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

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

Different outcomes of neonatal and adult Zika virus infection on startle reflex and prepulse inhibition in mice.

Zika virus (ZIKV) infection causes severe neurological consequences in both gestationally-exposed infants and adults. Sensorial gating deficits strongly correlate to the motor, sensorial and cognitive impairments observed in ZIKV-infected patients. However, no startle response or prepulse inhibition (PPI) assessment has been made in patients or animal models. In this study, we identified different outcomes according to the age of infection and sex in mice: neonatally infected animals presented an increase in PPI and delayed startle latency. However, adult-infected male mice presented lower startle amplitude, while a PPI impairment was observed 14 days after infection in both sexes. Our data further the understanding of the functional impacts of ZIKV on the developing and mature nervous system, which could help explain other behavioral and cognitive alterations caused by the virus. With this study, we support the startle reflex testing in ZIKV-exposed patients, especially infants, allowing for early detection of functional neuromotor damage and early intervention.

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.

The Existence of the StartReact Effect Implies Reticulospinal, Not Corticospinal, Inputs Dominate Drive to Motoneurons during Voluntary Movement.

Reaction time is accelerated if a loud (startling) sound accompanies the cue-the "StartReact" effect. Animal studies revealed a reticulospinal substrate for the startle reflex; StartReact may similarly involve the reticulospinal tract, but this is currently uncertain. Here we trained two female macaque monkeys to perform elbow flexion/extension movements following a visual cue. The cue was sometimes accompanied by a loud sound, generating a StartReact effect in electromyogram response latency, as seen in humans. Extracellular recordings were made from antidromically identified corticospinal neurons in primary motor cortex (M1), from the reticular formation (RF), and from the spinal cord (SC; C5-C8 segments). After loud sound, task-related activity was suppressed in M1 (latency, 70-200 ms after cue), but was initially enhanced (70-80 ms) and then suppressed (140-210 ms) in RF. SC activity was unchanged. In a computational model, we simulated a motoneuron pool receiving input from different proportions of the average M1 and RF activity recorded experimentally. Motoneuron firing generated simulated electromyogram, allowing reaction time measurements. Only if ≥60% of motoneuron drive came from RF (≤40% from M1) did loud sound shorten reaction time. The extent of shortening increased as more drive came from RF. If RF provided <60% of drive, loud sound lengthened the reaction time-the opposite of experimental findings. The majority of the drive for voluntary movements is thus likely to originate from the brainstem, not the cortex; changes in the magnitude of the StartReact effect can measure a shift in the relative importance of descending systems.SIGNIFICANCE STATEMENT Our results reveal that a loud sound has opposite effects on neural spiking in corticospinal cells from primary motor cortex, and in the reticular formation. We show that this fortuitously allows changes in reaction time produced by a loud sound to be used to assess the relative importance of reticulospinal versus corticospinal control of movement, validating previous noninvasive measurements in humans. Our findings suggest that the majority of the descending drive to motoneurons producing voluntary movement in primates comes from the reticulospinal tract, not the corticospinal tract.

Evolution of the acoustic startle response of Mexican cavefish.

The ability to detect threatening stimuli and initiate an escape response is essential for survival and under stringent evolutionary pressure. In diverse fish species, acoustic stimuli activate Mauthner neurons, which initiate a C-start escape response. This reflexive behavior is highly conserved across aquatic species and provides a model for investigating the neural mechanism underlying the evolution of escape behavior. Here, we characterize evolved differences in the C-start response between populations of the Mexican cavefish, Astyanax mexicanus. Cave populations of A. mexicanus inhabit an environment devoid of light and macroscopic predators, resulting in evolved differences in various morphological and behavioral traits. We find that the C-start is present in river-dwelling surface fish and multiple populations of cavefish, but that response kinematics and probability differ between populations. The Pachón population of cavefish exhibits an increased response probability, a slower response latency and speed, and reduction of the maximum bend angle, revealing evolved differences between surface and cave populations. Analysis of the responses of two other independently evolved populations of cavefish, revealed the repeated evolution of reduced angular speed. Investigation of surface-cave hybrids reveals a correlation between angular speed and peak angle, suggesting these two kinematic characteristics are related at the genetic or functional levels. Together, these findings provide support for the use of A. mexicanus as a model to investigate the evolution of escape behavior.

Effects of combined 5-HT2A and cannabinoid receptor modulation on a schizophrenia-related prepulse inhibition deficit in mice.

RATIONALE: Prepulse inhibition of the startle reflex (PPI) is disrupted in several psychiatric disorders including schizophrenia. Understanding PPI pharmacology may help elucidate the pathophysiology of these disorders and lead to better treatments. Given the advantages of multi-target approaches for complex mental illnesses treatment, we have investigated the interaction between receptors known to modulate PPI (5-HT1A and 5-HT2A) and the neuromodulatory endocannabinoid system. OBJECTIVES: To investigate serotonin and cannabinoid receptor (CBR) co-modulation in a model of PPI disruption relevant to schizophrenia METHODS: Male Swiss mice were pretreated with WIN 55,212-2 (CBR agonist), rimonabant (CB1R inverse agonist), 8-OH-DPAT (5-HT1A/7 agonist), and volinanserin (5-HT2A antagonist) or with a combination of a cannabinoid and a serotonergic drug. PPI disruption was induced by acute administration of MK-801. RESULTS: WIN 55,212-2 and rimonabant did not change PPI nor block MK-801-induced deficits. 8-OH-DPAT increased PPI in control mice and, in a higher dose, inhibited MK-801-induced impairments. Volinanserin also increased PPI in control and MK-801-treated mice, presenting an inverted U-shaped dose-response curve. Co-administration of either cannabinoid ligand with 8-OH-DPAT did not change PPI; however, the combination of volinanserin with rimonabant increased PPI in both control and MK-801-exposed mice. CONCLUSIONS: WIN 55,212-2 and rimonabant had similar effects in PPI. Moreover, serotonin and cannabinoid receptors interact to modulate PPI. While co-modulation of CBR and 5-HT1A receptors did not change PPI, a beneficial effect of 5-HT2A and CB1R antagonist combination was detected, possibly mediated through potentiation of 5-HT2A blockade effects by concomitant CB1R blockade.

Muscimol injection into the substantia nigra but not globus pallidus affects prepulse inhibition and startle reflex.

Behavioral arrest is an essential feature of an animal's survival. Acoustic startle reflex (ASR) is an involuntary whole-body contraction of the skeletal musculature to an unexpected auditory stimulus. This strong reaction can be decreased by prepulse inhibition (PPI) phenomenon; which, for example, is important in reducing distraction during the processing of sensory input. Several brainstem regions are involved in the PPI and startle reflex, but a previous study from our laboratory showed that the main input structure of Basal Ganglia (BG) - the striatum - modulates PPI. The pallidum and nigra are connected with striatum and these brainstem structures. Here, we investigated the role of these striatum outputs in the brain regions on startle amplitude, PPI regulation, and exploratory behavior in Wistar rats. The temporary bilateral inhibition of the globus pallidus (GP) by muscimol lead to motor impairment, without disturbing startle amplitude or PPI. Similarly, inhibition of the entopeduncular nucleus (EPN) specifically disrupted the exploratory behavior. On the other hand, the substantia nigra reticulata (SNr) inhibition interfered in all measured behaviors: decreased the PPI percentage, increased ASR and impaired the locomotor activity. The nigra is a key BG output structure which projects to the thalamus and brainstem. These findings extend our previous study showing that the striatum neurons expressing D1 receptors involvement in PPI occurs via the direct pathway to SNr, but not to the pallidum which more likely occurs by its connection with the caudal pontine nucleus, superior colliculus and/or pedunculopontine nucleus pivotal structures for startle reflex modulation.

Healthy Active Duty Military with Lifetime Experience of Mild Traumatic Brain Injury Exhibits Subtle Deficits in Sensory Reactivity and Sensory Integration During Static Balance.

Postural control and stress reactivity were investigated in active duty coast guard personnel to determine whether they are sensitive to lifetime effects of mild traumatic brain injury (mTBI). A custom-designed and validated virtual reality-based computerized posturography device was used to assess postural stability, whereas emotional reactivity was assessed using the acoustic startle response (ASR), and neurocognitive performance was assessed using the defense-automated neurobehavioral assessment (DANA). It was hypothesized that residual and subtle postural control imbalance and deficits in cognitive and sensory reactivity would be evident in those reporting multiple lifetime mTBI. Active duty military personnel (N = 36; 7 females and 29 males) with no Deployment Limiting Medical Condition were recruited and tested on all assessments. Medical history information provided a history of head injury. Thirty-nine percent of participants reported having a previous mTBI (nine reporting one and five reporting more than one incident). No participant had experienced a head injury within the past year and all were symptom free. A significant effect of number of mTBI was found in the postural assessment (p = 0.002). Lifetime mTBI was associated with suppressed ASR magnitude (p = 0.03) but did not affect neurocognitive performance. The current findings provide new insight into ongoing controversies concerning sensitivity to functional deficits following mTBI and when the window for treatment or restoration ends.

Dorsal striatum D1-expressing neurons are involved with sensorimotor gating on prepulse inhibition test.

Prepulse inhibition (PPI) is a behavioral test in which the startle reflex response to a high-intensity stimulus (pulse) is inhibited by the prior presentation of a weak stimulus (prepulse). The classic neural circuitry that mediates startle response is localized in the brainstem; however, recent studies point to the contribution of structures involved in higher cognitive functions in regulating the sensorimotor gating, particularly forebrain regions innervated by dopaminergic nuclei. The aim of the present study was to verify the role of dorsal striatum (DS) and dopaminergic transmitting mediated by D1 and D2 receptors on PPI test in rats. DS inactivation induced by muscimol injection did not affect PPI (%PPI and startle response), although it impaired the locomotor activity and caused catalepsy. Infusion of D1-like antagonist SCH23390 impaired %PPI but did not disturb the startle response and locomotor activity evaluated immediately after PPI test. D2 antagonist microinjection (sulpiride) did not affect %PPI and startle response, but impaired motor activity. These results point to an important role of DS, probably mediated by direct basal ganglia pathway, on modulation of sensorimotor gating, in accordance with clinical studies showing PPI deficits in schizophrenia, Tourette syndrome, and compulsive disorders - pathologies related to basal ganglia dysfunctions.

5-HT1A receptors of the prelimbic cortex mediate the hormonal impact on learned fear expression in high-anxious female rats.

Hormones highly influence female behaviors. However, research on this topic has not usually considered the variable hormonal status. The prelimbic cortex (PrL) is commonly engaged in fear learning. Connections from and to this region are known to be critical in regulating anxiety, in which serotonin (5-HT) plays a fundamental role, particularly through changes in 5-HT1A receptors functioning. Also, hormone fluctuations can greatly influence anxiety in humans and anxiety-related behavior in rodents, and this influence involves the functioning of 5-HT brain systems. The present investigation sought to determine whether fluctuations in ovarian hormones relative to the estrous cycle would influence the expression of learned fear in female rats previously selected as low- (LA) or high-anxious (HA). Furthermore, we investigate the role of the 5-HT system of the PrL, particularly the 5-HT1A receptors, as a possible modulator of estrous cycle influence on the expression of learned fear through intra-PrL microinjections of 5-HT itself or the full 5-HT1A agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamine)tetralin). Behavioral changes were assessed using the fear-potentiated startle (FPS) procedure. The results showed that fear intensity is associated with hormonal decay, being more accentuated during the estrus phase. This increase in fear levels was found to be negatively correlated with the expression of potentiated startle. In rats prone to anxiety and tested during the proestrus and estrus phases, 5-HT mechanisms of the PrL seem to play a regulatory role in the expression of learned fear. These results were not replicated in the LA rats. Similar but less intense results were found regarding the early and late diestrus. Our data indicate that future studies on this subject need to take into account the dissociation between low- and high-responsive females to understand how hormones affect emotional behavior.

Postnatal behavioral and inflammatory alterations in female pups prenatally exposed to valproic acid.

In Autism Spectrum Disorders (ASD), a bias to a higher incidence in boys than in girls has been reported. With the aim to identify biological mechanisms acting in female animals that could underlie this bias, we used an extensively validated mouse model of ASD: the prenatal exposure to valproic acid (VPA). We found postnatal behavioral alterations in female VPA pups: a longer latency in righting reflex at postnatal day (P) 3, and a delay in the acquisition of the acoustic startle response. We also analyzed the density of glial cells in the prefrontal cortex, hippocampus and cerebellum, in VPA and control animals. Female VPA pups showed alterations in the density of astrocytes and microglial cells between P21 and P42, with specific dynamics in each brain region. We also found a decrease in histone 3 acetylation in the cerebellum of female VPA pups at P14, suggesting that the changes in glial cell density could be due to alterations in the epigenetic developmental program. Finally, no differences in maternal behavior were found. Our results show that female VPA pups exhibit behavioral and inflammatory alterations postnatally, although they have been reported to have normal levels of sociability in adulthood. With our work, we contribute to the understanding of biological mechanisms underlying different effects of VPA on male and female rodents, and we hope to help elucidate whether there are factors increasing susceptibility to ASD in boys and/or resilience in girls.

Dopamine D2 receptors regulate unconditioned fear in deep layers of the superior colliculus and dorsal periaqueductal gray.

RATIONALE: Electrical and chemical stimulation of the dorsal periaqueductal gray (dPAG), deep layers of the superior colliculus (dlSC), and inferior colliculus (IC) causes freezing and escape behavior in rodents. Systemic injections of the selective dopamine D2 receptor antagonist sulpiride increased the number of switch-off responses (SORs) to light and auditory evoked potentials in response to loud sounds. Dopamine D2 receptor inhibition in the IC was shown to enhance unconditioned fear. Nevertheless, the role of dopamine receptors in the dlSC and dPAG in the mediation of unconditioned fear has not yet been demonstrated. OBJECTIVES: The purpose of the present study was to characterize the effects of sulpiride injections (4 and 8 µg/0.2 µl) in the dlSC and dPAG in rats that were subjected to unconditioned fear paradigms. METHODS: Switch-off responses to light and exploratory behavior in the elevated plus maze were used to evaluate unconditioned fear in rats. RESULTS: Intra-dlSC microinjections of sulpiride increased the number of SORs to light. Intra-dlSC and intra-dPAG injections of sulpiride reduced the number of entries into and time spent on the open arms and decreased end-arm exploration and head dipping in the elevated plus maze. CONCLUSION: These findings suggest that dopamine, through D2 receptors in the dlSC and dPAG, is involved in defense reactions that are organized in the midbrain tectum.

Assessments of Motor Abnormalities on the Grid-Walking and Foot-Fault Tests From Undernutrition in Wistar Rats.

This study was designed to verify whether different lactation conditions influenced nervous system development. The authors used motor tasks to verify changes in exploratory activity and muscle strength of weaned rats from different litter sizes and evaluated the applicability of the grid-walking test for assessing motor abnormalities caused by undernutrition. Alterations in litter size during the suckling period perturbed the nutritional status of pups, which exhibited body weight differences between the groups. Large-litter (L) pups showed significant delays in achieving developmental milestones and neurological reflexes compared to the small-litter (S) and medium-litter (M) pups. The S, M, and L group pups exhibited similar exploratory responses and muscle strength. In the grid-walking and foot-fault tests, the L group pups traveled shorter distances and, consequently, had less footsteps. However, the percentages of foot faults in the L group were higher than S and M groups. These results reflect delayed maturation of structures responsible for sensorimotor responses, such as the cerebellum, because much cerebellar maturation takes place postnatally. This is the first study to report that early undernutrition in pups resulted in suboptimal performances on the grid-walking and foot-fault tests and that the former test was sensitive to alterations caused by nutritional deficiency.

Possible evidence for re-regulation of HPA axis and brain reward systems over time in treatment in prescription opioid-dependent patients.

OBJECTIVES: There is growing evidence for a neuroadaptive model underlying vulnerability to relapse in opioid dependence. The purpose of this study was to evaluate clinical measures hypothesized to mirror elements of allostatic dysregulation in patients dependent on prescription opioids at 2 time points after withdrawal, compared with healthy control participants. METHODS: Recently withdrawn (n = 7) prescription opioid-dependent patients were compared with the patients in supervised residential care for 2 to 3 months (extended care; n = 7) and healthy controls (n = 7) using drug cue reactivity, affect-modulated startle response tasks, salivary cortisol, and 8 days of sleep actigraphy. Prefrontal cortex was monitored with functional near-infrared spectroscopy during the cue reactivity task. RESULTS: Startle response results indicated reduced hedonic response to natural rewards among patients recently withdrawn from opioids relative to extended care patients. The recently withdrawn patients showed increased activation to pill stimuli in right dorsolateral prefrontal cortex relative to extended care patients. Cortisol levels were elevated among recently withdrawn patients and intermediate for extended care relative to healthy controls. Actigraphy indicated disturbed sleep between recently withdrawn patients and extended care patients; extended care patients were similar to controls. Dorsolateral prefrontal cortex activation to drug and natural reward cues, startle responses to natural reward cues, day-time cortisol levels, time in bed, and total time spent sleeping were all correlated with the number of days since last drug use (ie, time in supervised residential treatment). CONCLUSIONS: These results suggest possible re-regulation of dysregulated hypothalamic-pituitary-adrenal axis and brain reward systems in prescription opioid-dependent patients over the drug-free period in residential treatment.

Medial prefrontal cortex serotonergic and GABAergic mechanisms modulate the expression of contextual fear: intratelencephalic pathways and differential involvement of cortical subregions.

Several lines of evidence indicate that the dorsal hippocampus (dH) and medial prefrontal cortex (mPFC) regulate contextual fear conditioning. The prelimbic (PrL), infralimbic (IL) and the anterior cingulate cortex (ACC) subregions of the mPFC likely play distinct roles in the expression of fear. Moreover, studies have highlighted the role of serotonin (5-hydroxytryptamine, 5-HT)- and γ-aminobutyric acid (GABA)-mediated mechanisms in the modulation of innate fear in the mPFC. The present study characterized dH-mPFC pathways and investigated the role of serotonergic and GABAergic mechanisms of the PrL, IL and ACC-area 1 (Cg1) in the elaboration of contextual fear conditioning using fear-potentiated startle (FPS) and freezing behavior in Rattus norvegicus. The results of neurotracing with microinjections of biotinylated dextran amine into the dH revealed a neural link of the dH with the PrL and ACC. Intra-PrL injections of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and the GABAA receptor-selective agonist muscimol reduced contextual FPS and freezing responses. Intra-Cg1 injections of muscimol but not 8-OH-DPAT decreased FPS and freezing responses. However, neither intra-IL injections of a 5-HT1A agonist nor of a GABAA agonist affected these defensive responses. Labeled neuronal fibers from the dH reached the superficial layers of the PrL cortex and spread to the inner layers of PrL and Cg1 cortices, supporting the pharmacological findings. The present results confirmed the involvement of PrL and Cg1 in the expression of FPS and freezing responses to aversive conditions. In addition, PrL serotoninergic mechanisms play a key role in contextual fear conditioning. This study suggests that PrL, IL and Cg1 distinctively contribute to the modulation of contextual fear conditioning.

Whole-body prepulse inhibition protocol to test sensorymotor gating mechanisms in monkeys.

Prepulse inhibition (PPI) is the decrease of startle reflex amplitude when a slight stimulus is previously generated. This paradigm may provide valuable information about sensorimotor gating functionality. Here we aimed at determining the inhibited and uninhibited startle response of capuchin monkeys (Sapajus spp.), and to evaluate the role of the superior colliculus in PPI. Capuchin monkeys were tested in a whole-body protocol, to determine the best startle amplitude and interstimuli interval. Additionally we tested two subjects with bilateral superior colliculus damage in this protocol. Results show that 115 dB auditory pulse has induced the best startle response. In contrast to reports in other species, no habituation to the auditory stimuli was observed here in capuchins. Also, startle reflex inhibition was optimal after 120 msec interstimuli interval. Finally, there was a downward tendency of percentage inhibition in superior colliculus-lesioned monkeys. Our data provides the possibility of further studies with whole-body protocol in capuchin monkeys and reinforces the importance of the superior colliculus in PPI.

D-serine prevents cognitive deficits induced by acute stress.

Increasing evidence indicates that acute stress disrupts cognitive functions mediated by glutamate-NMDA receptors, although the mechanisms are not fully understood. Here we investigated whether d-serine and glycine, the endogenous co-agonists of the NMDA receptor, are regulated by acute stress. We studied the biochemical and behavioral effects of acute restraint stress in C57BL/6 mice. Acute restraint stress decreased d-serine levels in the prefrontal cortex and glycine levels in the hippocampus. Behaviorally, acute stress impaired memory consolidation in the object recognition task and prepulse inhibition of the startle response. Importantly, d-serine administration (1 g/kg, i.p.) prevented both stress-induced impairments. Taken together, our results show for the first time an interplay between stress and d-serine and warrant further research on the role of d-serine in stress-related disorders.

Altered sedative effects of ethanol in mice with α1 glycine receptor subunits that are insensitive to Gßγ modulation.

Alcohol abuse and alcoholism are major health problems and one of the leading preventable causes of death. Before achieving better treatments for alcoholism, it is necessary to understand the critical actions of alcohol on membrane proteins that regulate fundamental functions in the central nervous system. After generating a genetically modified knock-in (KI) mouse having a glycine receptor (GlyR) with phenotypical silent mutations at KK385/386AA, we studied its cellular and in vivo ethanol sensitivity. Analyses with western blotting and immunocytochemistry indicated that the expression of α1 GlyRs in nervous tissues and spinal cord neurons (SCNs) were similar between WT and KI mice. The analysis of synaptic currents recorded from KI mice showed that the glycinergic synaptic transmission had normal properties, but the sensitivity to ethanol was significantly reduced. Furthermore, the glycine-evoked current in SCNs from KI was resistant to ethanol and G-protein activation by GTP-γ-S. In behavioral studies, KI mice did not display the foot-clasping behavior upon lifting by the tail and lacked an enhanced startle reflex response that are characteristic of other glycine KI mouse lines with markedly impaired glycine receptor function. The most notable characteristic of the KI mice was their significant lower sensitivity to ethanol (∼40%), expressed by shorter times in loss of righting reflex (LORR) in response to a sedative dose of ethanol (3.5 g/Kg). These data provide the first evidence to link a molecular site in the GlyR with the sedative effects produced by intoxicating doses of ethanol.

Conditioned fear in low- and high-anxious rats is differentially regulated by cortical subcortical and midbrain 5-HT(1A) receptors.

Interactions between the prelimbic cortex and the basolateral amygdala underlie fear memory processing, mostly through acquiring and consolidating the learning of a conditioned fear. More recently, studies highlighted the role of the dorsal periaqueductal gray (DPAG) in the modulation of learning fear responses. In addition, extensive data in the literature have signaled the importance of serotonin (5-HT) on fear and anxiety. In the present study, the role of 5-HT neurotransmission of the prelimbic cortex, basolateral amygdala or the DPAG on the unconditioned and conditioned fear responses in rats previously selected as low- (LA) or high-anxious (HA) were assessed through local infusions of 5-HT itself (10nmol/0.2µl) or the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT - 0.3µg/0.2µl). Behavioral analysis was conducted using the fear-potentiated startle (FPS) procedure. Dependent variables recorded were the latency and amplitude of the unconditioned startle response and FPS. Our findings suggest that, on the prelimbic cortex, 5-HT modulates the expression of conditioned fear response in HA rats and this modulation is dependent on 5-HT1A receptors. This is not true, however, for the basolateral amygdala or the DPAG. In these regions LA but not HA rats were susceptible to the anxiolytic-like effect of 5-HT1A receptor activation. It is thought that the expression of conditioned fear in HA subjects may be dependent on other 5-HT receptors, as the 5-HT1B subtype, and/or changes in other systems such as the GABA and glutamate neurotransmitters. These results increase our understanding of the rostrocaudal influence of 5-HT on the unconditioned and conditioned fear responses in LA and HA subjects and, to some extent, are in disagreement with the theoretical current that emphasizes the role of 5-HT on anxiety, mainly at the subcortical and midbrain levels.

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.