Elsevier

Brain Research

Volume 748, Issues 1–2, 14 February 1997, Pages 219-226
Brain Research

Research report
Effects of novelty, pain and stress on hippocampal extracellular acetylcholine levels in male rats

https://doi.org/10.1016/S0006-8993(96)01304-2Get rights and content

Abstract

In vivo microdialysis was used to assess the effects of Novelty, persistent pain (Formalin test) and stress (Restraint) on hippocampal acetylcholine (ACh) release. Experiments were carried out during the dark phase, i.e. during the active period of the animal, and consisted of four experimental phases: Baseline (30 min), Novelty (30 min), Formalin test (90 min) and Restraint (30 min); each animal was consecutively exposed to all phases. The extracellular levels of ACh in the dorsal hippocampus were estimated by measurement of its concentration in the perfusion fluid by high-performance liquid chromatography with electrochemical detection. The introduction to a new environment (Novelty) induced in all rats higher ACh levels than Baseline. Formalin treatment decreased ACh release only in animals considered `Inactive' during the Novelty phase while no modification in ACh release was observed in the `Active' ones. Restraint did not produce any modification of ACh release but increased Corticosterone plasma levels both in sham- and formalin-treated animals. Results indicate that Novelty, but not Formalin or Restraint, increases ACh release in the hippocampus and that the type of behavioral state displayed by the animal at the time of formalin injection determines the response of the septo-hippocampal cholinergic pathway.

Introduction

The hippocampal cholinergic system has long been implicated in several functions such as arousal and attention 31, 49, in certain aspects of learning and memory 7, 32and in the neuroendocrine and emotional responsiveness to aversive stimuli 19, 24, 40. The most important cholinergic innervation in the hippocampus is represented by the septo-hippocampal pathway 5, 8, 13, 17. While having the cell body in the medial septum, it is strongly modulated, directly or through the lateral septum, by inputs originating in regions related to hormonal and behavioral responses to aversive stimuli, such as the hypothalamus, the locus coeruleus and the raphe nuclei 14, 19.

The septo-hippocampal pathway has attracted particular attention in neurodegenerative diseases, such as dementia, where some symptoms have been related to the loss of its neurons 7, 12, 25. In experimental animals both stress and pain have been found to interact with the septo-hippocampal pathway. An increase in the synthesis and release of acetylcholine (ACh) occurs during restraint stress and these changes are dependent on the duration of the stressful stimuli 20, 23, 24, 29, 30. With regard to pain, electrophysiological data show that short-lasting nociceptive stimuli activate the septo-hippocampal cholinergic pathway 18, 45, 46, synchronize the hippocampal EEG [34], and induce long-term depression of the hippocampal pyramidal neurons 34, 35. The involvement of the central cholinergic system in pain modulation has been demonstrated by several pharmacological and behavioral studies 6, 11, 22, 26, 39. In freely-moving rabbits, persistent long-lasting nociceptive stimuli induced by a subcutaneous injection of formalin increase the frequency and decrease the percentages of the hippocampal theta rhythm [9]. Moreover, in rats, formalin decreases the hippocampal choline acetyltransferase (ChAT) activity in animals sacrificed 30 or 60 min after injection 2, 4.

It was the aim of our experiments to monitor the ACh release from the hippocampus, by means of the transversal microdialysis technique, in freely-moving rats after exposure to different types of emotional and somatic stimuli. All animals were consecutively exposed to the following experimental phases: (a) control in the home-cage (Baseline), (b) exposure to a new environment, the open-field (Novelty), (c) persistent nociceptive stimuli (Formalin test) and (d) immobilization stress (Restraint). We adopted the Formalin test, which is a commonly used tool to induce persistent pain in animals [48], since it provokes most of the symptoms described in humans after injury, such as sensitization and hyperalgesia. Since ACh release in the hippocampus is highly correlated with spontaneous motor activity 16, 41, 42and since rats introduced to a novel environment spend the first 15–20 min exploring the cage [3], the Formalin test was carried out after the animals had been habituated to the apparatus. Finally, Restraint was performed 90 min after formalin treatment to highlight the stress- and/or pain-induced effects on ACh release.

Section snippets

Animals

Male Wistar rats, weighing 280–300 g, were used (Nossan, Milano, Italy). The animals, housed in groups until surgery, were maintained on a 12:12-h light/dark schedule (lights off at 07.00 h), at 23°C and 53% humidity, with food and water available ad libitum. All experiments were carried out according to the International Association for the Study of Pain guidelines [50].

Surgery and ACh release

Extracellular ACh levels in the hippocampus were measured in vivo by means of the transversal microdialysis technique, as

Results

Hippocampal ACh release during the three 10-min Baseline periods was 0.51±0.08 pmol/min. The concentration of ACh was stable over these collection periods in all rats and the animals showed normal behavioral activity.

Discussion

The main results of the present experiment are: (a) the sharp and short-lasting increase of hippocampal ACh release in all experimental animals once introduced to a novel environment (Novelty phase); and (b) the inhibitory effect of formalin treatment on ACh release in the animals showing low levels of activity.

An increase in ACh release is commonly believed to be an index of activation of the septo-hippocampal pathway. In light of the studies carried out on this subject, several hypotheses

Acknowledgements

This work was supported by Ministero dell'Università e della Ricerca Scientifica e Tecnologica, MURST (40% and 60% funds).

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