Prepulse inhibition of the acoustic startle reflex impairment by 5-HT2A receptor activation in the inferior colliculus is prevented by GABAA receptor blockade in the pedunculopontine tegmental nucleus.

The relationship between serotonin dysfunction and schizophrenia commenced with the discovery of the effects of lysergic acid diethylamide (LSD) that has high affinity for 5-HT2A receptors. Activation of these receptors produces perceptual and behavioural changes such as illusions, visual hallucinations and locomotor hyperactivity. Using prepulse inhibition (PPI) of the acoustic startle, which is impaired in schizophrenia,we aimed to investigate:i) the existence of a direct and potentially inhibitory neural pathway between the inferior colliculus (IC) and the pedunculopontine tegmental nucleus (PPTg) involved in the mediation of PPI responses by a neural tract tracing procedure;ii) if the microinjection of the 5-HT2A receptors agonist DOI in IC would activate neurons in this structure and in the PPTg by a c-Fos protein immunohistochemistry study;iii) whether the deficits in PPI responses, observed after the administration of DOI in the IC, could be prevented by the concomitant microinjection of the GABAA receptor antagonist bicuculline in the PPTg.Male Wistar rats were used in this study. An IC-PPTg reciprocated neuronal pathway was identified by neurotracing. The number of c-Fos labelled cells was lower in the DOI group in IC and PPTg, suggesting that this decrease could be due to the high levels of GABA in both structures. The concomitant microinjections of bicuculline in PPTg and DOI in IC prevented the PPI deficit observed after the IC microinjection of DOI. Our findings suggest that IC 5-HT2A receptors may be at least partially involved in the regulation of inhibitory pathways mediating PPI response in IC and PPTg structures.

Tumoración en el colículo inferior como causa de acúfeno unilateral / Tumor at inferior colliculus causing unilateral tinnitus

Resumen La patología del sistema nervioso central, habitualmente, no provoca síntomas auditivos unilaterales, ya que la vía auditiva central está formada por una red de conexiones cruzadas entre los diferentes núcleos que la forman. Además, hay que considerar que una lesión pequeña puede extenderse a más de una estructura provocando varios déficits neurológicos debido a la proximidad de los tractos y núcleos nerviosos. Las lesiones unilaterales circunscritas en el colículo inferior son infrecuentes. No obstante, se han descrito casos en los que lesiones unilaterales de diversas etiologías en esta localización causaban síntomas auditivos. Ya que la vía auditiva central es cruzada, síntomas auditivos detectados con más frecuencia afectaban concretamente a la capacidad de localización del sonido o la comprensión verbal. Presentamos el caso de un hombre de 44 años con acúfeno unilateral derecho de larga evolución, sin otra clínica asociada quien fue diagnosticado de un tumor en el colículo inferior derecho mediante resonancia magnética cerebral. Se exponen los hallazgos clínicos y radiológicos del caso.

Abstract Central nervous system diseases usually do not cause auditory symptoms because the central auditory pathway consists on a network of crossed connections between the different nuclei that form it. In addition, we must consider that a small lesion might extend to more than one structure producing many neurologic symptoms due to the proximity of tracts and nuclei in the midbrain. Unilateral circumscribed lesions at inferior colliculus are rare. Nevertheless, lesions at this location causing auditory symptoms have been described. Because of the crossed central auditory pathway, the most commonly detected auditory symptoms specifically affected the ability to locate sound or verbal comprehension. We present the case of a 44-year-old man with a long-term monoaural right-sided tinnitus without other complaints who was diagnosed of a tumour at right inferior colliculus by neuroimaging. Clinical and radiological findings of this case are discussed.

Amygdala inhibition impairs fear conditioning but increases the stimulus-driven activity in the inferior colliculus.

It has been shown that fear conditioning improves the steady-state evoked potentials driven by a long lasting amplitude modulated tone in the inferior colliculus. In this work we tested the hypothesis that the amygdala modulates this effect, since it plays a crucial role in assessing the biological relevance of environmental stimuli. We inhibited the basolateral nucleus of the amygdala of rats by injecting a GABAa receptor agonist (muscimol) before the recall test session of an auditory fear conditioning paradigm and recorded the evoked activity in the central nucleus of the inferior colliculus. According to our results, the treatment with muscimol decreased the expression of freezing behavior during the recall test session, but did not impair the entrainment of the evoked activity in the inferior colliculus induced by fear conditioning. We repeated the injection protocol with another group of rats but without pairing the tone to an aversive stimulus and observed that the inhibition of the basolateral amygdala enhances the stimulus-driven activity in the inferior colliculus regardless of the conditioning task. Our findings suggest that the basolateral amygdala exerts a tonic modulation over the encoding of sensory information at the early stages of the sensory pathway.

The Blockade of µ1- and κ-Opioid Receptors in the Inferior Colliculus Decreases the Expression of Panic Attack-Like Behaviours Induced by Chemical Stimulation of the Dorsal Midbrain.

BACKGROUND: Gamma-aminobutyric acid (GABA)ergic and opioid systems play a crucial role in the neural modulation of innate fear organised by the inferior colliculus (IC). In addition, the IC is rich in GABAergic fibres and opioid neurons, which are also connected to other mesencephalic structures, such as the superior colliculus and the substantia nigra. However, the contribution of distinct opioid receptors (ORs) in the IC during the elaboration and expression of innate fear and panic-like responses is unclear. The purpose of the present work was to investigate a possible integrated action exerted by ORs and the GABAA receptor-mediated system in the IC on panic-like responses. METHODS: The effect of the blockade of either µ1- or κ-ORs in the IC was evaluated in the unconditioned fear-induced responses elicited by GABAA antagonism with bicuculline. Microinjections of naloxonazine, a µ1-OR antagonist, or nor-binaltorphimine (nor-BNI), a κ-OR antagonist, were made into the IC, followed by intramesencephalic administration of the GABAA-receptor antagonist bicuculline. The defensive behaviours elicited by the treatments in the IC were quantitatively analysed, recording the number of escapes expressed as running (crossing), jumps, and rotations, over a 30-min period in a circular arena. The exploratory behaviour of rearing was also recorded. RESULTS: GABAA-receptor blockade with bicuculline in the IC increased defensive behaviours. However, pretreatment of the IC with higher doses (5 µg) of naloxonazine or nor-BNI followed by bicuculline resulted in a significant decrease in unconditioned fear-induced responses. CONCLUSIONS: These findings suggest a role played by µ1- and κ-OR-containing connexions and GABAA receptor-mediated neurotransmission on the organisation of panic attack-related responses elaborated by the IC neurons.

Panicolytic-like effect of µ1-opioid receptor blockade in the inferior colliculus of prey threatened by Crotalus durissus terrificus pit vipers.

BACKGROUND: The endogenous opioid peptide system has been implicated in the neural modulation of fear and anxiety organised by the dorsal midbrain. Furthermore, previous results indicate a fundamental role played by inferior colliculus (IC) opioid mechanisms during the expression of defensive behaviours, but the involvement of the IC µ1-opioid receptor in the modulation of anxiety- and panic attack-related behaviours remains unclear. Using a prey-versus-snake confrontation paradigm, we sought to investigate the effects of µ1-opioid receptor blockade in the IC on the defensive behaviour displayed by rats in a dangerous situation. METHODS: Specific pathogen-free Wistar rats were treated with microinjection of the selective µ1-opioid receptor antagonist naloxonazine into the IC at different concentrations (1.0, 3.0 and 5.0 µg/0.2 µL) and then confronted with rattlesnakes ( Crotalus durissus terrificus). The defensive behavioural repertoire, such as defensive attention, flat back approach (FBA), startle, defensive immobility, escape or active avoidance, displayed by rats either during the confrontations with wild snakes or during re-exposure to the experimental context without the predator was analysed. RESULTS: The blockade of µ1-opioid receptors in the IC decreased the expression of both anxiety-related behaviours (defensive attention, FBA) and panic attack-related responses (startle, defensive immobility and escape) during the confrontation with rattlesnakes. A significant decrease in defensive attention was also recorded during re-exposure of the prey to the experimental apparatus context without the predator. CONCLUSION: Taken together, these results suggest that a decrease in µ1-opioid receptor signalling activity within the IC modulates anxiety- and panic attack-related behaviours in dangerous environments.

5-Hydroxytryptamine 2A receptors of the dorsal raphe nucleus modulate panic-like behaviours and mediate fear-induced antinociception elicited by neuronal activation in the central nucleus of the inferior colliculus.

It has been established that chemical stimulation of the inferior colliculus (IC) of laboratory animals evokes fear-related defensive responses, which are considered panic attack-like behaviours. In addition, there is evidence that defensive reactions provoked by chemical stimulation of midbrain tectum neurons may induce an antinociceptive response. Morphologically, the IC receives projections from other mesencephalic structures, such as the dorsal raphe nucleus (DRN), a region rich in serotonergic neurons that play a critical role in the control of defensive behaviours. Moreover, this monoaminergic brainstem reticular nucleus is suggested to comprise the endogenous pain modulatory system. The aim of the present study was to investigate the role of DRN 5-hydroxytryptamine 2A (5-HT2A) receptors in Wistar rats by local microinjection of R-96544 (a selective antagonist of the 5-HT2A receptor) at doses of 5, 10 or 15 nM on defensive reactions and fear-induced antinociception evoked by chemical stimulation of the central nucleus of the IC with NMDA (6, 9 or 12 nmol). Behavioural responses were analysed for 10 min, and then the nociceptive threshold was measured at 10 min intervals for 70 min. The dose of 12 nmol of NMDA was the most effective in causing panic attack-like defensive behaviours and much higher hypoalgesia. In addition, both effects were attenuated by pretreatment of the DRN with R-96544. These findings suggest the critical participation of DRN 5-HT2A receptors in the modulation of panic attack-like defensive behaviour and unconditioned fear-induced antinociception organised by neurons in the central nucleus of the IC.

Age-Related Changes in Ang II Receptor Localization and Expression in the Developing Auditory Pathway.

We studied Ang II receptor localization in different nuclei of the auditory system, by means of binding autoradiography, during brain development. The inferior colliculus (IC), a large midbrain structure which serves as an obligatory synaptic station in both the ascending and descending auditory pathways, exhibited high Ang II AT2 binding at all ages (P0, P8, P15, P30), being maximal at P15. These observations were confirmed by in situ hybridization and immunofluorescence at P15, demonstrating that AT2 receptor mRNA localized at the same area recognized by AT2 antibodies and anti ß III-tubulin suggesting the neuronal nature of the reactive cells. Ang II AT1 receptors were absent at early developmental ages (P0) in all nuclei of the auditory system and a low level was observed in the IC at the age P8. AT2 receptors were present at ventral cochlear nucleus and superior olivary complex, being higher at P15 and P8, respectively. We also explored the effect of prenatal administration of Ang II or PD123319 (AT2 antagonist) on binding of Ang II receptors at P0, P8, P15. Both treatments increased significantly the level of AT2 receptors at P0 and P8 in the IC. Although total binding in the whole IC from P15 animals showed no difference between treatments, the central nucleus of the IC exhibited higher binding. Our results supports a correlation between the timing of the higher expression of Ang II AT2 receptors in different nuclei, the onset of audition and the establishment of neuronal circuits of the auditory pathway.

The endogenous opioid system modulates defensive behavior evoked by Crotalus durissus terrificus: Panicolytic-like effect of intracollicular non-selective opioid receptors blockade.

BACKGROUND: There is a controversy regarding the key role played by opioid peptide neurotransmission in the modulation of panic-attack-related responses. AIMS: Using a prey versus rattlesnakes paradigm, the present work investigated the involvement of the endogenous opioid peptide-mediated system of the inferior colliculus in the modulation of panic attack-related responses. METHODS: Wistar rats were pretreated with intracollicular administration of either physiological saline or naloxone at different concentrations and confronted with rattlesnakes ( Crotalus durissus terrificus). The prey versus rattlesnake confrontations were performed in a polygonal arena for snakes. The defensive behaviors displayed by prey (defensive attention, defensive immobility, escape response, flat back approach and startle) were recorded twice: firstly, over a period of 15 min the presence of the predator and a re-exposure was performed 24 h after the confrontation, when animals were exposed to the experimental enclosure without the rattlesnake. RESULTS: The intramesencephalic non-specific blockade of opioid receptors with microinjections of naloxone at higher doses decreased both anxiety- (defensive attention and flat back approach) and panic attack-like (defensive immobility and escape) behaviors, evoked in the presence of rattlesnakes and increased non-defensive responses. During the exposure to the experimental context, there was a decrease in duration of defensive attention. CONCLUSIONS: These findings suggest a panicolytic-like effect of endogenous opioid receptors antagonism in the inferior colliculus on innate (panic attack) and conditioned (anticipatory anxiety) fear in rats threatened by rattlesnakes.

Behavioral and Auditory Electrophysiological Rebound as a Compensatory Response to the Reinforcing Effects of Morphine.

Auditory-evoked potentials (AEPs) can be modified by associative learning, where the appearance of learned compensatory responses (CCRs) may result in the emergence of drug withdrawal symptoms and relapse. Although CCRs' influence on later attentive and cognitive domains has been extensively examined, contextual conditioned tolerance occurring in preattentive mechanisms operating at earlier stages of information processing has remained largely unexplored. To extend our knowledge on this subject, compensatory changes on the motor and emotional aspects of behavior evoked by contextual cues were investigated with an electronic open field in morphine-pretreated rats challenged with two morphine overdoses (40 and 80 mg/kg). CCRs influence on the AEPs recorded in the central nucleus of the inferior colliculus (CIC) was analyzed with the help of a field potential recording device and a two-chamber shuttle box placed inside a Faraday cage system. The emergence of electrophysiological CCRs was analyzed by recording AEP latency and amplitude elicited in the central nucleus of the IC (CIC) with the aid of a field potential recording device and a two-chamber shuttle box placed inside a Faraday cage system. Behavioral analysis indicated that CCRs ensue in non-familiar contexts. Electrophysiological data revealed increases in the amplitude of AEPs evoked in a non-familiar context. Our results indicate that behavioral learning responses emerge following Pavlovian conditioning even with the use of low and regular doses of morphine over a short-term treatment. Changes in the CIC electrophysiology may indicate that the development of drug dependence occurs covertly in the early stages of sensory information processing.

Prelimbic NMDA receptors stimulation mimics the attenuating effects of clozapine on the auditory electrophysiological rebound induced by ketamine withdrawal.

Ketamine (KET) is a non-competitive N-Methyl-d-aspartate (NMDA) receptors antagonist that intensifies sensory experiences, prompts hallucinations and delusions, exacerbates previously installed psychosis and disrupts physiological evoked potentials (AEPs). Pharmacologically, KET stimulates glutamate efflux in the medial prefrontal cortex, mainly in the prelimbic (PrL) sub-region. Efferences from this region exert a top-down regulatory control of bottom-up sensory processes either directly or indirectly. In the midbrain, the central nucleus of the inferior colliculus (CIC) plays a fundamental role in the processing of auditory ascending information related to sound localization, sensorimotor gating, and preattentive event-related potentials. Auditory hallucinations elicited during a psychotic outbreak are accompanied by CIC neural activation. Thus, it is possible that NMDA-mediated glutamate neurotransmission in the PrL indirectly modulates CIC neuronal firing. The aim of the present study was to assess the effects of KET on the latency and amplitude of AEPs elicited in the CIC of rats tested during KET effects and following withdrawal from the chronic administration. Changes on emotionally induced by KET treatment were evaluated with the use of the elevated zero maze (EZM). Unlike typical neuroleptics, the atypical antipsychotic clozapine (CLZ) potently blocks the disruption of the sensorimotor gating induced by NMDA antagonists. Therefore, the effects of KET withdrawal on AEPs were challenged with a systemic injection of CLZ. In addition, we further investigated the role of NMDA receptors of the PrL on the AEPs expression recorded in the CIC through intra-PrL infusions of NMDA itself. Our results showed that the processing of sensory information in the CIC is under indirect control of PrL. These data suggest that the long-term KET treatment disrupts the collicular auditory field potentials, possibly through influencing PrL glutamate activity on intrinsic 5-HT mechanisms in the dorsal raphe and CIC.

Auditory fear conditioning modifies steady-state evoked potentials in the rat inferior colliculus.

The rat inferior colliculus (IC) is a major midbrain relay for ascending inputs from the auditory brain stem and has been suggested to play a key role in the processing of aversive sounds. Previous studies have demonstrated that auditory fear conditioning (AFC) potentiates transient responses to brief tones in the IC, but it remains unexplored whether AFC modifies responses to sustained periodic acoustic stimulation-a type of response called the steady-state evoked potential (SSEP). Here we used an amplitude-modulated tone-a 10-kHz tone with a sinusoidal amplitude modulation of 53.7 Hz-as the conditioning stimulus (CS) in an AFC protocol (5 CSs per day in 3 consecutive days) while recording local field potentials (LFPs) from the IC. In the preconditioning session (day 1), the CS elicited prominent 53.7-Hz SSEPs. In the training session (day 2), foot shocks occurred at the end of each CS (paired group) or randomized in the inter-CS interval (unpaired group). In the test session (day 3), SSEPs markedly differed from preconditioning in the paired group: in the first two trials the phase to which the SSEP coupled to the CS amplitude envelope shifted ~90°; in the last two trials the SSEP power and the coherence of SSEP with the CS amplitude envelope increased. LFP power decreased in frequency bands other than 53.7 Hz. In the unpaired group, SSEPs did not change in the test compared with preconditioning. Our results show that AFC causes dissociated changes in the phase and power of SSEP in the IC.NEW & NOTEWORTHY Local field potential oscillations in the inferior colliculus follow the amplitude envelope of an amplitude-modulated tone, originating a neural response called the steady-state evoked potential. We show that auditory fear conditioning of an amplitude-modulated tone modifies two parameters of the steady-state evoked potentials in the inferior colliculus: first the phase to which the evoked oscillation couples to the amplitude-modulated tone shifts; subsequently, the evoked oscillation power increases along with its coherence with the amplitude-modulated tone.

Specific immediate early gene expression induced by high doses of salicylate in the cochlear nucleus and inferior colliculus of the rat / Expressão específica de genes precoces imediatos induzida por doses elevadas de salicilato no núcleo coclear e colículo inferior de rato

Abstract Introduction: Salicylate at high doses induces tinnitus in humans and experimental animals. However, the mechanisms and loci of action of salicylate in inducing tinnitus are still not well known. The expression of Immediate Early Genes (IEG) is traditionally associated with long-term neuronal modifications but it is still not clear how and where IEGs are activated in animal models of tinnitus. Objectives: Here we investigated the expression of c-fos and Egr-1, two IEGs, in the Dorsal Cochlear Nucleus (DCN), the Inferior Colliculus (IC), and the Posterior Ventral Cochlear Nucleus (pVCN) of rats. Methods: Rats were treated with doses known to induce tinnitus in rats (300 mg/kg i.p. daily, for 3 days), and c-fos and Egr-1 protein expressions were analyzed using western blot and immunocytochemistry. Results: After administration of salicylate, c-fos protein expression increased significantly in the DCN, pVCN and IC when assayed by western blot. Immunohistochemistry staining showed a more intense labeling of c-fos in the DCN, pVCN and IC and a significant increase in c-fos positive nuclei in the pVCN and IC. We did not detect increased Egr-1 expression in any of these areas. Conclusion: Our data show that a high dose of salicylate activates neurons in the DCN, pVCN and IC. The expression of these genes by high doses of salicylate strongly suggests that plastic changes in these areas are involved in the genesis of tinnitus.

Resumo Introdução: Salicilato em doses elevadas induz zumbido nos seres humanos e em animais experimentais. No entanto, os mecanismos e loci de ação do salicilato na indução de zumbido ainda não são bem conhecidos. A expressão dos genes precoces imediatos (GPIs) está tradicionalmente associada a alterações neuronais em longo prazo, mas ainda não está claro como e onde os GPIs são ativados em modelos animais de zumbido. Objetivos: No presente estudo investigamos a expressão de c-fos e Egr-1, dois GPIs, no núcleo coclear dorsal (NCD), colículo inferior (CI) e núcleo coclear ventral posterior (NCVp) de ratos. Métodos: Os ratos foram tratados com doses que, conhecidamente, induzem zumbido em ratos (300 mg/kg IP/dia, por três dias) e as expressões das proteínas c-fos e Egr-1 foram analisadas por meio de Western blot e imunoistoquímica. Resultados: Após a administração de salicilato, a expressão da proteína c-fos aumentou significativamente no NCD, NCVp e CI, quando analisados por Western blot. A coloração imunoistoquímica mostrou uma marcação mais intensa de c-fos no NCD, NCVp e CI e um aumento significativo de núcleos positivos de c-fos no NCVp e CI. Não detectamos aumento da expressão de Egr-1 em qualquer dessas áreas. Conclusão: Nossos dados mostram que uma dose alta de salicilato ativa neurônios no NCD, NCVp e CI. A expressão desses genes por doses altas de salicilato sugere que as alterações plásticas nessas áreas estão envolvidas na gênese do zumbido.

Atypical antipsychotic clozapine reversed deficit on prepulse inhibition of the acoustic startle reflex produced by microinjection of DOI into the inferior colliculus in rats.

Dysfunctions of the serotonergic system have been suggested to be important in the neurobiology of schizophrenia. Patients with schizophrenia exhibit deficits in an operational measure of sensorimotor gating: prepulse inhibition (PPI) of startle. PPI is the normal reduction in the startle response caused by a low intensity non-startling stimulus (prepulse) which is presented shortly before the startle stimulus (pulse). The hallucinogen 2,5-dimethoxy-4-iodoamphetamine (DOI), a 5-hydroxytryptamine(HT)2 receptor agonist disrupted PPI in rats. The inferior colliculus (IC) is a critical nucleus of the auditory pathway mediating acoustic PPI. The activation of the IC by the acoustic prepulse reduces startle magnitude. The present study investigated the role of serotonergic transmission in the IC on the expression of acoustic PPI. For that we investigated whether 5-HT2A receptor activation or blockade would affect this response. Unilateral microinjection of DOI (10µg/0.3µl) into the IC disrupted PPI, while microinjection of the 5-HT2A receptor antagonist ritanserin (4µg/0.3µl), into this structure did not alter PPI. We also examined the ability of the atypical antipsychotic clozapine (5.0mg/kg; I.P.) to reverse the disruption of PPI produced by unilateral microinjections of DOI into the IC of rats. Pretreatment with clozapine blocked DOI-induced disruption of PPI. Altogether, these results suggest that serotonin-mediated mechanisms of the IC are involved in the expression of PPI in rodents and that this response is sensitive to atypical antipsychotic clozapine.

Specific immediate early gene expression induced by high doses of salicylate in the cochlear nucleus and inferior colliculus of the rat.

INTRODUCTION: Salicylate at high doses induces tinnitus in humans and experimental animals. However, the mechanisms and loci of action of salicylate in inducing tinnitus are still not well known. The expression of Immediate Early Genes (IEG) is traditionally associated with long-term neuronal modifications but it is still not clear how and where IEGs are activated in animal models of tinnitus. OBJECTIVES: Here we investigated the expression of c-fos and Egr-1, two IEGs, in the Dorsal Cochlear Nucleus (DCN), the Inferior Colliculus (IC), and the Posterior Ventral Cochlear Nucleus (pVCN) of rats. METHODS: Rats were treated with doses known to induce tinnitus in rats (300mg/kg i.p. daily, for 3 days), and c-fos and Egr-1 protein expressions were analyzed using western blot and immunocytochemistry. RESULTS: After administration of salicylate, c-fos protein expression increased significantly in the DCN, pVCN and IC when assayed by western blot. Immunohistochemistry staining showed a more intense labeling of c-fos in the DCN, pVCN and IC and a significant increase in c-fos positive nuclei in the pVCN and IC. We did not detect increased Egr-1 expression in any of these areas. CONCLUSION: Our data show that a high dose of salicylate activates neurons in the DCN, pVCN and IC. The expression of these genes by high doses of salicylate strongly suggests that plastic changes in these areas are involved in the genesis of tinnitus.

µ- and κ-Opioid receptor activation in the dorsal periaqueductal grey matter differentially modulates panic-like behaviours induced by electrical and chemical stimulation of the inferior colliculus.

It has been shown that electrical stimulation of the mesencephalic tectum (MT) provokes defensive responses in both humans and rodents. During an emotional aversive state, some convergent studies have also demonstrated the existence of a complex interaction between endogenous opioid peptide- and γ-aminobutyric acid (GABA)-containing connections during fear-induced responses. It has been proposed that opioid neurons exert an influence on GABAergic interneurons, which, in turn, exert inhibitory tonic control on the mesencephalic excitatory pathways. Thus, opioid peptides can disinhibit neurons that are tonically inhibited by GABA, therefore, modulating the expression of defensive behavioural reactions. In the present work, we used both electric stimulation and microinjections of the GABAA receptor antagonist bicuculline in the inferior colliculus (IC) of Wistar rats in combination with microinjections of µ- and κ-opioid receptor selective agonists into the dorsal columns of periaqueductal grey matter (dPAG) to evaluate the effects on panic-like behaviours elicited by IC electrical and chemical stimulation. The present results showed that neurochemical lesions of the dPAG caused a significant impairment in the organisation of defensive responses by IC neurons, reducing the duration [t(14)=3.0; p<0.01] of defensive immobility and the duration [t(14)=2.8; p<0.05] and frequency [t(14)=2.5; p<0.05] of escape. Paradoxically, treating the dPAG with the µ-opioid receptor agonist met-enkephalin caused a significant reduction of panic-like behaviours induced by both electrical and chemical stimulation of the IC, increasing the escape behaviour threshold [F(2,23)=13.5; p<0.001] and decreasing the frequency [F(3,36)=11.7; p<0.001] and duration [F(3,36)=11.6; p<0.001] of escape and the duration of defensive immobility [F(3,36)=16.1; p<0.05]. In contrast, treating the dPAG with the κ-opioid receptor agonist salvinorin-A increased the frequency [F(3,36)=12.4; p<0.01] and duration [F(3,34)=16.1; p<0.01] of defensive immobility induced by GABAA receptor blockade in the IC. The present results suggest the existence of a complex neuronal network in the MT in which endogenous opioid peptides and GABAergic pathways interact in the control of fear-related behavioural responses.

Distinct effects of haloperidol in the mediation of conditioned fear in the mesolimbic system and processing of unconditioned aversive information in the inferior colliculus.

Chemical and electrical stimulation of the inferior colliculus (IC) causes defensive behavior. Electrical stimulation of the IC at the escape threshold enhances dopamine (DA) release in the prefrontal cortex. Intra-ventral tegmental area injections of quinpirole at doses that act presynaptically reduce the release of DA in the terminal fields of the mesolimbic system and clearly reduce conditioned fear in several animal models of anxiety. However, little is known about the involvement of DA in the mediation of unconditioned fear, such as the reactivity to acute stressors. The present study investigated the neural substrates mediated by DA transmission associated with emotional changes triggered by the activation or inhibition of D2 receptors during conditioned and unconditioned fear. We examined the effects of systemic or local injections of the DA-receptor antagonist and agonist haloperidol and quinpirole, respectively, into the IC in rats subjected to fear-potentiated startle, a Pavlovian paradigm that uses loud sounds as the unconditioned stimulus and light previously paired with footshock as the conditioned stimulus. We also assessed auditory-evoked potentials (AEPs) recorded from electrodes implanted in the IC. Intraperitoneal haloperidol administration dose-dependently enhanced AEPs induced by loud tones and inhibited fear-potentiated startle. Intra-IC injections of quinpirole left AEPs unchanged, suggesting that an optimal level of postsynaptic D2 receptors in the IC may regulate the transmission of aversive information through the midbrain tectum. These findings provide evidence of opposing DA-mediated mechanisms in fear/anxiety processes that depend on the area under study. The activity of the neural substrates of conditioned fear was attenuated by haloperidol, whereas midbrain neural substrates of unconditioned fear were enhanced. Thus, DA appears to regulate unconditioned fear at the midbrain level, likely by reducing the sensory gating of aversive events and reducing conditioned fear by acting at more rostral levels of the brain.

Modulation of haloperidol-induced catalepsy in rats by GABAergic neural substrate in the inferior colliculus.

Not only is the inferior colliculus (IC) a highly important center of integration within the central auditory pathway, but it may also play a modulatory role in sensory-motor circuitry. Previous evidence from our laboratory relating the IC to motor behavior shows that glutamate-mediated mechanisms within the IC modulate haloperidol-induced catalepsy. The high density of GABAergic receptors in the IC led to this study of a possible link between these receptors, haloperidol-induced catalepsy, and a possible involvement of the blockade of dopaminergic receptors. Catalepsy was evaluated by positioning both forepaws of rats on an elevated horizontal wooden bar and recording the time that the animal maintained this position. The present study shows that haloperidol-induced catalepsy was enhanced by local microinjection into the IC of midazolam (20nmol/0.5µl), a benzodiazepine receptor agonist, whereas animals receiving a microinjection of bicuculline (40 or 80ng/0.5µl), a GABAergic antagonist, showed a reduction in the time of catalepsy. However, the microinjection of haloperidol (2.5 or 5.0µg/0.5µl) bilaterally into the IC did not induce catalepsy. Therefore, our results suggest the involvement of the IC in the modulation of catalepsy induced by haloperidol, even though the dopaminergic mechanisms of the IC are unable to induce catalepsy when blocked by the direct microinjection of haloperidol. It is thus possible that the IC plays a role in sensorimotor gating and that GABA-mediated mechanisms are involved.

Atypical antipsychotic olanzapine reversed deficit on prepulse inhibition of the acoustic startle reflex produced by microinjection of dizocilpine (MK-801) into the inferior colliculus in rats.

Patients with schizophrenia exhibit deficits in an operational measure of sensorimotor gating: prepulse inhibition (PPI) of startle. PPI is the normal reduction in the startle response caused by a low intensity non-startling stimulus (prepulse) which is presented shortly before the startle stimulus (pulse). MK-801 is an NMDA receptor-antagonist known to produce hyperactivity, deficits in prepulse inhibition and social withdrawal, behaviors which correlate well with some of the positive, cognitive and negative symptoms of schizophrenia. The inferior colliculus (IC) is a critical part of the auditory pathway mediating acoustic PPI. The activation of the IC by the acoustic prepulse reduces startle magnitude. Thus, the purpose of the present study was to elucidate the role of glutamatergic transmission in the IC on the expression of acoustic PPI. For that we investigated whether NMDA receptor stimulation or blockade would affect this response. Unilateral microinjections of NMDA (30 nmol/0.5 µL) into the IC did not alter PPI while microinjections of MK-801 (30 nmol/0.5 µL) into this structure disrupted PPI. We also examined the ability of the atypical antipsychotic olanzapine (5.0mg/kg; i.p.) to reverse the disruption of pre-pulse inhibition produced by unilateral microinjections of MK-801 into the IC of rats. Pretreatment with olanzapine blocked MK-801-induced disruption of PPI. Altogether, these results suggest that glutamate-mediated mechanisms of the IC are involved in the expression of PPI in rodents and that this response is sensitive to atypical antipsychotic olanzapine.

Wakefulness-promoting role of the inferior colliculus.

The inferior colliculus (IC) is a mesencephalic auditory nucleus involved in several functions including the analysis of the frequency and intensity of sounds as well as sound localization. In addition to auditory processes, the IC controls the expression of defensive responses. The objective of the present study was to test the hypothesis that the IC contributes to the maintenance of wakefulness. For this purpose, several experimental approaches were performed in urethane-anesthetized guinea pigs. Electrical or chemical stimulation of the IC resulted in electroencephalographic (EEG) desynchronization, theta rhythm in the hippocampus and an increase in heart rate; all of these effects suggest an arousal reaction. Furthermore, by means of extracellular unit recordings, we determined that most IC neurons increased their spontaneous and tone-evoked responses in association with EEG desynchronization. We also studied the effect on sleep and wakefulness of bilateral acute inhibition of the IC by microinjections of muscimol (a GABAA agonist), as well as the effect of bilateral IC lesions in chronically-instrumented (drug-free) guinea pigs. Acute (via muscimol microinjections), but not chronic (via electrolytic lesions) inhibition of the IC decreased wakefulness., We conclude that the IC plays an active role in the maintenance of wakefulness. Further, we propose that this nucleus may mediate arousal responses induced by biologically significant sounds.

Changes on auditory physiology in response to the inactivation of amygdala nuclei in high anxiety rats expressing learned fear.

The inferior colliculus (IC) is primarily involved in the processing of acoustic stimuli, including those emitted by prey and predators. The role of the central nucleus of the IC (CIC) in fear and anxiety has been suggested based on electrophysiological, behavioral and immunohistochemical studies. The reactivity of high-anxiety rats (HA) to diverse challenges is different from low-anxiety ones (LA). In humans and laboratory animals, pathological anxiety is often accompanied by heightened vigilance and alertness, hyperactivity of the amygdala (AM), and increased amplitude of the auditory evoked potentials (AEP) from the IC. This study aims to evaluate the influence of the inactivation of the central (CEA) and basolateral (BLA) nuclei of the amygdala, after local infusions of the full GABAA agonist muscimol (1nmol/0.2µl), on the AEP elicited in the CIC of rats tested under a learned fear state. Our results showed that both BLA and CEA inactivation change the expression of conditioned fear, in a paradigm using the context as the conditioned stimulus (CS). These changes are correlated to the innate anxiety levels of the animals. It is supposed that this shortcoming is in addition to the imbalance between the regulatory role of the top-down and bottom-up processes in the control of anxiety.