Matylda Macias

Functional anatomy of neural circuits regulating fear and extinction

The memory of fear extinction is context dependent: fear that is suppressed in one context readily renews in another. Understanding of the underlying neuronal circuits is, therefore, of considerable clinical relevance for anxiety disorders. Prefrontal cortical and hippocampal inputs to the amygdala have recently been shown to regulate the retrieval of fear memories, but the cellular organization of these projections remains unclear. By using anterograde tracing in a transgenic rat in which neurons express a dendritically-targeted PSD-95:Venus fusion protein under the control of a c-fos promoter, we found that, during the retrieval of extinction memory, the dominant input to active neurons in the lateral amygdala was from the infralimbic cortex, whereas the retrieval of fear memory was associated with greater hippocampal and prelimbic inputs. This pattern of retrieval-related afferent input was absent in the central nucleus of the amygdala. Our data show functional anatomy of neural circuits regulating fear and extinction, providing a framework for therapeutic manipulations of these circuits.

Long-Term Locomotor Training Up-Regulates TrkBFL Receptor-like Proteins, Brain-Derived Neurotrophic Factor, and Neurotrophin 4 with Different Topographies of Expression in Oligodendroglia and Neurons in the Spinal Cord

Neurotrophins are potent regulators of neuronal survival, maintenance, and synaptic strength. In particular, brain-derived neurotrophic factor (BDNF), acting through full-length TrkB receptor (TrkB(FL)), is implicated in the stimulation of neurotransmission. Physical activity has been reported to increase BDNF expression in the brain and spinal cord. In this study we have evaluated the hypothesis that activation of a spinal neuronal network, due to exercise, affects the entire spinal neurotrophin system acting via TrkB receptors by modulation of BDNF, neurotrophin 4 (NT-4), and their TrkB receptor proteins. We investigated the effect of treadmill walking (4 weeks, 1 km daily) on distribution patterns and response intensity of these proteins in the lumbar spinal cord of adult rats. Training enhanced immunoreactivity (IR) of both neurotrophins. BDNF IR increased in cell processes of spinal gray matter, mainly in dendrites. NT-4 IR was augmented in the white matter fibers, which were, in part, of astrocytic identity. Training strongly increased both staining intensity and number of TrkB(FL)-like IR small cells of the spinal gray matter. The majority of these small cells were oligodendrocytes, representing both their precursor and their mature forms. In contrast, training did not exert an effect on expression of the truncated form of TrkB receptor in the spinal cord. These results show that both neuronal and nonneuronal cells may be actively recruited to BDNF/NT-4/TrkB(FL) neurotrophin signaling which can be up-regulated by training. Oligodendrocytes of the spinal gray matter were particularly responsive to exercise, pointing to their involvement in activity-driven cross talk between neurons and glia.

Locomotor exercise alters expression of pro‐brain‐derived neurotrophic factor, brain‐derived neurotrophic factor and its receptor TrkB in the spinal cord of adult rats

Previous evidence indicates that locomotor exercise is a powerful means of increasing brain‐derived neurotrophic factor (BDNF) and its signal transduction receptor TrkB mRNA levels, immunolabeling intensity and number of BDNF‐ and TrkB‐immunopositive cells in the spinal cord of adult rats but the contribution of specific cell types to changes resulting from long‐term activity is unknown. As changes in BDNF protein distribution due to systemic stimuli may reflect either its in‐situ synthesis or its translocation from other sources, we investigated where BDNF and TrkB mRNA are expressed in the spinal lumbar segments. We report on the cell types defined by size, BDNF mRNA levels and number of cells with TrkB transcripts in sedentary and exercised animals following 28 days of treadmill walking. In the majority of cells, exercise increased perikaryonal levels of BDNF mRNA but did not affect TrkB transcript levels. Bidirectional changes in a number of TrkB mRNA‐expressing cells occurred in small groups of ventral horn neurons. An increase in BDNF transcripts was translated into changes in pro‐BDNF and BDNF levels. A 7‐day walking regimen increased BDNF protein levels similarly to 28‐day treadmill walking. Our observations indicate that long‐ and short‐term locomotor activity of moderate intensity produce stimuli sufficient to recruit a majority of spinal cells to increased BDNF synthesis, suggesting that continuous tuning of pro‐BDNF and BDNF levels permits spinal networks to undergo trophic modulation not requiring changes in TrkB mRNA supply.

Bcl-2 and Bax proteins are increased in neocortical but not in thalamic apoptosis following devascularizing lesion of the cerebral cortex in the rat: an immunohistochemical study.

The hypothesis that devascularization of somatosensory and motor cortex causes apoptosis in infarcted regions and in the linked thalamic nuclei was evaluated. To unravel whether Bcl-related proteins, known to regulate apoptosis, participate in neuronal and glial responses to devascularization, we analyzed immunohistochemically the distribution and intensity of staining of Bcl-2 and Bax proteins at different time points after lesion. Both early (up to 6 h) and late (1-7 days) responses were studied. Devascularization led to rapid (within hours) apoptosis in the cortex and to a delayed (within 3-7 days) apoptosis in thalamic nuclei. In control groups, Bcl-2 and Bax immunoreactivity (IR) was detected in neurons and oligodendrocytes but not in astrocytes or microglia. Following devascularization, Bcl-2 IR and Bax IR increased in neurons before the onset of the apoptosis. In the ischemic focus, the increase reached maximal values 3 h after the lesion. The increase was of slower onset in the penumbra zone (24 h and after), a region in which both proteins were induced in astrocytes also. The change of Bax IR intensity exceeded four times that of Bcl-2 at all time points investigated, indicating a diminution of Bcl-2/Bax ratio that may direct neurons to apoptotic pathway. In numerous neurons, an increase of IR in the cytoplasm was accompanied by induction of nuclear staining. No changes of Bcl-2 and Bax IR were found in thalamic nuclei. Our results point to different mechanisms underlying apoptosis of cortical and thalamic neurons. Nuclear appearance of Bcl-2 and Bax suggests they possess regulatory role of gene expression changes triggered by cortical infarct.