Maintenance of conditioned place avoidance induced by gastric malaise requires NMDA activity within the ventral hippocampus

Abstract

It has been reported that during chemotherapy treatment, some patients can experience nausea before pharmacological administration, suggesting that contextual stimuli are associated with the nauseating effects. There are attempts to reproduce with animal models the conditions under which this phenomenon is observed to provide a useful paradigm for studying contextual aversion learning and the brain structures involved. This manuscript assessed the hippocampus involvement in acquiring and maintaining long-term conditioned place avoidance (CPA) induced by a gastric malaise-inducing agent, LiCl. Our results demonstrate that a reliable induction of CPA is possible after one acquisition trial. However, CPA establishment requires a 20-min confinement in the compartment associated with LiCl administration. Interestingly, both hippocampal regions seem to be necessary for CPA establishment; nonetheless, inactivation of the ventral hippocampus results in a reversion of avoidance and turns it into preference. Moreover, we demonstrate that activation of dorsal/ventral hippocampal NMDA receptors after CS-US association is required for long-term CPA memory maintenance.

Figure 1.

Protocol schemes of Conditioned Place Avoidance protocol. (A) On the pretest session, the rat was placed at the middle compartment and freely explored the chambers over a period of 10 min period. On day 2, during the conditioning session, animals were injected i.p. with 0.4M LiCl (7.5 mL/kg) and confined to the black chamber for 10, 20, 40, or 60 min; 4 h later, rats were injected i.p, with 0.15 M NaCl (7.5 mL/kg) and confined to the white chamber for 10, 20, 40, or 60 min, respectively. On the post-test session, the rat was placed in the middle compartment and freely explored the chambers over a period of 10 min. (B) In a similar CPA protocol, during the infusion session on day 2, a volume of 0.5 µL (0.25 µL/min) of elected drug was bilaterally injected in the dorsal or ventral hippocampus. Infusion times were 15 min before the CS–US association or immediately after the CS–US association. Injected drugs were saline solution 0.9% (w/v), a GABA_A and GABA_B agonist cocktail (0.1 mM Muscimol and 1 mM Baclofen), or 10 µg/µL DL-2-amino-5-phosphonovaleric acid (APV). Accordingly to our results, 20-min confinement induces a robust CPA. Therefore, all pharmacological manipulations were conducted in 20-min association protocol.

Figure 2.

Conditioned place avoidance establishment. (A) A specific temporal condition of confinement is required for CPA establishment. (B) Administration of isotonic NaCl, hypertonic NaCl or isotonic LiCl fails to induce CPA. (*) P < 0.05, (**) P < 0.01 when compared with 20-min confinement or 0.4 M LiCl. Data are expressed as the mean of the score percentage ± SEM.

Figure 3.

Pharmacological blockade of the ventral hippocampal regions disrupts CPA formation. Evaluation of memory retrieval indicates that administration of GABA_A and GABA_B agonist cocktail (0.1 mM Muscimol and 1 mM Baclofen) impairs CPA memory formation. (**) P < 0.01 compared with SS treatment, (#) P < 0.05 between dorsal and ventral regions comparison. Data are expressed as the mean of the score percentage ± SEM.

Figure 4.

Pharmacological blockade of NMDA receptors within the ventral hippocampal region disrupts CPA memory maintenance. Evaluation of CPA memory retrieval indicates that administration of APV impairs CPA memory maintenance only if administered after CS–US association; (**) P < 0.01 compared with SS treatment in the ventral region. Data are expressed as the mean of the score percentage ± SEM.

Figure 5.

Histological representations of the microinfusion within the dorsal (A) and ventral (B) hippocampal regions.