Stress and IL-1beta contribute to the development of depressive-like behavior following peripheral nerve injury.

The physiological link between neuropathic pain and depression remains unknown despite a high comorbidity between these two disorders. A mouse model of spared nerve injury (SNI) was used to test the hypothesis that nerve injury precipitates depression through the induction of inflammation in the brain, and that prior exposure to stress exacerbates the behavioral and neuroinflammatory consequences of nerve injury. As compared with sham surgery, SNI induced mechanical allodynia, and significantly increased depressive-like behavior. Moreover, SNI animals displayed increased interleukin-1beta (IL-1beta) gene expression within the frontal cortex and concurrent increases in the expression of glial fibrillary acidic protein (GFAP) within the periaqueductal grey (PAG). Additionally, exposure to chronic restraint stress for 2 weeks before SNI exacerbated mechanical allodynia and depressive-like behavior, and resulted in an increase in IL-1beta gene expression in the frontal cortex and brain-derived neurotrophic factor (BDNF) gene expression in PAG. Treatment with metyrapone (MET), a corticosteroid synthesis inhibitor, before stress eliminated deleterious effects of chronic stress on SNI. Finally, this study showed that interference with IL-1beta signaling, through administration of IL-1 receptor antagonist (IL-1ra), ameliorated the effects of neuropathic pain on depressive-like behavior. Taken together, these data suggest that peripheral nerve injury leads to increased cytokine expression in the brain, which in turn, contributes to the development of depressive-like behavior. Furthermore, stress can facilitate the development of depressive-like behavior after nerve injury by promoting IL-1beta expression.

Neurobehavioral deficits in mice lacking the erythrocyte membrane cytoskeletal protein 4.1.

The erythrocyte membrane cytoskeletal protein 4.1 (4.1R) is a structural protein that confers stability and flexibility to erythrocytes via interactions with the cytoskeletal proteins spectrin and F-actin and with the band 3 and glycophorin C membrane proteins. Mutations in 4.1R can cause hereditary elliptocytosis, a disease characterized by a loss of the normal discoid morphology of erythrocytes, resulting in hemolytic anemia [1]. Different isoforms of the 4.1 protein have been identified in a wide variety of nonerythroid tissues by immunological methods [2-5]. The variation in molecular weight of these different 4.1 isoforms, which range from 30 to 210 kDa [6], has been attributed to complex alternative splicing of the 4.1R gene [7]. We recently identified two new 4.1 genes: one is generally expressed throughout the body (4. 1G) [8] and the other is expressed in central and peripheral neurons (4.1N) [9]. Here, we examined 4.1R expression by in situ hybridization analysis and found that 4.1R was selectively expressed in hematopoietic tissues and in specific neuronal populations. In the brain, high levels of 4.1R were discretely localized to granule cells in the cerebellum and dentate gyrus. We generated mice that lacked 4.1R expression; these mice had deficits in movement, coordination, balance and learning, in addition to the predicted hematological abnormalities. The neurobehavioral findings are consistent with the distribution of 4.1R in the brain, suggesting that 4.1R performs specific functions in the central nervous system.

Winter adaptations of male deer mice (Peromyscus maniculatus) and prairie voles (Microtus ochrogaster) that vary in reproductive responsiveness to photoperiod.

Individuals of many nontropical rodent species restrict breeding to the spring and summer. Seasonal reproductive quiescence putatively reflects the energetic incompatibility of breeding and thermoregulatory activities. However, so-called "out-of-season" breeding occurs in virtually all rodent populations examined, suggesting that the incompatibility can be resolved. Both reproductive inhibition and development of energy-saving adaptations are mediated by environmental photoperiod, but some individuals do not inhibit reproduction in short days. In order to assess the costs and benefits of winter breeding, the present study examined the extent to which male prairie voles (Microtus ochrogaster) and deer mice (Peromyscus maniculatus) that maintained summer reproductive function in winter-simulated daylengths also maintained summer thermoregulatory adaptations. Circadian locomotor activity patterns, basal metabolic rate, capacity for nonshivering thermogenesis, nest building, body mass, and daily food consumption were compared among short-day (LD 8:16) regressed males, short-day (LD 8:16) nonregressed males, and long-day (LD 16:8) males. Short-day nonregressed deer mice resembled long-day conspecifics in terms of body mass and nest-building activities; however, the locomotor activity pattern of short-day nonregressed deer mice was similar to that of their short-day regressed conspecifics. Short-day nonregressed prairie voles had body masses similar to those of long-day conspecifics. Regardless of their reproductive response to photoperiod, short-day prairie voles reduced their daily food consumption and wheel-running activity, compared to long-day voles. These results suggest that winter breeding has energetic costs, most likely resulting from maintaining a "summer-like" body mass relative to that of reproductively regressed animals. These costs may be ameliorated to some extent by the reduction in locomotor activity and nest-building behavior emitted by short-day animals, regardless of reproductive response to short days. Thus, the occurrence of winter breeding may be the result of sufficient numbers of reproductively photoperiod-nonresponsive morphs in the population and sufficiently mild ambient conditions to permit survival of these larger animals.

Physiological substrates of mammalian monogamy: the prairie vole model.

Prairie voles (Microtus ochrogaster) are described here as a model system in which it is possible to examine, within the context of natural history, the proximate processes regulating the social and reproductive behaviors that characterize a monogamous social system. Neuropeptides, including oxytocin and vasopressin, and the adrenal glucocorticoid, corticosterone, have been implicated in the neural regulation of partner preferences, and in the male, vasopressin has been implicated in the induction of selective aggression toward strangers. We hypothesize here that interactions among oxytocin, vasopressin and glucocorticoids could provide substrates for dynamic changes in social and agonistic behaviors, including those required in the development and expression of monogamy. Results from research with voles suggest that the behaviors characteristics of monogamy, including social attachments and biparental care, may be modified by hormones during development and may be regulated by different mechanisms in males and females.

Cognitive and behavioral assessment in experimental stroke research: will it prove useful?

Stroke in humans is associated with deficits in sensorimotor and cognitive function. Consequently, many stroke researchers recently have expanded their techniques to assess cognitive and behavioral correlates of histologically-determined stroke damage in animal models. Although the incorporation of functional outcome assessment represents an important step forward in stroke research, reports of middle cerebral artery occlusion (MCAO) induced behavioral deficits often conflict, and a significant correlation between post-stroke histology and behavior has been reported in few stroke studies. Discrepancies in behavioral outcomes among studies may be due to several factors, such as method of MCAO, duration of occlusion, strain, the timing and method of the behavioral testing and the laboratory environment. Furthermore, proper experimental and control groups, necessary to rule out potential confounding factors during cognitive testing, often are not incorporated. The goal of this review is: (1) to provide a description of the techniques most commonly employed to assess functional outcome after (MCAO) in rodents and (2) to identify potential confounding factors that may interfere with a clear interpretation of the behavioral data.

Anticonvulsant lamotrigine administered on reperfusion fails to improve experimental stroke outcomes.

BACKGROUND AND PURPOSE: Recent results suggest that selective inhibitors of presynaptic neuronal ion channels can diminish glutamate release during cerebral ischemia and modulate excitotoxic cell death. The aim of the present study was to evaluate lamotrigine (LTG), an antiepileptic that inhibits presynaptic sodium and voltage-sensitive calcium channels, as a potential stroke resuscitation agent in the rat. Three dosages of LTG were examined for effect on infarction volume and sensorimotor behavioral recovery after middle cerebral artery (MCA) occlusion. METHODS: Halothane-anesthetized male Wistar rats were subjected to 2 hours of MCA occlusion by the intraluminal occlusion technique. Physiological variables were controlled, and ipsilateral cortical perfusion was monitored by laser Doppler flowmetry throughout ischemia. At onset of reperfusion, rats received intravenous LTG 5, 10, or 20 mg/kg or PBS (n=9 to 11 per group) during 15 minutes. Behavioral assessment was completed at 3 and 7 days after stroke, and the brain was harvested for histology (triphenyltetrazolium chloride staining). RESULTS: Values are mean+/-SE. Cortical infarction volumes were unchanged in LTG-treated animals: 14+/-6% of contralateral cortex at 5 mg/kg LTG, 17+/-7% at 10 mg/kg, and 30+/-6% at 20 mg/kg, versus saline-treated cohorts (12+/-3%; P:=0.19; n=9). Caudate-putamen infarction injury was also unchanged (37+/-11% of contralateral caudate-putamen at 5 mg/kg LTG, 44+/-8% at 10 mg/kg, and 65+/-9% at 20 mg/kg versus saline (38+/-11%; P:=0.18). Total infarction was not different among groups (P:=0.15). Consistent with histology, behavioral outcomes were unimproved by treatment. CONCLUSIONS: Histological damage and behavioral recovery at 7 days after MCA occlusion was not altered by LTG treatment over the dosage range used in the present study.

Heteroaromatic analogs of 1-[2-(diphenylmethoxy)ethyl]- and 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazines (GBR 12935 and GBR 12909) as high-affinity dopamine reuptake inhibitors.

A new series of heteroaromatic GBR 12935 [1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)-piperazine] (I) and GBR 12909 [1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine] (2) analogs was synthesized and evaluated as dopamine transporter (DAT) ligands. Analogs 5-16, in which the benzene ring in the phenylpropyl side chain of the GBR molecule had been replaced with a thiophene, furan, or pyridine ring, exhibited high affinity and selectivity for the DAT vs serotonin transporter (SERT) and stimulated locomotor activity in rats in a manner similar to the parent compound 2. In cocaine and food self-administration studies in rhesus monkeys, both thiophene-containing (6 and 8) and pyridine-containing (14 and 16) derivatives displayed potency comparable to 2 in decreasing the cocaine-maintained responding at the doses tested (0.8, 1.7, and 3 mg/kg). However, these compounds did not produce the degree of separation between food- and cocaine-maintained responding that was seen with 2. Among the bicyclic fused-ring congeners 17-38, the indole-containing analog of 2, 22, showed the greatest affinity for binding to the DAT, with IC50 = 0.7 nM, whereas the corresponding indole-containing derivative of 1, 21, displayed the highest selectivity (over 600-fold) at this site vs the SERT site.

Reduced aggressive behaviour in mice with targeted disruption of the oxytocin gene.

Oxytocin (OT) has been reported to mediate aggressive and affiliative behaviours in several species. The behavioural role of OT has been established with physiological manipulations that potentially affected blood pressure, which may have indirectly affected the behaviours under study. To provide converging evidence of the physiological role of OT in aggressive behavior, wild type (WT), heterozygous (OT-/+), and homozygous (OT-/-) mutant mice were tested in two aggression paradigms. In general, there was no significant difference in aggressiveness between WT and OT-/+ mice. However, there were significant reductions in the duration of aggressive behaviors among OT-/- animals, especially in agonistic encounters within neutral arenas. The OT-/- mice did not exhibit any sensorimotor deficits or display any altered general anxiety levels that may have accounted for the observed reduction in aggressive behavior. These data indicate that aggression is mediated in part by OT in mice and that increased aggressiveness is not an obligatory phenotypic result of targeted genetic disruption of any gene.

The effects of oxytocin and vasopressin on partner preferences in male and female prairie voles (Microtus ochrogaster).

This study compared the effects of centrally administered oxytocin (OT) and arginine vasopressin (AVP) on partner preference formation and social contact in male and female prairie voles (Microtus ochrogaster). After 1 hr of cohabitation and pretreatment with either AVP or OT, both males and females exhibited increased social contact and significant preference for the familiar partner. After pretreatment with either an OT receptor antagonist (OTA) or an AVP (V1a) receptor antagonist (AVPA), neither OT nor AVP induced a partner preference. In addition, treatment with OT+OTA or AVP+AVPA was associated with low levels of social contact in both sexes. Either AVP or OT is sufficient to facilitate social contact if either the OT or AVP receptor is available. However, the formation of partner preferences may require access to both AVP and OT receptors.

Conditioned increases in anxiogenic-like behavior following exposure to contextual stimuli associated with cocaine are mediated by corticotropin-releasing factor.

Although cocaine is a powerful reinforcer, it has been reported to produce anxiety in humans and anxiogenic-like behavior in animals. The goal of this study was three-fold: (1) to determine the doses of cocaine that induce anxiogenic-like behavior in the elevated plus-maze in rats, (2) to determine if cocaine-associated contextual cues are capable of eliciting anxiogenic-like behavior in the absence of the drug, and (3) to identify possible mechanisms through which cocaine-associated cues affect behavior in the elevated plus-maze. Measurement of the amount of time that the animals spend exploring the open arms of the maze provides a sensitive index of anxiogenic-like behavior in rats. In experiment 1, rats were injected with 10 mg/kg, 20 mg/kg, or 30 mg/kg cocaine HCl or saline for 6 days. On day 6, the rats were tested in the elevated plus-maze 25 min after injection with cocaine or saline. The animals chronically treated with the three doses of cocaine exhibited a dose-dependent increase in anxiogenic-like behavior in the elevated plus-maze, compared to the saline-treated group. In experiment 2, cocaine-induced (30 mg/kg) conditioning was achieved using a simple contextual design. On the final day of the experiment (day 6), after 5 days of conditioning, the rats were exposed for 25 min to the cocaine-associated contextual cues, then placed in the elevated plus-maze. Animals that had been exposed to cocaine-associated contextual cues prior to being placed in the elevated plus-maze exhibited a significant increase in anxiogenic-like behavior compared to the control groups. However, pretreatment of the rats with the CRF antagonist, alpha-helical CRF9-41 (1 microg, i.c.v.), on the test day, prior to exposure to cocaine-associated contextual cues, attenuated the subsequent anxiogenic-like behavioral response in the elevated plus-maze (experiment 3). The results suggest that contextual cues associated with repeated treatment with 30 mg/kg cocaine are capable of eliciting anxiogenic-like behavior in the absence of the drug and that CRF mediates the expression of anxiogenic-like behaviors in the elevated plus-maze following exposure to cocaine-associated cues. The conditioned anxiogenic action elicited by cocaine-associated cues may have relevance for understanding the complex addictive nature of this drug and some of the clinical phenomena related to its use.

Adrenocorticoid hormones and the development and expression of mammalian monogamy.

Based on research with prairie voles, we hypothesize that the unusual patterns of reproduction and social behavior associated with mammalian monogamy may arise as a consequence of normal developmental exposure to high levels of glucocorticoids and/or other hormones of the HPA axis. Increased HPA activity could functionally inhibit some of the masculinizing processes expected during the perinatal period. We hypothesize that the unique behavioral, physiological, and anatomical changes associated with monogamy may reflect the adaptive consequences of reduced exposure to the masculinizing actions of HPG hormones, such as testosterone. Reproductively naive, unpaired adult prairie voles also show unusual patterns of adrenal activity, including a marked decline in corticosterone levels within minutes following exposure to novel animals of the opposite sex. In females, this decline in corticosterone may contribute to pair bonding, since corticosterone injections inhibit, and adrenalectomy is associated with a facilitation of pair bond formation. In males, corticosterone injections facilitate pair bonding and adrenalectomy has the opposite effect. In animals from established social pairs corticosterone levels also fluctuate according to the social environment of the animal; the absence of a familiar partner is associated with increased corticosterone secretion, and in the presence of the familiar partner corticosterone levels tend to decline. Prairie voles may offer a valuable source of information regarding the behavioral, anatomical, and physiological consequences of long-term and short-term exposure to high levels of adrenal activity in the absence of pathology.

Conditioned release of corticosterone by contextual stimuli associated with cocaine is mediated by corticotropin-releasing factor.

Elevated blood concentrations of corticosterone (CORT), an adrenal steroid associated with stress responses, is one of the endocrine correlates of cocaine treatment. Experiment 1 confirmed and extended previous findings that chronic cocaine treatment does not alter corticosteroid responses to cocaine. In Experiment 2, conditioned endocrine effects of cocaine were examined in three groups of rats after 7 consecutive days of treatment. Cocaine-induced conditioning was achieved using a simple contextual design. In group 1 (paired), rats were injected with cocaine (30 mg/kg), then immediately placed into a locomotor activity chamber for 30 min. One hour after the rats were returned to their home cages, they received an injection of saline. In group 2 (unpaired), rats were injected with saline, then immediately placed into a locomotor activity chamber for 30 min. One hour after the rats were returned to their home cages, they received an injection of cocaine (30 mg/kg). Rats in group 3 (control) received only saline injections, but otherwise were treated as animals in the other treatment groups. On the test day (Day 8), all rats were placed immediately into the locomotor apparatus for 30 min prior to collection of a blood sample. Blood CORT concentrations and locomotor activity in the paired group were significantly higher than in the unpaired and control groups. However, pretreatment of the rats in Experiment 3 with the corticotropin-releasing factor (CRF) antagonist, alpha-helical CRF9-41 (1 microg, i.c.v.), on the test day, prior to exposure to cocaine-associated contextual cues, attenuated the subsequent conditioned increase in blood CORT concentrations. These data represent the first demonstration of classical conditioning of a steroid hormone response to stimuli associated with a psychoactive drug in rats and suggest that the effect is mediated by endogenous CRF. Because the hypothalamic-pituitary-adrenal (HPA) axis has been implicated in modulating the actions of cocaine, it is plausible that such conditioned increases in CORT release by cocaine-associated cues may further predispose an organism to the reinforcing effects of the drug or enhance the susceptibility to drug-taking behavior. Alternatively, such conditioned effects may be related to the anxiogenic properties of cocaine. Further understanding of the conditioned effects of hormones in the development and expression of addictive behaviors may provide new insights into treatment of drug addiction.

Lipopolysaccharide facilitates partner preference behaviors in female prairie voles.

Exposure to proinflammatory cytokines (e.g., IL-1beta) or lipopolysaccharide (LPS) produces an acute activation of the immune response and results in a repertoire of behavioral patterns collectively termed sickness behaviors. Although nonspecific responses to pathogenic infection have traditionally been viewed as maladaptive effects of infection, sickness behaviors may have significant, adaptive value for the host. One set of adaptive behaviors affected by infection among mammals and birds is mate choice. In Experiment 1, female prairie voles exhibited the expected increase in blood corticosterone concentrations in response to a 0.1 cc i.p. LPS injection (50 microg), indicating activation of the endocrine system. A separate cohort of females was injected with LPS or saline and paired for 6 h with a novel, previously unpaired male. Following the cohabitation period, LPS-injected females spent significantly more time (p < 0.05) with the familiar partner when given a choice between familiar and unfamiliar males in a three-chamber apparatus designed to test partner preferences. Saline-injected females spent significantly more time with the unfamiliar male. In Experiment 2, males injected with LPS or saline spent equal amounts of time with familiar and unfamiliar females following a 6 h cohabitation with a naive female, and therefore, did not exhibit preferences. From a proximate perspective, this study provides evidence that sickness behaviors influence female, but not male, partner preference in prairie voles.

Targeted reduction of oxytocin expression provides insights into its physiological roles.

Oxytocin is a nonapeptide hormone that participates in the regulation of parturition and lactation. It has also been implicated in various behaviors, such as mating and maternal, and memory. To investigate whether or not oxytocin (OT) is essential for any of these functions, we eliminated, by homologous recombination, most of the first intron and the last two exons of the OT gene in mice. Those exons encode the neurophysin portion of the oxytocin preprohormone which is hypothesized to help in the packaging and transport of OT. The homozygous mutant mice have no detectable neurophysin or processed oxytocin in the paraventricular nucleus, supraoptic nucleus or posterior pituitary. Interestingly, homozygous mutant males and females are fertile and the homozygous mutant females are able to deliver their litters. However, the pups do not successfully suckle and die within 24 hours without milk in their stomachs. OT injection into the dams or rescue with the rat OT gene restores the milk ejection in response to suckling. OT is also needed for post-partum alveolar proliferation. These results indicate an absolute requirement for oxytocin for successful milk ejection, but not for mating, parturition and milk production, in mice. Furthermore, homozygous mutant mice show reduced aggression in some tests.

Effects of photoperiod and 2-deoxy-D-glucose-induced metabolic stress on immune function in female deer mice.

Nontropical rodents may experience large fluctuations in both food availability and energetic demands. The energy required for thermoregulation is high during the winter when energy availability is usually low. Winter conditions can induce a state of energetic stress that elevates circulating glucocorticoid levels and compromises immune function. Exposure to short days enhances immune function; the adaptive function of short-day enhancement of immune function may be to counteract the effects of stress-induced immunocompromise. To examine the role of energy availability in immune function, female deer mice were housed in either long (16:8-h light-dark cycle) or short (8:16-h light-dark cycle) days for 8 wk and then injected with either saline or 2-deoxy-D-glucose (2-DG), a glucose analog that inhibits cellular utilization of glucose and induces energetic stress. Long-day mice injected with 2-DG exhibited elevated corticosterone levels and reduced splenocyte proliferation compared with control mice. Short days buffered the animals against glucoprivation stress. Neither corticosterone levels nor splenocyte proliferation differed between 2-DG injected and control mice housed in short days. These data are consistent with the hypothesis that short days provide a buffer against metabolic stress.

The effects of stress on social preferences are sexually dimorphic in prairie voles.

Prairie voles (Microtus ochrogaster) are monogamous rodents that form pair bonds characterized by a preference for a familiar social partner. In male prairie voles, exposure to either the stress of swimming or exogenous injections of corticosterone facilitate the development of a social preference for a female with which the male was paired after injection or swimming. Conversely, adrenalectomy inhibits partner preference formation in males and the behavioral effects of adrenalectomy are reversed by corticosterone replacement. In female prairie voles, swim stress interferes with the development of social preferences and corticosterone treatments inhibit the formation of partner preferences, while adrenalectomized females form preferences more quickly than adrenally intact controls. Because sex differences in both behavior and physiology are typically reduced in monogamous species, we initially predicted that male and female prairie voles would exhibit similar behavioral responses to corticosterone. However, our findings suggest an unanticipated sexual dimorphism in the physiological processes modulating social preferences. This dimorphic involvement of stress hormones in pair bonding provides a proximate mechanism for regulating social organization, while permitting males and females to adapt their reproductive strategies in response to environmental challenges.