The distribution of fos immunoreactivity in rat brain following freezing and escape responses elicited by electrical stimulation of the inferior colliculus

Several sources of evidence indicate that the inferior colliculus also integrates acoustic information of an aversive nature besides its well-known role as a relay station for auditory pathways. Gradual increases of the electrical stimulation of this structure cause in a hierarchical manner alertness, freezing and escape behaviors. Independent groups of animals implanted with bipolar electrodes into the inferior colliculus received electrical stimulation at one of these aversive thresholds. Control animals were submitted to the same procedure but no current was applied. Next, analysis of Fos protein expression was used to map brain areas activated by the inferior colliculus stimulation at each aversive threshold and in the controls. Whereas alertness elicited by stimulation of the inferior colliculus did not cause any significant labeling in any structure studied in relation to the respective control, electrical stimulation applied at the freezing threshold increased Fos-like immunoreactivity in the central amygdaloid nucleus and entorhinal cortex. In contrast, escape response enhanced Fos-like immunoreactivity in the nucleus cuneiform and the dorsal periaqueductal gray matter of the mesencephalon. This evidence supports the notion that freezing and escape behaviors induced by electrical stimulation of the inferior colliculus activate different neural circuitries in the brain. Both defensive behaviors caused significant expression of c-fos in the frontal cortex, hippocampus and basolateral amygdaloid nucleus. This indistinct pattern of c-fos distribution may indicate a more general role for these structures in the modulation of fear-related behaviors. Therefore, the present data bring support to the notion that amygdala, dorsal hippocampus, entorhinal cortex, frontal cortex, dorsal periaqueductal gray matter and cuneiform nucleus altogether play a role in the integration of aversive states generated at the level of the inferior colliculus.     

Dual 5-HT mechanisms in basolateral and central nuclei of amygdala in the regulation of the defensive behavior induced by electrical stimulation of the inferior colliculus

Regulatory mechanisms in the basolateral nucleus of the amygdala (BLA) serves as a filter for unconditioned and conditioned aversive information that ascend to higher structures from the brainstem whereas the central nucleus (CeA) is the main output for the resultant defense reaction. We have shown that neural substrates in the inferior colliculus are activated by threatening stimuli of acoustic nature and have important functional links with the amygdala. In this work, we examined the influence of lesions with 5,7-dihydroxytryptamine (5,7-DHT) of these nuclei of amygdala on the aversive responses induced by electrical stimulation of the inferior colliculus. Thus, rats were implanted with an electrode in the CeA of the inferior colliculus for the determination of the thresholds of alertness, freezing and escape responses. Each rat also bore a cannula implanted in the BLA or CeA for injection of 5,7-DHT (8.0 microg/0.8 microl) or its vehicle. The data obtained show that CeA lesions increase the thresholds of aversive responses whereas BLA lesions decrease the thresholds of these responses. From this evidence it is suggested that defensive behavior induced by activation of the neural substrates of aversion in the inferior colliculus seems to depend on the integrity of the amygdala. BLA regulates the input and CeA functions as the output for these aversive states generated at brainstem level. It is likely that aversive information ascending from the inferior colliculus may receive either inhibitory or excitatory influences of 5-HT mechanisms in the BLA or CeA, respectively.     

Neuroanatomical and neuropharmacological study of opioid pathways in the mesencephalic tectum: effect of mu(1)- and kappa-opioid receptor blockade on escape behavior induced by electrical stimulation of the inferior colliculus

Deep layers of the superior colliculus (DLSC), the dorsal and ventral periaqueductal gray matter (PAG), and inferior colliculus (IC) are midbrain structures involved in the generation of defensive behavior. beta-Endorphin and Leu-enkephalin are some neurotransmitters that may modulate such behavior in mammals. Light microscopy immunocytochemistry with streptavidin method was used for the localization of the putative cells of defensive behavior with antibodies for endogenous opioids in rat brainstem. Midbrain structures showed positive neurons to beta-endorphin and Leu-enkephalin in similar distributions in the experimental animals, but we also noted the presence of varicose fibers positive to endogenous opioids in the PAG. Neuroanatomical techniques showed varicose fibers from the central nucleus of the inferior colliculus to ventral aspects of the PAG, at more caudal levels. Naloxonazine and nor-binaltorphimine, competitive antagonists that block mu(1)- and kappa-opioid receptors, were then used in the present work to investigate the involvement of opioid peptide neural system in the control of the fear-induced reactions evoked by electrical stimulation of the neural substrates of the inferior colliculus. The fear-like responses were measured by electrical stimulation of the central nucleus of the inferior colliculus, eliciting the escape behavior, which is characterized by vigorous running and jumping. Central administration of opioid antagonists (2.5 microg/0.2 microl and 5.0 microg/0.2 microl) was performed in non-anesthetized animals (Rattus norvegicus), and the behavioral manifestations of fear were registered after 10 min, 2 h, and 24 h of the pretreatment. Naloxonazine caused an increase of the defensive threshold, as compared to control, suggesting an antiaversive effect of the antagonism on mu(1)-opioid receptor. This finding was corroborated with central administration of nor-binaltorphimine, which also induced a decrease of the fear-like responses evoked by electrical stimulation of the inferior colliculus, since the threshold of the escape behavior was increased 2 and 24 h after the blockade of kappa-opioid receptor. These results indicate that endogenous opioids may be involved in the modulation of fear in the central nucleus of the inferior colliculus. Although the acute treatment (after 10 min) of both naloxonazine and nor-binaltorphimine causes nonspecific effect on opioid receptors, we must consider the involvement of mu(1)- and kappa-opioid receptors in the antiaversive influence of the opioidergic interneurons in the dorsal mesencephalon, at caudal level, after chronic (2-24 h) treatment of these opioid antagonists. The neuroanatomical study of the connections between the central nucleus of the inferior colliculus and the periaqueductal gray matter showed neuronal fibers with varicosities and with terminal bottons, both in the pericentral nucleus of the inferior colliculus and in ventral and dorsal parts of caudal aspects of the periaqueductal gray matter.     

Fos-like immunoreactive neurons following electrical stimulation of the dorsal periaqueductal gray at freezing and escape thresholds

Electrical stimulation of the dorsal regions of the periaqueductal gray (PAG) leads to defensive reactions characterized as freezing and escape responses. Until recently it was thought that this freezing behavior could be due to the recruitment of neural circuits in the ventrolateral periaqueductal gray (vlPAG), while escape would be mediated by other pathways. Nowadays, this view has been changing mainly because of evidence that freezing and escape behaviors thus elicited are not altered after lesions of the vlPAG. It has been suggested that there are at least two pathways for periaqueductal gray-mediated defensive responses, one involving the hypothalamus and the cuneiform nucleus (CnF) which mediates responses to immediate danger and another one involving the amygdala and vlPAG which mediates cue-elicited responses, either learned or innate. To examine this issue further we measured Fos protein expression in brain areas activated by electrical stimulation of the dorsolateral PAG (dlPAG) at the freezing and escape thresholds. The data obtained showed that freezing-provoking stimulation caused increases in Fos expression in the dorsomedial PAG (dmPAG), while escape-provoking stimulation led to increases at both dmPAG and dlPAG. Surprisingly, neither escape- nor freezing-provoking stimulations altered Fos expression in the central nucleus of amygdala (CeA). Escape-provoking stimulation caused increased Fos expression in the ventromedial hypothalamus (VMH), dorsal premammilary nucleus (PMd) and in the cuneiform nucleus. Significant increases in Fos labeling were found in the dmPAG and PMd following freezing-provoking stimulation. Therefore, the present data support the notion of a neural segregation for defensive behaviors in the dorsal columns of PAG, with increased Fos expression in the dmPAG following freezing, while dlPAG is affected by both freezing and escape responses. dlPAG, CnF, VMH and PMd are part of a brain aversion network activated by fear unconditioned stimuli. The present data also suggests that the defensive responses generated at the dlPAG level do not recruit the neural circuits of the vlPAG and CeA usually activated by conditioned fear stimuli.     

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

The amygdala and ventral portion of the periaqueductal gray (vPAG) are crucial for the expression of the contextual freezing behavior. However, it is still unclear whether the amygdala also plays a role in defensive behaviors induced by electrical stimulation of the dorsal periaqueductal gray (dPAG). In the present study, rats were implanted with electrodes into dPAG for determination of the thresholds for freezing and escape responses before and after sham or electrolytic lesions in the amygdala. Animals were then submitted to a context fear conditioning procedure. Amygdala lesions disrupted contextual freezing but did not affect defensive behaviors induced by dPAG electrical stimulation. These results indicate that contextual and unconditioned freezing behaviors are mediated by independent neural circuits.     

Connections between the central nucleus of the amygdala and the midbrain periaqueductal gray: Topography and reciprocity

Previous reports indicate that the midbrain periaqueductal gray and the central nucleus of the amygdala are interconnected but the organization of these projections has not been characterized. We have analyzed this reciprocal circuitry using anterograde and retrograde tracing methods and image analysis. Our findings reveal that innervation of periaqueductal gray from the central nucleus of the amygdala is extensive and discretely organized along the rostrocaudal axis of periaqueductal gray. In addition, the reciprocal projection from periaqueductal gray to the central nucleus of the amygdala is more extensive and more highly organized than previously suggested. Multiple or single discrete injections of wheatgerm agglutinin-horseradish peroxidase into several rostrocaudal levels of periaqueductal gray retrogradely labeled a substantial population of neurons, predominantly located in the medial division of the central nucleus of the amygdala. Tracer injections into the central nucleus revealed a high degree of spatial organization in the projection from this nucleus to periaqueductal gray. Two discrete longitudinally directed columns in dorsomedial and lateral/ventrolateral periaqueductal gray are heavily targeted by central amygdalar inputs throughout the rostral one-half to two-thirds of periaqueductal gray. Beginning at the level of dorsal raphe and continuing caudally, inputs from the central nucleus terminate more uniformly throughout the ventral half of periaqueductal gray. In addition, a substantial population of periaqueductal gray neurons were retrogradely labeled from the central nucleus of the amygdala; these were heterogeneously distributed along the rostrocaudal axis of periaqueductal gray, and included both raphe and non-raphe neurons. Thus, the present study demonstrates that periaqueductal gray receives heavy, highly organized projections from the central nucleus of the amygdala and in turn, has reciprocal connections with the central nucleus. Previous studies have demonstrated that longitudinally organized columns of output neurons located in dorsomedial and lateral/ventrolateral periaqueductal gray project to the ventral medulla. Thus, there may be considerable overlap between the two longitudinally organized terminal input columns from the central nucleus of the amygdala and the two longitudinal columns of descending projection neurons from periaqueductal gray to the ventral medulla. The central nucleus of the amygdala has been implicated in a variety of emotional/cognitive functions ranging from fear and orienting responses, defensive and aversive reactions, associative conditioning, cardiovascular regulation, and antinociception. Many of these same functions are strongly represented in the periaqueductal gray. It is noteworthy that the present results demonstrate that lateral periaqueductal gray, a preeminent central trigger site for behavioral and autonomic components of the defense/aversion response, is heavily targeted by inputs from the central nucleus of the amygdala at all levels of periaqueductal gray. Thus, the central nucleus of the amygdala to periaqueductal gray projection may be involved in the neural integration of behavioral, antinociceptive and autonomic responses with emotional state. In addition, the present demonstration of extensive reciprocal connections between the central nucleus of the amygdala and periaqueductal gray represents a route via which functional activity represented in periaqueductal gray may gain access to a forebrain structure long implicated m the integration of the cognitive and autonomic components of emotional behavior. Thus, the periaqueductal gray to central nucleus of the amygdala projection may provide a relatively direct linkage between critical species-preserving behavioral reactions and a forebrain structure capable of influencing multiple nodal points in the descending autonomic system.     

Distinct Fos expression in the brain following freezing behavior elicited by stimulation with NMDA of the ventral or dorsal inferior colliculus

The inferior colliculus (IC) is an important relay station for ascending auditory information to the medial geniculate nucleus (MGN) and temporal cortex. It has been reported that the ventral (ICv) and dorsal (ICd) regions of the IC are involved with the defensive reaction and audiogenic seizures, respectively. As freezing is the first response induced by stimulation of these IC nuclei with increasing doses of N-methyl-d-aspartate (NMDA), a question that arises is whether or not fear and audiogenic seizures generated at the IC level are interrelated processes. To address this issue, the Fos distribution in selected limbic structures following injections of NMDA into the ICv or ICd at freezing (7 nmol)- and escape (20 nmol)-producing doses was examined. Freezing behavior induced by intra-ICd NMDA caused an increase of Fos expression in the MGN, superior colliculus, dorsal columns of the periaqueductal gray and locus coeruleus while freezing induced by intra-ICv NMDA caused a significant Fos immunoreactivity in the prelimbic (PrL) and cingulate (Cg) cortices, basolateral and medial nuclei of the amygdala, ventrolateral periaqueductal gray, cuneiform nucleus and locus coeruleus. Escape behavior induced by NMDA injections into both nuclei caused a widespread Fos labeling in all limbic structures examined in this study. These results suggest that distinct circuits underlie the freezing behavior generated at the level of ICd and ICv. This is the first study to map Fos distribution associated with the stimulation of the ICv and ICd, regions supposed to be involved with fear and audiogenic seizures, respectively.     

Recruitment of striatonigral disinhibitory and nigrotectal inhibitory GABAergic pathways during the organization of defensive behavior by mice in a dangerous environment with the venomous snake Bothrops alternatus (Reptilia,Viperidae)

The neuropsychopharmacological basis of fear‐ or panic‐related behavior has been the focus of several studies. Some mesencephalic tectum (MT) structures, such as the superior colliculus (SC) and dorsal periaqueductal gray matter (dPAG), are considered to be responsible for the control of defensive responses evoked during threatening situations. Furthermore, the pars reticulata of the substantia nigra (SNpr) sends inputs to the SC that can work as a sensory channel to MT neurons fundamental for the elaboration of defensive responses. The purpose of the present study was to investigate the role of striato‐nigral GABAergic inputs in the activity of nigro‐tectal outputs during the elaboration of defensive behavior using a GABA_A receptor selective blockade in the MT of mice confronted pre‐treated with Bothrops alternatus. Mice with injections of physiological saline into the SNpr and treated with a GABA_A receptor selective antagonist in the MT displayed an increase in panic‐related behavior, expressed by an increase in the duration of freezing, frequency of nonoriented escape and frequency of total escape responses during the confrontation with the snake. However, intra‐SNpr injections of cobalt chloride followed by MT injections of bicuculline caused a significant decrease in the duration of freezing and total escape responses. In addition, intra‐SNpr injections of lidocaine followed by MT injections of bicuculline caused an increase in panic‐related behavior. The results highlight the involvement of SNpr and MT structures in the organization of defensive behaviors and suggest an inhibitory control of striatonigral‐nigrotectal pathways during the elaboration of fear‐ and panic‐related behavior. Synapse 69:299–313, 2015 . © 2015 Wiley Periodicals, Inc.     

Increases in extracellular levels of 5-HT and dopamine in the basolateral, but not in the central, nucleus of amygdala induced by aversive stimulation of the inferior colliculus

Consistent evidence has shown that dopamine release in the prefrontal cortex is increased by electrical stimulation of the inferior colliculus (IC) as unconditioned stimulus. Recent reports have also demonstrated that inactivation of the basolateral nucleus of the amygdala (BLA) with muscimol enhances the behavioural consequences of the aversive stimulation of the IC and reduces the dopamine release in the prefrontal cortex. Moreover, neurotoxic lesions of the BLA enhance whereas those of the central nucleus of the amygdala (CeA) reduce the aversiveness of the electrical stimulation of the IC. Based on these findings the present study examined the effects of the electrical stimulation of the IC on the extracellular levels of serotonin and dopamine in the BLA and CeA. To this end, rats implanted with a stimulation electrode in the IC also bore a microdialysis probe in the BLA or CeA for determination of the release of dopamine and serotonin. IC electrical stimulation at the freezing and escape thresholds increased the levels of serotonin ( approximately 70%) and dopamine ( approximately 60%) in the BLA related to the basal values. Similarly, the metabolites DOPAC and 5-HIAA increased in a parallel fashion in BLA. No significant changes could be detected in these biogenic amines and metabolites in CeA following IC aversive stimulation. These findings point to a differential role of serotonergic and dopaminergic mechanisms of the BLA and CeA in the setting up of adaptive responses to fear states generated at the inferior colliculus level.     

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 dorsal periaqueductal and basolateral amygdala are necessary for the expression of conditioned place avoidance induced by semicarbazide stimulation of the dorsal periaqueductal region

The amygdala is critically involved in the regulation of unconditioned and conditioned reactions to threatening stimuli. It has been suggested that a neural circuit responsible for the production of defensive behavior elicited by the dorsal periaqueductal gray (dPAG) stimulation may project through ascending fibers to forebrain structures such as the basolateral complex of the amygdala (BLA). The present study evaluates the involvement of the dPAG and BLA in the mediation of unconditioned and conditioned responses organized in the dPAG using the open field and the conditioned place aversion (CPA) tests. In both tests, the intra-dPAG injections of semicarbazide (SEM), an inhibitor of the GABA synthesizing enzyme, was used as unconditioned stimulus (US). Using the open field test, we examine the effects of BLA inactivation with the GABA-(A) receptor agonist muscimol (MUS) on the unconditioned fear. We also investigated, through the CPA test, the effects of BLA and/or dPAG inactivation with MUS on the acquisition and the expression of the fear conditioned response. Our results showed that intra-BLA injections of MUS did not change the unconditioned fear elicited by dPAG injections of SEM. As for the CPA test, intra-BLA and intra-dPAG injections of MUS impaired the expression of CPA behavior induced by SEM injections into the dPAG. However, this inactivation of BLA did not impair the acquisition of the CPA behavior induced by injections of SEM into the dPAG. Altogether, these findings suggest that BLA does not participate in the mediation of unconditioned fear induced by dPAG chemical stimulation or in the acquisition of CPA in which aversive stimulation of the dPAG was used as US. In contrast, our results indicate that the activation of the dPAG and BLA is essential to the expression of the conditioned aversive response.     

Predator odor fear conditioning: current perspectives and new directions

Predator odor fear conditioning involves the use of a natural unconditioned stimulus, as opposed to aversive electric foot-shock, to obtain novel information on the neural circuitry associated with emotional learning and memory. Researchers are beginning to identify brain sites associated with conditioned contextual fear such as the ventral anterior olfactory nucleus, dorsal premammillary nucleus, ventrolateral periaqueductal gray, cuneiform nucleus, and locus coeruleus. In addition, a few studies have reported an involvement of the basolateral and medial nucleus of the amygdala and hippocampus in fear conditioning. However, several important issues concerning the effectiveness of different predator odor unconditioned stimuli to produce fear conditioning, the precise role of brain nuclei in fear conditioning, and the general relation between the current predator odor and the traditional electric foot-shock fear conditioning procedures remain to be satisfactorily addressed. This review discusses the major behavioral results in the current predator odor fear conditioning literature and introduces two novel contextual and auditory fear conditioning models using cat odor. The new models provide critical information on the acquisition of conditioned fear behavior during training and the expression of conditioned responses in the retention test. Future studies adopting fear conditioning procedures that incorporate measures of both unconditioned and conditioned responses during training may lead to broad insights into predator odor fear conditioning and identify specific brain nuclei mediating conditioned stimulus-predator odor unconditioned stimulus associations.     

Neural segregation of Fos-protein distribution in the brain following freezing and escape behaviors induced by injections of either glutamate or NMDA into the dorsal periaqueductal gray of rats

Freezing and escape responses induced by gradual increases in the intensity of the electrical current applied to dorsal regions of the periaqueductal gray (dPAG) cause a distinct pattern of Fos distribution in the brain. From these studies, it has been suggested that a pathway involving the dPAG itself, dorsomedial hypothalamus and the cuneiform nucleus (CnF) would mediate responses to immediate danger and another one involving the amygdala and ventrolateral periaqueductal gray (vlPAG) would mediate cue-elicited responses. As electrical stimulation activates body cells and fibers of passage the need of studies with chemical stimulation of only post-synaptic fibers of the dPAG is obvious. To examine further this issue we measured Fos protein expression in brain areas activated by stimulation of the dPAG with glutamate (5 nmol/0.2 microL) and N-methyl-D-aspartate (NMDA) at doses that provoke either freezing (4 nmol/0.2 microL) or escape (7 nmol/0.2 microL) responses, respectively. The results showed that glutamate-induced freezing caused a selective increase in Fos expression in the superior and inferior colliculi as well as in the laterodorsal nucleus of the thalamus. On the other hand, NMDA-induced escape led to widespread increases in Fos labeling in almost all structures studied. Differently from glutamate, NMDA at doses provoking freezing caused significant increase of Fos labeling in the dPAG and CnF. Therefore, the present data support the notion that freezing behavior induced by activation of either non-NMDA or NMDA receptors in the dorsolateral periaqueductal gray (dlPAG) is neurally segregated: glutamate activates only structures that are mainly involved in the sensorial processing and NMDA-induced freezing structures involved in the motor output of defensive behavior. Therefore, the freezing elicited by the activation of non-NMDA receptors seem to be related to the acquisition of aversive information, whereas that resulting from the activation of NMDA receptors could serve as a preparatory response for flight.     

Fos-like immunoreactivity in the brain associated with freezing or escape induced by inhibition of either glutamic acid decarboxylase or GABAA receptors in the dorsal periaqueductal gray

GABAergic neurons exert tonic control over the neural substrates of aversion in the dorsal periaqueductal gray (dPAG). It has been shown that electrical stimulation of this region at freezing or escape thresholds activates different neural circuits in the brain. Since electrical stimulation activates cell bodies and fibers of passage, it is necessary to use chemical stimulation that activates only post-synaptic receptors. To investigate this issue further, reduction of GABA transmission was performed with local injections of either the GABA-A receptor antagonist bicuculline or the glutamic acid decarboxylase (GAD) inhibitor semicarbazide into the dorsolateral periaqueductal gray (dlPAG). Local infusions of semicarbazide (5.0 microg/0.2 microl) or bicuculline (40 ng/0.2 microl) into this region caused freezing and escape, respectively. The results obtained showed that freezing behavior induced by semicarbazide was associated with an increase in Fos expression in the laterodorsal nucleus of the thalamus (LD) and ventrolateral periaqueductal gray (vlPAG), while bicuculline-induced escape was related to widespread increase in Fos labeling, notably in the columns of the periaqueductal gray, hypothalamus nuclei, the central amygdaloid nucleus (Ce), the LD, the cuneiform nucleus (CnF) and the locus coeruleus (LC). Thus, the present data support the notion that freezing and escape behaviors induced by GABA blockade in the dlPAG are neurally segregated: freezing activates only structures that are mainly involved in sensory processing, and bicuculline-induced escape activates structures involved in both sensory processing and motor output of defensive behavior. Therefore, the freezing elicited by activation of dlPAG appears to be related to the acquisition of aversive information, whereas most brain structures involved in the defense reaction are recruited during escape.     

Parallel incentive processing: an integrated view of amygdala function

The amygdala is a heterogeneous structure that has been implicated in a wide variety of functions, most notably in fear conditioning. From this research, an influential serial model of amygdala processes has emerged in which aversive learning is mediated by the amygdala basolateral nucleus whereas performance, in this case of various defensive reflexes, is mediated by the central nucleus. By contrast, recent evidence from appetitive conditioning studies suggests that the basolateral and central nuclei operate in parallel to mediate distinct incentive processes: the basolateral nucleus encodes emotional events with reference to their particular sensory-specific features, whereas the central nucleus encodes their more general motivational or affective significance. Given that there is little if any direct behavioral evidence for the serial model, we suggest that more attention should be given to the claims of the parallel view.     

The defense system of fear: behavior and neurocircuitry.

Fear can be conceived as a functional defense behavior system representing a part of the innate species-specific behavioral repertoire (ethogram), basic to the survival of individuals and species. Its function is to protect living beings against dangerous, threatening and aversive situations. A distinction is made between anticipatory defense behaviors released by potential dangers and those elicited by effective dangers, especially predators. The neural mechanisms serving the defense system constitute a hierarchical network with the amygdala as point of convergence of the various threatening stimuli. The central nucleus of the amygdala projects to the midbrain periaqueductal gray (PAG), the hypothalamus and the brainstem which coordinate various defensive responses such as flight, defensive fight, freezing, avoidance reactions, submissive postures, tonic immobilization, hypoalgesia and autonomic arousal. These circuits may be activated either by unconditioned or conditioned stimuli.     

Bilateral lesions of the amygdala attenuate analgesia induced by diverse environmental challenges

This study was designed to evaluate the role of the amygdala, particularly its central nucleus, in the induction of analgesia elicited by environmental challenges. Rats with large, radiofrequency lesions centered in the central nucleus were found to display significantly attenuated analgesic responses to three different challenges: cat exposure, acute footshock, and re-exposure to an environment associated with footshock. These findings show that the amygdala plays an important role in the elicitation of analgesia by each of the environmental challenges tested. Since the amygdala has been shown to play a critical role in fear, these findings suggest that the analgesia elicited by these challenges involves a substantial fear component. Moreover, the finding that amygdala lesions significantly reduced the analgesia elicited by a non-noxious unconditional stimulus (cat exposure) strongly suggests that these lesions disrupt the expression of analgesia rather than producing a learning impairment. And finally, the findings of this study support the suggestion that fear-elicited analgesia is triggered by activation of a projection from amygdala to periaqueductal gray which forms one component of an integrated ‘defensive behavioral system’.     

The role of the dorsomedial part of the prefrontal cortex serotonergic innervation in rat responses to the aversively conditioned context: behavioral, biochemical and immunocytochemical studies

In this study we have explored differences in animal reactivity to conditioned aversive stimuli using the conditioned fear test (a contextual fear-freezing response), in rats subjected to the selective lesion of the prefrontal cortex serotonergic innervation, and differing in their response to the acute painful stimulation, a footshock (HS--high sensitivity rats, and LS--low sensitivity rats, selected arbitrarily according to their behavior in the 'flinch-jump' pre-test). Local administration of serotonergic neurotoxin (5,7-dihydroxytryptamine) to the dorsomedial part of the prefrontal cortex caused a very strong, structure and neurotransmitter selective depletion of serotonin concentration. In HS rats, the serotonergic lesion significantly disinhibited rat behavior controlled by fear, enhanced c-Fos expression in the dorsomedial prefrontal area, and increased the concentration of GABA in the basolateral amygdala, measured in vivo after the testing session of the conditioned fear test. The LS animals revealed an opposite pattern of behavioral and biochemical changes after serotonergic lesion: an increase in the duration of a freezing response, and expression of c-Fos in the basolateral and central nuclei of amygdala, and a lower GABA concentration in the basolateral amygdala. In control conditions, c-Fos expression did not differ in LS and HS, na?ve, not conditioned and not exposed to the test cage animals. The present study adds more arguments for the controlling role of serotonergic innervation of the dorsomedial part of the prefrontal cortex in processing emotional input by other brain centers. Moreover, it provides experimental data, which may help to better explain the anatomical and biochemical basis of differences in individual reactivity to stressful stimulation, and, possibly, to anxiolytic drugs with serotonergic or GABAergic profiles of action.     

Neural organization of different types of fear: implications for the understanding of anxiety

The dangerous stimuli may be potentially dangerous, distal or proximal and the recognition by the animals of each one of these conditions is determinant for the nature of the fear responses. In the present article a parallel with this particular process is drawn taking into account that different fear responses are generated by light, tones and contexts used as conditioned stimuli and by unconditioned stimulation of the dorsal periaqueductal gray (dPAG). In this review we summarize the efforts that have been made to characterize the neural circuits recruited in the organization of defensive reactions to the conditioned and unconditioned aversive stimulations, particularly evidence linking the brain's defense response systems to the concept of fear-stress-anxiety. The dPAG constitute the main neural substrates for the integration of aversive states in response to proximal aversive stimuli. In fact, panic-like behaviors often result when this structure is electrically or chemically stimulated. On the other hand, successful preparatory processes of danger-orientation and preparedness to flee seem to be linked to anxiety. The pre-frontal and cingulate cortex, median raphe nucleus, septum and hippocampus seem to be implicated in the elaboration and organization of these responses. As a working hypothesis, it is advanced that increasing the intensity and proximity of the danger may lead to an emotional shift. When the animals are submitted to this gradual increase in aversiveness there is a switch from the neural circuits responsible for the production of the orientated and organized motor patterns of appropriate defensive response to a conditioned stimulus towards the incomplete and uncoordinated defense responses related to panic attacks. The circuits in the amygdala and the medial hypothalamus responsible for the organization of the defense reaction may well subserve to this switch process.     

Neuroanatomical approaches of the tectum-reticular pathways and immunohistochemical evidence for serotonin-positive perikarya on neuronal substrates of the superior colliculus and periaqueductal gray matter involved in the elaboration of the defensive behavior and fear-induced analgesia

Deep layers of the superior colliculus, the dorsal periaqueductal gray matter and the inferior colliculus are midbrain structures involved in the generation of defensive behavior and fear-induced anti-nociception. Local injections of the GABA(A) antagonist bicuculline into these structures have been used to produce this defense reaction. Serotonin is thought to be the main neurotransmitter to modulate such defense reaction in mammals. This study is the first attempt to employ immunohistochemical techniques to locate serotonergic cells in the same midbrain sites from where defense reaction is evoked by chemical stimulation with bicuculline. The blockade of GABA(A) receptors in the neural substrates of the dorsal mesencephalon was followed by vigorous defensive reactions and increased nociceptive thresholds. Light microscopy immunocytochemistry with streptavidin method was used for the localization of the putative cells of defensive behavior with antibodies to serotonin in the rat's midbrain. Neurons positive to serotonin were found in the midbrain sites where defensive reactions were evoked by microinjection of bicuculline. Serotonin was localized to somata and projections of the neural networks of the mesencephalic tectum. Immunohistochemical studies showed that the sites in which neuronal perikarya positive to serotonin were identified in intermediate and deep layers of the superior colliculus, and in the dorsal and ventral columns of the periaqueductal gray matter are the same which were activated during the generation of defense behaviors, such as alertness, freezing, and escape reactions, induced by bicuculline. These findings support the contention that serotonin and GABAergic neurons may act in concert in the modulation of defense reaction in the midbrain tectum. Our neuroanatomical findings indicate a direct neural pathway connecting the dorsal midbrain and monoaminergic nuclei of the descending pain inhibitory system, with profuse synaptic terminals mainly in the pontine reticular formation, gigantocellularis nucleus, and nucleus raphe magnus. The midbrain tectum-gigantocellularis complex and midbrain tectum-nucleus raphe magnus neural pathways may provide an alternative output allowing the organization of the fear-induced anti-nociception by mesencephalic networks.