The behavioral effects of lesions of the hippocampus: a review

Race, Ethnicity, Family Socioeconomic Status, and Children's Hippocampus Volume

Introduction

The hippocampus has a significant role in memory, learning, and cognition. Although hippocampal size is highly susceptible to family socioeconomic status (SES) and associated stress, very little is known on racial and ethnic group differences in the effects of SES indicators on hippocampus volume among American children.

Purpose

This study explored the multiplicative effects of race, ethnicity, and family SES on hippocampus volume among American children.

Methods

Using data from the Adolescent Brain Cognitive Development (ABCD), we analyzed the functional Magnetic Resonance Imaging (fMRI) data of 9390 9-10 years old children. The main outcome was hippocampus volume. The predictor was parental education. Subjective family SES was the independent variable. Age, sex, and marital status were the covariates. Racial and ethnic group membership were the moderators. To analyze the data, we used regression models.

Results

High subjective family SES was associated with larger hippocampus volume. This effect was significantly larger for Whites than Black families.

Conclusions

The effect of subjective family SES on children's hippocampus volume is weaker in Black than White families.

Family Income Mediates the Effect of Parental Education on Adolescents' Hippocampus Activation During an N-Back Memory Task

Introduction: Hippocampus, a medial temporal lobe structure, has significant implications in memory formation and learning. Although hippocampus activity is believed to be affected by socioeconomic status (SES), limited knowledge exists on which SES indicators influence hippocampus function. Purpose: This study explored the separate and combined effects of three SES indicators, namely parental education, family income, and neighborhood income, on adolescents’ hippocampus activation during an N-Back memory task. As some of the effects of parental education may be through income, we also tested if the effect of parental education on hippocampus activation during our N-Back memory task is mediated by family or neighborhood income. Methods: The Adolescent Brain Cognitive Development (ABCD) study is a national multi-center investigation of American adolescents’ brain development. Functional magnetic resonance imaging (fMRI) data of a total sample of 3067 9–10-year-old adolescents were used. The primary outcome was left- hippocampus activation during the N-Back memory task (mean beta weight for N-Back run 1 2 back versus 0 back contrast in left hippocampus). The independent variable was parental education. Family income and neighborhood income were two possible mediators. Age, sex, and marital status were the covariates. To test mediation, we used hierarchical linear regression models first without and then with our mediators. Full mediation was defined according to Kenny. The Sobel test was used to confirm statistical mediation. Results: In the absence of family and neighborhood income in the model, higher parental educational attainment was associated with lower level of left hippocampus activation during the N-Back memory task. This effect was significant while age, sex, and marital status were controlled. The association between parental educational attainment and hippocampus activation during the N-Back memory task was no more significant when we controlled for family and neighborhood income. Instead, family income was associated with hippocampus activation during the N-Back memory task. These findings suggested that family income fully mediates the effect of parental educational attainment on left hippocampus activation during the N-Back memory task. Conclusions: The effect of parental educational attainment on adolescents’ hippocampus activation during an N-Back memory task is fully explained by family income. That means low family income is why adolescents with low-educated parents show highlighted hippocampus activation during an N-Back memory task. Given the central role of the hippocampus in learning and memory and as income is a modifiable factor by tax and economic policies, income-redistribution policies, fair taxation, and higher minimum wage may have implications for promotion of adolescent equality and social justice. There is a need to focus on family-level economic needs across all levels of neighborhood income.

Systems genetic analysis of hippocampal neuroanatomy and spatial learning in mice

Variation in hippocampal neuroanatomy correlates well with spatial learning ability in mice. Here, we have studied both hippocampal neuroanatomy and behavior in 53 isogenic BXD recombinant strains derived from C57BL/6J and DBA/2J parents. A combination of experimental, neuroinformatic and systems genetics methods was used to test the genetic bases of variation and covariation among traits. Data were collected on seven hippocampal subregions in CA3 and CA4 after testing spatial memory in an eight-arm radial maze task. Quantitative trait loci were identified for hippocampal structure, including the areas of the intra- and infrapyramidal mossy fibers (IIPMFs), stratum radiatum and stratum pyramidale, and for a spatial learning parameter, error rate. We identified multiple loci and gene variants linked to either structural differences or behavior. Gpc4 and Tenm2 are strong candidate genes that may modulate IIPMF areas. Analysis of gene expression networks and trait correlations highlight several processes influencing morphometrical variation and spatial learning.

Functional differentiation and cooperation among the hippocampal subregions in rats to effect spatial memory processes

We investigated the roles of the hippocampal subregions and intrahippocampal networks in effecting spatial reference and working processes. The results showed that the dentate gyrus plays a key role in encoding both types of spatial memory. Lesions in the dentate gyrus caused severe impairment in the acquisition of the Morris water maze and delayed matching-to-place tasks as compared to those in the other hippocampal subregions (Exp. 1). Further, there was functional cooperation between CA3 and CA1 via the Schaffer collaterals and the hippocampal commissure in the performance of both types of spatial memory tasks. Among the rats with intact hippocampal commissure, those with both contralateral and ipsilateral CA1/CA3 lesions showed a similar performance (Exp. 2A); however, among the rats with transected hippocampal commissure, those with the contralateral CA1/CA3 lesions showed a more disruptive performance than the rats with the ipsilateral CA1/CA3 lesions (Exp. 2B). This study suggests that the hippocampus is the functional unit for spatial reference and working memory processes, including differential functions and functional cooperation among the hippocampal subregions.

Hippocampal asymmetry in exploratory behavior to vasoactive intestinal polypeptide

The effects of vasoactive intestinal polypeptide (VIP) microinjected uni- or bilaterally into the CA1 hippocampal area of male Wistar rats at a dose of 10, 50 and 100 ng on exploratory behavior were examined. VIP microinjected bilaterally at a high dose (100 ng) significantly decreased the horizontal movements, while at low doses (10 and 50 ng) had no effect on the exploratory activity. Microinjections of VIP into the left hippocampal CA1 area at doses 50 and 100 ng suppressed the exploratory activity, while right-side VIP administration at a dose 100 ng significantly increased horizontal movements compared to the respective controls. Vertical activity was stimulated only by VIP administered into the right hippocampal CA1 area at the three doses used. Neither bilateral nor left injections of VIP induced changes in the vertical movements. The main finding was the presence of hippocampal asymmetry in exploratory behavior to unilateral microinjections of VIP depending on the dose and the microinjected hemisphere.

Spatial learning deficits in rats after injection of vincristine into the dorsal hippocampus

In the present study, performance in the Morris water escape task after bilateral lesioning of the dorsal hippocampus induced by the microtubule poison vincristine is discussed as a cognitive deficit model in rats. As we are especially interested in spontaneous or pharmacologically induced recovery processes after experimentally induced cognitive dysfunctions, the model should fulfil a number of criteria. Firstly, a clear dose-effect relationship between the dose of vincristine and the amount of spatial learning impairments should be present. Secondly, lesions must remain within the target area. Thirdly, there should be an observable behavioural recovery or compensation of the induced deficit. Two experiments evaluated the influence of the application volume (experiment 1) and the concentration of vincristine (experiment 2) on lesion location and size, and on spatial learning. The results of both experiments demonstrated that the effect of vincristine on the performance in the Morris water escape task seems to be characterized by an "all-or-none" relationship. Concentrations above a "threshold" value induced severe damage in the hippocampus and adjacent brain structures, whereas concentrations below the "threshold" value had marginal or no effects. The non-selective and highly toxic properties of vincristine make this neurotoxin an unsuitable tool for the establishment of a learning and memory deficit model.

The effects of lesions to thalamic lateral internal medullary lamina and posterior nuclei on learning, memory and habituation in the rat

The behavioral effects of radiofrequency lesions to the lateral internal medullary lamina region (IML) or the posterior region (Po: containing the parafascicular and posterior nuclei) of the thalamus were compared to sham operated controls. Subjects were pre-operatively trained and then tested for post-operative retention of a NMTP task. Whereas the Po-lesion group was impaired only on long delays (60, 90 s), the IML-lesion group was impaired on retention and re-acquisition and demonstrated lower performance at all delays (5-90 s) of the NMTP task. Post-operative training and testing was conducted on three additional tasks: Morris water maze, acoustic startle, and passive avoidance. The IML-lesion group was impaired in finding a hidden and visual platform in the Morris water maze, demonstrated a blunted response but normal habituation to an acoustic startle stimulus, and showed normal retention of a passive avoidance task. On those three tasks, the performance of the Po-lesion group was similar to controls. In the IML-lesion group, neuronal loss resulting from axotomy and/or transneuronal degeneration was observed within nuclei of the midline and anterior thalamus and the mammillary body. These results suggest that lesions to the IML region disrupt a range of cognitive functions and produce pathological destruction in distant brain regions; whereas damage to the posterior thalamus causes spatial delay-sensitive deficits.

NMDA and non-NMDA sensitive [L-3H]glutamate receptor binding in the brain of the Naples high- and low-excitability rats: an autoradiographic study

The Naples high-excitability (NHE) and low-excitability (NLE) rat lines, selectively bred for high and low activity in a Làt maze, respectively, are used as an animal model in the study of hippocampal functions. The aim of this study was to investigate the anatomical distribution of N-methyl-D-aspartate (NMDA) and non-NMDA sensitive [3H]glutamate receptor binding by quantitative autoradiography in the brain of the NHE and NLE rats with a randomly bred line (NRB) as controls. Twenty-micron-thick cryostat sagittal sections were incubated at 4 degrees C with 150 nM [L-3H]glutamate alone or in the presence of 100 microM NMDA or 2.5 microM quisqualate (QA). Non-specific binding was determined in the presence of 1 mM of non-labeled glutamate. The sections were exposed to tritium-sensitive films for 3 weeks at 4 degrees C. Quantitative analysis revealed: (1) higher levels of total binding in NHE than in NRB and NLE rats in all areas but the cerebellum; (2) fewer binding sites for both NMDA and QA receptors and larger binding sites for QA receptors in the hippocampus of NLE and NHE rats, respectively; (3) a positive correlation between total binding sites and activity level in a Làt maze in all areas, except the cerebellar molecular layer with NLE < NHE, which was due to differential contribution from NMDA and non-NMDA types. Thus, the brain of the NHE rats shows an imbalance between NMDA and non-NMDA sensitive [L-3H]glutamate receptors.

Immediate early genes and brain DNA remodeling in the Naples high- and low-excitability rat lines following exposure to a spatial novelty

The aim of these studies was to map the neural consequences of exposure to a spatial novelty on the expression of immediate gene (IEG) and on unscheduled brain DNA synthesis (UBDS) in two genetic models of altered activity and hippocampal functions, i.e., the Naples High- (NHE) and Low-excitability (NLE) rats. Adult male rats of NLE and NHE lines, and of a random-bred stock (NRB) were tested in a Làt-maze, and corner crossings, rearings, and fecal boli were counted during two 10-min tests 24 h apart. For IEG expression, rats were exposed to a Làt-maze with nonexposed or repeatedly exposed rats used as controls, and were sacrificed at different time intervals thereafter. For UBDS, rats were sacrificed immediately after the first or the second exposure o a Làt-maze. IEG expression was measured by immunocytochemistry for the FOS and JUN proteins. NRB rats exposed for the first time to the maze showed extensive FOS and JUN positive cells in the reticular formation, the granular and pyramidal neurons of hippocampus, the amygdaloid nuclei, all layers of somatosensory cortex, and the granule cells of the cerebellar cortex. The positivity, stronger in rats exposed for the first time, was present between 2 and 6 h and was prevented by the NMDA receptor antagonist CPP (5 mg/kg). The positivity was very low in NHE rats, and it was stronger in NLE compared to NRB rats. UBDS was measured in ex vivo homogenates of brain areas by the incorporation into DNA of 3H-[methyl]-thymidine given intraventricularly 15 min before test trial 1 or 2 (pulse of 0.5 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for and against the Naples high- and low-excitability rats as genetic model to study hippocampal functions

The Naples high- (NHE) and low-excitability (NLE) are two rat lines, selectively bred for high and low activity levels in a Làt-maze, respectively. Because the activity level in a novel environment depends mainly on the integrity of the hippocampal formation, and NLE and NHE rats differ with a similar background of emotionality, arterial blood pressure, and learning ability, they have been proposed as animal model to study hippocampal functions. Our aim is to prove evidence in favor and against this hypothesis. The evidence in favor indicates that NLE/NHE rats have a defective spatial processing, and pertains to (a) Differential activity in a spatial novelty situation (selection trait), proportional to the stimulus complexity rats are exposed to (NHE are hyper- and NLE-rats hypoactive); and (b) Impaired working memory in a six-arm non-reinforced tunnel maze in both lines compared to random-bred rats, that was reversed by the introduction of a reinforcer. In addition, multiple evidence of (i) lower intra- + infrapyramidal mossy fiber terminals in both NLE/NHE vs. controls; (ii) increased sensitivity of hippocampal elements to microinjections of vasopressin (but not oxytocin) and of "delta" (but not "mu") opioids; (iii) lower number of high-affinity glucocorticoid receptors; (iv) lower number of alpha- but not beta-adrenergic receptors in the hippocampus and hypothalamus of NHE rats only; and (v) the genotype-dependent behavior of a DNA fraction with fast turnover, suggest that both NHE/NLE are "disintegrated" at the hippocampal interface. Further, neurobehavioral covariations among individual differences reveal nonlinear, complex relationships, an evidence apparently against the hypothesis.(ABSTRACT TRUNCATED AT 250 WORDS)

What can genetic models tell us about behavioral plasticity?

Biological diversity and learning have played an essential interactive role in the evolution of species, as intra-specific individual differences have exerted a buffering effect towards environmental changes, and learning ability per se has allowed their maintenance. By exploiting biological diversity individuals with defective learning and memory have been produced that allow the study of the neural substrates of encoding mechanisms, as has been done in studies from Drosophila to rodents. Various aspects of this neurogenetic approach are reviewed and pitfalls are indicated. It is clear that genetic models need to be implemented by an integrated multidisciplinary top-down approach based on behavioral, electrophysiological, histochemical, immunocytochemical and neurochemical techniques. Examples are presented from some animal models that illustrate how a systems level analysis of the neural substrates of information processing can be carried out using such an integrated scheme.

A neurogenetic and morphogenetic approach to hippocampal functions based on individual differences and neurobehavioral covariations

To investigate the neural substrate of information processing, the within group inter-individual behavioral differences were related to the fine variations in some components of the architecture of the intact hippocampus by multivariate analyses of variance and correlative analyses. For, the extent of the intra/infrapyramidal mossy fibers, covering the hippocampal CA3-regio inferior (IIP-MF, revealed by Timm-staining), and the individual high-affinity maximal glucocorticoid receptor binding (HCB, measured by in vitro cytosol preparations with [3H]corticosterone as ligand), were assessed in adult male albino rats of the Naples High-Excitability (NHE) and Naples Low-Excitability (NLE) psychogenetically selected lines, and of a Sprague-Dawley random-bred stock (NRB) as unselected controls. The IIP-MF and the HCB were assumed as hippocampal hardware and software traits, respectively, and entered in a matrix with activity and defecation scores in a Làt-maze as behavioral covariates. Two dimensions were identified by discriminant function analyses tentatively labelled as "spatial" and "non-spatial" by the nature of the variables contributing with a high loading to the dimension. The IIP-MF and HCB contributed mostly to spatial processes and to a lower extent to emotional processes. The neuro-behavioral covariations of IIP-MF with arousal (A) and long-term habituation (LTH), computed by correlative analyses on the overall population (all rats combined), turned out to be of inverted-U type (quadratic function), i.e. positive in NLE, negative in NHE with no correlation in NRB. For HCB receptors, the covariations were quadratic with A, and of the S-type (cubic function), i.e. positive in NLE, negative in NRB and positive in NHE with LTH. Since these rat lines are located along a "continuum" with NLE < RB < NHE, they are assumed to represent entirely this subpopulation. For, the non-linear neuro-behavioral relationships might reveal (i) constraints on the expression of arousal and habituation to novelty; and (ii) that the hippocampus appears to be one such device exerting a modulatory role in the processing of "spatial" and "non-spatial" behavioral components.

Circadian activity, nociceptive thresholds, nigrostriatal and mesolimbic dopaminergic activity in the Naples High- and Low-Excitability rat lines

These experiments were designed to further characterize the differential phenotypic constellation of the Naples High- (NHE) and Naples Low-Excitability (NLE) lines. In order to determine possible differences between NHE and NLE rats in activity and circadian rhythms, besides reactivity to novelty (selection trait), adult male rats of both strains were tested during two 10-min exposures to a Làt-maze. They were then kept in activity cages continuously for 3 days. Moreover, nociceptive thresholds were measured with the hot-plate and the tail-flick test, to probe the possibility that these rats could be differentially sensitive to nociceptive stimuli. Further, the integrity of the nigro-striatal and mesolimbic system was investigated by measuring tyrosine-hydroxylase activity in the striatum and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the striatum as well as in the nucleus accumbens. In addition, TH activity was measured in the adrenals to probe the sympathetic section of the neurovegetative system. The results indicate that NHE and NLE rats differ by a factor of two in their phasic activity in a Làt-maze. In contrast, no differences in 24-h activity during the dark or light phase could be observed in the activity cages. However, NHE rats anticipated the light-on stimulus in the morning by reducing their activity 1 h earlier than NLE rats. Further, no difference could be found with the hot-plate and the tail-flick test. Finally, biochemical analyses revealed no difference in the NHE and NLE rats in the main terminal zone of mesolimbic system (n. accumbens) nor of nigrostriatal system (striatum) nor in the adrenal glands. In conclusion, since the only consistent difference between NHE and NLE rats appears to be reactivity to spatial novelty, an hippocampus-dependent behavioral trait (selection trait), independent of altered activity in the sympathetic system or dopaminergic activity in the major dopaminergic brain systems, the usefulness of these strains as genetic model to test current hypotheses of spatial processor device(s) in the mammalian brain is supported.

Development of monoamine systems after neonatal anoxia in rats

Neurochemical and morphological effects of neonatal anoxia on monoamine systems were studied after 100% N2 exposure for 25 min at 30 h postnatally (postnatal day 2-P2). At 20 min after anoxia, reductions of tissue levels of cerebellar noradrenaline (NA) and striatal dopamine (DA) and metabolites were seen, while 5-hydroxyindoleacetic acid (5-HIAA) was increased in cortex and cerebellum. At P7, NA increased in cerebellum, while serotonin (5-HT) and 5-HIAA decreased in cortex and cerebellum. At P21, increased hippocampal NA and striatal homovanillic acid (HVA) were found, while striatal 5-HT decreased and 5-HIAA increased in striatum and hippocampus. At P60, striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-HIAA levels were found to be enhanced. No effects were seen on 5-HT, tyrosine hydroxylase, or DARPP-32 immunostaining in cortex, hippocampus, and striatum. Thus, the neonatal anoxia induced both acute and persistent neurochemical abnormalities in monoamine systems that were not accompanied by morphological changes detectable with the methods used. The monoamine alterations found could be critically connected to the behavioral disturbances observed in rats after neonatal anoxia. The findings may also be of relevance to dysfunctions seen in humans after perinatal oxygen deficiency, e.g., the attention deficit hyperactivity disorder syndrome.

Nimodipine improves spatial working memory and elevates hippocampal acetylcholine in young rats

The calcium channel blocker nimodipine has been reported to improve cognitive performance in aged and brain-damaged animals. In the present study, the effects of nimodipine and placebo on spatial working memory and hippocampal acetylcholine were studied in young Fischer-344 rats. Nimodipine or placebo was administered via subcutaneously implanted, sustained-release pellets. Each active pellet contained 20 mg of nimodipine and released the drug over approximately 21 days. Two days after the drug or placebo pellets were implanted, training in the 8-arm radial maze started and continued for 12 days. Rats were required to learn a win-shift surgery. Nimodipine-treated animals learned the maze more rapidly than a placebo-treated group as indicated by the number of correct choices out of the first eight arms visited (p less than 0.001). Treated rats also made twice as many choices per unit time during the first week of training (p = 0.005). To assess hippocampal acetylcholine release, in vivo microdialysis was performed while animals were awake and unrestrained, 19-21 days after pellet implantation. A probe with a 3 mm semipermeable tip was placed in the hippocampus (CA1 and dentate gyrus), and individual microliters dialysate samples were collected at 2 microliters/min and immediately analyzed by high performance liquid chromatography with electrochemical detection. Significantly higher extracellular ACh levels were found in nimodipine-treated rats (71.4 +/- 3.6 nM; n = 4) compared to controls (52.5 +/- 2.5 nM; n = 5) (p = 0.003) and in another group of rats of the same age that received identical drug treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Swimming navigation and structural variations of the infrapyramidal mossy fibers in the hippocampus of the mouse

The extent of the infrapyramidal mossy fiber projection in CA3 (IIP-MF) at the midseptotemporal level correlates negatively with two-way avoidance learning and positively with performance in the radial maze, both tasks known to be sensitive to hippocampal lesions. If hippocampal structural variations are causing behavioral variations, one must predict positive correlations between the extent of the IIP-MF and performance in swimming navigation. Thus, the authors studied learning and reversal learning of swimming navigation in mice in which the size of the IIP-MF had been randomized by means of systematic crosses and in 2 mouse strains known for differential infrapyramidal projections (C57BL/6 and DBA/2). In 19 random-bred mice (9 male, 10 female), the extent of the IIP-MF showed negative correlations with swimming time after platform reversal (day 4: r = -0.50, P < .03; day 5 r = -0.73, P < .001), but none during acquisition of the task. In addition, statistical analysis suggested an influence of asymmetrically distributed mossy fiber projections during reversal learning. The strain comparison between 18 DBA/2 and 16 C57BL/6 male mice confirmed these results: no strain difference during days 1-3, and a significantly faster swimming time in the strain C57BL/6 (with large IIP-MF) at day 5 (second day of reversal), associated with significantly more crossings of the former platform location during the early phases of reversal learning. This latter measure was also negatively correlated with asymmetry of the IIP-MF in both strains. Finally, variations of the IIP-MF were correlated partially with adjustment of swimming speed that appeared to depend on size and asymmetry of CA4 as well. Thus, natural variations in the size of the IIP-MF distribution, and, perhaps, of CA4, appear to linearly influence processes directly involved in complex spatial learning.

Lesions of the amygdaloid complex impair acquisition of serial pattern learning in rats

Normal and amygdala-lesioned rats were trained in a runway to respond to a three-element pattern ending in 2, 12, or 0 Noyes pellets (45 mg), respectively. The amygdalectomized rats' performance was inferior to that of normal controls during the early and middle stages of training, that is, amygdalectomized rats ran more slowly on rewarded trials and developed patterned running later than control rats. In the later stages of training, the lesioned rats demonstrated tracking of the series elements, reflected by differential running times similar to those of the control animals. Results are discussed in terms of some of the current theories involving the role of the amygdaloid complex in learning and reinforcement.

Behavioral responses to novelty and structural variation of the hippocampus in mice. II. Multivariate genetic analysis

On the basis of results from lesion studies in rodents, covariations are expected to exist between naturally-occurring heritable variations in hippocampal morphology and exploratory behavior elicited by novel surroundings. For this reason, we set up a full diallel cross between five inbred mouse strains and analyzed the behavioral and the hippocampal anatomical variation in male animals from this cross. Employing a bivariate extension of the diallel-cross analysis, estimates were obtained for the phenotypical, environmental, and genetical correlations between the phenotypes studied. A factor analysis performed on the matrix of additive-genetic correlations revealed that variations in the size of the intra- and infrapyramidal mossy fiber terminal fields (iip-MF) are negatively related to open-field exploration and novelty-induced fear. These results indicate that having larger iip-MF projections promotes the collection and processing of information about a novel environment, entailing lower levels of exploration and fear.

Conditioning to time: evidence for a role of hippocampus from unit recording

Unit activity of the dorsal hippocampus was recorded in partially restrained but awake and undrugged rats during a "conditioning to time". In this type of conditioning, only one stimulus, the equivalent of the unconditioned stimulus of the usual procedures, is used. It is delivered at a constant interval which, in principle, is the conditioned stimulus. In our experiments, the unconditioned stimulus was a mechanical stimulation of a vibrissa; two successive unconditioned stimuli were separated by a 24-s interval. In 11/18 rats, anticipatory movements of a "trained" vibrissa developed at the end of the interstimulus interval. In a number of cases, in parallel to this conditioned behavior, there was a significant change in unit activity, either an increase or a decrease, during the last third of the interstimulus interval. Controls showed that these changes in unit activity did not merely reflect modifications of arousal state or of vibrissa and body movements. From autocorrelograms, it appeared that anticipatory increases in unit activity were associated with the development of a bursting mode of discharge. These data constitute one of the rare examples of a neurophysiological correlate of a "conditioning to time" at the unit level and the first recorded from the dorsal hippocampus of rats.

Strain-specific correlations between hippocampal structural traits and habituation in a spatial novelty situation

The rat strains Naples high-excitable (NHE) and Naples low-excitable (NLE) have been selectively bred since 1976 for behavioral arousal in a spatial novelty situation. The Timm-stained hippocampi of 20 NHE and 18 NLE rats were examined morphometrically for differences in the proportions (volume percentages) of terminal fields in the fascia dentata and CA3/CA4 at the mid-septotemporal level. Prior to histology animals were tested on 2 days for exploratory activity in a square alley system (Làt box). Overall, the two strains differed significantly in the percentage of stratum lacunosum molecular (NHE greater than NLE, P less than 0.001), of the intra/infrapyramidal mossy fiber (IIP-MF) projection (NLE greater than NHE, P less than 0.001), of the granule cell layer (NLE greater than NHE, P less than 0.05) and of the outer molecular layer (NHE greater than NLE, P less than 0.05). Both strains had an IIP-MF projection smaller than in any other rat strain. Most of the strain differences, however, appear to reflect genetic drift rather than a response to selective breeding. Unexpectedly, the highly active line (NHE) showed a strong correlation between the IIP-MF and overnight habituation; the larger the IIP-MF projection, the lesser the long-term habituation (r = -0.70, P less than 0.001). In the NLE rats, the IIP-MF correlated positively with overnight habituation (though not significantly, because of an outlier). In both lines, stratum oriens was negatively correlated with short-term habituation in the second exposure (NHE: r = -0.68, P less than 0.001; NLE: r = -0.66, P less than 0.01). Thus, hippocampal variability of non-genetic origin appears to be correlated with processes which modulate strain-characteristic responses to a spatial novelty situation.

A quantitative-genetic analysis of hippocampal variation in the mouse

This report analyses the genetic underpinnings of the proportions of the hippocampal terminal fields in the mouse at the midseptotemporal level. We used 5 inbred strains and all possible F1 crosses between them (diallel cross). Broad heritabilities ranged from 11 to 53%. Additive genetic variation was present for all phenotypes analyzed. Directional dominance was found for the relative size of the suprapyramidal mossy fiber terminal field only. For the stratum lacunosum-moleculare, ambidirectional dominance emerged. These findings suggest that, in evolutionary history, directional selection has operated for a proportionally large suprapyramidal terminal field. For all other hippocampal variables (viz. the relative sizes for the strata oriens, pyramidale, radiatum, lacunosum-moleculare, CA4, intra- and infrapyramidal mossy fiber terminal field and the absolute size of the regio inferior) past stabilizing selection was inferred.