Effects of interruption of limbic system pathways on different measures of activity

Hormones, genes and the structure of sexual arousal

Despite the inherent difficulty of connecting individual genes with integrated mammalian behaviors, it has been determined that a series of genes are turned on by estrogenic hormones acting in forebrain. Their products are, in turn, facilitatory for female reproductive behaviors such as lordosis. The causal routes by which two genes contribute to the control of lordosis behavior, the classical estrogen receptor gene (ER-alpha) and a thyroid hormone (TH) receptor gene (TR-beta), have been delineated. Beyond the mechanisms underlying the expression of concrete, specific natural behaviors, lies the question of sexual motivation. Required as an intervening variable to explain fluctuations in natural behaviors in the face of constant stimuli, motivational states have both general and specific features. Most theoretical and experimental approaches toward the general aspects of motivation have depended heavily on concepts of 'arousal.' Sexual arousal is likely to depend both on very general, broadly distributed neuronal influences and on specific affiliative and sexual tendencies. Is 'general arousal' a monolithic, undifferentiated process? In no way can a review at this time settle such issues, but the reasons behind six new experimental approaches to these questions are described.

Intrahippocampal grafts containing cholinergic and serotonergic fetal neurons ameliorate spatial reference but not working memory in rats with fimbria-fornix/cingular bundle lesions

Three-month-old Long-Evans female rats sustained aspirative lesions of the dorsal septohippocampal pathways and, 2 weeks later, received intrahippocampal suspension grafts containing cells from the mesencephalic raphe, cells from the medial septum and the diagonal band of Broca, or a mixture of both. Lesion-only and sham-operated rats were used as controls. All rats were tested for locomotor activity 1 week, 3 and 5 months after lesion surgery, for spatial working memory in a radial maze from 5 to 9 months, and for reference and working memory in a water tank during the 9th month after lesioning. Determination of hippocampal concentration of acetylcholine, noradrenaline, and serotonin was made after completion of behavioral testing. Compared to sham-operated rats, all rats with lesions, whether grafted or not, exhibited increased levels of locomotor activity and made more errors in the radial maze. The lesioned rats were also impaired in the probe trial (30 first seconds) of the water-tank test made according to a protocol requiring intact reference memory capabilities. While rats with septal or raphe grafts were also impaired, the rats with co-grafts showed performances not significantly different from those of sham-operated rats. With a protocol requiring intact working memory capabilities, all lesioned rats, whether grafted or not, were impaired in the water-tank test. In the dorsal hippocampus of lesion-only rats, the concentration of acetylcholine and serotonin was significantly reduced. In rats with septal grafts or co-grafts, the concentration of acetylcholine was close to normal, as was that of serotonin in rats with raphe grafts or co-grafts. These results confirm previous findings showing that co-grafts enabled the neurochemical properties of single grafts to be combined. Data from the water-tank test suggest that cholinergic and serotonergic hippocampal reinnervations by fetal cell grafts may induce partial recovery of spatial reference, but not working memory capabilities in rats.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

p-chlorophenylalanine-induced serotonin depletion: reduction in exploratory locomotion but no obvious sensory-motor deficits

Para-chlorophenylalanine (PCPA) depletes central serotonin (5-hydroxytryptamine, 5-HT) by inhibiting tryptophan hydroxylase, an enzyme necessary for the synthesis of 5-HT. The effects of a wide range of PCPA doses (150-1000 mg/kg) on spontaneous exploratory locomotor activity in a novel environment, activity in running wheels and a number of sensory-motor capacities were examined. Administration of 1000 mg/kg PCPA reduced whole brain levels of 5-HT and its metabolite 5-hydroxyindoleacetic acid to 9.4 and 8.2% of control levels, respectively. Treatment with PCPA produced a dose-dependent decrease in exploratory locomotion in an unfamiliar automated open field relative to vehicle-treated animals. Further, all measures of general, horizontal and vertical activity were suppressed by PCPA treatment. In contrast to previous work, hyperactivity of rats chronically housed in cages with running wheel access was not observed. In their home cages, some PCPA-treated rats exhibited hyper-reactivity to cutaneous stimulation. No other sensory-motor deficits were apparent. Previous theories of 5-HT function state that its action may be to inhibit motor activity or promote sleep. The present results challenge this view and suggest that 5-HT, at least in certain environments, may stimulate locomotor activity without directly controlling various sensory-motor capacities in rats.

The comparative developmental neurotoxicity of lead in humans and animals

The effects of lead on neurobehavioral development have been extensively investigated in humans as well as animals. This valuable lode of research findings offers a basis for comparing the developmental neurobehavioral toxicity of lead across species and for assessing the validity of animal models of developmental neurotoxicity. Comparisons of human and animal findings suggest that the greatest qualitative similarities involve relatively complex behavioral processes such as cognition and learning. Quantitative comparisons based on dose-response relationships for these endpoints are difficult to make because the relationships are sometimes nonmonotonic (U-shaped) and because blood lead levels may not be directly comparable between species. However, the lowest levels of exposure at which developmental neurobehavioral effects have been observed are similar: 10-15 micrograms/dl in children, less than 15 micrograms/dl in primates, and less than 20 micrograms/dl in rodents. Although the convergence between animal and human findings for other neurobehavioral endpoints is not as striking, sensory-evoked potentials and communicative processes offer two promising areas for continued investigation and cross-species comparison.

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 influence of congenital Toxoplasma infection on the spontaneous running activity of mice

Home-cage running-wheel activity of mice congenitally infected with Toxoplasma was recorded over 24 days. Infected mice were consistently more active than uninfected controls over the entire testing period. This finding extends previous studies and indicates that such increased activity levels occur not only in novel but also in familiar environments, and suggests that congenital toxoplasmosis tends to render mice "hyperactive'. If such behavioural alterations occur in wild mice, it is likely that infected mouse intermediate hosts would be more susceptible to predation by cats, the definitive hosts of Toxoplasma.

Activity and exploratory behavior after lesions of the medial entorhinal cortex in the woodmouse (Apodemus sylvaticus)

The effects of bilateral electrolytic lesions of the entorhinal cortex were studied in male adult woodmice. Experiments were designed to allow separate analysis of the basal activity level and exploratory behavior. Activity recording was conducted in three situations: (a) 24-hr wheel running in the home cage pre- and postoperatively; (b) 24-hr activity composition in a large enclosure over 4 days, 5 to 9 days postoperatively; and (c) sequence and duration of visits in a residential plus maze 11 to 14 days postoperatively. Medial entorhinal cortex lesion involving the para- and presubiculum increased the 24-hr amount of movements in the enclosure (b) without increasing wheel running in any situation (a or b). This lesion also enhanced the locomotor reactivity to being introduced into the plus maze and impaired exploratory behavior. This last effect was equally apparent when the whole situation was new or when part of the familiar maze was modified. Lesioned woodmice did notice the new element but did not show active focalization of their behavior on that element. Data showed that lesion induced hyperactivity and changes of exploratory behavior were not necessarily associated. Novelty detection was performed but it is not clear now on what information this discrimination was based.

Effects of lateral and medial septal lesions on exploratory behavior in the albino rat

The effects of selective medial and lateral septal lesions on exploratory behavior were studied in the rat. Three types of open-field test and two tests of object exploration were used. In the first experiment, medial but not lateral septal lesions abolished exploration of an open-field accessible from the home-cage. No effects of either of the lesions were found in a standard open-field (Experiment 2). In the third experiment, rats with medial septal lesions were less active and showed a preference for the smaller compartments of the testing apparatus. These results suggest that a change in exploratory behavior following medial septal lesions may be primarily due to their effect on emotionality (increased fear). Two tests of object exploration (Experiments 4 and 5) showed that both types of selective septal lesions enhanced approaches to the novel object placed in the home-cage, but they did not affect object exploration and object preference in other conditions (Experiment 5). The same experiments revealed an increased level of activity (rearing and ambulation) in the rats with lateral septal lesions. It is concluded that the effect of medial septal lesions on exploration is due to increased fear of novel places (but not objects). Lateral septal lesions, on the other hand, increase activity in the rats but there is no evidence that this activity represents enhanced exploration.

Light-dark patterns in running-wheel activity in rats during chronic administration of theophylline

Running-wheel activity for 24 hr and activity patterns were studied during chronic theophylline administration. Theophylline altered the normal relations between activity level and illumination. Dark-time activity was decreased to approximately 50% and 24 hr activity was unaffected. These observations were consistent with previous results showing that theophylline suppressed dark-time feeding but had no effect on 24 hr food intake. A possible mechanism to account for these results may be dependent on levels and turnover of brain norepinephrine.