Voluntary exercise improves both learning and consolidation of cued conditioned fear in C57 mice.

Exercise is associated with improved cognitive function in humans as well as improved learning across a range of tasks in rodents. Although these studies provide a strong link between exercise and learning, to date studies have largely focused on tasks that principally involve the hippocampus. However, exercise has been shown to produce alterations in other brain areas suggesting that the cognitive enhancing effects of exercise may be more general. Therefore we set out to examine the effects of voluntary exercise on cued Pavlovian fear conditioning, a form of learning that is critically dependent on the amygdala. In Experiment 1 we showed that mice given 2 weeks of access to a running wheel prior to tone and foot shock fear conditioning showed enhanced conditioned fear as measured by fear-potentiated startle. This effect was not the result of altered shock reactivity nor was it to due to reduced baseline startle amplitude in exercising mice. In subsequent experiments we sought to examine whether the enhanced cued conditioned fear was the result of an improvement in learning, consolidation or retrieval of conditioned fear. In separate groups of mice, two weeks of access to a running wheel was begun either prior to fear conditioning, immediately after fear conditioning (consolidation period) or 2 weeks after fear conditioning. Compared to sedentary mice, mice that exercised either prior to fear conditioning, or immediately after fear conditioning, showed enhanced cued conditioned fear. Fear conditioning was not enhanced in mice that began exercising 2 weeks after fear conditioning. Taken together these results suggest that voluntary exercise improves the learning and consolidation of cued conditioned fear but does not improve the retrieval or performance of conditioned fear. Because a great deal is known about the neural circuit for cued conditioned fear, it is now possible to examine the cellular, molecular and pharmacological changes associated with exercise in this well-understood neural circuit.

Voluntary exercise in C57 mice is anxiolytic across several measures of anxiety.

Voluntary wheel running in rodents is associated with a number of adaptive behavioral and physiological effects including improved learning, reduction in stress-associated behaviors, neurogenesis, angiogenesis, increases in neurotrophic factors, and changes in several signaling molecules. Exercise has also been reported to reduce anxiety-like behaviors. However, other studies have failed to find an anxiolytic effect of exercise. The inconsistencies in the literature may contribute to the scarcity of data examining the physiological correlates of the anxiolytic effect of exercise. Here we show that 2 weeks of voluntary exercise in male C57 mice is associated with reduced anxiety as measured with acoustic startle, stress-induced hyperthermia, social interaction, light-enhanced startle, and some, but not all, measures in the open field. A great deal is known about the neural circuits underlying anxiety. Given the consistency of the anxiolytic effect of voluntary exercise across several measures, it is now possible to begin a systematic analysis of the physiological basis of the anxiolytic effect of exercise.

Central CRF receptor antagonist a-helical CRF9-41 blocks reinstatement of extinguished fear: the role of the bed nucleus of the stria terminalis.

The present experiments assessed the necessity of central CRF in reinstatement of extinguished fear. Using the fear-potentiated startle procedure, rats were given light-shock pairings (fear conditioning) followed by light-alone extinction training. Rats were then given unsignaled shocks to reinstate fear to the light conditioned stimulus (CS). Intracerebroventricular administration of the CRF antagonist a-Helical CRF9-41 prior to reinstatement training dose-dependently prevented reinstatement. Further, a-Helical CRF9-41 administration prior to reinstatement training or the test for reinstatement of fear to the extinguished CS prevented reinstatement at both treatment times, suggesting that CRF activity is critical for this type of return of fear to an extinguished CS. The abolition of reinstatement by drug administration was not due to state-dependent learning, as rats treated with the drug prior to both reinstatement training or testing also failed a-Helical CRF9-41 in the bed nucleus of the stria terminalis suggested that this area is a site at which central CRF is involved in this form of relapse.

Exercise is associated with reduction in the anxiogenic effect of mCPP on acoustic startle.

Voluntary exercise has been associated with reduced anxiety across several animal models. Manipulation of central 5-HT can alter anxiety-like behaviors and administration of the 5-HT agonist metachlorophenylpiperazine (mCPP) increases anxiety in rodents and humans. To examine whether the anxiolytic effect of exercise is associated with an alteration in 5-HT systems, we examined the anxiogenic effect of mCPP in exercising and nonexercising mice. C57BL/6J mice were given 2 weeks of free access to either a functioning or nonfunctioning running wheel. Mice were then tested for acoustic startle following systemic injection of either 0, 0.1, 0.3, or 1 mg/kg of mCPP. Consistent with its anxiogenic properties, mCPP produced a dose-dependent increase in acoustic startle in nonexercising mice. However, this anxiogenic effect was blunted in exercising mice. These findings suggest that exercise may help to reduce anxiety by altering 5-HT systems, perhaps by down-regulating postsynaptic 5HT 2B/2C receptors.

Posttraining lesions of the auditory thalamus, but not cortex, disrupt the inhibition of fear conditioned to an auditory stimulus.

The purpose of this study was to examine the effects of lesions within the auditory system in an effort to disrupt the processing of the noise stimulus conditioned to inhibit fear. To accomplish this, three experiments were conducted in which rats were first given feature-negative discrimination training in which a noise was conditioned to inhibit fear to a light that signals danger. Following training, rats were given lesions of the medial geniculate body (MGB), auditory thalamus (ADT), or auditory cortex (CTX). Next, rats were tested for the ability to inhibit fear in the presence of the noise safety signal. The results of these experiments indicated that bilateral lesions of ADT disrupted the ability of the noise inhibitor to inhibit fear. In contrast, lesions largely restricted to the MGB or CTX did not disrupt the inhibition of fear. Along with past studies, these results suggest that an auditory pathway(s), which includes projections from the tectum to the ADT, is used to detect the safety properties previously conditioned to an auditory stimulus.

Prepulse inhibition and fear-potentiated startle are altered in tissue inhibitor of metalloproteinase-2 (TIMP-2) knockout mice.

The ability to discriminate between potential dangers and recall those stimuli is essential for survival. This emotional learning requires the involvement of higher brain structures, including the amygdala, hippocampus and related cortical structures. Long-term changes in synaptic transmission and structure are important for the establishment and consolidation of fear memory. The structural changes associated with this synaptic plasticity likely require alterations in the composition of the extracellular matrix (ECM). ECM integrity is maintained by the opposing action of matrix metalloproteinases (MMPs) and their specific inhibitors, tissue inhibitors of metalloproteinases (TIMPs). To date, no studies have examined the role of MMPs or TIMPs in conditioned fear. Here, we show that neither male nor female mice deficient in TIMP-2 (knockout) exhibit prepulse inhibition of the startle reflex, suggesting deficits in pre-attentional sensorimotor gating. In addition, knockout mice and mice expressing a mutant truncated TIMP-2 (knock-down) show deficits in fear-potentiated startle. This is the first report of a phenotype for the TIMP-2(-/-) mice and suggests that TIMP-2 may play a role in the synaptic plasticity underlying learning and memory.

The nucleus accumbens is not critically involved in mediating the effects of a safety signal on behavior.

Although considerable progress has been made towards understanding the neural systems mediating conditioned fear, little is known about the neural mechanisms underlying conditioned inhibitors of fear (or safety signals). The present series of experiments examined the involvement of the nucleus accumbens (NAC) in mediating the effects of safety signals on behavior using a conditioned inhibition of fear-potentiated startle paradigm. Neither increasing dopaminergic nor decreasing glutamatergic function in the NAC altered the magnitude of conditioned fear or conditioned inhibition of fear in rats. Furthermore, large pre- or post-training electrolytic lesions of the NAC did not affect acquisition or expression of fear-potentiated startle or conditioned inhibition of fear-potentiated startle. Taken together, these data suggest that the NAC is not critically involved in the acquisition or expression of fear-potentiated startle or conditioned inhibition of fear-potentiated startle. Previous research has implicated the NAC in 'reward-attenuated startle' in which presentation of a stimulus paired with food decreased startle responding. The present results, therefore, indicate important neural dissociations between the processing of appetitive and safety signals, even though behavioral studies and learning theories have suggested that these two forms of learning share some commonalities.

Destruction of the inferior colliculus disrupts the production and inhibition of fear conditioned to an acoustic stimulus.

The inferior colliculus (IC) is the major source of auditory information involved in processing the behavioral significance of acoustic stimuli. In the current study, we assessed whether the IC is a critical source of information which mediates the expression of fear and the inhibition of fear conditioned to an auditory stimulus. Fear and the inhibition of fear were tested by measuring fear-potentiated startle. In Experiment 1, we demonstrated that rats which received electrolytic lesions of the IC failed to show fear-potentiated startle in the presence of a noise previously conditioned to elicit fear. In Experiment 2, we demonstrated that rats with similarly placed lesions of the IC failed to inhibit fear-potentiated startle in the presence of a noise previously conditioned to inhibit fear to a light. Thus, in both Experiments 1 and 2, lesions of the IC disrupted the behavioral significance of the noise stimulus. Together with previous findings, these results are consistent with the view that the IC is a common source of diverging auditory information used to mediate the fear eliciting and safety signal properties conditioned to auditory stimuli.

Posttraining lesion of the superior colliculus interferes with feature-negative discrimination of fear-potentiated startle.

Though much is known about the neural circuits involved in the elicitation of fear, little is known about the neural circuits responsible for the reduction of fear. The present experiments investigated the contribution of the superior colliculus (SC) and the dorsal periacquaductal gray (dPAG) in the reduction of conditioned fear produced by an auditory feature trained in a feature-negative discrimination procedure. In this procedure, light plus foot shock training trials are interspersed with trials in which the light is preceded by a noise and this noise and light compound is not followed by foot shock. At the end of this feature-negative discrimination training, rats were given excitotoxic lesions of the SC or dPAG. Feature-negative discrimination of fear was assessed with the fear-potentiated startle effect in which conditioned fear is operationally defined as potentiated startle amplitude in the presence versus the absence of the light. Feature-negative discrimination of fear is evidenced by a reduction in fear-potentiated startle to the light when the noise feature accompanies the light. Lesions of the SC, but not the dPAG, interfered with feature-negative discrimination of fear-potentiated startle suggesting that the SC plays a role in feature-negative discrimination of fear. Both SC and dPAG lesions facilitated startle amplitude in the absence of the light suggesting that these structures may exert a tonic inhibition on the acoustic startle reflex. The SC receives polymodal sensory information and is known to project forebrain areas involved in the production of conditioned fear. Thus, the SC may be an important component of the feature-negative discrimination circuit.

Posttraining but not pretraining lesions of the hippocampus interfere with feature-negative discrimination of fear-potentiated startle.

Previous studies have suggested that the hippocampus may play an important role in some forms of inhibitory learning. The goal of the present study was to assess whether the hippocampus is also important for inhibition of fear acquired after serial feature-negative discrimination training. Rats were given aspiration lesions of the hippocampus either before or after training in which a target light was paired with shock when presented alone, but not paired with shock when presented in serial compound with a noise feature (light+/noise-->light-). Conditioned fear to the target stimulus and feature-target compound were measured with fear-potentiated startle. Pre-training lesion of the hippocampus did not disrupt feature-negative discrimination performance relative to sham-operated and cortical lesioned controls. In contrast, hippocampal lesions performed after training severely disrupted performance. Specifically, rats with hippocampal lesions failed to inhibit fear when the noise feature was presented in compound with the target. However, these rats could reacquire the feature-negative discrimination. These observations suggest that the hippocampus may normally be involved in retention or retrieval of serial feature-negative discrimination; however, in its absence feature-negative discrimination can still be acquired.

Fear-potentiated startle in mice.

Pavlovian fear conditioning is frequently used to assess the behavioral, physiological, genetic and molecular correlates of learning and memory. In the typical Pavlovian conditioned fear procedure a neutral stimulus, such as a tone, is paired with a mildly aversive stimulus such as a foot shock. The tone conditioned stimulus (CS) comes to elicit a variety of behaviors that are indicative of learned fear. One of the more prominent of these behaviors is a potentiated acoustic startle response. While fear-potentiated startle in mice is qualitatively similar to that in rats, the stimulus parameters and procedures for producing optimum fear-potentiated startle in mice differ considerably from those used in rats. Procedures outlined in this unit include initial assessment of startle, fear conditioning and fear-potentiated startle testing. Special attention is paid to the parameters that affect the magnitude of fear-potentiated startle and procedures designed to systematically examine these parameters are included.