Activation of the ventral medial prefrontal cortex during an uncontrollable stressor reproduces both the immediate and long-term protective effects of behavioral control

Timing in the absence of supraspinal input II: regularly spaced stimulation induces a lasting alteration in spinal function that depends on the NMDA receptor, BDNF release, and protein synthesis

The detection of temporal regularity allows organisms to predict the occurrence of future events. When events occur in an irregular manner, uncertainty is increased, and negative outcomes can ensue (e.g., stress). The present study shows that spinal neurons can discriminate between variable- and fixed-spaced stimulation and that the detection of regularity requires training and engages a form of NMDA receptor-mediated plasticity. The impact of stimulus exposure was assessed using a spinally mediated instrumental response, wherein spinally transected rats are given legshock whenever one hindlimb is extended. Over time, they learn to maintain the leg in a flexed position that minimizes net shock exposure. Prior exposure to 180-900 tailshocks given in a variable (unpredictable) manner inhibited this learning. A learning deficit was not observed when 900 tailshocks were applied using a fixed (predictable) spacing. Fixed-spaced stimulation did not have a divergent effect when fewer (180) shocks were presented, implying that the abstraction of temporal regularity required repeated exposure (training). Moreover, fixed-spaced stimulation both prevented and reversed the learning deficit. The protective effect of fixed-spaced shock lasted 48 h, and was prevented by pretreatment with the NMDA receptor antagonist MK-801. Administration of the protein synthesis inhibitor cycloheximide after training blocked the long-term effect. Inhibiting BDNF function, using TrkB-IgG, also eliminated the beneficial effect of fixed-spaced stimulation. The results suggest that spinal systems can detect regularity and that this type of stimulation promotes adaptive plasticity, which may foster recovery after spinal injury.

Behavioral control over shock blocks behavioral and neurochemical effects of later social defeat

Experience with behavioral control over tailshock (escapable shock, ES) has been shown to block the behavioral and neurochemical changes produced by later uncontrollable tail shock (inescapable shock, IS). The present experiments tested, in rats, whether the protective effect of control over tailshock extends beyond reducing the behavioral and neurochemical impact of a subsequent tailshock experience to stressors that are quite different. Social defeat (SD) was chosen as the second stress experience because it has few if any cues in common with tailshock. SD produced shuttlebox escape learning deficits ("learned helplessness") and reduced juvenile social investigation 24 h later, as does IS. IS is notable for inducing a large increase in dorsal raphe nucleus (DRN) serotonergic (5-HT) activity as measured by extracellular levels of 5-HT within the DRN, and SD did so as well. ES occurring 7 days before SD blocked this SD-induced DRN activation, as well as the SD-induced interference with shuttlebox escape and reduction in social investigation. Prior exposure to yoked IS did not reduce the DRN 5-HT activation or later behavioral effects produced by SD, and thus the proactive stress-blunting effects of ES can be attributed to it's controllability. Thus, ES confers a very general protection to the impact of a subsequent stress experience.

Selective activation of dorsal raphe nucleus-projecting neurons in the ventral medial prefrontal cortex by controllable stress

Exposure to uncontrollable stressors produces a variety of behavioral consequences (e.g. exaggerated fear, reduced social exploration) that do not occur if the stressor is controllable. In addition, an initial experience with a controllable stressor can block the behavioral and neural responses to a later uncontrollable stressor. The serotonergic (5-HT) dorsal raphe nucleus (DRN) has come to be viewed as a critical structure in mediating the behavioral effects of uncontrollable stress. Recent work suggests that the buffering effects of behavioral control on the DRN-dependent behavioral outcomes of uncontrollable stress require ventral medial prefrontal cortex (mPFCv) activation at the time of behavioral control. The present studies were conducted to directly determine whether or not controllable stress selectively activates DRN-projecting neurons within the mPFCv. To examine this possibility in the rat, we combined retrograde tracing (fluorogold iontophoresed into the DRN) with Fos immunohistochemistry, a marker for neural activation. Exposure to controllable, relative to uncontrollable, stress increased Fos expression in fluorogold-labeled neurons in the prelimbic region (PL) of the mPFCv. Furthermore, in a separate experiment, a prior experience with controllable stress led to potentiation of Fos expression in retrogradely labeled PL neurons in response to an uncontrollable stressor 1 week later. These results suggest that the PL selectively responds to behavioral control and utilizes such information to regulate the brainstem response to ongoing and subsequent stressors.

Medial prefrontal cortical activation modulates the impact of controllable and uncontrollable stressor exposure on a social exploration test of anxiety in the rat

The presence of behavioral control over a stressor can blunt many of the effects of the stressor. We have recently reported that uncontrollable stress (inescapable electric tailshock, IS) reduces later social exploration of a juvenile whereas controllable stress (escapable shock, ES) does not. Activation of the ventral medial prefrontal cortex (vmPFC) is crucial to blunting the effects of IS on later escape behavior (learned helplessness). The goal of the current study was to test the role of the vmPFC in modulating the effects of stressor controllability on anxiety in the social exploration test. Thus, adult male rats were implanted with cannula guides for drug microinjection into the vmPFC. In Experiment 1, temporary inactivation of the vmPFC with the GABA(A) agonist muscimol before exposure to ES prevented the protective effects of stress control, leading to reduced social exploration. In Experiment 2, excitation of the vmPFC prior to IS with the GABA-activated Cl(( - )) channel antagonist picrotoxin mimicked the stress resistance produced by control and prevented IS-induced reduction in social exploration. These results are consistent with prior work and identify the vmPFC as a critical component of the neural circuitry mediating the effects of stressor control on later behaviors. The relationship between the vmPFC, dorsal raphé nucleus, and other structures mediating stress-induced anxiety are discussed.

Treatment with escitalopram but not desipramine decreases escape latency times in a learned helplessness model using juvenile rats

RATIONALE

The pharmacological treatment of depression in children and adolescents is different from that of adults due to the lack of efficacy of certain antidepressants in the pediatric age group. Our current understanding of why these differences occur is very limited.

OBJECTIVES

To develop more effective treatments, a juvenile animal model of depression was tested to validate it as a possible model to specifically study pediatric depression.

MATERIALS AND METHODS

Procedures for use with juvenile rats at postnatal day (PND) 21 and 28 were adapted from the adult learned helplessness model in which, 24 h after exposure to inescapable stress, animals are unable to remove themselves from an easily escapable stressor. Rats were treated for 7 days with either the selective serotonin reuptake inhibitor escitalopram at 10 mg/kg or the tricyclic antidepressant desipramine at 3, 10, or 15 mg/kg to determine if treatment could decrease escape latency times.

RESULTS

Escitalopram treatment was effective at decreasing escape latency times in all ages tested. Desipramine treatment did not decrease escape latency times for PND 21 rats, but did decrease times for PND 28 and adult animals.

CONCLUSIONS

The learned helplessness model with PND 21 rats predicts the efficacy of escitalopram and the lack of efficacy of desipramine seen in the treatment of pediatric depression. These findings suggest that the use of PND 21 rats in a modified learned helplessness procedure may be a valuable model of human pediatric depression that can predict pediatric antidepressant efficacy and be used to study antidepressant mechanisms involved in pediatric depression.

A benchmarked feasibility study of a self-hypnosis treatment for depression in primary care

This investigation assessed the effectiveness of a self-help, self-hypnosis treatment in a primary-care setting in Edinburgh, UK. A partially randomized preference (PRP) study design was used, with benchmarking results to trials of CBT and counseling. Patients seeing their general practitioner for depression were offered randomization to, or their treatment preference of, either self-help (self-hypnosis) or antidepressant medication. Evaluation measures were Becks Depression Inventory, Brief Symptom Inventory, and SF-36. Of the 58 patients recruited, 50 chose self-hypnosis, 4 chose antidepressants, and 4 were randomized. The preference groups demonstrated similar demography, baseline measurements, and outcome effects to benchmarked trials. This feasibility study of a self-help, self-hypnosis program for depression showed promise for its future use in primary care. Benchmarking improved validity and reliability. A PRP study design appeared useful in a primary-care setting, where past studies have experienced problems of recruitment, concordance, and compliance.

Pain and learning in a spinal system: contradictory outcomes from common origins

The long-standing belief that the spinal cord serves merely as a conduit for information traveling to and from the brain is changing. Over the past decade, research has shown that the spinal cord is sensitive to response-outcome contingencies, demonstrating that spinal circuits have the capacity to modify behavior in response to differential environmental cues. If spinally transected rats are administered shock contingent on leg extension (controllable shock), they will maintain a flexion response that minimizes shock exposure. If, however, this contingency is broken, and shock is administered irrespective of limb position (uncontrollable shock), subjects cannot acquire the same flexion response. Interestingly, each of these treatments has a lasting effect on behavior; controllable shock enables future learning, while uncontrollable shock produces a long-lasting learning deficit. Here we suggest that the mechanisms underlying learning and the deficit may have evolved from machinery responsible for the spinal processing of noxious information. Experiments have shown that learning and the deficit require receptors and signaling cascades shown to be involved in central sensitization, including activation of NMDA and neurokinin receptors, as well as CaMKII. Further supporting this link between pain and learning, research has also shown that uncontrollable stimulation results in allodynia. Moreover, systemic inflammation and neonatal hindpaw injury each facilitate pain responding and undermine the ability of the spinal cord to support learning. These results suggest that the plasticity associated with learning and pain must be placed in a balance in order for adaptive outcomes to be observed.

Brain mediators of cardiovascular responses to social threat: part I: Reciprocal dorsal and ventral sub-regions of the medial prefrontal cortex and heart-rate reactivity

Social threat is a key component of mental "stress" and a potent generator of negative emotions and physiological responses in the body. How the human brain processes social context and drives peripheral physiology, however, is relatively poorly understood. Human neuroimaging and animal studies implicate the dorsal medial prefrontal cortex (MPFC), though this heterogeneous region is likely to contain multiple sub-regions with diverse relationships with physiological reactivity and regulation. We used fMRI combined with a novel multi-level path analysis approach to identify brain mediators of the effects of a public speech preparation task (social evaluative threat, SET) on heart rate (HR). This model provides tests of functional pathways linking experimentally manipulated threat, regional fMRI activity, and physiological output, both across time (within person) and across individuals (between persons). It thus integrates time series connectivity and individual difference analyses in the same path model. The results provide evidence for two dissociable, inversely coupled sub-regions of MPFC that independently mediated HR responses. SET caused activity increases in a more dorsal pregenual cingulate region, whose activity was coupled with HR increases. Conversely, SET caused activity decreases in a right ventromedial/medial orbital region, which were coupled with HR increases. Individual differences in coupling strength in each pathway independently predicted individual differences in HR reactivity. These results underscore both the importance and heterogeneity of MPFC in generating physiological responses to threat.

The medial prefrontal cortex regulates the differential expression of morphine-conditioned place preference following a single exposure to controllable or uncontrollable stress

Experiential factors, such as stress, are major determinants of vulnerability to drug addiction and relapse. The behavioral controllability of the stressor is a major determinant of how exposure to a stressor impacts addictive processes. Recent evidence suggests that controllable stressors, such as escapable shock (ES), activate ventral regions of the medial prefrontal cortex (mPFCv), whereas physically identical, but uncontrollable stress (inescapable shock, IS) does not. This activation is critical to the blunting effect that control has on neurochemical and behavioral sequelae of stress. Our laboratory has previously shown that IS, but not ES, potentiates morphine-conditioned place preference (CPP). However, the role of the mPFCv in this phenomenon is unknown. The present experiments investigated the role of the mPFCv during ES and IS in determining the effects of the stressor on subsequent morphine-CPP. Intra-mPFCv microinjection of the GABA(A) agonist muscimol 1 h before ES led ES to potentiate morphine-CPP, as does IS. Conversely, the potentiation of morphine-CPP normally observed in IS rats was blocked by intra-mPFCv microinjection of the GABA(A) antagonist picrotoxin 1 h before IS. These results suggest that during stress, activation of the mPFCv prevents subsequent potentiation of morphine-CPP, whereas inactivation of the mPFCv during stress does not. Thus, activation of the mPFCv during a stress experience is both necessary and sufficient to block the impact of stress on morphine-CPP, and control over stress blunts stress-induced potentiation of morphine effects by activating the mPFCv.

Prefrontal afferents to the dorsal raphe nucleus in the rat

The prefrontal cortex (PFC) receives strong inputs from monoaminergic cell groups in the brainstem and also sends projections to these nuclei. Recent evidence suggests that the PFC exerts a powerful top-down control over the dorsal raphe nucleus (DR) and that it may be involved in the actions of pharmaceutical drugs and drugs of abuse. In the light of these findings, the precise origin of prefrontal inputs to DR was presently investigated by using the cholera toxin subunit b (CTb) as retrograde tracer. All the injections placed in DR produced retrograde labeling in the medial, orbital, and lateral divisions of the PFC as well as in the medial part of the frontal polar cortex. The labeling was primarily located in layer V. Remarkably, labeling in the medial PFC was denser in its ventral part (infralimbic and ventral prelimbic cortices) than in its dorsal part (dorsal prelimbic, anterior cingulate and medial precentral cortices). After injections in the rostral or caudal DR, the largest number of labeled neurons was observed in the medial PFC, whereas after injections in the mid-rostrocaudal DR, the labeled neurons were more homogeneously distributed in the three main PFC divisions. A cluster of labeled neurons also was observed around the apex of the rostral pole of the accumbens, especially after rostral and mid-rostrocaudal DR injections. Overall, these results confirm the existence of robust prefrontal projections to DR, mainly derived from the ventral part of the medial PFC, and underscore a substantial contribution of the frontal polar cortex.

Stress-induced prefrontal reorganization and executive dysfunction in rodents

The prefrontal cortex (PFC) mediates a range of higher order 'executive functions' that subserve the selection and processing of information in such a way that behavior can be planned, controlled and directed according to shifting environmental demands. Impairment of executive functions typifies many forms of psychopathology, including schizophrenia, mood and anxiety disorders and addiction, that are often associated with a history of trauma and stress. Recent research in animal models demonstrates that exposure to even brief periods of intense stress is sufficient to cause significant structural remodeling of the principle projection neurons within the rodent PFC. In parallel, there is growing evidence that stress-induced alterations in PFC neuronal morphology are associated with deficits in rodent executive functions such as working memory, attentional set-shifting and cognitive flexibility, as well as emotional dysregulation in the form of impaired fear extinction. Although the molecular basis of stress-induced changes in PFC morphology and function are only now being elucidated, an understanding of these mechanisms could provide important insight into the pathophysiology of executive dysfunction in neuropsychiatric disease and foster improved strategies for treatment.