Hippocampal SSTR4 somatostatin receptors control ... - Springer Link

Psychopharmacology (2009) 202:153–163 DOI 10.1007/s00213-008-1204-x

ORIGINAL INVESTIGATION

Hippocampal SSTR4 somatostatin receptors control the selection of memory strategies François Gastambide & Cécile Viollet & Gabriel Lepousez & Jacques Epelbaum & Jean-Louis Guillou

Received: 6 March 2008 / Accepted: 12 May 2008 / Published online: 3 June 2008 # Springer-Verlag 2008

F. Gastambide : J.-L. Guillou (*) Université de Bordeaux, Centre de Neurosciences Intégratives et Cognitives, Université de Bordeaux 1, Avenue des facultés, 33405 Talence, France e-mail: [email protected]

trained to locate an escape platform based on either distal cues (place memory) or a visible proximal cue (cue memory). Retention was tested 24 h later on probe trials aimed at identifying which memory strategy was preferentially retained. Results Both SS14 and the SSTR4 agonist (L-803,087) dramatically impaired place memory formation in a dosedependent manner, whereas SSTR1 (L-797,591), SSTR2 (L-779,976), or SSTR3 (L-796,778) agonists did not yield any behavioral effects. However, unlike SS14, the SSTR4 agonist also dose-dependently enhanced cue-based memory formation. This effect was confirmed in another striataldependent memory task, the bar-pressing task, where L-803,087 improved memory of the instrumental response, whereas SS14 was once again ineffective. Conclusions These data suggest that hippocampal SSTR4 are selectively involved in the selection of memory strategies by switching from the use of hippocampus-based multiple associations to the use of simple dorsal striatumbased behavioral responses. Possible neural mechanisms and functional implications are discussed.

F. Gastambide : J.-L. Guillou CNRS UMR 5228, Talence, France

Keywords Memory systems . Hippocampus . Water maze . Spatial learning . Operant conditioning . Mice, sst4

C. Viollet : G. Lepousez : J. Epelbaum Université Paris Descartes, Faculté de médecine, IFR 77 Broca Sainte-Anne, Paris, France

Introduction

Abstract Rationale Somatostatin (SS14) has been implicated in various cognitive disorders, and converging evidence from animal studies suggests that SS14 neurons differentially regulate hippocampal- and striatal-dependent memory formation. Four SS14 receptor subtypes (SSTR1–4) are expressed in the hippocampus, but their respective roles in memory processes remain to be determined. Objectives In the present study, effects of selective SSTR1–4 agonists on memory formation were assessed in a water-maze task which can engage either hippocampus-dependent “place” and/or striatum-dependent “cue” memory formation. Materials and methods Mice received an intrahippocampal injection of one of each of the selective agonists and were then

C. Viollet : G. Lepousez : J. Epelbaum Inserm UMR 894, Centre de Psychiatrie et Neurosciences, Bât Paul Broca, 2 ter rue d’Alésia, 75014 Paris, France C. Viollet : G. Lepousez : J. Epelbaum Hôpital Saint Anne, Centre Paul Broca, 2 ter Rue Alésia, 75014 Paris, France

The tetradecapeptide somatostatin (SS14) was initially identified on the basis of its inhibitory effect on the pituitary secretion of growth hormone (Brazeau et al. 1973). However, in accordance with its widespread but nevertheless regionally selective distribution throughout the brain (Patel 1999), SS14 also acts as a neuromodulator influencing cognitive functions such as learning and memory (Vecsei et al. 1983;

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Schettini 1991; Matsuoka et al. 1995). Regionally selective modifications in somatostatin levels occur in various neurological disorders such as Alzheimer’s, Huntington’s, Parkinson’s disease, depression, and epilepsy (Bissette and Myers 1992; Epelbaum et al. 1994; Vecsei and Klivenyi 1995; Norris et al. 1996; Eve et al. 1997; Vezzani and Hoyer 1999). In both animal and human, this neuropeptide mediates its biological functions through five G protein coupled receptor subtypes (SSTR1–5), which trigger multiple transmembrane signaling pathways (Reisine and Bell 1995; Csaba and Dournaud 2001). These SSTRs are widely distributed in the brain (Thoss et al. 1995; 1996). In the hippocampus, SS14 is detected in interneurons which are mainly localized in the stratum oriens of the CA1 and CA3 regions and in the hilus of the DG region, and is co-localized with γ-aminobutyric acid (GABA) (Baraban and Tallent 2004; Jinno and Kosaka 2006). Four of the five somatostatin receptors (SSTR1–4) are expressed in this brain region (Hannon et al. 2002; Videau et al. 2003), predominantly in hippocampal pyramidal and granular cells (Csaba and Dournaud 2001). Based on this corpus of data, somatostatinergic neurons were proposed to modulate hippocampal neural networks subserving learning and memory processes. Indeed, previous experiments have provided evidence for a facilitatory effect of hippocampal SS14 on spatial learning. In particular, intrahippocampal injections of SS14 enhance the rate of acquisition of a spatial discrimination task in a radial maze but impaired flexibility in the spatial representation acquired (Guillou et al. 1993; Lamirault et al. 2001). Conversely, intrahippocampal injections of cysteamine—a depletor of SS14 stores—slow down the rate of acquisition in spatial tasks (Guillou et al. 1993; 1998) and improve the retention of a bar-pressing task (Guillou et al. 1999). Taken together, these findings supported the idea that the hippocampal somatostatinergic interneurons modulate learning and memory processes as a function of the task and/or the form of memory involved (DeNoble et al. 1989). According to the “Multiple Parallel Memory Systems” hypothesis, different forms of memory are mediated by independent brain systems. In this context, it is now widely accepted that “declarative”, “cognitive”, “relational” or “stimulus–stimulus” memory (S–S memory) depends on the hippocampal system, whereas “procedural”, “habit” or “stimulus–response” memory (S–R memory) depends on the dorsal striatal system (White and McDonald 2002). Another important implication of this notion is that multiple memory systems are coactivated in parallel during the very early stages of learning (Martel et al. 2007) and that these systems interact either synergistically (McDonald et al. 2004) or competitively (Poldrack and Packard 2003) to produce behavior. Thus, in rodents, hippocampus dysfunctions such as those produced by fimbriae–fornix lesion or temporary

Psychopharmacology (2009) 202:153–163

inactivation weaken the use of hippocampus-dependent spatial strategies and enhance the use of striatum-based S–R associations (Matthews and Best 1995; Packard and McGaugh 1996; Schroeder et al. 2002; Chang and Gold 2003a). Conversely, increased levels of glutamate or acetylcholine in the hippocampus were shown to promote the use of spatial strategies while impeding the use of alternative S–R strategies (Packard 1999; Chang and Gold 2003b). Based on the multiple memory systems hypothesis, the opposite effects of intrahippocampal injections of SS14 (or cysteamine) as a function of the task (Guillou et al. 1993; 1999; Lamirault et al. 2001) suggest that SS14 interneurons may modulate the selection of either the hippocampus or the dorsal striatum memory systems when coping with ambiguous problems is required. The aim of the present study was to investigate this issue and, in particular, to determine the respective roles of each hippocampal SSTR in these processes. Intrahippocampal injections of SS14 or different selective SSTRs agonists (Rohrer et al. 1998) were given before acquisition of a two-stage learning paradigm in a Morris water maze, which can ambiguously engage either hippocampus-dependent “place memory” and/or striatum-dependent “cue memory” (McDonald and White 1994; Devan et al. 1999; Martel et al. 2007). Then, 24-h retention performance was measured in probe trials aiming to determine which type of strategy was preferentially used. In addition, effects observed on “cue memory” were confirmed in another S–R memory task (i.e., the barpressing task) which was previously shown to also depend upon the striatum memory system (Kelley 2004).

Materials and methods Subjects Subjects were male mice (N=230) of the C57Bl/6ByJIco strain obtained from Charles River Laboratories (L’Arbresle, France) at the age of 8 weeks. They were housed collectively in an animal room equipped with air conditioning (23°C) and with an artificial 12-h light/dark cycle (from 7:00 A.M. on). At the age of 3–4 months (25–30 g), mice were housed in individual home cages and were given ad libitum access to food and water for 10 days before surgery. During the recovery period, they were handled (3 min per day) and weighed daily. For the bar-pressing task, the food ration was adjusted individually so that all mice had reached 82% of their ad libitum weight at the beginning and during all the experiments. Experiments were performed in compliance with the European Community Council Directive of 24 November 1986 (8616091 EEC) and French national laws on the care and use of laboratory animals in research.


Intrahippocampal injections Surgery Mice were implanted under general anesthesia—using a mixture of Ketamine (100 mg/kg i.p.) and Xylazine (2 mg/kg i.p.) with two guide cannulae (0.4 mm diameter, 8 mm long) aimed vertically toward the dorsal hippocampus. Guide cannulae were fixed to the skull bone with dental cement and fine bone screws. According to the atlas of Franklin and Paxinos (1996), stereotaxic coordinates were 2.0 mm posterior to the bregma, 1.3 mm each side of the sagittal suture, 0.9 mm ventral from the skull surface. After surgery, the mice remained in the animal room for a recovery period of 10 days before the experimental phase began. At the end of the experiments, mice were killed and placements of guide cannulae were verified histologically as being correctly above the dorsal hippocampus (see Fig. 1a). Drugs injections Somatostatin 14 (Sigma, France) was dissolved in artificial cerebrospinal fluid (aCSF, for details see Guillou et al. 1999) and administered based on previous reports (Guillou et al. 1993; Lamirault et al. 2001) at the dose of 0.3 nmol/ 0.5 μl per side (N=21). Control mice used in these

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experiments received 0.5 µl per side of aCSF (N=21). All SSTRs agonists were a generous gift from Merck Research Laboratories (NJ, USA). They were dissolved in aCSF containing dimethyl sulfoxide (DMSO, Sigma, France) which was concentrated at 25% in each dose except for the SSTR1 agonist for which each dose was dissolved in 100% DMSO. Control mice used in these respective experiments received the injection of the vehicle solution that was utilized to dissolve the drug (i.e., aCSF + 25% DMSO or DMSO for experiments with the SSTR1 agonist). The selective SSTR1 agonist L-797,591 was administered at doses of 0 (vehicle, N=10), 0.05 (N=10), or 0.5 nmol/ 0.5 μl per side (N=10). The selective SSTR2 agonist L-779,976 was administered at doses of 0 (vehicle, N=12), 0.05 (N=12), 0.5 (N=12), or 5 nmol/0.5 μl per side (N=12). The selective SSTR3 agonist L-796,778 was administered at doses of 0 (vehicle, N=10), 0.05 (N=10), 0.5 (N=10), or 5 nmol/0.5 μl per side (N=10). The selective SSTR4 agonist L-803,087 was administered at doses of 0 (vehicle, N=21), 0.05 (N=10), 0.1 (N=10), 0.5 (N=10), or 5 nmol/0.5 μl per side (N=19). Thirty minutes before each learning session, drugs were administered to freely moving mice via injection cannulae (0.2 mm diameter, 9 mm long) attached to 1 μl Hamilton syringes via polyethylene catheter tubing. The syringes were held in a constant-rate infusion pump, and injection

Fig. 1 a Histological controls showing representative placements of the cannulae tips (arrows) in the dorsal hippocampus. b Schematic representation of the experimental procedure (see “Materials and methods” section for explanation)

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was conducted over a 5-min period. In all cases, correct injection flow rates were visually controlled. The cannulae were left in place for a further 2-min period before removal. Water-maze task Apparatus The training apparatus was a circular pool (1.5 m diameter) filled with water that was heated to 21°C±1 and rendered opaque by the addition of white nontoxic latex paint. The pool was surrounded by numerous visual cues, which were kept in constant locations during the entire training period. A circular escape platform (13 cm diameter) was submerged 1 cm below the surface of the water. The escape platform was placed in the center of one of four imaginary quadrants of the pool and remained in this location throughout acquisition. For cued trials, a cylinder (15 cm high×1 cm diameter) with black–white striped patterns was positioned on the platform center. Swim times and distances were recorded and analyzed using an automated tracking system (Videotrack, Champagne au Mont d’Or, France). Between trials, the mice were dried and kept warm in a box equipped with dark lamps (night club lamps) providing an ambient temperature of 30°C inside the box. Behavioral procedure Animals underwent one training session comprised of ten trials (see Fig. 1b). Acquisition training involved a spatial and a visual cue-based reference memory task during which the platform remained in a fixed position and was marked by a cue (cued learning) or not (spatial learning). The number of cued trials (C), spatial trials (S), and their sequence (C-C-S-S-C-S-S-C-S-S) were determined based on Martel et al. (2006, 2007) along with further preliminary experiments so that retention performance, respectively, for the spatial representation and the cued representation were well-balanced in control animals. For each trial, the mouse was placed into the pool— facing the wall—from one of four randomly varied start positions located around the rim of the pool (N, S, W, E; such that the summed distances from the start point to the platform in every pair of two trials were equivalent) and was then allowed a maximum of 90 s to find the escape platform (NE). If the mouse found the platform, it was allowed to rest on it for 20 s before being removed from the pool and placed back into its home cage (inside the warming box). If a mouse failed to find the platform within 90 s, it was guided to it by hand. Mice were trained in squads of five animals with a 10- to 15-min intertrial interval (ITI) and fully counterbalanced with respect to groups. The next day (24 h later), retention for place


memory or cue memory was respectively assessed. First, the invisible platform was kept in the same location as in acquisition (NE) for two trials (spatial testing). Then, the platform was moved to the opposite quadrant (i.e. SW), and the cylinder was installed on it for one trial to probe cue memory (cued testing). The start positions used in retention were randomly NW and SE. Escape latency and path length to reach the escape platform were recorded. In addition, for the cued testing, the time spent in the quadrant where the platform was regularly located and the crossings of the virtual platform area were measured. Bar-pressing task Apparatus Behavioral testing was carried out in an operant test cage (12.5×13.5×18.5 cm) based on that described by Destrade et al. (1973) and made of translucent Plexiglas with a metal grid floor. A metal bar (5.8×3 cm) and a food cup extended from one wall and were separated by a 5-cm partition so that, after a bar-press, the mouse had to circumnavigate the partition to reach the food cup. The cage was equipped with photoelectric cells that detected the position of the animal in front of the bar and the food cup. This information was constantly transmitted to a computer, allowing automatic recording of each bar-press and consumption of the reinforcers. The test cage controls were programmed for continuous reinforcement (CRF1). An operant response was defined as a bar-press followed within 30 s by the consumption of a miniature (4 mg) decorticated seed of millet. Behavioral procedure For the acquisition session, the mouse was placed into the test cage and was allowed to perform 15 operant responses (see Fig. 1b). The next day (24 h later), the mouse was replaced in the test cage, and retention performance was measured during 30 min. Previous studies showed that the frequency of operant responses does not, or only slightly, increases over 15 trials, so that the acquisition of the task is only partial in control mice at the end of the first training session. However, performance in the 24-h retention test shows that mice have associated the pressing on the lever with the release of a pellet into the food cup and organize a motor pattern to collect food pellets as rapidly as possible (Jaffard et al. 1974; Martel et al. 2006). For each session, the number of operant responses over blocks of 5-min periods was recorded. In addition, memory was also assessed by measures of the time required to perform the last five responses in acquisition and the first five responses in retention.


Statistical analysis Data are presented as mean and standard error of the mean (SEM). Data were analyzed using two-way analysis of variance (ANOVA) with Statview 5.01 (SAS Institute, Cary, NC, USA). Post hoc paired comparisons were performed using the Scheffe F test when the effect of the main factors or their interaction was significant. Differences were considered significant at the P