J.M. Blasco-Ibáñez

Distribution, ultrastructure, and connectivity of calretinin‐immunoreactive mossy cells of the mouse dentate gyrus

Hilar mossy cells of the mouse were shown recently to display calretinin immunoreactivity (Liu et al. [1996] Exp Brain Res 108:389–403). The morphological and connectional characteristics of these cells are poorly understood. In the present study, we used immunohistochemical, electron microscopic, and neuronal tracing techniques to describe their distribution, morphology, and connectivity.

Chronic antidepressant treatment induces contrasting patterns of synaptophysin and PSA-NCAM expression in different regions of the adult rat telencephalon.

Structural modifications occur in the brain of severely depressed patients and they can be reversed by antidepressant treatment. Some of these changes do not occur in the same direction in different regions, such as the medial prefrontal cortex, the hippocampus or the amygdala. Differential structural plasticity also occurs in animal models of depression and it is also prevented by antidepressants. In order to know whether chronic fluoxetine treatment induces differential neuronal structural plasticity in rats, we have analyzed the expression of synaptophysin, a protein considered a marker of synaptic density, and the expression of the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a molecule involved in neurite and synaptic remodeling. Chronic fluoxetine treatment increases synaptophysin and PSA-NCAM expression in the medial prefrontal cortex and decreases them in the amygdala. The expression of these molecules is also affected in the entorhinal, the visual and the somatosensory cortices.

Enkephalin-containing interneurons are specialized to innervate other interneurons in the hippocampal CA1 region of the rat and guinea-pig

Enkephalins are known to have a profound effect on hippocampal inhibition, but the possible endogenous source of these neuropeptides, and their relationship to inhibitory interneurons is still to be identified. In the present study we analysed the morphological characteristics of met‐enkephalin‐immunoreactive cells in the CA1 region of the rat and guinea‐pig hippocampus, their coexistence with other neuronal markers and their target selectivity at the light and electron microscopic levels.

Chelation of synaptic zinc induces overexcitation in the hilar mossy cells of the rat hippocampus

Complete removal of synaptic zinc by the chelator dietyldithiocarbamate (DEDTC; 500 mg/kg i.p.) in rat was followed by convulsive behaviour including wet dog shakes alternating immobility. Histological analysis 1 day after DEDTC administration detected expression of heat shock protein in the hippocampus restricted to hilar cells. These cells colocalize the marker for neurons and the glutamate receptor GluR2/3 showing that they are excitatory neurons. Additionally, they projected to the contralateral dentate gyrus. Therefore, they correspond to hilar mossy cells. These data show that the synaptic zinc has a role in normal hippocampus avoiding overexcitation, that would impair functionality even in absence of pathological or exoexcitotoxic phenomena.

Effects of chronic fluoxetine treatment on the rat somatosensory cortex: activation and induction of neuronal structural plasticity.

Recent hypotheses support the idea that disruption of normal neuronal plasticity mechanisms underlies depression and other psychiatric disorders, and that antidepressant treatment may counteract these changes. In a previous report we found that chronic fluoxetine treatment increases the expression of the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a molecule involved in neuronal structural plasticity, in the somatosensory cortex. In the present study we intended to find whether, in fact, cell activation and neuronal structural remodeling occur in parallel to changes in the expression of this molecule. Using immunohistochemistry, we found that chronic fluoxetine treatment caused an increase in the expression of the early expression gene c-fos. Golgi staining revealed that this treatment also increased spine density in the principal apical dendrite of pyramidal neurons. These results indicate that, apart from the medial prefrontal cortex or the hippocampus, other cortical regions can respond to chronic antidepressant treatment undergoing neuronal structural plasticity.

Neural Overexcitation and Implication of NMDA and AMPA Receptors in a Mouse Model of Temporal Lobe Epilepsy Implying Zinc Chelation

Summary:  Purpose: Zinc chelation with diethyldithiocarbamate (DEDTC) during nondamaging kainic acid administration enhances excitotoxicity to the level of cell damage. The objective of this work was to study the developing of the lesion in this model of temporal lobe epilepsy and the implications of the different types of glutamate receptors.

GABAergic basal forebrain afferents innervate selectively GABAergic targets in the main olfactory bulb.

In this work we have analyzed the targets of the GABAergic afferents to the main olfactory bulb originating in the basal forebrain of the rat. We combined anterograde tracing of 10 kD biotinylated dextran amine (BDA) injected in the region of the horizontal limb of the diagonal band of Broca that projects to the main olfactory bulb, with immunocytochemical detection of GABA under electron microscopy or vesicular GABA transporter (vGABAt) under confocal fluorescent microscopy. GABAergic afferents were identified as double labeled BDA-GABA boutons. Their targets were identified by their ultrastructure and GABA content. We found that GABAergic afferents from the basal forebrain were distributed all over the bulbar lamination, but were more abundant in the glomerular and inframitral layers (i.e. internal plexiform layer and granule cell layer). The fibers had thick varicosities with abundant mitochondria and large perforated synaptic specializations. They contacted exclusively GABAergic cells, corresponding to type 1 periglomerular cells in the glomerular layer, and to granule cells in inframitral layers. This innervation will synchronize the bulbar inhibition and consequently the response of the principal cells to the olfactory input. The effect of the activation of this pathway will produce a disinhibition of the bulbar principal cells. This facilitation might occur at two separate levels: first in the terminal tufts of mitral and tufted cells via inhibition of type 1 periglomerular cells; second at the level of the firing of the principal cells via inhibition of granule cells. The GABAergic projection from the basal forebrain ends selectively on interneurons, specifically on type 1 periglomerular cells and granule cells, and is likely to control the activity of the olfactory bulb via disinhibition of principal cells. Possible similarities of this pathway with the septo-hippocampal loop are discussed.

SYNAPTIC CONNECTIVITY OF SEROTONERGIC AXONS IN THE OLFACTORY GLOMERULI OF THE RAT OLFACTORY BULB

Although the major mode of transmission for serotonin in the brain is volume transmission, previous anatomical studies have demonstrated that serotonergic axons do form synaptic contacts. The olfactory glomeruli of the olfactory bulb of mammals receive a strong serotonergic innervation from the dorsal and medial raphe nuclei. In the present report, we investigate the synaptic connectivity of these serotonergic axons in the glomerular neuropil of the rat olfactory bulb. Our study shows that serotonergic axons form asymmetrical synaptic contacts on dendrites within the glomerular neuropil. Analyzing the neurochemical nature of the synaptic targets, we have found that 55% of the synapses were on GABA-immunopositive profiles and 45% on GABA-immunonegative profiles. These data indicate that barely half of the contacts were found in GABA-immunonegative profiles and half of the synapses in GABA-positive dendrites belonging to type 1 periglomerular cells. Synaptic contacts from serotonergic axons on dendrites of principal cells cannot be excluded, since some of the GABA-immunonegative postsynaptic profiles contacted by serotonergic axons had the typical ultrastructural features of bulbar principal cell dendrites. Altogether, our results suggest a complex action of the serotonergic system in the modulation of the bulbar circuitry.

Characterization of a population of tyrosine hydroxylase-containing interneurons in the external plexiform layer of the rat olfactory bulb.

The olfactory bulb (OB) of mammals contains the major endogenous dopamine-producing system in the forebrain. The vast majority of dopaminergic neurons consists of juxtaglomerular cells, which innervate the olfactory glomeruli and modulate the entrance of sensory information to the OB. Although dopaminergic juxtaglomerular cells have been widely investigated, the presence of dopaminergic interneurons other than juxtaglomerular cells has been largely unexplored. In this study, we analyze a population of tyrosine hydroxylase (TH)-containing interneurons located in the external plexiform layer (EPL) of the rat OB. These interneurons are GABAergic and morphologically heterogeneous. They have an axon and two to four dendrites running throughout the EPL. Frequently, they have appendages similar to spines in the dendrites and, sometimes, the distal portions of the dendritic branches show enlargements or swellings similar to varicosities. Contrary to other interneurons of the EPL, the TH-containing ones do not form dendro-dendritic synapses on principal cells and do not receive dendro-dendritic synapses from them. In fact, no synapses were found from the dendrites of these interneurons. When their dendrites are involved in synaptic contacts, they are always the postsynaptic element. They receive symmetrical and asymmetrical synapses from GABAergic and non-GABAergic axons of unidentified origin. Our data indicate that the local circuits of the EPL are more complex than previously thought. Although most of the interneurons of this layer establish dendro-dendritic synaptic relationships with principal cells, the TH-containing interneurons constitute an exception to this rule, resembling interneurons from other cortical areas.

DISTRIBUTION OF THE A3 SUBUNIT OF THE CYCLIC NUCLEOTIDE-GATED ION CHANNELS IN THE MAIN OLFACTORY BULB OF THE RAT

Previous data suggest that cyclic GMP (cGMP) signaling can play key roles in the circuitry of the olfactory bulb (OB). Therefore, the expression of cGMP-selective subunits of the cyclic nucleotide-gated ion channels (CNGs) can be expected in this brain region. In the present study, we demonstrate a widespread expression of the cGMP-selective A3 subunit of the cyclic nucleotide-gated ion channels (CNGA3) in the rat OB. CNGA3 appears in principal cells, including mitral cells and internal, medium and external tufted cells. Moreover, it appears in two populations of interneurons, including a subset of periglomerular cells and a group of deep short-axon cells. In addition to neurons, CNGA3-immunoreactivity is found in the ensheathing glia of the olfactory nerve. Finally, an abundant population of CNGA3-containing cells with fusiform morphology and radial processes is found in the inframitral layers. These cells express doublecortin and have a morphology similar to that of the undifferentiated cells that leave the rostral migratory stream and migrate radially through the layers of the OB. Altogether, our results suggest that CNGA3 can play important and different roles in the OB. Channels composed of this subunit can be involved in the processing of the olfactory information taking place in the bulbar circuitry. Moreover, they can be involved in the function of the ensheathing glia and in the radial migration of immature cells through the bulbar layers.