Sex-Dependent Effects of Stress on Immobility Behavior and VTA Dopamine Neuron Activity: Modulation by Ketamine.
ABSTRACT: Stress constitutes a risk factor across several psychiatric disorders. Moreover, females are more susceptible to stress-related disorders, such as depression, than males. Although dopamine system underactivation is implicated in the pathophysiology of depression, little is known about the female dopamine system at baseline and post-stress.The effects of chronic mild stress were examined on ventral tegmental area dopamine neuron activity and forced swim test immobility by comparing male and female rats. The impact of a single dose of the rapid antidepressant ketamine (10 mg/kg, i.p.) on forced swim test immobility and ventral tegmental area function was then tested.Baseline ventral tegmental area dopamine activity was comparable in both sexes. At baseline, females exhibited roughly double the forced swim test immobility duration than males, which corresponded to ~50% decrease in ventral tegmental area dopamine population activity compared with similarly treated (i.e., post-forced swim test) males. Following chronic mild stress, there was greater immobility duration in both sexes and reduced ventral tegmental area dopamine neuron activity by approximately 50% in males and nearly 75% in females. Ketamine restored behavior and post-forced swim test ventral tegmental area dopamine activity for up to 7 days in females as well as in both male and female chronic mild stress-exposed rats.These data suggest increased female susceptibility to depression-like phenotypes (i.e., greater immobility, ventral tegmental area hypofunction) is associated with higher dopamine system sensitivity to both acute and repeated stress relative to males. Understanding the neural underpinnings of sex differences in stress vulnerability will provide insight into mechanisms of disease and optimizing therapeutic approaches in both sexes.
Project description:Stress facilitates reinstatement of addictive drug seeking in animals and promotes relapse in humans. Acute stress has marked and long-lasting effects on plasticity at both inhibitory and excitatory synapses on dopamine neurons in the ventral tegmental area (VTA), a key region necessary for drug reinforcement. Stress blocks long-term potentiation at GABAergic synapses on dopamine neurons in the VTA (LTPGABA), potentially removing a normal brake on activity. Here we show that blocking kappa opioid receptors (KORs) prior to forced-swim stress rescues LTPGABA. In contrast, blocking KORs does not prevent stress-induced potentiation of excitatory synapses nor morphine-induced block of LTPGABA. Using a kappa receptor antagonist as a selective tool to test the role of LTPGABA in vivo, we find that blocking KORs within the VTA prior to forced-swim stress prevents reinstatement of cocaine seeking. These results suggest that KORs may represent a useful therapeutic target for treatment of stress-triggered relapse in substance abuse.
Project description:Anhedonia, or diminished interest or pleasure in rewarding activities, characterizes depression and reflects deficits in brain reward circuitries. Social stress induces anhedonia and increases risk of depression, although the effect of social stress on brain reward function is incompletely understood.This study assessed the following: 1) brain reward function in rats (using the intracranial self-stimulation procedure) and protein levels of brain-derived neurotrophic factor and related signaling molecules in response to chronic social defeat, 2) brain reward function during social defeat and long-term treatment with the antidepressants fluoxetine (5 mg/kg/day) and desipramine (10 mg/kg/day), and 3) forced swim test behavior after social defeat and fluoxetine treatment.Social defeat profoundly and persistently decreased brain reward function, reflecting an enduring anhedonic response, in susceptible rats, whereas resilient rats showed no long-term brain reward deficits. In the ventral tegmental area, social defeat, regardless of susceptibility or resilience, decreased brain-derived neurotrophic factor and increased phosphorylated AKT, whereas only susceptibility was associated with increased phosphorylated mammalian target of rapamycin. Fluoxetine and desipramine reversed lower, but not higher, stress-induced brain reward deficits in susceptible rats. Fluoxetine decreased immobility in the forced swim test, as did social defeat.These results suggest that the differential persistent anhedonic response to psychosocial stress may be mediated by ventral tegmental area signaling molecules independent of brain-derived neurotrophic factor and indicate that greater stress-induced anhedonia is associated with resistance to antidepressant treatment. Consideration of these behavioral and neurobiological factors associated with resistance to stress and antidepressant action may promote the discovery of novel targets to treat stress-related mood disorders.
Project description:The beneficial effects of omega (?)-3 polyunsaturated fatty acid (PUFA) supplementation on major depressive disorder have been actively studied, but the underlying mechanism remains unknown. The present study examined the involvement of the nucleus accumbens (NAc) dopaminergic systems in behavioral changes in mice fed a diet high in ?-3 PUFAs. Mice fed a diet containing about double the amount of ?-3 PUFAs (krill oil (KO) diet) exerted shorter immobility times in the forced swim test (FST) than mice fed a control diet, containing only ?-linolenic acid (ALA) as ?-3 PUFAs. The shorter immobility times were observed in both male and female mice. A dopamine metabolite, 3,4-dihydroxyphenylacetic acid, increased in the NAc in male mice fed the KO diet when compared with those fed the control diet. In addition, dopamine, 3-methoxytyramine, and homovanillic acid increased in the NAc in female mice fed the KO diet. Notably, the effects of the KO diet on the immobility time in the FST were abolished by microinjection of sulpiride, an antagonist of D2-like receptors, into the NAc. A similar microinjection of an antagonist selective for D1-like receptors, SKF83566, also abolished the reduction in immobility in the FST. Moreover, we found that tyrosine hydroxylase-positive cells increased in the ventral tegmental area (VTA) in mice fed the KO diet. These results suggest that modulation of the VTA-NAc dopaminergic pathway is one of the mechanisms by which a KO diet rich in ?-3 PUFAs reduces the immobility behavior in the mouse FST.
Project description:Dopaminergic neurons in the ventral tegmental area of the brain are an important site of convergence of drugs and stress. We previously identified a form of long-term potentiation of gamma-aminobutyric acid (GABA)ergic synapses on these neurons (LTPGABA). Our studies have shown that exposure to acute stress blocks this LTP and that reversal of the block of LTPGABA is correlated with prevention of stress-induced reinstatement of cocaine-seeking behavior.Sprague-Dawley rats were subjected to cold-water swim stress. Midbrain slices were prepared following stress, and whole-cell patch clamp recordings of inhibitory postsynaptic currents were performed from ventral tegmental area dopamine neurons. Antagonists of glucocorticoid receptors and kappa opioid receptors (?ORs) were administered at varying time points after stress. Additionally, the ability of a kappa antagonist administered following stress to block forced swim stress-induced reinstatement of cocaine self-administration was tested.We found that an acute stressor blocks LTPGABA for 5 days after stress through a transient activation of glucocorticoid receptors and more lasting contribution of ?ORs. Even pharmacological block of ?ORs beginning 4 days after stress has occurred reversed the block of LTPGABA. Administration of a ?ORs antagonist following stress prevents reinstatement of cocaine-seeking behavior.A brief stressor produces changes in the reward circuitry lasting several days. Our findings reveal roles for glucocorticoid receptors and ?ORs as mediators of the lasting effects of stress on synaptic plasticity. ?ORs antagonists reverse the neuroadaptations underlying stress-induced drug-seeking behavior and may be useful in the treatment of cocaine addiction.
Project description:There is growing evidence that kappa opioid receptor (KOR) antagonists could be a useful class of therapeutics for treating depression and anxiety. However, the overwhelming majority of preclinical investigations examining the behavioral effects of KOR antagonists have been in male rodents. Here, we examined the effects of the long-acting KOR antagonist nor-binaltophimine (norBNI) on immobility in the forced swim test in males and females of two different rodent species (C57Bl/6J and California mice). Consistent with previous reports, norBNI (10 mg/kg) decreased immobility in the forced swim test for male C57Bl/6J and California mice. Surprisingly, dose-response studies in female C57Bl/6J and California mice showed that norBNI did not reduce immobility. Pharmacokinetic analyses showed that metabolism and brain concentrations of norBNI were similar in male and female C57Bl/6J. In the nucleus accumbens of male but not female C57Bl/6J, norBNI increased phosphorylation of c-Jun N-terminal kinase (pJNK), a putative mechanism for norBNI action. However, no differences in pJNK were observed in male or female California mice. Together, these results suggest that immobility in the forced swim test is less dependent on endogenous KOR signaling in female rodents and highlight the importance of examining the effects of possible therapeutic agents in both males and females.
Project description:The leptin receptor (Lepr) is expressed on midbrain dopamine neurons. However, the specific role of Lepr signaling in dopamine neurons remains to be clarified. In the present study, we generated a line of conditional knockout mice lacking functional Lepr selectively on dopamine neurons (Lepr(DAT-Cre)). These mice exhibit normal body weight and feeding. Behaviorally, Lepr(DAT-Cre) mice display an anxiogenic-like phenotype in the elevated plus-maze, light-dark box, social interaction and novelty-suppressed feeding tests. Depression-related behaviors, as assessed by chronic stress-induced anhedonia, forced swim and tail-suspension tests, were not affected by deletion of Lepr in dopamine neurons. In vivo electrophysiological recordings of dopamine neurons in the ventral tegmental area revealed an increase in burst firing in Lepr(DAT-Cre) mice. Moreover, blockade of D1-dependent dopamine transmission in the central amygdala by local microinjection of the D1 antagonist SCH23390 attenuated the anxiogenic phenotype of Lepr(DAT-Cre) mice. These findings suggest that Lepr signaling in midbrain dopamine neurons has a crucial role for the expression of anxiety and for the dopamine modulation of amygdala function.
Project description:Ciliary neurotrophic factor (CNTF) is produced by astrocytes and promotes neurogenesis and neuroprotection. Little is known about the role of CNTF in affective behavior. We investigated whether CNTF affects depressive- and anxiety-like behavior in adult mice as tested in the forced swim, sucrose preference and elevated-T maze tests. Female wild type CNTF+/+ mice more readily developed behavioral despair with increased immobility time and decreased latency to immobility in the forced swim test than male CNTF+/+ littermates. The lack of CNTF in CNTF-/- mice had an opposite effect on depressive-like behavior in female mice (reduced immobility time and increased sucrose preference) vs. male mice (increased immobility time). Female wildtype mice expressed more CNTF in the amygdala than male mice. Ovariectomy increased CNTF expression, as well as immobility time, which was significantly reduced in CNTF-/- mice, suggesting that CNTF mediates overiectomy-induced immobility time, possibly in the amygdala. Progesterone but not 17-? estradiol inhibited CNTF expression in cultured C6 astroglioma cells. Progesterone treatment also reduced CNTF expression in the amygdala and decreased immobility time in female CNTF+/+ but not in CNTF-/- mice. Castration did not alter CNTF expression in males nor their behavior. Lastly, there were no effects of CNTF on the elevated T-maze, a behavioral test of anxiety, suggesting that a different mechanism may underlie anxiety-like behavior. This study reveals a novel CNTF-mediated mechanism in stress-induced depressive-like behavior and points to opportunities for sex-specific treatments for depression, e.g. progesterone in females and CNTF-stimulating drugs in males.
Project description:Drug addiction is a chronic disorder characterized by a cycle composed of drug seeking, intoxication with drug taking and withdrawal associated with negative affect. Numerous studies have examined withdrawal/negative affect after chronic use; however, very few have examined the effect of acute administration on the negative affective state after acute drug withdrawal. One dose of amphetamine was injected into Sprague-Dawley rats. Despair behavior using the modified forced swim test (FST) and dopamine (DA) activity in the ventral tegmental area using in vivo electrophysiological recordings were studied 18, 48 and 72 h after injection of amphetamine. The effects of inactivation of the basolateral amygdala (BLA) and ketamine administration on VTA DA neuron activity and passivity in the modified FST were examined. Eighteen hours following amphetamine withdrawal, there was a substantial decrease in the number of active DA neurons, as well as an increase in time spent immobile in the modified FST, which returned to baseline after 72 h. Inactivation of the BLA after acute amphetamine prevented the decrease in DA neuron tonic activity. Injection of ketamine also prevented the decrease in DA population activity but had no effect on immobility measured in the modified FST. The data support a model in which the negative affective state following acute amphetamine withdrawal is associated with a decrease in DA neuron population activity, driven by hyperactivity of the BLA. Although ketamine reversed the hypodopaminergic state following withdrawal, the failure to reduce immobility in the modified FST indicates that different processes underlying negative emotional state may exist between depression and drug withdrawal.
Project description:Chronic stress drives behavioral and physiological changes associated with numerous psychiatric disease states. In rodents, the vast majority of chronic stress models involve imposition of external stressors, whereas in humans stress is often driven by internal cues, commonly associated with a sense of loss. We previously exposed groups of rats to environmental enrichment (EE) for a protracted period (1 month), followed by removal of enrichment (ER), to induce an experience of loss in male rats. ER enhanced immobility in the forced swim test (FST), led to hypothalamic pituitary adrenal (HPA) axis hypoactivity, and caused hyperphagia relative to continuously enriched (EE), single-housed (Scon) and pair-housed (Pcon) groups, most of which were reversible by antidepressant treatment (Smith et al., 2017). Here, we have applied the same approach to study enrichment loss in female rats. Similar to the males, enrichment removal in females led to an increase in the time spent immobile in the FST and increased daytime food intake compared to the single and pair-housed controls. Unlike males, ER females showed decreased sucrose preference, and showed estrus cycle-dependent HPA axis hyperactivity to an acute restraint stress. The increase in passive coping (immobility), anhedonia-like behavior in the sucrose preference test and HPA axis dysregulation suggest that enrichment removal produces a loss phenotype in females that differs from that seen in males, which may be more pronounced in nature.
Project description:Synaptic plasticity in the ventral tegmental area (VTA) is modulated by drugs of abuse and stress and is hypothesized to contribute to specific aspects of addiction. Both excitatory and inhibitory synapses on dopamine neurons in the VTA are capable of undergoing long-term changes in synaptic strength. While the strengthening or weakening of excitatory synapses in the VTA has been widely examined, the role of inhibitory synaptic plasticity in brain reward circuitry is less established. Here, we investigated the effects of drugs of abuse, as well as acute stress, on long-term potentiation of GABAergic synapses onto VTA dopamine neurons (LTP(GABA)). Morphine (10 mg/kg i.p.) reduced the ability of inhibitory synapses in midbrain slices to express LTP(GABA) both at 2 and 24 h after drug exposure but not after 5 days. Cocaine (15 mg/kg i.p.) impaired LTP(GABA) 24 h after exposure, but not at 2 h. Nicotine (0.5 mg/kg i.p.) impaired LTP(GABA) 2 h after exposure, but not after 24 h. Furthermore, LTP(GABA) was completely blocked 24 h following brief exposure to a stressful stimulus, a forced swim task. Our data suggest that drugs of abuse and stress trigger a common modification to inhibitory plasticity, synergizing with their collective effect at excitatory synapses. Together, the net effect of addictive substances or stress is expected to increase excitability of VTA dopamine neurons, potentially contributing to the early stages of addiction.