Damage of GABAergic neurons in the medial septum impairs spatial working memory and extinction of active avoidance: effects on proactive interference

Abstract

The medial septum and diagonal band (MSDB) are important in spatial learning and memory. On the basis of the excitotoxic damage of GABAergic MSDB neurons, we have recently suggested a role for these neurons in controlling proactive interference. Our study sought to test this hypothesis in different behavioral procedures using a new GABAergic immunotoxin. GABA-transporter-saporin (GAT1-SAP) was administered into the MSDB of male Sprague-Dawley rats. Following surgery, rats were trained in a reference memory water maze procedure for 5 days, followed by a working memory (delayed match to position) water maze procedure. Other rats were trained in a lever-press avoidance procedure after intraseptal GAT1-SAP or sham surgery. Intraseptal GAT1-SAP extensively damaged GABAergic neurons while sparing most cholinergic MSDB neurons. Rats treated with GAT1-SAP were not impaired in acquiring a spatial reference memory, learning the location of the escape platform as rapidly as sham rats. In contrast, GAT1-SAP rats were slower than sham rats to learn the platform location in a delayed match to position procedure, in which the platform location was changed every day. Moreover, GAT1-SAP rats returned to previous platform locations more often than sham rats. In the active avoidance procedure, intraseptal GAT1-SAP impaired extinction but not acquisition of the avoidance response. Using a different neurotoxin and behavioral procedures than previous studies, the results of this study paint a similar picture that GABAergic MSDB neurons are important for controlling proactive interference.

Figure 1

Photomicrographs of the medial septum and diagonal band of Broca following a sham surgery (A, C) and GAT1-saporin administration (B, D). Immunoreactivity for parvalbumin (A, B) and choline acetyltransferase (C, D) was used to visualize GABAergic and cholinergic MSDB neurons, respectively.

Figure 2

Intraseptal GAT1-saporin reduced the number of paravalbumin-ir and GAD67-ir neurons by 86% and 84%, respectively. In contrast, cholinergic neurons (ChAT) were only mildly affected by GAT1-saporin (28% reduction).

Figure 3

Spatial reference memory was not altered by intraseptal GAT1-saporin. GAT1-saporin treated rats learned the location of an escape platform at a similar rate and to a similar degree as sham treated rats (left). One day after session 4, rats were tested in a probe trial without the escape platform (60 s maximum). Sham and GAT1-saporin rats swam in the target quadrant to a similar extent during this probe trial (right).

Figure 4

Intraseptal GAT1-saporin impaired spatial working memory in a delayed match to position procedure. One of two configurations used for the delayed match to position task (A). The letter O refers to the platform position during the reference memory phase. Numbers refer to the platform location during each daily session of the working memory phase. The circles surrounding the square platform depicts the annulus (20 cm) used in the analysis. The other configuration was similar. At the start of each daily session, the escape platform was moved to a new location. Sham and GAT1-SAP rats learned the location of the platform within a session, as assessed by escape latency (B) or path length (not shown). However, GAT1-saporin treated rats were slower than sham rats to learn the new location.

Figure 5

Rats treated with GAT1-SAP searched previous platform locations more often than sham rats. Annuli (20 cm) were constructed around previous and future platform locations. GAT1-SAP rats made more entries into the reference memory and previous session annuli compared to sham rats, but entries into the next session annulus were similar between treatment groups (A). Mean entries per trial per session were determined from trials 2 – 4; trial 1 was not included in the analysis because it was a sampling trial on which the rat learned the new platform location. When examined by trials, treatment differences were especially apparent on trials 2 and 3 for entries into the reference memory (B) and the previous session annuli (C). In contrast, GAT1-SAP and sham rats made similar number of entries into the next session annulus for all trials (D). The enhanced entries made by GAT1-SAP rats into previous goal locations but not future goal locations demonstrate a search strategy biased to previously rewarded locations and is consistent with increased proactive interference following GABAergic MSDB damage.

Figure 6

Acquisition of lever press avoidance was not altered by intraseptal GAT1-saporin. Proportion of trials with avoidance responses (left) and response latency (right) were used to determine acquisition of avoidance behavior. The proportion of avoidance responses increased across sessions and the response latency decreased across trials. Neither measure was different between treatments. Avoidance responses were responses with latencies less than 60 s (dotted line in right panel).

Figure 7

Intraseptal GAT1-saporin impaired extinction of lever press avoidance. Extinction sessions were identical to acquisition sessions, except the foot shock was never delivered. Although there was no foot shock to “avoid” during extinction trials, all responses with latencies less than 60 s (dotted line in right panel) were designated avoidance responses, similar to the acquisition phase. The proportion of trials with avoidance responses decreased across extinction sessions (left) and response latency increased across trials (right). Rats treated with GAT1-saporin were slower to extinguish avoidance responding in both measures.

Figure 8

GAT1-SAP treatment did not alter intertrial interval responding during avoidance learning. Intertrial interval responses increased through the first half, then decreased during the latter half of the acquisition phase (A). However, treatment groups did not differ. During the extinction phase, intertrial interval responses decreased throughout, and treatment groups were similar.