Research reportTemporary inhibition of dorsal or ventral hippocampus by muscimol: Distinct effects on measures of innate anxiety on the elevated plus maze, but similar disruption of contextual fear conditioning
Introduction
Studies examining the effects of lesion or pharmacological manipulations of the hippocampus in rats have provided compelling evidence that the hippocampus is important for unconditioned anxiety/fear responses, as well as the formation and expression of conditioned fear responses to elemental (e.g., auditory) and contextual stimuli1 Moreover, the weight of evidence from studies using separate ventral or dorsal hippocampal manipulations suggests that the ventral hippocampus plays a rather general role in unconditioned anxiety and conditioned fear, whereas dorsal hippocampal contributions are more restricted to specific mnemonic aspects of fear conditioning, such as context learning; this is consistent with the ventral hippocampus featuring stronger direct connectivity to amygdala and hypothalamus, key components of the brain's anxiety and fear circuit, whereas the dorsal hippocampus is more closely linked to parts of the entorhinal cortex that are implicated in visuo-spatial information encoding [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].
The present paper reports three experiments, in which we compared further the contributions of dorsal and ventral hippocampus to unconditioned anxiety and conditioned fear. We examined the effects of bilateral functional inhibition of neurons within dorsal or ventral hippocampus by local microinfusion of the GABA-A agonist muscimol (0.5 μg/0.5 μl/side) on measures of unconditioned anxiety on the elevated plus maze (Experiment 1) and on the formation of conditioned fear (measured as freezing) to a tone or the conditioning context (Experiments 2 and 3). The elevated plus maze experiment (Experiment 1) addressed the hypothesis that ventral hippocampal muscimol would cause more pronounced anxiolytic effects than dorsal muscimol. This hypothesis is consistent with the idea that the ventral hippocampus plays a more important role in unconditioned anxiety than the dorsal hippocampus, which is based on wide range of evidence [6], [8], [10]). More specifically, ventral cytotoxic lesions have been found to cause more pronounced anxiolytic effects than dorsal lesions on a variety of measures of innate anxiety, including elevated plus maze measures [4], [6], [13], and ventral infusion of the local anesthetic lidocaine (a sodium channel blocker inactivating neurons and fibers of passage) significantly increased the proportion of open-arm entries on the elevated plus maze test, whereas dorsal lidocaine had no significant effect [14]. Moreover, ventral, but not dorsal, hippocampal muscimol reduced unconditioned fear, as assessed by the shock-probe burying test [15]. However, even though one study reported that dorsal hippocampal muscimol reduced measures of unconditioned anxiety on the elevated plus maze [16], the effects of dorsal and ventral hippocampal muscimol infusions on the elevated plus maze remain to be compared directly. Furthermore, in the present study, the effects of dorsal and ventral hippocampal muscimol infusions are examined alongside the effects of these manipulations on fear conditioning (Experiments 2 and 3), allowing a direct comparison. In the fear conditioning experiments (Experiment 2 and 3), we aimed to corroborate our previous finding that ventral hippocampal muscimol (1 μg/0.5 μl/side) disrupts contextual, but not tone, fear conditioning [17] and to extend this finding by demonstrating similar effects of dorsal hippocampal muscimol. Such an outcome would be consistent with the idea that contextual fear conditioning requires dorsal hippocampal mechanisms mediating the formation of context representations, and ventral hippocampal mechanisms relating the context representations to fear processing via subcortical structures, including the amygdala [2], [5], [6], [11], [17], [18]. While the ventral hippocampus has also been implicated in tone fear conditioning [3], [6], ventral hippocampal muscimol did not significantly reduce tone fear conditioning in our previous study (even though there was a numerical reduction), and we argued that partial inhibition of neuronal activity in the ventral hippocampus via GABA-A receptor stimulation may not sufficiently interfere with ventral hippocampal processing to affect tone fear conditioning (in contrast, more general ventral hippocampal inactivation by the sodium channel blocker tetrodotoxin markedly impaired tone fear conditioning) [17]. Following our initial 2001 study [17], a number of studies examined the effects of hippocampal muscimol infusions on fear conditioning, with somewhat discrepant outcomes. Maren and Holt [7] reported that ventral hippocampal muscimol (0.25 μg/0.25 μl/side) disrupted tone, but not contextual (background), fear conditioning, whereas dorsal infusions had no effect. Consistent with two main findings by Maren and Holt [7], additional studies reported that dorsal hippocampal muscimol (0.5 μg/0.5 μl/side; [19]) and muscimol infusion into the ventral hippocampus (subiculum; 0.5 μg/1 μl/side; [20]) did not cause anterograde contextual fear conditioning deficits. Such absence of anterograde contextual fear conditioning deficits following hippocampal muscimol (and also lesions) was explained by the competition hypothesis [20], [21], [22]. This hypothesis suggests that, while hippocampal mechanisms are normally important for contextual fear conditioning, they compete with an extra-hippocampal system that can also support contextual fear conditioning, albeit less efficiently; the hippocampus normally suppresses the alternative system, but this suppression is released during hippocampal inactivation or inhibition, so that the extra-hippocampal system can support contextual fear conditioning. Most recently, however, Esclassan et al. [23] reported that ventral hippocampal muscimol (0.25 μg/0.25 μl/side) disrupted both tone and contextual fear conditioning, whereas dorsal muscimol selectively reduced contextual fear conditioning, and Wang et al. [24] also reported that dorsal hippocampal muscimol (0.5 μg/0.5 μl/side) impaired contextual fear conditioning [24]. Considering the different findings made in different laboratories, we found it important to re-examine the anterograde effects of ventral hippocampal muscimol infusions [17] and to compare directly the effects of ventral and dorsal hippocampal muscimol on fear conditioning in our laboratory.
Section snippets
Subjects
A total of 40 adult male Wistar rats (Zur:WIST[HanIbm], Research Unit Schwerzenbach, Schwerzenbach, Switzerland), weighing about 250 g and aged about 2–3 months at the time of surgery, were used in this study. They were housed in groups of four per cage under a reversed light–dark cycle (lights on: 19:00–07:00) in a temperature (21 ± 1 °C) and humidity (55 ± 5%) controlled room. All animals were allowed free access to food and water. Eighteen rats received bilateral implantation of guide cannulae
Histology
In all 40 cannulated rats, the tips of the infusion cannulae were located in the targeted areas within the border of the dorsal or ventral hippocampus (Fig. 1). Visible tissue damage was restricted to the area immediately surrounding the guide and infusion cannulae.
Experiment 1: Distinct effects of muscimol infusion into the dorsal or ventral hippocampus on the elevated plus maze
During the experiment, one ventral hippocampal muscimol rat fell off the maze and was, therefore, not included in the behavioral analysis, leaving final group sizes of: n = 6 for dorsal hippocampal muscimol, n = 5 for ventral hippocampal
Discussion
In the elevated plus maze experiments, ventral and dorsal hippocampal muscimol infusions had distinct effects. Ventral hippocampal muscimol infusions decreased movement only on the closed arms, consistent with decreased locomotion found in a previous study [17], but left open arm measures unaffected; the latter may reflect anxiolytic effects (which would increase movement in open arms) countering the locomotor suppressing effects of ventral hippocampal muscimol. In contrast, dorsal hippocampal
Conclusions
On the elevated plus maze, the effects of ventral hippocampal muscimol infusion were consistent with a reduction in locomotor activity, possibly accompanied by anxiolytic effects. In contrast, the effects of dorsal hippocampal inhibition by muscimol were more consistent with anxiogenic effects. Our fear conditioning experiments, corroborate that both ventral and dorsal hippocampus are required for contextual fear conditioning; a comparison with previous studies using dorsal and ventral
Acknowledgements
This research was supported by grants from the Swiss National Science Foundation and the Swiss Federal Institute of Technology (ETH) Zurich. W.Z. is currently supported by Jiangsu University (Nr. 13JDG001). At the Laboratory for Behavioural Neurobiology, ETH, the authors thank the animal facility team for their care of the animals, Ms Liz Weber for her histology preparations, Mr. Peter Schmid for the design and maintenance of the computerized systems for behavioral analysis.
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