Influence of hippocampectomy on gastric ulcer in rats

Functional differentiation in the hippocampus

The hippocampus is critically involved in certain kinds of memory. During memory formation, it may operate as an integrated unit, or isolated parts may be responsible for different functions. Recent evidence suggests that the hippocampus is functionally differentiated along its dorsoventral (septotemporal) axis. The cortical and subcortical connections of the dorsal and ventral hippocampus are different, with information derived from the sensory cortices entering mainly in the dorsal two-thirds or three-quarters of the dentate gyrus. Rats can acquire a spatial navigation task if small tissue blocks are spared within this region, but equally large blocks at the ventral end are not capable of supporting spatial learning. In primates, the posterior hippocampus (corresponding to the dorsal hippocampus of rodents) appears to be more important than anterior areas for encoding of spatial memory and certain forms of nonspatial memory. The ventral (or anterior) hippocampal formation is to some extent disconnected from the rest of the structure both in terms of intrahippocampal and extrahippocampal connections and may be performing functions that are qualitatively different from, and independent of, those of the dorsal hippocampal formation.

Potentiation of inputs from the posterolateral amygdala to the dentate gyrus and resistance to stress ulcers formation in rats

Physical restraint was found to increase the activity of a number of multiple units in the lateral amygdala of rats. High-frequency electrical stimulation of units in the posterolateral amygdala increased the amplitudes of granule cell potentials in the dentate gyrus. This bilateral long-term potentiation (LTP) of inputs from posterior areas of the lateral amygdala also attenuated the severity of stress ulcers produced by physical restraint. This effect was reversed by intraventricular injections of the selective N-methyl-D-aspartate receptor blocker, aminophosphonovaleric acid. LTP in this pathway also reduced "struggling" behaviour during restraint. The data were interpreted to indicate that LTP in this temporal lobe pathway increased the coping ability because of faster habituation to stressors.

Hippocampal pathway to the amygdala and stress ulcer development

The objective of the first study was to localize the reported aggravation of stress ulcers found after large bilateral hippocampal lesions in rats. Lesions in the ventral hippocampus produced a similar increase in the severity of gastric erosions after cold-restraint, as was seen after large bilateral lesions. Dorsal hippocampal damage produced no differential effects. In the second experiment, high-frequency electrical stimulation of the ventral CA1 region of the hippocampus, a procedure known to induce long-term potentiation, increased the evoked potentials in the lateral central nucleus of the amygdala, and in adjacent parts of the lateral and basolateral nuclei. The increase in the efficacy of synaptic transmission in this pathway attenuated stress ulcer development. It was concluded that the ventral hippocampus is part of a coping system, and a strengthening of synaptic connections with the central amygdala increases the coping ability of rats under stress conditions.

Electrical stimulation of the medial amygdala facilitates gastric acid secretion in conscious rats

One hundred and twenty-seven conscious rats prepared with chronic gastric fistula were studied to investigate the effect of stimulation of the medial amygdala on gastric acid secretion. Gastric acid output was significantly increased by electrical stimulation of the medial amygdala in normal rats and the increase in acid secretion was completely abolished by vagotomy. Vagotomized rats, with or without amygdaloid stimulation, showed comparable levels of gastric acid output which were significantly lower than in controls. These results indicate that the amygdala effect on gastric acid secretion is carried via the vagus nerve. Subcutaneous injections of high doses of histamine increased gastric acid secretion which was further increased by amygdaloid stimulation. Plasma levels of gastrin were not significantly changed by stimulation of the medial amygdala with or without vagotomy. From the above results, we concluded that in conscious rats the medial amygdala plays a significant role in stimulating gastric acid secretion, the vagus nerve is involved in this process, but it is not mediated by release of either histamine or gastrin.

Granule cell potentials in the dentate gyrus of the hippocampus: coping behavior and stress ulcers in rats

Evoked population potentials of the granule cells in the dentate gyrus of the hippocampus were increased in stress-resistant rats and decreased in stress-susceptible rats, as indexed by restraint-induced gastric ulcers. Inescapable, uncontrollable shock stimulation also suppressed granule cell population spikes and interfered with subsequent coping responses when escape was possible, i.e. the so-called helplessness effect. The data were interpreted to indicate that the hippocampus is part of a coping system in stressful situations.

Synaptic efficacy in the entorhinal-dentate pathway and stress ulcers in rats

High-frequency electrical stimulation in the entorhinal-dentate pathway of male Wistar rats was found to increase the amplitudes of the population spikes recorded from granule cells located in the dentate gyrus of the hippocampal formation. This increase in synaptic efficacy attenuated the gastric ulceration produced by cold restraint. It was suggested that this limbic system structure modulates the ability to cope with environmental demands.

The behavioral effects of lesions of the septum: a review

Many investigations have been conducted in an effort to deduce the nature of septal function. This paper is an overview of the work done by several researchers in their attempt to find the possible connections between overt behaviors and septal structures in the rat.

The anterior cingulate cortex and stress: effects of chlordiazepoxide on unit-activity and stimulation-induced gastric pathology in rats

Multiple unit-activity in the anterior cingulate cortex of rats indicated that during immobilization-stress some units increased responding, and others decreased their response rates. Changes in unit-behavior were also found when a stimulus was presented that previously had been paired with the restraint treatment. Pretreatment with chlordiazepoxide prevented the stress-induced increases in unit-activity, whereas, suppressed units were not affected by the drug treatment. Injections of chlordiazepoxide also attenuated the gastric pathology which was found following low-level electrical stimulation of the area of the units which had been identified to respond to restraint-stress.

The bed nucleus of the stria terminalis and immobilization-stress: unit activity, escape behaviour, and gastric pathology in rats

Multiple unit-activity in the bed nucleus of the stria terminalis was increased or decreased, relative to baselines, during physical restraint in rats. Changes in unit-activity were also obtained by presenting an auditory stimulus that had been paired with the immobilization treatment. The animals escaped from that stimulus in behavioural tests, and bilateral lesions in the bed nucleus reduced the latencies of escape responses. The lesion also increased the severity of restraint-induced mucosal erosions. The latter effect was most pronounced when the damage was in the lateral portion of the bed nucleus. It was concluded that the bed nucleus of the stria terminalis is part of a coping system which responds when the organism is placed in a stressful situation.

Effects of catecholamine agonist and antagonist drugs on acute stomach ulceration induced by medial hypothalamic lesions in rats

In order to investigate the involvement of catecholamines (CAs) in acute stomach ulceration induced by hypothalamic lesions, rats were given bilateral electrolytic anodal lesions in the medial hypothalamus followed by a single subcutaneous injection of CA agonist or antagonist drugs. As in previous studies, lesioned rats that received no post operative drug treatment showed extensive gastric damage when examined 24 hr after the brain lesion. Chlorpromazine, amphetamine, desipramine and isoproterenol caused significant reductions in the extent (total length) and/or number of erosions induced by the brain lesion. Haloperidol and propranolol did not seem to affect ulcer formation. Clozapine increased the number but not the total length of ulcers. Phentolamine, alone or in combination with propranolol, significantly increased both the number and total length of lesion-induced ulcers. Similarities between these results and those reported for most of these drugs in the context of ulcers induced by various experimental stress procedures suggest a degree of commonality between acute stress ulcers and ulcers induced by hypothalamic lesions. The overall pattern of results obtained is also consistent with evidence indicating a protective role for catecholamines in acute ulcer formation.

Mucosal damage following electrical stimulation of the anterior cingulate cortex and pretreatment with atropine and cimetidine

Bipolar electrical stimulation of a restricted area of the anterior cingulate cortex in anesthetized rats produced stomach erosions. Pretreatment with atropine sulfate prevented the pathological effects of stimulation. Cimetidine was not effective in preventing the gastric damage. It was concluded that the pathological effects of electrical stimulation were cholinergically mediated, whereas, histamine-2 receptors were not involved.

The telencephalic limbic system and experimental gastric pathology: a review

The effects of lesions and stimulations of the telencephalic limbic system on experimental gastric ulcers and erosions are reviewed. It is concluded that the centromedial amygdala and the anterior cingulate gyrus are facilitatory structures, whereas the medial prefrontal cortex, posterior cingulate cortex, entorhinal cortex, hippocampus and posterolateral amygdala are inhibitory areas during stressful experiences, e.g., immobilization. Both the centromedial amygdala and the anterior cingulate gyrus may be part of an "ancillary" pain system, mediating the affective components of aversive experiences. The inhibitory structures, on the other hand, are assumed to be part of a "preventive" mechanism which is initiated by the selective nuclear binding of glucocorticoids when under stress.

The cingulate cortex and gastric pathology

Bilateral lesions in the anterior or posterior cingulate cortex were performed in rats. After a recovery period, the rats were immobilized for 24 hours. Results showed that posterior lesions increased the severity of the stomach pathology under restraint and non-restraint conditions. Anterior lesions, on the other hand, attenuated the effects of immobilization on gastric pathology. The data were discussed in reference to telencephalic limbic mechanisms and stress.

Hippocampal deafferentation and deefferentation and gastric pathology in rats

Previous studies have shown that large hippocampal lesions increase the gastric pathology found in restrained and unrestrained animals. The present experiments investigated the effects of lesions in the two major afferent and efferent pathways of the hippocampus, i.e., the dorsal pathway through the fimbria-fornix and the ventral pathway in the entorhinal region, on gastric pathology in restrained and unrestrained rats. Result demonstrated that lesions in the ventral pathway produced an increased incidence of pathology whereas lesions in the fimbria-fornix did not. Neuroanatomical pathways which may mediate these effects are described