Cellular basis for the effects of substance P in the periaqueductal gray and dorsal raphe nucleus

Enhanced responsiveness to hypoxic panicogenic challenge in female rats in late diestrus is suppressed by short-term, low-dose fluoxetine: Involvement of the dorsal raphe nucleus and the dorsal periaqueductal gray

BACKGROUND

Acute hypoxia, which is panicogenic in humans, also evokes panic-like behavior in male rats. Panic disorder is more common in women and susceptibility increases during the premenstrual phase of the cycle.

AIMS

We here investigated for the first time the impact of hypoxia on the expression of panic-like escape behavior by female rats and its relationship with the estrous cycle. We also evaluated functional activation of the midbrain panic circuitry in response to this panicogenic stimulus and whether short-term, low-dose fluoxetine treatment inhibits the hyper-responsiveness of females in late diestrus.

METHODS

Male and female Sprague Dawley rats were exposed to 7% O₂. Females in late diestrus were also tested after short-term treatment with fluoxetine (1.75 or 10 mg/kg, i.p.). Brains were harvested and processed for c-Fos and tryptophan hydroxylase immunoreactivity in the periaqueductal gray matter (PAG) and dorsal raphe nucleus (DR).

RESULTS

Acute hypoxia evoked escape in both sexes. Overall, females were more responsive than males and this is clearer in late diestrus phase. In both sexes, hypoxia induced functional activation (c-Fos expression) in non-serotonergic cells in the lateral wings of the DR and dorsomedial PAG, which was greater in late diestrus than proestrus (lowest behavioral response to hypoxia). Increased responding in late diestrus (behavioral and cellular levels) was prevented by 1.75, but not 10 mg/kg fluoxetine.

DISCUSSION

The response of female rats to acute hypoxia models panic behavior in women. Low-dose fluoxetine administered in the premenstrual phase deserves further attention for management of panic disorders in women.

Brain neurokinin-1 receptor availability in never-medicated patients with major depression - A pilot study

BACKGROUND

Neurotransmitter substance P (SP) and its preferred neurokinin-1 receptor (NK1R) have been implicated in the treatment of affective and addiction disorders. Despite promising preclinical data on antidepressant action, the clinical trials of NK1R antagonists in major depression have been disappointing. There are no direct in vivo imaging studies on NK1R characteristics in patients with a major depressive disorder (MDD).

METHODS

In this cross-sectional case-control study, we recruited nine never-medicated patients with moderate to severe MDD and nine matched healthy controls. NK1R availability (NK1R binding potential, BP_ND) was measured with in vivo 3-D positron emission tomography and a specific NK1 receptor tracer [¹⁸F]SPA-RQ. Clinical symptoms were assessed with the 17-item Hamilton Rating Scale for Depression (HAM-D17).

RESULTS

NK1R-BP_ND did not differ statistically significantly between patients with MDD and healthy controls. HAM-D17 total scores (range 21-32) correlated positively with NK1R-BP_ND in cortical and limbic areas. HAM-D17 subscale score for anxiety symptoms correlated positively with NK1R-BP_ND in specific brain areas implicated in fear and anxiety.

LIMITATIONS

Small sample size. Low variability in the clinical HAM-D subscale ratings may affect the observed correlations.

CONCLUSIONS

Our preliminary results do not support a different baseline expression of NK1Rs in a representative sample of never-medicated patients with MDD during a current moderate/severe depressive episode. The modulatory effect of NK1Rs on affective symptoms is in line with early positive results on antidepressant action of NK1 antagonists. However, the effect is likely to be too weak for treatment of MDD with NK1R antagonists alone in clinical practice.

Activation of orexin-1 receptors in the ventrolateral periaqueductal grey matter (vlPAG) modulates pulpal nociception and the induction of substance P in vlPAG and trigeminal nucleus caudalis

AIM

To characterize the role of orexin-1 receptors (OX1Rs) in ventrolateral periaqueductal grey matter (vlPAG) on modulation of capsaicin-induced pulpal nociception in rats.

METHODOLOGY

Sixty-six adult male Wistar rats (2 months old) weighing between 230 and 260 g were used. The animals were cannulated for microinjection of drugs into the vlPAG matter. Pulpalgia was induced by intradental application of capsaicin solution (100 μg) into the incisor teeth of the rats. Ten min prior to capsaicin application, orexin-A (50, 100 and 150 pmol L^-1 per rat) was administered. Orexin-A (150 pmol L^-1 ) was also co-administrated with SB-334867 (40 nmol L^-1 per rat), an OX1Rs antagonist; or bicuculline (1 μg per rat), a GABAA receptors antagonist. Moreover, treatment effects on the release of pro-nociceptive modulator substance P (SP) in vlPAG and trigeminal nucleus caudalis (Vc) of rats were explored using an immunofluorescence technique. One-way analysis of variance was used for the statistical analysis.

RESULTS

Orexin-A dose-dependently decreased capsaicin-induced nociceptive behaviour. However, SB-334867 (40 nmol L^-1 per rat) pretreatment (P < 0.05), but not bicuculline (1 μg per rat), attenuated the analgesic effect of orexin-A (150 pmol L^-1 ). The level of SP was significantly increased in Vc and decreased in vlPAG of capsaicin-treated rats (P < 0.05). Capsaicin-induced changes in SP levels, however, were prohibited by orexin-A treatment (150 pmol L^-1 ) (P < 0.05).

CONCLUSIONS

Orexin-A administration into the vlPAG was associated with an inhibitory effect on capsaicin-induced pulpal nociception and bidirectional effects on the induction of SP in vlPAG and Vc of rats. Central activation of OX1Rs is a potential therapeutic tool for pulpalgia.

The potential role of substance P in brainstem homeostatic control in the pathogenesis of sudden infant death syndrome (SIDS)

Victims of sudden infant death syndrome (SIDS) are believed to have an underlying dysfunction in medullary homeostatic control that impairs critical responses to life threatening challenges such as hypoxia, hypercarbia and asphyxia, often during a sleep period. This failure is thought to result from abnormalities in a network of neural pathways in the medulla oblongata that control respiration, chemosensitivity, autonomic function and arousal. Studies have mainly focused on the role of serotonin, 5-hydroxytyptamine (5HT), although the neuropeptide substance P (SP) has also been shown to play an integral role in the modulation of medullary homeostatic function, often in conjunction with 5-HT. Actions of SP include regulation of respiratory rhythm generation, integration of cardiovascular control, modulation of the baroreceptor reflex and mediation of the chemoreceptor reflex in response to hypoxia. Abnormalities in SP neurotransmission may, therefore, also play a significant role in homeostatic dysfunction of the neurotransmitter network in SIDS. This review focuses on the pathways within the medulla involving SP and its tachykinin NK1 receptor, their potential relationship with the medullary 5-HT system, and possible involvement in the pathogenesis of SIDS.

Cognitive disruptions in stress-related psychiatric disorders: A role for corticotropin releasing factor (CRF)

This article is part of a Special Issue "SBN 2014". Stress is a potential etiology contributor to both post-traumatic stress disorders (PTSD) and major depression. One stress-related neuropeptide that is hypersecreted in these disorders is corticotropin releasing factor (CRF). Dysregulation of CRF has long been linked to the emotion and mood symptoms that characterize PTSD and depression. However, the idea that CRF also mediates the cognitive disruptions observed in patients with these disorders has received less attention. Here we review literature indicating that CRF can alter cognitive functions. Detailed are anatomical studies revealing that CRF is poised to modulate regions required for learning and memory. We also describe preclinical behavioral studies that demonstrate CRF's ability to alter fear conditioning, impair memory consolidation, and alter a number of executive functions, including attention and cognitive flexibility. The implications of these findings for the etiology and treatment of the cognitive impairments observed in stress-related psychiatric disorders are described.

Increased neuronal expression of neurokinin-1 receptor and stimulus-evoked internalization of the receptor in the rostral ventromedial medulla of the rat after peripheral inflammatory injury

This study examined possible mechanisms by which Substance P (Sub P) assumes a pronociceptive role in the rostral ventromedial medulla (RVM) under conditions of peripheral inflammatory injury, in this case produced by intraplantar (ipl) injection of complete Freund's adjuvant (CFA). In saline- and CFA-treated rats, neurokinin-1 receptor (NK1R) immunoreactivity was localized to neurons in the RVM. Four days after ipl injection of CFA, the number of NK1R-immunoreactive neurons in the RVM was increased by 30%, and there was a concomitant increase in NK1R-immunoreactive processes in CFA-treated rats. Although NK1R immunoreactivity was increased, tachykinin-1 receptor (Tacr1) mRNA was not increased in the RVM of CFA-treated rats. To assess changes in Sub P release, the number of RVM neurons that exhibited NK1R internalization was examined in saline- and CFA-treated rats following noxious heat stimulation of the hind paws. Only CFA-treated rats that experienced noxious heat stimulation exhibited a significant increase in the number of neurons showing NK1R internalization. These data suggest that tonic Sub P release is not increased as a simple consequence of peripheral inflammation, but that phasic or evoked release of Sub P in the RVM is increased in response to noxious peripheral stimulation in a persistent inflammatory state. These data support the proposal that an upregulation of the NK1R in the RVM, as well as enhanced release of Sub P following noxious stimulation, underlie the pronociceptive role of Sub P under conditions of persistent inflammatory injury.

The role of lateral habenula-dorsal raphe nucleus circuits in higher brain functions and psychiatric illness

Serotonergic neurons in the dorsal raphe nucleus (DRN) play an important role in regulation of many physiological functions. The lateral nucleus of the habenular complex (LHb) is closely connected to the DRN both morphologically and functionally. The LHb is a key regulator of the activity of DRN serotonergic neurons, and it also receives reciprocal input from the DRN. The LHb is also a major way-station that receives limbic system input via the stria medullaris and provides output to the DRN and thereby indirectly connects a number of other brain regions to the DRN. The complex interactions of the LHb and DRN contribute to the regulation of numerous important behavioral and physiological mechanisms, including those regulating cognition, reward, pain sensitivity and patterns of sleep and waking. Disruption of these functions is characteristic of major psychiatric illnesses, so there has been a great deal of interest in how disturbed LHb-DRN interactions may contribute to the symptoms of these illnesses. This review summarizes recent research related to the roles of the LHb-DRN system in regulation of higher brain functions and the possible role of disturbed LHb-DRN function in the pathogenesis of psychiatric disorders, especially depression.

Functional topography of serotonergic systems supports the Deakin/Graeff hypothesis of anxiety and affective disorders

Over 20 years ago, Deakin and Graeff hypothesized about the role of different serotonergic pathways in controlling the behavioral and physiologic responses to aversive stimuli, and how compromise of these pathways could lead to specific symptoms of anxiety and affective disorders. A growing body of evidence suggests these serotonergic pathways arise from topographically organized subpopulations of serotonergic neurons located in the dorsal and median raphe nuclei. We argue that serotonergic neurons in the dorsal/caudal parts of the dorsal raphe nucleus project to forebrain limbic regions involved in stress/conflict anxiety-related processes, which may be relevant for anxiety and affective disorders. Serotonergic neurons in the "lateral wings" of the dorsal raphe nucleus provide inhibitory control over structures controlling fight-or-flight responses. Dysfunction of this pathway could be relevant for panic disorder. Finally, serotonergic neurons in the median raphe nucleus, and the developmentally and functionally-related interfascicular part of the dorsal raphe nucleus, give rise to forebrain limbic projections that are involved in tolerance and coping with aversive stimuli, which could be important for affective disorders like depression. Elucidating the mechanisms through which stress activates these topographically and functionally distinct serotonergic pathways, and how dysfunction of these pathways leads to symptoms of neuropsychiatric disorders, may lead to the development of novel approaches to both the prevention and treatment of anxiety and affective disorders.

Substance P receptor antagonist in lateral habenula improves rat depression-like behavior

Substance P (SP) levels are closely related with the pathogenesis of depression. Recent work has focused on antidepressive effect of substance P receptor antagonist (SPA), however, its action site and mechanism remain largely unresolved. Our previous results showed that the lateral habenula (LHb) plays a key role in the pathogenesis of depression. The current study investigated the effects of SPA microinjected into LHb on the behavioral responses of two rat models that exhibit depression-like behavior. To produce adult rats that exhibit depression-like behavior, rats were either exposed to chronic mild stress (CMS), or chronically administered clomipramine (CLI), a tricyclic antidepressant, during the neonatal state of life. The forced-swimming test (FST) was used to evaluate behavioral responses. Furthermore, we measured serotonin (5-HT) levels in dorsal raphe nucleus (DRN) using microdialysis. The FST showed a decreased immobility time and an increased climbing time after SPA injection into the LHb of depression-like behavior rats. In addition, 5-HT levels in DRN increased after SPA was microinjected into LHb of the rats that exhibited depression-like behavior. This study demonstrates that LHb mediates antidepressive effect of SPA by increasing 5-HT levels in the DRN, suggesting that the LHb may be a potential target of antidepressant.

Sex differences in stress-related receptors: ″micro″ differences with ″macro″ implications for mood and anxiety disorders

Stress-related psychiatric disorders, such as unipolar depression and post-traumatic stress disorder (PTSD), occur more frequently in women than in men. Emerging research suggests that sex differences in receptors for the stress hormones, corticotropin releasing factor (CRF) and glucocorticoids, contribute to this disparity. For example, sex differences in CRF receptor binding in the amygdala of rats may predispose females to greater anxiety following stressful events. Additionally, sex differences in CRF receptor signaling and trafficking in the locus coeruleus arousal center combine to make females more sensitive to low levels of CRF, and less adaptable to high levels. These receptor differences in females could lead to hyperarousal, a dysregulated state associated with symptoms of depression and PTSD. Similar to the sex differences observed in CRF receptors, sex differences in glucocorticoid receptor (GR) function also appear to make females more susceptible to dysregulation after a stressful event. Following hypothalamic pituitary adrenal axis activation, GRs are critical to the negative feedback process that inhibits additional glucocorticoid release. Compared to males, female rats have fewer GRs and impaired GR translocation following chronic adolescent stress, effects linked to slower glucocorticoid negative feedback. Thus, under conditions of chronic stress, attenuated negative feedback in females would result in hypercortisolemia, an endocrine state thought to cause depression. Together, these studies suggest that sex differences in stress-related receptors shift females more easily into a dysregulated state of stress reactivity, linked to the development of mood and anxiety disorders. The implications of these receptor sex differences for the development of novel pharmacotherapies are also discussed.

Development of the serotonergic cells in murine raphe nuclei and their relations with rhombomeric domains

The raphe nuclei represent the origin of central serotonergic projections. The literature distinguishes seven nuclei grouped into rostral and caudal clusters relative to the pons. The boundaries of these nuclei have not been defined precisely enough, particularly with regard to developmental units, notably hindbrain rhombomeres. We hold that a developmental point of view considering rhombomeres may explain observed differences in connectivity and function. There are twelve rhombomeres characterized by particular genetic profiles, and each develops between one and four distinct serotonergic populations. We have studied the distribution of the conventional seven raphe nuclei among these twelve units. To this aim, we correlated 5-HT-immunoreacted neurons with rhombomeric boundary landmarks in sagittal mouse brain sections at different developmental stages. Furthermore, we performed a partial genoarchitectonic analysis of the developing raphe nuclei, mapping all known serotonergic differentiation markers, and compared these results, jointly with others found in the literature, with our map of serotonin-containing populations, in order to examine regional variations in correspondence. Examples of regionally selective gene patterns were identified. As a result, we produced a rhombomeric classification of some 45 serotonergic populations, and suggested a corresponding modified terminology. Only a minor rostral part of the dorsal raphe nucleus lies in the midbrain. Some serotonergic neurons were found in rhombomere 4, contrary to the conventional assumption that it lacks such neurons. We expect that our reclassification of raphe nuclei may be useful for causal analysis of their differential molecular specification, as well as for studies of differential connectivity and function.

Motor cortex stimulation inhibits thalamic sensory neurons and enhances activity of PAG neurons: possible pathways for antinociception

Motor cortex stimulation is generally suggested as a therapy for patients with chronic and refractory neuropathic pain. However, the mechanisms underlying its analgesic effects are still unknown. In a previous study, we demonstrated that cortical stimulation increases the nociceptive threshold of naive conscious rats with opioid participation. In the present study, we investigated the neurocircuitry involved during the antinociception induced by transdural stimulation of motor cortex in naive rats considering that little is known about the relation between motor cortex and analgesia. The neuronal activation patterns were evaluated in the thalamic nuclei and midbrain periaqueductal gray. Neuronal inactivation in response to motor cortex stimulation was detected in thalamic sites both in terms of immunolabeling (Zif268/Fos) and in the neuronal firing rates in ventral posterolateral nuclei and centromedian-parafascicular thalamic complex. This effect was particularly visible for neurons responsive to nociceptive peripheral stimulation. Furthermore, motor cortex stimulation enhanced neuronal firing rate and Fos immunoreactivity in the ipsilateral periaqueductal gray. We have also observed a decreased Zif268, δ-aminobutyric acid (GABA), and glutamic acid decarboxylase expression within the same region, suggesting an inhibition of GABAergic interneurons of the midbrain periaqueductal gray, consequently activating neurons responsible for the descending pain inhibitory control system. Taken together, the present findings suggest that inhibition of thalamic sensory neurons and disinhibition of the neurons in periaqueductal gray are at least in part responsible for the motor cortex stimulation-induced antinociception.

Supraspinal injection of Substance P attenuates allodynia and hyperalgesia in a rat model of inflammatory pain

The neuropeptide Substance P (SP), that has a high affinity for the neurokinin 1 (NK1) receptor, is involved in modulation of pain transmission. Although SP is thought to have excitatory actions and promote nociception in the spinal cord, the peptide induces analgesia at the supraspinal level. The aim of this study was to evaluate the role of supraspinal SP and the NK1 receptor in inflammatory pain induced by injection of carrageenan in the hind paw of the rat. There are two nociceptive behavioral responses associated with this pain state: mechanical allodynia and heat hyperalgesia. Because the NK1 receptor colocalizes with the MOP receptor in supraspinal sites involved in pain modulation, we also decided to study the possible involvement of the opioid system on SP-induced analgesia. We found that treatment with SP, at doses of 3.5, 5 and 7 μg/5 μl/rat i.c.v., clearly showed inhibition of allodynia and hyperalgesia. Pretreatment with the selective NK1 antagonist L-733,060 (10mg/kg i.p.) blocked the SP-induced analgesia, suggesting the involvement of the NK1 receptor. This SP-induced analgesia was significantly reduced by administration of the opioid antagonist naloxone (3mg/kg s.c.). This reduction occurred when SP was administered either before or after the carrageenan injection. These results suggest a significant antinociceptive role for SP and the NK1 receptor in inflammatory pain at the supraspinal level, possibly through the release of endogenous opioids.

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.

Effects of neurokinin-1 and 3-receptor antagonists on the defensive behavior induced by electrical stimulation of the dorsal periaqueductal gray

The dorsal periaqueductal gray (dPAG) is the main output structure for the defensive response to proximal aversive stimulation. Panic-like responses, such as freezing and escape behaviors, often result when this structure is electrically stimulated. Freezing also ensues after termination of the dPAG stimulation (post-stimulation freezing (PSF)). GABA and 5-HT have been proposed as the main neuromediators of these defense reactions. Neurokinins (NKs) also play a role in the defense reaction; however, it is unclear how the distinct types of NK receptors are involved in the expression of these fear responses. This study investigated the role of NK-1 and NK-3 receptors in the unconditioned defensive behaviors induced by electrical stimulation of the dPAG of rats, with and without previous experience with contextual fear conditioning (CFC). Spantide (100 ρmol/0.2 μl) and SB 222200 (50 and 100 ρmol/0.2 μl), selective antagonists of NK-1 and NK-3 receptors, respectively, were injected into the dPAG. Injection of spantide had antiaversive effects as determined by stimulation of the dPAG in naive animals and in animals subjected previously to CFC. SB 222200 also increased these aversive thresholds but only at doses that caused a motor deficit. Moreover, neither spantide nor SB 222200 influenced the PSF. The results suggest that NK-1 receptors are mainly involved in the mediation of the defensive behaviors organized in the dPAG. Because dPAG-evoked PSF was not affected by intra-dPAG injections of either spantide or SB 222200, it is suggested that neurokinin-mediated mechanisms are not involved in the processing of ascending aversive information from the dPAG.

Glutamatergic drive of the dorsal raphe nucleus

The dorsal raphe nucleus (DR) contains the majority of serotonin (5-hydroxytryptamine, 5-HT) neurons in the brain that regulate neural activity in forebrain regions through their widespread projections. DR function is linked to stress and emotional processing, and is implicated in the pathophysiology of affective disorders. Glutamatergic drive of the DR arises from many different brain areas with the capacity to inform the nucleus of sensory, autonomic, endocrine and metabolic state as well as higher order neural function. Imbalance of glutamatergic neurotransmission could contribute to maladaptive 5-HT neurotransmission and represents a potential target for pharmacotherapy. Within the DR, glutamate-containing axon terminals can be identified by their content of one of three types of vesicular glutamate transporter, VGLUT1, 2 or 3. Each of these transporters is heavily expressed in particular brain areas such that their content within axons correlates with the afferent's source. Cortical sources of innervation to the DR including the medial prefrontal cortex heavily express VGLUT1 whereas subcortical sources primarily express VGLUT2. Within the DR, many local neurons responsive to substance P contain VGLUT3, and these provide a third source of excitatory drive to 5-HT cells. Moreover VGLUT3 is present, with or without 5-HT, in output pathways from the DR. 5-HT and non-5-HT neurons receive and integrate glutamatergic neurotransmission through multiple subtypes of glutamate receptors that have different patterns of expression within the DR. Interestingly, excitatory drive provided by glutamatergic neurotransmission is closely opposed by feedback inhibition mediated by 5-HT1A receptors or local GABAergic circuits. Understanding the intricacies of these local networks and their checks and balances, may help identify how potential imbalances could cause psychopathology and illuminate strategies for therapeutic manipulation.

Anxiolytic-like effects of the neurokinin 1 receptor antagonist GR-205171 in the elevated plus maze and contextual fear-potentiated startle model of anxiety in gerbils

Gerbils show a neurokinin (NK)1 receptor pharmacological profile, which is similar to that observed in humans, and thus have become a commonly used species to test efficacy of NK1 receptor antagonists. The aim of this study was to determine whether systemic administration of the NK1 receptor antagonist GR-205171 produced anxiolytic-like effects in the elevated plus maze and in a novel contextual conditioned fear test using fear-potentiated startle (FPS). On the elevated plus maze, treatment with GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing produced anxiolytic-like effects in an increasing dose-response manner as measured by the percentage of open arm time and percentage of open arm entries. For contextual fear conditioning, gerbils were given 10 unsignaled footshocks (0.6 mA) at a 2-min variable interstimulus interval in a distinctive training context. Twenty-four hours after training, gerbils received treatment of GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing in which startle was elicited in the same context in which they were trained. Contextual FPS was defined as an increase in startle over pretraining baseline values. All drug dose levels (0.3, 1.0, and 5.0 mg/kg) significantly attenuated contextual FPS when compared with the vehicle control group. A control group, which received testing in a different context, showed little FPS. These findings support other evidence for anxiolytic activity of NK1 receptor antagonists and provide a novel conditioned fear test that may be an appropriate procedure to test other NK1 antagonists for preclinical anxiolytic activity in gerbils.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Chemical neuroanatomy of the dorsal raphe nucleus and adjacent structures of the mouse brain

Serotonin neurons play a major role in many normal and pathological brain functions. In the rat these neurons have a varying number of cotransmitters, including neuropeptides. Here we studied, with histochemical techniques, the relation between serotonin, some other small-molecule transmitters, and a number of neuropeptides in the dorsal raphe nucleus (DRN) and the adjacent ventral periaqueductal gray (vPAG) of mouse, an important question being to establish possible differences from rat. Even if similarly distributed, the serotonin neurons in mouse lacked the extensive coexpression of nitric oxide synthase and galanin seen in rat. Although partly overlapping in the vPAG, no evidence was obtained for the coexistence of serotonin with dopamine, substance P, cholecystokinin, enkephalin, somatostatin, neurotensin, dynorphin, thyrotropin-releasing hormone, or corticotropin-releasing hormone. However, some serotonin neurons expressed the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase (GAD). Work in other laboratories suggests that, as in rat, serotonin neurons in the mouse midline DRN express the vesicular glutamate transporter 3, presumably releasing glutamate. Our study also shows that many of the neuropeptides studied (substance P, galanin, neurotensin, dynorphin, and corticotropin-releasing factor) are present in nerve terminal networks of varying densities close to the serotonin neurons, and therefore may directly or indirectly influence these cells. The apparently low numbers of coexisting messengers in mouse serotonin neurons, compared to rat, indicate considerable species differences with regard to the chemical neuronatomy of the DRN. Thus, extrapolation of DRN physiology, and possibly pathology, from rat to mouse, and even human, should be made with caution.

Spinal cord mechanisms mediating behavioral hyperalgesia induced by neurokinin-1 tachykinin receptor activation in the rostral ventromedial medulla

Hyperalgesia in animal injury models is linked to activation of descending raphespinal modulatory circuits originating in the rostral ventromedial medulla (RVM). A neurokinin-1 (NK-1) receptor antagonist microinjected into the RVM before or after inflammation produced by complete Freund's adjuvant (CFA) resulted in an attenuation of thermal hyperalgesia. A transient (acute) or a continuous infusion of Substance P (SP) microinjected into the RVM of non-inflamed animals led to similar pain hypersensitivity. Intrathecal pretreatment or post-treatment of a 5-HT3 receptor antagonist (Y-25130 or ondansetron) blocked the SP-induced hyperalgesia. The SP-induced hyperalgesia was both GABA(A) and NMDA receptor-dependent after pre- and post-treatment with selective antagonists at the spinal level. A microinjection of SP into the RVM also led to increased NMDA NR1 receptor subunit phosphorylation in spinal cord tissue. The GABA(A) receptor-mediated hyperalgesia involved a shift in the anionic gradient in dorsal horn nociceptive neurons and an increase in phosphorylated NKCC1 protein (isoform of the Na-K-Cl cotransporter). Following a low dose of SP infused into the RVM, intrathecal muscimol (GABA(A) agonist) increased SP-induced thermal hyperalgesia, phosphorylated NKCC1 protein expression, and NMDA NR1 subunit phosphorylation in the spinal cord. The thermal hyperalgesia was blocked by intrathecal gabazine, the GABA(A) receptor antagonist, and MK-801, the NMDA receptor channel blocker. These findings indicate that NK-1 receptors in the RVM are involved in SP-induced thermal hyperalgesia, this hyperalgesia is 5-HT3-receptor dependent at the spinal level, and involves the functional interaction of spinal GABA(A) and NMDA receptors.

S41744, a dual neurokinin (NK)1 receptor antagonist and serotonin (5-HT) reuptake inhibitor with potential antidepressant properties: a comparison to aprepitant (MK869) and paroxetine

Though neurokinin(1) (NK(1)) receptors are implicated in depressed states and their treatment, selective antagonists have disappointed in clinical trials. Accordingly, we designed a novel ligand, S41744 (2-piperazin-1-yl-indan-2-carboxylic-acid-(3-chloro-5-fluoro-benzyl)-methyl-amide), which both blocks NK(1) receptors and interferes with serotonin (5-HT) reuptake. S41744 mimicked the selective antagonist aprepitant in binding human (h)NK(1) receptors and in antagonising Substance-P-mediated Extracellular-Regulated-Kinase phosphorylation (pK(B), 7.7). Further, it dose-dependently (0.63-40.0 mg/kg, i.p.) displaced ex vivo [(3)H]-[Sar(9),Met(O(2))(11)]-Substance P binding to gerbil striatum, attenuated formalin-induced hind-paw licking in gerbils, and antagonised locomotion induced by i.c.v. administration of the NK(1) agonist GR73632 to guinea pigs. Like paroxetine, S41744 recognised h5-HT transporters, reduced synaptosomal uptake of 5-HT (pK(B), 7.9), and dose-dependently (0.63-10.0 mg/kg) elevated dialysis levels of 5-HT in the hippocampus and frontal cortex of freely-moving guinea pigs. Further, S41744 increased extracellular levels of 5-HT in frontal cortex and hippocampus of rats to a greater extent than paroxetine, and its inhibitory influence upon serotonergic perikarya was blunted relative to its affinity for 5-HT transporters. S41744 more potently blocked stress-induced vocalizations in guinea pigs than aprepitant and paroxetine, and it was active in forced-swim and marble-burying procedures of putative antidepressant properties in mice. While aprepitant displayed anxiolytic actions in stress-induced foot-tapping and social interaction tests in gerbils, paroxetine was anxiogenic and S41744 "neutral", reflecting balanced NK(1) antagonism and suppression of 5-HT reuptake. Moreover, S41744 shared anxiolytic actions of aprepitant in the rat Vogel Conflict Test. In conclusion, S41744 is an innovative NK(1) antagonist/5-HT reuptake inhibitor justifying further evaluation for treatment of stress-related disorders.

VGLUT3 (vesicular glutamate transporter type 3) contribution to the regulation of serotonergic transmission and anxiety

Three different subtypes of H(+)-dependent carriers (named VGLUT1-3) concentrate glutamate into synaptic vesicles before its exocytotic release. Neurons using other neurotransmitter than glutamate (such as cholinergic striatal interneurons and 5-HT neurons) express VGLUT3. It was recently reported that VGLUT3 increases acetylcholine vesicular filling, thereby, stimulating cholinergic transmission. This new regulatory mechanism is herein designated as vesicular-filling synergy (or vesicular synergy). In the present report, we found that deletion of VGLUT3 increased several anxiety-related behaviors in adult and in newborn mice as early as 8 d after birth. This precocious involvement of a vesicular glutamate transporter in anxiety led us to examine the underlying functional implications of VGLUT3 in 5-HT neurons. On one hand, VGLUT3 deletion caused a significant decrease of 5-HT(1A)-mediated neurotransmission in raphe nuclei. On the other hand, VGLUT3 positively modulated 5-HT transmission of a specific subset of 5-HT terminals from the hippocampus and the cerebral cortex. VGLUT3- and VMAT2-positive serotonergic fibers show little or no 5-HT reuptake transporter. These results unravel the existence of a novel subset of 5-HT terminals in limbic areas that might play a crucial role in anxiety-like behaviors. In summary, VGLUT3 accelerates 5-HT transmission at the level of specific 5-HT terminals and can exert an inhibitory control at the raphe level. Furthermore, our results suggest that the loss of VGLUT3 expression leads to anxiety-associated behaviors and should be considered as a potential new target for the treatment of this disorder.

The structure of the dorsal raphe nucleus and its relevance to the regulation of sleep and wakefulness

Serotonergic (5-HT) cells in the rat dorsal raphe nucleus (DRN) appear in topographically organized groups. Based on cellular morphology, expression of other neurotransmitters, afferent and efferent connections and functional properties, 5-HT neurons of the DRN have been grouped into six cell clusters. The subdivisions comprise the rostral, ventral, dorsal, lateral, caudal and interfascicular parts of the DRN. In addition to 5-HT cells, neurons containing γ-aminobutyric acid (GABA), glutamate, dopamine, nitric oxide and the neuropeptides corticotropin-releasing factor, substance P, galanin, cholecystokinin, neurotensin, somatostatin, vasoactive intestinal peptide, neuropeptide Y, thyrotropin-releasing hormone, growth hormone, leu-enkephalin, met-enkephalin and gastrin have been characterized in the DRN. Moreover, numerous brain areas have neurons that project to the DRN and express monoamines (norepinephrine, histamine), amino acids (GABA, glutamate), acetylcholine or neuropeptides (orexin, melanin-concentrating hormone, corticotropin-releasing factor and substance P) that directly or indirectly, through local circuits, regulate the activity of 5-HT cells. The 5-HT cells predominate along the midline of the rostral, dorsal and ventral subdivisions of the DRN and outnumber the non-5-HT cells occurring in the raphe nucleus. The GABAergic and glutamatergic neurons are clustered mainly in the lateral and dorsal subdivisions of the DRN, respectively. The 5-HT(1A) receptor is located on the soma and the dendrites of 5-HT neurons and at postsynaptic sites (outside the DRN). It is expressed, in addition, by non-5-HT cells of the DRN. The 5-HT(1B) receptor is located at presynaptic and postsynaptic sites (outside the boundaries of the DRN). It has been described also in the ventromedial DRN where it is expressed by non-5-HT cells. The 5-HT(2A) and 5-HT(2C) receptors are located within postsynaptic structures. At the level of the DRN the 5-HT(2A) and 5-HT(2C) receptor-containing cells are predominantly GABAergic interneurons and projection neurons. Within the boundaries of the DRN the 5-HT(3) receptor is expressed by, among others, glutamatergic interneurons. 5-HT(7) receptors in the DRN are not localized to serotonergic neurons but, at least in part, to GABAergic cells and terminals. The complex structure of the DRN may have important implications for neural mechanisms underlying 5-HT modulation of wakefulness and REM sleep.

Neuronal pathways linking substance P to drug addiction and stress

Accumulating evidence suggests that the neuropeptide substance P (SP) and its principal receptor neurokinin 1 (NK1) play a specific role in the behavioral response to opioids and stress that may help to initiate and maintain addictive behavior. In animal models, the NK1 receptor is required for opioids to produce their rewarding and motivational effects. SP neurotransmission is also implicated in the behavioral response to stress and in the process of drug sensitization, potentially contributing to vulnerability to addiction or relapse. However, SP neurotransmission only plays a minor role in opioid-mediated antinociception and the development of opioid tolerance. Moreover, the effects of SP on addiction-related behavior are selective for opioids and evidence supporting a role in the response to cocaine or psychostimulants is less compelling. This review will summarize the effects of SP neurotransmission on opioid-dependent behaviors and correlate them with potential contributing neural pathways. Specifically, SP neurotransmission within components of the basal forebrain particularly the nucleus accumbens and ventral pallidum as well as actions within the ascending serotonin system will be emphasized. In addition, cellular- or network-level interactions between opioids and SP signaling that may underlie the specificity of their relationship will be reviewed.

Fractalkine/CX3CL1 enhances GABA synaptic activity at serotonin neurons in the rat dorsal raphe nucleus

Serotonin (5-hydroxytryptamine; 5-HT) has an important role in mood regulation, and its dysfunction in the central nervous system (CNS) is associated with depression. Reports of mood and immune disorder co-morbidities indicate that immune-5-HT interactions may mediate depression present in immune compromised disease states including HIV/AIDS, multiple sclerosis, and Parkinson's disease. Chemokines, immune proteins that induce chemotaxis and cellular adhesion, and their G-protein coupled receptors distribute throughout the CNS, regulate neuronal patterning, and mediate neuropathology. The purpose of this study is to investigate the neuroanatomical and neurophysiological relationship between the chemokine fractalkine/CX3CL1 and its receptor CX3CR1 with 5-HT neurons in the rat midbrain raphe nuclei (RN). Immunohistochemistry was used to examine the colocalization of CX3CL1 or CX3CR1 with 5-HT in the RN, and whole-cell patch-clamp recordings in rat brain slices were used to determine the functional impact of CX3CL1 on 5-HT dorsal raphe nucleus (DRN) neurons. Greater than 70% of 5-HT neurons colocalize with CX3CL1 and CX3CR1 in the RN. CX3CL1 localizes as discrete puncta throughout the cytoplasm, whereas CX3CR1 concentrates to the perinuclear region of 5-HT neurons and exhibits microglial expression. CX3CL1 and CX3CR1 also colocalize with one another on individual RN cells. Electrophysiology studies indicate a CX3CL1-mediated enhancement of spontaneous inhibitory postsynaptic current (sIPSC) amplitude and dose-dependent increase of evoked IPSC (eIPSC) amplitude without affecting eIPSC paired-pulse ratio, a finding observed selectively in 5-HT neurons. CX3CL1's effect on eIPSC amplitude is blocked by pretreatment with an anti-CX3CL1 neutralizing antibody. Thus, CX3CL1 enhances postsynaptic GABA receptor number or sensitivity on 5-HT DRN neurons under conditions of both spontaneous and synaptically-evoked GABA release. CX3CL1 may indirectly inhibit 5-HT neurotransmission by increasing the sensitivity of 5-HT DRN neurons to GABA inputs. Therapies targeting CX3CL1 may treat serotonin related mood disorders, including depression experienced by patients with compromised immune systems.

Receptor-selective agonists induce emesis and Fos expression in the brain and enteric nervous system of the least shrew (Cryptotis parva)

Research on the mechanisms of emesis has implicated multiple neurotransmitters via both central (dorsal vagal complex) and peripheral (enteric neurons and enterochromaffin cells) anatomical substrates. Taking advantage of advances in receptor-specific agonists, and utilizing Fos expression as a functional activity marker, this study demonstrates a strong, but incomplete, overlap in anatomical substrates for a variety of emetogens. We used cisplatin and specific agonists to 5-HT(3) serotonergic, D(2)/D(3) dopaminergic, and NK(1) tachykininergic receptors to induce vomiting in the least shrew (Cryptotis parva), and quantified the resulting Fos expression. The least shrew is a small mammal whose responses to emetic challenges are very similar to its human counterparts. In all cases, the enteric nervous system, nucleus of the solitary tract, and dorsal motor nucleus of the vagus demonstrated significantly increased Fos immunoreactivity (Fos-IR). However, Fos-IR induction was notably absent from the area postrema following the dopaminergic and NK(1) receptor-specific agents. Two brain nuclei not usually discussed regarding emesis, the dorsal raphe nucleus and paraventricular thalamic nucleus, also demonstrated increased emesis-related Fos-IR. Taken together, these data suggest the dorsal vagal complex is part of a common pathway for a variety of distinct emetogens, but there are central emetic substrates, both medullary and diencephalic, that can be accessed without directly stimulating the area postrema.

Substance P drives endocannabinoid-mediated disinhibition in a midbrain descending analgesic pathway

Substance P is thought to play an essential role in several forms of supraspinally mediated analgesia. The actions of substance P on synaptic transmission within descending analgesic pathways, however, are largely unknown. Here, we used whole-cell recordings from rat midbrain slices to examine the effects of substance P on GABAergic and glutamatergic transmission within the periaqueductal gray (PAG), a key component of a descending analgesic pathway that projects via the rostral ventromedial medulla (RVM) to the spinal cord dorsal horn. We found that substance P reversibly decreased the amplitude and increased the paired-pulse ratio of evoked IPSCs recorded from identified PAG-RVM projection neurons and from unidentified PAG neurons. Substance P had no effect on miniature IPSCs, implying an indirect mode of action. The effects of substance P were abolished by metabotropic glutamate type 5 and cannabinoid CB1 receptor antagonists, but unaltered by NMDA, GABA(B), mu,delta-opioid, adenosine A(1), and 5HT(1A) receptor antagonists. Consistent with a role for endogenous glutamate in this process, substance P increased the frequency of action potential-dependent spontaneous EPSCs. Moreover, the effect of substance P on evoked IPSCs was mimicked and occluded by a glutamate transport inhibitor. Finally, these effects were dependent on postsynaptic G-protein activation and diacylglycerol lipase activity, suggesting the requirement for retrograde signaling by the endocannabinoid 2-arachidonoylglycerol. Thus, substance P may facilitate descending analgesia in part by enhancing glutamate-mediated excitation and endocannabinoid-mediated disinhibition of PAG-RVM projection neurons.

Substance P enhances excitatory synaptic transmission on spinally projecting neurons in the rostral ventromedial medulla after inflammatory injury

It has been proposed, but not directly tested, that persistent inflammatory nociception enhances excitatory glutamatergic inputs to neurons in the rostral ventromedial medulla (RVM), altering the activity and function of these neurons. This study used whole cell patch-clamp methods to record evoked excitatory postsynaptic currents (eEPSCs) in spinally projecting RVM neurons from rats injected with saline or complete Freund's adjuvant (CFA) 3-4 days earlier and to examine the role of substance P (SP) in modulating excitatory synaptic transmission. Input-output relationships demonstrated that CFA treatment facilitated fast excitatory glutamatergic inputs to type 1 and type 2 nonserotonergic spinally projecting RVM neurons, but not to type 3 neurons. The facilitation in type 1 and 2 neurons was dependent on neurokinin-1 (NK1) and N-methyl-d-aspartate (NMDA) receptors and prevented by the PKC inhibitor GF109203X. In a subset of neurons from naïve rats, SP mimicked the effects of CFA and increased the potency and efficacy of glutamatergic synaptic transmission. The facilitation was prevented by 10 microM GF109203X, but not by 10 microM KN93, a CaMKII inhibitor. SP (0.3-3 microM) by itself produced concentration-dependent inward currents in most nonserotonergic, but not serotonergic neurons. The present study is the first demonstration, at the cellular level, that persistent inflammatory nociception leads to a sustained facilitation of fast excitatory glutamatergic inputs to RVM neurons by an NK1 and NMDA receptor-dependent mechanism that involves PKC. Further, it demonstrates that the facilitation is restricted to specific populations of RVM neurons that by inference may be pain facilitatory neurons.

Locally collateralizing glutamate neurons in the dorsal raphe nucleus responsive to substance P contain vesicular glutamate transporter 3 (VGLUT3)

The serotonergic dorsal raphe nucleus (DR) is an area enriched with cell bodies and axons containing the third vesicular glutamate transporter, VGLUT3. However, the role of VGLUT3-containing axons in modulating activity of serotonin (5-HT) neurons within the DR remains poorly understood. In this study, neurochemical features and topography of VGLUT3-containing cell bodies and axons in the DR were examined. Since many 5-HT cells have been reported to express VGLUT3, the distribution of dually labeled axons was examined within the DR. Axons containing both VGLUT3 and 5-HT immunolabeling had a topographic distribution: they innervated the ependyma and ramified within the caudal DR at the base of the aqueduct, an area known to give rise to ependymal innervation. Thus VGLUT3 is only present in a specific subcomponent of recurrent 5-HT axon collaterals. Remaining VGLUT3 axons were only rarely dually immunolabeled for markers of monoamines, GABA, or acetycholine, suggesting these axons contain a predominance of glutamate-filled vesicles. Since the substance P receptor, neurokinin 1 (NK1), has previously been associated with local glutamate neurons in the DR, the relationship between NK1, 5-HT and VGLUT3 cells was examined using triple-immunolabeling. Results indicate that the majority of non-5-HT VGLUT3-containing cell bodies in the DR contain NK1 immunolabeling. Taken together, these findings indicate locally collateralizing glutamate neurons responsive to substance P contain VGLUT3.

Evidence for mediation of nociception by injection of the NK-3 receptor agonist, senktide, into the dorsal periaqueductal gray of rats

RATIONALE

Ultrasound vocalizations (USVs) at approximately 22 kHz are usual components of the defensive response of rats. However, depending on the neural substrate that is activated, such as the dorsal periaqueductal gray (dPAG), USV emissions may be reduced. Activation of neurokinin-1 (NK-1)-mediated mechanisms of the dPAG causes analgesia, reduced 22 kHz USVs, and anxiogenic-like effects in rats exposed to the elevated plus maze (EPM). Involvement of other types of neurokinin receptors in this activation has not yet been evaluated.

OBJECTIVES

The present study examined whether local injections of the selective NK-3 agonist senktide (1-100 pmol/0.2 microL) into the dPAG can (1) cause anxiogenic effects in the EPM, (2) influence novelty-induced 22 kHz USVs, or (3) change nociceptive reactivity in the tail-flick test.

RESULTS

Senktide elicited a significant increase in exploratory behavior, an effect accompanied by hyperalgesia and an increase in the number of 22 kHz USVs. The nociceptive effects, increased locomotor activity, and USV emissions elicited by local injections of senktide (50 pmol/0.2 microL) were reduced by prior injections of the selective NK-3 receptor antagonist SB222200 (50 pmol/0.2 microL) into the dPAG.

CONCLUSIONS

These findings show that NK-3 receptors in the dPAG mediate nociceptive responses in this area, contrasting with the known fear-related processes mediated by NK-1 receptors in the dPAG. Both hyperalgesia and fear-related processes are accompanied by emissions of 22 kHz USVs.

Neurotensin inhibition of GABAergic transmission via mGluR-induced endocannabinoid signalling in rat periaqueductal grey

Neurotensin modulates pain via its actions within descending analgesic pathways which include brain regions such as the midbrain periaqueductal grey (PAG). The aim of this study was to examine the cellular actions of neurotensin on PAG neurons. Whole cell patch clamp recordings were made from rat midbrain PAG slices in vitro to examine the postsynaptic effects of neurotensin and its effects on GABA(A) mediated inhibitory postsynaptic currents (IPSCs). Neurotensin (100-300 nM) produced an inward current in subpopulations of opioid sensitive and insensitive PAG neurons which did not reverse over membrane potentials between -50 and -130 mV. The neurotensin induced current was abolished by the NTS1 and NTS1/2 antagonists SR48692 (300 nM) and SR142948A (300 nM). Neurotensin also produced a reduction in the amplitude of evoked IPSCs, but had no effect on the rate and amplitude of TTX-resistant miniature IPSCs. The neurotensin induced inhibition of evoked IPSCs was reduced by the mGluR5 antagonist MPEP (5microM) and abolished by the cannabinoid CB(1) receptor antagonist AM251 (3 microM). These results suggest that neurotensin produces direct neuronal depolarisation via NTS1 receptors and inhibits GABAergic synaptic transmission within the PAG. The inhibition of synaptic transmission is mediated by neuronal excitation and action potential dependent release of glutamate, leading to mGluR5 mediated production of endocannabinoids which activate presynaptic CB(1) receptors. Thus, neurotensin has cellular actions within the PAG which are consistent with both algesic and analgesic activity, some of which are mediated via the endocannabinoid system.

An NK1 receptor antagonist microinjected into the periaqueductal gray blocks lateral hypothalamic-induced antinociception in rats

Substantial data are accumulating that implicate the lateral hypothalamus (LH) as part of the descending pain modulatory system. The LH modifies nociception in the spinal cord dorsal horn partly through connections with the periaqueductal gray (PAG), an area known to play a central role in brainstem modulation of nociception. Early work demonstrated a putative substance P connection between the LH and the PAG, but the connection is not fully defined. To determine whether LH-induced antinociception mediated by the PAG is neurokinin1 (NK1) receptor-dependent, we conducted behavioral experiments in which the cholinergic agonist carbachol (125 nmol) was microinjected into the LH of lightly anesthetized female Sprague-Dawley rats (250-350 g) and antinociception was obtained on the tail flick or foot withdrawal tests. Cobalt chloride (100 nM), which reversibly blocks synaptic activation, blocked LH-induced antinociception. In another set of experiments, the specific NK1 receptor antagonist L-703,606 (5 microg) was microinjected in the PAG following LH stimulation with carbachol abolished LH-induced antinociception as well. Microinjection of cobalt chloride or L-703,606 in the absence of LH stimulation had no effect. These behavioral experiments coupled with earlier work provide converging evidence to support the hypothesis that antinociception produced by activating neurons in the LH is mediated in part by the subsequent activation of neurons in the PAG by NK1 receptors.

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.

Association and linkage analysis of candidate genes GRP, GRPR, CRHR1, and TACR1 in panic disorder

Panic disorder (PD) is a debilitating anxiety disorder, characterized by recurrent episodes of intense fear that are accompanied by autonomic and psychological symptoms leading to behavioral impairment. Basic research implicates neuropeptide-signaling genes in the modulation of anxiety and stress. The genes encoding corticotropin releasing hormone receptor 1 (CRHR1), tachykinin receptor 1 (TACR1), gastrin releasing peptide (GRP), and gastrin releasing peptide receptor (GRPR) were selected as candidates for PD based on their biology. Linkage and association analysis in 120 multiplex U.S. PD pedigrees was performed using 21 single nucleotide polymorphisms (SNPs). Parametric and non-parametric linkage tests in pedigrees, for single point and multipoint analysis, revealed limited support for genetic linkage to TACR1 (parametric and non-parametric lod scores approximately 1). The family-based association test (FBAT) generated nominal support for allelic association in TACR1 (P = 0.02), and GRP (P = 0.02), findings which must be considered in the light of multiple comparisons. Further exploration of the GRP and TACR1 findings in large case-control PD samples may provide more definitive evidence implicating these loci in the genetic etiology of PD.

Effects of substance P in the amygdala, ventromedial hypothalamus, and periaqueductal gray on fear-potentiated startle

The neural pathways through which substance P (SP) influences fear and anxiety are poorly understood. However, the amygdala, a brain area repeatedly implicated in fear and anxiety processes, is known to contain large numbers of SP-containing neurons and SP receptors. Several studies have implicated SP neurotransmission within the amygdala in anxiety processes. In the present study, we evaluated the effects of site-specific infusions of an SP receptor antagonist, GR 82334, on conditioned fear responses using the fear-potentiated startle paradigm. GR 82334 infusion into the basolateral (BLA) or the medial (MeA) nuclei of the amygdala, but not into the central nucleus of the amygdala (CeA), dose dependently reduced fear-potentiated startle. Similar effects were obtained with GR 82334 infusion into the ventromedial nucleus of the hypothalamus (VMH), to which the MeA projects, and into the rostral dorsolateral periaqueductal gray (PAG), to which the VMH projects, but not into the deep layers of the superior colliculus/deep mesencephalic nucleus (dSC/DpMe), an output of the CeA previously shown to be important for fear-potentiated startle. Consistent with previous findings, infusion of the AMPA receptor antagonist, NBQX, into the dSC/DpMe, but not into the PAG, did disrupt fear-potentiated startle. These findings suggest that multiple outputs from the amygdala play a critical role in fear-potentiated startle and that SP plays a critical, probably modulatory role, in the MeA to VMH to PAG to the startle pathway based on these and data from others.

Substance P Neurokinin 1 Receptor Activation within the Dorsal Raphe Nucleus Controls Serotonin Release in the Mouse Frontal Cortex

Preclinical studies suggest that substance P (SP) neurokinin 1 (NK1) receptor antagonists are efficient in the treatment of anxiety and depression. This therapeutic activity could be mediated via stimulation of serotonin (5-HT) neurons located in the dorsal raphe nucleus (DRN), which receive important SP-NK1 receptor immunoreactive innervations. The present study examined the effects of intraraphe injection of SP on extracellular 5-HT levels in the frontal cortex, ventral hippocampus, and DRN by using intracerebral microdialysis in conscious mice. Intraraphe SP injection dose dependently decreased cortical 5-HT release, whereas no effects were detected in the ventral hippocampus. Cortical effects were blocked by the selective NK1 receptor antagonist N-[[2-methoxy-5-[5-(trifluoromethyl)tetrazol-1-yl]phenyl]methyl]-2-phenylpiperidin-3-amine (GR205171) and completely dampened in mice lacking NK1 receptors. Furthermore, genetic (in knockout 5-HT1A(-/-) mice) or pharmacological inactivation of 5-HT1A autoreceptors blocked cortical responses to SP. Contrasting with its cortical effects, intraraphe SP injection increased 5-HT outflow in the DRN in wild-type mice; this effect was potentiated by a local perfusion of the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635). Finally, SP-induced changes in frontal cortex and DRN dialysate 5-HT levels were blocked by the DRN perfusion of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate ionotropic receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These data support the hypothesis that SP-induced over-activation of 5-HT1A autoreceptors within the DRN limits cortical 5-HT release. A better knowledge of the complex relationship between tachykininergic, serotonergic, and glutamatergic systems within the DRN might help better understand the pathophysiology and subsequent treatment of depression.

A histologically derived stereotaxic atlas and substance P immunohistochemistry in the brain of the least shrew (Cryptotis parva) support its role as a model organism for behavioral and pharmacological research

Chemotherapy is an effective treatment but difficult to tolerate due to side effects like vomiting. Studies on the etiology of chemotherapy-related emesis have implicated brainstem nuclei and the neurotransmitter substance P, among other substrates. Since rodents do not vomit, other species have been necessary as alternative models of chemotherapy-induced emesis. Of these, the least shrew (Cryptotis parva) has proven valuable due to its small size, hardiness, and close phylogenetic relationship with primates. However, very little neuroanatomical data on C. parva exist. We used histological and immunohistochemical techniques to provide neuroanatomical data to help validate C. parva as a model organism, especially for emesis research. Brains were sectioned and stained for Nissl substance or myelin, or immunofluorescently labeled for substance P. Sections were photographed, traced, and reconstructed with standardized zero points, and these data used to create a stereotaxic atlas. The brain of C. parva was similar to but smaller than other mammalian brains, with the cerebellum and hippocampus demonstrating the biggest differences. Differences appeared to be related to the small size of the brain and the metabolic compromises required of such a small mammal. Substance P-like immunoreactivity (SPL-IR) was semiquantitatively mapped, and correlated very well with SPL-IR observed in other species. Dense SPL-IR areas included the periaqueductal grey, trigeminal nuclei, dorsal raphe, and emesis-related brainstem nuclei including the area postrema and solitary tract nucleus. These data demonstrate that the anatomical differences between C. parva and other mammals will not preclude its use as a model organism.

Selective 5-HT receptor inhibition of glutamatergic and GABAergic synaptic activity in the rat dorsal and median raphe

The dorsal (DR) and median (MR) raphe nuclei contain 5-hydroxytryptamine (5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain. The DR and MR have differential roles in mediating stress, anxiety and depression. Glutamate and GABA activity sculpt putative 5-HT neuronal firing and 5-HT release in a seemingly differential manner in the MR and DR, yet isolated glutamate and GABA activity within the DR and MR has not been systematically characterized. Visualized whole-cell voltage-clamp techniques were used to record excitatory and inhibitory postsynaptic currents (EPSC and IPSC) in 5-HT-containing neurons. There was a regional variation in action potential-dependent (spontaneous) and basal [miniature (m)] glutamate and GABAergic activity. mEPSC activity was greater than mIPSC activity in the DR, whereas in the MR the mIPSC activity was greater. These differences in EPSC and IPSC frequency indicate that glutamatergic and GABAergic input have distinct cytoarchitectures in the DR and MR. 5-HT(1B) receptor activation decreased mEPSC frequency in the DR and the MR, but selectively inhibited mIPSC activity only in the MR. This finding, in concert with its previously described function as an autoreceptor, suggests that 5-HT(1B) receptors influence the ascending 5-HT system through multiple mechanisms. The disparity in organization and integration of glutamatergic and GABAergic input to DR and MR neurons and their regulation by 5-HT(1B) receptors may contribute to the distinction in MR and DR regulation of forebrain regions and their differential function in the aetiology and pharmacological treatment of psychiatric disease states.

Immunocytochemical evidence for the existence of substance P receptor (NK1) in serotonin neurons of rat and mouse dorsal raphe nucleus

In addition to its neurotransmitter/modulator role in pain perception, substance P (SP) is involved in a regulation of mood, as antagonists of its neurokinin-1 receptor (NK1r) have been found to have antidepressant-like effects in humans. In rodents, treatment with NK1r antagonists has been shown to increase the firing of dorsal raphe nucleus (DRN) serotonin (5-hydroxytryptamine, 5-HT) neurons and to induce a desensitization of their 5-HT1A autoreceptors, suggesting local interactions between the SP and 5-HT systems. To search for the presence of NK1r on 5-HT neurons of the DRN, we used light and electron microscopic immunocytochemistry, as well as confocal microscopy, after single- and double-labelling of NK1r and of the biosynthetic enzyme of 5-HT, tryptophan hydroxylase (TpOH). A significant number of 5-HT (TpOH-positive) cell bodies and dendrites endowed with NK1r were thus demonstrated in the caudal part of rat and mouse DRN. As visualized by electron microscopy after gold immunolabelling, NK1r was mostly cytoplasmic in 5-HT neurons, while predominating on the plasma membrane in the case of TpOH-negative dendrites. The proportion of NK1r observed on the plasma membrane of 5-HT neurons was, however, slightly higher in mouse than rat. Thus, in both rat and mouse DRN, a subpopulation of 5-HT neurons is endowed with NK1r receptors and may be directly involved in the antidepressant-like effects of NK1r antagonists. These 5-HT neurons represent a new element in the neuronal circuitry currently proposed to account for the role of SP in mood regulation.

Lack of interaction between NMDA and cholecystokinin-2 receptor-mediated neurotransmission in the dorsolateral periaqueductal gray in the regulation of rat defensive behaviors

Several neurotransmitters, including GABA, serotonin, glutamate, and cholecystokinin, modulate defensive behaviors in the dorsolateral periaqueductal gray (dlPAG). Although both glutamate and cholecystokinin have been shown to facilitate these behaviors, a possible interaction between them remains to be examined. The present study investigates whether activation or antagonism of N-methyl-D-aspartic acid (NMDA) glutamate and cholecystokinin 2 (CCK(2)) receptors located in the dlPAG would interact in animals tested in the elevated T-maze. The effect of the NMDA (50 pmol) was evaluated in rats pretreated with the CCK(2) receptor antagonist LY225910 (0.05 nmol). In addition, the effect of the CCK(2) receptor agonist CCK-4 (0.08 nmol) was evaluated in rats pretreated with the NMDA receptor antagonist AP-7 (1.0 nmol). Intra-dlPAG injection of NMDA increased risk assessment and inhibitory avoidance behaviors. This NMDA anxiogenic-like effect was unaltered by the pretreatment with LY225910. Similarly, the shortening of escape latencies induced by CCK-4 was unaffected by AP-7. No drug changed the general exploratory activity as assessed in the open-field. These results, showing that the activation of dlPAG NMDA or CCK(2) receptors facilitate anxiety- and fear-related behaviors, further implicate glutamate and cholecystokinin-mediated neurotransmission in this midbrain area on modulation of defensive behaviors. However, the regulatory action of these two excitatory neurotransmitters seems to be exerted through independent mechanisms.

The role of substance P in stress and anxiety responses

Substance P (SP) is one of the most abundant peptides in the central nervous system and has been implicated in a variety of physiological and pathophysiological processes including stress regulation, as well as affective and anxiety-related behaviour. Consistent with these functions, SP and its preferred neurokinin 1 (NK1) receptor has been found within brain areas known to be involved in the regulation of stress and anxiety responses. Aversive and stressful stimuli have been shown repeatedly to change SP brain tissue content, as well as NK1 receptor binding. More recently it has been demonstrated that emotional stressors increase SP efflux in specific limbic structures such as amygdala and septum and that the magnitude of this effect depends on the severity of the stressor. Depending on the brain area, an increase in intracerebral SP concentration (mimicked by SP microinjection) produces mainly anxiogenic-like responses in various behavioural tasks. Based on findings that SP transmission is stimulated under stressful or anxiety-provoking situations it was hypothesised that blockade of NK1 receptors may attenuate stress responses and exert anxiolytic-like effects. Preclinical and clinical studies have found evidence in favour of such an assumption. The status of this research is reviewed here.

Activity-dependent heterogeneous populations of nitric oxide synthase neurons in the rat dorsal raphe nucleus

The brainstem dorsal raphe nucleus (DRN) contains an abundant distribution of nitric oxide (NO) synthase (NOS)-containing neuronal profiles in two distinct populations: faint- and intense-immunoreactive cells in midline (ventromedial and dorsomedial) and lateral wing subregions, respectively. This study tested the hypothesis that different functional dynamics underlie the topography of NOS-containing cells in the DRN rostrocaudal and mediolateral neuraxis by using a capsaicin challenge paradigm (50 mg/kg, subcutaneous). Compared with vehicle, capsaicin significantly and preferentially increased nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d, an index of constitutive NOS) reactivity in the rostral midline and caudal lateral wing subregions. Furthermore, capsaicin activated more Fos-positive cells than vehicle within all subregions of the DRN but with a caudal versus rostral predominance in activation pattern. In addition, a high proportion of capsaicin-induced Fos cells in the midline but almost none in lateral wing stained for NADPH-d. These observations suggest the existence of two functionally distinct populations of NOS neurons in the DRN. Furthermore, capsaicin increased galanin immunoreactivity with predominant staining in cell soma and fiber processes in midline and lateral wing subregions of the nucleus, respectively. The total capsaicin-induced galanin immunoreactivity was higher in rostral versus caudal DRN, and a high proportion of galanin-positive cells in the midline also contained NADPH-d and neuronal NOS, thus suggesting a potential NO-galanin interaction in these neurons. The differential pattern of Fos/NADPH-d colocalization across the nucleus suggests that midline and lateral wing NOS neurons of the DRN express their neuromodulatory actions on discrete efferent targets via different intracellular mechanisms.

Postsynaptic actions of substance P on rat periaqueductal grey neurons in vitro

The postsynaptic actions of substance P on rat midbrain periaqueductal grey (PAG) neurons were examined using whole-cell patch-clamp recordings in brain slices. Substance P produced an inward current in a subpopulation (60%) of PAG neurons. The substance P induced current was concentration dependent (EC50=27 nM) and was reduced by the NK1, NK2 and NK3 antagonists L-732,138 (20 microM), GR 159897 (3 microM) and SB 218795 (3 microM). The selective NK1, NK2 and NK3 agonists [Sar9,Met(O2)11]-Substance P (100 nM), GR 64349 (300-500 nM) and senktide (300 nM) also produced inward currents in subpopulations of neurons. A greater proportion of substance P-sensitive neurons (70%) than substance P-insensitive neurons (31%) responded to the mu/delta opioid agonist met-enkephalin (10 microM). Substance P reduced the outward current produced by met-enkephalin. The reversal potential of the substance P induced current varied from -5 mV to below -140 mV in the absence of met-enkephalin, and was -105 mV in the presence of met-enkephalin. These results indicate that substance P acts via NK1, NK2 and NK3 receptors to excite subpopulations of opioid-sensitive and insensitive PAG neurons by increasing a non-selective cation conductance and by reducing a K+ current. In addition, substance P has anti-opioid actions that are largely mediated by a reduction in the opioid induced K+ current.

Distribution of extracellular signal-regulated kinase 1- and 2-activated neurons in the rat periaqueductal gray matter after noxious stimulation

The periaqueductal gray matter (PAG), the midbrain region made up of neuronal columns encircling the cerebral aqueduct, plays a key role in nociception. As the extracellular signal-regulated kinases (ERKs) 1 and 2 are activated after noxious stimulation, we analyzed the distribution of ERK-activated neurons in the PAG after visceral noxious stimulation. Ether- and urethane-anesthetized rats received an intraperitoneal injection of acetic acid or were left untreated and were perfused after 2 hr. Serial sections immunoreacted with an antibody selective for the activated ERKs. Significant ERK activation occurred only in the ether-anesthetized noxious stimulated rats. In these rats, we evaluated the number of ERK-activated neurons and their density as the ratio of the number of immunolabeled neurons to the extension of the region where they were located. ERK-activated neurons were more numerous in the lateral (LPAG) and ventrolateral (VLPAG) columns, but without significant differences. No ERK activation was seen in neurons of the most rostral PAG. The ERK-activated neurons were significantly denser at the intermediate level of the PAG. At the caudal level, they were denser in the LPAG and VLPAG columns, and in the DPAG column at the intermediate and rostral level. These findings suggest that noxious stimulation activates ERKs in neurons involved in the different functional activities related to nociception, overlapping in the PAG columns, and strengthens the role of PAG in integration.

Substance P receptor antagonists in psychiatry: rationale for development and therapeutic potential

Increasing evidence suggests that substance P (SP) and its receptor (neurokinin [NK]-1 receptor [NK1R]) might play an important role in the modulation of stress-related, affective and/or anxious behaviour. First, SP and NK1R are expressed in brain regions that are involved in stress, fear and affective response (e.g. amygdala, hippocampus, hypothalamus and frontal cortex). Second, the SP content in these areas changes upon application of stressful stimuli. Third, the central administration of SP produces a range of fear-related behaviours. In addition, the SP/NK1R system shows significant spatial overlap with neurotransmitters such as serotonin and noradrenaline (norepinephrine), which are known to be involved in the regulation of stress, mood and anxiety. Therefore, it was hypothesised that blockade of the NK1R might have anxiolytic as well as antidepressant effects. Preclinical studies investigating the effects of genetic or pharmacological NK1R inactivation on animal behaviour in assays relevant to depression and anxiety revealed that the behavioural changes resemble those seen with reference antidepressant or anxiolytic drugs. Furthermore, antagonism or genetic inactivation of the NK1R causes alterations in serotonin and norepinephrine neuronal transmission that are likely to contribute to the antidepressant/anxiolytic activity of NK1R antagonists but that are--at least partially--distinct from those produced by established antidepressant drugs. This underlines the conceivable unique mechanism of action of this new class of compounds. In three independent clinical trials with three different compounds (aprepitant [MK-869], L-759274 and CP-122721), an antidepressant effect of NK1R antagonists could be demonstrated. These results, however, have been challenged by recent failed studies with aprepitant. There are numerous indications from preclinical studies that, in addition to SP and NK1R, other neurokinins and/or neurokinin receptors might also be involved in the modulation of stress-related behaviour and that exclusive blockade of the NK1R might not be sufficient to produce consistent anxiolytic and antidepressant effects. One such candidate is the neurokinin-2 receptor (NK2R), and clinical trials to assess the antidepressant effects of NK2R antagonists are currently underway. Of special interest might also be substances that block more than one receptor type such as NK1/2R antagonists or NK1/2/3R antagonists. These compounds may be more efficacious in antagonising the effects of SP than compounds that only block the NK1R.

Effect of neurokinin-1 receptor antagonists on serotoninergic, noradrenergic and hippocampal neurons: comparison with antidepressant drugs

Neurokinin-1 (NK1) receptor antagonists have been reported to possess antidepressant and anxiolytic properties in controlled trials. Since antidepressant and anxiolytic drugs act mainly by enhancing serotonin (5-HT) and norepinephrine (NE) neurotransmission in forebrain areas, the main focus of the present review is to critically examine the electrophysiological effects of NK1 receptor antagonists on serotoninergic and noradrenergic neurons, and then hippocampal neurons. It is concluded that NK1 antagonists increase the firing and burst activity of 5-HT neurons, increase burst activity of NE neurons, and modulate postsynaptic transmission at the hippocampus level. Further research is needed in order to develop more selective ligands for the human NK1 receptor and to gain better knowledge of required brain penetration and optimal pharmacodynamic conditions for their use in patients.

The impact of chronic nandrolone decanoate administration on the NK1 receptor density in rat brain as determined by autoradiography

Adult male Sprague-Dawley rats were treated with the anabolic androgenic steroid nandrolone decanoate (15 mg/kg day) or oil vehicle (sterile arachidis oleum) during 14 days. The effect on the densities of the neurokinin NK1 receptor in brain was examined with autoradiography. An overall tendency of attenuation of NK1 receptor density was observed after completed treatment with nandrolone decanoate. The density of the NK1 receptor was found to be significantly lower compared to control animals in the nucleus accumbens core (37% density reduction), in dentate gyrus (26%), in basolateral amygdaloid nucleus (23%), in ventromedial hypothalamic nucleus (36%), in dorsomedial hypothalamic nucleus (43%) and finally in the periaqueductal gray (PAG) (24%). In the cortex region, no structures exhibited any significant reduction of NK1 receptor density. This result provides additional support to the hypothesis that substance P and the NK1 receptor may be involved as important components that participate in mediating physiological responses including the adverse behaviors often associated with chronically administrated anabolic androgenic steroids in human.

Alterations in stress-associated behaviors and neurochemical markers in adult rats after neonatal short-lasting local inflammatory insult

Recently, there has been a growing interest in long-term consequences of neonatal pain because modern neonatal intensive care units routinely employ procedures that cause considerable pain and may be followed by local inflammation and hyperalgesia lasting for several hours or even days. To address this question, we developed a rat model of short lasting (<2 days) early local inflammatory insult produced by a single injection of 0.25% carrageenan (CAR) into the plantar surface of a hindpaw. Previously, we demonstrated that rats receiving this treatment within the first week after birth grow into adults with a global reduction in responsiveness to acute pain. Here, we report that these animals also manifest a low anxiety trait associated with reduced emotional responsiveness to stress. This conclusion is based in the following observations: (a) rats in our model display reduced anxiety on an elevated plus-maze; (b) in the forced swim test, these rats exhibit behavioral characteristics associated with stronger ability for stress coping; and (c) these animals have reduced basal and stress-induced plasma levels of such stress-related neuroendocrine markers as corticotropin-releasing factor, vasopressin, and adrenocorticotrophic hormone. In addition, we used DNA microarray and real-time reverse-transcriptase polymerase chain reaction to profile long-term changes in gene expression in the midbrain periaqueductal gray (PAG; a region involved in both stress and pain modulation) in our animal model. Among the affected genes, serotonergic receptors were particularly well represented. Specifically, we detected increase in the expression of 5-HT1A, 5-HT1D, 5-HT2A, 5-HT2C and 5-HT4 receptors. Several of these receptors are known to be involved in the anxiolytic and analgesic activity of the PAG. Finally, to determine whether neonatal inflammatory insult induces elevation in maternal care, which may play a role in generating long-term behavioral alterations seen in our model, we examined maternal behavior for 3 days following CAR injection. Indeed, we observed a substantial increase in maternal attention to the pups at the time of inflammation, but this increase was not without its cost: a period of significant maternal neglect afterward.