Somatostatin receptors (SSTR1-5) on inhibitory interneurons in the barrel cortex.

Inhibitory interneurons in the cerebral cortex contain specific proteins or peptides characteristic for a certain interneuron subtype. In mice, three biochemical markers constitute non-overlapping interneuron populations, which account for 80-90% of all inhibitory cells. These interneurons express parvalbumin (PV), somatostatin (SST), or vasoactive intestinal peptide (VIP). SST is not only a marker of a specific interneuron subtype, but also an important neuropeptide that participates in numerous biochemical and signalling pathways in the brain via somatostatin receptors (SSTR1-5). In the nervous system, SST acts as a neuromodulator and neurotransmitter affecting, among others, memory, learning, and mood. In the sensory cortex, the co-localisation of GABA and SST is found in approximately 30% of interneurons. Considering the importance of interactions between inhibitory interneurons in cortical plasticity and the possible GABA and SST co-release, it seems important to investigate the localisation of different SSTRs on cortical interneurons. Here, we examined the distribution of SSTR1-5 on barrel cortex interneurons containing PV, SST, or VIP. Immunofluorescent staining using specific antibodies was performed on brain sections from transgenic mice that expressed red fluorescence in one specific interneuron subtype (PV-Ai14, SST-Ai14, and VIP-Ai14 mice). SSTRs expression on PV, SST, and VIP interneurons varied among the cortical layers and we found two patterns of SSTRs distribution in L4 of barrel cortex. We also demonstrated that, in contrast to other interneurons, PV cells did not express SSTR2, but expressed other SSTRs. SST interneurons, which were not found to make chemical synapses among themselves, expressed all five SSTR subtypes.

Somatostatin receptors in the brain.

Somatostatin is a peptide that participates in numerous biochemical and signaling pathways. It functions via receptors (SSTRs1-5), which belong to the family of receptors coupled with protein G. All somatostatin receptors are characterized by a certain degree of homology in molecular structure. The cell effects of their agonists in peripheral tissues rely mainly on the inhibition of the hormones release. Somatostatin is also an important neuromodulator and neurotransmitter. SSTRs may affect other receptors, forming structural and functional homodimers and heterodimers. SSTRs play also role in the regulation of physiological processes, such as itching and pain, reproductive functions, regulation of feeding or mood. Besides physiological functions, SSTRs contribute also to the pathogenesis of glial tumors, neurodegenerative diseases, or post hemorrhagic stroke changes. Recent years of research have provided new data regarding the role of somatostatin receptor signaling pathways in the brain and the knowledge in this field is developing rapidly.

Inhibition of Tnf-α R1 signaling can rescue functional cortical plasticity impaired in early post-stroke period.

Tumor necrosis factor-α (TNF-α) is one of the key players in stroke progression and can interfere with brain functioning. We previously documented an impairment of experience-dependent plasticity in the cortex neighboring the stroke-induced lesion, which was accompanied with an upregulation of Tnf-α level in the brain of ischemic mice 1 week after the stroke. Because TNF receptor 1 (TnfR1) signaling is believed to be a major mediator of the cytotoxicity of Tnf-α through activation of caspases, we used an anti-inflammatory intervention aimed at Tnf-α R1 pathway, in order to try to attenuate the detrimental effect of post-stroke inflammation, and investigated if this will be effective in protecting plasticity in the infarct proximity. Aged mice (12-14 months) were subjected to the photothrombotic stroke localized near somatosensory cortex, and immediately after ischemia sensory deprivation was introduced to induce plasticity. Soluble TNF-α R1 (sTNF-α R1), which competed for TNF-α with receptors localized in the brain, was delivered chronically directly into the brain tissue for the whole period of deprivation using ALZET Micro-Osmotic pumps. We have shown that such approach undertaken simultaneously with the stroke reduced the level of TNF-α in the peri-ischemic tissue and was successful in preserving the post-stroke deprivation-induced brain plasticity.

Npas4 expression in two experimental models of the barrel cortex plasticity.

Npas4 has recently been identified as an important factor in brain plasticity, particularly in mechanisms of inhibitory control. Little is known about Npas4 expression in terms of cortical plasticity. In the present study expressions of Npas4 and the archetypal immediate early gene (IEG) c-Fos were investigated in the barrel cortex of mice after sensory deprivation (sparing one row of whiskers for 7 days) or sensory conditioning (pairing stimulation of one row of whiskers with aversive stimulus). Laser microdissection of individual barrel rows allowed for analysis of IEGs expression precisely in deprived and nondeprived barrels (in deprivation study) or stimulated and nonstimulated barrels (in conditioning study). Cortex activation by sensory conditioning was found to upregulate the expression of both Npas4 and c-Fos. Reorganization of cortical circuits triggered by removal of selected rows of whiskers strongly affected c-Fos but not Npas4 expression. We hypothesize that increased inhibitory synaptogenesis observed previously after conditioning may be mediated by Npas4 expression.

Experience-dependent plasticity of the barrel cortex in mice observed with 2-DG brain mapping and c-Fos: effects of MMP-9 KO.

Modifications of properties of the adult sensory cortex by elimination of sensory input (deprivation) serves as a model for studying plasticity in the adult brain. We studied the effects of short- and long-term deprivation (sparing one row of vibrissae) upon the barrel cortex. The response to stimulation (exploration of a new environment) of the spared row was examined with [14C]-2-deoxyglucose autoradiography and c-Fos immunohistochemistry. Both methods found large increases of the functional cortical representation of the spared row of vibrissae, extending into parts of the barrel cortex previously activated by the deprived vibrissae. With both methods, the greatest expansion of spared input was observed in cortical layer IV. In this way, we established a model, which was applied for examining involvement of matrix metalloproteinase 9 (MMP-9), upon experience-dependent cortical plasticity. MMP-9 is an enzyme implicated in plastic modification of the neuronal connections. We found that MMP-9 activity was increased in response to stimulation, and furthermore, MMP-9 knockout mice showed a modest but significant decrease of plasticity in layer IV with 2-DG mapping and in layers II/III with c-Fos mapping. Thus, in adult mouse brain experience-dependent plasticity is in part supported by the activity of MMP-9.

Increased excitability of cortical neurons induced by associative learning: an ex vivo study.

In adult mice, classical conditioning in which whisker stimulation is paired with an electric shock to the tail results in a decrease in the frequency of head movements, induces expansion of the cortical representation of stimulated vibrissae and enhances inhibitory synaptic interactions within the 'trained' barrels. We investigated whether such a simple associative learning paradigm also induced changes in neuronal excitability. Using whole-cell recordings from ex vivo slices of the barrel cortex we found that layer IV excitatory cells located in the cortical representation of the 'trained' row of vibrissae had a higher frequency of spikes recorded at threshold potential than neurons from the 'untrained' row and than cells from control animals. Additionally, excitatory cells within the 'trained' barrels were characterized by increased gain of the input-output function, lower amplitudes of fast after-hyperpolarization and decreased effect of blocking of BK channels by iberiotoxin. These findings provide new insight into the possible mechanism for enhanced intrinsic excitability of layer IV excitatory neurons. In contrast, the fast spiking inhibitory cells recorded in the same barrels did not change their intrinsic excitability after the conditioning procedure. The increased excitability of excitatory neurons within the 'trained' barrels may represent the counterpart of homeostatic plasticity, which parallels enhanced synaptic inhibition described previously. Together, the two mechanisms would contribute to increase the input selectivity within the conditioned cortical network.

Involvement of retrosplenial cortex in classical conditioning.

The cingulate cortex, which comprises of two major subdivisions - anterior cingulate cortex (CG) and retrosplenial cortex (RSP), is implicated in many cognitive functions. The RSP is an important node in the systemic integration network. Studies point to its role in learning that involves spatial stimuli and navigation. Relatively little is known about its involvement in simple learning such as classical conditioning. We examined the involvement of the two cytoarchitectonic divisions, agranular and granular, of the rostral and caudal RSP in a delay conditioning, where stimulation of the facial vibrissae was paired with a tail shock. During the conditioning session the [(14)C]-2-deoxyglucose (2DG) brain mapping was performed. Effectiveness of conditioning was assessed with frequency of head movements, which decreased in the course of the conditioning. 2DG uptake in RSP and additionally in CG was examined in conditioned, pseudoconditioned and stimulated control groups. The metabolic labeling was elevated in caudal and rostral both RSP and CG in the conditioned group, but not in animals which received CS or UCS alone. Comparison between conditioned and pseudoconditioned groups showed the specific activation by associative learning in both divisions of the rostral RSP and rostral CG. Counts of c-Fos expressing nuclei confirmed activation of the rostral RSP in the CS+UCS group. These data support the concept of RSP as structure that, besides its recognized role in visuospatial learning, monitors and reacts to activity of brain systems responsible for other types of learning and, together with CG, subserve cognitive processes, with simple associative learning among them.

Transplantation of a novel human cord blood-derived neural-like stem cell line in a rat model of cortical infarct.

Umbilical cord blood can be a rich source of stem/progenitor cells, not only for hematopoetic but also for other tissue-specific lineages. Recently, we have developed a novel, self-renewed neural-like stem cell line named HUCB-NSC from human cord blood. To test if HUCB-NSCs can supply brain in need of regeneration, we injected these cells into immunosuppressed intact rat forebrain and to animals suffering from a photothrombotic cortical lesion at 48 h after injury. The survival, migration, and differentiation of the transplanted HUCB-NSCs were measured at 7 and 30 days post-transplantation by immunohistochemical methods. Results show survival and extensive migration of transplanted neural-like progenitors into damaged brain cortex during the first week of post-stroke recovery. The donor cells accumulated mainly in peri-infarct area and then differentiated showing a strong co-expression of neuronal (NF-200) but only moderate of astrocytic (GFAP) cell markers. However, the paucity of HUCB-NSCs detected within post-ischemic rat brain at the end of a 1 month period, as well as acute rejection of grafted cells by intact, yet cyclosporin A (CsA) immunosuppressed, rat brain tissue, suggests development of a severe adverse host reaction to the presence of alien donor cells and an urgent need for further study of the immunological response evoked by xenotransplantations of human cord blood-derived cells in animal experimental models.

[Estrogens and synaptic plasticity]. / Estrogeny i plastycznosc synapsy.

Estrogens influence morphology of the brain not only in structures linked to reproductive cycle and reproductive behavior but also structure engaged in memory and cognitive functions. Estrogens stimulate synaptogenesis in pyramidal neurons of CA1 field of hippocampus. Increase in the number of spines on apical dendrites in rats occurs in the prostures phase of the cycle as well as exogenous estradiol application in ovariectomized females. The new synapses are enriched in NMDA receptor and it was found that their generation involves activation of NMDA receptors, PKA and CREB. Estradiol-induced synaptogenesis is accompanied by facilitation of LTP induction. Estradiol affects pyramidal cells of CA1 probably by inhibiting GABA-ergic interneurons. It also modulates unspecific activatory systems, which contribute significantly to neuroplasticity.