Metabotropic Glutamate Receptors at the Aged Mossy Fiber - CA3 Synapse of the Hippocampus.

Metabotropic glutamate receptors (mGluRs) are a group of G-protein-coupled receptors that exert a broad array of modulatory actions at excitatory synapses of the central nervous system. In the hippocampus, the selective activation of the different mGluRs modulates the intrinsic excitability, the strength of synaptic transmission, and induces multiple forms of long-term plasticity. Despite the relevance of mGluRs in the normal function of the hippocampus, we know very little about the changes that mGluRs functionality undergoes during the non-pathological aging. Here, we review data concerning the physiological actions of mGluRs, with particular emphasis on hippocampal area CA3. Later, we examine changes in the expression and functionality of mGluRs during the aging process. We complement this review with original data showing an array of electrophysiological modifications observed in the synaptic transmission and intrinsic excitability of aged CA3 pyramidal cells in response to the pharmacological stimulation of the different mGluRs.

Cognitive training increases dendritic arborization in the dorsal hippocampal CA1 and CA3 neurons of female and male Long-Evans rats.

Experiences influence the development of the central nervous system. Cognitive training promotes changes in the structure of the brain, such as in its weight and number of cells, as well as ability to perform dendritic remodeling. The present study was designed to detect possible differences in the neuronal morphology of the dorsal hippocampus between female and male Long-Evans rats after cognitive training (CT). CT was promoted through three learning and memory tests: the Morris water maze, the Barnes circular maze, and Novel object recognition tests. Our data revealed no differences in learning or memory capacities between female and male rats; rats of the two sexes solved the behavioral test with equal efficiency. CT caused an increase in the basilar and apical dendritic arborization of CA1 neurons in male rats, whereas female rats that underwent CT presented only remodeling in the apical arbors of CA1 neurons. The basilar arbors of CA3 neurons of female rats showed an increase in arborization, but their apical arbors were not modified; the arbors of CA3 neurons of male rats submitted to CT were not modified. Total dendritic length was modified by CT in the apical arbors of CA1 neurons of female and male rats and in the basilar CA1 arbors of male rats. There was a significant increase in dendritic spine density in all arbors of CA1 and CA3 neurons of females and males subjected to CT. These results suggest that dendritic remodeling after CT is similar between female and male rats.

Intracerebroventricular streptozotocin induces impaired Barnes maze spatial memory and reduces astrocyte branching in the CA1 and CA3 hippocampal regions.

Sporadic Alzheimer's disease (SAD) is the most common form of dementia; therefore, there is an urgent need for a model that recapitulates the main pathologic hallmarks of this disease. The intracerebroventricular (icv) injection of streptozotocin (icv-STZ) in rats constitutes a promising model, and thus, icv-STZ rats develop insulin-resistant brain state and cognitive impairments. Even though a great piece of studies has hitherto described this system as a model for SAD, further behavioral and morphometric studies are still needed to fully characterize it. In this study, using Sprague Dawley rats, we evaluated short-term effects on behavior and hippocampus morphometry of the icv-STZ injection at two doses: 1 (STZ1) and 3 mg/kg (STZ3). We found that, following icv-STZ injection, STZ3 animals, but not STZ1, exhibited impairments in spatial reference learning and memory (Barnes maze test) and in recognition memory (object recognition test). Furthermore, the results from behavioral and morpho-histochemical data are compatible. STZ3 rats displayed Stratum Radiatum volume reduction and a decreased NeuN immunoreactivity (neuron loss) in hippocampal CA1 region, together with an increased immunoreactivity for microglial (Iba1) and astroglial (GFAP) markers (neuroinflammation). Sholl analysis revealed the vulnerability of hippocampal astrocytes to STZ in CA1 and CA3. Thus, both doses induced a reduction in process length and in the number of main processes, accompanied by a frank decrease in branching complexity. The present study provides important knowledge of this AD rat model. Overall, we found that the only high STZ dose induced severe and acute neurodegenerative lesions, associated with an inflammation process.

Increased dendritic length in CA1 and CA3 hippocampal neurons during the metestrus phase in Wistar rats.

Studies have shown that changes in ovarian hormone concentrations promote natural fluctuations in the density of dendritic spines of hippocampal neurons in female Sprague-Dawley rats, without changes in dendritic length, throughout the estrous cycle. However, it is still unknown whether these fluctuations are present in other rat strains. Due to our interest in Wistar rats, the objective of the present study was to determine if there is natural dendritic remodeling in the female Wistar rat throughout the estrous cycle. This study analyzed the dendritic arborization of pyramidal neurons CA1 and CA3 of the dorsal hippocampus in each phase of the estrous cycle. We used the Golgi-Cox staining method and Sholl analysis to evaluate the dendritic length and density of dendritic spines. Our results showed that the dendritic length of the basilar and apical trees of CA1 neurons was longer in the metestrus phase. In CA3 neurons, only the apical dendritic trees showed longer dendritic length during metestrus. There was no variation in the density of dendritic spines in relation to any of the phases of the estrous cycle. Taken together, these results indicated that pyramidal neurons of the CA1 and CA3 regions of the dorsal hippocampus in the Wistar rat exhibited changes in dendritic length in the metestrus phase of the estrous cycle. Together, these data are important when considering the use of these organisms in behavioral studies.

Flavonoid profile of Lupinus mexicanus germinated seed extract and evaluation of its neuroprotective effect.

The aim of this study was to determine the flavonoid profile of Lupinus mexicanus germinated seed extract (PE) and to evaluate its effect as a phytoestrogen on the morphometric parameters of CA3 hippocampal neurons of ovariectomized rats (OVX). L. mexicanus seeds, germinated for 48 h, were homogenized and macerated using an 80% ethanol solution. The extract was analyzed by HPLC/MS-MS. Thirty young Wistar strain female rats (200±10 g) were randomly distributed into four groups: sham operated (S) treated with dimethyl sulfoxide (vehicle); ovariectomized and treated with 1250 µg of PE extract (OVX-PE); ovariectomized and treated with 5 µg estradiol benzoate (OVX-EB); and ovariectomized and vehicle treated (OVX). All substances were injected subcutaneously daily for 28 days. On day 29, the animals were sacrificed, perfused, and fixed to obtain the brains for histological processing. Each brain was cut and stained with hematoxylin and eosin. The thickness of the stratum oriens (SO), the nuclear diameter, and the neuronal density were measured in the hippocampus CA3 area. Nine different flavonoids and one non-identified compound were detected. The histological analysis demonstrated that the thickness of the SO was higher in the OVX-EB and S groups than in the OVX-PE and OVX groups (p⟨0.05); in addition, the nuclear diameters of the neurons in the OVX-EB and S groups were higher compared with the other groups (p⟨0.05). The OVX group had the highest cellular density among groups (p⟨0.05). Based on our results, the PE obtained did not have beneficial effects on CA3 hippocampal neurons.

Dissociation of CA3 pyramidal cells with attached, functional, identified mossy fiber and interneuronal boutons for studying glutamatergic and GABAergic synaptic transmission.

Pyramidal cells of CA3 area receive glutamatergic signals from the mossy fibers (MFs), perforant path and collaterals of other pyramidal cells, as well as GABAergic inputs from interneurons. In hippocampal slices, an extracellular stimulation electrode is often used to activate the MFs, with the disadvantage of possibly activating fibers other than MFs. We set-up a preparation that allows the analysis of the glutamatergic input from identified, giant MF boutons as well as of GABAergic inputs from boutons of interneurons on single CA3 pyramidal cells. Mossy fiber boutons were labeled by exposing hippocampal slices to a zinc-reactive fluorescent dye, or by injecting a fluorescent dye in the granule cell layer and allowing its transport along the MFs to their terminals in CA3 area. After conducting an enzyme-free, mechanical dissociation of CA3 area, we obtained pyramidal cells containing fluorescent, giant MF boutons attached to their apical dendrites, as well as boutons of interneuronal origin. Whole cell recordings were then performed, whereby synaptic responses could be evoked by selective stimulation of the identified boutons. The synaptic currents evoked by stimulation of MF boutons, unlike those evoked by stimulation of interneuronal boutons, underwent strong frequency potentiation and were depressed by activation of metabotropic glutamate receptors, which are characteristics of transmission of MF origin. Combination of fluorophores can be used to label different tracts/boutons allowing the study of the different characteristics of neurotransmitter release from a variety of sources on single target cells.

Hippocampal plasticity in rats submitted to a gastric restrictive procedure.

Bariatric surgery has been the most effective therapeutic intervention for morbidly obese patients. However, recent evidence has shown that this procedure may cause serious neurological complications such as Wernicke encephalopathy, depression, and memory impairment. With this in mind, we conducted an experimental study to investigate whether weight-reduction surgery would promote morphological changes in the hippocampal formation, a brain region linked to cognitive and emotional processes. To do so, the present study evaluated the hippocampal expression of parvalbumin interneurons in rats submitted to a gastric restrictive procedure (experimental phytobezoar). Our results demonstrated that rats with gastric-reduced capacity presented a significant increase in the expression of the parvalbumin interneurons in the hippocampal CA1 and CA3 subfields. These data are the first experimental evidence that restrictive bariatric surgery may alter hippocampal cytoarchitecture.

Excitation-inhibition balance in the CA3 network--neuronal specificity and activity-dependent plasticity.

Activation of the axons of the granule cells, the mossy fibers, excites pyramidal cells and interneurons in the CA3 area, which, in turn, inhibit pyramidal cells. The integration of the various inputs that converge onto CA3 cells has been studied by pharmacological dissection of either the excitatory or inhibitory components. This strategy has the disadvantage of partially isolating the recorded cell from the network, ignoring the sources and the impact of concurrent inputs. To overcome this limitation, we dissociated excitatory and inhibitory synaptic conductances by mathematical extraction techniques, and analysed the dynamics of the integration of excitatory and inhibitory inputs in pyramidal cells and stratum lucidum interneurons (Sl-Ints) of CA3. We have uncovered a shunting mechanism that decreases the responsiveness of CA3 output cells to mossy fiber input after a period of enhanced excitability. The activation of the dentate gyrus (DG) after applying a kindling-like protocol in vitro, or after producing one or several seizures in vivo, results in a graded and reversible increase of inhibitory conductances in pyramidal cells, while in Sl-Ints, an increase of excitatory conductances occurs. Thus, interneurons reach more depolarized membrane potentials on DG activation yielding a high excitatory postsynaptic potential-spike coupling, while the contrary occurs in pyramidal cells. This effective activation of feedforward inhibition is synergized by the emergence of direct DG-mediated inhibition on pyramidal cells. These factors force the synaptic conductance to peak at a potential value close to resting membrane potential, thus producing shunt inhibition and decreasing the responsiveness of CA3 output cells to mossy fiber input.