Action of tachykinins in the hippocampus: Facilitation of inhibitory drive to GABAergic interneurons

Behavioral sensitization induced by dorsal periaqueductal gray electrical stimulation is counteracted by NK1 receptor antagonism in the ventral hippocampus and central nucleus of the amygdala

A single threatening experience may change the behavior of an animal in a long-lasting way and elicit generalized behavioral responses to a novel threatening situation that is unrelated to the original aversive experience. Electrical stimulation (ES) of the dorsal periaqueductal gray (dPAG) produces a range of defensive reactions, characterized by freezing, escape, and post-stimulation freezing (PSF). The latter reflects the processing of ascending aversive information to prosencephalic structures, including the central nucleus of the amygdala (CeA), which allows the animal to evaluate the consequences of the aversive situation. This process is modulated by substance P (SP) and its preferred receptor, neurokinin 1 (NK1). The ventral hippocampus (VH) has been associated with the processing of aversive information and expression of emotional reactions with negative valence, but the participation of the VH in the expression of these defensive responses has not been investigated. The VH is rich in NK1 receptor expression and has a high density of SP-containing fibers. The present study examined the role of NK1 receptors in the VH in the expression of defensive responses and behavioral sensitization that were induced by dPAG-ES. Rats were implanted with an electrode in the dPAG for ES, and a cannula was implanted in the VH or CeA for injections of vehicle (phosphate-buffered saline) or the NK1 receptor antagonist spantide (100 pmol/0.2 μL. Spantide reduced the duration of PSF that was evoked by dPAG-ES, without changing the aversive freezing or escape thresholds. One and 7 days later, exploratory behavior was evaluated in independent groups of rats in the elevated plus maze (EPM). dPAG-ES in rats that received vehicle caused higher aversion to the open arms of the EPM compared with rats that did not receive dPAG stimulation at both time intervals. Injections of spantide in the VH or CeA prevented the proaversive effects of dPAG-ES in the EPM only 1 day later. These findings suggest that NK1 receptors are activated in both the VH and CeA during the processing of aversive information that derives from dPAG-ES. As previously shown for the CeA, SP/NK1 receptors in the VH are recruited during PSF that is evoked by dPAG-ES, suggesting that a 24-h time window is susceptible to interventions with NK1 antagonists that block the passage of aversive information from the dPAG to higher brain areas.

Substance P induces plasticity and synaptic tagging/capture in rat hippocampal area CA2

The hippocampal area Cornu Ammonis (CA) CA2 is important for social interaction and is innervated by Substance P (SP)-expressing supramammillary (SuM) nucleus neurons. SP exerts neuromodulatory effects on pain processing and central synaptic transmission. Here we provide evidence that SP can induce a slowly developing NMDA receptor- and protein synthesis-dependent potentiation of synaptic transmission that can be induced not only at entorhinal cortical (EC)-CA2 synapses but also at long-term potentiation (LTP)-resistant Schaffer collateral (SC)-CA2 synapses. In addition, SP-induced potentiation of SC-CA2 synapses transforms a short-term potentiation of EC-CA2 synaptic transmission into LTP, consistent with the synaptic tagging and capture hypothesis. Interestingly, this SP-induced potentiation and associative interaction between the EC and SC inputs of CA2 neurons is independent of the GABAergic system. In addition, CaMKIV and PKMζ play a critical role in the SP-induced effects on SC-CA2 and EC-CA2 synapses. Thus, afferents from SuM neurons are ideally situated to prime CA2 synapses for the formation of long-lasting plasticity and associativity.

Excitatory/inhibitory imbalance in autism spectrum disorders: Implications for interventions and therapeutics

OBJECTIVES

Imbalance between excitation and inhibition and increased excitatory-inhibitory (E-I) ratio is a common mechanism in autism spectrum disorders (ASD) that is responsible for the learning and memory, cognitive, sensory, motor deficits, and seizures occurring in these disorders. ASD are very heterogeneous and better understanding of E-I imbalance in brain will lead to better diagnosis and treatments.

METHODS

We perform a critical literature review of the causes and presentations of E-I imbalance in ASD.

RESULTS

E-I imbalance in ASD is due primarily to abnormal glutamatergic and GABAergic neurotransmission in key brain regions such as neocortex, hippocampus, amygdala, and cerebellum. Other causes are due to dysfunction of neuropeptides (oxytocin), synaptic proteins (neuroligins), and immune system molecules (cytokines). At the neuropathological level E-I imbalance in ASD is presented as a "minicolumnopathy". E-I imbalance alters the manner by which the brain processes information and regulates behaviour. New developments for investigating E-I imbalance such as optogenetics and transcranial magnetic stimulation (TMS) are presented. Non-invasive brain stimulation methods such as TMS for treatment of the core symptoms of ASD are discussed.

CONCLUSIONS

Understanding E-I imbalance has important implications for developing better pharmacological and behavioural treatments for ASD, including TMS, new drugs, biomarkers and patient stratification.

Delta Opioid Receptors: Learning and Motivation

Delta opioid receptor (DOR) displays a unique, highly conserved, structure and an original pattern of distribution in the central nervous system, pointing to a distinct and specific functional role among opioid peptide receptors. Over the last 15 years, in vivo pharmacology and genetic models have allowed significant advances in the understanding of this role. In this review, we will focus on the involvement of DOR in modulating different types of hippocampal- and striatal-dependent learning processes as well as motor function, motivation, and reward. Remarkably, DOR seems to play a key role in balancing hippocampal and striatal functions, with major implications for the control of cognitive performance and motor function under healthy and pathological conditions.

Impaired hippocampus-dependent and facilitated striatum-dependent behaviors in mice lacking the δ opioid receptor

Pharmacological data suggest that delta opioid receptors modulate learning and memory processes. In the present study, we investigated whether inactivation of the delta opioid receptor modifies hippocampus (HPC)- and striatum-dependent behaviors. We first assessed HPC-dependent learning in mice lacking the receptor (Oprd1(-/-) mice) or wild-type (WT) mice treated with the delta opioid antagonist naltrindole using novel object recognition, and a dual-solution cross-maze task. Second, we subjected mutant animals to memory tests addressing striatum-dependent learning using a single-solution response cross-maze task and a motor skill-learning task. Genetic and pharmacological inactivation of delta opioid receptors reduced performance in HPC-dependent object place recognition. Place learning was also altered in Oprd1(-/-) animals, whereas striatum-dependent response and procedural learning were facilitated. Third, we investigated the expression levels for a large set of genes involved in neurotransmission in both HPC and striatum of Oprd1(-/-) mice. Gene expression was modified for several key genes that may contribute to alter hippocampal and striatal functions, and bias striatal output towards striatonigral activity. To test this hypothesis, we finally examined locomotor effects of dopamine receptor agonists. We found that Oprd1(-/-) and naltrindole-treated WT mice were more sensitive to the stimulant locomotor effect of SKF-81297 (D1/D5), supporting the hypothesis of facilitated striatonigral output. These data suggest, for the first time, that delta receptor activity tonically inhibits striatal function, and demonstrate that delta opioid receptors modulate learning and memory performance by regulating the HPC/striatum balance.

Neurokinin-1 receptor deletion modulates behavioural and neurochemical alterations in an animal model of depression

The substance P/NK1 receptor system plays an important role in the regulation of stress and emotional responding and as such had been implicated in the pathophysiology of anxiety and depression. The present study investigated whether alterations in the substance P/NK1 receptor system in brain areas which regulate emotional responding accompany the depressive behavioural phenotype observed in the olfactory bulbectomised (OB) mouse. The effect of NK1 receptor deletion on behavioural responding and monoamine levels in discrete brain regions of the OB model, were also examined. Substance P levels in the frontal cortex and NK1 receptor expression in the amygdala and hippocampus were enhanced following olfactory bulbectomy. Although NK1 receptor knockout (NK1-/-) mice did not exhibit altered behavioural responding in the open field test, noradrenaline levels were enhanced in the frontal cortex, amygdala and hippocampus, as were serotonin levels in the frontal cortex. Locomotor activity and exploratory behaviour were enhanced in wild type OB mice, indicative of a depressive-like phenotype, an effect attenuated in NK1-/- mice. Bulbectomy induced a decrease in noradrenaline and 5-HIAA in the frontal cortex and an increase in serotonin in the amygdala, effects attenuated in OB NK1-/- mice. The present studies indicate that alterations in substance P/NK1 receptor system underlie, at least in part, the behavioural and monoaminergic changes in this animal model of depression.

Excitatory actions of substance P in the rat lateral posterior nucleus

The lateral posterior nucleus (LP) receives inputs from both neocortex and superior colliculus (SC), and is involved with integration and processing of higher-level visual information. Relay neurons in LP contain tachykinin receptors and are innervated by substance P (SP)-containing SC neurons and by layer V neurons of the visual cortex. In this study, we investigated the actions of SP on LP relay neurons using whole-cell recording techniques. SP produced a graded depolarizing response in LP neurons along the rostro-caudal extent of the lateral subdivision of LP nuclei (LPl), with a significantly larger response in rostral LPl neurons compared with caudal LPl neurons. In rostral LPl, SP (5-2000 nm) depolarized nearly all relay neurons tested (> 98%) in a concentration-dependent manner. Voltage-clamp experiments revealed that SP produced an inward current associated with a decreased conductance. The inward current was mediated primarily by neurokinin receptor (NK)(1) tachykinin receptors, although significantly smaller inward currents were produced by specific NK2 and NK3 receptor agonists. The selective NK(1) receptor antagonist RP67580 attenuated the SP-mediated response by 71.5% and was significantly larger than the attenuation of the SP response obtained by NK2 and NK3 receptor antagonists, GR159897 and SB222200, respectively. The SP-mediated response showed voltage characteristics consistent with a K+ conductance, and was attenuated by Cs+, a K+ channel blocker. Our data suggest that SP may modulate visual information that is being processed and integrated in the LPl with inputs from collicular sources.

Coincidence of neurokinin 1 receptor with the vesicular glutamate transporter 3 (VGLUT3) in the rat forebrain

The third vesicular glutamate transporter (VGLUT3) is expressed in a subset of cholinergic and GABAergic neurons in the forebrain. In this study the distribution of VGLUT3 was mapped in relation to the receptor for substance P, neurokinin 1 (NK1), which has been independently reported within cholinergic and GABAergic neurons in a similar distribution. Dual immunofluorescence labeling techniques were used, sometimes in combination with triple labeling for the vesicular acetylcholine transporter (VAChT), to identify cholinergic cells. Virtually all cells immunolabeled for VGLUT3 in the nucleus accumbens core and shell regions, ventral pallidum, olfactory tubercle and caudate putamen were cholinergic and also contained immunolabeling for the NK1 receptor. In the hippocampal formation where VGLUT3 has been described in GABAergic neurons, colocalization between NK1 and VGLUT3 was also common but less complete. Cells double labeled for NK1 and VGLUT3 were most prevalent in stratum radiatum in the CA1 subfield. In the habenula VGLUT3 was also found within NK1 receptor immunolabeled neurons. However, there were some areas where neurons containing these two proteins were separate populations including the cerebral cortex and median raphe nucleus. These results reveal a trend for VGLUT3 to localize within neurons containing the NK1 receptor in several areas of the forebrain.