition of GABAergic nigrotectal neurons modulating fear in defensive behavior-related structures of the cranial mesencephalon, in a short link, and through a major neural circuit, also in GABA-containing perikarya and axons of nigrotectal neurons.

Effects of acute and chronic fluoxetine and diazepam on freezing behavior induced by electrical stimulation of dorsolateral and lateral columns of the periaqueductal gray matter

The defensive responses induced by electrical stimulation of the dorsal periaqueductal gray matter (dPAG) of the rat have been proposed as a model of panic attacks in humans. In the present study we investigated the acute and chronic effects of fluoxetine and diazepam on freezing and escape reactions elicited by electrical stimulation of the dorsolateral (dlPAG) and lateral (lPAG) columns of the periaqueductal gray matter (PAG). The frequencies of crossing, rearing, bouts of micturition and fecal boli were also recorded. Electrodes were unilaterally implanted in the brainstem aimed at the PAG. Drug treatments were given daily for 2 weeks with fluoxetine (5, 10 and 20 mg/kg ip), a selective inhibitor of serotonin reuptake, diazepam (1, 2 and 4 mg/kg ip), or saline. Drug effects were assessed acutely (15 min after the first injection) and chronically (15 min after the 14th injection). Chronic, but not acute, administration of fluoxetine caused a significant increase in the threshold of freezing without affecting the escape response elicited by dlPAG/lPAG stimulation. This characteristic pattern of effects could not be attributed to motor deficit, since this drug did not change the number of crossings and rearings. In contrast, no significant threshold changes were observed following acute and chronic treatment with diazepam. These data give further evidence for (a) an antiaversive effect of chronic treatment with fluoxetine, which caused a selective reduction in freezing behavior and neurovegetative responses associated with fearlike reaction elicited by dlPAG/lPAG electrical stimulation; (b) the involvement of the dlPAG and lPAG in the generation and organization of defensive responses and that freezing may probably be associated with panic attacks; and (c) the lack of effect of diazepam in this model is in line with its inefficacy as a panicolytic drug. The study of the unconditioned freezing behavior evoked by dlPAG/lPAG stimulation may constitute a new and interesting model for the study of panic disorder.

Defense reaction mediated by NMDA mechanisms in the inferior colliculus is modulated by GABAergic nigro-collicular pathways

Electrical stimulation of the inferior colliculus (IC) causes a behavioral activation together with autonomic responses similar to fear reactions to threatening situations. GABAergic mechanisms exert a tonic inhibitory control on the neural substrates of aversion in the IC insofar as local injections of GABA agonists or antagonists inhibit or mimic these defensive behaviors, respectively. Recently, we have shown that systemic injections of the GABA-A receptor agonist muscimol unexpectedly enhanced the freezing and escape responses provoked by gradual increases in the intensity of the electrical stimulation of the IC. Taking into account that the neural circuits mediated by excitatory amino acids (EAA) in the IC may be responsible for the integration of fear states, in the present study we examined whether the defensive behavior induced by local injections of NMDA into the IC is influenced by prior treatment with systemic muscimol and also whether this GABAergic control could be exerted by GABAergic fibers that project to the inferior colliculus from the substantia nigra pars reticulata (SNpr). Rats were implanted with two guide-cannulae aimed at the IC and SNpr through which drug microinfusions with glass micropipette could be made with reduced brain damage. One week after surgery, the animals received either NMDA (7 nmol/0.2 microl) or saline into the IC and were placed into the middle of an enclosure where behavioral responses such as freezing, crossings, jumping, rearing, and turnings could be measured as an indirect index of unconditioned fear. These animals were pretreated either with saline or muscimol (0.5 mg/kg, IP) or with brain injections of saline or muscimol (1 nmol/0.2 ìl into SNpr). NMDA applied into the IC produced a behavioral activation with significant increases in all behavioral measures. IP injections of muscimol or into the SNpr enhanced the defense reaction caused by microinjections of NMDA into the IC. These findings give support to the idea that unconditioned defensive responses generated in the IC may be mediated by NMDA mechanisms. Additionally, a reduction of the inhibitory control exerted by nigrocollicular GABAergic neurons seems to be responsible for the unexpected pro-aversive action of systemic injections of muscimol on the neural substrates of aversion mediated by NMDA in the IC.

The relevance of neuronal substrates of defense in the midbrain tectum to anxiety and stress: empirical and conceptual considerations

The medial hypothalamus, amygdala, and dorsal periaqueductal gray constitute the main neural substrates for the integration of aversive states in the brain. More recently, some regions of the mesencephalon, such as the superior and inferior colliculi have also been proposed as part of this system. In fact, fear-like behaviors often result when these sites are electrically or chemically stimulated. 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 gamma-aminobutyric acid (GABA)-mediated neurotransmission, which exerts a tonic inhibitory control on the neural circuits responsible for the so-called defense behavior repertoire. Besides GABA, also 5-hydroxy tryptamine serotonin (5-HT), opioids, neuropeptides, histaminergic and excitatory amino acids have all been implicated in the regulation of anxiety-related behaviors induced by stimulation of midbrain tectum. Efforts have been made to characterize how these neurotransmitters interact with each other in the organization of these reactions to aversive stimulation. In this review, we summarize the evidence linking the brain's defense response systems to the concept of fear-anxiety. Furthermore, a case is made for the consideration of the relevance of this body of data to the search for the physiological underpinnings of depression and its consequences.

Neuroanatomical and psychopharmacological evidence for interaction between opioid and GABAergic neural pathways in the modulation of fear and defense elicited by electrical and chemical stimulation of the deep layers of the superior colliculus and dorsal periaqueductal gray matter

The effects of central administration of opioid antagonists on the aversive responses elicited by electrical (at the freezing and escape thresholds) or chemical stimulation (crossings, rearings, turnings and jumps, induced by microinjections of bicuculline) of the midbrain tectum were determined. Central microinjections of naloxone and naltrexone in the mesencephalic tectum caused a significant increase in the freezing and escape thresholds elicited by electrical midbrain tectum stimulation. Furthermore, both opioid antagonists caused a significant decrease in the mean incidence of aversive behavioral responses induced by microinjections of bicuculline in the deep layers of the superior colliculus (DLSC) and in dorsal aspects of the periaqueductal gray matter (DPAG), as compared with controls. These findings suggest an opioid modulation of the GABAergic inhibitory inputs controlling the aversive behavior elicited by midbrain tectum stimulation. In fact, immunohistochemical evidence suggests that the dorsal mesencephalon is rich in beta-endorphin-containing neurons and fibers with varicosities. Iontophoretical microinjections of the neurotracer biodextran in the substantia nigra, pars reticulata (SNpr), show nigro-tectal pathways connecting SNpr with the same neural substrate of the DPAG rich in neuronal cells immunoreactive for opioid peptides. Labeled neurons of the DLSC and periaqueductal gray matter send inputs with varsicosities to ipsi- and contralateral DPAG and ipsilateral SNpr. These findings, in addition to the psychopharmacological evidence for the interaction between opioid and GABAergic mechanisms, offer a neuroanatomical basis of a possible presynaptic opioid inhibition of GABAergic nigro-tectal neurons modulating the fear in aversive structures of the cranial mesencephalon, in a short link, and maybe through a major neural circuit, also in GABA-containing perikarya of nigro-tectal neurons.

Opposite effects of substance P fragments C (anxiogenic) and N (anxiolytic) injected into dorsal periaqueductal gray

Recent findings implicating neurokinins in the expression of anxiety-like behaviors have stimulated interest in the participation of these neuropeptides in the dorsal periaqueductal gray matter (dPAG), one of the main output regions of the brainstem for the expression of defense reaction. Studies on the behavior of rats submitted to the elevated plus-maze test in this laboratory have shown that microinjections of substance P into the dorsal periaqueductal gray produce anxiogenic-like effects. Now, we analyze what portion of the molecule of substance P is responsible for these effects through the examination of the action of its C- and N-terminus fragments (6-11 and 1-7) in the elevated plus-maze. We also investigated whether these effects are influenced by prior treatment with the tachykinin NK(1) receptor antagonist 17-beta-hydroxy-17-alpha-ethynyl-5alpha-androstanol[3,2-b]pyrimido[1,2-a]benzimidazole (WIN51,708). To this end, rats were implanted with a cannula in the dorsal periaqueductal gray and injected 1 week later with equimolar doses (17.5 and 35 pmol) of either C- or N-fragments of substance P and tested in the elevated plus-maze. The results show that the C-terminal fragment has an anxiogenic profile of effects, including reduction in the number of entries and time spent in the open arms of the maze, plus increases in scanning, stretched-attend posture, head dipping and flat-back approach. On the other hand, the N-terminal fragment produced opposite effects, namely, an increase in the number of entries and time spent in the open arms of the maze accompanied by an increase in end-arm activity, rearing and head dipping. The tachykinin NK(1) receptor antagonist WIN51,708 (20 mg/kg, i.p.) inhibited the effects of the carboxy-terminal of substance P while it did not change the effects of the N-terminal fragment. Microinjection of WIN51,708 (20 mg/kg, i.p.), by its own, did not produce any significant effects. Therefore, the results indicate that the anxiogenic effects of substance P injected into the dorsal periaqueductal gray are encoded by its carboxy-terminal sequence and due to its action on tachykinin NK(1) receptors.

Place aversion induced by microinjections of C-fragment of substance P into the dorsal periaqueductal gray of rats is mediated by tachykinin NK1 receptors

Neural circuits in the dorsal periaqueductal gray matter (dPAG) play an important role in the integration of defensive behavior. The neurokinin substance P causes conditioned place aversion when administered into this region. The present study examined whether these effects may be mimicked by its carboxy-terminal amino acid sequence and whether they are influenced by prior treatment with the tachykinin NK1 receptor antagonist WIN51,708. The behavioral testing apparatus is a circular open field consisting of 4 uniform quadrants that are equally preferred by the rats prior to drug treatments. For conditioning, rats received drug injections on three consecutive days and were placed into their assigned quadrant. The carboxy-terminal analog (17.5 pmol/0.2 microl) applied into the dPAG produced place aversion effects with reduced time spent in the drug-paired quadrant on the testing day. The effects of the carboxy-terminal analog was antagonized by pretreatment with WIN51,708 (20 mg/kg, i.p.). Microinjection of WIN51,708 (20 mg/kg, i.p.), by its own, did not produce significant effects. These findings suggest that previous reports showing conditioned place aversion effects of SP injected into the dPAG are encoded by its carboxy-terminal sequence and due to its action on tachykinin NK1 receptors.

Place aversion induced by blockade of mu or activation of kappa opioid receptors in the dorsal periaqueductal gray matter

Neural circuits in the dorsal periaqueductal grey matter (DPAG) play an important role in the integration of defensive behaviour. As considerable numbers of mu and kappa opioid receptors have been found in this region, we studied the effects of morphine, [3H]-[H-D-Phe-Cys-Tyr- D-Trp-Orn-Thr-Pen-Thr-NH2] (CTOP), a selective peptide antagonist for mu opioid receptors, U-50488H, a specific agonist for kappa opioid receptors, and nor-binaltorphimine (nor-BNI), a long-lasting selective antagonist for kappa opioid receptors, injected into the DPAG of rats submitted to the corral method, a conditioned place preference test. The behavioural testing apparatus was a circular open field consisting of four uniform quadrants that were equally preferred by the rats prior to drug treatments. For conditioning, rats received drug injections on three consecutive days and were placed into their assigned quadrant. Injection of 40 nmol of morphine into the DPAG produced place aversion effects, with reduced time spent in the drug-paired quadrant on the testing day. These place aversion effects were not inhibited by previous DPAG microinjection of CTOP (1 nmol) but were significantly reduced by prior systemic injections of nor-BNI (2 mg / kg). Microinjection of CTOP alone produced a clear decrease in the time spent in the treatment quadrant, whereas nor-BNI alone did not. Similarly, microinjection into the DPAG of the kappa agonist U-50488H (10 nmol) mimicked the effects of morphine, also producing place aversion for the drug-paired quadrant. These findings suggest that blockade of mu opioid receptors or activation of kappa opioid receptors in the DPAG may produce conditioned place aversion.

Blockade of mu- and activation of kappa-opioid receptors in the dorsal periaqueductal gray matter produce defensive behavior in rats tested in the elevated plus-maze

We studied the effects of morphine injected into the dorsal periaqueductal gray using conventional and novel ethological measures of the behavior of rats submitted to the elevated plus-maze test. Morphine (20 and 40 nmol) applied into the dorsal periaqueductal gray produced dose dependent aversive effects with reduced entries and time spent in the open arms. Freezing behavior was the most prominent novel ethological measure produced by microinjections of these doses of morphine. These pro-aversive effects were not inhibited by previous dorsal periaqueductal gray microinjection of [D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2)](CTOP) (1 nmol), a selective peptide antagonist for mu-opioid receptors. On one hand, microinjection of CTOP produced a dose dependent increase in scanning and stretched attended postures, by its own. On the other hand, the aversive effects of morphine into the dorsal periaqueductal gray microinjections were significantly reduced by systemic administration of nor-binaltorphimine, an opioid receptor antagonist with a tardive and selective action at kappa-opioid receptors. These findings suggest that mechanisms mediated by mu-opioid receptors in the dorsal periaqueductal gray may be involved in the control of risk assessment behavior. On the other hand, the pro-aversive effects produced by microinjections of morphine into the dorsal periaqueductal gray are probably mediated by kappa-opioid receptors.

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.

Escapable and inescapable stress differentially and selectively alter extracellular levels of 5-HT in the ventral hippocampus and dorsal periaqueductal gray of the rat

The effects of escapable and yoked inescapable electric tailshocks on extracellular levels of serotonin (5-HT) in the ventral hippocampus and dorsal periaqueductal gray (dPAG) were measured by in vivo microdialysis. Inescapable, but not escapable shock increased extracellular 5-HT in the ventral hippocampus relative to restrained controls. Basal levels of 5-HT were elevated 24 h after inescapable shock, and previously inescapably shocked subjects exhibited an exaggerated 5-HT response to 2 brief footshocks. In contrast, escapable, but not inescapable shock, increased extracellular 5-HT in the dPAG, increased basal 5-HT in the dPAG 24 h later, and led to an enhanced 5-HT response to subsequent brief footshock.

Regulation of contextual conditioning by the median raphe nucleus

The median raphe nucleus (MRN) has been suggested as the origin of a behavioral inhibition system that projects to the septum and hippocampus. Electrical stimulation of this mesencephalic area causes behavioral and autonomic manifestations characteristic of fear such as, freezing, defecation and micturition. In this study we extend these observations by analyzing the behavioral and autonomic responses of rats with lesions in the MRN submitted to a contextual conditioning paradigm. The animals underwent electrolytic or sham lesions of the median raphe nucleus. One day (acute) or 7 days (chronic) later they were tested in an experimental chamber where they received 10 foot-shocks (0.7 mA, 1 s with 20-s interval). The next day, sham and MRN-lesioned animals were tested again either in the same or in a different experimental chamber. During this, the duration of freezing, rearings, bouts of micturition and number of fecal boli were recorded. Sham-operated rats placed in the same chamber showed more freezing than rats exposed to a different context. This freezing behavior was clearly suppressed in rats with acute or chronic lesions in the MRN. MRN lesions also reduced the bouts of micturition and number of fecal boli. These rats showed a reduced number of rearings than sham-lesioned rats. This effect is probably the result of the displacement effect provoked by freezing since no significant differences in the number of rearings could be observed between these animals and the NMR-lesioned rats tested in an open field. This lesion produced higher horizontal locomotor activity in this test than the controls (sham-lesioned rats). These results point to the importance of the median raphe nucleus in the processing of fear conditioning with freezing being the most salient feature of it. Behavioral inhibition is also under control of MRN but its neural substrate seems to be dissociated from that of contextual fear.

Immunohistochemical distribution of cannabinoid CB1 receptors in the rat central nervous system

Immunohistochemical distribution of cannabinoid receptors in the adult rat brain was studied using specific purified antibodies against the amino-terminus of the CB1 receptor. Our results generally agree well with the previous studies using CB1 receptor autoradiography and messenger RNA in situ hybridization. However, because of its greater resolution, immunohistochemistry allowed identification of particular neuronal cells and fibers that possess cannabinoid receptors. CB1-like immunoreactivity was found in axons, cell bodies and dendrites, where it appeared as puncta in somata and processes. Both intensely and moderately or lightly stained neurons were observed. The intensely stained neurons were dispersed and only occur in cortical structures including hippocampal formation and olfactory bulb. Moderately or lightly stained neurons were found in caudate-putamen and amygdala. In the hippocampal formation only intensely stained neurons were observed. The cell bodies of pyramidal neurons in CA1 and CA3 fields appeared to be unstained but surrounded by a dense plexus of immunoreactive fibers. The granule cells in the dentate area were also immunonegative. Many intensely stained neurons were located at the base of the granule cell layer. CB1-like immunoreactive neurons and fibers were also found in the somatosensory, cingulate, perirhinal, entorhinal and piriform cortices, in claustrum, amygdaloid nuclei, nucleus accumbens and septum. Beaded immunoreactive fibers were detected in periaqueductal gray, nucleus tractus solitarius, spinal trigeminal tract and nucleus, dorsal horn and lamina X of the spinal cord. A triangular cap-like mass of immunoreactivity was found to surround the basal part of the Purkinje cell body in the cerebellum. Only small, lightly stained cells were found in the molecular layer in the cerebellum close to the Purkinje cell layer. The CB1 receptor is widely distributed in the forebrain and has a more restricted distribution in the hindbrain and the spinal cord. It appears to be expressed on cell bodies, dendrites and axons. According to the location and morphology, many, but not all, CB1-like immunoreactive neurons appear to be GABAergic. Therefore, cannabinoids and cannabinoid receptors may play a role in modulating GABAergic neurons.

Effects of 5-HT2 receptors blockade on fear-induced analgesia elicited by electrical stimulation of the deep layers of the superior colliculus and dorsal periaqueductal gray

The deep layers of the superior colliculus (DLSC) and the dorsal periaqueductal gray matter (DPAG) have been implicated in the control of defensive-like behaviors. Electrical and chemical stimulation of these structures elicits fear and escape behaviour, expressed by immobility (freezing) and wild running, followed by jumps and rapid rotations. There is evidence that the neural substrates responsible for defensive behavior in this level of the midbrain tectum (MT) may also be responsible for fear-induced analgesia. This study was aimed at examining the characteristics of the analgesia that follows the defense-oriented reactions induced by electrical midbrain tectum stimulation at freezing and escape thresholds. The animals were submitted to the tail-flick test, following the induction of the defense behavioral responses. The obtained results show that the antinociception that follows the freezing and escape behaviors were not antagonized by MT microinjections of the opioid antagonist naltrexone. These results emphasize previous data showing the non-opioid nature of this analgesia. On the other hand, the fear-induced analgesia was inhibited by microinjections of the serotonergic blockers, methysergide and ketanserin in the MT. Since methysergide is a non-specific antagonist of 5-HT receptors and ketanserin acts with a high degree of specificity at 5-HT2 receptors the present results suggest that activation of 5-HT2 receptors may be implicated in the antinociception induced by midbrain tectum stimulation.

Effects of the blockade of opioid receptor on defensive reactions elicited by electrical stimulation within the deep layers of the superior colliculus and DPAG

The effects of peripheral administration of naloxone and naltrexone on aversive thresholds (freezing and escape reactions) elicited by electrical stimulation of the midbrain tectum were determined. Naloxone caused a significant increase in the freezing and flight thresholds elicited by electrical stimulation in the deep layers of the superior colliculus and of dorsal regions of the periaqueductal grey matter, as compared with controls. These effects were confirmed by the peripheral administration of naltrexone. These findings suggest that opioid receptors can modulate aversive behaviour elicited by midbrain tectum stimulation.

Effects of microinjections of mu and kappa receptor agonists into the dorsal periaqueductal gray of rats submitted to the plus maze test

Several lines of evidence have shown that aversive states are under the influence of opioid mechanisms in the dorsal periaqueductal gray (DPAG). In order to characterize the type of opioid receptors involved in these effects in this work we injected DAMGO and U50,488H, mu and kappa selective agonists, respectively, directly in this structure. Rats implanted with chemitrode in the DPAG were submitted to the elevated plus maze test for 5 min. The effects of DAMGO (0.1-1 nmol/0.2 microliter) and U50,488H (1-10 nmol/0.2 microliter) following administration into DPAG were studied. Low doses of DAMGO (0.1 and 0.3 nmol) caused dose-dependent increases in the number of entries and time spent in the open arms while an overall deficit in the exploratory activity was produced by the higher dose used (1.0 nmol). Clear aversive effects were observed following the administration of U50,488H in the DPAG. The antiaversive effects of 0.3 nmol DAMGO were inhibited by the intraperitoneal administration of the mu receptor antagonist naltrexone (2.0 mg/kg, IP) whereas the aversive effects of 5.0 nmol U50,488H were antagonized by the selective kappa receptor antagonist nor-binaltorphimine (1.0 mg/kg, IP). It is suggested that activation of mu receptors inhibit and kappa receptors enhance the neural substrate of aversion in the DPAG.

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.

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)

Involvement of opioid mechanisms in the dorsal periaqueductal gray in drug abuse

The opiate abstinence syndrome is comprised of motivational symptoms and physical correlates. In rats, physical correlates have been characterized as intense hyperactivity together with jumping, ptosis, teeth chattering, wet dog shakes, and diarrhea. Motivational symptoms have often been associated with the development of place aversions. The search for the neural substrates for the motivational symptoms and physical correlates of the opiate withdrawal syndrome has not reached a consensus as to the existence of a common or separate neural basis responsible for them. In our laboratory, we have gathered evidence that microinjection of morphine into the DPAG inhibits defensive behaviors. However, DPAG microinjections of high doses may produce a 'fearful' hyperactivity as measured in a circular arena or produce anxiogenic-like effects as measured in the elevated plus maze. The antiaversive effects of morphine result from its action on mu receptors and the aversive ones may be associated with its action on other opiate receptors. These findings implicate the PAG as an important structure of the neurobiological substrate of the emotional and physical correlates of the opiate abstinence syndrome.

Aversive and antiaversive effects of morphine in the dorsal periaqueductal gray of rats submitted to the elevated plus-maze test

The dorsal periaqueductal gray (DPAG) is a well-known region for processing defensive behavior in the brainstem. Rats implanted with cannulae in the DPAG were submitted to the elevated plus-maze test for 5 min. The effects of morphine following systemic (0.1-1.0 mg/kg) or DPAG administration (5-30 nmol) were compared with the benzodiazepine compound midazolam injected similarly (1-10 mg/kg, IP, and 10-80 nM, DPAG). Morphine and midazolam caused dose-dependent increases in the number of entries and time spent in the open arms. A systemic injection of naloxone in doses that block mu-opioid receptors reversed the effects of centrally administered morphine. Higher doses of morphine (70 nmol) induced a non-naloxone-reversible "fearful" hyperreactivity. It is suggested that low doses of morphine inhibit the neural substrate of aversion in the DPAG, probably through activation of mu-receptors, and that microinjections of higher doses of morphine cause proaversive actions not mediated by these opioid receptors.

Evidence for the involvement of serotonin in the antinociception induced by electrical or chemical stimulation of the mesencephalic tectum

A great deal of evidence has shown that electrical stimulation or microinjections of GABAA blockers, such as bicuculline, into the midbrain tectum (MT) produce escape behavior, which has been associated to fear. This study was aimed to examine the characteristics of the analgesia that follows the escape behavior induced by electrical (freezing and escape thresholds) and chemical (bicuculline microinjections) stimulation of the midbrain tectum. Immediately after the expression of the aversive responses the rats were submitted to the tail-flick test. The obtained results show that analgesia always follows aversive responses integrated at the MT level regardless of the kind of stimulation applied. The antinociceptive effects induced by either electrical or chemical stimulation of the MT were not antagonized by central microinjections of naloxone. On the other hand, the non-specific serotonin antagonist methysergide microinjected into the MT was effective in antagonizing the analgesia induced by any of the aversive stimulations. Based on these results we suggest that serotonin, but not opioid mechanisms, may be involved in the integration of antinociceptive responses to stimulation of the midbrain tectum.

Opioid mediation of the antiaversive and hyperalgesic actions of bradykinin injected into the dorsal periaqueductal gray of the rat

Reported evidence indicates that the dorsal region of the periaqueductal gray matter (PAG) is involved in the modulation of both pain and aversion, and that opioid mechanisms, among others, participate in their modulation. Since many central actions of bradykinin (BK) have been shown to be similar to those of morphine, the present was undertaken to measure the effects of microinjection of BK into the PAG on the thresholds of aversive electrical stimulation of the same brain area and of dental pulp electrical stimulation. Bradykinin, injected into the dorsal PAG, induced a dose-dependent increase in the aversive threshold, an effect similar to that reported by others for morphine. Also, as reported for morphine, the antiaversive effect of BK was antagonized by naloxone injected intraperitoneally. Whereas subcutaneously administered morphine induced marked analgesia, intra-PAG administration of BK caused a small but significant hyperalgesia. Similarly, morphine injected into the dorsal PAG tended to cause hyperalgesia instead of analgesia. Furthermore, the hyperalgesic effect of BK also appears to involve opioid mechanisms since it was blocked by naloxone. As in previously reported studies, intracerebroventricularly injected BK raised the pain threshold. These results indicate that BK mobilizes opioid mechanisms in the dorsal PAG that inhibit aversion but not pain.

Antiaversive action of benzodiazepines on escape behavior induced by electrical stimulation of the inferior colliculus

In the present work, evidence is presented for the involvement of inferior colliculus in the generation and elaboration of aversive responses which suggests that this structure may be part of a brain system that commands aversive states. Electrical stimulation of the inferior colliculus of rats placed inside an open field allowed the determination of thresholds for the escape response. Afterward these rats were placed inside a shuttle box and submitted to a switch-off paradigm. Electrical stimulation of the inferior colliculus was applied at a current intensity 5% below the escape threshold. This electrical stimulation showed clear aversive properties: the rats quickly learned to interrupt it. Systemic administration (3 and 5.6 mg/kg) as well as inferior colliculus microinjections (10 and 20 nmol) 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 GABA-A agonist muscimol (0.1 and 0.5 nmol). These results suggest that neural substrates commanding defensive behavior in the inferior colliculus may be depressed by benzodiazepines as part of the anxiolytic action of these compounds. This antiaversive action may be produced by the enhancement of GABA-A mechanisms.