Mid-adolescent stress differentially affects binge-like intake of sucrose across estrous cycles in female rats✰.

Binge eating disorder (BED), characterized by excessive food consumption within a discrete period of time, is the most prevalent of all eating disorders, with higher rates in women than men. Chronic stress, particularly during adolescence, is a significant risk factor for BED in women, but the mechanism underlying this relationship remains elusive. We investigated the phenomenon by testing the impact of mid-adolescent intermittent physical stress (IPS) on binge-like intake of sucrose in adult female rats, assessing how the behavior changed across reproductive cycles. One hundred and nineteen Long-Evans rats were exposed to IPS (n = 59) or no stress (NS; n = 60) for 12 days during mid-adolescence (PD35-46). Binge-like eating was induced in adult animals using an intermittent access protocol: animals were provided with 12 h or 24 h access to sucrose, 12 h access to saccharin, or 12 h access to food over 28 days. After 1- or 28-day abstinence, compulsive responding for sucrose was measured using a conditioned suppression paradigm. Rats given 12 h access to sucrose developed binge-like intake, measured as increased consumption during the first hour; the effect was magnified in IPS animals and most pronounced during proestrous. Solution intake in IPS rats was predicted by open arm entries in the elevated plus maze, suggesting that increased risk-taking behavior is associated with greater binge-like eating. IPS blocked conditioned suppression after 28 days of abstinence, pointing to a role of mid-adolescent stress in compulsivity. Collectively, these findings emphasize the impact of stress on the emergence of binge eating in females and suggest that intervention programs for women with a history of adolescent adversity should be investigated as a means to reduce risk for BED.

Adversity in early adolescence promotes an enduring anxious phenotype and increases serotonergic innervation of the infralimbic medial prefrontal cortex.

Stress during early development produces lasting effects on psychopathological outcomes. We analysed the impact of prior intermittent, physical stress (IPS) during early adolescence (PD 22-33) on anxiety-like behaviour of female rats in adulthood. After behavioural testing, we used immunohistochemistry for the 5-HT transporter (SERT) to evaluate 5-HT innervation profiles in the medial prefrontal cortex (mPFC) and ventral hippocampus (VH). Administration of IPS (i.e., water immersion, elevated platform, foot shock) in early adolescence increased rats' anxiety-like behaviour in the elevated plus-maze but had no effects in the shock-probe burying test. In the social interaction test, IPS decreased social interaction, and this effect was driven by selective decreases in the frequency of playfighting with no evident changes in contact and investigative behaviours. Selective stress-induced increases in the density of SERT-ir positive fibres were found in the infralimbic (IL) subregion of the mPFC but not in the cingulate or prelimbic (PL) subregions. IPS in early adolescence did not affect 5-HT innervation profiles in any sub-fields of the VH. Our findings confirm and extend on earlier evidence that stress during early adolescence promotes the emergence of an anxious phenotype and provide novel evidence that these effects are associated with increased 5-HT innervation of the IL mPFC.

The lateral septum and anterior hypothalamus act in tandem to regulate burying in the shock-probe test but not open-arm avoidance in the elevated plus-maze.

Both the lateral septum (LS) and anterior hypothalamus (AHA) regulate behavioural defense. We tested whether those two interconnected structures act in serial in that regard. Infusions of the GABAA agonist muscimol into one side of the LS and the contralateral (but not ipsilateral) AHA suppressed rats' burying in the shock-probe test whereas none of our muscimol infusion approaches altered their open-arm avoidance in the elevated plus-maze. These results suggest that the LS-AHA circuit serves a specialized role in defensive responses towards discrete, localizable threat stimuli but not towards potential threats.

The lateral septum as a regulator of hippocampal theta oscillations and defensive behavior in rats.

Hippocampal theta oscillations are linked to various processes, including locomotion, learning and memory, and defense and affect. The lateral septum (LS) has been implicated in the generation of the hippocampal theta rhythm, but its precise role in this process is not well understood. Here, we investigated the effects of direct pharmacological inhibition or disinhibition of the dorsal LS (dLS) on the frequency of hippocampal theta activity elicited by stimulation of the reticular formation in urethane-anesthetized rats. We found that bilateral infusions of the GABAA receptor agonist muscimol into the dLS significantly increased theta frequency. Strikingly, intra-dLS infusions of the GABAA receptor antagonist GABAzine largely abolished reticularly elicited theta activity. We also locally injected these same compounds into the medial septum (MS) to test for neuroanatomical specificity. In contrast to the effects seen in the dLS, intra-MS infusions of muscimol had no effect on theta frequency, whereas intra-MS infusions of GABAzine increased theta frequency. Given the hypothesized role of hippocampal theta in behavioral defense, we also examined the effects of intra-dLS application of muscimol in two models of anxiety, the elevated plus maze and the novelty-induced suppression of feeding paradigm; both tests revealed clear, anxiolytic-like effects following muscimol infusions. The fact that dLS-muscimol increased theta frequency while also reducing anxiety-like behaviors challenges the influential theta suppression model of anxiolysis, which predicts a slowing of theta with anxiolytic compounds. More importantly, the experiments reveal a novel role of the LS, especially its dorsal aspects, as an important gating mechanism for the expression of theta oscillations in the rodent hippocampus.

Infusions of muscimol into the lateral septum do not reduce rats' defensive behaviors toward a cat odor stimulus.

The lateral septum (LS) is implicated in behavioral defense. We tested whether bilateral infusions of the GABAA receptor agonist muscimol into the LS suppress rats' defensive responses to cat odor. Rats received intra-LS infusions of either saline or muscimol (40 ng/rat) and were exposed to either a piece of a cat collar that had been previously worn by a cat or to a control (cat odor free) collar. Rats exposed to the cat odor collar displayed more head-out postures, while intra-LS application of muscimol reduced the number of head-out postures. However, this reduction was also present in rats exposed to a control (cat odor free) collar. This latter finding suggests that despite its involvement in other defensive behaviors (e.g., open arm avoidance in the elevated plus maze), the LS does not selectively regulate rats' receptor defensive responding to the olfactory cues present in our cat odor stimulus.

Behavioral anxiolysis without reduction of hippocampal theta frequency after histamine application in the lateral septum of rats.

Hippocampal theta activity is linked to various processes, including locomotion, learning and memory, and defense and affect (i.e., fear and anxiety). Interestingly, all classes of clinically effective anxiolytics, as well as experimental compounds that decrease anxiety in pre-clinical animal models of anxiety, reduce the frequency of hippocampal theta activity elicited by stimulation of the reticular formation in freely behaving or anesthetized animals. In the present experiments, we found that bilateral histamine infusions (0.5 µg/hemisphere) into the lateral septum (LS) of rats decreased anxiety-like responses in two models of anxiety, the elevated plus maze and novelty-induced suppression of feeding test. Surprisingly, these same infusions significantly increased hippocampal theta frequency elicited by reticular stimulation in urethane-anesthetized rats. In contrast to these findings, additional experiments showed that the clinically effective anxiolytic buspirone (40 mg/kg, i.p.) reduced theta frequency, confirming previous observations. Taken together, the dissociation of behavioral anxiolysis and theta frequency reduction noted here suggest that hippocampal theta frequency is not a direct index of anxiety levels in rodents. Further, the mechanisms underlying the behavioral and physiological effects elicited by histamine in the LS require further study.

The histaminergic H1, H2, and H3 receptors of the lateral septum differentially mediate the anxiolytic-like effects of histamine on rats' defensive behaviors in the elevated plus maze and novelty-induced suppression of feeding paradigm.

The neural histaminergic system is involved in a wide range of physiological processes, including anxiety. Histaminergic neurons are localized in the tuberomammillary nucleus of the posterior hypothalamus and share bidirectional connections with the lateral septum, an area well implicated in anxiety. The current study examined whether the histaminergic system of the lateral septum regulates rats' defensive behaviors in two animal models of anxiety, the elevated plus maze (EPM) and novelty-induced suppression of feeding paradigm (NISF). We found that bilateral infusions of histamine (1.0 µg and 5.0 µg) into the lateral septum selectively decreased rats' defensive behaviors in the EPM (both doses) and NISF (1.0 µg only). Follow-up studies found that pre-infusions of the H1 and H2 antagonists, pyrilamine (20 µg) and ranitidine (20 µg) respectively, reversed the anxiolytic-like effects of intra-LS histamine (1.0 µg) in the NISF but not in the EPM, while pre-infusions of the H3 antagonist ciproxifan (200 pg) attenuated the anxiolytic-like effects of intra-LS histamine in the EPM but not in the NISF. This double dissociation suggests that H1 and H2 receptors in the lateral septum, likely via a post-synaptic mechanism, mediate the anxiolytic-like effects of histamine in the NISF but not in the EPM. In contrast, lateral septal H3 receptors mediate, likely pre-synaptically, the anxiolytic-like effects of histamine in the EPM but not in the NISF. Our findings indicate that these receptors differentially contribute to rats' specific defensive behaviors in the EPM and NISF, that is, avoidance of open spaces and neophagia respectively.

Lateral septal infusions of the neuropeptide Y Y2 receptor agonist, NPY(13-36) differentially affect different defensive behaviors in male, Long Evans rats.

The lateral septum has been extensively implicated in regulating anxiety-related defensive behaviors in the rat. Neuropeptide Y (NPY) contributes to anxiety, likely through activity at the NPY Y1 and/or Y2 receptor binding sites. Although the lateral septum contains the highest density of Y2 receptors in brain, the involvement of this receptor in anxiety-related defensive behaviors is not clear. Thus, the purpose of the current study was to characterize lateral septal Y2 receptor contributions to rats' defensive responses to threat and/or potentially threatening environments. We investigated this by infusing the NPY Y2 agonist NPY13-36 into the lateral septum and testing rats across a battery of animal models of anxiety (Experiment 1). To verify the role of Y2 in mediating the observed effects, rats were pre-infused with the potent and highly selective Y2 antagonist BIIE 0246 prior to infusion with NPY13-36 (Experiment 2). Infusions of NPY13-36 into the lateral septum increased rats' open-arm exploration in the elevated plus-maze test (p<0.01) and decreased the proportion of rats' that buried (p<0.05) as well as their latency to initiate burying in the shock-probe burying test (p<0.01). By contrast, NPY13-36 did not affect either anxiety- or appetite-related responses in the novelty-induced suppression of feeding test (all ps>0.3; Experiment 1). Pre-treatment with the Y2 antagonist BIIE 0246 prevented the anxiolytic-like actions of NPY13-36 in the plus-maze but not in the shock-probe test (Experiment 2). Thus, it appears that the anxiolytic-like actions of lateral septal NPY13-36 are mediated by the Y2 receptor in a test-specific manner.

Intermittent physical stress during early- and mid-adolescence differentially alters rats' anxiety- and depression-like behaviors in adulthood.

Prior work showed that exposing rats to stress from weaning through to late adolescence (PD23-51) increased anxiety- and depression-related responses in adulthood. In the current study, we tested the hypothesis that the outcome of adolescent stress depends on the specific timing of adversity in adolescence. Male and female rats were exposed to intermittent, physical stress during either early (PD22-33) or mid -(PD35-46) adolescence, and then their anxiety- and depression-related responses to acute stressors were tested in adulthood. Early adolescent stress decreased rats' open-arm exploration in the elevated plus-maze in both male and female rats. Early adolescent stress also increased the duration of time rats spent burying in the shock-probe test and the duration of time they spent immobile in the forced swim test, but these effects were only seen in males. Stress in mid-adolescence did not increase rats' anxiety-related responding in adulthood. Instead, we observed paradoxical increases in open-arm exploration and only modest increases in shock-probe burying that failed to reach significance. Mid-adolescent stress also tended to increase depression-related immobility in the swim test. Thus, the current findings underscore the importance of timing of adolescent adversity to long-term outcomes. It appears that stress in early adolescence leads to a broader range of outcomes in adulthood, at least in male rats. By contrast, stress in mid-adolescence might have more predominant effects on risk-taking behavior (indexed by increases in open-arm activity), a possibility that merits further investigation.

Infusions of neuropeptide Y into the lateral septum reduce anxiety-related behaviors in the rat.

Neuropeptide Y (NPY) is one of the most abundant peptides in mammalian brain and NPY-like-immunoreactivity is highly expressed in the lateral septum, an area extensively involved in anxiety regulation. NPY counteracts the neurochemical and behavioral responses to acute threat in animal models, and intracerebroventricular (i.c.v.) administration of NPY at low doses is anxiolytic. Less is known about the specific contributions of the lateral septum to NPY-mediated anxiety regulation. In Experiment 1, the effects of infusions of NPY (1.5 µg) into the lateral septum were investigated in three animal models of anxiety: the elevated plus-maze, novelty-induced suppression of feeding, and shock-probe burying tests. Experiment 2 examined the role of the NPY Y1 receptor in these models by co-infusing the Y1 antagonist BIBO 3304 (0.15 µg, 0.30 µg) with NPY into the lateral septum. In the elevated plus-maze, there were no changes in rats' open arm exploration, the index of anxiety reduction in this test. In the novelty-induced suppression of feeding test, rats infused with NPY showed decreases in the latency to consume a palatable snack in a novel (but not familiar) environment, suggesting a reduction in anxiety independent of increases in appetite. This anxiolysis was attenuated by co-infusion with BIBO 3304 (0.30 µg) in Experiment 2. Lastly, rats infused with NPY showed decreases in the duration of burying behavior in the shock-probe burying test, also indicative of anxiety reduction. However, unlike in the feeding test, BIBO 3304 did not attenuate the NPY-induced anxiolysis in the shock-probe test. It is concluded that NPY produces anxiolytic-like actions in the lateral septum in two animal models of anxiety: the novelty-induced suppression of feeding, and shock-probe burying tests, and that this anxiolysis is dependent on Y1 receptor activation in the feeding test.

Lesions of the dorsal lateral septum do not affect neophagia in the novelty induced suppression of feeding paradigm but reduce defensive behaviours in the elevated plus maze and shock probe burying tests.

Past studies have shown that the lateral septum is involved in anxiety. Here, we tested whether the dorsal lateral septum contributes to neophagia by using the novelty induced suppression of feeding (NISF) paradigm. We found that while lesions of the dorsal lateral septum did not affect home or novel cage responding in the NISF test, they did decrease open arm avoidance in the elevated plus maze and burying in the shock probe burying test. Our results suggest that the dorsal lateral septum does not regulate neophagia in the NISF, but further experiments are needed to determine if the same is true for the intermediate and ventral lateral septum.

The ventral hippocampus and the lateral septum work in tandem to regulate rats' open-arm exploration in the elevated plus-maze.

Lesions or pharmacological inhibition of the ventral hippocampus or the lateral septum suppress rats' defensive responses in various rat models of anxiety. Although these two structures are extensively connected, it was not clear whether they regulate anxiety in a parallel (independent) or serial (integrated) fashion. In Experiment 1, bilateral infusions of the GABA-A receptor agonist muscimol (5 ng/side) into the lateral septum increased rats' open-arm exploration in the elevated plus-maze test, whereas unilateral infusions of muscimol did not. Similar anxiolytic-like effects were observed in Experiment 2, following bilateral infusions of muscimol (500 ng/side) into the ventral hippocampus. In Experiment 3, we confirmed that unilateral infusions of muscimol into either the lateral septum (5 ng) or the ventral hippocampus (500 ng) did not alter rats' normal open-arm avoidance. Importantly, dramatic increases in open-arm exploration were evident when muscimol was co-infused into one side of the lateral septum (5 ng) and the contralateral ventral hippocampus (500 ng). By contrast, open-arm exploration was not altered when these same doses of muscimol were co-infused into one side of the lateral septum and the ipsilateral ventral hippocampus. These results support the contention that the ventral hippocampus and the lateral septum regulate rats' open-arm exploration in a serial fashion, and that this involves ipsilateral projections from the former to the latter site.

Post-weaning social isolation increases activity in a novel environment but decreases defensive burying and subchronic MK-801 enhances the activity but not the burying effect in rats.

Subchronic treatment with a non-competitive glutamate NMDA-receptor antagonist [e.g., MK-801 or phencyclidine] or social isolation (SI) from weaning (age 21 days) to adulthood (age 56 days) produce deficits similar to some of the positive and negative symptoms of schizophrenia. Few studies have evaluated the effects of these treatments on emotional behavior. We hypothesized that subchronic MK-801, post-weaning SI or the two in combination would alter activity in a novel environment, anxiety-like behaviors in the elevated plus-maze, coping responses in the defensive burying paradigm and social behavior. In experiment 1, SI rats (n=17) showed increased locomotor activity when exposed to a novel environment, no change in plus-maze behavior and decreased defensive burying when compared to group housed rats (n=16). Subchronic MK-801 enhanced the increase in activity but not the decrease in burying in SI rats. Experiment 2 evaluated the effects on social behavior of post-weaning SI. The locomotor and burying results of experiment 1 were replicated and SI rats (n=9) were found to decrease orientation towards a novel conspecific social target when compared to group housed rats (n=8). The behavioral abnormalities of SI rats may be a manifestation of GABAergic dysfunction that has recently become evident in schizophrenia.

The lateral hypothalamus and anterior hypothalamic nucleus differentially contribute to rats' defensive responses in the elevated plus-maze and shock-probe burying tests.

Lesions or pharmacological inhibition of the lateral septum reduce rats' open-arm avoidance in the elevated plus-maze and their burying behavior in the shock-probe test. The current study examined whether hypothalamic areas that receive direct input from the lateral septum also influence open-arm avoidance and defensive burying. Bilateral infusions of the GABA-A receptor agonist muscimol (20 ng) into the lateral hypothalamus selectively increased rats' open-arm avoidance without affecting shock-probe burying. In contrast, infusions of muscimol into the anterior hypothalamic nucleus suppressed burying without affecting rats' open-arm avoidance. These dissociations suggest that the lateral hypothalamus contributes to the exploration of potentially threatening environments, whereas the anterior hypothalamus influences defensive responses to proximal discrete threat stimuli.

Electrical stimulation of dorsal, but not ventral hippocampus reduces behavioral defense in the elevated plus maze and shock-probe burying test in rats.

Recent evidence indicates that the hippocampus contributes to the control of defensive behaviors in rodents. Here, electrical stimulation (1s, 60 Hz) of the rat dorsal hippocampus inhibited defense in the elevated plus maze (increased open arm exploration) and shock-probe burying test (reduced burying duration), while ventral hippocampal stimulation had no effects. Thus, the dorsal hippocampus may play an important role in modulating a range of defensive strategies.

Repeated exposure to stress across the childhood-adolescent period alters rats' anxiety- and depression-like behaviors in adulthood: The importance of stressor type and gender.

This research tests the hypothesis that specific forms of adversity in early life map onto behavioral signs analogous to depression versus anxiety in later life. Male and female rats were exposed to either severe sporadic stress or chronic mild stress during the childhood-adolescent period, and their behavior was tested in adulthood. Males in the severe sporadic stress group showed exaggerated anxiety-related behaviors, as indicated by increases in shock-probe burying and escape-like responses (jumps) from the open arms of the elevated plus-maze. Females exposed to severe sporadic stress displayed no change in burying behavior but did display increases in escape behavior. These same females also exhibited behaviors analogous to depression that manifested as decreased sucrose consumption. The chronic mild stress regime produced effects only in females, including reduced burying, decreased sucrose consumption, and an exaggerated corticosterone response to cold-water immersion stress. Findings reiterate the importance of early life experience to the development of adult psychopathologies and emphasize the need to consider both the type of early experience and gender differences in these analyses.

Variations in maternal care influence vulnerability to stress-induced binge eating in female rats.

Binge eating disorder (BED) is characterized by intermittent, discrete periods of uncontrollable consumption during which huge quantities of high-fat food are eaten. The onset of BED occurs most frequently in adolescent or young adult females and is often associated with a history of dieting and psychological stress. Animal research suggests the importance of two synergistic factors in the aetiology of binge eating: a history of restriction-refeeding cycles (i.e., "yo-yo" dieting) and exposure to acute stress. In the rat, natural variations in maternal licking and grooming (LG) of pups during the first week of life are associated with long-lasting individual differences in offspring sensitivity to stress. The current set of experiments examined the effects of restriction--refeeding--footshock cycles on intake of highly palatable (HP) food in adolescent and adult female offspring of Low, Mid, and High LG dams. Following cycles of food restriction or unlimited food access, sated rats were exposed to footshock and their intake of HP food and chow was measured at 2, 4, and 22 h post-shock. Adolescent offspring of Low LG mothers displayed shock-induced binge eating, regardless of food-restriction history. In contrast, adolescent female offspring of Mid and High LG mothers failed to exhibit shock-induced increases in food intake. We saw no evidence of binge eating when shock was introduced in adulthood. The data suggest that low levels of maternal care in early life are associated with greater vulnerability to the later development of stress-related binge eating and further that this heightened vulnerability manifests during the adolescent period.