Behavioral effects of microinjections of SR 95103, a new GABA-A antagonist, into the medial hypothalamus or the mesencephalic central gray

Orchestration of innate and conditioned defensive actions by the periaqueductal gray

The midbrain periaqueductal gray (PAG) has been recognized for decades as having a central role in the control of a wide variety of defensive responses. Initial discoveries relied primarily on lesions, electrical stimulation and pharmacology. Recent developments in neural activity imaging and in methods to control activity with anatomical and genetic specificity have revealed additional streams of data informing our understanding of PAG function. Here, we discuss both classic and modern studies reporting on how PAG-centered circuits influence innate as well as learned defensive actions in rodents and humans. Though early discoveries emphasized the PAG's role in rapid induction of innate defensive actions, emerging new data indicate a prominent role for the PAG in more complex processes, including representing behavioral states and influencing fear learning and memory. This article is part of the Special Issue on "Fear, Anxiety and PTSD".

Optogenetic Stimulation of the Superior Colliculus Confers Retinal Neuroprotection in a Mouse Glaucoma Model

Glaucoma is characterized by a progressive loss of retinal ganglion cells (RGCs) in the eye, which ultimately results in visual impairment or even blindness. Because current therapies often fail to halt disease progression, there is an unmet need for novel neuroprotective therapies to support RGC survival. Various research lines suggest that visual target centers in the brain support RGC functioning and survival. Here, we explored whether increasing neuronal activity in one of these projection areas could improve survival of RGCs in a mouse glaucoma model. Prolonged activation of an important murine RGC target area, the superior colliculus (SC), was established via a novel optogenetic stimulation paradigm. By leveraging the unique channel kinetics of the stabilized step function opsin (SSFO), protracted stimulation of the SC was achieved with only a brief light pulse. SSFO-mediated collicular stimulation was confirmed by immunohistochemistry for the immediate-early gene c-Fos and behavioral tracking, which both demonstrated consistent neuronal activity upon repeated stimulation. Finally, the neuroprotective potential of optogenetic collicular stimulation was investigated in mice of either sex subjected to a glaucoma model and a 63% reduction in RGC loss was found. This work describes a new paradigm for optogenetic collicular stimulation and a first demonstration that increasing target neuron activity can increase survival of the projecting neurons.SIGNIFICANCE STATEMENT Despite glaucoma being a leading cause of blindness and visual impairment worldwide, no curative therapies exist. This study describes a novel paradigm to reduce retinal ganglion cell (RGC) degeneration underlying glaucoma. Building on previous observations that RGC survival is supported by the target neurons to which they project and using an innovative optogenetic approach, we increased neuronal activity in the mouse superior colliculus, a main projection target of rodent RGCs. This proved to be efficient in reducing RGC loss in a glaucoma model. Our findings establish a new optogenetic paradigm for target stimulation and encourage further exploration of the molecular signaling pathways mediating retrograde neuroprotective communication.

Panicolytic-like effects caused by substantia nigra pars reticulata pretreatment with low doses of endomorphin-1 and high doses of CTOP or the NOP receptors antagonist JTC-801 in male Rattus norvegicus

RATIONALE

Gamma-aminobutyric acid (GABA)ergic neurons of the substantia nigra pars reticulata (SNpr) are connected to the deep layers of the superior colliculus (dlSC). The dlSC, in turn, connect with the SNpr through opioid projections. Nociceptin/orphanin FQ peptide (N/OFQ) is a natural ligand of a Gi protein-coupled nociceptin receptor (ORL1; NOP) that is also found in the SNpr. Our hypothesis is that tectonigral opioid pathways and intranigral orphanin-mediated mechanisms modulate GABAergic nigrotectal connections.

OBJECTIVES

Therefore, the aim of this work was to study the role of opioid and NOP receptors in the SNpr during the modulation of defence reactions organised by the dlSC.

METHODS

The SNpr was pretreated with either opioid or NOP receptor agonists and antagonists, followed by dlSC treatment with bicuculline.

RESULTS

Blockade of GABAA receptors in the dlSC elicited fear-related defensive behaviour. Pretreatment of the SNpr with naloxone benzoylhydrazone (NalBzoH), a μ-, δ-, and κ1-opioid receptor antagonist as well as a NOP receptor antagonist, decreased the aversive effect of bicuculline treatment on the dlSC. Either μ-opioid receptor activation or blockade by SNpr microinjection of endomorphin-1 (EM-1) and CTOP promoted pro-aversive and anti-aversive actions, respectively, that modulated the defensive responses elicited by bicuculline injection into the dlSC. Pretreatment of the SNpr with the selective NOP receptor antagonist JTC801 decreased the aversive effect of bicuculline, and microinjections of the selective NOP receptor agonist NNC 63-0532 promoted the opposite effect.

CONCLUSIONS

These results demonstrate that opioid pathways and orphanin-mediated mechanisms have a critical role in modulating the activity of nigrotectal GABAergic pathways during the organisation of defensive behaviours.

Dissociation between the panicolytic effect of cannabidiol microinjected into the substantia nigra, pars reticulata, and fear-induced antinociception elicited by bicuculline administration in deep layers of the superior colliculus: The role of CB1-cannabinoid receptor in the ventral mesencephalon

Many studies suggest that the substantia nigra, pars reticulata (SNpr), a tegmental mesencephalic structure rich in γ-aminobutyric acid (GABA)- and cannabinoid receptor-containing neurons, is involved in the complex control of defensive responses through the neostriatum-nigral disinhibitory and nigro-tectal inhibitory GABAergic pathways during imminently dangerous situations. The aim of the present work was to investigate the role played by CB1-cannabinoid receptor of GABAergic pathways terminal boutons in the SNpr or of SNpr-endocannabinoid receptor-containing interneurons on the effect of intra-nigral microinjections of cannabidiol in the activity of nigro-tectal inhibitory pathways. GABAA receptor blockade in the deep layers of the superior colliculus (dlSC) elicited vigorous defensive behaviour. This explosive escape behaviour was followed by significant antinociception. Cannabidiol microinjection into the SNpr had a clear anti-aversive effect, decreasing the duration of defensive alertness, the frequency and duration of defensive immobility, and the frequency and duration of explosive escape behaviour, expressed by running and jumps, elicited by transitory GABAergic dysfunction in dlSC. However, the innate fear induced-antinociception was not significantly changed. The blockade of CB1 endocannabinoid receptor in the SNpr decreased the anti-aversive effect of canabidiol based on the frequency and duration of defensive immobility, the frequency of escape expressed by running, and both the frequency and duration of escape expressed by jumps. These findings suggest a CB1 mediated endocannabinoid signalling in cannabidiol modulation of panic-like defensive behaviour, but not of innate fear-induced antinociception evoked by GABAA receptor blockade with bicuculline microinjection into the superior colliculus, with a putative activity in nigro-collicular GABAergic pathways.

Collateral pathways from the ventromedial hypothalamus mediate defensive behaviors

The ventromedial hypothalamus (VMH) was thought to be essential for coping with threat, although its circuit mechanism remains unclear. To investigate this, we optogenetically activated steroidogenic factor 1 (SF1)-expressing neurons in the dorsomedial and central parts of the VMH (VMHdm/c), and observed a range of context-dependent somatomotor and autonomic responses resembling animals' natural defensive behaviors. By activating independent pathways emanating from the VMHdm/c, we demonstrated that VMHdm/c projection to the dorsolateral periaqueductal gray (dlPAG) induces inflexible immobility, while the VMHdm/c to anterior hypothalamic nucleus (AHN) pathway promotes avoidance. Consistent with the behavior changes induced by VMH to AHN pathway activation, direct activation of the AHN elicited avoidance and escape jumping, but not immobility. Retrograde tracing studies revealed that nearly 50% of PAG-projecting VMHdm/c neurons send collateral projection to the AHN and vice versa. Thus, VMHdm/c neurons employ a one-to-many wiring configuration to orchestrate multiple aspects of defensive behaviors.

Chronic unpredictable mild stress alters an anxiety-related defensive response, Fos immunoreactivity and hippocampal adult neurogenesis

Previous results show that elevated T-maze (ETM) avoidance responses are facilitated by acute restraint. Escape, on the other hand, was unaltered. To examine if the magnitude of the stressor is an important factor influencing these results, we investigated the effects of unpredictable chronic mild stress (UCMS) on ETM avoidance and escape measurements. Analysis of Fos protein immunoreactivity (Fos-ir) was used to map areas activated by stress exposure in response to ETM avoidance and escape performance. Additionally, the effects of the UCMS protocol on the number of cells expressing the marker of migrating neuroblasts doublecortin (DCX) in the hippocampus were investigated. Corticosterone serum levels were also measured. Results showed that UCMS facilitates ETM avoidance, not altering escape. In unstressed animals, avoidance performance increases Fos-ir in the cingulate cortex, hippocampus (dentate gyrus) and basomedial amygdala, and escape increases Fos-ir in the dorsolateral periaqueductal gray and locus ceruleus. In stressed animals submitted to ETM avoidance, increases in Fos-ir were observed in the cingulate cortex, ventrolateral septum, hippocampus, hypothalamus, amygdala, dorsal and median raphe nuclei. In stressed animals submitted to ETM escape, increases in Fos-ir were observed in the cingulate cortex, periaqueductal gray and locus ceruleus. Also, UCMS exposure decreased the number of DCX-positive cells in the dorsal and ventral hippocampus and increased corticosterone serum levels. These data suggest that the anxiogenic effects of UCMS are related to the activation of specific neurobiological circuits that modulate anxiety and confirm that this stress protocol activates the hypothalamus-pituitary-adrenal axis and decreases hippocampal adult neurogenesis.

Effects of chronic treatment with corticosterone and imipramine on fos immunoreactivity and adult hippocampal neurogenesis

In a previous study we showed that rats chronically treated with corticosterone (CORT) display anxiogenic behavior, evidenced by facilitation of avoidance responses in the elevated T-maze (ETM) model of anxiety. Treatment with the tricyclic antidepressant imipramine significantly reversed the anxiogenic effects of CORT, while inhibiting ETM escape, a response related to panic disorder. To better understand the neurobiological mechanisms underlying these behavioral effects, analysis of c-fos protein immunoreactivity (fos-ir) was used here to map areas activated by chronic CORT (200 mg pellets, 21-day release) and imipramine (15 mg/kg, IP) administration. We also evaluated the number of cells expressing the neurogenesis marker doublecortin (DCX) in the hippocampus and measured plasma CORT levels on the 21st day of treatment. Results showed that CORT increased fos-ir in the ventrolateral septum, medial amygdala and paraventricular hypothalamic nucleus and decreased fos-ir in the lateral periaqueductal gray. Imipramine, on the other hand, increased fos-ir in the medial amygdala and decreased fos-ir in the anterior hypothalamus. CORT also decreased the number of DCX-positive cells in the ventral and dorsal hippocampus, an effect antagonized by imipramine. CORT levels were significantly higher after treatment. These data suggest that the behavioral effects of CORT and imipramine are mediated through specific, at times overlapping, neuronal circuits, which might be of relevance to a better understanding of the physiopathology of generalized anxiety and panic disorder.

Acute restraint differently alters defensive responses and fos immunoreactivity in the rat brain

Results from a previous study show that rats exposed to acute restraint display anxiogenic-like behavior, evidenced by facilitation of avoidance responses in the elevated T-maze (ETM) model of anxiety. In contrast, escape responses were unaltered by stress exposure. Since ETM avoidance and escape tasks seem to activate distinct sets of brain structures, it is possible that the differences observed with acute restraint are due to particularities in the neurobiological mechanisms which modulate these responses. In the present study, analysis of fos protein immunoreactivity (fos-ir) was used to map areas activated by exposure of male Wistar rats to restraint stress (30 min) previously (30 min) to the ETM. Corticosterone levels were also measured in stressed and non-stressed animals. Confirming previous observations restraint facilitated avoidance performance, an anxiogenic result, while leaving escape unaltered. Performance of the avoidance task increased fos-ir in the frontal cortex, intermediate lateral septum, basolateral amygdala, basomedial amygdala, lateral amygdala, anterior hypothalamus and dorsal raphe nucleus. In contrast, performance of escape increased fos-ir in the ventromedial hypothalamus, dorsolateral periaqueductal gray and locus ceruleus. Both behavioral tasks also increased fos-ir in the dorsomedial hypothalamus. Restraint significantly raised corticosterone levels. Additionally after restraint, fos-ir was predominantly seen in the basolateral amygdala and dorsal raphe of animals submitted to the avoidance task. This data confirms that different sets of brain structures are activated by ETM avoidance and escape tasks and suggests that acute restraint differently alters ETM behavior and the pattern of fos activation in the brain.

Cannabinoid and GABA modulation of sympathetic nerve activity and blood pressure in the dorsal periaqueductal gray of the rat

Sympathoexcitation and increased blood pressure evoked by central networks integrating defensive behavior are fundamental to the acute stress response. A balance between excitatory glutamatergic and inhibitory GABAergic neurotransmission in the dorsal periaqueductal gray (dPAG) results in a tonic level of activity in the alerting system. Neuromodulators such as endocannabinoids have been shown to influence the sympathoexcitatory and pressor components of acute stress in the dPAG, exemplified by the defense response as a model, but the mechanism of integration remains unknown. The present study examines the role of GABA and its interaction with endocannabinoids in modulating sympathetic nerve activity and blood pressure related to the defense response. Microinjection of the broad-spectrum excitatory amino acid dl-homocysteic acid (DLH) identified sites of the defense pathway in the dPAG from which an increase in renal sympathetic nerve activity and blood pressure could be evoked, and subsequent microinjections were made at the same site through a multibarrelled micropipette. Blockade of GABAA receptors or microinjection of the cannabinoid 1 receptor agonist anandamide elicited a renal sympathoexcitation and pressor response. Prior microinjection of the GABAA receptor antagonist gabazine attenuated the sympathoexcitation and pressor response associated with anandamide microinjection. In contrast, the sympathetic response to DLH was enhanced by GABAA receptor blockade. These data demonstrate that sympathoexcitatory neurons in the dPAG are under tonic inhibition by GABA and that endocannabinoids modulate this GABAergic neurotransmission to help regulate components of the defense response.

Relationships between the superior colliculus and hippocampus: Neural and behavioral considerations

Theories of superior collicular and hippocampal function have remarkable similarities. Both structures have been repeatedly implicated in spatial and attentional behaviour and in inhibitory control of locomotion. Moreover, they share certain electrophysiological properties in their single unit responses and in the synchronous appearance and disappearance of slow wave activity. Both are phylogenetically old and the colliculus projects strongly to brainstem nuclei instrumental in the generation of theta rhythm in the hippocampal EEC

On the other hand, close inspection of behavioural and electrophysiological data reveals disparities. In particular, hippocampal processing mainly concerns stimulus ambiguity, contextual significance, and spatial relations or other subtle, higher order characteristics. This requires the use of largely preprocessed sensory information and mediation of poststimulus investigation. Although collicular activity must also be integrated with that of “higher” centres (probably to a varying degree, depending on the nature of stimuli being processed and the task requirements), its primary role in attention is more “peripheral” and specific in controlling orienting/localisation via eye and body movements toward egocentrically labelled spatial positions. In addition, the colliculus may exert a nonspecific influence in alerting higher centres to the imminence of information potentially worthy of focal attention. Nevertheless, it is noteworthy that collicular and hippocampal lesions produce deficits on similar tasks, although the type of deficit is usually different (often opposite) in each case. Functional overlap between hippocampus and colliculus (i.e., strategically synchronised or mutually interdependent activity) is virtually certain vis-à-vis stimulus sampling, for example in the acquisition of information via vibrissal movements and visual scanning. In addition, insofar as stimulus significance is a factor in collicular orienting mechanisms, the hippocampus — cingulate – cortex — colliculus pathway may play a significant role, modulating collicular responsiveness and thus ensuring an attentional strategy appropriate to current requirements (stimulus familiarity, stage of learning). A tentative “reciprocal loop” model is proposed which bridges physiological and behavioural levels of analysis and which would account for the observed degree and nature of functional overlap between the superior colliculus and hippocampus.

GABA/benzodiazepine receptors in the ventromedial hypothalamic nucleus regulate both anxiety and panic-related defensive responses in the elevated T-maze

It has been shown that facilitation of GABA-mediated neurotransmission in the medial nucleus of the amygdala and the dorsal periaqueductal gray (dPAG) inhibits the escape, but not the inhibitory avoidance response generated in the elevated T-maze test of anxiety (ETM). These defensive behaviors have been associated with panic and generalized anxiety, respectively. Previous evidence indicates that the dorsomedial part of the ventromedial hypothalamus (VMHdm), which is interconnected with these two brain areas, is also part of the neurobiological substrate controlling escape behavior. In the present study, we investigated in male Wistar rats whether the intra-VMHdm injection of GABA-modulating drugs differently affect the two defensive tasks measured in the ETM. The results showed that the microinjection of the benzodiazepine (BZD) receptor agonist midazolam (10, 20 and 40 nmol), the GABA(A) receptor agonist muscimol (2, 4 and 8 nmol) or the GABA(B) receptor agonist baclofen (2, 4 and 8 nmol) impaired inhibitory avoidance and escape performance, an anxiolytic and panicolytic-like effect, respectively. On the other hand, local administration of the BZD inverse agonist FG 7142 (20, 40 and 80 pmol) facilitated both behaviors, suggesting anxiogenic and panicogenic-like effects. These results were not due to motor alterations, since the drugs did not affect exploratory behavior in an open field. The data suggest that GABA(A)/BZD and GABA(B) receptors within the VMHdm are involved not only in the control of panic-related, but also of anxiety-related behaviors.

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.

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.

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.

Neuroanatomical and neuropharmacological study of opioid pathways in the mesencephalic tectum: effect of μ1- and κ-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.

GABAergic regulation of REM sleep in reticularis pontis oralis and caudalis in rats

The nucleus reticularis pontis oralis (RPO) and nucleus reticularis pontis caudalis (RPC) are implicated in the generation of rapid eye movement sleep (REM). Work in cats has indicated that GABA in RPO plays a role in the regulation of REM. We assessed REM after local microinjections into RPO and RPC of the gamma-aminobutyric acid-A (GABA(A)) agonist, muscimol (MUS), and the GABA(A) antagonist, bicuculline (BIC). Rats (90-day-old male Sprague-Dawley) were implanted with electrodes for recording electroencephalographs (EEG) and electromyographs (EMG). Guide cannulae were aimed into RPO (n = 9) and RPC (n = 8) for microinjecting MUS (200, 1,000.0 microM) and BIC (0.056, 0.333, 1.0, 1,000.0, and 10,000.0 microM). Animals received bilateral microinjections of saline, MUS, and BIC (0.2 microl microinjected at 0.1 microl/min) into each region followed by 6-h sleep recordings. In RPO, MUS (1,000.0 microM) suppressed REM and BIC (1,000.0 microM) enhanced REM. In RPC, MUS (200, 1,000.0 microM) suppressed REM, but BIC (1,000.0 microM and less) did not significantly affect REM. Higher concentrations of BIC (10,000.0 microM) injected into RPO (n = 9) and RPC (n = 4) produced wakefulness and escape behavior. The results indicate that GABA in RPO/RPC is involved in the regulation of REM and suggest site-specific differences in this regulation.

The medial hypothalamic defensive system: hodological organization and functional implications

The hypothalamus is a relatively small division of the vertebrate forebrain that plays especially important roles in neural mechanisms assuring homeostasis, defense, and reproduction. Previous studies from our laboratory have suggested a distinct circuit in the medial hypothalamic zone as critically involved in the organization of innate defensive behavior. Thus, after exposure to a natural predator known to elicit innate defensive responses, increased Fos levels in the medial zone of the hypothalamus have been found restricted to the anterior hypothalamic nucleus, dorsomedial part of the ventromedial nucleus, and dorsal premammillary nucleus (PMd). Previous anatomical studies have shown that these Fos-responsive cell groups in the medial hypothalamus are interconnected in a distinct neural system, in which the PMd appears to be a critical element for the expression of defensive responses elicited by the presence of a predator. The purpose of this review is to provide an overview of what is currently known about the functional and hodological organization of this hypothalamic circuit subserving defensive responses.

Tracing from the dorsal premammillary nucleus prosencephalic systems involved in the organization of innate fear responses

The dorsal premammillary nucleus (PMd) is thought to play a critical role in the expression of fear responses to environmental threats. We have previously reported that, during an encounter with a predator, the PMd presents an impressive increase in Fos levels and cell body-specific chemical lesions therein virtually eliminated the expression of escape and freezing responses. Therefore, the PMd may be viewed as a strategic starting point to delineate prosencephalic circuits seemingly critical for the organization of innate fear responses. In the present review, we provide a comprehensive examination of the neural circuits putatively involved in influencing this hypothalamic site, and supplement this analysis with recent observations from our laboratory on the expression of Fos protein in the central nervous system of rats exposed to a live predator.

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

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

Changes in the auditory-evoked potentials induced by fear-evoking stimulations

It is long established that the inferior colliculus is involved in conveying all kinds of auditory information to higher cortical structures. Moreover, gradual increases in the electrical stimulation of this structure produces progressive aversive responses from vigilance, through freezing, until escape. Recently, we have shown that microinjections of the excitatory amino acids, N-methyl-D-aspartate (NMDA) and glutamate, into the inferior colliculus mimic these aversive effects. In the present study, we extend these observations showing that unilateral microinjections of 5 nmol of glutamate into the inferior colliculus--a dose that causes freezing behavior--in rats with bilateral recording electrodes into this structure produce an increase in the magnitude of the collicular-evoked potential in the ipsilateral side of the injection in relation to saline-injected animals. Besides, the application of two kinds of fear-evoking stimulations--light as a conditioned stimuli (CS) and ultrasound signals at the frequency of 22 kHz--also produced an increase in the amplitude of the evoked potentials recorded from the inferior colliculus in comparison to control situations without aversive stimuli presentations. These data support previous reports showing that fast-acting excitatory amino acid receptors in this midbrain region are involved in the processing of auditory information. Moreover, fear-eliciting stimulations, such as light-CS and ultrasound signals, increase acoustically evoked firing of neurons in the central nucleus of the inferior colliculus of rats.

The posterior hypothalamic area: chemoarchitecture and afferent connections

This study provides an analysis of the chemoarchitecture of the posterior hypothalamic area (PHA) and a retrograde transport analysis of inputs to the PHA in the rat. The chemoarchitectural analysis reveals that the majority of PHA neurons contain glutamate. Hypocretin, melanin concentrating hormone, tyrosine hydroxylase, neuropeptide Y and gamma-aminobutyric acid are also found in subsets of PHA neurons, and fibers immunoreactive for these substances as well as for serotonin, dopamine-beta-hydroxylase and met-enkephalin are observed in the area and aid in the delineation of its borders. The retrograde tracing study demonstrates that the PHA receives input from multiple, diverse neuron populations. Descending projections to the PHA arise from the limbic forebrain (cingulate cortex and lateral septum) and both the medial and lateral hypothalamus. Subcortical visual nuclei, including the ventral lateral geniculate nucleus and intergeniculate leaflet, pretectal area, and superior colliculus, and the subthalamus (zona incerta, fields of Forel) also project to the PHA. Ascending projections to the PHA arise from brainstem cholinergic nuclei, the reticular formation, midbrain raphe nuclei, periaqueductal gray and parabrachial nucleus. Retrograde transport studies using the psuedorabies virus (PRV) demonstrate that the PHA receives input indirectly from the hippocampus, amygdala and suprachiasmatic nucleus through circuits including nuclei in the limbic forebrain and hypothalamus. These data suggest that the PHA is important in the neural control of behavioral state, modulating aspects of hippocampal, autonomic and cortical function as they relate to the elaboration of adaptive behavior.

Anxiogenic effects of substance P and its 7-11 C terminal, but not the 1-7 N terminal, injected into the dorsal periaqueductal gray

The dorsal periaqueductal gray matter (DPAG) is one of the main output regions of the brainstem for the expression of defense reaction. Recent findings implicating neurokinins in the expression of fear or anxiety-like behaviors, have stimulated interest in the participation of these neuropeptides in the generation of aversive states in the dorsal periaqueductal gray matter. Analyses of traditional measures of the behavior of rats submitted to the elevated plus-maze test in this laboratory have shown that microinjections of substance P (SP) into the DPAG produce anxiogenic-like effects. The present study employs an ethological analysis of the behavior of animals in this test to investigate the involvement of substance P (SP) and its C- and N- fragments (7-11 and 1-7) in the expression of the different aspects of fear upon injection into the DPAG. To this end, rats were implanted with a cannula in the DPAG and injected one week later with 35 and 70 pmol of either substance P, or C- or N- SP fragments and tested immediately afterwards in the elevated plus-maze. The results show that SP and its C terminal fragment, produced increases in scanning, stretched attend posture, head dipping and flat-back approach, whereas the fragment N terminal produced only an increase in rearing. Therefore, the effects of SP and its C terminal fragment were associated to risk assessment behavior, whereas those of N terminal fragment were related to vertical exploratory activity. The results indicate that SP produces anxiogenic effects through activation of neural substrates of aversion in the DPAG and that this effect is probably related to its C terminal fragment.

Descending projections of the posterior nucleus of the hypothalamus: Phaseolus vulgaris leucoagglutinin analysis in the rat

No previous report in any species has systematically examined the descending projections of the posterior nucleus of the hypothalamus (PH). The present report describes the descending projections of the PH in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin. PH fibers mainly descend to the brainstem through two routes: dorsally, within the central tegmental tract, and ventromedially, within the mammillo-tegmental tract and its caudal extension, ventral reticulo-tegmental tracts. PH fibers were found to distribute densely to several nuclei of the brainstem. They are (from rostral to caudal) 1) lateral/ ventrolateral regions of the diencephalo-mesopontine periaqueductal gray (PAG); 2) the peripeduncular nucleus; 3) discrete nuclei of pontomesencephalic central gray (dorsal raphe nucleus, laterodorsal tegmental nucleus, and Barrington's nucleus); 4) the longitudinal extent of the central core of the mesencephalic through meduallary reticular formation (RF); 5) the ventromedial medulla (nucleus gigantocellularis pars alpha, nucleus raphe magnus, and nucleus raphe pallidus); 6) the ventrolateral medulla (nucleus reticularis parvocellularis and the rostral ventrolateral medullary region); and 7) the inferior olivary nucleus. PH fibers originating from the caudal PH distribute much more heavily than those from the rostral PH to the lower brainstem. The PH has been linked to the control of several important functions, including respiration, cardiovascular activity, locomotion, antinociception, and arousal/wakefulness. It is likely that descending PH projections, particularly those to the PAG, the pontomesencephalic RF, Barrington's nucleus, and parts of the ventromedial and ventrolateral medulla, serve a role in a PH modulation of complex behaviors involving integration of respiratory, visceromotor, and somatomotor activity.

Effects of microinjections of the neuropeptide substance P in the dorsal periaqueductal gray on the behaviour of rats in the plus-maze test

Currently, the participation of neuropeptides in the generation of aversive states in the dorsal periaqueductal gray matter (DPAG) is poorly understood. The elevated plus maze (EPM) is widely used for studying the neurobiological mechanisms of anxiety in the laboratory. One difficulty with this test has been to evaluate the involvement of GABA mechanisms in the DPAG substrates of aversion, because microinjections of GABA receptor blockers in this region cause an intense behavioral activation. In this study, we examined in the EPM the effects of semicarbazide, a drug that acts indirectly on GABA neurotransmission through inhibition of the glutamic acid decarboxylase, and substance P (SP) following microinjections into the dorsal periaqueductal gray. Semicarbazide caused a clear decrease in the number of entries and time spent in the open arms. These results confirm previous data showing that GABA has a modulatory role in the DPAG, probably through reduction of tonic inhibitory mechanisms on neural substrates of aversion. A similar pattern of behavioral responses was observed with SP. However, these effects were more pronounced with intermediate doses of SP (25 ng), confirming the characteristic bell-shaped dose-effect function of this neuropeptide. The proaversive effects observed with DPAG microinjections of SP in the present study gain further relevance when combined with previous reports that have shown unconditioned and conditioned aversive effects following DPAG microinjections of SP in other animal models of aversion.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

Fear induced by the blockade of GABAA-ergic transmission in the hypothalamus of the cat: behavioral and neurochemical study

Intrahypothalamic injections of d-Tubocurarine (DT) and bicuculline (BM) in the cat produced a fear reaction characterized by terrific mewing, increased locomotor activity, jumps and attempt to escape from the chamber, pupillary dilatation, increased respiratory rate, and sometimes urination and defecation. HPLC analysis showed a significant increase in the noradrenergic system activity in the emotional brain areas (hypothalamus, midbrain, amygdala) and frontal cortex at the time of the fear drive. No changes in the cat's behavior and in the monoaminergic systems activity occurred after muscimol+d-Tubocurarine injections into the hypothalamus. Similar behavioral and neurochemical effects evoked by DT and BM suggest that the fear response evoked by DT does not result from the blockade of N-cholinergic transmission but rather from their action on GABAA receptor complex. The results obtained indicate that the central triggering mechanism for fear drive depends on the blockade of GABAA-ergic transmission.

Neural substrate of defensive behavior in the midbrain tectum

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

GABAergic nigro-collicular pathways modulate the defensive behaviour elicited by midbrain tectum stimulation

Midbrain tectum (MT) structures such as the dorsal periaqueductal gray matter and deep layers of superior colliculus are well-known for the organization and generation of defensive behaviour. Electrical stimulation or microinjection of GABA antagonists into these structures produce aversive behaviour. In order to determine whether the nigrocollicular GABAergic fibers exert some control over this behaviour, rats bearing neurochemical lesions with kainic acid in the substantia nigra, pars reticulata (SNpr) and compacta (SNpc), were submitted to MT microinjections of bicuculline or electrical stimulation at aversive thresholds. The same procedure was carried out after enhancement or inhibition of GABAergic transmission in SNpr through microinjections of muscimol or bicuculline, respectively. Animals with SNpr neurochemical lesion exhibited a significant decrease in the aversive thresholds and an increase in the responsiveness to bicuculline microinjections. An opposite effect was observed following microinjections of bicuculline into the SNpr. The enhancement of the GABAergic transmission into the SNpr following microinjection of muscimol mimicked the effects produced by the lesion with kainic acid. These results suggest an inhibitory control of GABAergic fibers from the substantia nigra, pars reticulata, on aversive behaviour induced by midbrain stimulation.

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

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

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

Wild running elicited by microinjections of bicuculline or morphine into the inferior colliculus of rats: lack of effect of periaqueductal gray lesions

Bicuculline methiodide, a GABAA receptor antagonist, or a high dose of morphine was injected at the same site within the inferior colliculus (IC) of rats. Both drugs elicited the same behavioral activity (wild running). However, the time course and magnitude of the effects of the two drugs differed. Since the behavioral activation elicited was reminiscent of what was found with microinjections of bicuculline methiodide or morphine into the periaqueductal gray (PAG), we lesioned the PAG in another group of rats. It was found that extensive lesions of the PAG including those extending to the medial part of the superior colliculus did not significantly reduce the wild running.

Hypothalamic, midbrain and bulbar areas involved in the defense reaction in rabbits

The present study mapped neuroanatomical sites in the hypothalamus and periaqueductal gray (PAG) of the rabbit which, when stimulated electrically, evoked the cardiorespiratory components of the defense reaction (CRDR). This included increases in heart rate, blood pressure, hindlimb blood flow and respiration rate. All of the components of the CRDR were elicited by electrical stimulation of the posterior hypothalamus, at sites dorsal and medial to the fornix. Although there were regions throughout the PAG in which electrical stimulation elicited concomitant increases in blood pressure, hindlimb blood flow and respiration rate, only stimulation of the dorsal PAG evoked tachycardia. Injection of horseradish peroxidase into the rostral ventrolateral medulla (RVLM) led to heavy retrograde and anterograde labeling in the region of the hypothalamus that yielded the CRDR when stimulated electrically. Heavy labeling was also observed in the dorsal and ventral PAG. The results of this study provide evidence that the posterior hypothalamus and the dorsal PAG are nodal structures in the mediation of the CRDR and that cells in posterior hypothalamus, dorsal PAG and ventral PAG make monosynaptic connections with the RVLM.

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

Electrical stimulation of the mesencephalic tectum elicits behavioral and autonomic responses similar to those following peripheral noxious stimulation. Benzodiazepine and opioid compounds attenuate escape behavior induced by electrical stimulation of the dorsal periaqueductal gray (PAG) and deep layers of the superior colliculus (SC). The present study determines if microinjections of midazolam and morphine applied to these PAG-SC sites affect both responsiveness to peripheral noxious stimulation and to aversive PAG-SC stimulation. Both aversive brain stimulation or foot-shocks applied at threshold intensities caused running or jumps concomitant with increases in mean arterial blood pressure (MBP) and heart rate (HR). Microinjection of both drugs attenuated the behavioral reaction and increases in MBP and HR induced by mesencephalic tectum stimulation, while attenuating only the increase in heart rate induced by peripheral painful stimulation. These results suggest that the neural substrates of the behavioral and autonomic effects of stimulating the mesencephalic tectum and peripheral nociceptors are different although they may partially overlap.

Differential pharmacological reactivity of aversion induced by stimulation of periaqueductal gray or mesencephalic locomotor region

Rats were trained to switch-off aversive electrical brain stimulations applied to the periaqueductal gray (PAG) or mesencephalic locomotor region (MLR) by pressing a bar (switch-off behavior). We investigated the effects of IP injections of the benzodiazepine (BZ) receptor inverse agonist FG 7142 (2.5, 5, 10 mg/kg) or BZ receptor agonist chlordiazepoxide (CDP: 5 mg/kg) on the switch-off latency, i.e., the time elapsed between the onset of the stimulation and its offset by a press of the bar. It was found that FG 7142 decreased, whereas CDP increased the mean switch-off latency for electrical stimulation of the PAG, which is interpreted as a potentiating effect of FG 7142 and a reducing effect of CDP on the electrically induced aversive state. By contrast, neither FG 7142 nor CDP were found to affect the mean switch-off latency for MLR stimulations. These results suggest a difference in the pharmacological sensitivity to BZ receptor ligands between aversive states elicited by electrical stimulation of the PAG or MLR.

Excitatory amino acid projections to the periaqueductal gray in the rat: a retrograde transport study utilizing D[3H]aspartate and [3H]GABA

The afferents to the periaqueductal gray utilizing excitatory amino acid transmitters have been described in rat brain by autoradiography following microinfusion and retrograde transport of D[3H]aspartate. Parallel experiments employing injections of [3H]GABA established that the retrograde labelling found with D[3H]aspartate was transmitter-selective. Following infusion of D[3H]aspartate, perikaryal labelling was found in nine subcortical areas, particularly infralimbic and cingulate cortices, with a predominance of ipsilateral labelled perikarya. Heaviest cortical labelling was localized in perirhinal cortex, in an extensive band of cells adjoining the rhinal sulcus. The hypothalamus contained the heaviest perikaryal labelling within brain: D[3H]aspartate labelled cells in 11 hypothalamic and mammillary nuclei. Intense bilateral labelling was obtained in ventromedial hypothalamus, although the number of perikarya was lower contralaterally. D[3H]Aspartate also produced heavy ipsilateral labelling of perikarya in posterior hypothalamus. Labelling patterns in cortex and hypothalamus were precise and topographic, and [3H]GABA never labelled cells in these regions. Other telencephalic and diencephalic areas containing prominent, retrogradely labelled cells were the lateral septum, amygdala, zona incerta and lateral habenula. The relative density of labelled cells in mesencephalic areas was much lower than that found in cortex and hypothalamus, although D[3H]aspartate labelled a moderate number of perikarya in the inferior colliculus and cuneiform nucleus. A smaller number of heavily labelled cells was found in the parabrachial nuclei, Kolliker-Fuse nucleus and laterodorsal tegmental nucleus. Only occasional labelled perikarya were observed in the myencephalon. Low densities of labelled cells were found after the injection of [3H]GABA into the periaqueductal gray, and the only regions in which a small number of perikarya were labelled by both [3H]GABA and D[3H]aspartate were the dorsal raphe and parabrachial nuclei. Overall, the retrograde transport of D[3H]aspartate revealed a complex topographic and convergent network of afferent pathways to the periaqueductal gray likely to utilize an excitatory amino acid transmitter. Our findings confirm the selectivity of this neurochemical mapping technique and provide evidence that hypothalamic, habenular, subthalamic and cuneiform afferents to the periaqueductal gray utilize an acidic amino acid as their transmitter. They also confirm that corticofugal afferents to periaqueductal gray utilize an excitatory amino acid.

Conditioned place aversion produced by microinjections of semicarbazide into the periaqueductal gray of the rat

Previous studies have shown that the blockade of GABA-ergic neurotransmission in the periaqueductal gray (PAG) of the rat induce flight reactions. The present study examined whether a negative affective state was produced by such a blockade. Microinjections of semicarbazide, a GABA synthesis inhibitor, into the PAG were found to produce a conditioned place aversion. In a second experiment, it was found that the potent GABA agonist muscimol antagonized the effects of semicarbazide, without producing a conditioned place preference or aversion by itself. These results suggest that the blockade of the tonic inhibition exerted by GABAergic terminals in the PAG results in both an aversive experience and an overt flight reaction.

Flight behavior induced by microinjection of GABA antagonists into periventricular structures in detelencephalated rats

Behavioral effects of unilateral microinjections into periventricular structures of bicuculline, a classic GABA-A antagonist, and semicarbazide, a glutamic acid decarboxylase blocker, were studied in detelencephalated rats. These drugs produced a behavioral activation together with jumps. However, the characteristics of this behavioral activation differed as the injections were made in dorsal periaqueductal gray matter or medial hypothalamus. These data show close similarities to those observed with intact animals suggesting that GABA-A receptors are involved in the neural control of expression of flight behavior and functions in an intact manner and possibly independent of influences from forebrain structures. At variance with intact animals, these drugs produced contralateral turning behavior when locally injected into MH, pointing to some kind of inhibitory control exerted by telencephalic structures on the expression of circling behavior from diencephalic regions.

Hypothalamic substrates for brain stimulation-induced patterns of locomotion and escape jumps in the rat

The hypothalamic response area for electrically induced locomotion was determined using moveable electrodes and discriminant analysis as an appropriate statistical technique. At 241 out of 641 stimulated sites locomotion was induced. The distribution of locomotion sites is relatively diffuse. Discriminant analysis of both positive and negative electrode localizations yields areas with high, intermediate or low probability of inducing the response. The response is considered to be mediated by fibres of the subpallido-pedunculopontine system, which includes the mesencephalic locomotor region. Different categories of exploratory and flight-directed locomotion were distinguished, and response areas for both categories were determined. In addition the response area for escape jumps was delimited. Exploratory locomotion is mainly induced from the lateral hypothalamus, while flight-directed locomotion and escape jumps are evoked from the medial hypothalamus. The response area for exploratory locomotion reflects the lateral hypothalamic distribution of the subpallidal projection to the mesencephalic locomotor region. A diffuse substrate for flight behavior seems to occupy almost the entire medial hypothalamus. It is concluded that a locomotor subroutine subserving different behavioural mechanisms can be activated at many hypothalamic sites.

Defense reaction induced by microinjections of bicuculline into the inferior colliculus

Behavioral and autonomic effects of microinjections of bicuculline--a GABA receptor antagonist--into ventral aspects of the inferior colliculus (IC) produced a behavioral activation together with jumps. This activation was similar to escape behavior that has been induced from periventricular structures although it was neither as explosive as that observed from the dorsal periaqueductal gray matter (DPAG) nor as coordinated as that obtained from the medial hypothalamus (MH). In anesthetized rats, microinjection of bicuculline into the IC produced a clear rise in mean arterial blood pressure and in heart rate. These effects reached a maximum after 10 min and subsided within 20 min after injection. GABA receptor blockade in the IC of detelencephalated rats also resulted in escape behavior qualitatively similar to that observed in intact animals. These results suggest that in addition to the central gray and medial hypothalamus, GABA also exerts a tonic inhibitory action on neurons of the IC implicated in the generation or elaboration of aversive responses.

GABA-benzodiazepine modulation of aversion in the medial hypothalamus of the rat

Earlier results indicate that the neurons of the midbrain central gray (CG) responsible for the elaboration and/or expression of aversive states are tonically inhibited by the GABA-benzodiazepine system. In the present study, chemitrodes were implanted in the medial hypothalamus (MH) of the rat, another aversive area of the brain deeply interrelated with the dorsal CG. Microinjection of the benzodiazepine receptor agonist midazolam raised the aversive threshold of electrical stimulation of the MH in a dose-dependent way, though in only about half of the animals tested. In the remaining rats, midazolam was ineffective. Similar antiaversive effects were caused by the GABA-A receptor agonist THIP. In contrast, microinjection of the GABA-A receptor blocker bicuculline induced aversive-like behavioral and autonomic changes. The effects of bicuculline were antagonized by pretreatment with either THIP or midazolam, the latter being counteracted by the competitive benzodiazepine receptor blocker Ro 15-1788. These results extend to the MH, the hypothesis of GABA-benzodiazepine modulation of neurons integrating aversive motivational states.

Behavioural evidence for a selective GABA-A antagonistic activity of SR 95103 and SR 42641 after intrastriatal injection in mice

Two pyridazinyl GABA derivatives, SR 95103 and SR 42641 have recently been described as selective GABAA receptor antagonists. We have now investigated the behavioural effects of SR 95103 and SR 42641 after intrastriatal injection in mice. When injected into the right striatum, SR 95103 (0.01-0.5 microgram), SR 42641 (0.0001-0.01 microgram) and bicuculline methiodide (0.005-0.05 microgram) induced contralateral rotations which were antagonized by intraperitoneal injection of muscimol. In contrast, the intrastriatal injection of the GABAA receptor agonist muscimol induced ipsilateral rotations. Muscimol-induced turning was antagonized by SR 95103 (10-30 mg/kg), SR 42641 (1-10 mg/kg) and (+)-bicuculline (0.125-0.5 mg/kg) injected intraperitoneally, but not by strychnine. Intrastriatal glycine also induced ipsilateral rotations which were antagonized by strychnine (0.01-0.3 mg/kg i.p.) but not by (+)-bicuculline, SR 95103 or SR 42641. These results suggest that SR 95103 and SR 42641 induce turning through a selective blockade of GABAA receptors within the striatum.