Role of amygdala in conditioned and unconditioned fear generated in the periaqueductal gray

The modulatory role of serotonin-1A receptors of the basolateral amygdala and dorsal periaqueductal gray on the impact of hormonal variation on the conditioned fear response

Despite significant advances in the study of fear and fear memory formation, little is known about fear learning and expression in females. This omission has been proven surprising, as normal and pathological behaviors are highly influenced by ovarian hormones, particularly estradiol and progesterone. In the current study, we investigated the joint influence of serotonin (5-HT) neurotransmission and estrous cycle phases (low or high levels of estradiol and progesterone) on the expression of conditioned fear in a group of female rats that were previously divided according to their response to stressful stimuli into low or high anxiety-like subjects. The baseline amplitude of the unconditioned acoustic startle responses was high in high-anxiety female rats, with no effect on the estrous cycle observed. Data collected during the proestrus-estrus phase revealed that low-anxiety rats had startle amplitudes similar to those of high-anxiety rats. It is supposed that high-anxiety female rats benefit from increased estradiol and progesterone levels to achieve comparable potentiated startle amplitudes. In contrast, female rats experienced a significant decrease in hormone levels during the Diestrus phase. This decrease is believed to play a role in preventing them from displaying a heightened startle response when faced with strongly aversive stimuli. Data collected after 5-HT and 8-OH-DPAT were administered into the basolateral nuclei and dorsal periaqueductal gray suggest that 5-HT neurotransmission works with progesterone and estrogen to reduce startle potentiation, most likely by activating the serotonin-1A receptor subtype.

Pharmacological and Physiological Correlates of the Bidirectional Fear Phenotype of the Carioca Rats and Other Bidirectionally Selected Lines

The Carioca rat lines originated from the selective bidirectional breeding of mates displaying extreme defense responses to contextual conditioned fear. After three generations, two distinct populations could be distinguished: the Carioca High- and Low-conditioned Freezing rats, CHF, and CLF, respectively. Later studies identified strong anxiety-like behaviors in the CHF line, while indications of impulsivity and hyperactivity were prominent in the CLF animals. The present review details the physiological and pharmacological-related findings obtained from these lines. The results discussed here point towards a dysfunctional fear circuitry in CHF rats, including alterations in key brain structures and the serotoninergic system. Moreover, data from these animals highlight important alterations in the stress-processing machinery and its associated systems, such as energy metabolism and antioxidative defense. Finally, evidence of an alteration in the dopaminergic pathway in CLF rats is also debated. Thus, accumulating data gathered over the years, place the Carioca lines as significant animal models for the study of psychiatric disorders, especially fear-related ones like anxiety.

Behavioral sensitization induced by dorsal periaqueductal gray electrical stimulation is counteracted by NK1 receptor antagonism in the ventral hippocampus and central nucleus of the amygdala

A single threatening experience may change the behavior of an animal in a long-lasting way and elicit generalized behavioral responses to a novel threatening situation that is unrelated to the original aversive experience. Electrical stimulation (ES) of the dorsal periaqueductal gray (dPAG) produces a range of defensive reactions, characterized by freezing, escape, and post-stimulation freezing (PSF). The latter reflects the processing of ascending aversive information to prosencephalic structures, including the central nucleus of the amygdala (CeA), which allows the animal to evaluate the consequences of the aversive situation. This process is modulated by substance P (SP) and its preferred receptor, neurokinin 1 (NK1). The ventral hippocampus (VH) has been associated with the processing of aversive information and expression of emotional reactions with negative valence, but the participation of the VH in the expression of these defensive responses has not been investigated. The VH is rich in NK1 receptor expression and has a high density of SP-containing fibers. The present study examined the role of NK1 receptors in the VH in the expression of defensive responses and behavioral sensitization that were induced by dPAG-ES. Rats were implanted with an electrode in the dPAG for ES, and a cannula was implanted in the VH or CeA for injections of vehicle (phosphate-buffered saline) or the NK1 receptor antagonist spantide (100 pmol/0.2 μL. Spantide reduced the duration of PSF that was evoked by dPAG-ES, without changing the aversive freezing or escape thresholds. One and 7 days later, exploratory behavior was evaluated in independent groups of rats in the elevated plus maze (EPM). dPAG-ES in rats that received vehicle caused higher aversion to the open arms of the EPM compared with rats that did not receive dPAG stimulation at both time intervals. Injections of spantide in the VH or CeA prevented the proaversive effects of dPAG-ES in the EPM only 1 day later. These findings suggest that NK1 receptors are activated in both the VH and CeA during the processing of aversive information that derives from dPAG-ES. As previously shown for the CeA, SP/NK1 receptors in the VH are recruited during PSF that is evoked by dPAG-ES, suggesting that a 24-h time window is susceptible to interventions with NK1 antagonists that block the passage of aversive information from the dPAG to higher brain areas.

Amygdala selectively modulates defensive responses evoked from the superior colliculus in non-human primates

Brain circuitry underlying defensive behaviors includes forebrain modulatory sites, e.g. the amygdala and hypothalamus, and midbrain effector regions, such as the deep/intermediate layers of the superior colliculus (DLSC). When disinhibited, this network biases behavior towards reflexive defense reactions. While well characterized in rodent models, little is known about this system in the primate brain. Employing focal pharmacological manipulations, we have previously shown that activation of the DLSC triggers reflexive defensive responses, including cowering, escape behaviors and defensive vocalizations. Here, we show that activation of the DLSC also disrupts normal dyadic social interactions between familiar pairs of monkeys. When the basolateral complex of the amygdala (BLA) was inhibited concurrent with DLSC activation, cowering behavior was attenuated, whereas escape behaviors and defensive vocalizations were not. Moreover, inhibition of the BLA, previously shown to produce a profound increase in dyadic social interactions, was unable to normalize the decrease in social behavior resulting from DLSC activation. Together these data provide an understanding of forebrain-midbrain interactions in a species and circuit with translational relevance for the psychiatry of anxiety and post-traumatic stress disorders.

Dorsal periaqueductal gray post-stimulation freezing is counteracted by neurokinin-1 receptor antagonism in the central nucleus of the amygdala in rats

Electrical stimulation of the dorsal periaqueductal gray (dPAG) in rats generates defensive responses that are characterized by freezing and escape behaviors, followed by post-stimulation freezing that resembles symptoms of panic attacks. dPAG post-stimulation freezing involves the processing of ascending aversive information to prosencephalic centers, including the amygdala, which allows the animal to evaluate the consequences of stressful situations. The basolateral nucleus of the amygdala (BLA) is thought to act as a filter for innate and learned aversive information that is transmitted to higher structures. The central (CeA) and medial (MeA) nuclei of the amygdala constitute an output for the expression of fear reactions through projections to limbic and brainstem regions. Neurokinin (NK) receptors are abundant in the CeA, MeA, and BLA, but their role in the expression of defensive responses and processing of aversive information that is evoked by electrical stimulation of the dPAG is still unclear. In the present study, we examined the role of NK1 receptors in these amygdala nuclei in the expression of defensive responses induced by electrical stimulation of the dPAG in rats and fear memory of this aversive stimulation. Rats were implanted with an electrode into the dPAG for electrical stimulation and one cannula in the CeA, MeA, or BLA for injections of vehicle (phosphate-buffered saline) or the NK1 receptor antagonist spantide (SPA; 100 pmol/0.2 μl). Injections of SPA into the CeA but not BLA or MeA reduced the duration of post-stimulation freezing evoked by electrical stimulation of the dPAG, without changing the aversive thresholds of freezing or escape. Twenty-four hours later, exploratory behavior was evaluated in the elevated plus maze test (EPM) in the CeA group of rats. Electrical stimulation of the dPAG rats that received vehicle exhibited higher aversion to the open arms of the EPM than sham rats that did not receive any dPAG stimulation. SPA injections into the CeA prevented the proaversive effects of electrical stimulation of the dPAG assessed in the EPM 24 h later. The present results suggest that neurokininergic modulation via NK1 receptors in the CeA but not BLA or MeA is involved in the processing of aversive information derived from dPAG stimulation. The long-lasting consequences of electrical stimulation of the dPAG may be prevented by NK1 receptor antagonism in the CeA.

Effect of dorsal and ventral hippocampal lesions on contextual fear conditioning and unconditioned defensive behavior induced by electrical stimulation of the dorsal periaqueductal gray

The dorsal (DH) and ventral (VH) subregions of the hippocampus are involved in contextual fear conditioning. However, it is still unknown whether these two brain areas also play a role in defensive behavior induced by electrical stimulation of the dorsal periaqueductal gray (dPAG). In the present study, rats were implanted with electrodes into the dPAG to determine freezing and escape response thresholds after sham or bilateral electrolytic lesions of the DH or VH. The duration of freezing behavior that outlasted electrical stimulation of the dPAG was also measured. The next day, these animals were subjected to contextual fear conditioning using footshock as an unconditioned stimulus. Electrolytic lesions of the DH and VH impaired contextual fear conditioning. Only VH lesions disrupted conditioned freezing immediately after footshock and increased the thresholds of aversive freezing and escape responses to dPAG electrical stimulation. Neither DH nor VH lesions disrupted post-dPAG stimulation freezing. These results indicate that the VH but not DH plays an important role in aversively defensive behavior induced by dPAG electrical stimulation. Interpretations of these findings should be made with caution because of the fact that a non-fiber-sparing lesion method was employed.

Participation of NK1 receptors of the amygdala on the processing of different types of fear

The amygdala, medial hypothalamus, dorsal periaqueductal gray (dPAG), superior and inferior colliculus together constitutes the encephalic aversion system which has been considered the main neural substrate for the integration of unconditioned aversive behavioral states. Within the amygdala the basolateral nucleus (BLA) is thought to act as a filter for innate and learned aversive information to higher structures, whereas the central nucleus (CeA) is considered the main output for the expression of fear reactions through projections to limbic and brainstem regions. Although neurokinin (NK) receptors are abundant in the amygdala, their role in the processing and expression of fear is yet unclear. In this study, we examined the role of SP/NK1 receptor system of the CeA and BLA on the expression of defensive responses elaborated by Wistar rats submitted to elevated plus maze (EPM) and to electrical stimulation (ES) of the dPAG. For EPM test, cannulae were implanted in the CeA and BLA for injections of substance P (SP - 10 and 100pmol/0.2μL) and spantide (SPA - 10, 100 and 500pmol/0.2μL). For ES of dPAG, aversive thresholds for freezing and escape responses as well as post-stimulation freezing (PSF) were measured in rats treated with PBS and SPA (100pmol/0.2μL) in CeA. Injections of SP into the CeA, but not the BLA, produced anxiogenic-like effects in the EPM test. SPA injected into the CeA had no effect on the exploratory behavior of rats submitted to the EPM but blocked the effects of SP. The duration of dPAG-PSF was also reduced significantly following injection of SPA in CeA but had no effect on thresholds for freezing and escape responses. The EPM gives the animal a control over its environment i.e. the option to choose or not to enter into the open arm and dPAG-PSF is thought to reflect a period when the animal evaluates the significance of dPAG-evoked aversion once the unconditioned responses of freezing and escape were elicited. The data indicate that SP may be involved in mediating responses of the animal in only certain types of aversive behavior and suggests a differential participation of the NK1 receptors in the processing of distinct types of fear in the amygdala.

Stimulation of the dorsal periaqueductal gray enhances spontaneous recovery of a conditioned taste aversion

Due to its relevance to clinical practice, extinction of learned fears has been a major focus of recent research. However, less is known about the means by which conditioned fears re-emerge (i.e., spontaneously recover) as time passes or contexts change following extinction. The periaqueductal gray represents the final common pathway mediating defensive reactions to fear and we have reported previously that the dorsolateral PAG (dlPAG) exhibits a small but reliable increase in neural activity (as measured by c-fos protein immunoreactivity) when spontaneous recovery (SR) of a conditioned taste aversion (CTA) is reduced. Here we extend these correlational studies to determine if inducing dlPAG c-fos expression through electrical brain stimulation could cause a reduction in SR of a CTA. Male Sprague-Dawley rats acquired a strong aversion to saccharin (conditioned stimulus; CS) and then underwent CTA extinction through multiple non-reinforced exposures to the CS. Following a 30-day latency period after asymptotic extinction was achieved; rats either received stimulation of the dorsal PAG (dPAG) or stimulation of closely adjacent structures. Sixty minutes following the stimulation, rats were again presented with the saccharin solution as we tested for SR of the CTA. The brain stimulation evoked c-fos expression around the tip of the electrodes. However, stimulation of the dPAG failed to reduce SR of the previously extinguished CTA. In fact, dPAG stimulation caused rats to significantly suppress their saccharin drinking (relative to controls) - indicating an enhanced SR. These data refute a cause-and-effect relationship between enhanced dPAG c-fos expression and a reduction in SR. However, they highlight a role for the dPAG in modulating SR of extinguished CTAs.

Patterns of phosphorylated tyrosine hydroxylase vary with song production in female starlings

Vocal signal production in male songbirds is well studied, but the neural correlates of female song production are poorly understood. In European starlings, females sing to defend nesting resources, and song can be considered agonistically motivated. Across vertebrates catecholamines strongly influence motivated, agonistic social behaviors. The present study was designed to provide insight into a possible role for catecholamine activity in territorial song in female starlings. We presented females that were defending nest-cavities with an unfamiliar female and assessed song production. We then measured immunolabeling for phosphorylated tyrosine hydroxylase (pTH-ir), the rate-limiting enzyme for catecholamine synthesis, in brain regions in which catecholamines stimulate agonistic behavior. Females that sang had higher pTH-ir in the caudomedial ventral tegmental area and the lateral septum than females that did not sing. Furthermore, the number of songs produced correlated positively with pTH-ir in the medial preoptic nucleus. Phosphorylation of TH is thought to occur after catecholamine release, so these results link increased catecholamine activity in several brain regions governing agonistic behavior to territorial song production in females.

Intra-amygdala infusion of the protein kinase Mzeta inhibitor ZIP disrupts foreground context fear memory

Protein kinase Mzeta has been the subject of much recent interest, as it is the only molecule currently identified to maintain memory. Despite the wealth of studies investigating PKMζ in memory, questions remain about which types of memory PKMζ supports. Further, it is unclear how long the inhibitor of PKMz, ζ-pseudosubstrate inhibitory peptide (ZIP) remains in the brain after infusion. Here, we demonstrate that foreground context fear memory requires PKMζ activity in the amygdala. We also show that ZIP is fully cleared from the brain by 24h after infusion. These data contribute to a growing body of literature that demonstrates that PKMζ plays a key role in maintaining amygdala-dependent memory and provides new information about the degradation timecourse of the most commonly used inhibitor of PKMζ, ZIP.

Central, but not basolateral, amygdala involvement in the anxiolytic-like effects of midazolam in rats in the elevated plus maze

The role of the amygdala in the mediation of fear and anxiety has been extensively investigated. However, how the amygdala functions during the organization of the anxiety-like behaviors generated in the elevated plus maze (EPM) is still under investigation. The basolateral (BLA) and the central (CeA) nuclei are the main input and output stations of the amygdala. In the present study, we ethopharmacologically analyzed the behavior of rats subjected to the EPM and the tissue content of the monoamines dopamine (DA) and serotonin (5-HT) and their metabolites in the nucleus accumbens (NAc), dorsal hippocampus (DH), and dorsal striatum (DS) of animals injected with saline or midazolam (20 and 30 nmol/0.2 µL) into the BLA or CeA. Injections of midazolam into the CeA, but not BLA, caused clear anxiolytic-like effects in the EPM. These treatments did not cause significant changes in 5-HT or DA contents in the NAc, DH, or DS of animals tested in the EPM. The data suggest that the anxiolytic-like effects of midazolam in the EPM also appear to rely on GABA-benzodiazepine mechanisms in the CeA, but not BLA, and do not appear to depend on 5-HT and DA mechanisms prevalent in limbic structures.

Involvement of midbrain tectum neurokinin-mediated mechanisms in fear and anxiety

Electrical stimulation of midbrain tectum structures, particularly the dorsal periaqueductal gray (dPAG) and inferior colliculus (IC), produces defensive responses, such as freezing and escape behavior. Freezing also ensues after termination of dPAG stimulation (post-stimulation freezing). These defensive reaction responses are critically mediated by _Y-aminobutyric acid and 5-hydroxytryptamine mechanisms in the midbrain tectum. Neurokinins (NKs) also play a role in the mediation of dPAG stimulation-evoked fear, but how NK receptors are involved in the global processing and expression of fear at the level of the midbrain tectum is yet unclear. The present study investigated the role of NK-1 receptors in unconditioned defensive behavior induced by electrical stimulation of the dPAG and IC of male Wistar rats. Spantide (100 pmol/0.2 µL), a selective NK-1 antagonist, injected into these midbrain structures had anti-aversive effects on defensive responses and distress ultrasonic vocalizations induced by stimulation of the dPAG but not of the IC. Moreover, intra-dPAG injections of spantide did not influence post-stimulation freezing or alter exploratory behavior in rats subjected to the elevated plus maze. These results suggest that NK-1 receptors are mainly involved in the mediation of defensive behavior organized in the dPAG. Dorsal periaqueductal gray-evoked post-stimulation freezing was not affected by intra-dPAG injections of spantide, suggesting that NK-1-mediated mechanisms are only involved in the output mechanisms of defensive behavior and not involved in the processing of ascending aversive information from the dPAG.

Effects of neurokinin-1 and 3-receptor antagonists on the defensive behavior induced by electrical stimulation of the dorsal periaqueductal gray

The dorsal periaqueductal gray (dPAG) is the main output structure for the defensive response to proximal aversive stimulation. Panic-like responses, such as freezing and escape behaviors, often result when this structure is electrically stimulated. Freezing also ensues after termination of the dPAG stimulation (post-stimulation freezing (PSF)). GABA and 5-HT have been proposed as the main neuromediators of these defense reactions. Neurokinins (NKs) also play a role in the defense reaction; however, it is unclear how the distinct types of NK receptors are involved in the expression of these fear responses. This study investigated the role of NK-1 and NK-3 receptors in the unconditioned defensive behaviors induced by electrical stimulation of the dPAG of rats, with and without previous experience with contextual fear conditioning (CFC). Spantide (100 ρmol/0.2 μl) and SB 222200 (50 and 100 ρmol/0.2 μl), selective antagonists of NK-1 and NK-3 receptors, respectively, were injected into the dPAG. Injection of spantide had antiaversive effects as determined by stimulation of the dPAG in naive animals and in animals subjected previously to CFC. SB 222200 also increased these aversive thresholds but only at doses that caused a motor deficit. Moreover, neither spantide nor SB 222200 influenced the PSF. The results suggest that NK-1 receptors are mainly involved in the mediation of the defensive behaviors organized in the dPAG. Because dPAG-evoked PSF was not affected by intra-dPAG injections of either spantide or SB 222200, it is suggested that neurokinin-mediated mechanisms are not involved in the processing of ascending aversive information from the dPAG.

Ventrolateral periaqueductal gray lesion attenuates nociception but does not change anxiety-like indices or fear-induced antinociception in mice

The exposure of rodents to an open elevated plus-maze (oEPM: four open arms raised from the floor) elicits naloxone-insensitive antinociception. Midazolam infusion into the dorsal portion of the periaqueductal gray (dPAG), a structure of the descending inhibitory system of pain, failed to alter oEPM-induced antinociception. Chemical lesion of dorsomedial and dorsolateral PAG attenuated defensive behavior in the standard EPM (sEPM), an animal model of anxiety, but failed to change oEPM-induced antinociception. The present study investigated the effects of bilateral lesion, with the injection of NMDA (N-methyl-D-aspartic acid), of the ventrolateral column of PAG (vlPAG) (i) on nociceptive response induced by 2.5% formalin injected into the right hind paw (nociception test) in mice exposed to the enclosed EPM (eEPM: four enclosed arms - a non-aversive situation) or to the oEPM and (ii) on anxiety indices in mice exposed to the sEPM without prior formalin injection. Results showed that oEPM-induced antinociception was not altered by lesion of vlPAG. Nevertheless, the lesion reduced the nociceptive response in mice exposed to the eEPM and increased general locomotor activity during the eEPM and oEPM exposure. Furthermore, vlPAG lesion did not alter anxiety-like indices in mice exposed to the sEPM. The results suggest that vlPAG does not play a role in oEPM-induced antinociception or in defensive reactions assessed in the sEPM. Moreover, vlPAG inactivation induces pain inhibition in mice not exposed to an aversive situation and seems to increase general activity.

Midazolam reduces the selective activation of the rhinal cortex by contextual fear stimuli

Independent brain circuits appear to underlie different forms of conditioned fear, depending on the type of conditioning used, such as a context or explicit cue paired with footshocks. Several clinical reports have associated damage to the medial temporal lobe (MTL) with retrograde amnesia. Although a number of studies have elucidated the neural circuits underlying conditioned fear, the involvement of MTL components in the aversive conditioning paradigm is still unclear. To address this issue, we assessed freezing responses and Fos protein expression in subregions of the rhinal cortex and ventral hippocampus of rats following exposure to a context, light or tone previously paired with footshock (Experiment 1). A comparable degree of freezing was observed in the three types of conditioned fear, but with distinct patterns of Fos distribution. The groups exposed to cued fear conditioning did not show changes in Fos expression, whereas the group subjected to contextual fear conditioning showed selective activation of the ectorhinal (Ect), perirhinal (Per), and entorhinal (Ent) cortices, with no changes in the ventral hippocampus. We then examined the effects of the benzodiazepine midazolam injected bilaterally into these three rhinal subregions in the expression of contextual fear conditioning (Experiment 2). Midazolam administration into the Ect, Per, and Ent reduced freezing responses. These findings suggest that contextual and explicit stimuli endowed with aversive properties through conditioning recruit distinct brain areas, and the rhinal cortex appears to be critical for storing context-, but not explicit cue-footshock, associations.

Metabotropic glutamate receptor 7: at the interface of cognition and emotion

Understanding the complex interaction between stress and genetics that leads to the manifestation of disorders such as depression, anxiety, and cognitive dysfunction is one of the key areas of research in modern neuroscience. Growing evidence suggests that the glutamatergic system may be a relevant therapeutic target for such disorders. Glutamate is the neurotransmitter at the vast majority of excitatory synapses in the brain, and metabotropic glutamate (mGlu) receptor subtypes (mGlu(1) receptor-mGlu(8) receptor) act as important pre- and postsynaptic regulators of neurotransmission in the central nervous system (CNS), providing a mechanism by which fast synaptic responses through ligand-gated cation channels can be fine-tuned. Thus mGlu receptors are poised to participate in a wide variety of functions of the CNS. The presynaptic mGlu(7) receptor shows the highest evolutionary conservation within the family and it is thought to regulate neurotransmitter release. The mGlu(7) receptor is also the most widely distributed of the presynaptic mGlu receptors and is present at a broad range of synapses that are postulated to be critical for both normal CNS function and a range of psychiatric and neurological disorders. A growing body of evidence suggests that the mGlu(7) receptor is a key player in shaping synaptic responses at glutamatergic synapses as well as being a key regulator of inhibitory GABAergic transmission. The development of selective pharmacological and genetic tools has allowed for the unravelling of mGlu(7) receptor function in a host of physiological and behavioural processes. Knockout mice and siRNA knockdown has pointed to a role of the mGlu(7) receptor in anxiety, extinction of fear and aversion learning, spatial memory and the hormonal response to stress. In addition, these studies are largely supported by pharmacological manipulation of mGlu(7) receptor using the selective modulator N,N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082), although paradoxical effects with this agonist have also emerged. Together these data suggest that the mGlu(7) receptor is an important regulator of glutamatergic function, of fear and aversion and cognition and thus this receptor represents an innovative therapeutic target for stress-related disorders at the interface of cognition and anxiety.

Conditioned and unconditioned fear organized in the inferior colliculus are differentially sensitive to injections of muscimol into the basolateral nucleus of the amygdala

Chemical stimulation of the inferior colliculus (IC) with semicarbazide--an inhibitor of the gamma aminobutyric acid synthesizing enzyme--functions as an unconditioned stimulus in the conditioned place aversion test (CPA), and electrolytic lesions of the basolateral amygdala (BLA) enhance the aversiveness of the IC stimulation. This study examined the effects of inactivation of the BLA with muscimol on the conditioned and unconditioned fear using semicarbazide injections into the IC of rats subjected to conditioned (CPA) or unconditioned (open field) fear tests. In both tests, the rats were injected with semicarbazide or saline into the IC and muscimol or saline into the BLA. Muscimol decreased the CPA and increased the unconditioned fear elicited by IC injections of semicarbazide. These findings indicate that distinct modulatory mechanisms in the BLA are recruited during the conditioned and unconditioned fear triggered by IC activation.

Brief neonatal maternal separation alters extinction of conditioned fear and corticolimbic glucocorticoid and NMDA receptor expression in adult rats

Neonatal maternal separation alters adult HPA axis responsiveness to stress, adult emotionality, and glucocorticoid receptor (GR) concentrations in forebrain regions such as hippocampus. To investigate effects of neonatal maternal separation on emotion regulation and its neural substrates, we assessed acquisition and extinction of conditioned fear in adult rats that underwent neonatal maternal separation. Corticolimbic structures including basolateral amygdala and medial prefrontal cortex are critical for acquisition and extinction of conditioned fear, and such learning is N-methyl-D-aspartic acid (NMDA) receptor-dependent. Thus, we used immunohistochemistry to assess expression of the GR and the NR1 subunit of the NMDA receptor in basolateral amygdala and medial prefrontal cortex. On postnatal days 2-14, pups underwent control rearing or maternal separation for 15 min per day. Fear conditioning and extinction in adulthood were then assessed in male rats. Rats received five tone-alone habituation trials, then seven tone/footshock pairings. After 1 h, rats received tone-alone extinction trials to criterion, and 15 recall of extinction trials the next day. Brains were processed for immunohistochemical labeling of GR and NR1, and staining was quantified. Brief maternal separation did not alter acquisition or initial extinction, but impaired extinction recall. Brief maternal separation did not alter GR or NR1 expression in basolateral amygdala. However, brief maternal separation increased GR and decreased NR1 expression specifically in the infralimbic region of medial prefrontal cortex, consistent with work implicating this area in extinction recall. Thus, brief maternal separation impaired extinction recall and altered GR and NR1 expression in its neural substrate in adults.

Amygdaloid lesions produced similar contextual fear conditioning disruption in the Carioca high- and low-conditioned freezing rats

Rats selectively bred for high or low levels of emotionality represent an important and powerful tool to investigate the role of genetic variables in the occurrence of different anxiety disorders. In the present study, albino rats were selectively bred for differences in defensive freezing behavior in response to contextual cues previously associated with footshock, an animal model of general anxiety disorder. The results indicate that these two new lines of rats, which we refer to as Carioca High-Freezing (CHF) and Carioca Low-Freezing (CLF), show a reliable difference in conditioned freezing after three generations of selection. CHF and CLF rats did not present any differences during baseline or post-shock periods. Males from both lines consistently exhibit more conditioned freezing to contextual cues than females. A second experiment used male rats from the fourth generation to investigate the participation of the amygdala during contextual fear conditioning in the CHF and CLF lines. The results indicate that post-training amygdaloid electrolytic lesions lead to similar disruptions in conditioned freezing behavior in both animal lines.

Involvement of the midbrain tectum in the unconditioned fear promoted by morphine withdrawal

The midbrain tectum structures, dorsal periaqueductal gray (dPAG) and inferior colliculus (IC), are involved in the organization of fear and anxiety states during the exposure to dangerous stimuli. Since opiate withdrawal is associated with increased anxiety in both humans and animals, this study aimed to investigate the possible sensitization of the neural substrates of fear in the midbrain tectum and its influence on the morphine withdrawal-induced anxiety. For the production of drug withdrawal, rats received morphine injections (10 mg/kg; s.c.) twice daily during 10 days. Forty-eight hours after the interruption of the chronic treatment, independent groups were probed in the elevated plus-maze and open-field tests. Additional groups of animals were implanted with a bipolar electrode into the dPAG or the IC and submitted to the electrical stimulation of these structures for the determination of the freezing and escape thresholds after 48 h of withdrawal. Our results showed that the morphine withdrawal promoted clear-cut levels of anxiety without the somatic signs of opiate withdrawal. Moreover, morphine-withdrawn rats had an increase in the reactivity to the electrical stimulation of the dPAG and the IC. These findings suggest that the increased anxiety induced by morphine withdrawal is associated with the sensitization of the neural substrates of fear in the dPAG and the IC. So, the present results give support to the hypothesis that withdrawal from chronic treatment with morphine leads to fear states possibly engendered by activation of the dPAG and IC, regardless of the production of somatic symptoms.

Aspects of learned fear related to the hippocampus are sleep-dependent

Reduced sleep interferes with contextual but not cued learned fear, and it was suggested that this selectivity reflects underlying neural substrates. The apparent lack of contextual fear in sleep-deprived animals, however, could be secondary to hyperactivity. Also, changing the parameters of cued conditioning can change the neural pathways involved, such that some types of cued fear might be sensitive to sleep loss. To address these issues, we measured fear expressed with conditioned defecation as well as behavior, and used a trace cued learning paradigm. Using the platform-over-water method, male Sprague-Dawley rats were continuously sleep-deprived for 3 days, or for 20 h/day for 3 days. Animals then underwent fear conditioning, and were tested for learning the next day. Sleep-deprived or -restricted animals showed a lack of contextual fear at testing, as conditioned freezing and defecation were minimal. Sleep deprivation also blocked cued fear after trace conditioning. Therefore, reduced sleep impairs contextual learning, and impairs cued learning only when the hippocampus is involved. The data support a model in which sleep loss interferes with hippocampal function while sparing amygdala function.

Serotonergic mechanisms in the basolateral amygdala differentially regulate the conditioned and unconditioned fear organized in the periaqueductal gray

The amygdala is an important filter for unconditioned and conditioned aversive information. The amygdala synthesizes the stimuli input from the environment and then signals the degree of threat that they represent to the dorsal periaqueductal gray (dPAG), which would be in charge of selecting, organizing and executing the appropriate defense reaction. In this study, we examined the influence of fluoxetine microinjections (1.75 and 3.5 nmol/0.2 microL) into the lateral (LaA) and basolateral (BLA) amygdaloid nuclei on the freezing and escape responses induced by electrical stimulation of the dPAG. Freezing behavior was also measured after the interruption of the electrical stimulation of the dPAG. On the following day, these rats were also submitted to a contextual fear paradigm to examine whether these microinjections would affect the conditioned freezing to contextual cues previously associated with foot shocks. Fluoxetine injections into both amygdaloid nuclei did not change the freezing and escape thresholds, but disrupted the dPAG-post-stimulation freezing. Moreover, the conditioned freezing was enhanced by fluoxetine. Whereas 5-HT mechanisms in the amygdala facilitate the acquisition of conditioned fear they inhibit the dPAG-post-stimulation freezing. However, the unconditioned fear triggered by activation of the dPAG is produced downstream of the amygdala. These findings have important implications for the understanding of the neurochemical substrates that underlie panic and generalized anxiety disorders.

Conditioned place aversion organized in the dorsal periaqueductal gray recruits the laterodorsal nucleus of the thalamus and the basolateral amygdala

The amygdala-ventral periaqueductal gray circuit is crucial for the expression of contextual conditioned fear. However, little is known about the neural circuits activated when the stimulation of the dorsal periaqueductal gray (dPAG) is used as unconditioned stimulus (US) in conditioned fear paradigms. The present paper examines the Fos-protein distribution in the brain of rats submitted to a conditioned place aversion (CPA) paradigm using the dPAG chemical stimulation with semicarbazide (SMC), an inhibitor of the GABA synthesizing enzyme, as US and the quadrant of an arena where the drug was injected as the paired neutral stimulus. Our results show that CPA associated with SMC injections caused a significant Fos labeling in the laterodorsal nucleus of the thalamus (LD), basolateral nucleus of amygdala (BLA) and in the dorsomedial PAG (dmPAG). This pattern of brain activation is clearly different from the neural substrates of the classical fear conditioning reported in the literature. Moreover, this paper shows that CPA with the use of chemical stimulation of the dPAG could be used as an experimental model of panic disorder with agoraphobia in the extent that panic attacks repeatedly associated with specific contexts may turn in this condition in the clinics. This condition activates the BLA probably through the LD. Besides, it indicates that the dPAG can be the link between amygdala and the brainstem motor regions that controls CPA when dPAG stimulation is used as US instead of footshocks. From this evidence we suggest that a loop dPAG-LD-BLA-dPAG is activated during the panic disorder with agoraphobia.

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.

5-HT2 receptor mechanisms of the dorsal periaqueductal gray in the conditioned and unconditioned fear in rats

RATIONALE

It is well known that 5-HT(2) mechanisms modulate the defensive behavior produced by the stimulation of the dorsal periaqueductal gray (dPAG). However, in spite of the notion that past stressful experiences play a role in certain types of anxiety, only studies with the stimulation of the dPAG of rats without previous aversive experience have been conducted so far.

OBJECTIVES

We investigated the mediation of 5-HT(2) receptors of the dPAG in rats previously submitted to contextual fear conditioning (CFC). Defensive behaviors induced by the activation of the dPAG were assessed by measuring the lowest intensity of electric current applied to this structure (threshold) able to produce freezing and escape responses during the testing sessions of CFC in which animals were placed in a context previously paired to footshocks. The 5-HT(2) function of the dPAG in this condition was evaluated by local injections of alpha-methyl-5-HT (20 nmol/0.2 mul) and ketanserin (5 and 10 nmol/0.2 mul), selective agonist and antagonist of 5-HT(2) receptors, respectively.

RESULTS

In accordance with previous studies, alpha-methyl-5-HT increased the aversive thresholds (antiaversive effects) in naive rats, and injection of ketanserin into the dPAG did not produce significant effects. On the other hand, ketanserin decreased in a dose-dependent manner the freezing threshold (proaversive effect) determined by the dPAG electrical stimulation, whereas alpha-methyl-5-HT continued to show antiaversive effects in animals under CFC.

CONCLUSIONS

The present results suggest that past stressful experience can produce changes in the synaptic function of 5-HT(2) receptors within the dPAG with important impact on the expression of defensive behaviors.

Exploratory behaviour of rats in the elevated plus-maze is differentially sensitive to inactivation of the basolateral and central amygdaloid nuclei

The amygdala has a crucial role in detecting motivationally significant inputs and in communicating relevant information to other limbic structures. Behavioural studies have shown that the central (CeA) and basolateral (BLA) nuclei of amygdala differentially regulate conditioned and unconditioned fear. Indeed, much evidence has accumulated suggesting that regulatory mechanisms in the BLA serve as a filter for unconditioned and conditioned aversive information that ascends to higher structures from the brainstem, whereas the CeA is the main output for the autonomic and somatic components of fear reaction through major projections to other limbic regions. It is still unclear, however, how amygdaloid nuclei function in high and open spaces so as to determine the characteristic exploratory behaviour of rats submitted to the elevated plus-maze test (EPM). In the present study, we carried out an ethopharmacological analysis of the behaviour of rats submitted to the elevated plus-maze test together with analysis of the tissue content of monoamine dopamine (DA) and serotonin (5-HT) and their metabolites in the dorsal hippocampus (DH), nucleus accumbens (NAC) and dorsal striatum (DS) of animals injected with saline or muscimol (1.0 nmol/0.2 microL) into the BLA or CeA. The data obtained show that injections of muscimol into the CeA, but not into the BLA, caused anxiolytic-like effects in the EPM. Such effects of muscimol into the CeA were accompanied by increases in 5-HT content of the DH, whereas corresponding injections into the BLA caused a reduction in the DA content of the NAC. There was no change in the turnover rates of these monoamines. These data suggest that the BLA and CeA have distinct roles in the exploratory behaviour of rodents in the EPM. While BLA appears to be related to the detection and validation of threatening stimuli, the CeA appears to be involved in the expression of fear behaviours in the EPM.

5-HT2- and D1-mechanisms of the basolateral nucleus of the amygdala enhance conditioned fear and impair unconditioned fear

The inferior colliculus (IC) is involved in processing of auditory information, but also integrates acoustic information of aversive nature. In fact, chemical stimulation of the IC with semicarbazide (SMC) - an inhibitor of the GABA synthesizing enzyme glutamic acid decarboxylase - has been found to cause defensive behavior in an open-field test and functions as an unconditioned stimulus in the place conditioned aversion test (PCA). A question has arisen regarding whether the basolateral nucleus of the amygdala (BLA) is involved in the acquisition of the aversive information ascending from the IC and whether dopaminergic and serotoninergic mechanisms of the BLA regulate this process. Recent evidence has shown that inactivation of the BLA with muscimol inhibits the PCA and causes an increase in the aversiveness of the chemical stimulation of the IC. Based on this, we examined the effects of ketanserin and SCH-23390, antagonists of the 5HT(2) and D(1) receptors, respectively, on the conditioned and unconditioned fear elicited by IC stimulation with SMC. The results obtained confirm the crucial role of 5-HT(2)- and D(1)-mechanisms of the BLA on conditioned fear in that ketanserin and SCH-23390 injections into the BLA caused a reduction in the PCA. On the other hand, ketanserin and SCH-23390 injections into the BLA enhanced the aversiveness of the IC injections of SMC. These findings suggest that while 5-HT(2) and DA(1) mechanisms in the BLA appear to facilitate the conditioned fear they inhibit the unconditioned fear triggered by IC activation.

Pharmacological assessment of the freezing, antinociception, and exploratory behavior organized in the ventrolateral periaqueductal gray

Opioid and serotonergic mechanisms of the ventrolateral periaqueductal gray (vlPAG) are recruited by conditioned freezing and antinociception. However, it is unclear whether freezing and antinociception induced by stimulation of the vlPAG are interrelated. To address this issue we looked at the effects of the opioid antagonist naltrexone, the 5-HT2 antagonist ketanserin, and the benzodiazepine agonist midazolam injected into the vlPAG on the freezing and antinociception induced by electrical stimulation of this region. This antinociception was evaluated by the tail-flick and formalin tests. To further characterize the involvement of the vlPAG in unconditioned fear, the effects of intra-vlPAG injections of midazolam on the exploratory behavior were also assessed in independent groups of rats submitted to the elevated plus-maze test (EPM). The data obtained showed that: (i) electrical stimulation of the vlPAG causes freezing blocked by midazolam but not by naltrexone and ketanserin; (ii) antinociception generated at the level of the vlPAG is inhibited by naltrexone, ketanserin, and midazolam; (iii) activation of benzodiazepine-mediated mechanisms in the vlPAG increased the exploratory behavior of rats in the closed arms but not the avoidance behavior of open arms of the EPM. Thus, freezing and antinociception generated in the vlPAG are dissociated pharmacologically. Whereas antinociception is a multimediated process sensitive to naltrexone, ketanserin, and midazolam, the freezing induced by vlPAG stimulation was reversed only by the benzodiazepine compound. As injections of midazolam into the vlPAG do not cause anxiolytic effects in the EPM, the aversive stimuli inherent of this test seem to bypass the vlPAG.

Padrões de respostas defensivas de congelamento associados a diferentes transtornos de ansiedade

Embora os transtornos de ansiedade sejam tipicamente humanos, eles apresentam correlações com determinadas reações de defesa de animais em situações de perigo. Este trabalho apresenta algumas relações entre determinados padrões da resposta defensiva de congelamento e diferentes formas de transtornos de ansiedade. Em particular, destaca-se o isomorfismo entre a resposta de congelamento a estímulos contextuais associados a um estímulo aversivo e o transtorno de ansiedade generalizado. Evidências indicam também que a resposta de congelamento induzida pela estimulação elétrica da matéria cinzenta periaqueductal dorsal (MCPD) constitui um excelente modelo animal de ataque de pânico. A resposta de congelamento que surge imediatamente após estimulação da MCPD, capaz de produzir uma resposta de fuga, parece estar associada ao transtorno de pânico. Finalmente, é possível que a resposta de congelamento a estímulos previamente associados à estimulação elétrica da MCPD seja um modelo animal para o transtorno de pânico com agorafobia.

The involvement of glucocorticoids and interleukin-1 in the regulation of brain prostaglandin production in response to surgical stress

BACKGROUND

This study examined the role of glucocorticoids (GC) and interleukin-1 (IL-1) in regulating the production of brain prostaglandin E(2) (PGE(2)) in response to surgical stress.

METHODS

Surgical stress was induced in rats by laparotomy or exploration of the carotid. PGE(2) ex vivo production was measured in the frontal cortex or central amygdala of adrenalectomized rats, or of rats treated with either the GC type II receptor blocker (RU38486) or synthetic GC (dexamethasone). IL-1 involvement in mediating PGE(2) response to surgical stress was examined in IL-1 receptor type I deficient (IL-1rKO) mice.

RESULTS

Surgical stress elevated serum corticosterone and increased PGE(2) production by the frontal cortex and the central amygdala. A more pronounced PGE(2) response was found in adrenalectomized rats and in rats treated with RU38486, whereas administration of dexamethasone inhibited stress-induced PGE(2) production. IL-1rKO mice exhibited lower PGE(2) production in the frontal cortex under basal condition and failed to increase PGE(2) production in response to surgical stress.

CONCLUSIONS

Surgical stress-induced production of brain PGE(2) is specifically regulated by GC via the mediation of type II corticosteroid receptors. Normal IL-1 signaling is required for the production of brain PGE(2) under basal conditions and in response to surgical stress.

Conditioned and unconditioned fear organized in the periaqueductal gray are differentially sensitive to injections of muscimol into amygdaloid nuclei

The lateral and basolateral nuclei of the amygdala (LaA and BLA, respectively) serve as a filter for unconditioned and conditioned aversive information that ascends to higher structures from the brainstem, whereas the central nucleus of the amygdala (CeA) is considered to be the main output for the defense reaction. It has been shown that the dorsal periaqueductal gray (dPAG) is activated by threatening stimuli and has important functional links with the amygdala through two-way anatomical connections. In this work, we examined the influence of chemical inactivation of these nuclei of amygdala on the freezing and escape responses induced by electrical stimulation through electrodes implanted in the dPAG of Wistar rats. Each rat also bore a cannula implanted in the LaA, BLA or CeA for injections of muscimol (0.5 microg/0.5 microL) or its vehicle. The duration of freezing behavior that outlasts electrical stimulation of the dPAG was also measured. On the following day, these animals were submitted to a contextual fear-conditioning using foot shocks as unconditioned stimulus. Conditioned freezing to contextual cues previously associated with foot shocks was also inhibited by injections of muscimol into these amygdaloid nuclei. The contextual conditioned freezing behavior is generated in the neural circuits of conditioned fear in the amygdala. The data obtained also show that injections of muscimol into the three amygdaloid nuclei did not change the aversive threshold of freezing, but disrupted the dPAG post-stimulation freezing. Previous findings that the latter freezing results directly from dPAG stimulation and that it is not sensitive to a context shift suggest that it is unconditioned in nature. Thus, the amygdala can affect some, but not all, aspects of unconditioned freezing. Post-stimulation freezing may reflect the process of transferring aversive information from dPAG to higher brain structures.

Effects of surgical stress on brain prostaglandin E2 production and on the pituitary–adrenal axis: Attenuation by preemptive analgesia and by central amygdala lesion

Surgical stress is the combined result of tissue injury, anesthesia, and postoperative pain. It is characterized by elevated levels of adrenocorticotropin (ACTH), corticosterone (CS), and elevated levels of prostaglandin E2 (PGE2) in the periphery and in the spinal cord. The present study examined the effects of perioperative pain management in rats undergoing laparotomy on serum levels of ACTH, CS, and on the production of PGE2 in several brain regions, including the amygdala. The amygdala is known to modulate the pituitary-adrenal axis response to stress. We, therefore, also examined the effects of bilateral lesions in the central amygdala (CeA) on laparotomy-induced activation of the pituitary-adrenal axis in rats. In the first experiment, rats either underwent laparotomy or were not operated upon. Half the rats received preemptive analgesia extended postoperatively, the other received saline. ACTH, CS serum levels, and ex vivo brain production of PGE2 were determined. In the second experiment, rats underwent bilateral lesions of the CeA. Ten days later, rats underwent laparotomy, and ACTH and CS serum levels were determined. Laparotomy significantly increased amygdala PGE2 production, and CS and ACTH serum levels. This elevation was markedly attenuated by perioperative analgesia. Bilateral CeA lesions also attenuated the pituitary-adrenal response to surgical stress. The present findings suggest that the amygdala plays a regulatory role in mediating the neuroendocrine response to surgical stress. Effective perioperative analgesia attenuated the surgery-induced activation of pituitary-adrenal axis and PGE2 elevation. The diminished elevation of PGE2 may suggest a mechanism by which pain relief mitigates pituitary-adrenal axis activation.

Gabaergic regulation of the neural organization of fear in the midbrain tectum

In midbrain tectum (MT) structures, such as the dorsal periaqueductal gray (dPAG), the superior colliculus (SC) and the inferior colliculus (IC) GABAergic neurons exert a tonic control on the neural substrates involved in the expression of defensive reactions. In this review, we summarize behavioral, immunohistochemical (brain Fos distribution) and electrophysiological (auditory evoked potentials) data obtained with the reduction of GABA transmission by local injections of a GABA receptor blocker (bicuculline, BIC) or a glutamic acid decarboxylase inhibitor (semicarbazide, SMC) into the MT. Distinct patterns of Fos distribution were obtained following the freezing and escape reactions induced by MT injections of SMC and BIC, respectively. While only the laterodorsal nucleus of the thalamus was labeled after SMC-induced freezing, a widespread increase in Fos expression in the brain occurred after BIC-induced escape. Also, injections of SMC into the IC increased the auditory evoked potentials recorded from this structure. It is suggested that GABAergic mechanisms of MT are also called into play when sensory gating of the MT is activated during different emotional states.

Neural mechanisms of freezing and passive aversive behaviors

Cues that predict aversive outcomes often produce marked inhibitions of behavior known as freezing, but it is unknown exactly what neural pathways cause this inhibition. The amygdala and bed nucleus of the stria terminalis, along with their projections to the periaqueductal gray, are strongly implicated in freezing, but it is not known how these structures inhibit motor output. The median raphe nucleus (MRN), which contains a major population of serotonin neurons, has also been implicated in freezing, but the serotonin neurons themselves do not seem to be involved, leaving it uncertain which neurons in this area promote freezing. Our recent work suggests that GABAergic neurons just lateral to the MRN, but not within the MRN, regulate freezing via projections to midbrain dopamine neurons. Because freezing pathways may control a variety of other passive aversive behaviors, their elucidation may help understand the mechanisms of addictions and compulsions, which involve a failure of aversive outcomes to inhibit behavior.