GluN2B-containing NMDA receptors and AMPA receptors in medial prefrontal cortex are necessary for odor span in rats

Abstract

Working memory is a type of short-term memory involved in the maintenance and manipulation of information essential for complex cognition. While memory span capacity has been extensively studied in humans as a measure of working memory, it has received considerably less attention in rodents. Our aim was to examine the role of the N-methyl-D-aspartate (NMDA) and α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors in odor span capacity using systemic injections or infusions of receptor antagonists into the medial prefrontal cortex (mPFC). Long Evans rats were trained on a well-characterized odor span task (OST). Initially, rats were trained to dig for a food reward in sand followed by training on a non-match to sample discrimination using sand scented with household spices. The rats were then required to perform a serial delayed non-match to sample procedure which was their odor span. Systemic injection of the broad spectrum NMDA receptor antagonist 3-(2-Carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) (10 mg/kg) or the GluN2B-selective antagonist Ro 25-6981 (10 mg/kg but not 6 mg/kg) significantly reduced odor span capacity. Infusions of the GluN2B- selective antagonist Ro 25-6981 (2.5 μg/hemisphere) into mPFC reduced span capacity, an effect that was nearly significant (p = 0.069). Infusions of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (1.25 μg/hemisphere) into mPFC reduced span capacity and latency for the rats to make a choice in the task. These results demonstrate span capacity in rats depends on ionotropic glutamate receptor activation in the mPFC. Further understanding of the circuitry underlying span capacity may aid in the novel therapeutic drug development for persons with working memory impairments as a result of disorders such as schizophrenia and Alzheimer's disease.

Keywords: CNQX; CPP; Ro 25-6981; glutamate; odor span task; working memory.

Figure 1

(A) Illustration of the OST. See text for details. Odors are indicated with letters. On subsequent trials for a given span, the bowl (black circle) that contains the novel odor is rewarded (+) while all previously encountered stimuli are not (−). Bowls are added one at a time to the platform until an error is made. The span is calculated as the number of bowls on the platform for the last error free trial minus 1. Note that all bowls are moved around the platform before each new trial. (B) Mean odor spans during the 7 days of training immediately prior to the first treatment (3-(2-Carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP); n = 8). (C) Mean odor spans on the baseline sessions before each treatment in the within subjects design (see Table 1). The GluN2B-selective NMDA receptor antagonist Ro 25-6981 was given three times: first, 6 mg/kg (i.p.); second, 10 mg/kg (i.p.); and third, 2.5 μg/hemisphere. * Refers significantly lower span relative to all other conditions (p < 0.05). Ro, Ro 25-6981.

Figure 2

Effects of systemic NMDA receptor antagonism on performance of the OST (n = 8). (A) Mean spans of the rats following saline or CPP treatment (10 mg/kg, i.p.). (B) The mean latency of the rats to begin digging in a bowl (saline or CPP). (C) Mean spans for the rats following vehicle (Veh) or Ro 25-6981 (Ro) treatment (6 mg/kg, i.p.) (D) The mean latency of the rats to begin digging in a bowl during the tests conducted in C. (E) Mean spans for the rats following each treatment with either Veh or Ro 25-6981 (10 mg/kg, i.p.). (F) The mean latency of the rats to begin digging in a bowl (Veh or Ro, 10 mg/kg). * Refers to a significant difference between treatments (p < 0.05).

Figure 3

OST performance following infusions of either Ro 25-6981 or CNQX into mPFC (n = 7). (A) Mean spans of the rats following vehicle (Veh) or Ro 25-6981 infusions into mPFC. (B) The mean latency of the rats to begin digging in a bowl for the treatments in A. (C) Mean spans for the rats following treatment with either Veh or CNQX into the mPFC. (D) The mean latency for the rats to begin digging in a bowl (Veh or CNQX; mPFC infusion). (E) Infusion sites in the mPFC. Numbers refer to the anterior-posterior location of the plates relative to bregma. * Refers to a significant difference between treatments (p < 0.05).