Faculty Bibliography

Spike timing-dependent modulation of synaptic efficacy is concomitant with changes in dendritic structure (Engert and Bonhoeffer 1999), a process thought to require actin polymerization. Recent studies have indicated both that plasticity-inducing activity modulates spine actin dynamics (Star and Murthy 2002) and that actin assembly is necessary for the maintenance of long-term potentiation (Krucker et al. 2000). We report here that actin dynamics are necessary for spike timing-dependent plasticity (STDP) in visual cortex. Whole-cell recordings were made from L2/3 pyramids and EPSPs were evoked by extracellular stimulation at 0.2 Hz. When the postsynaptic cell spiked less than 25 s before presynaptic activation, LTD was induced (-38+-4%, n=22). When presynaptic stimulation was followed less than 12 ms by a postsynaptic AP, LTP was induced (59+-9%, n=21). However, when 20 muM cytochalasin D, an inhibitor of actin polymerization, was included in the whole-cell pipette, neither LTP (n=4, p>0.4) nor LTD (n=3, p>0.4) could be induced. Latrunculin A, which sequesters monomeric actin, and phalloidin, which stabilizes polymeric actin, were applied intracellularly (10 muM and 100 muM) and prevented induction of LTP (n=3, p>0.6; n=3, p>0.25). These results suggest that dynamic actin turnover is necessary for spike-timing dependent plasticity

Spike timing-dependent plasticity (STDP) has been described in many preparations (Bi and Poo, 2001). A question remains as to whether the mechanisms underlying STDP are similar to those responsible for long-term potentiation (LTP) and depression (LTD) induced by conventional protocols such as high-frequency stimulation. We report here that Ca2+ influx is necessary for STDP induction in rat visual cortical slices. Whole-cell recordings were made from L2/3 pyramids and EPSPs were evoked by extracellular stimulation at 0.2 Hz. When the postsynaptic cell spiked <25ms before presynaptic activation, LTD was induced (-38+-4%,n=22). When presynaptic stimulation was followed <12ms by a postsynaptic AP, LTP was induced (59+-9%,n=21). When 5 mM BAPTA was included in the whole-cell pipette, neither LTP (n=4, p>0.8) nor LTD (n=4, p>0.4) could be induced. Ca2+ influx seems to come from two sources, NMDA receptors and L-type Ca2+ channels. Blockade of NMDA receptors with bath application of 100 muM APV prevented induction of both LTP (n=6, p>0.6) and LTD (n=5, p>0.8). Likewise, when L-type Ca2+ channels were blocked with 10 muM nimodipine, LTP (n=5, p>0.2) and LTD (n=4, p>0.5) were both prevented. These experiments suggest that induction of STDP in the visual cortex requires postsynaptic Ca2+ influx, similar to traditional protocols for induction of LTP and LTD

Recent studies in visual cortex have examined the effects of complex spike trains using a spike timing-dependent plasticity (STDP) induction protocol (Sjostrom et al. 2001, Froemke and Dan 2002). Here we report that the effect of a second postsynaptic action potential (AP) on STDP is suppressed by the preceding postsynaptic AP, and that suppression is altered when A-type K+ channels are blocked. Whole-cell recordings were made from L2/3 in slices of the rat visual cortex. EPSPs were evoked by extracellular stimulation at 0.2 Hz. Under control conditions, when the postsynaptic cell spiked <25 ms before presynaptic activation, long-term depression (LTD) was induced (-38+-4%, n=22). When presynaptic stimulation was followed <12 ms by a postsynaptic AP, long-term potentiation (LTP) was induced (59+-9%, n=21). However, when the postsynaptic cell spiked both <25 ms before and <10 ms after presynaptic activation, LTD was induced (-33+-7%, n=14). This contrasts with the expected LTP that should result assuming independent contributions of both AP pairs, indicating that the first AP pair is suppressing the effect of the second. To determine if postsynaptic suppression was due to activation of dendritic A-type K+ channels, we introduced 4-AP into the postsynaptic cell via the patch electrode. STDP was intact after postsynaptic K+ channel block, but when the postsynaptic cell fired both before and after presynaptic activation, LTP was unmasked (60+-20%, n=9). This suggests that postsynaptic K+ channels play a crucial role in determining sign and magnitude of STDP by complex spike trains

Local recurrence remains a major obstacle to achieving cure of many locally advanced solid tumors treated with definitive radiation therapy. The microenvironment of solid tumors is hypoxic compared with normal tissue, and this hypoxia is associated with decreased radiosensitivity. Recent preclinical data also suggest that intratumoral hypoxia, particularly in conjunction with an acid microenvironment, may be directly or indirectly mutagenic. Investigations of the prognostic significance of the pretreatment oxygenation status of tumors in patients with head and neck or cervical cancer have demonstrated that increased hypoxia, typically designated in these studies as pO(2) levels below 2.5-10 mm Hg, is associated with decreased local tumor control and lower rates of disease-free and overall survival. Hypoxia-directed therapies in the radiation oncology setting include treatment using hyperbaric oxygen, fluosol infusion, carbogen breathing, and electron-affinic and hypoxic-cell sensitizers. These interventions have shown the potential to increase the effectiveness of curative-intent radiation therapy, demonstrating that the strategy of overcoming hypoxia may be a viable and important approach. Anemia is common in the cancer population and is suspected to contribute to intratumoral hypoxia. A review of the literature reveals that a low hemoglobin level before or during radiation therapy is an important risk factor for poor locoregional disease control and survival, implying that a strong correlation could exist between anemia and hypoxia (ultimately predicting for a poor outcome). While having a low hemoglobin level has been shown to be detrimental, it is unclear as to exactly what the threshold for "low" should be (studies in this area have used thresholds ranging from 9-14.5 g/dl). Optimal hemoglobin and pO(2) thresholds for improving outcomes may vary across and within tumor types, and this is an area that clearly requires further evaluation. Nonetheless, the correction of anemia may be a worthwhile strategy for radiation oncologists to improve local control and survival.

The flow of activity in the cortical microcircuitry is poorly understood. We use calcium imaging to reconstruct, with millisecond and single-cell resolution, the spontaneous activity of populations of neurons in unstimulated slices from mouse visual cortex. We find spontaneous activity correlated among networks of layer 5 pyramidal cells. Synchronous ensembles occupy overlapping territories, often share neurons, and are repeatedly activated. Sets of neurons are also sequentially activated numerous times. Network synchronization and sequential correlations are blocked by glutamatergic antagonists, even though spontaneous firing persists in many 'autonomously active' neurons. This autonomous activity is periodic and depends on hyperpolarization-activated cationic (H) and persistent sodium (Na(p)) currents. We conclude that the isolated neocortical microcircuit generates spontaneous activity, mediated by a combination of intrinsic and circuit mechanisms, and that this activity can be temporally precise

Computer-assisted guidance technology represents the next step in the application of stereotactic techniques to skull base surgery. Use of this 3-D technique reduces operative time and complications as the technique allows a more direct approach with precise real-time anatomical guidance. We present seven cases of transsphenoidal parasellar surgery where this technique has been employed. The pathology included lesions of the sella turcica, parasellar region and the petrous apex

BACKGROUND: Successful rehabilitation after ablative surgery requires not only the reconstruction of 3-dimensional form but also the restoration of physiologic function. OBJECTIVE: To assess sensory recovery of reinnervated radial forearm flaps used for tongue reconstruction. PATIENTS AND METHODS: Seventeen patients, who underwent reconstruction of glossectomy defects with reinnervated radial forearm free flaps, formed the study group. Recovery of sensation was measured by both subjective and detailed objective tests 8 months after surgery. Sensory function of the flap was compared with that of the normal residual tongue or the adjacent oral mucosa and the contralateral forearm donor site. RESULTS: All patients involved in this study had tongue defects of hemiglossectomy or greater and adjacent floor of the mouth. Sensory recovery was observed in all of the 17 patients within 8 months. Detailed sensory testing showed that median static 2-point discrimination, moving 2-point discrimination, and pressure sensitivity (1.2 cm, 0.8 cm, and 3.7 psi, respectively) were subjectively greater in the innervated forearm flaps than in the contralateral forearm donor site (2.3 cm, 1.7 cm, and 4.6 psi, respectively) (P= .064) and similar to those of the normal tongue (0.9 cm, 0.5 cm, and 3.6 psi). CONCLUSIONS: In all modalities examined, sensate free flaps proved superior in sensory fidelity to the native forearm donor site and closely approached that of the normal tongue. Microsurgical reinnervation of flaps should be considered in tongue reconstruction