Startle gating in rats is disrupted by chemical inactivation but not D2 stimulation of the dorsomedial thalamus

Roles of the medial prefrontal cortex, mediodorsal thalamus, and their combined circuit for performance of the odor span task in rats: analysis of memory capacity and foraging behavior

Working memory (WM), the capacity for short-term storage of small quantities of information for immediate use, is thought to depend on activity within the prefrontal cortex. Recent evidence indicates that the prefrontal neuronal activity supporting WM is driven by thalamocortical connections arising in mediodorsal thalamus (mdThal). However, the role of these connections has not been studied using olfactory stimuli leaving open the question of whether this circuit extends to all sensory modalities. Additionally, manipulations of the mdThal in olfactory memory tasks have yielded mixed results. In the present experiment, we investigated the role of connections between the rat medial prefrontal cortex (mPFC) and mdThal in the odor span task (OST) using a pharmacological contralateral disconnection technique. Inactivation of either the mPFC or mdThal alone both significantly impaired memory performance in the OST, replicating previous findings with the mPFC and confirming that the mdThal plays an essential role in intact OST performance. Contralateral disconnection of the two structures impaired OST performance in support of the idea that the OST relies on mPFC-mdThal connections, but ipsilateral control infusions also impaired performance, complicating this interpretation. We also performed a detailed analysis of rats’ errors and foraging behavior and found a dissociation between mPFC and mdThal inactivation conditions. Inactivation of the mdThal and mPFC caused a significant reduction in the number of approaches rats made per odor, whereas only mdThal inactivation or mPFC-mdThal disconnection caused significant increases in choice latency. Our results confirm that the mdThal is necessary for performance of the OST and that it may critically interact with the mPFC to mediate OST performance. Additionally, we have provided evidence that the mPFC and mdThal play dissociable roles in mediating foraging behavior.

Manganese-Enhanced MRI Reflects Both Activity-Independent and Activity-Dependent Uptake within the Rat Habenulomesencephalic Pathway

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful technique for assessing the functional connectivity of neurons within the central nervous system. Despite the widely held proposition that MEMRI signal is dependent on neuronal activity, few studies have directly tested this implicit hypothesis. In the present series of experiments, MnCl2 was injected into the habenula of urethane-anesthetized rats alone or in combination with drugs known to alter neuronal activity by modulating specific voltage- and/or ligand-gated ion channels. Continuous quantitative T1 mapping was used to measure Mn2+ accumulation in the interpeduncular nucleus, a midline structure in which efferents from the medial habenula terminate. Microinjection of MnCl2 into the habenular complex using a protocol that maintained spontaneous neuronal activity resulted in a time-dependent increase in MEMRI signal intensity in the interpeduncular nucleus consistent with fast axonal transport of Mn2+ between these structures. Co-injection of the excitatory amino-acid agonist AMPA, increased the Mn2+-enhanced signal intensity within the interpeduncular nucleus. AMPA-induced increases in MEMRI signal were attenuated by co-injection of either the sodium channel blocker, TTX, or broad-spectrum Ca2+ channel blocker, Ni2+, and were occluded in the presence of both channel blockers. However, neither Ni2+ nor TTX, alone or in combination, attenuated the increase in signal intensity following injection of Mn2+ into the habenula. These results support the premise that changes in neuronal excitability are reflected by corresponding changes in MEMRI signal intensity. However, they also suggest that basal rates of Mn2+ uptake by neurons in the medial habenula may also occur via activity-independent mechanisms.

Mapping brain regions in which deep brain stimulation affects schizophrenia-like behavior in two rat models of schizophrenia

BACKGROUND AND OBJECTIVES

The development of more efficient treatment remains a major unmet need in the realm of schizophrenia disease. Using the maternal immune stimulation and the pubertal cannabinoid administration rat model of schizophrenia, the present study aimed at testing the hypothesis that deep brain stimulation (DBS) serves as a novel therapeutic technique for this disorder.

METHODS

Adult offspring of dams, treated with the immune activating agent poly I:C (4 mg/kg, n = 50) or saline (n = 50), underwent bilateral stereotactic electrode implantation into one of the following brain regions: subthalamic nucleus (STN, n = 12/10), entopeduncularis nucleus (EP, n = 10/11), globus pallidus (GP, n = 10/10), medial prefrontal cortex (mPFC, n = 8/8), or dorsomedial thalamus (DM, n = 10/11). Adult rats treated with the CB1 receptor agonist WIN 55,212-2 (WIN, n = 16) or saline (n = 12) during puberty were bilaterally implanted with electrodes into either the mPFC (n = 8/6) or the DM (n = 8/6). After a post-operative recovery period of one week, all rats were tested on a well-established cross-species phenomenon that is disrupted in schizophrenia, the pre-pulse inhibition (PPI) of the acoustic startle reflex (ASR) under different DBS conditions.

RESULTS

Poly I:C induced deficits in PPI of the ASR were normalized upon DBS. DBS effects depended on both stimulation target and stimulation parameters. Most prominent effects were found under DBS at high frequencies in the mPFC and DM. These effects were replicated in the pubertal WIN administration rat model of schizophrenia.

CONCLUSIONS

Brain regions, in which DBS normalized PPI deficits, might be of therapeutic relevance to the treatment of schizophrenia. Results imply that DBS could be considered a plausible therapeutic technique in the realm of schizophrenia disease.

Nitric oxide modulates dopaminergic regulation of prepulse inhibition in the basolateral amygdala

Systemic injection of the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (LNO) prevents the disruptive effect of amphetamine (Amph) on prepulse inhibition (PPI), a sensorimotor gating model in which the amplitude of the acoustic startle response (ASR) to a startling sound (pulse) is reduced when preceded immediately by a weaker stimulus (prepulse). Given that dopamine (DA) projections to the basolateral amygdala (BLA) are involved in the control of information processing, our aim was to investigate if intra-BLA administration of LNO would modify the disruption caused by the DA agonists, Amph, apomorphine (Apo) and quinpirole (QNP), on PPI. Male Wistar rats received bilateral intra-BLA microinjections (0.2 µL/min/side) of combined treatments (saline or LNO 11 µg followed by saline, QNP 3 µg, Apo 10 µg or Amph 30 µg). PPI was disrupted by intra-BLA Apo, QNP or Amph but not by LNO. Prior bilateral intra-BLA injection of LNO prevented the Apo- and QNP-induced disruption of PPI but did not affect that caused by Amph. APO- or QNP-induced increases in ASR to prepulse + pulse were also restored by LNO. Since local inhibition of NO formation affected the effects of direct, but not indirect, DA agonists, the results suggest that this modulation is not occurring at the level of DA release but may involve complex interactions with other neurotransmitter systems.

Discrete forebrain neuronal networks supporting noradrenergic regulation of sensorimotor gating

Prepulse inhibition (PPI) refers to the reduction in the startle response when a startling stimulus is preceded by a weak prestimulus, and is an endophenotype of deficient sensorimotor gating in several neuropsychiatric disorders. Emerging evidence suggests that norepinephrine (NE) regulates PPI, however, the circuitry involved is unknown. We found recently that stimulation of the locus coeruleus (LC), the primary source of NE to the forebrain, induces a PPI deficit that is a result of downstream NE release. Hence, this study sought to identify LC-innervated forebrain regions that mediate this effect. Separate groups of male Sprague-Dawley rats received a cocktail solution of the α1-NE receptor agonist phenylephrine plus the β-receptor agonist isoproterenol (equal parts of each; 0, 3, 10, and 30 μg) into subregions of the medial prefrontal cortex (mPFC), nucleus accumbens (NAcc), extended amygdala, mediodorsal thalamus (MD-thalamus), or the dorsal hippocampus (DH) before PPI testing. NE agonist infusion into the posterior mPFC, NAcc shell, bed nucleus of the stria terminalis, basolateral amygdala, and the MD-thalamus disrupted PPI, with particularly strong effects in MD-thalamus. Sites in which NE receptor stimulation did not disrupt PPI (anterior mPFC, NAcc core, central amygdala, and DH) did support PPI disruptions with the dopamine D2 receptor agonist quinpirole (0, 10 μg). This pattern reveals new pathways in the regulation of PPI, and suggests that NE transmission within distinct thalamocortical and ventral forebrain networks may subserve the sensorimotor gating deficits that are seen in disorders such as schizophrenia, Tourette syndrome, and post-traumatic stress disorder.

Realistic expectations of prepulse inhibition in translational models for schizophrenia research

INTRODUCTION

Under specific conditions, a weak lead stimulus, or "prepulse", can inhibit the startling effects of a subsequent intense abrupt stimulus. This startle-inhibiting effect of the prepulse, termed "prepulse inhibition" (PPI), is widely used in translational models to understand the biology of brainbased inhibitory mechanisms and their deficiency in neuropsychiatric disorders. In 1981, four published reports with "prepulse inhibition" as an index term were listed on Medline; over the past 5 years, new published Medline reports with "prepulse inhibition" as an index term have appeared at a rate exceeding once every 2.7 days (n=678). Most of these reports focus on the use of PPI in translational models of impaired sensorimotor gating in schizophrenia. This rapid expansion and broad application of PPI as a tool for understanding schizophrenia has, at times, outpaced critical thinking and falsifiable hypotheses about the relative strengths vs. limitations of this measure.

OBJECTIVES

This review enumerates the realistic expectations for PPI in translational models for schizophrenia research, and provides cautionary notes for the future applications of this important research tool.

CONCLUSION

In humans, PPI is not "diagnostic"; levels of PPI do not predict clinical course, specific symptoms, or individual medication responses. In preclinical studies, PPI is valuable for evaluating models or model organisms relevant to schizophrenia, "mapping" neural substrates of deficient PPI in schizophrenia, and advancing the discovery and development of novel therapeutics. Across species, PPI is a reliable, robust quantitative phenotype that is useful for probing the neurobiology and genetics of gating deficits in schizophrenia.

Frontal dopamine D(2/3) receptor binding in drug-naive first-episode schizophrenic patients correlates with positive psychotic symptoms and gender

BACKGROUND

The aim of the study was to examine extrastriatal dopamine D(2/3) receptor binding and psychopathology in schizophrenic patients, and to relate binding potential (BP) values to psychopathology.

METHODS

Twenty-five drug-naive schizophrenic patients and 20 healthy controls were examined with single-photon emission computerized tomography (SPECT) using the D(2/3)-receptor ligand [123I]epidepride.

RESULTS

In the hitherto largest study on extrastriatal D(2/3) receptors we detected a significant correlation between frontal D(2/3) BP values and positive schizophrenic symptoms in the larger group of male schizophrenic patients, higher frontal BP values in male (n = 17) compared to female (n = 8) patients, and - in accordance with this - significantly fewer positive schizophrenic symptoms in the female patients. No significant differences in BP values were observed between patients and controls; the patients, however, had significantly higher BP in the right compared to the left thalamus, whereas no significant hemispheric imbalances were observed in the healthy subjects.

CONCLUSIONS

The present data are the first to confirm a significant correlation between frontal D(2/3) receptor BP values and positive symptoms in male schizophrenic patients. They are in agreement with the hypothesis that frontal D(2/3) receptor activity is significant for positive psychotic symptoms. Additionally, the data support a thalamic hemispheric imbalance in schizophrenia.

Neonatal neurosteroid administration results in development-specific alterations in prepulse inhibition and locomotor activity: neurosteroids alter prepulse inhibition and locomotor activity

RATIONALE

Early life exposure to stress and to GABAA receptor modulators have well-defined and persistent behavioral effects. A single neonatal injection of the GABAergic neurosteroid allopregnanolone (3alpha-hydroxy,5alpha-pregnane-20-one, 10 mg/kg, i.p.) alters the localization of prefrontal cortex (PFC) interneurons in adulthood. Such displacement could result in disinhibited behavior associated with impaired development of the mesocortical dopamine system.

OBJECTIVES

To determine if there is a critical window in which allopregnanolone levels may impact the development and mature function of the mesocorticolimbic circuitry.

METHODS

Behavioral measures, including prepulse inhibition (PPI) and total locomotor activity, after amphetamine exposure were assessed at postnatal day 20 (P20) (prepuberty), P40 (puberty), P60 (postpuberty), and P80 (adulthood) in animals previously exposed to allopregnanolone (10 mg/kg) on P2 and P5. PFC tyrosine hydroxylase immunoreactivity was stereologically measured.

RESULTS

P2 administration of allopregnanolone resulted in an increased locomotor response to amphetamine (14, 28% on P20 and P80, respectively) and reduced PPI (28, 22% on P20 and P80, respectively) at P20 and P80, whereas allopregnanolone administration on P5 increased locomotor response to amphetamine (20%) and reduced PPI (37%) at P80. Clozapine (7.5 mg/kg) pretreatment reversed the PPI deficit in P2-exposed animals. The total length of tyrosine hydroxylase immunopositive fibers in PFC was not altered by neonatal neurosteroid exposure, but more fibers were located in layers V/VI vs I-III.

CONCLUSIONS

Altering neonatal allopregnanolone levels disrupts PFC-dependent behavior, indicating that allopregnanolone might be important for normal PFC circuitry development. The temporal exposure differences (P2 vs P5) and ontological-dependent effects (P20 and P80, but not P40 or P60) suggest critical windows of vulnerability to neurosteroid insult across development.

The role of extrastriatal dopamine D2 receptors in schizophrenia

Despite numerous studies on extrastriatal regions involved in schizophrenia, studies on the functional implications of dopamine (DA) D2 receptors in the extrastriatal regions, including the cortex and thalamus, are limited. We review postmortem and in vivo human imaging studies as well as animal studies, focusing on the function of extrastriatal DA D2 receptors and their role in the pathophysiology of schizophrenia. Based on recent findings, cortical DA D2 receptors may interact with the gamma-aminobutyric acid system to modulate DA transmission, and thalamic DA D2 receptors are likely to participate in sensory gating function into the prefrontal cortex. We have found decreased DA D2 receptors in the anterior cingulate cortex and thalamic subregions of patients with schizophrenia. These observations may suggest that alterations of extrastriatal DA D2 receptors are involved in dysregulation of DA transmission and sensory signals from the thalamus to the cortex. Excessive excitatory signals from the thalamus might flow into the cortical neurotransmission system, aggravating dysregulation of DA transmission in both the striatal and extrastriatal regions in schizophrenia. These notions suggest the need for future investigations of extrastriatal DA D2 receptor function to gain important clues regarding the pathogenesis and of possible treatments for schizophrenia.

Perinatal allopregnanolone influences prefrontal cortex structure, connectivity and behavior in adult rats

Cortical neurosteroid levels vary dramatically across development; during the second week of life elevated levels of allopregnanolone are associated with decreased GABA(A) receptor function. Since GABA(A) receptor modulation plays a role in proliferative regulation in developing neocortex, it is possible that endogenous neurosteroids such as allopregnanolone, acting through GABA(A) receptors, modulate cortical development. We augmented normally low levels with exogenous administration of allopregnanolone (10 mg/kg) during the first week of rodent life. The localization of parvalbumin-labeled cells was markedly altered; the ratio of cell number in the deep (layers V-VI) vs. superficial (layers I-III) layers of adult prefrontal cortex increased two-fold in rats administered allopregnanolone on postnatal day 1 or 5. The mechanism underlying these anatomical changes likely involves GABA(A) receptors because similar changes in interneuron placement were observed after neonatal benzodiazepine administration. Measures of mature cortical function were also altered after neonatal neurosteroid administration, including [(3)H]MK-801 binding, prepulse inhibition and amphetamine-induced locomotor activity. Moreover, neonatal allopregnanolone administration increases the number of parvalbumin-expressing neurons in medial dorsal nucleus of the thalamus while the total neuron number is decreased. These findings suggest that connectivity between the medial dorsal nucleus of the thalamus and prefrontal cortex is likely altered by neonatal neurosteroid administration and may result in a disinhibited frontal cortex. Disinhibition in the prefrontal cortex is associated with behavioral changes relevant to human psychosis and developmental disorders. If neurosteroids play a role in normal development of prefrontal/medial dorsal patency as suggested by these studies, then alterations in neurosteroid levels may contribute to abnormal neurodevelopment.

Prefrontal D1 and ventral hippocampal N-methyl-D-aspartate regulation of startle gating in rats

BACKGROUND

Sensorimotor gating, as measured by prepulse inhibition of the startle reflex, is deficient in schizophrenia patients, and in rats after specific manipulations of limbic cortico-striato-pallido-thalamic circuitry. For example, prepulse inhibition in rats is disrupted after D1 blockade in the medial prefrontal cortex, and after N-methyl-D-aspartate infusion into the ventral hippocampus. In the present study, we examined whether these two substrates form part of an integrated circuit regulating sensorimotor gating, which might contribute to the loss of prepulse inhibition in patient populations.

METHODS

Prepulse inhibition was assessed in male Sprague-Dawley rats after systemic or intra-medial prefrontal cortex administration of the D1 antagonist, SCH 23390. Separate rats received intra-medial prefrontal cortex infusion of the retrograde transported label Fluoro-Gold. In rats with sham or electrolytic lesions of the medial prefrontal cortex, prepulse inhibition was tested after infusion of N-methyl-D-aspartate or vehicle into ventral hippocampus regions that were determined to send projections to the medial prefrontal cortex.

RESULTS

Prepulse inhibition was disrupted after systemic SCH 23390 treatment and after infusion of SCH 23390 into medial prefrontal cortex sites within the prelimbic and cingulate cortices. Fluoro-Gold infusion into these medial prefrontal cortex sites labeled cells in the ventral hippocampus complex, including regions CA1 and entorhinal cortex. N-methyl-D-aspartate infusions into these ventral hippocampus regions disrupted prepulse inhibition in rats after sham but not electrolytic lesions of the medial prefrontal cortex.

CONCLUSIONS

Prepulse inhibition appears to be regulated by interacting substrates within the ventral hippocampus and MPFC. Specifically, NMDA activation of the ventral hippocampus appears to disrupt prepulse inhibition in a manner that is dependent on the integrity of infralimbic or cingulate cortical regions that also support a D1-mediated regulation of prepulse inhibition. Conceivably, dysfunction within these hippocampal-frontal circuits may contribute to sensorimotor gating deficits in schizophrenia.

Using animal models to develop therapeutics for Tourette Syndrome

The science of Tourette Syndrome (TS) is advancing at multiple levels of analysis and will be enhanced through the use of animal models. Particular challenges in the development of TS animal models reflect complex features of this disorder, including its waxing and waning course and its "invisible" sensory and psychic symptoms. Animal models can achieve face, predictive, or construct validity based on their particular features. Predictive validity, of most direct relevance to drug development for TS, is achieved to some degree by a several animal models, although the reliance of most of these models on measures of motor suppression may ultimately limit their utility. Other models achieve construct validity with proposed pathophysiological mechanisms related to the immune and neural circuit etiologies of TS. One model-deficient sensorimotor gating of the startle reflex-is discussed in terms of its present and future applications towards advancing our understanding of the pathophysiology and treatment of TS. In addition to models that will advance the pharmacotherapy of TS, other animal models may enhance the utility of nonpharmacologic TS treatments, ranging from behavior therapy to deep brain stimulation (DBS).

P50 and acoustic startle gating are not related in healthy participants

Although P50 event-related potential (ERP) suppression and acoustic startle prepulse inhibition are conceptualized as measures of sensory and sensorimotor gating, respectively, the relationship between these measures is unclear. In the present study, P50 and prepulse inhibition trials were interleaved in a single testing session to determine their relationship. Thirty-one healthy participants were presented with startle- and P50-eliciting stimuli across six trial blocks. Lead stimuli (i.e., the prepulse to the acoustic startle and the first click in the dual click ERP paradigm) resulted in significant gating, or amplitude attenuation, of responses to the startle probe and second paired click. There were no meaningful correlations between the P50 and prepulse inhibition variables, indicating that P50 suppression and acoustic startle prepulse inhibition measure distinct neural mechanisms. The implications of these findings for operationally defining the psychological construct of gating with these psychophysiological measures are discussed.