Involuntary, unreinforced (pure) spatial learning is impaired by fimbria-fornix but not by dorsal hippocampus lesions

Exploring time-dependent changes in conditioned place preference for food reward and associated changes in the nucleus accumbens

The conditioned place preference (CPP) procedure has been used to study the incubation of craving phenomenon with rewarding drugs such as cocaine and methamphetamine. The present study examined whether rats trained in a CPP behavioral design would display an incubation of craving response for chocolate-flavored pellets or milk chocolate chips at the behavioral and neural levels. Rats were conditioned using an unbiased CPP design then underwent abstinence from food reward for 24 hs, 7, 14, or 28 days at which point they were tested for CPP. Brains underwent immunohistochemical staining for c-Fos and FosB as well as Golgi staining to assess dendritic spine density in the nucleus accumbens (NAc). A time-dependent increase in CPP and entries into the previously paired compartment was observed in the chocolate-flavored pellet group but not the milk chocolate group. Time-dependent neural changes were not directly associated with behavioral outcomes but c-Fos labelling was higher in the chocolate pellet group than controls at the 7-day abstinence period. The behavioral results show that chocolate pellets are rewarding and are associated with long-term behavioral changes but, as evidenced by limited neural changes, these food rewards do not have the same effects on the NAc as drugs of abuse.

Volume of hippocampal subfields in patients with alcohol dependence

Alcohol-induced hippocampal atrophy has been well documented in many studies and is known to affect various subfields. Given the functional heterogeneity of these subfields, we investigated the precise effects of alcohol-induced damage in these areas. Twenty-six male patients with alcohol dependence (alcohol group) and twenty-six age-matched male healthy social drinkers were recruited from a mental health hospital and the community respectively, with the aim of comparing the hippocampal subfields between groups. Each participant underwent a 3T MRI scan. Hippocampal subfield volumes were estimated using an automated procedure and drinking history recorded using Lifetime Drinking History, Alcohol Use Disorder Identification Test, and the Brief Michigan Alcoholism Screening Test. The alcohol group showed a lower total hippocampus volume, specifically in the left presubiculum, fimbria, and bilateral subiculum. Regression analysis assessing the influence of age and group showed that group was a more significant factor than age in most subfields. Our findings suggest that alcohol dependence alters hippocampal subfield volumes. Further longitudinal studies on the interaction of structural and neurocognitive changes would improve our understanding of brain structural changes resulting from long-term alcohol consumption.

Rat intra-hippocampal NMDA infusion induces cell-specific damage and changes in expression of NMDA and GABAA receptor subunits

Excessive stimulation of NMDA receptors with glutamate or other potent agonists such as NMDA leads to excitotoxicity and neural injury. In this study, we aimed to provide insight into an animal model of brain excitotoxic damage; single unilateral infusion of NMDA at mild dose into the hippocampal formation. NMDA infusion induced chronic, focal neurodegeneration in the proximity of the injection site. The lesion was accompanied by severe and progressive neuroinflammation and affected preferentially principal neurons while sparing GABAergic interneurons. Furthermore, the unilateral lesion did not cause significant impairment of spatial learning abilities. Finally, GluN1 and GluN2B subunits of NMDA receptor were significantly upregulated up to 3 days after the NMDA infusion, while GABAA α5 subunit was downregulated at 30 days after the lesion. Taken together, a single infusion of NMDA into the hippocampal formation represents an animal model of excitotoxicity-induced chronic neurodegeneration of principal neurons accompanied by severe neuroinflammation and subunit specific changes in NMDA and GABAA receptors.

Age-related changes in diffusion tensor imaging metrics of fornix subregions in healthy humans

OBJECTIVE

White matter diffusivity measures of the fornix change with aging, which likely relates to changes in memory and cognition in older adults. Subregional variations in forniceal diffusivity may exist, given its heterogeneous anatomy and connectivity; however, these have not been closely examined in vivo. We examined diffusivity parameters (fractional anisotropy, FA; radial diffusivity, RD; axial diffusivity, AD) in forniceal subregions of healthy subjects and correlated them with age and hippocampal volume.

METHODS

Diffusion-weighted imaging and streamline tractography of the fornix were performed on 20 healthy, right-handed females (23-66 years). Six anatomical subregions were defined: midline (body, column, precommissural fornix) or lateral (fimbria, crura, postcommissural fornix). Regression analysis was performed comparing diffusivities against age. Hippocampal and ventricular volumes were also compared.

RESULTS

Diffusivity values revealed statistical changes with age in both midline and lateralized subregions. The fornix body and left crus showed age-related alterations in all metrics (FA, RD, AD), whereas only right crus FA was altered. There was no significant change in hippocampal volumes, suggesting that forniceal changes may precede hippocampal age-related changes.

CONCLUSIONS

Age-related changes in fornix diffusivity measures appear subregion dependent and asymmetrical. Specific subregion diffusivity measures may be a more sensitive aging marker than hippocampal volume change.

The effects of acute, chronic, and withdrawal from chronic nicotine on novel and spatial object recognition in male C57BL/6J mice

RATIONALE

Spatial and novel object recognition learning is different from learning that uses aversive or appetitive stimuli to shape acquisition because no overt contingencies are needed. While this type of learning occurs on a daily basis, little is known about how nicotine administration affects it.

OBJECTIVES

To determine the effects of acute, chronic, and withdrawal from chronic nicotine on two related but distinct incidental learning tasks, novel and spatial object recognition.

METHODS

In C57BL/6J mice, the effects of acute (0.045-0.18 mg/kg), chronic (6.3 mg/kg/day), and withdrawal from chronic nicotine on novel and spatial object recognition were examined.

RESULTS

With a 48-h delay between training and testing, acute nicotine enhanced spatial (difference score, saline = 3.34 s, nicotine = 7.71 s, p = 0.029) but resulted in a deficit in novel object recognition (difference score, saline = 8.76 s, nicotine = 4.48 s, p = 0.033). Chronic nicotine resulted in a strong trend towards a deficit in spatial object recognition (difference score, saline = 4.01 s, nicotine = 1.81 s, p = 0.059) but had no effect on novel object recognition, and withdrawal from chronic nicotine disrupted spatial object recognition (difference score, saline = 3.00 s, nicotine = 0.17 s, p = 0.004) but had no effect on novel object recognition.

CONCLUSIONS

The effects of nicotine on spatial object recognition shift from enhancement to deficit as administration changes from acute to chronic and withdrawal. These effects were specific for spatial object recognition, which may be due to differing underlying neural substrates involved in these tasks. Understanding how nicotine alters learning has implications for understanding diseases associated with altered cholinergic function.

A novel, rapidly acquired and persistent spatial memory task that induces immediate early gene expression

Background

The Morris water maze task is a hippocampus-dependent learning and memory test that typically takes between 3 days to 2 weeks of training. This task is used to assess spatial learning and induces the expression of genes known to be crucial to learning and memory in the hippocampus. A major caveat in the protocol is the prolonged duration of training, and difficulty of assessing the time during training in which animals have learned the task. We introduce here a condensed version of the task that like traditional water maze tasks, creates lasting hippocampus-dependent spatial cognitive maps and elicits gene expression following learning.

Methods

This paradigm was designed for rats to quickly acquire a hippocampus-dependent spatial cognitive map and retain this memory for at least 24 hours. To accomplish this, we interspersed visible and hidden training trials, delivering them in a massed fashion so training takes a maximum of 15 minutes. Learning was assessed based on latencies to the platform during each training trial, as well as time spent in the goal quadrant during probe testing 30 minutes and 24 hours after training. Normal rats were compared to two impaired cohorts (rats with fimbria-fornix lesions and rats administered NMDA receptor antagonist (CPP)). To quantitate hippocampal expression of known learning genes, real-time polymerase chain reaction (RT-PCR) was performed on hippocampal cDNA.

Results

We show that massed training using alternating visible and hidden training trials generates robust short-term working and long-term reference memories in rats. Like the traditional Morris water maze paradigm, this task requires proper hippocampal function, as rats with fimbria-fornix lesions and rats administered CPP fail to learn the spatial component of the task. Furthermore, training in this paradigm elicits hippocampal expression of genes upregulated following learning in a variety of spatial tasks: homer1a, cfos and zif268.

Conclusions

We introduce here a condensed version of the Morris water maze, which is like a traditional water maze paradigm, in that it is hippocampus-dependent, and elicits hippocampal expression of learning genes. However, this task is administered in 15 minutes and induces spatial memory for at least 24 hours.

Roles of learning and motivation in preference behavior: Mediation by entorhinal cortex, dorsal and ventral hippocampus

In the latent cue preference (LCP) task, water-deprived rats alternately drink a salt solution in one distinctive compartment of a conditioned cue preference (CCP) apparatus and water in the other compartment over 8 days (training trials). They are then given a choice between the two compartments with no solutions present (preference test). Previous findings showed that this training procedure results in two parallel forms of learning: conditioning to water-paired cues (a water-CCP) and latent learning of an association between salt and salt-paired compartment cues (a salt-LCP). Experiment 1 examined these two types of learning in isolation. Results showed that expression of the salt-LCP required salt deprivation during testing, but expression of the water-CCP did not require a deprivation state during testing. Other results showed that salt-LCP learning itself involves two distinct components: (1) the latent association among neutral cues in the salt-paired compartment, and (2) motivational information about salt deprivation during testing. Previous findings also demonstrated roles for the dorsal hippocampus (DH), ventral hippocampus (VH), and entorhinal cortex (EC) in salt-LCP learning. Experiment 2 examined the involvement of these structures during acquisition or expression of salt-LCP learning. Rats with cannulas aimed at DH, VH, or EC were given infusions of muscimol, either before exposure to the salt-paired, but not the water-paired, compartment during training or before the preference test. Inactivation of the DH or EC impaired both acquisition and expression of the association between salt and salt-paired compartment cues, while inactivation of the VH disrupted the influence of motivational information about salt deprivation required to express the salt-LCP. These results suggest unique roles for the EC-DH circuit and VH in salt-LCP learning, as well as a functional dissociation between the DH and VH.

Fornix integrity and hippocampal volume in male schizophrenic patients

BACKGROUND

The hippocampus has been shown to be abnormal in schizophrenia. The fornix is one of the main fiber tracts connecting the hippocampus with other brain regions. Few studies have evaluated the fornix in schizophrenia, however. A focus on fornix abnormalities and their association with hippocampal abnormalities might figure importantly in our understanding of the pathophysiology of schizophrenia.

METHODS

Line-scan diffusion tensor imaging (DTI) was used to evaluate diffusion in the fornix in 24 male patients with chronic schizophrenia and 31 male control subjects. Maps of fractional anisotropy (FA) and mean diffusivity (D(m)), which are indices sensitive to white-matter integrity, were generated to quantify diffusion within the fornix. We used high spatial resolution magnetic resonance imaging (MRI) to measure hippocampal volume.

RESULTS

FA and cross-sectional area of the fornix were significantly reduced in patients compared with control subjects. D(m) was significantly increased, whereas hippocampal volume was bilaterally reduced in patients. Reduced hippocampal volume was correlated with increased mean D(m) and reduced cross-sectional area of the fornix for patients. Patients also showed a significant correlation between reduced scores on neuropsychologic measures of declarative-episodic memory and reduced hippocampal volumes.

CONCLUSIONS

These findings demonstrate a disruption in fornix integrity in patients with schizophrenia.

Neural circuits mediating latent learning and conditioning for salt in the rat

Male Long-Evans rats alternately drank a salt solution in one distinctive compartment of a conditioned cue preference (CCP) apparatus and water in a different compartment over 8 days (training trials) and were then given a choice between the two compartments with no solutions present (test trial). Rats that were water deprived during training, then salt+water deprived during testing, spent more time in their salt-paired compartments, a salt latent cue preference (LCP). Rats that were water-only deprived during training and testing spent more time in their water-paired compartments, a water CCP. Rats that were salt+water deprived during both training and testing spent more time in their salt-paired compartments, a salt CCP. Bilateral, pre-training lesions of the lateral amygdala impaired the water and salt CCPs but not the salt LCP, reflecting the role of the amygdala in Pavlovian conditioning. Lesions of the dorsal or ventral hippocampus impaired the salt LCP and the water and salt CCPs, possibly reflecting the role of the hippocampus in contextual learning. Lesions of the fimbria-fornix impaired the water and salt CCPs but not the salt LCP, while lesions of the entorhinal cortex impaired the salt LCP but not the CCPs. This suggests that the LCP depends on a circuit that includes dorsal and ventral hippocampus and entorhinal cortex, a major conduit of sensory information from the cortex. In contrast, the CCPs depend on the amygdala and a circuit that includes the hippocampus and fimbria-fornix, possibly as a conduit of motivational information from subcortical structures.

Dorsal hippocampus function in learning and expressing a spatial discrimination

Learning to discriminate between spatial locations defined by two adjacent arms of a radial maze in the conditioned cue preference paradigm requires two kinds of information: latent spatial learning when the rats explore the maze with no food available, and learning about food availability in two spatial locations when the rats are then confined in one arm with food and the other with no food. Previous research showed that a functional dorsal hippocampus is not required for latent learning. The present experiments show that it is required for learning about food availability, and during retrieval of both types of information.

Inactivation of the dorsal hippocampus does not affect learning during exploration of a novel environment

The conditioned cue preference (CCP) task was used to study the ability of rats to discriminate between spatial locations. Food-deprived rats explored an eight-arm radial maze with no food present (pre-exposure). On subsequent days, they were alternately confined in one arm of the maze with food and in another arm with no food (training), followed by a preference test with no food present, to determine if they had learned to discriminate between the two arm locations. No injections were given during the two latter phases. With adjacent radial maze arms, rats given three 10-min pre-exposure sessions and four food-pairing trials exhibited a preference for their food-paired arms; rats not pre-exposed did not exhibit this preference. Rats pre-exposed 30 min after dorsal hippocampus injections of muscimol exhibited the preference. With widely separated maze arms, rats given two training trials with no pre-exposure exhibited a preference for the food-paired arm; rats that were given one pre-exposure session did not. Rats pre-exposed 30 min after dorsal hippocampus injections of muscimol did not exhibit the preference. The same intrahippocampal muscimol injections that failed to affect the influence of pre-exposure on CCP learning with both arm configurations impaired win-shift performance, a standard test of spatial learning. These findings suggest that a functional dorsal hippocampus is not required for the (incidental or latent) learning that occurs during unreinforced exploration of a novel environment. The information acquired during this activity subsequently produces a latent learning effect if it is used to discriminate between two ambiguous locations (adjacent arms) or a latent inhibition--like effect if it is used to discriminate between two unambiguous locations (separated maze arms).

Place Versus Response Learning Revisited: Tests of Blocking on the Radial Maze

Neurobiological and behavioral research indicates that place learning and response learning occur simultaneously, in parallel. Such findings seem to conflict with theories of associative learning in which different cues compete for learning. The authors conducted place+response training on a radial maze and then tested place learning and response learning separately by reconfiguring the maze in various ways. Consistent with the effects of manipulating place and response systems in the brain (M. G. Packard & J. L. McGaugh, 1996), well-trained rats showed strong place learning and strong response learning. Three experiments using associative blocking paradigms indicated that prior response learning interferes with place learning. Blocking and related tests can be used to better understand how memory systems interact during learning.

The role of stimulus ambiguity and movement in spatial navigation: A multiple memory systems analysis of location discrimination

This paper reviews recent findings about how rats navigate by learning to discriminate among locations. The assumption underlying the experiments and their interpretation is that the information required to do this is learned by three independent, parallel memory systems. One system processes cognitive information (or "knowledge"), a second system processes reinforced stimulus-response associations and a third processes Pavlovian conditioned responses in the form of stimulus-affect associations. The information stored in each system produces behavior that, in some cases, results in a location discrimination. The present experiments focus on three factors that influence what each system learns and whether the resulting memory produces behavior that results in a location discrimination. One factor is whether the locations to be discriminated can be identified by unique, unambiguous stimuli or whether they are ambiguously associated with the same stimuli. The second factor is whether the stimuli are observed passively or whether the rats move among them, voluntarily or involuntarily. The third factor is whether or not the rats perform specific reinforced responses in the presence of the stimuli. Instances of co-operative behavioral outputs from memory systems that facilitate location discriminations and of competitive outputs that impede discriminations are described.

Effects of fimbria-fornix, hippocampus, and amygdala lesions on discrimination between proximal locations

The conditioned cue preference (CCP) task was used to study the information required to discriminate between spatial locations defined by adjacent arms of an 8-arm radial maze. Normal rats learned the discrimination after 3 unreinforced preexposure (PE) sessions and 4 food paired-unpaired training trials. Fimbria-fornix lesions made before, but not after, PE, and hippocampus lesions made at either time, blocked the discrimination, suggesting that the 2 structures processed different information. Lateral amygdala lesions made before PE facilitated the discrimination. This amygdala-mediated interference with the discrimination was the result of a conditioned approach response that did not discriminate between the 2 arm locations. A hippocampus/fimbria-fornix system and an amygdala system process different information about the same learning situation simultaneously and in parallel.