The separate and combined effects of a dangerous context and an epinephrine injection on sensory preconditioning in rats

Four experiments examined the effects of a dangerous context and a systemic epinephrine injection on sensory preconditioning in rats. In each experiment, rats were exposed to presentations of a tone and light in stage 1, light-shock pairings in stage 2, and test presentations of the tone alone and light alone in stage 3. Presentations of the tone and light in stage 1 occurred in either a safe or a previously shocked context, and/or under a systemic injection of epinephrine. Experiment 1 showed that a trace interval of 20 sec between presentations of the tone and light produced sensory preconditioning of the tone in a previously shocked context but not in a safe context, while experiment 2 provided evidence that this trace preconditioning was associative, due to the formation of a tone-light association. Experiment 3 showed that, in a safe context, exposure to the trace protocol under the influence of an epinephrine injection also produced sensory preconditioning of the tone, while experiment 4 provided evidence that a shocked context and an epinephrine injection have additive effects on trace preconditioning. These findings are discussed in relation to theories of trace conditioning. They suggest that the release of epinephrine by danger enhances attention and/or working memory processes, and thereby associative formation across a trace interval.

Animals learn about stimuli that signal motivationally significant events such as the availability of food or imminence of danger and use this information to guide food-seeking or defensive behaviors (Pearce and Bouton 2001). Animals also learn about the relations between affectively neutral stimuli, but do not always express this learning in behavior; presumably because the events lack motivational significance. A protocol used to reveal this learning is what Brogden (1939) termed sensory preconditioning. This protocol consists in three stages. In stage 1, subjects (e.g., rats) are exposed to a signaling relation between two neutral stimuli, such as a sound whose presentations are followed by a light. In stage 2, rats are exposed to a signaling relation between one of these stimuli, for example, the light, and a motivationally significant event, such as aversive shock unconditioned stimulus (US). In stage 3, rats exhibit defensive responses (e.g., freezing [Holmes et al. 2013; Wong et al. 2019] or suppression of appetitively rewarded lever pressing [Rescorla 1980]) when tested with the conditioned visual stimulus (CS) and when tested with the sensory preconditioned auditory stimulus, even though the latter stimulus was never paired with the aversive US. Control groups have confirmed that the defensive responses elicited by the sensory preconditioned sound are due to its association with the light in stage 1 rather than to generalization of such responses from the light, and to the association between the light and the aversive US in stage 2 rather than to any unconditioned ability of the light to imbue the tone with fear-eliciting properties (Rizley and Rescorla 1972).Our previous work used the sensory preconditioning protocol to examine the substrates of the association produced by pairing two affectively neutral, auditory and visual stimuli, specifically focusing on the roles of two regions in the medial temporal lobe, the perirhinal cortex (PRh), and basolateral amygdala complex (BLA). This work showed that the nature of the experience in the context before the pairings determines whether the PRh or BLA is selected for forming the association between the paired stimuli. When rats are exposed to the pairings in a familiar, safe context, formation of the association requires neuronal activity, including activation of N-methyl-D-aspartate (NMDA) receptors, in the PRh but not the BLA (Holmes et al. 2013). In contrast, when rats are exposed to the pairings in an equally familiar but previously shocked (and hence dangerous) context, formation of the association requires neuronal activity, including NMDA receptor activation, in the BLA but not the PRh (Holmes et al. 2013).Our previous work using the sensory preconditioning protocol also showed that the circumstances of associative formation differed when the auditory and visual stimuli (a pure tone sound and flashing light, respectively) were presented in a safe or dangerous context (Holmes and Westbrook 2017). Rats were placed into a safe or a previously shocked context and exposed to presentations of a sound and a light. For half of the rats in each group, every presentation of the sound was followed immediately by presentation of the light, whereas for the remaining rats in each group, every presentation of the sound was followed 20 sec later by presentation of the light. The 20-sec trace interval was selected to reduce the likelihood of associative formation among rats exposed to tone-light pairings in the safe context, and thereby, maximize our capacity to detect any effect of the dangerous context on associative formation. All rats then received pairings of the light and foot shock, and, finally, test presentations of both the sound and the conditioned light. There were no differences among the four groups in the levels of defensive/fear responses (freezing) elicited by the conditioned light, but there were such differences in the levels of freezing elicited by the sound. Specifically, rats that had been exposed to the 20-sec trace interval between the sound and light in a dangerous context froze more to the sound than rats that had been exposed to the trace interval in a safe context, and critically, just as much as rats exposed to the contiguous relation between the sound and light in either the safe or dangerous context (which did not differ).A potential mechanism by which the dangerous context might have enabled associative formation across the trace interval rests in its activation of peripheral and central adrenergic systems. There is considerable evidence that exposure to foot shock in a distinctive context increases levels of epinephrine in the periphery, and via its effects on systems that respond to stress, norepinephrine in the amygdala (Hatfield et al. 1999; McIntyre et al. 2002; McGaugh 2004). Increased epinephrine and norepinephrine levels are thought to enhance learning about cues that signal danger, including auditory and visual stimuli paired with foot shock (e.g., Rodrigues et al. 2009). Increases in these levels have also been identified with enhanced memory for affectively neutral experiences. For example, object recognition memory is enhanced by BLA infusions of norepinephrine, and this enhancement is blocked by coadministration of the β-adrenergic receptor antagonist, propranolol (Roozendaal et al. 2006, 2008). Such evidence suggests that a systemic injection of epinephrine may influence sensory preconditioning in the same way as a dangerous context; and hence that a 20-sec trace interval between the auditory and visual stimuli under a systemic injection of epinephrine will support sensory preconditioning in a safe context, as does such an interval in a dangerous context.The present study tested this suggestion. It had two specific aims. The first aim was to replicate our previous finding that test presentations of the tone elicit freezing among rats exposed to the trace interval between tone and light presentations in a dangerous, but not in a safe context (experiment 1). We additionally sought to show that danger acts directly on associative formation between the tone and the light in the trace protocol, thereby enabling the tone to elicit freezing at test (experiment 2). The second aim was to determine whether a systemic injection of epinephrine prior to placement in a safe context functions like a dangerous context to enable associative formation in the trace protocol, as indexed by the test levels of freezing to the tone (experiment 3). We additionally sought to determine whether an epinephrine injection and a dangerous context have additive effects on associative formation in the trace protocol (experiment 4).     

A Prospective Study of Adolescent Suicidal Behavior Following Hospitalization

Identification of factors associated with adolescent suicidal behavior following psychiatric hospitalization would facilitate development of effective aftercare interventions. This prospective study identified specific predictors of suicidal behavior in 100 adolescents during a 6-month follow-up period. Standardized baseline assessments and structured follow-up telephone interviews were conducted with adolescents and their guardians. Eighteen percent of adolescents reported suicidal behavior during the follow-up period, primarily of low medical lethality and minimal or ambivalent suicidal intent. This behavior was associated with suicidal thoughts, family dysfunction, and dysthymia. It was not associated with initial posthospitalization treatment compliance. Implications of these findings are discussed.