Male and female rats exhibit comparable gaping behavior but activate brain regions differently during expression of conditioned nausea

Lipopolysaccharide (LPS) attenuates the primary conditioning of lithium chloride (LiCl)-induced context aversion but not the secondary conditioning of context aversion or taste avoidance

A first-order association can be formed between toxin-induced nausea and a context, as well as nausea and a taste cue. However, comparatively little is understood about second-order associations. The present study examined if the bacterial endotoxin, LPS, could impair the first- and second-order conditioning of context aversion (anticipatory nausea paradigm) and subsequent conditioned taste avoidance (two-bottle task). Adult male Long Evans rats were treated with LiCl (127 mg/kg, intraperitoneal [i.p.]) or vehicle control (NaCl) and then exposed to a distinct context for 4 first-order conditioning trials. LPS (200 μg/kg, i.p.) or NaCl were administered 24 h after each trial. Seventy-two h after the final first-order conditioning trial, rats underwent 2 second-order conditioning trials where they were treated with 2% saccharin (i.p.) and then exposed to the same context. Twenty-four h after the final second-order conditioning trial, rats were tested in a two-bottle task (2 trials), where they were given a choice between water and a palatable 0.2% saccharin solution. LiCl-treated rats demonstrated a context aversion by the 3rd conditioning trial in the anticipatory nausea paradigm. Rats previously exposed to LiCl also displayed a conditioned taste avoidance of saccharin within the two-bottle task. LPS attenuated first-order context aversion but did not alter either second-order context aversion or conditioned taste avoidance in the two-bottle task. This study demonstrated that a secondary association formed within an aversive context could result in a conditioned taste avoidance. Further, LPS may be able to attenuate primary conditioning, but not secondary conditioning.

A new experimental rat model of nocebo-related nausea involving double mechanisms of observational learning and conditioning

AIM

The nocebo effect, such as nausea and vomiting, is one of the major reasons patients discontinue therapy. The underlying mechanisms remain unknown due to a lack of reliable experimental models. The goal of this study was to develop a new animal model of nocebo-related nausea by combining observational learning and Pavlovian conditioning paradigms.

METHODS

Male Sprague-Dawley rats with nitroglycerin-induced migraine were given 0.9% saline (a placebo) or LiCl (a nausea inducer) following headache relief, according to different paradigms.

RESULTS

Both strategies provoked nocebo nausea responses, with the conditioning paradigm having a greater induction impact. The superposition of two mechanisms led to a further increase in nausea responses. A preliminary investigation of the underlying mechanism revealed clearly raised peripheral and central cholecystokinin (CCK) levels, as well as specific changes in the 5-hydroxytryptamine and cannabinoid systems. Brain networks related to emotion, cognition, and visceral sense expressed higher c-Fos-positive neurons, including the anterior cingulate cortex (ACC), insula, basolateral amygdala (BLA), thalamic paraventricular nucleus (PVT), hypothalamic paraventricular nucleus (PVN), nucleus tractus solitarius (NTS), periaqueductal gray (PAG), and dorsal raphe nucleus-dorsal part (DRD). We also found that nausea expectances in the model could last for at least 12 days.

CONCLUSION

The present study provides a useful experimental model of nocebo nausea that might be used to develop potential molecular pathways and therapeutic strategies for nocebo.

Immune activation attenuates memory acquisition and consolidation of conditioned disgust (anticipatory nausea) in rats

Anticipatory nausea is a classically conditioned response to cues (e.g. contexts) that have been previously paired with a nauseating stimulus, such as chemotherapy in humans. In rodents, anticipatory nausea can be modeled by pairing a novel context with lithium chloride (LiCl), which leads to conditioned disgust behaviours (such as gaping) when exposed to the context alone. Growing evidence suggests that selective immune activation attenuates various forms of learning and memory. The present study investigated the effects of the endotoxin lipopolysaccharide (LPS) on LiCl-induced anticipatory nausea across critical stages of associative memory including acquisition, consolidation, and extinction. Adult male Long Evans rats were subject to intraperitoneal (i.p.) LiCl (127 mg/kg) or vehicle control (NaCl) paired with a 30 min conditioning trial in a distinct context for a total of 4 trials. To study acquisition, rats were administered either LPS or NaCl (200 μg/kg, i.p.) 90 mins before the conditioning trials. To study consolidation, different rats were administered either LPS or NaCl (200 μg/kg, i.p.) immediately after the conditioning trials. These trials were followed by 4 drug-free extinction trials within the same context. LPS significantly reduced conditioned gaping behaviours by the 4th conditioning trial and on the 1st drug-free extinction trial when administered 90 mins before or immediately after the conditioning trials. LPS had no significant effect on extinction. The present study provides strong evidence for the attenuating effects of LPS exposure on the acquisition and consolidation of LiCl-induced anticipatory nausea.