Jean-Louis Guillou

Somatostatin, Alzheimer's disease and cognition: An old story coming of age?

In mammalian brain, the somatostatin (SRIF: somatotropin release-inhibiting factor) family is composed of two peptides: SRIF and cortistatin (CST), which interact with five different receptor subtypes, sst(1-5). This review summarizes the properties of these receptors, the involvement of somatostatinergic systems in Alzheimers disease (SRIF/acetylcholine (Ach), SRIF/amyloid beta peptides, and SRIF/tau interactions) and their role in cognition from early studies using cysteamine as an SRIF depleting substance to the use of subtype selective analogues and knockout mice, and modulation of synaptic plasticity. The current SRIF story illustrates how cognition and emotion are intimately integrated in brain function.

The role of Ca2+/calmodulin-stimulable adenylyl cyclases as molecular coincidence detectors in memory formation

Abstract. Evidence from systems as diverse as mollusks, insects and mammals has revealed that adenylyl cyclase, cyclic adenosine 3′,5′-monophosphate (cAMP) cascade, cAMP-dependent protein kinases and their substrates are required for the cellular events underlying the short-term and long-term forms of memory. In Aplysia and Drosophila models, the coincident activation of independent paths converge to produce a synergistic activation of Ca2+/calmodulin-stimulable adenylyl cyclase, thereby enhancing the cAMP level that appears as the primary mediator of downstream events that strengthen enduring memory. In mammals, in which long-term memories require hippocampal function, our understanding of the role of adenylyl cyclases is still fragmentary. Of the differently regulated isoforms present in the hippocampus, the susceptibility of type 1 and type 8 to stimulation by the complex Ca2+/calmodulin and their expression in the hippocampus suggest a role for these two isoforms as a molecular coincidence device for hippocampus-related memory function. Here, we review the key features of Ca2+/calmodulin stimulable adenylyl cyclases, as well as the involvement of cAMP-regulated signaling pathway in the processes of learning and memory.

Impairment of spatial memory consolidation in APP751SL mice results in cue-guided response

APP(751SL) mice of 5-6- and 7-8-month-old and their wild-type littermates were submitted to one-session learning in a water-maze with three levels of training (4, 12 or 22 trials). Training consisted in finding a submerged platform with a fixed location and marked by a cue. During testing two platforms were presented: one consistent with the spatial location allowing place-response (PR) and the other signaled by the cue enabling cued-response (CR). When testing occurred 24h after training, wild-type and 5-6-month-old APP(751SL) mice exhibited a shift in response strategy as a function of training level, by executing CR when trained with 4 trials and PR when trained with 12 trials, but 7-8-month-old APP(751SL) mice executed only CR. However, they displayed PR when tested 1h after 12- and 22-trial, suggesting a consolidation deficit. Zif268 imaging showed plasticity impairment of the hippocampal-dependent memory system but not of the dorsolateral caudate nucleus. Moreover, in these APP(751SL) mice, the deficit selectively affecting hippocampal function cannot be directly related to the onset of beta-amyloid deposits.

Intrahippocampal injections of somatostatin dissociate acquisition from the flexible use of place responses

Previous studies showed that injections of somatostatin (SS‐14) into the hippocampus facilitate the acquisition of spatial tasks in mice. The present study was aimed at better understanding the learning and memory processes that could be affected by hippocampal SS‐14 stimulation. Balb/c mice were submitted to a two‐stage learning paradigm. In stage 1, they were trained for acquisition of a spatial discrimination task in a radial maze and, in stage 2, were submitted to a probe test aiming at evaluating their ability to use flexibly their previously acquired knowledge. Injections of vehicle or SS‐14 were given during the acquisition phase and/or before the probe test using a 2 × 2 factorial design. Mice receiving SS‐14 during acquisition failed to succeed in the probe test despite showing a trend to faster acquisition of the initial spatial discrimination task. By contrast, when given only prior to probe trials, SS‐14 did not yield any behavioural effects. Thus, SS‐14 interfered with the establishment of a flexible form of memory, not with its expression per se, and therefore dissociated the acquisition of place responses from their flexible use. The theoretical issues raised by the present findings are discussed.

Spatial learning induces differential changes in calcium/calmodulin-stimulated (ACI) and calcium-insensitive (ACII) adenylyl cyclases in the mouse hippocampus.

Several lines of evidence indicate that Ca2+/calmodulin-stimulated isoforms of adenylyl cyclase (AC) are involved in long-term potentiation and in certain forms of learning. Recently, we found that training in different types of learning task differentially activates Ca2+-sensitive versus Ca2+-insensitive AC activities in certain brain regions, indicating that AC species other than those stimulated by Ca2+/calmodulin may play an important role in learning processes (Guillou, Rose, & Cooper, 1999). Here, we report the effects of spatial reference memory training in a radial arm maze on the levels of AC1 and AC2 mRNA in the dorsal hippocampus of C57BL/6 mice. Acquisition of the task was associated with a learning-specific and time-dependent increase of AC1 mRNA expression selectively in subfields CA1-CA2. In contrast, AC2 mRNA levels were either reduced or not reliably affected depending on the stage of acquisition. Moreover, no significant changes in AC expression were observed either in the dorsal hippocampus of mice trained in a non-spatial (procedural) version of the task or in cortical regions of mice learning the spatial or procedural task. The regional specificity of these effects indicates that the formation of spatial and non-spatial memory requires distinct contributions from Ca2+-sensitive and Ca2+-insensitive AC in the hippocampus. It is suggested that downregulation of AC2 throughout all hippocampal subfields may play a permissive role during the acquisition of spatial learning whereas an upregulation of AC1 specifically in subfield CA1, may be critical to accurately encode, store or use spatial information.

Cooperation between hippocampal somatostatin receptor subtypes 4 and 2: functional relevance in interactive memory systems.

The hippocampal somatostatin (sst) receptor subtype 4 (sst4) modulates memory formation by diminishing hippocampus‐based spatial function while enhancing striatum‐dependent behaviors. sst4‐mediated regulations on neuronal activity in the hippocampus appear to depend on both competitive and cooperative interactions with sst receptor subtype 2 (sst2). Here, we investigated whether interactions with sst2 receptors are required for sst4‐mediated effects on hippocampus‐dependent spatial memory and striatum‐dependent cued memory in a water maze paradigm. Competition was assessed in mice by intrahippocampal injections of the sst4 agonist L‐803,087 alone or combined with sst2 agonists (L‐779,976 or octreotide). Effects of L‐803,087 were also tested in sst2 knockout mice to assess for receptor cooperation. Finally, sst2a and sst4 localizations within hippocampal subregions were analyzed by immunohistochemistry and expression levels of sst2a and sst2b were quantified by real‐time qPCR. Hippocampal injections of L‐803,087 impaired spatial memory but enhanced cued memory. The latter effect was lost not only in sst2 knockout mice but also in the presence of sst2 agonists, whereas the former effect remained unaffected by sst2 agonists or gene deletion. Octreotide and L‐779,976 did not yield memory effects but reduced swim velocity throughout the acquisition trials suggesting that stimulation of sst2 affected motivation and/or anxiety. sst2a and sst4 were respectively detected in the dentate gyrus (DG) and the CA1 subfield suggesting that their functional interactions are not mediated by direct receptor coupling. Hippocampus sst2a expression was 36‐fold higher than sst2b. Possible neural mechanisms and functional significances for interaction between memory systems in relationship with stress‐induced changes in hippocampal functions are discussed.

Intrahippocampal injections of cysteamine improve the retention of a bar-pressing task in mice

Cysteamine was used as a tool aimed at investigating the role of central somatostatin (SS-14) and was shown to modulate learning in a task-dependent manner. However, direct arguments have not yet been provided to support the hypothesis that impairments and facilitation of learning produced by cysteamine are both mediated by the hippocampus. Mice were given daily intrahippocampal injections of artificial cerebrospinal fluid (CSF) or cysteamine at doses of either 2.5 microg/0.2 microl or 25 microg/0.2 microl 1 h prior to each learning session of a bar-pressing task, for which the acquisition was previously shown to be improved by systemic injections. The results showed that, with respect to CSF, the mice injected with cysteamine learned the bar pressing task faster whereas no evidence of changes in locomotor activity was provided. Moreover, the results showed that retention was specifically increased in the two groups injected with cysteamine. It is argued that the action of cysteamine on the hippocampus is sufficient to modulate specifically learning-memory processes in a task-dependent manner. In conclusion, the blockade of some hippocampal information processing function by cysteamine is discussed to understand the bidirectional effects of drugs on learning and memory.

Effects of intrahippocampal injections of somatostatin and cysteamine on spatial discrimination learning in mice

The involvement of hippocampal somatostatin (SS-14) in spatial discrimination learning in mice was evaluated by bilaterally administering either SS-14 or its depletor, cysteamine, into the hippocampus through chronically implanted guide cannulae. Two separate experiments were performed using an eight-arm radial maze. In Experiment 1, mice were trained for 4 days on a discrimination task in which only three arms (A, B, and C) were used. In each daily session, subjects were each presented in 10 trials with the pair AB and the pair BC, with arm B consistently baited over trials (e.g., A − B + and B + C −). Immediately after completion of the fourth daily session with these regular trials, they were given 10 additional probe trials with free access to all three arms (e.g., A − B + C −). In Experiment 2, animals had free access to all eight arms. They were trained to discriminate the three arms that were consistently baited over trials (8 trials per day for 7 days). Intrahippocampal injections were given each day, 30 min before testing. The results of both experiments showed that, compared with saline, SS-14 administration (0.2 µg/0.2 µl per hippocampus) resulted in faster acquisition rates. In contrast, animals treated with cysteamine (20 µg) were dramatically impaired. In Experiment 1, however, the SS-14 treatment group was significantly impaired relative to both the saline and the intact control groups when testing was shifted from regular to probe trials. The impaired ability of somatostatin-treated mice to adapt to probe trials, together with their improved performance on both types of spatial discrimination task, suggests that this peptide produced a change in the strategy the animals used to solve these tasks. Notwithstanding, the present findings seem difficult to reconcile with current theories about hippocampal function.

Identification of hippocampus-dependent and hippocampus independent memory components in step-down inhibitory avoidance tasks.

The aim of this study was to determine if the memory of the association between a step-down response and a foot-shock can be dissociated from the memory of the context in which the shocking experience occurred. To test this, two versions of the step-down inhibitory avoidance task were used: a standard version, in which animals were given one trial with a weak exposure to the context and a new version, in which animals were given a stronger exposure to the context. A retention test was performed with the platform placed either in the same conditioning chamber as during the acquisition phase or in a new context. Our results demonstrate that the step-down inhibitory avoidance can actually be solved without a functional hippocampus. Specifically, the results show that hippocampus-lesioned mice and sham controls can express similar level of memory performance but use two different strategies which were distinguished by assessing retention in a new context. Hippocampus-lesioned mice and mice injected with forskolin (adenylyl cyclase activator) 3h after acquisition use a memory strategy which is independent of the context of acquisition. In addition, our results confirm that the cAMP signaling pathway is a key step in memory consolidation processing.

Roles of Hippocampal Somatostatin Receptor Subtypes in Stress Response and Emotionality.

Altered brain somatostatin functions recently appeared as key elements for the pathogenesis of stress-related neuropsychiatric disorders. The hippocampus exerts an inhibitory feedback on stress but the mechanisms involved remain unclear. We investigated herein the role of hippocampal somatostatin receptor subtypes in both stress response and behavioral emotionality using C57BL/6, wild type and sst2 or sst4 knockout mice. Inhibitory effects of hippocampal infusions of somatostatin agonists on stress-induced hypothalamo-pituitary-adrenal axis (HPA) activity were tested by monitoring peripheral blood and local hippocampus corticosterone levels, the latter by using microdialysis. Anxiolytic and antidepressant-like effects were determined in the elevated-plus maze, open field, forced swimming, and stress-sensitive beam walking tests. Hippocampal injections of somatostatin analogs and sst2 or sst4, but not sst1 or sst3 receptor agonists produced rapid and sustained inhibition of HPA axis. sst2 agonists selectively produced anxiolytic-like behaviors whereas both sst2 and sst4 agonists had antidepressant-like effects. Consistent with these findings, high corticosterone levels and anxiety were found in sst2KO mice and depressive-like behaviors observed in both sst2KO and sst4KO strains. Both hippocampal sst2 and sst4 receptors selectively inhibit stress-induced HPA axis activation but mediate anxiolytic and antidepressive effects through distinct mechanisms. Such results are to be accounted for in development of pathway-specific somatostatin receptor agents in the treatment of hypercortisolism (Cushing’s disease) and stress-related neuropsychiatric disorders.