Strain specific alterations in hippocampal cholinergic function following acute footshock

Contextual fear conditioning is associated with an increase of acetylcholine release in the hippocampus of rat

The effects of contextual fear conditioning on the release of acetylcholine (ACh) in the hippocampus of freely moving rats was assessed using microdialysis. Measures were carried out during both acquisition and retention testing (re-exposure to the conditioning chamber) and compared between animals that either received foot-shocks as unconditioned stimulus (conditioned group) or no foot-shocks (control group) during acquisition. Results showed that during acquisition, hippocampal ACh extracellular level was increased with respect to baseline but that this increase was of similar magnitude in both groups. By contrast, re-exposure to the conditioning chamber the day after (retention testing) produced a significantly greater increase in ACh extracellular level in the conditioned (that, otherwise, displayed conditioned freezing behavior to contextual cues), than in the control group (which displayed virtually no freezing). This enhanced hippocampal ACh release seems to result from the greater hippocampal processing of contextual stimuli in conditioned animals with respect to controls.

Time-related effects of stress on cholinergic sensitivity

The effect of the administration of the muscarinic cholinergic agonist oxotremorine on locomotor activity was investigated in DBA/2 mice subjected to chronic restraint stress of different durations (120 min daily for 10, 14 or 18 days). Oxotremorine induced a depressant effect on locomotion, which was reduced after 10 and 14 days of restraint, but not after a 18-day restraint stress. Acetylcholine (ACh) content was significantly reduced in prefrontal cortex after 10 and 14 days of stress but returned to control values after 18 days of restraint. No changes in ACh content were observed in nucleus accumbens and striatum. These results are discussed in terms of possible changes in muscarinic receptor sensitivity.

Inescapable stress enhances extracellular acetylcholine in the rat hippocampus and prefrontal cortex but not the nucleus accumbens or amygdala

A number of experimental results has pointed to a cholinergic involvement in the stress response. Recently, analytical techniques have become available to measure acetylcholine release in vivo during exposure to various stressors. In these experiments, microdialysis was used to monitor acetylcholine output every 15 min in the dorsal hippocampus, amygdala, nucleus accumbens and prefrontal cortex before, during and after 1 h of restraint, including a 15-min session of intermittent tail-shock (1/min, 1 mA, 1-s duration) in rats. In response to the stressful event, acetylcholine release was significantly increased in the prefrontal cortex (186%; p < 0.01) and hippocampus (168%; P < 0.01) but not in the amygdala or nucleus accumbens. The sole effects observed in the amygdala and nucleus accumbens occurred upon release from the restrainer, at which point acetylcholine levels were significantly elevated in both areas (amygdala: 150%; P < 0.05; nucleus accumbens: 13%; P < 0.05). An enhanced acetylcholine release was also evident during this sample period in the hippocampus and prefrontal cortex. These data demonstrate an enhancement of cholinergic activity in response to stress in two acetylcholine projection systems (hippocampus and prefrontal cortex) but not in the intrinsic acetylcholine system of the nucleus accumbens or the extrinsic innervation of the amygdala. Moreover, the data showed that relief from stress was accompanied by a more ubiquitous acetylcholine response that extended to each site tested.

Reduction of oxotremorine-induced analgesia after chronic but not acute restraint stress

The analgesic response (tail-flick latency) induced by the muscarinic cholinergic agonist oxotremorine was investigated in DBA/2 mice exposed to acute (a single 2 h session) and chronic (2 h once daily for 10 days) restraint stress. While a single exposure to stress did not influence the antinociceptive effects of the cholinergic agonist, chronic stress induced a clear-cut reduction of the oxotremorine-induced analgesia. The results show an involvement of cholinergic mechanisms in the adaptive modulation of nociception after chronic stressful events.

Effects of acute and chronic stress and of genotype on oxotremorine-induced locomotor depression of mice

The locomotor behavior of unstressed and stressed mice of two inbred strains, DBA/2 and C57/BL6, was investigated. Animals were tested in a toggle-floor box apparatus, 30 min after saline or oxotremorine treatment (ip). A dose of oxotremorine that did not depress the activity of naïve mice (0.01 mg/kg) was chosen. Stressed mice were injected 24 h after either a single 2-h stress session (acute stress) or the last of 14 daily stress sessions of tube restraining (chronic stress). Acute stress did not modify the depressant effect of oxotremorine on locomotor behavior in either strain. On the contrary, chronic stress induced a clear sensitization of DBA but not C57 mice to the depressant effect of oxotremorine. These findings show that chronic stress may result in modifications of the cholinergic function, and its behavioral correlates, and that these changes are modulated by the genetic makeup.

Norepinephrine depletion reduces the effects of social and olfactory experience

Control juvenile rats adapted normally to a new home-cage bedding odor if they were caged with rats neonatally treated with 6-hydroxydopa, but not DSP-4. Neither social nor olfactory experience influenced preferences of NE-depleted rats. In some forebrain regions of controls caged with DSP-4 rats, monoamine concentrations were depressed and a metabolite elevated, suggesting the situation was stressful. DSP-4 treatment decreased the effect of footshock on hippocampal cholinergic activity, implying that NE depletion reduced sensitivity to stress. Thus, norepinephrine may modulate the biobehavioral effects of the postweaning olfactory and social environment.

Region selective increase in activities of CNS cholinergic marker enzymes during learning of memory tasks in aged rats

The effects of learning memory tasks on activities of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) in the frontal cortex (FC), hippocampus (HC) and cerebellum of aged rat brains were studied in comparison with those of young adult rats. Aged rats were significantly inferior than young adult rats in both active avoidance (two-way shuttle box) and water-filled multiple T-maze learning. ChAT activity in the FC of aged rats was significantly increased after 5 days of training in an active-avoidance learning task. ChAT activity in the HC of aged rats was also significantly increased after 6 days of training in a water-filled multiple T-maze. These changes did not occur in young adult rats after either 2 or 5 days of active avoidance training, or in aged rats after 10 days of training, both of which were after the maximum level of learning of active avoidance task had been attained. AChE activity was significantly lower in the FC and HC of nontrained aged rats when compared with that of nontrained young adult rats. The reduced activity of AChE in both brain regions of nontrained aged rats rose to almost the same level as that in young adult rats in nontrained and trained states in an active avoidance task. From these findings, it is hypothesized that the task-dependent elevation in the activities of the central nervous system (CNS) cholinergic marker enzymes in trained aged rats may be compensatory changes to keep a relevant level of neurotransmission in the face of specific motor and/or cognitive insults.

Alterations of muscarinic cholinergic receptors in the hippocampal formation of stressed rat: in vitro quantitative autoradiographic analysis

Alterations in muscarinic cholinergic (mACh) binding sites in the hippocampal formation of rats after immobilization stress (IM-stress) lasting 0.5 or 3 h were quantitated using a computer-assisted image analysis system on autoradiograms following labeling of frozen sections with [3H]quinuclidinyl benzilate (QNB). The concentration of acetylcholine (ACh) in the rat hippocampus after IM-stress was also measured and showed no significant change compared to control. IM-stress for 0.5 h produced significant increases in the concentration of mACh binding sites in the whole hippocampal formation and in two subdivisions studied (CA1 plus CA2 and dentate gyrus); after 3 h of stress, the increase remained significant only in the dentate gyrus. These findings suggest that IM-stress induce a hypersensitivity of the septo-hippocampal cholinergic system and the 'up-regulation' of postsynaptic mACh receptors is more lasting in the dentate gyrus than in the hippocampus as a whole.

An age-related spatial learning deficit: choline uptake distinguishes "impaired" and "unimpaired" rats

A functional decline in the hippocampal formation may underlie the emergence of spatial learning deficits in aged rodents. In this study, sodium-dependent high-affinity choline uptake (HACU) was used to monitor hippocampal function in response to training on a spatial task. The subjects were male Long-Evans rats at either 4 months or 22-24 months of age. Animals were trained to locate a camouflaged escape platform in the Morris water maze. Each animal that received place training had a yoked counterpart that was exposed to swimming in the maze but was not required to learn the task. Animals, both young and aged, were sacrificed after attaining a criterion performance. Relative to animals in the yoked condition, place training significantly reduced HACU in both the young rats and in a subpopulation of the aged animals that learned the task rapidly. In contrast, for aged rats that had an impaired rate of acquisition, no effect of place training on HACU was observed. These results provide evidence for a relationship between the behavioral capacities of aged rats and changes in the status of hippocampal function.

Immobilization stress and direct glucocorticoid effects on rat septohippocampus

The rat septohippocampal cholinergic system to a large extent regulates the adaptive physiological and behavioral response to stress. The mesoseptal dopaminergic (DA) system, one of the converging inputs to the lateral septum, exerts a tonic inhibitory action on the septohippocampal cholinergic neurons. High concentrations of pituitary-adrenocortical hormones in plasma may activate the septohippocampal cholinergic system. We have sought to determine whether this mode of activation may be directly initiated by hormonal action on the cholinergic terminals, or indirectly induced through an alteration in the DA septal inputs. The results indicate that stress initiates rapid and transient changes in DA uptake by septal DA terminals, changes which probably contribute to the initial transient activation of the hippocampal cholinergic system. While the effects of glucocorticoids, observed in vitro, may mimic the enhanced ACh release in stress, they do not mimic the increased choline uptake. Nevertheless, high glucocorticoid concentrations may act directly on septal dopaminergic terminals to reduce their DA uptake capacity. These results imply that the septohippocampal cholinergic activity represents an integrative pathway for neuronal and hormonal signals of stress.

The stress-induced response of the septo-hippocampal cholinergic system. A vectorial outcome of psychoneuroendocrinological interactions

Considerable data have emerged which strongly indicate that the septohippocampal cholinergic system is involved in the adaptive response to stress. Neurotransmitter regulatory mechanisms in cholinergic synaptic terminals of this part of the limbic system undergo adaptive changes in response to stress and recover slowly after stress. The initial stress-induced response is characterized by activation of hippocampal cholinergic terminals within minutes, as indicated by a rapid and transient elevation in high affinity choline uptake and increased newly synthesized acetylcholine release. The response of this cholinergic system to stress is influenced by both neuronal and hormonal stimuli. Among the several neuronal systems converging in the septum, terminals of the dopaminergic mesolimbic system have been found to be selectively involved in the early response to stress. Pharmacological interference with dopaminergic neurotransmission, with agonist and antagonist treatments, revealed that changes in the tonic inhibitory influence of septal dopaminergic terminals can modulate the response of hippocampal cholinergic terminals to stress. A similar activation of hippocampal cholinergic terminals as after short-term stress was observed after treatments with a large dose of either adrenocorticotropic hormone or corticosterone. Furthermore, glucocorticoids and not adrenocorticotropic hormone can directly enhance acetylcholine release, but only from excited terminals. This indicates that stress-induced activation of the septo-hippocampal system may occur secondary to, but not directly by, increased levels of pituitary-adrenocortical hormones. Yet, it is possible that under stressful conditions the increased glucocorticoid levels may modulate the activity of the stimulated hippocampal cholinergic terminals. Together the findings support the notion that the stress-induced response of the septo-hippocampal cholinergic system represents an integrated output of converging neuronal and hormonal stimuli which convey signals of stress to this limbic brain region.

Stress-induced immobility in rats with cholinergic supersensitivity

Immobility during forced swimming or after mild footshock (1 mA for 2 sec) was observed in five groups of rats. The Flinders Sensitive Line (FSL) of rats, known to be more sensitive to cholinergic agonists, exhibited the greatest degree of immobility in the forced swim test. Rats chronically treated with, and subsequently withdrawn from, either scopolamine (2 mg/kg, once daily) or amitriptyline (10 mg/kg, once daily) were also significantly more immobile than either a control group treated chronically with isotonic saline or the Flinders Resistant Line (FRL) of rats in the forced swim test. Similar trends were observed for locomotor depression in the open field following exposure to footshock. Receptor binding studies indicated significantly greater concentrations of muscarinic acetylcholine receptors in the hippocampus of the scopolamine, and amitriptyline, withdrawn rats. These findings indicate that rats with increased cholinergic sensitivity are more sensitive to the immobility-inducing effects of mild stressors. Thus, they may prove to be useful models for studying the relationship between affective disorders and the cholinergic system.

Stress-induced activation of the hippocampal cholinergic system and the pituitary-adrenocortical axis

The septo-hippocampal cholinergic system in rats undergoes rapid activation after acute stress. This is expressed by rapid increases both in high affinity choline uptake and newly synthesized acetylcholine release. Administration of ACTH or corticosterone at a high dose led 10 min later to changes comparable to those observed after acute stress. Choline uptake and acetylcholine release were also elevated 2 days after adrenalectomy. The effects of adrenalectomy could be attenuated by corticosterone, but not by ACTH treatment. The results demonstrate that (a) after short term stress the septo-hippocampal cholinergic system is activated secondary to activation of the pituitary-adrenocortical axis and (b) major changes in circulating corticosterone can modulate the activity of the hippocampal cholinergic synapse.

Dynamics of cholinergic synaptic mechanisms in rat hippocampus after stress

Changes in high affinity [3H]choline uptake, newly synthesized [3H]acetylcholine release and [3H]quinuclidinylbenzilate (QNB) binding were characterized in crude synaptosomal preparations from rat hippocampus immediately after different intervals of immobilization stress and at different times following chronic intermittent stress (2h once daily for 5 days). Choline uptake was increased to 125% of unhandled controls after 10 min of stress, after 2 h it returned to control levels and after chronic stress uptake was reduced to 75% of control. Acetylcholine release was enhanced after all stress intervals. Maximal muscarinic (QNB) binding capacity (Bmax) was increased to 135% of control only after chronic stress, with no change in Kd values. Following chronic stress the changes observed in cholinergic synaptic mechanisms all persist for up to 2 days. Recovery occurred only by the 7th post-stress day. We conclude: presynaptic hippocampal cholinergic terminals are rapidly activated by stressful stimuli and this is expressed by an increase in choline uptake and newly synthesized acetylcholine release; after prolonged periods of stress adaptive changes in the cholinergic terminals are expressed by a reduction in choline uptake and an elevation in the number of muscarinic binding sites; and the chronic stress-induced changes are slow to recover. The results demonstrate that the septo-hippocampal cholinergic system is an integral part of the adaptive response to stress.

Comparison of regional CNS ligand binding in two inbred rat strains: effects of chronic morphine

Male rats of the F-344 and BUF inbred strains were given free access to a 10% sucrose solution containing 0.5 mg/ml morphine sulfate (controls received sucrose only) as their sole source of fluids. The daily intake of morphine averaged 101 +/- 13 mg/kg. After 18 days on this regimen, animals were sacrificed and assayed for 3H-clonidine (alpha-2 adrenergic), 3H-dihydroalprenolol (DHA, beta 1 and 2 adrenergic) and 3H-spiperone (SPD, 5-HT2 and D2) binding in several brain regions. In the absence of morphine treatment, BUF rats displayed higher levels of SPD binding in brainstem, as compared with the F-344 strain. In contrast, untreated F-344 rats exhibited higher levels of DHA binding in hypothalamus and SPD binding in striatum than BUF rats. Chronic morphine resulted in an increase in clonidine and DHA binding in the brainstem and hippocampus respectively of BUF, but not F-344 rats, suggesting a greater sensitivity of adrenergic function to opiate treatment in the BUF strain. The two strains differed qualitatively in the effect of morphine on striatal SPD binding, with BUF rats exhibiting a decrease, and F-344 rats an increase. The one consistent change observed in both strains was a quantitatively similar increase in hippocampal SPD binding after chronic morphine. The results demonstrate that despite strain-dependent differences in binding characteristics, chronic morphine elicits a strain-independent alteration in hippocampal 5-HT2 binding. On the basis of these preliminary findings, it may be speculated that this particular neurochemical consequence contributes to morphine-induced behaviors which are observed independent of rat strain.

Strain specific cholinergic changes in response to stress: analysis of a time-dependent avoidance variation

Investigators have established that the performance of an incompletely learned avoidance task is a U shaped function of the time since the original partial acquisition. Thus rats perform more poorly when retested at intermediate time intervals (1-8 hr) after training than they do when tested at longer post-acquisition intervals (24-48 hr). Studies have suggested that such time-dependent deficits are not related to changes in learning ability, but rather result from shock-induced motor suppression which interferes with active avoidance responding. Pharmacological studies utilizing drugs which effect cholinergic function have indicated that an inhibitory cholinergic system may be involved in mediating post-shock motor suppression. To obtain direct biochemical evidence for possible cholinergic mediation of post-shock motor suppression, measurements of high affinity choline uptake and acetylcholine turnover were made at varying time intervals following partial active avoidance training in F-344 rats. An increase in cholinergic function was found in the dorsal, but not the ventral hippocampus 30 min, 1 hr and 4 hr following acquisition training. These biochemical alterations were temporally correlated with deficits in active avoidance responding. We have reported that the immediate behavioral suppression observed in another rat strain (Sprague-Dawley, Zivic Miller Laboratories), which exhibits inferior active avoidance performance, is similarly correlated with cholinergic activation in the dorsal hippocampus [17]. These data support the hypothesis that the dorsal-hippocampal cholinergic system is involved in the mediation of stress-induced behavioral suppression.(ABSTRACT TRUNCATED AT 250 WORDS)