Conditioned odor potentiation of startle in rats

Quantifying defence cascade responses as indicators of pig affect and welfare using computer vision methods

Affective states are key determinants of animal welfare. Assessing such states under field conditions is thus an important goal in animal welfare science. The rapid Defence Cascade (DC) response (startle, freeze) to sudden unexpected stimuli is a potential indicator of animal affect; humans and rodents in negative affective states often show potentiated startle magnitude and freeze duration. To be a practical field welfare indicator, quick and easy measurement is necessary. Here we evaluate whether DC responses can be quantified in pigs using computer vision. 280 video clips of induced DC responses made by 12 pigs were analysed by eye to provide 'ground truth' measures of startle magnitude and freeze duration which were also estimated by (i) sparse feature tracking computer vision image analysis of 200 Hz video, (ii) load platform, (iii) Kinect depth camera, and (iv) Kinematic data. Image analysis data strongly predicted ground truth measures and were strongly positively correlated with these and all other estimates of DC responses. Characteristics of the DC-inducing stimulus, pig orientation relative to it, and 'relaxed-tense' pig behaviour prior to it moderated DC responses. Computer vision image analysis thus offers a practical approach to measuring pig DC responses, and potentially pig affect and welfare, under field conditions.

An Evolutionarily Threat-Relevant Odor Strengthens Human Fear Memory

Olfaction is an evolutionary ancient sense, but it remains unclear to what extent it can influence routine human behavior. We examined whether a threat-relevant predator odor (2-methyl-2-thiazoline) would contextually enhance the formation of human fear memory associations. Participants who learned to associate visual stimuli with electric shock in this predator odor context later showed stronger fear responses to the visual stimuli than participants who learned in an aversiveness-matched control odor context. This effect generalized to testing in another odor context, even after extinction training. Results of a separate experiment indicate that a possible biological mechanism for this effect may be increased cortisol levels in a predator odor context. These results suggest that innate olfactory processes can play an important role in human fear learning. Modulatory influences of odor contexts may partly explain the sometimes maladaptive persistence of human fear memory, e.g., in post-traumatic stress disorders.

Acquisition of Pavlovian fear conditioning using β-adrenoceptor activation of the dorsal premammillary nucleus as an unconditioned stimulus to mimic live predator-threat exposure

In the present work, we sought to mimic the internal state changes in response to a predator threat by pharmacologically stimulating the brain circuit involved in mediating predator fear responses, and explored whether this stimulation would be a valuable unconditioned stimulus (US) in an olfactory fear conditioning paradigm (OFC). The dorsal premammillary nucleus (PMd) is a key brain structure in the neural processing of anti-predatory defensive behavior and has also been shown to mediate the acquisition and expression of anti-predatory contextual conditioning fear responses. Rats were conditioned by pairing the US, which was an intra-PMd microinjection of isoproterenol (ISO; β-adrenoceptor agonist), with amyl acetate odor-the conditioned stimulus (CS). ISO (10 and 40 nmol) induced the acquisition of the OFC and the second-order association by activation of β-1 receptors in the PMd. Furthermore, similar to what had been found for contextual conditioning to a predator threat, atenolol (β-1 receptor antagonist) in the PMd also impaired the acquisition and expression of OFC promoted by ISO. Considering the strong glutamatergic projections from the PMd to the dorsal periaqueductal gray (dPAG), we tested how the glutamatergic blockade of the dPAG would interfere with the OFC induced by ISO. Accordingly, microinjections of NMDA receptor antagonist (AP5, 6 nmol) into the dPAG were able to block both the acquisition, and partially, the expression of the OFC. In conclusion, we have found that PMd β-1 adrenergic stimulation is a good model to mimic predatory threat-induced internal state changes, and works as a US able to mobilize the same systems involved in the acquisition and expression of predator-related contextual conditioning.

Hypnotherapeutic olfactory conditioning (HOC): case studies of needle phobia, panic disorder, and combat-induced PTSD

The authors developed a technique, which they call hypnotherapeutic olfactory conditioning (HOC), for exploiting the ability of scents to arouse potent emotional reactions. During hypnosis, the patient learns to associate pleasant scents with a sense of security and self-control. The patient can subsequently use this newfound association to overcome phobias and prevent panic attacks. This may be especially effective for posttraumatic stress disorder (PTSD) with episodes of anxiety, flashbacks, and dissociation triggered by smells. The authors present 3 cases, patients with needle phobia, panic disorder, and combat-induced PTSD who were successfully treated with the HOC technique.

Animal models of anxiety and anxiolytic drug action

Animal models of anxiety attempt to represent some aspect of the etiology, symptomatology, or treatment of human anxiety disorders, in order to facilitate their scientific study. Within this context, animal models of anxiolytic drug action can be viewed as treatment models relevant to the pharmacological control of human anxiety. A major purpose of these models is to identify novel anxiolytic compounds and to study the mechanisms whereby these compounds produce their anxiolytic effects. After a critical analysis of "face," "construct," and "predictive" validity, the biological context in which animal models of anxiety are to be evaluated is specified. We then review the models in terms of their general pharmacological profiles, with particular attention to their sensitivity to 5-HTIA agonists and antidepressant compounds. Although there are important exceptions, most of these models are sensitive to one or perhaps two classes of anxiolytic compounds, limiting their pharmacological generality somewhat, but allowing in depth analysis of individual mechanisms of anxiolytic drug action (e.g., GABAA agonism). We end with a discussion of possible sources of variability between models in response to 5-HTIA agonists and antidepressant drugs.

Appetitive odor-cue conditioning attenuates the acoustic startle response in rats

We here show that a neutral odor previously paired with a positive emotional context is an effective stimulus for attenuating the acoustic startle response (ASR) in rats. Olfactory cues can, therefore, be effectively used in the startle probe procedure for appetitive conditioning. This cue-induced reduction in ASR is not related to attentional alterations or a more general arousal by odor presentation, the conditioned olfactory cue rather elicits a pleasant emotional state during which the ASR is inhibited. This odor conditioned "pleasure" attenuation of the startle response might, therefore, provide a new effective operational measure for the hedonic aspects of reward.

Differential regional expression of brain-derived neurotrophic factor following olfactory fear learning

We examined brain-derived neurotrophic factor (BDNF) mRNA expression across the olfactory system following fear conditioning. Mice received 10 pairings of odor with footshock or equivalent unpaired odors and shocks. We found increased BDNF mRNA in animals receiving paired footshocks in the multiple regions examined including the posterior piriform cortex (PPC) and basolateral amygdala (BLA). This was in contrast to the unpaired and odor-alone treatments, where BDNF mRNA was increased in the olfactory bulb (OB) and anterior piriform cortex (APC) only, but not the higher olfactory areas. We propose that odor exposure increases expression of BDNF in the OB and APC while the PPC and BLA increase BDNF mRNA only when associative learning occurs.

Conditioned fear extinction and reinstatement in a human fear-potentiated startle paradigm

The purpose of this study was to analyze fear extinction and reinstatement in humans using fear-potentiated startle. Participants were fear conditioned using a simple discrimination procedure with colored lights as the conditioned stimuli (CSs) and an airblast to the throat as the unconditioned stimulus (US). Participants were extinguished 24 h after fear conditioning. Upon presentation of unsignaled USs after extinction, participants displayed significant fear reinstatement. In summary, these procedures produced robust fear-potentiated startle, significant CS+/CS- discrimination, within-session extinction, and significant reinstatement. This is the first demonstration of fear extinction and reinstatement in humans using startle measures.

Mild interoceptive stressors affect learning and reactivity to contextual cues: toward understanding the development of unexplained illnesses

Contextual learning is evident with repeated experiences with agents and treatments that induce frank illness and interoceptive stress. Here, we examined whether acute treatment with mild interoceptive stressors (low doses of pyridostigmine bromide (PB), neostigmine bromide (NB), and interleukin (IL)-1beta) may serve as unconditional stimuli supporting contextual learning. Rats were exposed to interoceptive and exteroceptive stressors in contexts distinguished by visual or olfactory cues. Acoustic startle responses (ASRs) were measured the day following exposure and 2 weeks thereafter, without delivery of the unconditional stimuli. The appearance, form, and duration of startle potentiation depended on the distinguishing features of the context and the nature of the interoceptive stressor. Rats given cholinesterase inhibitors (PB and NB), but not IL-1beta or exposed to an exteroceptive stressor, exhibited exaggerated ASRs in a novel context distinguished by visual cues. Treatment with either PB or IL-1beta led to potentiated ASRs in the presence of odors congruent with those experiences during exposure to the stressor. Startle potentiation by odor was still apparent 2 weeks after treatment. For contexts differentiated by visual stimuli, cholinomimetics transiently alter reactivity within novel contexts. In the case of contexts differentiated by odors, learning is apparent at least 2 weeks after acute treatment of cholinomimetics and IL-1beta. Contextual learning and changes in reactivity consequent to mild interoceptive stressors such as PB may play a role in the development of nonspecific symptoms typical of unexplained illnesses, such as Gulf War Illness.

Mapping the neural circuit activated by alarm pheromone perception by c-Fos immunohistochemistry

We previously reported that the alarm pheromones released from stressed male rats exaggerated both behavioral and autonomic (stress-induced hyperthermia) responses in recipient rats that were introduced into a novel environment. Subsequent experiments provided evidence that these alarm pheromones could be divided into two functionally different categories based on the site specificity and testosterone dependency of their production. However, the neural mechanisms underlying these behavioral and physiological responses remain unknown. In the present study, we examined Fos expression in 26 brain sites of the recipient rat 60 min after the exposure to the pheromone that aggravated stress-induced hyperthermia. The alarm pheromone-exposed rats showed a concurrent increase in Fos expression, in contrast to control odor-exposed rats in the anterior division lateral and medial group of the bed nucleus of the stria terminalis, paraventricular nucleus, dorsomedial hypothalamic nucleus, anterodorsal medial, lateral and basolateral amygdaloid nucleus, ventrolateral periaqueductal gray, laterodorsal tegmental nucleus, and locus coeruleus. These results provide information about the neural mechanisms in response to a non-sexual pheromone, i.e., an alarm pheromone, and suggest that the perception of the alarm pheromone is related to stress-responsive brains structures, including the hypothalamus and brainstem, as well as to the amygdaloid nuclei.

Behavioral expression of learned fear: Updating of early memories

The expression of learned fear emerges in a response-specific sequence where freezing occurs before fear potentiated startle (FPS) to an odor conditioned stimulus (CS; Postnatal Day [PN] 16 vs. PN 23; e.g., Hunt, 1997; Richardson, Paxinos, & Lee, 2000). Studies have shown that learned fear is expressed in a manner appropriate to the animal's age at training and not its age at test (Richardson & Fan, 2002; Richardson et al., 2000). Specifically, animals trained with an odor CS at PN 16 exhibit avoidance but not FPS when tested at PN 23. The present study shows that subsequent training with a different CS can "update" an early memory, allowing it to be expressed in a manner appropriate to the animal's age at test. This updating effect appears to be modality specific, whereby the subsequent training must involve a CS of the same sensory modality as the original training.

Olfactory fear conditioning induces field potential potentiation in rat olfactory cortex and amygdala

The widely used Pavlovian fear-conditioning paradigms used for studying the neurobiology of learning and memory have mainly used auditory cues as conditioned stimuli (CS). The present work assessed the neural network involved in olfactory fear conditioning, using olfactory bulb stimulation-induced field potential signal (EFP) as a marker of plasticity in the olfactory pathway. Training consisted of a single training session including six pairings of an odor CS with a mild foot-shock unconditioned stimulus (US). Twenty-four hours later, the animals were tested for retention of the CS as assessed by the amount of freezing exhibited in the presence of the learned odor. Behavioral data showed that trained animals exhibited a significantly higher level of freezing in response to the CS than control animals. In the same animals, EFPs were recorded in parallel in the anterior piriform cortex (aPC), posterior piriform cortex (pPC), cortical nucleus of the amygdala (CoA), and basolateral nucleus of the amygdala (BLA) following electrical stimulation of the olfactory bulb. Specifically, EFPs recorded before (baseline) and after (during the retention test) training revealed that trained animals exhibited a lasting increase (present before and during presentation of the CS) in EFP amplitude in CoA, which is the first amygdaloid target of olfactory information. In addition, a transient increase was observed in pPC and BLA during presentation of the CS. These data indicate that the olfactory and auditory fear-conditioning neural networks have both similarities and differences, and suggest that the fear-related behaviors in each sensory system may have at least some distinct characteristics.

Pretraining Inactivation of the Caudal Pontine Reticular Nucleus Impairs the Acquisition of Conditioned Fear-Potentiated Startle to an Odor, but Not a Light

Recent data from developing rats suggest that structures downstream from the amygdala are involved in the acquisition of conditioned fear-potentiated startle (FPS). The authors tested this idea in adult rats by temporarily inactivating the structure critical for FPS, the caudal pontine reticular nucleus (PnC), during fear conditioning. When the conditioned stimulus (CS) was an odor, rats displayed freezing, but not FPS, at test. This effect was not due to a decrease in footshock sensitivity. Further, no savings were evident on retraining. When the CS was a light, inactivation of the PnC had no effect on the acquisition of FPS. Thus, the PnC may be crucial for the acquisition of conditioned FPS to an odor, but not a light.

Latent inhibition of conditioned odor potentiation of startle: a developmental analysis

We conducted a two-part study of age and latent inhibition in the rat. In the first part of the study, rats given odor-shock pairings at 23 or 75 days of age exhibited a potentiated startle response in the presence of the odor the following day. This effect did not occur in rats trained at 16 or 20 days of age. Odor pre-exposure on the day prior to conditioning markedly reduced the odor potentiation of startle effect in 23- and 75-day-old rats but had no effect in 16 and 20-day-olds. In the second part of the study, rats were pre-exposed to the odor at 16 or 20 days of age and then conditioned at 23 days of age. When tested the day after conditioning, these pre-exposed rats exhibited a disruption in the odor potentiation of startle effect. We compare our results with other studies of latent inhibition, and with recent studies on whether conditioned responses are appropriate to the animal's age at training or their age at test.

Effects of an odor paired with illness on startle, freezing, and analgesia in rats

The data reported in this experiment provide the first systematic exploration of the effectiveness of an odor previously paired with an aversive reinforcer other than shock on eliciting various behavioral expressions of fear in the rat. Specifically, we measured potentiation of the acoustic startle response, freezing, and analgesia in the presence of an odor previously paired with an illness-inducing agent (lithium chloride; LiCl). We found that this odor elicited freezing and analgesia, but failed to potentiate the startle response. The results are discussed in terms of (1). potential threshold differences for various expressions of learned fear and (2). the possibility that the content of the learning established by odor-shock pairings differ from those established by odor-illness pairings.

Second-order olfactory-mediated fear-potentiated startle

Recently, we reported that discrete (4-sec) olfactory cues paired with footshock serve as effective conditioned stimuli (CSs) for potentiating the acoustic startle response in rats using the fear-potentiated startle paradigm. Because odors are such salient cues for the rat, and because of the robust olfactory conditioning observed previously, the current studies investigated second-order fear conditioning using olfactory and visual cues. In Experiments 1 and 2, we used a small number of first-order and second-order training trials on separate days to investigate second-order fear-potentiated startle. Significant potentiated startle was observed in animals receiving Paired/Paired training in both studies, but surprisingly, control animals in the Unpaired/Paired group (Exp. 1) also showed significant potentiated startle to a light S2 at testing. These findings are addressed in the Discussion. Overall, the results of both experiments suggest that olfactory cues serve as efficient S1 and S2 stimuli in second-order fear-potentiated startle paradigms when only a small number of first and second-order training trials are presented.

Behavioral expression of learned fear in rats is appropriate to their age at training, not their age at testing

Recent research has shown that learned fear emerges in a response-specific sequence. For example, an odor conditioned stimulus (CS) previously paired with shock elicits behavioral expressions of fear like avoidance at a younger age than it elicits other behavioral expressions of fear like potentiation of the startle response (Richardson, Paxinos, & Lee, 2000). In the present study, the question of whether learned fear is expressed in a manner appropriate to the animal's age at training or its age at testing was explored in three experiments, all using a within-subjects design. The results suggest that learned fear is expressed in a manner appropriate to the rat's age at training, not its age at testing. The Discussion section focuses on the implications of these findings for (1) the developmental analysis of memory and (2) the idea that an aversive CS elicits a central state of fear.

Olfactory-mediated fear-potentiated startle

Recently, R. Richardson, A. Vishney, and J. Lee (1999) reported that ambient odor cues that were previously paired with footshock potentiate the acoustic startle response in rats. The authors of the present study extend those findings by using a discrete 4-s amyl acetate odor paired with footshock to address several parametric issues that might be important for using odorants as conditioned stimuli (CSs) in this paradigm. Amyl acetate (5%) had no significant effect on startle in untrained rats but did potentiate startle in rats that received 1, 2, 5, or 10 odor-shock pairings. Fear-potentiated startle decreased but was still significant up to 40 days after conditioning and could be measured in test trials separated by as little as 30 s. The magnitude of potentiated startle decreased with decreasing concentrations of amyl acetate (5%-5 x 10-9%). The anxiolytic compound buspirone (10 mg/kg) significantly attenuated olfactory-mediated fear-potentiated startle.

Centrally administered corticotropin-releasing hormone and peripheral injections of strychnine hydrochloride potentiate the acoustic startle response in preweanling rats

Attempts to condition fear potentiation of startle (FPS) in rats younger than 23 days of age have not been successful, regardless of the type of aversively conditioned stimulus used (P. S. Hunt, R. Richardson, & B. A. Campbell, 1994; R. Richardson, G. Paxinos, & J. Lee, 2000; R. Richardson & A. Vishney, 2000). In the present study, the authors report that peripheral injections of strychnine hydrochloride, a glycine receptor antagonist, and intracerebroventricular infusions of corticotropin releasing hormone (CRH) both potentiated the acoustic startle response (ASR) in 16-18-day-old rats. Because strychnine and CRH have distinct sites of activation in the primary startle pathway, it can be concluded that this pathway is functional and modifiable in rats younger than 23 days of age. This finding suggests that the failure to observe conditioned FPS in preweanling rats is due to an immaturity of the secondary fear circuit responsible for enhancing the ASR during a fear state.

Shock sensitization of startle: learned or unlearned fear?

Following shock, rats exhibit a potentiated startle response to a sudden, loud noise. It has been suggested that this shock sensitization of the startle response can be used as a model preparation for studying unlearned fear. After reviewing the theoretical and empirical bases for this claim, the results of several recent studies that show that the shock sensitization of startle effect is actually mediated by contextually conditioned fear are presented. From this, it is concluded that the shock sensitization of startle procedure is an appropriate model preparation for studying contextual conditioning but is not an appropriate procedure for studying unlearned fear. Several other procedures that have potential as model preparations for studying unlearned fear are discussed briefly.