Three-dimensional quantification of mossy-fiber presynaptic boutons in living hippocampal slices using confocal microscopy

Retrograde axonal degeneration "dieback" in the corticospinal tract after transection injury of the rat spinal cord: a confocal microscopy study

Axonal dieback is a process in which axons in spinal tracts retract away from the initial site of injury. The purpose of this project is to study the dynamics of dieback in corticospinal tract (CST) axons after various time intervals post-injury, to find the optimal spatial-temporal window for regenerative treatment. Rats received transection injuries at the T8 spinal level and were sacrificed at different time periods (1, 2, 4, 8, and 16 weeks). Three weeks prior to sacrifice, DiI crystals were implanted in the sensorimotor cortex and produced excellent CST labeling, and clear delineation of the terminal bulbs of transected axons. With DiI and confocal microscopy, we visualized axons along the entire length of the CST, and quantified the temporal and spatial features of dieback in axons of the CST based on the location of the terminal bulbs. We found that the majority of axons stopped dieing back 4 weeks after injury by which time they were approximately 2.5 mm from the site of injury. However, at 8 and 16 weeks after injury, some terminal bulbs were more than 10 and 19 mm, respectively, from the site of injury.

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.

Dual mechanism for presynaptic modulation by axonal metabotropic glutamate receptor at the mouse mossy fibre-CA3 synapse

1. To investigate mechanisms responsible for the presynaptic inhibitory action mediated by the axonal group II metabotropic glutamate receptor (mGluR) at the mossy fibre-CA3 synapse, we used a quantitative fluorescence measurement of presynaptic Ca2+ in mouse hippocampal slices. 2. Bath application of the group II mGluR-specific agonist (2S,1'R,2'R,3'R)-2-(2, 3-dicarboxycyclopropyl)glycine (DCG-IV, 1 microM) reversibly suppressed the presynaptic Ca2+ influx (to 55.2 +/- 4.6 % of control, n = 5) as well as field EPSPs recorded simultaneously (to 3.1 +/- 2.0%). Presynaptic fibre volley was not affected by 1 microM DCG-IV. 3. A quantitative analysis of the inhibition of presynaptic Ca2+ influx and field EPSP suggested that DCG-IV suppressed the field EPSP to a greater extent than would be expected if the suppression were solely due to a decrease in the presynaptic Ca2+ influx. 4. DCG-IV at 1 microM suppressed the mean frequency (to 73.8 +/- 3.9% of control, n = 11), but not the mean amplitude (to 97.0 +/- 3.5%), of miniature EPSCs recorded from CA3 neurones using the whole-cell patch-clamp technique. 5. These results suggest that group II mGluR-mediated suppression is due both to a reduction of presynaptic Ca2+ influx and downregulation of the subsequent exocytotic machinery.

Complex synaptic current waveforms evoked in hippocampal pyramidal neurons by extracellular stimulation of dentate gyrus

Excitatory postsynaptic currents (EPSCs) evoked in hippocampal CA3 pyramidal neurons by extracellular stimulation of the dentate gyrus typically exhibit complex waveforms. They commonly have inflections or notches on the rising phase; the decay phase may exhibit notches or other obvious departures from a simple monoexponential decline; they often display considerable variability in the latency from stimulation to the peak current; and the rise times tend to be long. One hypothesis is that these complex EPSC waveforms might result from excitation via other CA3 pyramidal cells that were recruited antidromically or trans-synaptically by the stimulus due to the complex anatomy of this region. An alternative hypothesis is that EPSC complexity does not emerge from the functional anatomy but rather reflects an unusual physiological property, intrinsic to excitation-secretion coupling in mossy-fiber (mf) synaptic terminals, that causes asynchronous quantal release. We evaluated certain predictions of our anatomic hypothesis by adding a pharmacological agent to the normal bathing medium that should suppress di- or polysynaptic responses. For this purpose we used baclofen (3 microM), a selective agonist for the gamma-aminobutyric acid B receptor. The idea was that baclophen should discriminate against polysynaptic versus monosynaptic inputs by hyperpolarizing the cells, bringing them further from spike threshold and possibly also through inhibitory presynaptic actions. Whole cell recordings were done from visually preselected CA3 pyramidal neurons and EPSCs were evoked by fine bipolar electrodes positioned into the granule cell layer of the dentate. To the extent that the EPSC complexity reflects di- or polysynaptic responses, we predicted baclofen to reduce the number of notches on the rising and decay phases, reduce the variance in latency to peak of the EPSCs, decrease the amplitudes and rise times of the individual and averaged EPSCs, and increase the apparent failures in evoked EPSCs. All of these predictions were confirmed, in support of the hypothesis that these complex EPSC waveforms commonly reflect di- or polysynaptic responses. We also documented a distinctly different, intermittent, form of EPSC complexity, which also is predicted and easily explained by our anatomic hypothesis. In particular, the results were in accord with the suggestion that stimulation of the dentate gyrus might antidromically stimulate axon collaterals of CA3 neurons that make recurrent synapses onto the recorded cell. We conclude that the overall pattern of results is consistent with expectations based on the functional anatomy. The explanation does not demand a special type of intrinsic asynchronous mechanism for excitation-secretion coupling in the mf synapses.

Intravital imaging of green fluorescent protein using two-photon laser-scanning microscopy

Imaging a fluorophore in a living tissue presents several unique problems. The fluorescence from the labeled cell(s) may be weak, the labeled cells may be buried deep within tissue and the presence of a fluorophore may render the cells photo-sensitive. Two-photon laser-scanning microscopy (TPLSM) offers several advantages in meeting these challenges. We show that TPLSM provides greater sensitivity, better resolution and less photo-bleaching, as compared to confocal laser-scanning microscopy. The dramatically reduced photo-bleaching makes it possible to image cells continuously for long periods of time. Therefore, TPLSM allows a safer and higher-resolution means of imaging living cells labeled with a variety of fluorophores, including green fluorescent protein.

Confocal imaging of dendritic Ca2+ transients in hippocampal brain slices during simultaneous current- and voltage-clamp recording

Changes in the intracellular Ca2+ concentration ([Ca2+]i) within CA1 hippocampal pyramidal neurons were imaged using confocal laser scanning microscopy in conjunction with Ca(2+)-sensitive fluorescent indicators. The imaging was performed in thick hippocampal brain slices while simultaneously measuring or controlling electrical activity with sharp microelectrodes or whole-cell patch-clamp electrodes. The combination of imaging and electrophysiology was essential for interpreting the changes in [Ca2+]i. We compared the increases in [Ca2+]i produced by either of two methods--direct depolarization of the cell via the somatic electrode or high-frequency stimulations of synaptic inputs. The increases in [Ca2+]i in the soma and proximal dendrites caused by both methods were of comparable magnitude and they always decayed within seconds in healthy cells. However, the spatial patterns of distal Ca2+ increases were different. Separate sets of synaptic inputs to the same cell resulted in different spatial patterns of [Ca2+]i transients. We isolated and observed what appeared to be a voltage-independent component of the synaptically mediated [Ca2+]i transients. This work demonstrates that the combination of neurophysiology and simultaneous confocal microscopy is well suited for visualizing and analyzing [Ca2+]i changes within highly localized regions of neurons in thick brain slices. The approach should allow further analysis of the relative contribution of voltage- and agonist-dependent influences on [Ca2+]i within neurons throughout the CNS and it raises the possibility of routinely relating subcellular [Ca2+]i changes to structural and functional modifications.