The expression of calretinin in transfected PC12 cells provides no protection against Ca(2+)-overload or trophic factor deprivation

Calcium-Binding Proteins as Determinants of Central Nervous System Neuronal Vulnerability to Disease

Neuronal subpopulations display differential vulnerabilities to disease, but the factors that determine their susceptibility are poorly understood. Toxic increases in intracellular calcium are a key factor in several neurodegenerative processes, with calcium-binding proteins providing an important first line of defense through their ability to buffer incoming calcium, allowing the neuron to quickly achieve homeostasis. Since neurons expressing different calcium-binding proteins have been reported to be differentially susceptible to degeneration, it can be hypothesized that rather than just serving as markers of different neuronal subpopulations, they might actually be a key determinant of survival. In this review, we will summarize some of the evidence that expression of the EF-hand calcium-binding proteins, calbindin, calretinin and parvalbumin, may influence the susceptibility of distinct neuronal subpopulations to disease processes.

Melatonin Influences Structural Plasticity in the Axons of Granule Cells in the Dentate Gyrus of Balb/C Mice

Melatonin, the main product synthesized by the pineal gland, acts as a regulator of the generation of new neurons in the dentate gyrus (DG). Newborn neurons buffer the deleterious effects of stress and are involved in learning and memory processes. Furthermore, melatonin, through the regulation of the cytoskeleton, favors dendrite maturation of newborn neurons. Moreover, newborn neurons send their axons via the mossy fiber tract to Cornu Ammonis 3 (CA3) region to form synapses with pyramidal neurons. Thus, axons of newborn cells contribute to the mossy fiber projection and their plasticity correlates with better performance in several behavioral tasks. Thus, in this study, we analyzed the impact of exogenous melatonin (8 mg/kg) administered daily for one- or six-months on the structural plasticity of infrapyramidal- and suprapyramidal mossy fiber projection of granule cells in the DG in male Balb/C mice. We analyzed the mossy fiber projection through the staining of calbindin, that is a calcium-binding protein localized in dendrites and axons. We first found an increase in the number of calbindin-positive cells in the granular cell layer in the DG (11%, 33%) after treatment. Futhermore, we found an increase in the volume of suprapyramidal (>135%, 59%) and infrapyramidal (>128%, 36%) mossy fiber projection of granule neurons in the DG after treatment. We also found an increase in the volume of CA3 region (>146%, 33%) after treatment, suggesting that melatonin modulates the structural plasticity of the mossy fiber projection to establish functional synapses in the hippocampus. Together, the data suggest that, in addition to the previously reported effects of melatonin on the generation of new neurons and its antidepressant like effects, melatonin also modulates the structural plasticity of axons in granule cells in the DG.

Melatonin maintains calcium-binding calretinin-positive neurons in the dentate gyrus during aging of Balb/C mice

Melatonin, the main product synthesized by the pineal gland, modulates several brain functions through different mechanisms, some of them involving the activation or participation of calcium binding intracellular proteins, such as the alpha calcium dependent protein kinase C and calmodulin. Another calcium-binding protein is calretinin, which exerts an essential role for adult hippocampal neurogenesis. Melatonin favors calretinin-positive neurons in the dentate gyrus (DG) of young mice but hippocampal neurogenesis and plasma levels of melatonin decrease during aging. Thus, in this study, we analyzed the impact of exogenous supplementation with melatonin in calretinin-neurons and their distribution along the dorsal-ventral DG in the hippocampus at three different time points (1, 3, or 6 months) after daily treatment with melatonin (8 mg/kg) in male Balb/C mice. We found an increase in the number of calretinin-positive neurons in the DG after treatment (>66%). Although a significant decline in the number of calretinin-neurons was found in both treated (~60.46-69.56%) and untreated mice (~68.81-70.34%) with respect to the youngest mice analyzed, melatonin still maintained higher number of cells in the DG. Also, the distribution of calretinin-neurons along the dorsal-ventral DG significantly showed more cells in the ventral-DG of mice treated with melatonin. Together, the data suggest that melatonin also acts on calretinin in the DG, supporting it as a molecule connecting calcium signaling and neuronal development.

Calretinin: from a "simple" Ca(2+) buffer to a multifunctional protein implicated in many biological processes

The hexa-EF-hand Ca(2+)-binding protein calretinin (CR) is predominantly expressed in specific neurons of the central and peripheral nervous system. However, CR expression is also observed in non-neuronal cells, e.g., during embryonic development and in mesothelioma cells. Of the 6 EF-hand domains, 5 are functional; the first 4 domains form 2 pairs showing high cooperativity within a pair that results in non-linear modulation of intracellular Ca(2+) signals by CR. EF-hand domain 5 has a low affinity and represents the identified interaction site with CR-binding partners present in mouse cerebellar granule cells. CR binding to other targets including the pore-forming α1 subunit of the Ca(2+) channel Ca V 2.1, as well as to huntingtin indicates additional Ca(2+) sensor functions besides the well-known Ca(2+)-buffering functions. The absence of CR in cerebellar granule cells of CR(-/-) mice results in increased excitability and altered firing of Purkinje cells and promotes cerebellar 160-Hz oscillations impairing motor coordination. The putative role of CR in neuroprotection is still highly discussed. Altogether, CR emerges as a multi-functional protein also associated with development, i.e., cell proliferation, differentiation, and cell death.

Calbindin 2 (CALB2) regulates 5-fluorouracil sensitivity in colorectal cancer by modulating the intrinsic apoptotic pathway

The role of the calcium binding protein, Calbindin 2 (CALB2), in regulating the response of colorectal cancer (CRC) cells to 5-Fluorouracil (5-FU) was investigated. Real-time RT-PCR and Western blot analysis revealed that CALB2 mRNA and protein expression were down-regulated in p53 wild-type and p53 null isogenic HCT116 CRC cell lines following 48 h and 72 h 5-FU treatment. Moreover, 5-FU-induced apoptosis was significantly reduced in HCT116 and LS174T CRC cell lines in which CALB2 expression had been silenced. Further investigation revealed that CALB2 translocated to the mitochondria following 5-FU treatment and that 5-FU-induced loss of mitochondrial membrane potential (Δψ(m)) was abrogated in CALB2-silenced cells. Furthermore, CALB2 silencing decreased 5-FU-induced cytochrome c and smac release from the mitochondria and also decreased 5-FU-induced activation of caspases 9 and 3/7. Of note, co-silencing of XIAP overcame 5-FU resistance in CALB2-silenced cells. Collectively, these results suggest that following 5-FU treatment in CRC cell lines, CALB2 is involved in apoptosis induction through the intrinsic mitochondrial pathway. This indicates that CALB2 may be an important mediator of 5-FU-induced cell death. Moreover, down-regulation of CALB2 in response to 5-FU may represent an intrinsic mechanism of resistance to this anti-cancer drug.

Long-term regulation in calretinin staining in the rat inferior colliculus after unilateral auditory cortical ablation

In this study we analyzed the effects in the inferior colliculus of a unilateral ablation of the auditory cortex in rats. Variations in both calretinin immunoreactivity and protein levels determined by Western blot suggest that such lesions induce changes in the regulation of this calcium-binding protein. Stereological counts of calretinin-immunoreactive neurons in the inferior colliculus 15, 90, and 180 days after the lesion showed a progressive increase in the number of immunoreactive neurons, with a parallel increase in the intensity of staining. Two hundred forty days after the cortical lesion, both the number of immunoreactive neurons and the staining intensity had returned to control values. The effects of the cortical lesion on calretinin regulation are more intense in those inferior colliculus subdivisions more densely innervated by the corticocollicular projection. This finding, along with the time course of calretinin regulation suggests that degeneration of the descending projection is linked to calretinin regulation in the inferior colliculus. We hypothesize, based on the role of calretinin, that the observed increase in immunoreactivity levels seen in the inferior colliculus after lesioning of the auditory cortex may be related to altered excitability in deafferented neurons. Our finding, may reflect adaptive mechanisms to changes in calcium influx and excitability in inferior colliculus neurons induced by lesions of the descending projection from the cortex to the inferior colliculus.

Immunohistochemical study on the expression of calcium binding proteins (calbindin-D28k, calretinin, and parvalbumin) in the cerebellum of the nNOS knock-out(-/-) mice

Nitric Oxide (NO) actively participates in the regulation of neuronal intracellular Ca²⁺ levels by modulating the activity of various channels and receptors. To test the possibility that modulation of Ca²⁺ buffer protein expression level by NO participates in this regulatory effect, we examined expression of calbindin-D28k, calretinin, and parvalbumin in the cerebellum of neuronal NO synthase knock-out (nNOS^(-/-)) mice using immunohistochemistry. We observed that in the cerebellar cortex of the nNOS^(-/-) mice, expression of calbindin-D28k and parvalbumin were significantly increased while expression of calretinin was significantly decreased. These results suggest another mechanism by which NO can participate in the regulation of Ca²⁺ homeostasis.

Calretinin gene promoter activity is differently regulated in neurons and cancer cells. Role of AP2-like cis element and zinc ions

Calretinin (CR) is an EF-hand calcium binding protein expressed in a tissue-specific manner. CR gene is active in some neurons, such as cerebellar granule neurons and in some tumor cells such as colon adenocarcinoma or epithelial mesothelioma. Our aim was to understand the mechanism(s) involved in tissue-specific regulation of CR gene transcription. Our recent paper showed that an "AP2-like" sequence located between -90/-80 bp of the CR gene promoter is important for specific expression of this gene in neurons differentiating from DEV multipotent cells [Biochim. Biophys. Acta (1577) 2000 412]. We now show by electrophoretic mobility shift assay (EMSA) that in cerebellar granule neurons, there is a nuclear protein that interacts with the "AP2-like" sequence. This interaction results in an increased CR gene promoter activity. In contrast, the "AP2-like" sequence does not play such a role in regulation of the CR gene transcription in adenocarcinoma and mesothelioma cancer cells. In these cells mutation in the "AP2-like" element did not affect transcriptional activity of the CR gene promoter and no protein binding to this element was observed by EMSA. These data clearly show that expression of CR gene in neurons and cancer cells is regulated by different mechanisms. On the other hand, in both cell types, the activity of CR gene promoter can be stimulated by zinc ions.

'New' functions for 'old' proteins: the role of the calcium-binding proteins calbindin D-28k, calretinin and parvalbumin, in cerebellar physiology. Studies with knockout mice

Calretinin (CR), calbindin D-28k (CB) and parvalbumin (PV) belong to the large family of EF-hand calcium-binding proteins, which comprises more than 200 members in man. Structurally these proteins are characterized by the presence of a variable number of evolutionary well-conserved helix-loop-helix motives, which bind Ca2+ ions with high affinity. Functionally, they fall into two groups: by interaction with target proteins, calcium sensors translate calcium concentrations into signaling cascades, whereas calcium buffers are thought to modify the spatiotemporal aspects of calcium transients. Although CR, CB and PV are currently being considered calcium buffers, this may change as we learn more about their biology. Remarkable differences in their biophysical properties have led to the distinction of fast and slow buffers and suggested functional specificity of individual calcium buffers. Evaluation of the physiological roles of CR, CB and PV has been facilitated by the recent generation of mouse strains deficient in these proteins. Here, we review the biology of these calcium-binding proteins with distinct reference to the cerebellum, since they are particularly enriched in specific cerebellar neurons. CR is principally expressed in granule cells and their parallel fibres, while PV and CB are present throughout the axon, soma, dendrites and spines of Purkinje cells. PV is additionally found in a subpopulation of inhibitory interneurons, the stellate and basket cells. Studies on deficient mice together with in vitro work and their unique cell type-specific distribution in the cerebellum suggest that these calcium-binding proteins have evolved as functionally distinct, physiologically relevant modulators of intracellular calcium transients. Analysis of different brain regions suggests that these proteins are involved in regulating calcium pools critical for synaptic plasticity. Surprisingly, a major role of any of these three calcium-binding proteins as an endogenous neuroprotectant is not generally supported.

AP2-like cis element is required for calretinin gene promoter activity in cells of neuronal phenotype differentiated from multipotent human cell line DEV

Calretinin (CR) is an EF-hand calcium binding protein expressed at high level in neurons. To identify regulatory elements in CR gene promoter, cultured rat cortical cells were transfected with constructs containing its 5'-end deletion mutants and the luciferase reporter gene. A fragment ending at -115 bp upstream of the transcription start site had high promoter activity and was able to induce expression of luciferase specifically in neuronal cells of cortical cultures. The wild type sequence of -115/+54 CR promoter fragment preferentially drove the expression of green fluorescent protein analog in cells of neuronal phenotype differentiated from multipotent human cell line DEV. Electrophoretic mobility shift assays (EMSA) revealed that the -115/-71 CR gene promoter region contains a binding site for a factor present in brain nuclear extract. Among oligonucleotides containing consensus binding sites for transcription factors within this region, the one representing AP2 binding site was able to compete formation of a protein complex. Mutations of this site prevented the binding between brain protein(s) and the -115/+54 CR gene promoter region and abolished the preferential expression of reporter gene in neuronal cells of DEV line. Thus, the AP2-like element seems to be essential for the neuron-specific activity of the CR gene promoter.

Calretinin and calbindin D-28k, but not parvalbumin protect against glutamate-induced delayed excitotoxicity in transfected N18-RE 105 neuroblastoma-retina hybrid cells

Excitotoxic effects leading to neuronal cell degeneration are often accompanied by a prolonged increase in the intracellular level of Ca(2+) ions and L-glutamate-induced toxicity is assumed to be mediated via a Ca(2+)-dependent mechanism. Due to their buffering properties, EF-hand Ca(2+)-binding proteins (CaBPs) can affect intracellular Ca(2+) homeostasis and a neuroprotective role has been attributed to some of the family members including calretinin, calbindin D-28k and parvalbumin. We have stably transfected N18-RE 105 neuroblastoma-retina hybrid cells with the cDNAs for the three CaBPs and investigated the effect of these proteins on the L-glutamate-induced, Ca(2+)-dependent cytotoxicity. Several clones for each CaBP were selected according to immunocytochemical staining and characterization of the overexpressed proteins by Western blot analysis. In calretinin- and parvalbumin-expressing clones, expression levels were quantitatively determined by ELISA techniques. Cytotoxicity of transfected clones was quantified by measurement of the activity of lactate dehydrogenase (LDH) that was released into the medium after L-glutamate (10 mM) exposure as a result of necrotic cell death. In untransfected and parvalbumin-transfected cells, LDH released into the medium progressively increased (starting from the 20th hour) reaching maximum levels after 28-30 h of glutamate application. In contrast, LDH release in both, calretinin and calbindin D-28k-transfected clones, was not significantly different from unstimulated transfected or untransfected cells over the same period of time. The results indicate that the 'fast' Ca(2+)-buffers calretinin and calbindin D-28k, but not the 'slow' buffer parvalbumin can protect N18-RE 105 cells from this type of Ca(2+)-dependent L-glutamate-induced delayed cytotoxicity.

Monoclonal antibodies recognizing epitopes of calretinins: dependence on Ca2+-binding status and differences in antigen accessibility in colon cancer cells

Monoclonal antibodies are very helpful tools to investigate the localization and sometimes even the function of specific proteins in cells and tissues. By generating monoclonal antibodies against calretinin-22k (CR-22k), a C-terminally truncated isoform of calretinin (CR) as a result of alternative splicing of the CR mRNA, we envisaged that screening multiple monoclonal antibodies would allow the identification of CR-22k as well as CR. Both proteins share the first 178 amino acids, but have different C-termini. All three antibodies 10C10, 6B3 and 2H4 recognize recombinant CR-22k and the specificity to also recognize CR was demonstrated in brain extracts of different species and human tumour cells, which express CR. All monoclonal antibodies did not crossreact with the closely related protein calbindin D-28k. Antibody binding was depending on the Ca2+-binding status of both forms of calretinin. Generally, the Ca2+-bound form was better recognized than the Ca2+-free form. Carboxy- and amino-terminally truncated CR proteins were expressed in E. coli in order to characterize the epitopes recognized by the three antibodies. Additionally, tryptic and cyanogen bromide fragments were produced to further narrow down the sequences recognized by the three antibodies. 10C10 recognizes an epitope consisting of the linker region between EF-hand domains I and II and the N-terminal part of EF-hand II, while the others (6B3, 2H4) bind to a region including the linker between EF-hand domains III and IV. These antibodies are valuable tools to further investigate the distribution and eventually the specific function of these two proteins in the nervous tissue and under pathological conditions, e.g. in colon tumours and mesotheliomas.

Calretinin and calbindin D-28k delay the onset of cell death after excitotoxic stimulation in transfected P19 cells

In some neurological diseases, injury to neurones reflects an over-stimulation of their receptors for excitatory amino acids. This response may disturb the Ca(2+)-homeostasis and lead to a pronounced and sustained increase in the intracellular concentration of this ion. On the basis of data derived from correlative studies, calcium-binding proteins have been postulated to play a protective role in these pathologies. We tested, directly, the capacity of the three calcium-binding proteins calretinin (CR), calbindin D-28k (CB) and parvalbumin (PV) to buffer [Ca(2+)], and to protect cells against excitotoxic death. We used P19 murine embryonic carcinoma cells, which can be specifically induced (by retinoic acid) to transform into nerve-like ones. The differentiated cells express functional glutamate-receptors and are susceptible to excitotoxic shock. Undifferentiated P19-cells were stably transfected with the cDNA for CR, CB or PV, induced to differentiate, and then exposed to NMDA, a glutamate-receptor agonist. The survival rates of clones expressing CR, CB or PV were compared with those of untransfected P19-cells using the lactate-dehydrogenase assay. CR- and CB-expressing cells were protected from death during the first 2 h of exposure to NMDA. This protection was, however, transient, and did not suffice to rescue P19-cells after prolonged stimulation. Two of the three PV-transfected clones raised were vulnerable to NMDA-induced excitotoxicity; the third, which expressed the lowest level of PV, was protected to a similar degree as that found for the CR- and CB-transfected clones. Our results indicate that in the P19-cell model, CR and CB can help to delay the onset of cell death after excitotoxic stimulation.

Calretinin modifies presynaptic calcium signaling in frog saccular hair cells

To determine whether the concentrations of calcium-binding proteins present in some neurons and sensory cells are sufficient to influence presynaptic calcium signaling, we studied the predominant calcium-binding protein in a class of sensory hair cells in the frog ear. Based on antibody affinity and molecular weight, we identified this protein as calretinin. We measured its cytoplasmic concentration to be approximately 1.2 mM, sufficient to bind approximately 6 mM Ca2+. Calcium signaling was altered when the diffusible cytoplasmic components were replaced by an intracellular solution lacking any fast calcium buffer, and was restored by the addition of 1.2 mM exogenous calretinin to the intracellular solution. We conclude that calretinin, when present at millimolar concentration, can serve as a diffusionally mobile calcium buffer/transporter capable of regulating calcium signaling over nanometer distances at presynaptic sites.

Calretinin-containing axons and neurons are resistant to an intrastriatal 6-hydroxydopamine lesion

Relative preservation of dopaminergic axons in patches and a subcallosal layer was observed in the dorsal, lateral and caudal striatum 4 weeks after intrastriatal injection of 6-hydroxydopamine (6-OHDA), a neurotoxin selective for catecholaminergic neurons. Since calcium binding proteins are reported to provide neuroprotective influence in neurons, differences in the distribution of the calcium binding proteins might be related to the different vulnerabilities of dopaminergic neurons and axons to neurotoxins. To address this possibility, we characterized patches of relatively dense tyrosine hydroxylase-immunoreactive (TH-IR) axons in intrastriatal 6-OHDA lesioned rats, focusing on two calcium binding proteins, calbindin (CB) and calretinin (CR). The patches and subcallosal layer of preserved dopaminergic axons in the striatum of rats lesioned with 6-OHDA contained CR, a 31-kDa calcium-binding protein, but interestingly not CB. Dopaminergic neurons containing CR in the substantia nigra pars compacta (SNpc) were relatively spared compared to those that did not contain CR. Taken together, our data indicate that dopaminergic axons and neurons containing CR in the nigrostriatal pathway are more resistant to 6-OHDA lesion than those that do not contain CR.

Green fluorescent protein as a quantitative tool

Manipulating the expression of a protein can provide a powerful tool for understanding its function, provided that the protein is expressed at physiologically-significant concentrations. We have developed a simple method to measure (1) the concentration of an overexpressed protein in single cells and (2) the covariation of particular physiological properties with a protein's expression. As an example of how this method can be used, teratocarcinoma cells were transfected with the neuron-specific calcium binding protein calretinin (CR) tagged with green fluorescent protein (GFP). By measuring GFP fluorescence in microcapillaries, we created a standard curve for GFP fluorescence that permitted quantification of CR concentrations in individual cells. Fura-2 measurements in the same cells showed a strong positive correlation between CR-GFP fusion protein expression levels and calcium clearance capacity. This method should allow reliable quantitative analysis of GFP fusion protein expression.

Measurements of [Ca2+] using fura-2 in glioma C6 cells expressing calretinin with GFP as a marker of transfection: no Ca2+-buffering provided by calretinin

Glioma C6 cells were transfected with a plasmid containing the calretinin (CR) and green fluorescent protein (GFP) coding regions to analyze the effect of CR's presence on [Ca2+]i. Positive transfectants were identified by the detection of GFP and [Ca2+]i was measured using fura-2 as a probe. We found that neither the basic [Ca2+]i nor activated [Ca2+]i achieved by exposure to ionomycin, ADP or thapsigargin were affected by CR's presence in transfected cells, despite the ability of CR to bind Ca2+ as part of fusion protein. The level of expressed CR was estimated as at least 1 microM. The presented results suggest that CR's function is unlikely to be an intracellular Ca2+-buffer and support the hypothesis that CR might be involved in a specific Ca2+-dependent process. The results of this work also show that the S65T mutant of GFP is compatible with fura-2 measurements of intracellular [Ca2+]. We have demonstrated that the presence of GFP, as a transfection marker of glioma C6 cells, does not disturb fura-2 fluorescence, the basal or activated [Ca2+]i in these cells.

Expression of calbindin-D28k in C6 glial cells stabilizes intracellular calcium levels and protects against apoptosis induced by calcium ionophore and amyloid beta-peptide

The calcium binding protein, calbindin-D28k is normally present in neurons. Recently we reported that brain injury and tumor necrosis factors (TNFs) induce calbindin-D28k in astrocytes. TNF-treated calbindin expressing astrocytes were resistant to acidosis and calcium ionophore toxicity, suggesting that calbindin may have a cytoprotective role in astrocytes in the injured brain (M.P. Mattson, B. Cheng, S.A. Baldwin, V.L. Smith-Swintosky, J. Keller, J. Geddes, Scheff, J.W., Christakos, S., Brain injury and tumor necrosis factors induce calbindin-D28k in astrocytes: evidence for a cytoprotective response, J. Neurosci. Res., 42 (1995) 257). In order to obtain direct evidence for a role of calbindin, using the eukaryotic expression vector pREP4, rat calbindin-D28k was stably expressed in C6 rat astocytoma glial cells. Cytotoxicity in response to calcium ionophore or amyloid beta-peptide (which accumulates in the brain in Alzheimer's disease and has been reported to be neurotoxic) was measured by MTT reduction in vector transfected cells and in calbindin transfected clones. Stably expressed calbindin resulted in increased cell survival in the presence of calcium ionophore (1-10 microM) or amyloid beta-peptide (10-100 microM). In addition, the calcium ionophore or amyloid beta-peptide mediated rise in intracellular calcium in vector transfected cells was significantly attenuated in calbindin expressing cells. Apoptotic cell death was detected by the Hoechst method in vector transfected C6 glial cells treated with calcium ionophore or beta-amyloid (34-36% apoptotic cells/culture). The number of apoptotic nuclei was significantly attenuated in similarly treated calbindin-D28k transfected clones (10-13% apoptotic cells/culture; p<0.01). Our results support the involvement of calcium fluxes in apoptosis and suggest that calbindin-D28k, by buffering calcium, can suppress death in apoptosis susceptible cells in the central nervous system.

Calretinin immunoreactivity in the developing thalamus of the rat: a marker of early generated thalamic cells

The present work was aimed to study the immunocytochemical localization of the calcium-binding protein, calretinin, in the rat thalamus from embryonic day 14 to the third postnatal week. In the adult rat thalamus, calretinin immunoreactivity is intensely expressed in some intralaminar and midline nuclei, as well as in selected regions of the reticular nucleus. At embryonic day 14, calretinin was expressed by immature and migrating neurons and fibres laterally to the neuroepithelium of the diencephalic vesicle in the region identified as reticular neuroepithelium. At embryonic day 16, immunoreactive neurons were present in the primordium of the reticular nucleus and in the region of the reticular thalamic migration, where neurons showed the morphology of migratory cells. At the end of embryonic development and in the first postnatal week, calretinin-positive neurons were observed in selected region of the reticular nucleus and it was intensely expressed in some intralaminar and midline nuclei. Bands of immunopositive fibres were also observed crossing the thalamus. During the second postnatal week, the immunolabelling in the reuniens, rhomboid, paraventricular and central medial thalamic nuclei remains very intense while a decrease of immunoreactivity in mediodorsal, centrolateral and laterodorsal nuclei was observed. The immunostaining of fibres, particularly evident in the perinatal period, progressively decreased and it was no longer visible by the end of the second postnatal week when the distribution and intensity of calretinin immunostaining was similar to that observed in the adult rat thalamus. The present findings indicate that the immunolocalization of calretinin can be used to identify subsets of thalamic neuronal population during pre- and postnatal maturation allowing also the detection of the migratory pattern of early generated reticular thalamic neurons.