Sexual dimorphism in the response to N-methyl-D-aspartate receptor antagonists and morphine on behavior and c-Fos induction in the rat brain

Morphine exposure alters Fos expression in a sex-, age-, and brain region-specific manner during adolescence

Data in both humans and preclinical animal models clearly indicate drug exposure during adolescence, when the "reward" circuitry of the brain develops, increases the risk of substance use and other mental health disorders later in life. Human data indicate that different neural and behavioral sequelae can be observed in early versus late adolescence. However, most studies with rodent models examine a single adolescent age compared to a mature adult age, and often only in males. Herein, we sought to determine whether the acute response to the opioid morphine would also differ across adolescence, and by sex. By quantifying Fos positive cells, a proxy for neural activity, at different stages during adolescence (pre-, early, mid-, and late adolescence) and in multiple reward regions (prefrontal cortex, nucleus accumbens, caudate/putamen), we determined that the neural response to acute morphine is highly dependent on adolescent age, sex, and brain region. These data suggest that heterogeneity in the consequences of adolescent opioid exposure may be due to age- and sex-specific developmental profiles in individual reward processing regions. In future studies, it will be important to add age within adolescence as an independent variable for a holistic view of healthy or abnormal reward-related neural development.

Rostral Intralaminar Thalamus Engagement in Cognition and Behavior

The thalamic rostral intralaminar nuclei (rILN) are a contiguous band of neurons that include the central medial, paracentral, and central lateral nuclei. The rILN differ from both thalamic relay nuclei, such as the lateral geniculate nucleus, and caudal intralaminar nuclei, such as the parafascicular nucleus, in afferent and efferent connectivity as well as physiological and synaptic properties. rILN activity is associated with a range of neural functions and behaviors, including arousal, pain, executive function, and action control. Here, we review this evidence supporting a role for the rILN in integrating arousal, executive and motor feedback information. In light of rILN projections out to the striatum, amygdala, and sensory as well as executive cortices, we propose that such a function enables the rILN to modulate cognitive and motor resources to meet task-dependent behavioral engagement demands.

22q11.2 deletion syndrome and schizophrenia

22q11.2 deletion is a common microdeletion that causes an array of developmental defects including 22q11.2 deletion syndrome (22q11DS) or DiGeorge syndrome and velocardiofacial syndrome. About 30% of patients with 22q11.2 deletion develop schizophrenia. Mice with deletion of the ortholog region in mouse chromosome 16qA13 exhibit schizophrenia-like abnormal behaviors. It is suggested that the genes deleted in 22q11DS are involved in the pathogenesis of schizophrenia. Among these genes, COMT, ZDHHC8, DGCR8, and PRODH have been identified as schizophrenia susceptibility genes. And DGCR2 is also found to be associated with schizophrenia. In this review, we focused on these five genes and reviewed their functions in the brain and the potential pathophysiological mechanisms in schizophrenia, which will give us a deeper understanding of the pathology of schizophrenia.

The thalamic midline nucleus reuniens: potential relevance for schizophrenia and epilepsy

Anatomical, electrophysiological and behavioral studies in rodents have shown that the thalamic midline nucleus reuniens (RE) is a crucial link in the communication between hippocampal formation (HIP, i.e., CA1, subiculum) and medial prefrontal cortex (mPFC), important structures for cognitive and executive functions. A common feature in neurodevelopmental and neurodegenerative brain diseases is a dysfunctional connectivity/communication between HIP and mPFC, and disturbances in the cognitive domain. Therefore, it is assumed that aberrant functioning of RE may contribute to behavioral/cognitive impairments in brain diseases characterized by cortico-thalamo-hippocampal circuit dysfunctions. In the human brain the connections of RE are largely unknown. Yet, recent studies have found important similarities in the functional connectivity of HIP-mPFC-RE in humans and rodents, making cautious extrapolating experimental findings from animal models to humans justifiable. The focus of this review is on a potential involvement of RE in schizophrenia and epilepsy.

Puberty marks major changes in the hippocampal and cortical c-Fos activation pattern induced by NMDA receptor antagonists

Non-selective and subunit (GluN2B)-specific N-methyl-d-aspartate receptor (NMDAR) antagonists represent promising alternative antidepressant drugs with fast onset of the therapeutic action. The neuronal activation pattern induced by NMDAR antagonists is well characterized by c-Fos expression analysis only in the adult rodent brain. In contrast, there is little information available regarding their effects during postnatal development. Here we performed a systematic c-Fos brain mapping of the non-selective NMDAR antagonist MK-801 and the GluN2B-specific antagonist Ro 25-6981 from postnatal day 16 (P16) to P40. We found significant regional differences with gender-specificity in the activation pattern compared to the adult. Surprisingly, in the hippocampus, MK-801 triggered at pre-pubertal stages (especially at P24) very strong c-Fos expression, followed by low levels after P30, the approximate time point of puberty onset in mice. The cortical distribution of MK-801-triggered c-Fos expression before puberty differed also substantially from the adult brain, showing high levels only in deep cortical layers at pre-pubertal stages. In comparison, the cortical activation induced by Ro 25-6981 diminished from high pre-pubertal levels and was in comparison with that triggered by MK-801 low in the hippocampus. These results reveal highly dynamic changes in the c-Fos activation pattern induced by NMDAR antagonists during puberty. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Father absence in the monogamous california mouse impairs social behavior and modifies dopamine and glutamate synapses in the medial prefrontal cortex

The role of the father in psycho-affective development is indispensable. Yet, the neurobehavioral effects of paternal deprivation (PD) are poorly understood. Here, we examined the behavioral consequences of PD in the California mouse, a species displaying monogamous bonding and biparental care, and assessed its impact on dopamine (DA), serotonin (5-HT), and glutamate (GLU) transmission in the medial prefrontal cortex (mPFC). In adult males, deficits in social interaction were observed, when a father-deprived (PD) mouse was matched with a PD partner. In adult females, deficits were observed when matching a PD animal with a non-PD control, and when matching 2 PD animals. PD also increased aggression in females. Behavioral abnormalities in PD females were associated with a sensitized response to the locomotor-activating effect of amphetamine. Following immunocytochemical demonstration of DA, 5-HT, and GLU innervations in the mPFC, we employed in vivo electrophysiology and microiontophoresis, and found that PD attenuated the basal activity of low-spiking pyramidal neurons in females. PD decreased pyramidal responses to DA in females, while enhancing responses to NMDA in both sexes. We thus demonstrate that, during critical neurodevelopmental periods, PD leads to sex-dependent abnormalities in social and reward-related behaviors that are associated with disturbances in cortical DA and GLU neurotransmission.

Neurocircuitry for modeling drug effects

The identification and functional understanding of the neurocircuitry that mediates alcohol and drug effects that are relevant for the development of addictive behavior is a fundamental challenge in addiction research. Here we introduce an assumption-free construction of a neurocircuitry that mediates acute and chronic drug effects on neurotransmitter dynamics that is solely based on rodent neuroanatomy. Two types of data were considered for constructing the neurocircuitry: (1) information on the cytoarchitecture and neurochemical connectivity of each brain region of interest obtained from different neuroanatomical techniques; (2) information on the functional relevance of each region of interest with respect to alcohol and drug effects. We used mathematical data mining and hierarchical clustering methods to achieve the highest standards in the preprocessing of these data. Using this approach, a dynamical network of high molecular and spatial resolution containing 19 brain regions and seven neurotransmitter systems was obtained. Further graph theoretical analysis suggests that the neurocircuitry is connected and cannot be separated into further components. Our analysis also reveals the existence of a principal core subcircuit comprised of nine brain regions: the prefrontal cortex, insular cortex, nucleus accumbens, hypothalamus, amygdala, thalamus, substantia nigra, ventral tegmental area and raphe nuclei. Finally, by means of algebraic criteria for synchronizability of the neurocircuitry, the suitability for in silico modeling of acute and chronic drug effects is indicated. Indeed, we introduced as an example a dynamical system for modeling the effects of acute ethanol administration in rats and obtained an increase in dopamine release in the nucleus accumbens-a hallmark of drug reinforcement-to an extent similar to that seen in numerous microdialysis studies. We conclude that the present neurocircuitry provides a structural and dynamical framework for large-scale mathematical models and will help to predict chronic drug effects on brain function.

Sex and gender differences in pain and analgesia

It is a clinical reality that women make up the large majority of chronic pain patients, and there is now consensus from laboratory experiments that when differences are seen, women are more sensitive to pain than men. Research in this field has now begun to concentrate on finding explanations for this sex difference. Although sex differences in sociocultural, psychological, and experiential factors likely play important roles, evidence largely from animal studies has revealed surprisingly robust and often qualitative sex differences at low levels of the neuraxis. Although not yet able to affect clinical practice, the continued study of sex differences in pain may have important implications for the development of new analgesic strategies.

Sex does matter: comments on the prevalence of male-only investigations of drug effects on rodent behaviour

Despite abundant evidence of sex differences in the effects of drugs on nonsexual behaviour in rats and mice, most researchers continue to investigate male animals exclusively. This was evident from a survey of all relevant research reports published during the period February 2005-September 2006 (inclusive) in recent issues of five representative behavioural pharmacological journals. Reasons for excluding female animals from most studies are discussed along with attempts to justify the use of either male or female animals only, and the value of including both sexes (especially when a drug effect is poorly understood). Although there are other factors that can influence the effects of drugs, such as strain, age and social density, the sex of experimental animals is the easiest to control and thus is well suited to inclusion in pharmacological investigations. It is accordingly suggested that, as has been recommended many times in the past, animals' sex should play a more important part in future research than is still currently the case.

Sex differences in NMDA antagonist enhancement of morphine antihyperalgesia in a capsaicin model of persistent pain: comparisons to two models of acute pain

In acute pain models, N-methyl-D-aspartate (NMDA) antagonists enhance the antinociceptive effects of morphine to a greater extent in males than females. The purpose of this investigation was to extend these findings to a persistent pain model which could be distinguished from acute pain models on the basis of the nociceptive fibers activated, neurochemical substrates, and duration of the nociceptive stimulus. To this end, persistent hyperalgesia was induced by administration of capsaicin in the tail of gonadally intact F344 rats, following which the tail was immersed in a mildly noxious thermal stimulus, and tail-withdrawal latencies measured. For comparison, tests were conducted in two acute pain models, the hotplate and warm water tail-withdrawal procedures. In males, the non-competitive NMDA antagonist dextromethorphan enhanced the antihyperalgesic effect of low to moderate doses of morphine in a dose-and time-dependent manner. Across the doses and pretreatment times examined, enhancement was not observed in females. Enhancement of morphine antinociception by dextromethorphan was seen in both males and females in the acute pain models, with the magnitude of this effect being greater in males. These findings demonstrate a sexually-dimorphic interaction between NMDA antagonists and morphine in a persistent pain model that can be distinguished from those observed in acute pain models.

Gabapentin completely attenuated the acute morphine induced c-Fos expression in the rat striatum

The neuro-anatomical sites and molecular mechanism of action of gabapentin (GBP)-morphine interaction to prevent and reverse morphine side effects as well as enhancement of the analgesic effect of morphine is not known. Therefore, we examined the combined effects of GBP-Morphine on acute morphine induced c-Fos expression in rat striatum. The combined effect of GBP-Morphine was examined by means of c-Fos immunohistochemistry. A single intraperitoneal injection (i.p.) of morphine (10 mg/kg), saline (control), co-injection of GBP (150 mg/kg) with morphine (10 mg/kg) was administered under anaesthesia. Ninety minutes after drugs administration the deeply anesthetized rats were perfused transcardially with 4% paraformaldehyde. Serial 40 mum thick sections of brain were cut and processed by immunohistochemistry to locate and quantify the sites and number of neurons with c-Fos immunoreactivity. Detection of c-Fos protein was performed using the peroxidase-antiperoxidase (PAP) detection protocol. Our present study demonstrated that, administration of GBP (150 mg/kg, i.p.) in combination with morphine (10 mg/kg, i.p.) significantly (p < 0.01) attenuated the acute morphine (10 mg/kg, i.p.) induced c-Fos expression in the rat striatum. Present results showed that GBP-morphine combination action prevented the acute morphine induced c-Fos expression in rat striatum. Moreover, this study provides first evidence of neuro-anatomical site and that GBP neutralized the morphine induced activation of rat striatum.

Greater vulnerability to the amnestic effects of ketamine in males

RATIONALE

Gender differences both in response to ketamine in animals and general cognitive functioning in humans have been observed and suggested to be related to modulatory effects of sex hormones on N-methyl-D: -aspartate receptor (NMDA-R) functioning.

OBJECTIVES

The current study aimed to determine whether there were gender differences in response to ketamine in humans.

METHODS

Behavioral data including positive and negative symptoms (Brief Psychiatric Rating Scale), perceptual alterations (Clinician-Administered Dissociative States Scale, CADSS), and "high" and "anxiety" states (Visual Analog Scale) from 295 subjects who participated in a total of 11 placebo-controlled ketamine studies were analyzed. In a subset of subjects, memory (Hopkins Verbal Learning Task: HVLT, n=108) and attention (continuous performance task, n=177) data were also analyzed.

RESULTS

Male participants showed a greater performance decrement on the HVLT after ketamine administration compared to women. Men also reported a greater subjective sense of memory impairment on a CADSS subscale. No other gender differences in behavioral or cognitive measures were observed.

CONCLUSIONS

Men showed a greater vulnerability to the amnestic effects of ketamine than women. Possible explanations of these findings are neuroanatomical and cognitive differences in processing of words in men and women and interactions between sex hormones and NMDA-R function.

Bromocriptine and clozapine regulate dopamine 2 receptor gene expression in the mouse striatum

In a previous study, we showed that the psychoactive drug caffeine alters the expression of the dopamine 2 receptor (D2R) gene in vitro and in vivo. Here, we report that acute administration of antipsychotic and anti-parkinsonian drugs also regulate D2R gene expression in PC12 cells and in the mouse striatum. Treatment of PC12 cells with the atypical antipsychotic and specific 5-HT antagonist clozapine (60 microM) reduced D2R/luciferase reporter expression by 46% after 24 h. However, male and female mice treated with a clinical dose of clozapine (10 mg/kg) showed no changes in striatal D2R mRNA expression when assayed by quantitative RT-PCR. Treatment of PC12 cells with the specific D2R agonist anti-parkinsonian drug, bromocriptine mesylate (BCM; 5 microM) also resulted in decreased D2R/luciferase reporter activity (27%). In contrast to clozapine, a clinical dose of BCM (16 mg/kg) led to a 21% decrease and a 45% increase in striatal D2R mRNA expression in male and female mice, respectively, after 24 h. Coadministration of clozapine and BCM in PC12 cells resulted in a synergistic decrease in D2R/luciferase reporter expression (68%), and coadministration of these drugs in vivo led to decreases in striatal D2R mRNA expression in both male and female mice (45% and 22%, respectively). Collectively, these results indicate that clozapine, BCM, or a combination of these drugs have differential effects on dopamine receptor gene expression and might also affect striatal physiology in a sexually dimorphic manner.

Morphine-induced c-fos mRNA expression in striatofugal circuits: modulation by dose, environmental context, and drug history

Opiates and psychostimulants produce many shared behavioral and neurobiological adaptations, such as behavioral sensitization and the induction of immediate early genes in the caudate-putamen (CPu). Previous studies indicate that factors such as dose, the environmental context surrounding drug administration and drug history can influence both morphine- and psychostimulant-induced behavioral sensitization. In addition, these factors can modulate the ability of psychostimulants to engage striatofugal circuits in the CPu. The present study, therefore, sought to examine whether these factors have similar influences over the ability of morphine to engage cortico-striatofugal circuits. We report that, when given in the home cage, morphine produced a small, but significant increase in the number of c-fos+ striatonigral cells and c-fos+ cells in cingulate cortex, but had no effect on the number of c-fos+ striatopallidal cells. When given in a novel test environment, however, morphine dramatically increased the number of c-fos+ striatonigral cells in a dose-dependent fashion, and this effect was maintained following repeated treatment. Unexpectedly, morphine treatment in a novel environment produced a dose-dependent reduction in the number of c-fos+ striatopallidal cells and c-fos+ cells in cingulate cortex, relative to exposure to novelty alone-effects that were reversed by repeated morphine treatment. We suggest that alterations in c-fos expression patterns in striatofugal circuits following morphine administration may be involved in drug-experience-dependent plasticity.

Sexual dimorphism and the NMDA receptor in alloparental behavior in juvenile prairie voles (Microtus ochrogaster)

The prairie vole (Microtus ochrogaster) exhibits parental behavior in both males and females and extensive alloparenting in juveniles. The authors studied the effects on juvenile alloparenting of antagonists for the PCP, glycine, and glutamate sites on the N-methyl-D-aspartate (NMDA) receptor. In male voles, all 3 drugs had an inverted-U dose-response curve. This change could not be attributed to fear of the pup or a nonspecific impairment of cognition, level of locomotor activity, or motor coordination. The PCP site antagonist had a U-shaped dose-response curve in females, the opposite of that in males, but neither of the other drugs changed female alloparental behavior. Both male and female voles exhibit alloparental behavior, but its neurobiological underpinnings are sexually dimorphic in juveniles.

Sex and estrogen influence drug abuse

Evidence is accumulating that the etiology, epidemiology, consequences and mechanisms that underlie drug abuse are different in males and females. In this review, we present examples of sex differences in all phases of drug abuse, including acquisition, steady-state maintenance, escalation, dysregulation, withdrawal, relapse and treatment. Most reported findings are based on laboratory research in animals, but there are corroborating reports from human clinical and epidemiological studies. In all phases of drug abuse, females seem to be more sensitive to the rewarding effects of drugs than males, and estrogen is a major factor that underlies these sex differences.

Evidence that the gene encoding ZDHHC8 contributes to the risk of schizophrenia

Using a relatively dense genetic map of 72 single-nucleotide polymorphisms (SNPs) distributed across the entire 1.5-Mb locus on chromosome 22q11 associated with susceptibilit to schizophrenia, we previously identified two subregions that were consistently associated with the disease. In the distal subregion, we detected an association signal with five neighboring SNPs distributed over a haplotypic block of 80 kb encompassing six known genes. One of these five SNPs, rs175174, had the strongest association of all 72 SNPs that we tested. Here we show that rs175174 regulates the level of the fully functional transcript by modulating the retention of intron 4 of the gene ZDHHC8, which encodes a putative transmembrane palmitoyltransferase. Zdhhc8-knockout mice had a sexually dimorphic deficit in prepulse inhibition, a gene dosage-dependent decrease in exploratory activity in a new environment and a decreased sensitivity to the locomotor stimulatory effects of the psychomimetic drug dizocilpine (MK801). SNP rs175174 shows differences in transmission distortion between sexes in individuals with schizophrenia. Our results indicate that there is an unexpected connection between impaired palmitate modification of neuronal proteins and the psychiatric phenotypes associated with microdeletions of chromosome 22q11.

Behavioral and neurochemical vulnerability during adolescence in mice: studies with nicotine

People are very likely to start psychoactive drug use during adolescence, an earlier onset being associated with a higher risk of developing addiction later in life. In experiment I, Pre- (postnatal day (pnd) 23-35), Mid- (pnd 36-48), or Post- (pnd 49-61) adolescent mice underwent a restricted-drinking period (2 h/day for 12 days), one bottle containing water and the other containing nicotine (10 mg/l) or water. After this period, Mid-adolescents showed prominent exploration and reduced anxiety in the plus-maze. This ontogenetic profile was dampened by nicotine consumption. After 2 months, these mice were tested in a novel environment (30 min/day for 3 days). Locomotor-habituation profiles were specifically disrupted by nicotine consumption during Mid-adolescence, suggesting this age as a critical period. In experiment II, Mid-adolescent (pnd 35-44) and adult (pnd > 70) mice were pretreated with nicotine (0, 0.03, 0.10, 0.30 mg/kg/day for 10 days). Acute nicotine administration had opposite effects on anxiety in adolescents and adults. At 2 months after pretreatment, we measured levels of AMPA GluR2/3 subunits, thought to be involved in the control of addictive behaviors. Nicotine exposure during Mid-adolescence dose-dependently downregulated these subunits in the striatum and hippocampus, but comparable exposure during adulthood had either opposite or no effects. NMDA NR2A/B subunits were affected by nicotine, but without age-related differences. The present data identified a nicotine-vulnerable age window, characterized by long-term disruption of locomotor habituation and downregulation of AMPA receptors. These findings support neurobiological vulnerability to drugs in adolescent humans.

Stomach-brain communication by vagal afferents in response to luminal acid backdiffusion, gastrin, and gastric acid secretion

Vagal afferents play a role in gut-brain signaling of physiological and pathological stimuli. Here, we investigated how backdiffusion of luminal HCl or NH(4)OH and pentagastrin-stimulated acid secretion interact in the communication between rat stomach and brain stem. Rats were pretreated intraperitoneally with vehicle or appropriate doses of cimetidine, omeprazole, pentagastrin, dexloxiglumide (CCK(1) receptor antagonist), and itriglumide (CCK(2) receptor antagonist) before intragastric administration of saline or backdiffusing concentrations of HCl or NH(4)OH. Two hours later, neuronal activation in the nucleus of the solitary tract (NTS) and area postrema was visualized by c-Fos immunohistochemistry. Exposure of the rat gastric mucosa to HCl (0.15-0.5 M) or NH(4)OH (0.1-0.3 M) led to a concentration-dependent expression of c-Fos in the NTS, which was not related to gender, gastric mucosal injury, or gastropyloric motor alterations. The c-Fos response to HCl was diminished by cimetidine and omeprazole, enhanced by pentagastrin, and left unchanged by dexloxiglumide and itriglumide. Pentagastrin alone caused an omeprazole-resistant expression of c-fos, which in the NTS was attenuated by itriglumide and prevented by dexloxiglumide but in the area postrema was reduced by dexloxiglumide and abolished by itriglumide. We conclude that vagal afferents transmit physiological stimuli (gastrin) and pathological events (backdiffusion of luminal HCl or NH(4)OH) from the stomach to the brain stem. These communication modalities interact because, firstly, acid secretion enhances afferent signaling of gastric acid backdiffusion and, secondly, gastrin activates NTS neurons through stimulation of CCK(1) receptors on vagal afferents and of CCK(2) receptors on area postrema neurons projecting to the NTS.

Caffeine regulates neuronal expression of the dopamine 2 receptor gene

The psychoactive drug caffeine influences neuronal physiology; however, it is unknown whether it can dynamically alter the expression of genes that influence neurotransmission. Here, we report that caffeine stimulates transcription of the dopamine 2 receptor (D2R) gene in PC-12 cells and primary striatal cultures and increases D2R protein expression in the striatum. Physiological doses of caffeine and the specific adenosine 2A receptor antagonist 8-(3-chlorostyryl) caffeine both increased the activity of a D2R/luciferase reporter construct within 24 h, and simultaneous treatment with 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680), a specific adenosine 2A receptor agonist, eliminated this effect. Tests of additional constructs revealed that specific regions of the D2R promoter (-117/-75) and 5'-untranslated region (+22/+317) were required for activation of D2R gene expression by caffeine. In primary striatal cultures, caffeine increased spontaneous firing of neurons between 12 and 80 min after treatment, whereas it increased D2R mRNA expression after only 4 h. These results indicate that regulation of D2R gene expression by caffeine occurs after the initial physiological response has subsided. In vivo, female mice treated with a dose of caffeine (50 mg/kg) showed 1.94- and 2.07-fold increases in D2R mRNA and protein expression, respectively. In contrast, male mice exhibited a 31% decrease in D2R mRNA expression and showed no changes in D2R protein expression. Collectively, these results demonstrate for the first time that caffeine alters D2R expression in neurons. They also suggest that caffeine consumption can lead to sexually dimorphic patterns of gene expression in the brain.

Morphine actions in the rat forebrain: role of protein kinase C

Acute administration of morphine induces expression of the immediate-early gene (IEG) c-Fos in dorsomedial striatum, portions of cerebral cortex, and in several midline-intralaminar thalamic nuclei, partly via a trans-synaptic mechanism that involves activation of glutamate receptors. Because activation of protein kinase C (PKC) may occur following the activation of glutamate receptors, we determined whether pharmacological inhibition of PKC would attenuate morphine-induced c-Fos expression, and whether acute administration of morphine would induce translocation of PKC. The selective PKC antagonist NPC 15437 given 30 min prior to morphine significantly decreased morphine-induced c-Fos expression in striatum and cingulate cortex, but not in centrolateral thalamus. In another experiment, rats were given an acute dose of morphine, and immunocytochemical analysis was performed for the betaI and betaII isoforms of PKC. Morphine induced a rapid and transient translocation of PKC betaII, but not betaI, from perinuclear spots to plasma membrane in numerous cortical and striatal neurons. Prior administration of naloxone blocked this response. Ultrastructural studies confirmed translocation from Golgi apparatus to plasma membrane 15 min after morphine injection. Double immunocytochemistry at the light microscopic level demonstrated co-localization of translocated PKC betaII and c-Fos in some cortical neurons 90 min after morphine injection. These results support a role for PKC, especially PKC betaII, in the rapid effects of morphine on the brain.

Long-term behavioural, molecular and morphological effects of neonatal NMDA receptor antagonism

Brief N-methyl-D-aspartate (NMDA) receptor blockade in neonatal rats has been reported to increase neuronal apoptosis. We replicated this finding using MK-801 (0.5 mg/kg) administered twice on postnatal day 7, and then studied the long-term consequences. In adulthood, treated rats showed reduced volume and neuronal number within the hippocampus, and altered hippocampal NMDA receptor (NR1 subunit) expression. Synaptophysin mRNA was decreased in the thalamus (laterodorsal nucleus). Adult MK-801-treated females had prepulse inhibition deficits and increased locomotor activity. The data show that a transient and limited glutamatergic intervention during development can have chronic behavioural, structural and molecular effects. The effects are reminiscent of alterations reported in schizophrenia and, as such, are consistent with hypotheses advocating a role for NMDA receptor hypofunction, and aberrant apoptosis, in the neurodevelopmental pathogenesis of the disorder.

Morphine induction of c-fos expression in the rat forebrain through glutamatergic mechanisms: role of non-n-methyl-D-aspartate receptors

Acute injection of morphine induces expression of the immediate-early genes c-Fos and JunB in several forebrain regions of the rat, in part through an N-methyl-D-aspartate (NMDA) receptor-dependent mechanism. Because membrane depolarization through (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors is believed to be necessary for full activation of NMDA receptors, we determined the role of AMPA receptors in morphine-induced c-Fos expression. Rats were given the AMPA receptor antagonist GYKI-52466 (12.9 mg/kg, i.p.) 15 min before morphine (10 mg/kg, s.c.), or the AMPA receptor enhancer CX516 (30 mg/kg, i.p.) 5 min after morphine. The c-Fos response was attenuated by the antagonist and augmented by the enhancer. Using double immunocytochemistry, we found that morphine induced c-Fos in neurons containing the GluR2/3, but not the GluR1 and rarely the GluR4, subunits of the AMPA receptor. Double immunocytochemistry for mu opioid receptor and c-Fos showed that c-Fos expression was mainly absent in the patch compartment of the striatum, which is enriched in mu opioid receptors. The glutamatergic synapse often contains metabotropic receptors as well as ionotropic receptors. Type I metabotropic glutamate receptors are coupled to activation of protein kinase C, which has also been shown to mediate the immediate-early gene response to morphine. To determine if activation of metabotropic glutamate receptors is involved in rapid effects of morphine on the brain, rats were given the type I metabotropic glutamate receptor antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 0.2 mg/kg, i.p.) or vehicle 30 min before morphine treatment. Pretreatment with AIDA completely blocked morphine-induced c-Fos expression in the caudate-putamen.Taken together, these results demonstrate involvement of both AMPA and type I metabotropic glutamate receptors in the acute effects of morphine on the forebrain, supporting an important role for glutamatergic neurotransmission mediated by non-NMDA glutamate receptors in morphine's actions.

Effects of postnatal PCP treatment on locomotor behavior and striatal D2 receptor

Exposing the developing brain to the N-methyl-D-aspartate (NMDA) receptor antagonist phencyclidine (PCP) has been shown to cause deficits in neurobehavioral functions, particularly on learning and memory and seizure sensitivity. Besides acting as a noncompetitive NMDA antagonist, PCP at high doses is known to affect the dopaminergic system. The present study assessed the effect of postnatal PCP treatment on locomotor activity and striatal dopamine (DA) D(2) receptor. Male and female rat pups were injected intraperitoneally (i.p.) with one of three doses of PCP (1, 3 and 5 mg/kg) or saline from postnatal day (PD) 5 to PD 15. Control and PCP-treated rats were given a challenge dose of PCP (10 mg/kg) as adults, and their locomotor behaviors--locomotion, stereotypy and ataxia--were scored. Postnatal PCP treatment did not have any significant effect in either sex on any of the PCP-induced locomotor behavioral paradigms studied. Separate groups of male and female rats were treated daily with saline or PCP (5 mg/kg i.p.) from PD 5 to PD 15 and sacrificed either as juveniles (PD 21) or adults, and D(2) receptor binding was measured in their striata. Striatal D(2) receptor density in juvenile and adult male postnatal PCP-treated rats did not differ from saline-treated controls. Adult female PCP-treated rats showed a slight but significant reduction in the maximal binding of striatal D(2) receptors. There was no effect of postnatal PCP on striatal D(2) receptor binding in female juvenile rats. These results support the hypothesis that blocking the developing NMDA receptor minimally affects PCP-induced locomotor behavior and the striatal D(2) receptor.

Modulation of glutamate receptor expression by gonadal steroid hormones in the rat striatum

The non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist dizocilpine (MK-801) attenuates morphine-induced immediate-early gene expression in the rat striatum in a sexually dimorphic manner that depends in part on gonadal steroids. To determine if this effect was dependent on modulation of glutamate receptor gene expression, we studied the effect of gonadal hormones on levels of the NR1 subunit of NMDA receptor and the GluR2 subunit of the AMPA-subtype of glutamate receptor in the rat striatum, using autoradiographic immunocytochemistry. We found that ovariectomy decreased GluR2 immunoreactivity in the striatum, but no changes were seen in levels of NR1 following gonadectomy in either sex. Thus, the effects of gonadal steroids on NMDA receptor-mediated responses are not due to regulation of NR1 expression.

mu-opioid receptor-mediated antinociceptive responses differ in men and women

Sex differences in the experience of clinical and experimental pain have been reported. However, the neurobiological sources underlying the variability in pain responses between sexes have not been adequately explored, especially in humans. The endogenous opioid neurotransmitters and mu-opioid receptors are centrally implicated in responses to stress, in the suppression of pain, and in the action of opiate analgesic drugs. Here we examined sex differences in the activation of the mu-opioid system in response to an intensity-controlled sustained deep-tissue pain challenge with positron emission tomography and a mu-opioid receptor-selective radiotracer. Twenty-eight young healthy volunteers (14 men and 14 women) were studied during saline control and pain conditions using a double-blind, randomized, and counterbalanced design. Women were scanned during the early follicular phase of their menstrual cycles after ovulatory cycles. Significant sex differences in the regional activation of the mu-opioid system in response to sustained pain were detected compared with saline controls. Men demonstrated larger magnitudes of mu-opioid system activation than women in the anterior thalamus, ventral basal ganglia, and amygdala. Conversely, women demonstrated reductions in the basal state of activation of the mu-opioid system during pain in the nucleus accumbens, an area previously associated with hyperalgesic responses to the blockade of opioid receptors in experimental animals. These data demonstrate that at matched levels of pain intensity, men and women during their follicular phase differ in the magnitude and direction of response of the mu-opioid system in distinct brain nuclei.

Sexually dimorphic effects of morphine and MK-801: sex steroid-dependent and -independent mechanisms

Rats show gender differences in responses to morphine and the N-methyl-D-aspartate receptor antagonist dizocilpine (MK-801); the role of sex steroids in mediating these differences is unclear. We tested the overall hypothesis that circulating gonadal steroids determine the gender differences in morphine- and MK-801-induced behavior and c-Fos expression. Morphine caused a greater expression of c-Fos in the striatum of intact males than of that females, which was independent of sex steroids. MK-801 completely inhibited morphine-induced c-Fos in intact females but only caused partial inhibition in intact males; castrated males showed complete inhibition, which was reversed by testosterone, but gonadal steroids had no effect on this response in females. In thalamus, there was a large sex difference in the response to MK-801 that was independent of gonadal steroids. Behavioral responses to morphine were greater in males, but responses to MK-801 were greater in females; both were sex steroid independent. These findings show significant sex differences in response to morphine and MK-801 that are mediated by sex steroid-dependent and -independent mechanisms, which may be important in treatment outcomes of drug addiction.

Gender differences in spontaneous and MK-801-induced activity after striatal lesions

At different times post-lesion, the excitotoxically lesioned striatum has been shown to undergo significant neuroanatomical and neurochemical changes, which could be expressed behaviorally. Gender and dose of excitotoxin are other variables that may modify the behavioral effects of the lesion. Consequently, the purpose of this study was to determine the effect of dose, gender, and time post-lesion on spontaneous and drug-induced locomotor behavior after intrastriatal KA lesions. Results showed that dose and time post-lesion had a significant effect on the deficits observed. Hyperactivity induced by the lesion with KA (5 nm) subsided as time post-lesion increased. Both the pattern of spontaneous and MK-801-induced locomotor activity were different for male and female rats. In female animals with KA lesions (5 nm), MK-801 did not stimulate ambulatory activity nor reduce vertical activity. Both female and male rats lesioned with KA (5 nm) showed an exaggerated response to amphetamine, at a time when spontaneous locomotor activity was reduced to control levels. Haloperidol significantly reduced locomotor activity in all groups.

Involvement of nitric oxide in morphine-induced c-Fos expression in the rat striatum

Induction of expression of immediate-early gene c-Fos in the striatum is a common effect of many drugs of abuse, including morphine. Previous studies have shown that the morphine-mediated c-Fos response is attenuated by antagonists of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. Other evidence suggests that the NDMA receptor may be coupled to the enzyme neuronal nitric oxide synthase (nNOS). NMDA receptor-mediated increases in intracellular calcium can activate nNOS, which catalyzes the formation of the signaling molecule nitric oxide. Because activation of NMDA receptors mediates morphine-induced c-Fos expression, we tested the hypothesis that activation of nNOS is involved in this cascade. Male rats were injected with the nNOS-selective inhibitor 7-nitroindazole (7-NI) or vehicle 30 min prior to injection of morphine sulfate or vehicle. Two hours later they were perfused with fixative and the brains removed for immunocytochemical analysis for c-Fos. Morphine induced c-Fos expression in the striatum, cerebral cortex, and midline/intralaminar nuclei of thalamus. Expression in the striatum, but not thalamus or cortex, was significantly blocked by 7-NI. Double-label immunocytochemistry revealed no co-localization of c-Fos and nNOS in any brain region. These results support a role for nNOS in the neural circuits activated by morphine.

Cooperation of NMDA and tachykinin NK(1) and NK(2) receptors in the medullary transmission of vagal afferent input from the acid-threatened rat stomach

Noxious challenge of the rat gastric mucosa by hydrochloric acid (HCl) is signaled to the nucleus tractus solitarii (NTS) and area postrema (AP). This study examined the participation of glutamate and tachykinins in the medullary transmission process. Activation of neurons was visualized by in situ hybridization autoradiography of c-fos messenger RNA (mRNA) 45 min after intragastric (IG) administration of 0.5 M HCl or saline. IG HCl caused many neurons in the NTS and some neurons in the AP to express c-fos mRNA. The NMDA glutamate receptor antagonist MK-801 (2 mg/kg), the NK(1) tachykinin receptor antagonist GR-205,171 (3 mg/kg) and the NK(2) receptor antagonist SR-144,190 (0.1 mg/kg) failed to significantly reduce the NTS response to IG HCl, whereas the triple combination of MK-801, GR-205,171 and SR-144,190 inhibited it by 45--50%. Only in rats that had been preexposed IG to HCl 48 h before the experiment was MK-801 alone able to depress the NTS response to IG HCl. In contrast, the c-fos mRNA response in the AP was significantly augmented by MK-801, an action that was prevented by coadministration of GR-205,171 plus SR-144,190. Inhibition of neuronal nitric oxide synthase with 7-nitroindazole (45 mg/kg) was without effect on the IG HCl-evoked c-fos mRNA expression in the NTS and AP. Our data show that glutamate acting via NMDA receptors and tachykinins acting via NK(1) and NK(2) receptors cooperate in the vagal afferent input from the acid-threatened stomach to the NTS and participate in the processing of afferent input to the AP in a different and complex manner. These opposing interactions in the AP and NTS and the increase in NMDA receptor function in the NTS after a gastric acid insult are likely to have a bearing on the neuropharmacology of dyspepsia.

A comparison of morphine analgesic tolerance in male and female mice

Studies comparing morphine tolerance in males and females are rare, and all studies to date have utilized the rat. To generalize from findings with rats morphine tolerance was investigated in male and female mice using the tail-withdrawal test. Three and 7 days of systemic morphine injections produced significant but unequal rightward shifts in the morphine dose-response curve such that females displayed greater increases in analgesic ED(50) values when compared to males. In a separate experiment, males and females displayed similar reductions in morphine analgesic sensitivity when %MPE (maximum possible effect) and %total (area under the curve) were compared after 3 days of morphine. Differences in initial morphine sensitivity between sexes were not observed in either study. The data demonstrate that, in contrast to rats, female mice undergo greater reductions in morphine analgesia relative to males following chronic morphine, but this sex difference may depend on the method of assessing analgesia. Furthermore, the duration and/or cumulative dose of morphine treatment does not affect the expression of sex differences in morphine tolerance.

Androgenic-anabolic steroids blunt morphine-induced c-fos expression in the rat striatum: possible role of beta-endorphin

Self-administration of large doses of androgenic-anabolic steroids (AAS) in a significant portion of the population suggests that these agents are drugs of abuse. However, acute administration of AAS did not induce striatal immediate-early genes (IEG) expression in male rats, indicating that AAS do not share a common mechanism of action with other drugs of abuse. Surveys have indicated that people who abuse AAS are more likely to self-administer other drugs of abuse than do people who do not take AAS. In the present study, chronic administration of AAS blunted the striatal c-fos response to morphine, indicating that AAS can alter the molecular responses to at least one drug of abuse. Chronic administration of AAS also increased the content of beta-endorphin in the midline thalamus, suggesting a possible mechanism by which AAS may modulate the response to morphine through regulation of thalamo-striatal neurons.

Infusion of beta-FNA into the thalamus attenuates morphine-induced c-Fos induction in the rat caudate putamen

The medial thalamus contains mu opioid receptors and sends a glutamatergic projection to the caudate putamen (CPu) in rat. Morphine-induced c-Fos expression in the CPu has been shown to be blocked by pretreatment with antagonists to N-methyl-D-aspartate receptors, indicating the involvement of glutamate in this morphine-induced response. The importance of the glutamatergic projections from the thalamus was assessed by infusing the mu opioid receptor antagonist, beta-funaltrexamine (beta-FNA), prior to systemic morphine injection. Infusion of beta-FNA near specific medial thalamic nuclei attenuated morphine-induced c-Fos expression in the CPu.

Chronic administration of morphine alters immediate-early gene expression in the forebrain of post-dependent rats

Previous studies from this laboratory have demonstrated that acute, systemic administration of morphine results in an induction of the immediate-early gene (IEG) proteins, c-Fos and Jun-B, in the dorsomedial portion of the rat caudate-putamen (CPu). These studies have also shown that morphine can induce c-Fos in the central medial nucleus of the thalamus (CM). To determine whether this response is altered in post-dependent rats, twice-daily injections of an ascending dose of morphine were administered for 5 days, followed by a withdrawal period of 7 or 14 days. A challenge injection of morphine (10 mg/kg) was administered on the last day of withdrawal. As compared to an acute dose of morphine in a naive animal, the induction of c-Fos was increased in the dorsolateral CPu following challenge injection at 7 days, but not at 14 days. Induction of c-Fos in the CM following the challenge injection was blunted following 7 day, but not at 14 days, of withdrawal. An increase in the IEG protein, Jun-B, was also seen following 7 but not 14 days of withdrawal in both the dorsomedial and dorsolateral CPu. These findings demonstrate that a chronic treatment of morphine can result in altered patterns of IEG expression upon challenge with acute morphine, in a time-dependent manner, within the rat CPu and CM.