Activation of neurokinin-1 receptors promotes GABA release at synapses in the rat entorhinal cortex

Perspective role of Substance P in Amyotrophic Lateral Sclerosis: From neuronal vulnerability to neuroprotection

The neuropeptide Substance P (SP) and its preferred Neurokinin1 Receptor (NK1R) are known to participate in the physiopathology of neurodegenerative diseases and mainly exert a neuroprotective role. In the present work, we have described the involvement of SP and NK1R in Amyotrophic Lateral Sclerosis (ALS). This was demonstrated by the detection of altered levels of SP in the brain, spinal cord and cerebrospinal fluid (CSF) of patients and preclinical models of ALS, and by its ability to inhibit excitotoxicity-induced neurodegeneration in ALS animal models. These data are supported by results indicating an excitatory effect of SP at the motor neuron (MN) level, which promotes locomotor activity. ALS patients are characterized by a differential susceptibility to MNs degeneration, since sphincters and extraocular muscles are classically spared. It is hypothesized that SP may play a role in the maintenance of the ocular system and the innervation of the pelvic floor by contributing directly or indirectly to the selective resistance of this subset of MNs.

Honokiol for the Treatment of Neonatal Pain and Prevention of Consequent Neurobehavioral Disorders

This study examined the short- and long-term neuroprotective and analgesic activity of honokiol (a naturally occurring lignan isolated from Magnolia) on developing brains in neonates exposed to inflammatory pain, known to cause neuronal cell death. Postnatal day 4 (P4) neonatal rat pups were subjected to intraplantar formalin injection to four paws as a model of severe neonatal pain. Intraperitoneal honokiol (10 mg/kg) or corn oil vehicle control was administered 1 h prior to formalin insult, and animals were maintained on honokiol through postnatal day 21 (P21). Behavioral tests for stress and pain were performed after the painful insult, followed by morphological examinations of the brain sections at P7 and P21. Honokiol significantly attenuated acute pain responses 30 min following formalin insult and decreased chronic thermal hyperalgesia later in life. Honokiol-treated rats performed better on tests of exploratory behavior and performed significantly better in tests of memory. Honokiol treatment normalized hippocampal and thalamic c-Fos and hippocampal alveus substance P receptor expression relative to controls at P21. Together, these findings support that (1) neonatal pain experiences predispose rats to the development of chronic behavioral changes and (2) honokiol prevents and reduces both acute and chronic pathological pain-induced deteriorations in neonatal rats.

New insights into the regulation of synaptic plasticity from an unexpected place: hippocampal area CA2

The search for molecules that restrict synaptic plasticity in the brain has focused primarily on sensory systems during early postnatal development, as critical periods for inducing plasticity in sensory regions are easily defined. The recent discovery that Schaffer collateral inputs to hippocampal area CA2 do not readily support canonical activity-dependent long-term potentiation (LTP) serves as a reminder that the capacity for synaptic modification is also regulated anatomically across different brain regions. Hippocampal CA2 shares features with other similarly "LTP-resistant" brain areas in that many of the genes linked to synaptic function and the associated proteins known to restrict synaptic plasticity are expressed there. Add to this a rich complement of receptors and signaling molecules permissive for induction of atypical forms of synaptic potentiation, and area CA2 becomes an ideal model system for studying specific modulators of brain plasticity. Additionally, recent evidence suggests that hippocampal CA2 is instrumental for certain forms of learning, memory, and social behavior, but the links between CA2-enriched molecules and putative CA2-dependent behaviors are only just beginning to be made. In this review, we offer a detailed look at what is currently known about the synaptic plasticity in this important, yet largely overlooked component of the hippocampus and consider how the study of CA2 may provide clues to understanding the molecular signals critical to the modulation of synaptic function in different brain regions and across different stages of development.

Antagonism of L-type Ca(v) channels with nifedipine differentially affects performance of wildtype and NK1R-/- mice in the 5-Choice Serial Reaction-Time Task

Mice with functional ablation of the substance P-preferring receptor gene ('Nk1r' in mice ('NK1R-/-'), 'TACR1' in humans) display deficits in cognitive performance that resemble those seen in patients with Attention Deficit Hyperactivity Disorder (ADHD): namely, inattentiveness, impulsivity and perseveration. A recent report suggested that the L-type Ca(v) channel blocker, nifedipine, can ameliorate behavioral abnormalities of this type in humans. In light of evidence that NK1R antagonists modulate the opening of these L-type channels, we investigated whether nifedipine modifies %premature responses (impulsivity), perseveration or %omissions (inattentiveness) in the 5-Choice Serial Reaction-Time Task (5-CSRTT) and whether the response differs in NK1R-/- and wildtype mice. %Premature responses and perseveration were reduced in both genotypes, although wildtype mice were more sensitive to the effects of nifedipine than NK1R-/- mice. By contrast, nifedipine greatly increased %omissions but, again, was more potent in wildtypes. %Accuracy and locomotor activity were unaffected in either genotype. We infer that behavior of mice in the 5-CSRTT depends on the regulation of striato-cortical networks by L-type Ca(v) channels and NK1R. We further suggest that disruption of NK1R signaling in patients with ADHD, especially those with polymorphisms of the TACR1 gene, could lead to compensatory changes in the activity of L-type channels that underlie or exacerbate their problems. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Cortical nNOS neurons co-express the NK1 receptor and are depolarized by Substance P in multiple mammalian species

We have previously demonstrated that Type I neuronal nitric oxide synthase (nNOS)-expressing neurons are sleep-active in the cortex of mice, rats, and hamsters. These neurons are known to be GABAergic, to express Neuropeptide Y (NPY) and, in rats, to co-express the Substance P (SP) receptor NK1, suggesting a possible role for SP in sleep/wake regulation. To evaluate the degree of co-expression of nNOS and NK1 in the cortex among mammals, we used double immunofluorescence for nNOS and NK1 and determined the anatomical distribution in mouse, rat, and squirrel monkey cortex. Type I nNOS neurons co-expressed NK1 in all three species although the anatomical distribution within the cortex was species-specific. We then performed in vitro patch clamp recordings in cortical neurons in mouse and rat slices using the SP conjugate tetramethylrhodamine-SP (TMR-SP) to identify NK1-expressing cells and evaluated the effects of SP on these neurons. Bath application of SP (0.03–1 μM) resulted in a sustained increase in firing rate of these neurons; depolarization persisted in the presence of tetrodotoxin. These results suggest a conserved role for SP in the regulation of cortical sleep-active neurons in mammals.

Residual performance impairments in adult rats trained on an object discrimination task subsequent to cocaine administration during adolescence

The present study was conducted to determine whether cognitive impairments in adult rats treated with cocaine during adolescence demonstrated in previous investigations extend to tests of object discrimination learning. Accordingly, 30-day-old male Long-Evans rats were injected subcutaneously with either 10 or 20 mg/kg cocaine or received control injections of saline for 7-8 consecutive days. An extended abstinence period was then introduced (mean = 70.7 ± 9.8 days) before subjects, who were now young adults (mean = 106.3 ± 10.2 days old), were assessed for acquisition of a two-choice object discrimination task. Using a correctional learning procedure conducted in a water maze, subjects were trained (eight trials per day for 10 days) to approach one of two multi-dimentional 'junk' objects. Although all animals acquired the discrimination to a reasonable extent, cocaine-treated subjects exhibited lower percentages of correct choices over the course of training (10 mg/kg = 59.6 ± 7.2% and 20 mg/kg = 59.4 ± 4.9%) relative to the saline control group (67.5 ± 4.9%). Further analyses revealed that saline-treated subjects acquired proficient discrimination performance earlier during the course of training, achieving an approximate 72% performance rate after only 3 days of training. This was in contrast to the two cocaine-treated groups needing 7 days of training to achieve comparable levels of performance. In addition, saline-treated subjects required significantly fewer trials (20.8 ± 8.9) than either cocaine-treated group (10 mg/kg = 52.2 ± 11.9 and 20 mg/kg = 63.3 ± 8.7) to reach an 87.5% correct response criterion (i.e. 7-correct-out-of-8-consecutive-trials) and performed at a higher above-chance level (13.5%) than either cocaine-treated group (3.6% and 5.3% for the 10 and 20 mg/kg cocaine groups, respectively). These findings demonstrate the existence of cognitive impairments in adulthood subsequent to cocaine exposure during adolescence despite a prolonged drug-free interval. Speculation regarding the neurobiological basis for this effect, especially with regard to alterations to prefrontal circuitry, is provided.

Role of neurokinin receptors and ionic mechanisms within the respiratory network of the lamprey

We have suggested that in the lamprey, a medullary region called the paratrigeminal respiratory group (pTRG), is essential for respiratory rhythm generation and could correspond to the pre-Bötzinger complex (pre-BötC), the hypothesized kernel of the inspiratory rhythm-generating network in mammals. The present study was performed on in vitro brainstem preparations of adult lampreys to investigate whether some functional characteristics of the respiratory network are retained throughout evolution and to get further insights into the recent debated hypotheses on respiratory rhythmogenesis in mammals, such as for instance the "group-pacemaker" hypothesis. Thus, we tried to ascertain the presence and role of neurokinins (NKs) and burst-generating ion currents, such as the persistent Na(+) current (I(NaP)) and the Ca(2+)-activated non-specific cation current (I(CAN)), described in the pre-Bötzinger complex. Respiratory activity was monitored as vagal motor output. Substance P (SP) as well as NK1, NK2 and NK3 receptor agonists (400-800 nM) applied to the bath induced marked increases in respiratory frequency. Microinjections (0.5-1 nl) of SP as well as the other NK receptor agonists (1 microM) into the pTRG increased the frequency and amplitude of vagal bursts. Riluzole (RIL) and flufenamic acid (FFA) were used to block I(NaP) and I(CAN), respectively. Bath application of either RIL or FFA (20-50 microM) depressed, but did not suppress respiratory activity. Coapplication of RIL and FFA at 50 microM abolished the respiratory rhythm that, however, was restarted by SP microinjected into the pTRG. The results show that NKs may have a modulatory role in the lamprey respiratory network through an action on the pTRG and that I(NaP) and I(CAN) may contribute to vagal burst generation. We suggest that the "group-pacemaker" hypothesis is tenable for the lamprey respiratory rhythm generation since respiratory activity is abolished by blocking both I(NaP) and I(CAN), but is restored by enhancing network excitability.

Is subcortical-cortical midline activity in depression mediated by glutamate and GABA? A cross-species translational approach

Major depressive disorder has recently been characterized by abnormal resting state hyperactivity in anterior midline regions. The neurochemical mechanisms underlying resting state hyperactivity remain unclear. Since animal studies provide an opportunity to investigate subcortical regions and neurochemical mechanisms in more detail, we used a cross-species translational approach comparing a meta-analysis of human data to animal data on the functional anatomy and neurochemical modulation of resting state activity in depression. Animal and human data converged in showing resting state hyperactivity in various ventral midline regions. These were also characterized by abnormal concentrations of glutamate and gamma-aminobutyric acid (GABA) as well as by NMDA receptor up-regulation and AMPA and GABA receptor down-regulation. This cross-species translational investigation suggests that resting state hyperactivity in depression occurs in subcortical and cortical midline regions and is mediated by glutamate and GABA metabolism. This provides insight into the biochemical underpinnings of resting state activity in both depressed and healthy subjects.

Neurokinin1 antagonists potentiate antidepressant properties of serotonin reuptake inhibitors, yet blunt their anxiogenic actions: a neurochemical, electrophysiological, and behavioral characterization

Though neurokinin(1) (NK(1)) receptor antagonists are active in experimental models of depression, clinical efficacy has proven disappointing. This encourages interest in association of NK(1) receptor blockade with inhibition of serotonin (5-HT) reuptake. The selective NK(1) antagonist, GR205171, dose-dependently enhanced citalopram-induced elevations of extracellular levels of 5-HT in frontal cortex, an action expressed stereospecifically vs its less active distomer, GR226206. Further, increases in 5-HT levels in dorsal hippocampus, basolateral amygdala, nucleus accumbens, and striatum were likewise potentiated, and GR205171 similarly facilitated the influence of fluoxetine upon levels of 5-HT, as well as dopamine and noradrenaline. In parallel electrophysiological studies, the inhibitory influence of citalopram and fluoxetine upon raphe-localized serotonergic neurones was stereospecifically blunted by GR205171. Antidepressant actions of citalopram in a forced-swim test in mice were stereospecifically potentiated by GR205171, and it also enhanced attenuation by citalopram of stress-related ultrasonic vocalizations in rats. Further, GR205171 and citalopram additively abrogated the advance in circadian rhythms provoked by exposure to light in hamsters. By contrast, GR205171 stereospecifically blocked anxiogenic actions of citalopram in social interaction procedures in rats and gerbils, and stereospecifically abolished facilitation of fear-induced foot tapping by fluoxetine in gerbils. By analogy to GR205171, a further NK(1) antagonist, RP67580, enhanced the influence of citalopram upon frontocortical levels of 5-HT and potentiated its actions in the forced swim test. In conclusion, NK(1)receptor blockade differentially modulates functional actions of SSRIs: antidepressant properties are reinforced, whereas anxiogenic effects are attenuated. Combined NK(1) receptor antagonism/5-HT reuptake inhibition may offer advantages in the management of depressed and anxious states.

Central mechanisms I: plasticity of central pathways

Cough is the most common symptom for which individuals seek medical attention and spend health-care dollars. Despite the burden induced by cough, the current treatments for cough are only partially effective. Delineating the sites and mechanisms in the cough central network for changes in the cough reflex could lead to new therapeutic strategies and drug target sites for more effective treatments. The first synaptic target in the CNS for the cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS); these neurons reorganize the primary sensory information into a coherent output. The NTS neurons have been shown to undergo neuroplasticity under a variety of conditions, such as respiratory disorders, stress, and exposures to environmental pollutants. The NTS contains a rich innervation of substance P immunoreactive nerve terminals, suggesting that substance P might be important in altered cough reflex response. This chapter summarizes our current findings on the role of substance P in enhanced cough reflex as well as the potential NTS targets for the action of substance P.

Neuropeptides in depression: role of VGF

The monoamine hypothesis of depression is increasingly called into question by newer theories that revolve around changes in neuronal plasticity, primarily in the hippocampus, at both the structural and the functional levels. Chronic stress negatively regulates hippocampal function while antidepressants ameliorate the effects of stress on neuronal morphology and activity. Both stress and antidepressants have been shown to affect levels of brain-derived neurotrophic factor (BDNF) whose transcription is dependent on cAMP response element binding protein (CREB). BDNF itself has antidepressant-like actions and can induce transcription of a number of molecules. One class of genes regulated by both BDNF and serotonin (5-HT) are neuropeptides including VGF (non-acryonimic) which has a novel role in depression. Neuropeptides are important modulators of neuronal function but their role in affective disorders is just emerging. Recent studies demonstrate that VGF, which is also a CREB-dependent gene, is upregulated by antidepressant drugs and voluntary exercise and is reduced in animal models of depression. VGF enhances hippocampal synaptic plasticity as well as neurogenesis in the dentate gyrus but the mechanisms of antidepressant-like actions of VGF in behavioral paradigms are not known. We summarize experimental data describing the roles of BDNF, VGF and other neuropeptides in depression and how they may be acting through the generation of new neurons and altered synaptic activity. Understanding the molecular and cellular changes that underlie the actions of neuropeptides and how these adaptations result in antidepressant-like effects will aid in developing drugs that target novel pathways for major depressive disorders.

An immunohistochemical study on a unique colocalization relationship between substance P and GABA in the central nucleus of amygdala

Substance P (SP) is a neuropeptide contained in axon terminals. Various classical neurotransmitters coexist with SP in mammalian brains, but there has been no information on the colocalizing substances in the central nucleus of amygdala (CeA), where both SP and its specific receptor are highly concentrated. The present study aimed at determining the colocalizing neurotransmitter in SP terminals in CeA by multi-label immunohistochemistry combined with digitized quantitative analysis. Unexpectedly, most of SP-containing boutons did not show immunoreactivities for any of the transmitters or their marker proteins examined (GABA, glycine, glutamate, acetylcholine, serotonin, or dopamine). Electron microscopy demonstrated small clear vesicles in addition to dense core vesicles within SP-positive terminals that formed symmetrical synapses, indicating the presence of some classical neurotransmitter, most likely GABA. Therefore tissues were fixed by zinc-aldehyde to enhance immunoreactivity for a low level of glutamic acid decarboxylase (GAD), the GABA synthetic enzyme. This led to weak but consistent labeling for GAD in the majority of SP-positive boutons in CeA. By contrast, definite GAD-immunoreactivity was confirmed in SP-containing boutons in the substantia nigra pars reticulata even in specimens treated with a conventional fixative, indicating that negligible GAD labeling in CeA is not ascribed to methodological problems such as interference by the presence of SP but actually reflects low GAD content. These data suggest a unique mode of synaptic transmission at amygdalar SP-containing terminals where slowly-acting SP is concentrated but both GABA and its synthetic enzyme are maintained at low levels, possibly underlying long-lasting responses in emotions.

DeltaFosB induction in orbitofrontal cortex mediates tolerance to cocaine-induced cognitive dysfunction

Current cocaine users show little evidence of cognitive impairment and may perform better when using cocaine, yet withdrawal from prolonged cocaine use unmasks dramatic cognitive deficits. It has been suggested that such impairments arise in part through drug-induced dysfunction within the orbitofrontal cortex (OFC), yet the neurobiological mechanisms remain unknown. We observed that chronic cocaine self-administration increased expression of the transcription factor deltaFosB within both medial and orbitofrontal regions of the rat prefrontal cortex. However, the increase in OFC deltaFosB levels was more pronounced after self-administered rather than experimenter-administered cocaine, a pattern that was not observed in other regions. We then used rodent tests of attention and decision making to determine whether deltaFosB within the OFC contributes to drug-induced alterations in cognition. Chronic cocaine treatment produced tolerance to the cognitive impairments caused by acute cocaine. Overexpression of a dominant-negative antagonist of deltaFosB, deltaJunD, in the OFC prevented this behavioral adaptation, whereas locally overexpressing deltaFosB mimicked the effects of chronic cocaine. Gene microarray analyses identified potential molecular mechanisms underlying this behavioral change, including an increase in transcription of metabotropic glutamate receptor subunit 5 and GABA(A) receptors as well as substance P. Identification of deltaFosB in the OFC as a mediator of tolerance to the effects of cocaine on cognition provides fundamentally new insight into the transcriptional modifications associated with addiction.

Immunohistochemical characterization of substance P receptor (NK(1)R)-expressing interneurons in the entorhinal cortex

It has been reported that application of substance P (SP) to the medial portion of the entorhinal cortex (EC) induces a powerful antiepileptic effect (Maubach et al. [1998] Neuroscience 83:1047-1062). This effect is presumably mediated via inhibitory interneurons expressing the neurokinin-1 receptor (NK(1)R), but the existence of NK(1)R-expressing inhibitory interneurons in the EC has not yet been reported. The present immunohistochemical study was performed in the rat to examine the existence and distribution of NK(1)R-expressing neurons in the EC as well as any co-expression of other neurotransmitters/neuromodulators known to be associated with inhibitory interneurons: gamma-aminobutyric acid (GABA), parvalbumin (PARV), calretinin (CT), calbindin (CB), somatostatin (SST), and neuropeptide Y (NPY). Our results indicated that NK(1)R-positive neurons were distributed rather sparsely (especially in the medial EC), primarily in layers II, V, and VI. The results of our double-immunohistochemical staining indicated that the vast majority of NK(1)R-expressing neurons also expressed GABA, SST, and NPY. In addition, CT was co-expressed in a weakly stained subgroup of NK(1)R-expressing neurons, and CB was co-expressed very rarely in the lateral EC, but not in the medial EC. In contrast, SP-immunopositive axons with fine varicosities were distributed diffusely throughout all layers of the EC, appearing to radiate from the angular bundle. SP may be released in a paracrine manner to activate a group of NK(1)R-expressing entorhinal neurons that co-express GABA, SST, and NPY, exerting a profound inhibitory influence on synchronized network activity in the EC.

Morphology and synaptic input of substance P receptor-immunoreactive interneurons in control and epileptic human hippocampus

Substance P (SP) is known to be a peptide that facilitates epileptic activity of principal cells in the hippocampus. Paradoxically, in other models, it was found to be protective against seizures by activating substance P receptor (SPR)-expressing interneurons. Thus, these cells appear to play an important role in the generation and regulation of epileptic seizures. The number, distribution, morphological features and input characteristics of SPR-immunoreactive cells were analyzed in surgically removed hippocampi of 28 temporal lobe epileptic patients and eight control hippocampi in order to examine their changes in epileptic tissues. SPR is expressed in a subset of inhibitory cells in the control human hippocampus, they are multipolar interneurons with smooth dendrites, present in all hippocampal subfields. This cell population is considerably different from SPR-positive cells of the rat hippocampus. The CA1 (cornu Ammonis subfield 1) region was chosen for the detailed morphological analysis of the SPR-immunoreactive cells because of its extreme vulnerability in epilepsy. The presence of various neurochemical markers identifies functionally distinct interneuron types, such as those responsible for perisomatic, dendritic or interneuron-selective inhibition. We found considerable colocalization of SPR with calbindin but not with parvalbumin, calretinin, cholecystokinin and somatostatin, therefore we suppose that SPR-positive cells participate mainly in dendritic inhibition. In the non-sclerotic CA1 region they are mainly preserved, whereas their number is decreased in the sclerotic cases. In the epileptic samples their morphology is considerably altered, they possessed more dendritic branches, which often became beaded. Analyses of synaptic coverage revealed that the ratio of symmetric synaptic input of SPR-immunoreactive cells has increased in epileptic samples. Our results suggest that SPR-positive cells are preserved while principal cells are present in the CA1 region, but show reactive changes in epilepsy including intense branching and growth of their dendritic arborization.

NK1 receptor antagonists under investigation for the treatment of affective disorders

Substance P-neurokinin-1 (NK1) receptor pathways have been repeatedly implicated in the pathophysiology of affective disorders. Anatomical studies in humans have shown a high expression of NK1 receptors in brain regions that are important for the regulation of affective behaviours and stress responses. A large body of evidence that has been generated from animal experiments indicates that treatment with a selective NK1 receptor antagonist might be effective in the treatment of certain forms of anxiety and depressive disorders. Accordingly, numerous NK1 receptor antagonists have either been synthesised and are under clinical development, or have already been tested in clinical trials. However, the initial encouraging clinical results were followed by repeated demonstrations of a lack of effectiveness, thus disappointment and doubt currently surrounds the idea that these compounds may become effective antidepressants. Research continues and novel molecules may show better pharmacokinetic and pharmacodynamic properties and, therefore, may achieve therapeutic success. Furthermore, NK1 receptor antagonists that are ineffective in the treatment of mood disorders may still prove to be effective in the treatment of anxiety problems.

Stereoselective and region-specific induction of immediate early gene expression in rat parietal cortex by blockade of neurokinin 1 receptors

Antagonists at neurokinin 1 (NK1) receptors are attracting attention as potential treatments for depressive states in light of their actions in behavioural models predictive of antidepressant properties, their modulation of corticolimbic monoaminergic transmission, and their influence upon neural plasticity. Here, we evaluated the influence of NK1 receptor blockade upon two immediate early genes, Arc and c-fos, implicated in mechanisms of synaptic plasticity. Administration of the selective NK1 receptor antagonist, GR 205,171 (40, but not 1, 5 or 10 mg/kg i.p.), elicited a pronounced elevation in mRNA encoding Arc in both outer and inner layers of the parietal cortex of rat brain. This action was region-specific inasmuch as Arc expression did not change in other cortical territories examined including frontal cortex, nor in CA1, CA3 and the dentate gyrus of the hippocampus. In comparison to GR 205,171, its less active isomer GR 226,206 (1-40 mg/kg) did not significantly modify Arc gene expression in parietal cortex or other cortical areas. GR 205,171 (40 mg/kg) also increased the abundance of c-fos mRNA in outer and inner parietal cortex and caused a corresponding increase in c-fos immunoreactivity in this region. GR 226,206 (40 mg/kg i.p.) had no effect on either c-fos mRNA or protein in parietal cortex. In conclusion, administration of GR 205,171 elicits a stereospecific increase in Arc and c-fos expression in rat parietal cortex but not in other cortical regions. These data suggest that the parietal cortex plays a role in the central actions of NK1 receptor antagonists.

Neurokinin-1 receptor antagonism by SR140333: enhanced in vivo ACh in the hippocampus and promnestic post-trial effects

Substance P (SP) has memory-promoting, reinforcing and anxiolytic-like effects when applied systemically or centrally. Such effects may be mediated by the neurokinin-1 (NK-1) receptor, since SP preferentially binds to this receptor. We measured the effects of a selective non-peptide NK-1 receptor antagonist, SR140333 (1, 3 and 9 mg/kg i.p.) on ACh levels in frontal cortex, amygdala and hippocampus by microdialysis and HPLC. Levels of ACh in the hippocampus increased dose-dependently immediately after treatment. The same doses of SR140333 given post-trial had minor facilitative effects on inhibitory avoidance learning and open-field habituation, but did not have reinforcing effects in a conditioned place preference (CPP) task. The selective action of NK-1 receptor antagonism on hippocampal ACh may be related to its positive influence on learning.

NK1 receptor activation leads to enhancement of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Substance P (SP) is a well-established pain messenger in the spinal cord, although its role in substantia gelatinosa (lamina II) still remains elusive. We carried out patch-clamp recordings on lamina II neurons from transverse mouse spinal cord slices (P8-12), using the selective NK1 receptor agonist [Sar9,Met(O2)11]-SP (SM-SP, 3-5 microM) in the presence of NBQX. Activation of NK1 receptors was confirmed after pre-incubation with selective NK1 antagonist L732,138 (4 microM) that consistently blocked the effects of SM-SP (nine neurons). After SM-SP challenge and spontaneous inhibitory post-synaptic current (sIPSC) analysis, 50% of recorded neurons (15 out of 30) were found to display a transient increase in frequency; in five neurons this was also associated with increase of peak amplitude. Five out of eight neurons displayed pure GABAA microM) receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After miniature IPSC analysis, a significant increase in frequency was observed in three out of 14 SM-SP responsive neurons. At least four different morphological types were apparent among NK1-responsive neurons after filling with Lucifer Yellow/biocytin: fusiform with dorso-ventral dendritic arbors (i); round-to-oval with dendritic arborization mainly directed to lamina I (ii) or III (iii), and round-to-oval with dendrites sparsely distributed all around the cell body (iv). Thus, there was no correlation between morphology and electrophysiological properties of responsive neurons. Our observations provide new insights on the processing of sensory neurotransmission in spinal cord, and indicate that activation of NK1 receptors is involved in the maintenance of the inhibitory tone of substantia gelatinosa interneurons.

NK-3 receptor activation depolarizes and induces an after-depolarization in pyramidal neurons in gerbil cingulate cortex

The involvement of tachykinins in cortical function is poorly understood. To study the actions of neurokinin-3 (NK3) receptor activation in frontal cortex, whole cell patch clamp recordings were performed from pyramidal neurons in slices of cingulate cortex from juvenile gerbils. Senktide (500nM), a selective NK3 receptor agonist, induced a transient increase in spontaneous EPSPs in layer V pyramidal neurons, accompanied by a small depolarization ( approximately 4 mV). EPSPs during senktide had a larger amplitude and faster 10-90% rise time than during control. Senktide induced a transient depolarization in layer II/III pyramidal neurons, which often reached threshold for spikes. The depolarization ( approximately 6 mV) persisted in TTX, and was accompanied by an increase in input resistance. Senktide also transiently induced a slow after-depolarization, which appeared following a depolarizing pulse. The slow after-depolarization persisted in TTX. These data suggest that activation of NK3 receptors on layer II/III pyramidal neurons induce post-synaptic depolarization and an after-depolarization, which could be mediated by blockade of a leak potassium conductance and a non-selective cation conductance, respectively.

Neurokinin-1 receptors in the rat nucleus tractus solitarius: pre- and postsynaptic modulation of glutamate and GABA release

Neurokinins such as substance P and neurokinin A have long been thought to act as neurotransmitters or modulators in the nucleus tractus solitarius. However, the role and location of the receptors for these peptides have remained unclear. We examined the consequences of activation of the neurokinin-1 (NK1) receptor subtype in the rat nucleus tractus solitarius using whole-cell patch clamp recordings in brain slices. Application of delta-Ala-Phe-Phe-Pro-MeLeu-D-Pro[spiro-gamma-lactam]-Leu-Trp-NH2 (a specific NK1 agonist) or neurokinin A resulted in depolarization, evident as a slow inward current, mediated by direct postsynaptic NK1 receptor activation. The effect was conserved in the presence of tetrodotoxin, and protein kinase C-dependent since it was blocked by 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide, a specific protein kinase C inhibitor. In addition, an increase in the frequency and amplitude of spontaneous excitatory postsynaptic currents was observed, reflecting increased glutamate release induced by NK1 receptor activation. This effect was abolished by tetrodotoxin, suggesting that it resulted from increased firing in afferent neurons, subsequent to somatodendritic excitation via NK1 receptors. Furthermore, spontaneous inhibitory postsynaptic currents were increased in frequency and amplitude showing that GABA release was promoted by NK1 receptor activation. However, amplitude of miniature inhibitory postsynaptic currents was unaltered by NK1 receptor activation, but the increase in frequency persisted. These findings suggest that NK1 receptors are located on presynaptic terminals as well as at somatodendritic sites of GABAergic neurons. The increase in GABA release was also shown to be protein kinase C-dependent. The data presented here show NK1 receptors in the rat nucleus tractus solitarius are present both excitatory and inhibitory neurons. Activation of these receptors can result in increases in release of both GABA and glutamate, suggesting a crucial modulatory role for NK1 receptors in the rat nucleus tractus solitarius.

Action of tachykinins in the rat hippocampus: modulation of inhibitory synaptic transmission

Substance P and other neuropeptides of the tachykinin family can powerfully excite CA1 hippocampal interneurons present in the CA1 region. In the present work we show that, by exciting hippocampal interneurons, tachykinins can indirectly inhibit pyramidal neurons. We found that tachykinins caused a decrease in the inhibitory synaptic current interval and an increase in the inhibitory synaptic current amplitude in almost all pyramidal neurons tested. This effect was tetrodotoxin sensitive. Tachykinins did not alter the frequency or amplitude of miniature inhibitory synaptic currents and were without effect on evoked inhibitory synaptic currents. Thus, these neuropeptides acted at the somatodendritic membrane of GABAergic interneurons, rather than at their axon terminals. The effect of substance P on spontaneous inhibitory synaptic currents could be mimicked by a selective agonist of NK1 receptors, but not by selective agonists of NK2 and NK3 receptors. It was suppressed by an NK1 receptor antagonist. In CA1 interneurons located in stratum radiatum, substance P generated a sustained tetrodotoxin-insensitive inward current or induced membrane depolarization and action potential firing. This direct excitatory action was mediated by NK1 receptors. Current-voltage relationships indicate that the net tachykinin-evoked current reversed in polarity at or near the K+ equilibrium potential, suggesting that a suppression of a resting K+ conductance was involved. By increasing the excitability of CA1 GABAergic interneurons, tachykinins can powerfully facilitate the inhibitory synaptic input to pyramidal neurons. This indirect inhibition could play a role in regulating short-term and/or long-term synaptic plasticity, promoting neuronal circuit synchronization or, in some physiopathological situations, influencing epileptogenesis.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Neurokinin-receptor-mediated depolarization of cortical neurons elicits an increase in glutamate release at excitatory synapses

Using whole-cell patch-clamp recordings of spontaneous synaptic activity, we have previously shown that activation of neurokinin-1 (NK1) but not NK3 receptors leads to increased GABA release onto principal cells in the rat entorhinal cortex. In the present study, we examine the effect of activation of these receptors on spontaneous excitatory synaptic responses mediated by glutamate. Both neurokinin B (NKB) and the specific NK3 receptor agonist, senktide, increased the spontaneous release of glutamate, and a similar effect was also seen with substance P (SP) and other NK1 receptor agonists. The increased release induced by either SP or senktide was absent in the presence of tetrodotoxin, demonstrating that it was likely to occur via activation of presynaptic excitatory neurons. Current-clamp recordings confirmed that principal neurons were depolarized by both NK3 and NK1 agonists. However, the response to the former but not the latter persisted in tetrodotoxin, allowing us to conclude that NK3 receptor activation provoked glutamate release via recurrent collaterals between principal neurons, whereas the NK1 receptors may be localized to excitatory interneurons. Finally, the increased release induced by senktide, but not SP, was reduced by an antagonist of group III metabotropic glutamate receptors. Thus, glutamate release from recurrent collaterals is facilitated by a presynaptic group III autoreceptor [Evans, D.I.P., Jones, R.S.G. & Woodhall, G.L. (2000) J. Neurophysiol.,83, 2519-2525], whereas the terminals of neurons responsible for the NK1-receptor induced glutamate release may not bear these receptors. These results have implications for control of activity and epileptogenesis in cortical networks.