Phase delays between mouse globus pallidus neurons entrained by common oscillatory drive arise from their intrinsic properties, not their coupling

A hallmark of Parkinson's disease is the appearance of correlated oscillatory discharge throughout the cortico-basal ganglia (BG) circuits. In the primate globus pallidus (GP), where the discharge of GP neurons is normally uncorrelated, pairs of GP neurons exhibit oscillatory spike correlations with a broad distribution of pairwise phase delays in experimental parkinsonism. The transition to oscillatory correlations is thought to indicate the collapse of the normally segregated information channels traversing the BG. The large phase delays are thought to reflect pathological changes in synaptic connectivity in the BG. Here we study the structure and phase delays of spike correlations measured from neurons in the mouse external GP (GPe) subjected to identical 1-100 Hz sinusoidal drive but recorded in separate experiments. First, we find that spectral modes of a GPe neuron's empirical instantaneous phase response curve (iPRC), elucidate at what phases of the oscillatory drive the GPe neuron locks when it is entrained, and the distribution of phases at which it spikes when it is not. Then, we show that in this case the pairwise spike cross-correlation equals the cross-correlation function of these spike phase distributions. Finally, we show that the distribution of GPe phase delays arises from the diversity of iPRCs, and is broadened when the neurons become entrained. Modeling GPe networks with realistic intranuclear connectivity demonstrates that the connectivity decorrelates GPe neurons without affecting phase delays. Thus, common oscillatory input gives rise to GPe correlations whose structure and pairwise phase delays reflect their intrinsic properties captured by their iPRCs.Significance Statement The external globus pallidus (GPe) is a hub in the basal ganglia, whose neurons impose a barrage of inhibitory synaptic currents on neurons of the subthalamic nucleus, substantia nigra and internal globus pallidus. GPe neurons normally fire independently, but in experimental parkinsonism, they become correlated in the frequency range associated with the pathological rhythms seen in human Parkinson's disease, raising the possibility that they may be generators of the pathological oscillation. We drove individual pallidal neurons with an oscillatory input over a wide range of frequencies. Cross-correlations of these neurons reproduced many of the features seen in parkinsonism, suggesting that their correlated oscillations might derive from a shared input rather than internal interconnections.

Striatal Neurons Are Recruited Dynamically into Collective Representations of Self-Initiated and Learned Actions in Freely Moving Mice

Striatal spiny projection neurons are hyperpolarized-at-rest (HaR) and driven to action potential threshold by a small number of powerful inputs-an input-output configuration that is detrimental to response reliability. Because the striatum is important for habitual behaviors and goal-directed learning, we conducted a microendoscopic imaging in freely moving mice that express a genetically encoded Ca2+ indicator sparsely in striatal HaR neurons to evaluate their response reliability during self-initiated movements and operant conditioning. The sparse expression was critical for longitudinal studies of response reliability, and for studying correlations among HaR neurons while minimizing spurious correlations arising from contamination by the background signal. We found that HaR neurons are recruited dynamically into action representation, with distinct neuronal subsets being engaged in a moment-by-moment fashion. While individual neurons respond with little reliability, the population response remained stable across days. Moreover, we found evidence for the temporal coupling between neuronal subsets during conditioned (but not innate) behaviors.

Acetylcholine waves and dopamine release in the striatum

Striatal dopamine encodes reward, with recent work showing that dopamine release occurs in spatiotemporal waves. However, the mechanism of dopamine waves is unknown. Here we report that acetylcholine release in mouse striatum also exhibits wave activity, and that the spatial scale of striatal dopamine release is extended by nicotinic acetylcholine receptors. Based on these findings, and on our demonstration that single cholinergic interneurons can induce dopamine release, we hypothesized that the local reciprocal interaction between cholinergic interneurons and dopamine axons suffices to drive endogenous traveling waves. We show that the morphological and physiological properties of cholinergic interneuron - dopamine axon interactions can be modeled as a reaction-diffusion system that gives rise to traveling waves. Analytically-tractable versions of the model show that the structure and the nature of propagation of acetylcholine and dopamine traveling waves depend on their coupling, and that traveling waves can give rise to empirically observed correlations between these signals. Thus, our study provides evidence for striatal acetylcholine waves in vivo, and proposes a testable theoretical framework that predicts that the observed dopamine and acetylcholine waves are strongly coupled phenomena.

Non-uniform distribution of dendritic nonlinearities differentially engages thalamostriatal and corticostriatal inputs onto cholinergic interneurons

The tonic activity of striatal cholinergic interneurons (CINs) is modified differentially by their afferent inputs. Although their unitary synaptic currents are identical, in most CINs cortical inputs onto distal dendrites only weakly entrain them, whereas proximal thalamic inputs trigger abrupt pauses in discharge in response to salient external stimuli. To test whether the dendritic expression of the active conductances that drive autonomous discharge contribute to the CINs’ capacity to dissociate cortical from thalamic inputs, we used an optogenetics-based method to quantify dendritic excitability in mouse CINs. We found that the persistent sodium (NaP) current gave rise to dendritic boosting, and that the hyperpolarization-activated cyclic nucleotide-gated (HCN) current gave rise to a subhertz membrane resonance. This resonance may underlie our novel finding of an association between CIN pauses and internally-generated slow wave events in sleeping non-human primates. Moreover, our method indicated that dendritic NaP and HCN currents were preferentially expressed in proximal dendrites. We validated the non-uniform distribution of NaP currents: pharmacologically; with two-photon imaging of dendritic back-propagating action potentials; and by demonstrating boosting of thalamic, but not cortical, inputs by NaP currents. Thus, the localization of active dendritic conductances in CIN dendrites mirrors the spatial distribution of afferent terminals and may promote their differential responses to thalamic vs. cortical inputs.

A tonic nicotinic brake controls spike timing in striatal spiny projection neurons

Striatal spiny projection neurons (SPNs) transform convergent excitatory corticostriatal inputs into an inhibitory signal that shapes basal ganglia output. This process is fine-tuned by striatal GABAergic interneurons (GINs), which receive overlapping cortical inputs and mediate rapid corticostriatal feedforward inhibition of SPNs. Adding another level of control, cholinergic interneurons (CINs), which are also vigorously activated by corticostriatal excitation, can disynaptically inhibit SPNs by activating α4β2 nicotinic acetylcholine receptors (nAChRs) on various GINs. Measurements of this disynaptic inhibitory pathway, however, indicate that it is too slow to compete with direct GIN-mediated feedforward inhibition. Moreover, functional nAChRs are also present on populations of GINs that respond only weakly to phasic activation of CINs, such as parvalbumin-positive fast-spiking interneurons (PV-FSIs), making the overall role of nAChRs in shaping striatal synaptic integration unclear. Using acute striatal slices from mice we show that upon synchronous optogenetic activation of corticostriatal projections blockade of α4β2 nAChRs shortened SPN spike latencies and increased postsynaptic depolarizations. The nAChR-dependent inhibition was mediated by downstream GABA release, and data suggest that the GABA source was not limited to GINs that respond strongly to phasic CIN activation. In particular, the observed decrease in spike latency caused by nAChR blockade was associated with a diminished frequency of spontaneous inhibitory postsynaptic currents in SPNs, a parallel hyperpolarization of PV-FSIs, and was occluded by pharmacologically preventing cortical activation of PV-FSIs. Taken together, we describe a role for tonic (as opposed to phasic) activation of nAChRs in striatal function. We conclude that tonic activation of nAChRs by CINs maintains a GABAergic brake on cortically-driven striatal output by ‘priming’ feedforward inhibition, a process that may shape SPN spike timing, striatal processing, and synaptic plasticity.

α-Synuclein-induced Kv4 channelopathy in mouse vagal motoneurons drives nonmotor parkinsonian symptoms

No disease-modifying therapy is currently available for Parkinson's disease (PD), the second most common neurodegenerative disease. The long nonmotor prodromal phase of PD is a window of opportunity for early detection and intervention. However, we lack the pathophysiological understanding to develop selective biomarkers and interventions. By using a mutant α-synuclein selective-overexpression mouse model of prodromal PD, we identified a cell-autonomous selective Kv4 channelopathy in dorsal motor nucleus of the vagus (DMV) neurons. This functional remodeling of intact DMV neurons leads to impaired pacemaker function in vitro and in vivo, which, in turn, reduces gastrointestinal motility, a common early symptom of prodromal PD. We identify a chain of events from α-synuclein via a biophysical dysfunction of a specific neuronal population to a clinically relevant prodromal symptom. These findings will facilitate the rational design of clinical biomarkers to identify people at risk for developing PD.

Population dynamics and entrainment of basal ganglia pacemakers are shaped by their dendritic arbors

The theory of phase oscillators is an essential tool for understanding population dynamics of pacemaking neurons. GABAergic pacemakers in the substantia nigra pars reticulata (SNr), a main basal ganglia (BG) output nucleus, receive inputs from the direct and indirect pathways at distal and proximal regions of their dendritic arbors, respectively. We combine theory, optogenetic stimulation and electrophysiological experiments in acute brain slices to ask how dendritic properties impact the propensity of the various inputs, arriving at different locations along the dendrite, to recruit or entrain SNr pacemakers. By combining cable theory with sinusoidally-modulated optogenetic activation of either proximal somatodendritic regions or the entire somatodendritic arbor of SNr neurons, we construct an analytical model that accurately fits the empirically measured somatic current response to inputs arising from illuminating the soma and various portions of the dendritic field. We show that the extent of the dendritic tree that is illuminated generates measurable and systematic differences in the pacemaker’s phase response curve (PRC), causing a shift in its peak. Finally, we show that the divergent PRCs correctly predict differences in two major features of the collective dynamics of SNr neurons: the fidelity of population responses to sudden step-like changes in inputs; and the phase latency at which SNr neurons are entrained by rhythmic stimulation, which can occur in the BG under both physiological and pathophysiological conditions. Our novel method generates measurable and physiologically meaningful spatial effects, and provides the first empirical demonstration of how the collective responses of SNr pacemakers are determined by the transmission properties of their dendrites. SNr dendrites may serve to delay distal striatal inputs so that they impinge on the spike initiation zone simultaneously with pallidal and subthalamic inputs in order to guarantee a fair competition between the influence of the monosynaptic direct- and polysynaptic indirect pathways.

The substantia nigra pars reticulata (SNr) is a main output nucleus of the basal ganglia (BG), where inputs from the competing direct and indirect pathways converge onto the same neurons. Interestingly, these inputs are differentially distributed with direct and indirect pathway projections arriving at distal and proximal regions of the dendritic arbor, respectively. We employ a novel method combining theory with electrophysiological experiments and optogenetics to study the distinct effects of inputs arriving at different locations along the dendrite. Our approach represents a useful compromise between complexity and reduction in modelling. Our work addresses the question of high fidelity encoding of inputs by networks of neurons in the new context of pacemaking neurons, which are driven to fire by their intrinsic dynamics rather than by a network state. We provide the first empirical demonstration that dendritic delays can introduce latencies in the responses of a population of neurons that are commensurate with synaptic delays, suggesting a new role for SNr dendrites with implications for BG function.

Thinking Outside the Box (and Arrow): Current Themes in Striatal Dysfunction in Movement Disorders

The basal ganglia are an intricately connected assembly of subcortical nuclei, forming the core of an adaptive network connecting cortical and thalamic circuits. For nearly three decades, researchers and medical practitioners have conceptualized how the basal ganglia circuit works, and how its pathology underlies motor disorders such as Parkinson's and Huntington's diseases, using what is often referred to as the "box-and-arrow model": a circuit diagram showing the broad strokes of basal ganglia connectivity and the pathological increases and decreases in the weights of specific connections that occur in disease. While this model still has great utility and has led to groundbreaking strategies to treat motor disorders, our evolving knowledge of basal ganglia function has made it clear that this classic model has several shortcomings that severely limit its predictive and descriptive abilities. In this review, we will focus on the striatum, the main input nucleus of the basal ganglia. We describe recent advances in our understanding of the rich microcircuitry and plastic capabilities of the striatum, factors not captured by the original box-and-arrow model, and provide examples of how such advances inform our current understanding of the circuit pathologies underlying motor disorders.

Selective remodeling of glutamatergic transmission to striatal cholinergic interneurons after dopamine depletion

The widely held view that the pathophysiology of Parkinson's disease arises from an under-activation of the direct pathway striatal spiny neurons (dSPNs) has gained support from a recently described weakening of the glutamatergic projection from the parafascicular nucleus (PfN) to dSPNs in experimental parkinsonism. However, the impact of the remodeling of the thalamostriatal projection cannot be fully appreciated without considering its impact on cholinergic interneurons (ChIs) that themselves preferentially activate indirect pathway spiny neurons (iSPNs). To study this thalamostriatal projection, we virally transfected with Cre-dependent channelrhodopsin-2 (ChR2) the PfN of Vglut2-Cre mice that were dopamine-depleted with 6-hydroxydopamine (6-OHDA). In parallel, we studied the corticostriatal projection to ChIs in 6-OHDA-treated transgenic mice expressing ChR2 under the Thy1 promoter. We found the 6-OHDA lesions failed to affect short-term synaptic plasticity or the size of unitary responses evoked optogenetically in either of these projections. However, we found that NMDA-to-AMPA ratios at PfN synapses-that were significantly larger than NMDA-to-AMPA ratios at cortical synapses-were reduced by 6-OHDA treatment, thereby impairing synaptic integration at PfN synapses onto ChIs. Finally, we found that application of an agonist of the D₅ dopamine receptors on ChIs potentiated NMDA currents without affecting AMPA currents or short-term plasticity selectively at PfN synapses. We propose that dopamine depletion leads to an effective de-potentiation of NMDA currents at PfN synapses onto ChIs which degrades synaptic integration. This selective remodeling of NMDA currents at PfN synapses may counter the selective weakening of PfN synapses onto dSPNs in parkinsonism.

Transient Activation of GABAB Receptors Suppresses SK Channel Currents in Substantia Nigra Pars Compacta Dopaminergic Neurons

Dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) are richly innervated by GABAergic neurons. The postsynaptic effects of GABA on SNc DA neurons are mediated by a mixture of GABAA and GABAB receptors. Although activation of GABAA receptors inhibits spike generation, the consequences of GABAB receptor activation are less well characterized. To help fill this gap, perforated patch recordings were made from young adult mouse SNc DA neurons. Sustained stimulation of GABAB receptors hyperpolarized SNc DA neurons, as previously described. However, transient stimulation of GABAB receptors by optical uncaging of GABA did not; rather, it reduced the opening of small-conductance, calcium-activated K+ (SK) channels and increased the irregularity of spiking. This modulation was attributable to inhibition of adenylyl cyclase and protein kinase A. Thus, because suppression of SK channel activity increases the probability of burst spiking, transient co-activation of GABAA and GABAB receptors could promote a pause-burst pattern of spiking.

Cholinergic Interneurons Amplify Corticostriatal Synaptic Responses in the Q175 Model of Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder characterized by deficits in movement control that are widely viewed as stemming from pathophysiological changes in the striatum. Giant, aspiny cholinergic interneurons (ChIs) are key elements in the striatal circuitry controlling movement, but whether their physiological properties are intact in the HD brain is unclear. To address this issue, the synaptic properties of ChIs were examined using optogenetic approaches in the Q175 mouse model of HD. In ex vivo brain slices, synaptic facilitation at thalamostriatal synapses onto ChIs was reduced in Q175 mice. The alteration in thalamostriatal transmission was paralleled by an increased response to optogenetic stimulation of cortical axons, enabling these inputs to more readily induce burst-pause patterns of activity in ChIs. This adaptation was dependent upon amplification of cortically evoked responses by a post-synaptic upregulation of voltage-dependent Na⁺ channels. This upregulation also led to an increased ability of somatic spikes to invade ChI dendrites. However, there was not an alteration in the basal pacemaking rate of ChIs, possibly due to increased availability of Kv4 channels. Thus, there is a functional "re-wiring" of the striatal networks in Q175 mice, which results in greater cortical control of phasic ChI activity, which is widely thought to shape the impact of salient stimuli on striatal action selection.

Functional segregation of voltage-activated calcium channels in motoneurons of the dorsal motor nucleus of the vagus

Calcium influx elevates mitochondrial oxidant stress (mOS) in dorsal motor nucleus of the vagus (DMV) neurons that are prone to Lewy body pathologies in presymptomatic Parkinson's disease (PD) patients. In experimental PD models, treatment with isradipine, the dihydropyridine with the highest affinity to Cav1.3 channels, prevents subthreshold calcium influx via Cav1.3 channels into midbrain dopamine neurons and protects them from mOS. In DMV neurons, isradipine is also effective in reducing mOS despite overwhelming evidence that subthreshold calcium influx is negligible compared with spike-triggered influx. To solve this conundrum we combined slice electrophysiology, two-photon laser scanning microscopy, mRNA profiling, and computational modeling. We find that the unusually depolarized subthreshold voltage trajectory of DMV neurons is positioned between the relatively hyperpolarized activation curve of Cav1.3 channels and that of other high-voltage activated (HVA) calcium channels, thus creating a functional segregation between Cav1.3 and HVA calcium channels. The HVA channels flux the bulk of calcium during spikes but can only influence pacemaking through their coupling to calcium-activated potassium currents. In contrast, Cav1.3 currents, which we show to be more than an order-of-magnitude smaller than the HVA calcium currents, are able to introduce sufficient inward current to speed up firing. However, Kv4 channels that are constitutively open in the subthreshold range guarantee slow pacemaking, despite the depolarizing action of Cav1.3 and other pacemaking currents. We propose that the efficacy of isradipine in preventing mOS in DMV neurons arises from its mixed effect on Cav1.3 channels and on HVA Cav1.2 channels.

Spectral reconstruction of phase response curves reveals the synchronization properties of mouse globus pallidus neurons

The propensity of a neuron to synchronize is captured by its infinitesimal phase response curve (iPRC). Determining whether an iPRC is biphasic, meaning that small depolarizing perturbations can actually delay the next spike, if delivered at appropriate phases, is a daunting experimental task because negative lobes in the iPRC (unlike positive ones) tend to be small and may be occluded by the normal discharge variability of a neuron. To circumvent this problem, iPRCs are commonly derived from numerical models of neurons. Here, we propose a novel and natural method to estimate the iPRC by direct estimation of its spectral modes. First, we show analytically that the spectral modes of the iPRC of an arbitrary oscillator are readily measured by applying weak harmonic perturbations. Next, applying this methodology to biophysical neuronal models, we show that a low-dimensional spectral reconstruction is sufficient to capture the structure of the iPRC. This structure was preserved reasonably well even with added physiological scale jitter in the neuronal models. To validate the methodology empirically, we applied it first to a low-noise electronic oscillator with a known design and then to cortical pyramidal neurons, recorded in whole cell configuration, that are known to possess a monophasic iPRC. Finally, using the methodology in conjunction with perforated-patch recordings from pallidal neurons, we show, in contrast to recent modeling studies, that these neurons have biphasic somatic iPRCs. Biphasic iPRCs would cause lateral somatically targeted pallidal inhibition to desynchronize pallidal neurons, providing a plausible explanation for their lack of synchrony in vivo.

Calcium entry induces mitochondrial oxidant stress in vagal neurons at risk in Parkinson's disease

Mitochondrial oxidant stress is widely viewed as being critical to pathogenesis in Parkinson's disease. But the origins of this stress are poorly defined. One possibility is that it arises from the metabolic demands associated with regenerative activity. To test this hypothesis, we characterized neurons in the dorsal motor nucleus of the vagus (DMV), a population of cholinergic neurons that show signs of pathology in the early stages of Parkinson's disease, in mouse brain slices. DMV neurons were slow, autonomous pacemakers with broad spikes, leading to calcium entry that was weakly buffered. Using a transgenic mouse expressing a redox-sensitive optical probe targeted to the mitochondrial matrix, we found that calcium entry during pacemaking created a basal mitochondrial oxidant stress. Knocking out DJ-1 (also known as PARK7), a gene associated with early-onset Parkinson's disease, exacerbated this stress. These results point to a common mechanism underlying mitochondrial oxidant stress in Parkinson's disease and a therapeutic strategy to ameliorate it.

Muscarinic modulation of striatal function and circuitry

Striatal cholinergic interneurons are pivotal modulators of the striatal circuitry involved in action selection and decision making. Although nicotinic receptors are important transducers of acetylcholine release in the striatum, muscarinic receptors are more pervasive and have been more thoroughly studied. In this review, the effects of muscarinic receptor signaling on the principal cell types in the striatum and its canonical circuits will be discussed, highlighting new insights into their role in synaptic integration and plasticity. These studies, and those that have identified new circuit elements driven by activation of nicotinic receptors, make it clear that temporally patterned activity in cholinergic interneurons must play an important role in determining the effects on striatal circuitry. These effects could be critical to the response to salient environmental stimuli that serve to direct behavior.

Adenosine A2a receptor antagonists attenuate striatal adaptations following dopamine depletion

The motor symptoms of Parkinson's disease (PD) are widely thought to arise from an imbalance in the activity of the two major striatal efferent pathways following the loss of dopamine (DA) signaling. In striatopallidal, indirect pathway spiny projection neurons (iSPNs), intrinsic excitability rises following the loss of inhibitory D2 receptor signaling. Because these receptors are normally counterbalanced by adenosine A2a adenosine receptors, antagonists of these receptors are being examined as an adjunct to conventional pharmacological therapies. However, little is known about the effects of sustained A2a receptor antagonism on striatal adaptations in PD models. To address this issue, the A2a receptor antagonist SCH58261 was systemically administered to DA-depleted mice. After 5 days of treatment, the effects of SCH58261 on iSPNs were examined in brain slices using electrophysiological and optical approaches. SCH58261 treatment did not prevent spine loss in iSPNs following depletion, but did significantly attenuate alterations in synaptic currents, spine morphology and dendritic excitability. In part, these effects were attributable to the ability of SCH58261 to blunt the effects of DA depletion on cholinergic interneurons, another striatal cell type that co-expresses A2a and D(2) receptors. Collectively, these results suggest that A2a receptor antagonism improves striatal function in PD models by attenuating iSPN adaptations to DA depletion.

The origins of oxidant stress in Parkinson's disease and therapeutic strategies

Parkinson's disease (PD) is a major world-wide health problem afflicting millions of the aged population. Factors that act on most or all cell types (pan-cellular factors), particularly genetic mutations and environmental toxins, have dominated public discussions of disease etiology. Although there is compelling evidence supporting an association between disease risk and these factors, the pattern of neuronal pathology and cell loss is difficult to explain without cell-specific factors. This article focuses on recent studies showing that the neurons at greatest risk in PD-substantia nigra pars compacta dopamine neurons-have a distinctive physiological phenotype that could contribute to their vulnerability. The opening of L-type calcium channels during autonomous pacemaking results in sustained calcium entry into the cytoplasm of substantia nigra pars compacta dopamine neurons, resulting in elevated mitochondrial oxidant stress and susceptibility to toxins used to create animal models of PD. This cell-specific stress could increase the negative consequences of pan-cellular factors that broadly challenge either mitochondrial or proteostatic competence. The availability of well-tolerated, orally deliverable antagonists for L-type calcium channels points to a novel neuroprotective strategy that could complement current attempts to boost mitochondrial function in the early stages of the disease.

Thalamic gating of corticostriatal signaling by cholinergic interneurons

Salient stimuli redirect attention and suppress ongoing motor activity. This attentional shift is thought to rely upon thalamic signals to the striatum to shift cortically driven action selection, but the network mechanisms underlying this interaction are unclear. Using a brain slice preparation that preserved cortico- and thalamostriatal connectivity, it was found that activation of thalamostriatal axons in a way that mimicked the response to salient stimuli induced a burst of spikes in striatal cholinergic interneurons that was followed by a pause lasting more than half a second. This patterned interneuron activity triggered a transient, presynaptic suppression of cortical input to both major classes of principal medium spiny neuron (MSN) that gave way to a prolonged enhancement of postsynaptic responsiveness in striatopallidal MSNs controlling motor suppression. This differential regulation of the corticostriatal circuitry provides a neural substrate for attentional shifts and cessation of ongoing motor activity with the appearance of salient environmental stimuli.

What causes the death of dopaminergic neurons in Parkinson's disease?

The factors governing neuronal loss in Parkinson's disease (PD) are the subject of continuing speculation and experimental study. In recent years, factors that act on most or all cell types (pan-cellular factors), particularly genetic mutations and environmental toxins, have dominated public discussions of disease aetiology. Although there is compelling evidence supporting an association between disease risk and these factors, the pattern of neuronal pathology and cell loss is difficult to explain without cell-specific factors. This chapter focuses on recent studies showing that the neurons at greatest risk in PD--substantia nigra pars compacta (SNc) dopamine (DA) neurons--have a distinctive physiological phenotype that could contribute to their vulnerability. The opening of L-type calcium channels during autonomous pacemaking results in sustained calcium entry into the cytoplasm of SNc DA neurons, resulting in elevated mitochondrial oxidant stress and susceptibility to toxins used to create animal models of PD. This cell-specific stress could increase the negative consequences of pan-cellular factors that broadly challenge either mitochondrial or proteostatic competence. The availability of well-tolerated, orally deliverable antagonists for L-type calcium channels points to a novel neuroprotective strategy that could complement current attempts to boost mitochondrial function in the early stages of the disease.

Statistical properties of pauses of the high-frequency discharge neurons in the external segment of the globus pallidus

The neurons of many basal ganglia nuclei, including the external and internal globus pallidus (GPe and GPi, respectively) and the substantia nigra pars reticulata (SNr) are characterized by their high-frequency (50-100 spikes/s) tonic discharge (HFD). However, the high firing rate of GPe neurons is interrupted by long pauses. We studied the extracellularly recorded spiking activity of 212 well-isolated HFD GPe and 52 GPi/SNr neurons from five monkeys during different states of behavioral activity. An algorithm that maximizes the surprise function was used to detect pauses and pauser cells ("pausers"). Only 6% of the GPi/SNr neurons versus as many as 56% of the GPe neurons were classified as pausers. The GPe average pause duration equals 0.62 s. The interpause intervals follow a Poissonian distribution with a frequency of 13 pauses/minute. No linear relationship was found between pause parameters (duration or frequency) and the firing rate of the cell. Pauses were preceded by various changes in firing rate but not dominantly by a decrease. The average amplitude and duration of the spike waveform was modulated only after the pause but not before it. Pauses of pairs of cells that were recorded simultaneously were not correlated. The probability of GPe cells to pause spontaneously was extremely variable among monkeys (30-90%) and inversely related to the degree of the monkey's motor activity. These findings suggest that spontaneous GPe pauses are related to low-arousal periods and are generated by a process that is independent of the discharge properties of the cells.

Response Properties and Synchronization of Rhythmically Firing Dendritic Neurons

The responsiveness of rhythmically firing neurons to synaptic inputs is characterized by their phase-response curve (PRC), which relates how weak somatic perturbations affect the timing of the next action potential. The shape of the somatic PRC is an important determinant of collective network dynamics. Here we study theoretically and experimentally the impact of distally located synapses and dendritic nonlinearities on the synchronization properties of rhythmically firing neurons. By combining the theories of quasi-active cables and phase-coupled oscillators we derive an approximation for the dendritic responsiveness, captured by the neuron's dendritic PRC (dPRC). This closed-form expression indicates that the dPRCs are linearly filtered versions of the somatic PRC and that the filter characteristics are determined by the passive and active properties of the dendrite. The passive properties induce leftward shifts in the dPRCs and attenuate them. Our analysis yields a single dimensionless parameter that classifies active dendritic conductances as either regenerative conductances that counter the passive properties by boosting the dPRCs or restorative conductances that high-pass filter the dPRCs. Thus dendritic properties can generate a qualitative difference between the somatic and dendritic PRCs. As a result collective dynamics can be qualitatively different depending on the location of the synapse, the neuronal firing rates, and the dendritic nonlinearities. Finally, we use dual whole cell recordings from the soma and apical dendrite of cortical pyramidal neurons to test these predictions and find that empirical dPRCs are shifted leftward, as predicted, but may also display high-pass characteristics resulting from the restorative dendritic HCN (h) current.

Dopamine replacement therapy does not restore the full spectrum of normal pallidal activity in the 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine primate model of Parkinsonism

Current physiological studies emphasize the role of neuronal oscillations and synchronization in the pathophysiology of Parkinson's disease; however, little is known about their specific roles in the neuronal substrate of dopamine replacement therapy (DRT). We investigated oscillatory activity and correlations throughout the different states of levodopa-naive parkinsonism as well as "Off-On" and dyskinetic states of DRT in the external globus pallidum (GPe) of tremulous (vervet) and rigid-akinetic (macaque) monkeys and in the internal globus pallidum (GPi) of the vervet monkey. We found that, although oscillatory activity of cells and interneuronal correlation in both pallidal segments increases after induction of parkinsonism with 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine (MPTP) and decreases in response to DRT, important differences exist between the two pallidal segments. In the GPi, the fraction of oscillatory cells and relative power of oscillations were significantly higher than in the GPe, and the dominant frequency was within the range of 7.5-13.5 Hz compared with a range of 4.5-7.5 Hz within the GPe. The interneuronal correlations were mostly oscillatory in the GPi, whereas at least half are non-oscillatory in the GPe. We demonstrate that the tremor characteristics after exposure to DRT do not resemble those of the normal or the levodopa-naive state. Moreover, although DRT reverses the MPTP-induced neuronal changes (rate, pattern, and pairwise correlations), the balance between GPe and GPi fails to restore. We therefore suggest that this imbalance reflects additional abnormal organization of the basal ganglia networks in response to dopamine replacement and may constitute the physiological substrate of the limitations and side effects of chronic DRT.

RGS4-dependent attenuation of M4 autoreceptor function in striatal cholinergic interneurons following dopamine depletion

Parkinson disease is a neurodegenerative disorder whose symptoms are caused by the loss of dopaminergic neurons innervating the striatum. As striatal dopamine levels fall, striatal acetylcholine release rises, exacerbating motor symptoms. This adaptation is commonly attributed to the loss of interneuronal regulation by inhibitory D(2) dopamine receptors. Our results point to a completely different, new mechanism. After striatal dopamine depletion, D(2) dopamine receptor modulation of calcium (Ca(2+)) channels controlling vesicular acetylcholine release in interneurons was unchanged, but M(4) muscarinic autoreceptor coupling to these same channels was markedly attenuated. This adaptation was attributable to the upregulation of RGS4-an autoreceptor-associated, GTPase-accelerating protein. This specific signaling adaptation extended to a broader loss of autoreceptor control of interneuron spiking. These observations suggest that RGS4-dependent attenuation of interneuronal autoreceptor signaling is a major factor in the elevation of striatal acetylcholine release in Parkinson disease.

Control of spontaneous firing patterns by the selective coupling of calcium currents to calcium-activated potassium currents in striatal cholinergic interneurons

The spontaneous firing patterns of striatal cholinergic interneurons are sculpted by potassium currents that give rise to prominent afterhyperpolarizations (AHPs). Large-conductance calcium-activated potassium (BK) channel currents contribute to action potential (AP) repolarization; small-conductance calcium-activated potassium channel currents generate an apamin-sensitive medium AHP (mAHP) after each AP; and bursts of APs generate long-lasting slow AHPs (sAHPs) attributable to apamin-insensitive currents. Because all these currents are calcium dependent, we conducted voltage- and current-clamp whole-cell recordings while pharmacologically manipulating calcium channels of the plasma membrane and intracellular stores to determine what sources of calcium activate the currents underlying AP repolarization and the AHPs. The Cav2.2 (N-type) blocker omega-conotoxin GVIA (1 microM) was the only blocker that significantly reduced the mAHP, and it induced a transition to rhythmic bursting in one-third of the cells tested. Cav1 (L-type) blockers (10 microM dihydropyridines) were the only ones that significantly reduced the sAHP. When applied to cells induced to burst with apamin, dihydropyridines reduced the sAHPs and abolished bursting. Depletion of intracellular stores with 10 mM caffeine also significantly reduced the sAHP current and reversibly regularized firing. Application of 1 microM omega-conotoxin MVIIC (a Cav2.1/2.2 blocker) broadened APs but had a negligible effect on APs in cells in which BK channels were already blocked by submillimolar tetraethylammonium chloride, indicating that Cav2.1 (Q-type) channels provide the calcium to activate BK channels that repolarize the AP. Thus, calcium currents are selectively coupled to the calcium-dependent potassium currents underlying the AHPs, thereby creating mechanisms for control of the spontaneous firing patterns of these neurons.

Origin of the slow afterhyperpolarization and slow rhythmic bursting in striatal cholinergic interneurons

Striatal cholinergic interneurons recorded in slices exhibit three different firing patterns: rhythmic single spiking, irregular bursting, and rhythmic bursting. The rhythmic single-spiking pattern is governed mainly by a prominent brief afterhyperpolarization (mAHP) that follows single spikes. The mAHP arises from an apamin-sensitive calcium-dependent potassium current. A slower AHP (sAHP), also present in these neurons, becomes prominent during rhythmic bursting or driven firing. Although not apamin sensitive, the sAHP is caused by a calcium-dependent potassium conductance. It is not present after blockade of calcium current with cadmium or after calcium is removed from the media or when intracellular calcium is buffered with bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. It reverses at the potassium equilibrium potential. It can be generated by subthreshold depolarizations and persists after blockade of sodium currents by tetrodotoxin. It is slow, being maximal approximately 1 s after depolarization onset, and takes several seconds to decay. It requires >300-ms depolarizations to become maximally activated. Its voltage sensitivity is sigmoidal, with a half activation voltage of -40 mV. We conclude the sAHP is a high-affinity apamin-insensitive calcium-dependent potassium conductance, triggered by calcium currents partly activated at subthreshold levels. In combination with those calcium currents, it accounts for the slow oscillations seen in a subset of cholinergic interneurons exhibiting rhythmic bursting. In all cholinergic interneurons, it contributes to the slowdown or pause in firing that follows driven activity or prolonged subthreshold depolarizations and is therefore a candidate mechanism for the pause response observed in cholinergic neurons in vivo.

Spike synchronization in the cortex/basal-ganglia networks of Parkinsonian primates reflects global dynamics of the local field potentials

Cortical local field potentials (LFPs) reflect synaptic potentials and accordingly correlate with neuronal discharge. Because LFPs are coherent across substantial cortical areas, we hypothesized that cortical spike correlations could be predicted from them. Because LFPs recorded in the basal ganglia (BG) are also correlated with neuronal discharge and are clinically accessible in Parkinson's disease patients, we were interested in testing this hypothesis in the BG, as well. We recorded LFPs and unit discharge from multiple electrodes, which were placed in primary motor cortex or in the basal ganglia (striatum and pallidum) of two monkeys before and after rendering them parkinsonian with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. We used the method of partial spectra to construct LFP predictors of the spike cross-correlation functions (CCFs). The predicted CCF is an estimate of the correlation between two neurons under the assumption that their association is explained solely by the association of each with the LFP recorded on a third electrode. In the normal condition, the predictors account for cortical rate covariations but not for the association among the tonically active neurons of the striatum. In the parkinsonian condition, with the appearance of 10 Hz oscillations throughout the cortex-basal ganglia networks, the LFP predictors account remarkably better for the CCFs in both the cortex and the basal ganglia. We propose that, in the parkinsonian condition, the cortex-basal ganglia networks are more tightly related to global modes of brain dynamics that are echoed in the LFP.

Patterns of Ongoing Activity and the Functional Architecture of the Primary Visual Cortex

Ongoing spontaneous activity in the cerebral cortex exhibits complex spatiotemporal patterns in the absence of sensory stimuli. To elucidate the nature of this ongoing activity, we present a theoretical treatment of two contrasting scenarios of cortical dynamics: (1) fluctuations about a single background state and (2) wandering among multiple "attractor" states, which encode a single or several stimulus features. Studying simplified network rate models of the primary visual cortex (V1), we show that the single state scenario is characterized by fast and high-dimensional Gaussian-like fluctuations, whereas in the multiple state scenario the fluctuations are slow, low dimensional, and highly non-Gaussian. Studying a more realistic model that incorporates correlations in the feed-forward input, spatially restricted cortical interactions, and an experimentally derived layout of pinwheels, we show that recent optical-imaging data of ongoing activity in V1 are consistent with the presence of either a single background state or multiple attractor states encoding many features.

Globus pallidus discharge is coincident with striatal activity during global slow wave activity in the rat

The emergence of bursting and oscillations in the basal ganglia under normal and pathological conditions has attracted considerable interest, but the neural substrate of these patterns is poorly understood. Here we use multisite recordings in anesthetized rats to examine the relationship of globus pallidus (GP) spiking and striatal activity in relation to cortical slow-wave activity. We found that GP neurons displayed increased spike rates or bursts coincident with cortical activation and striatal up states. Furthermore, the onset of GP bursts typically coincides with transitions to striatal up states that precede striatal spiking. These data indicate that GP activity is driven by excitatory corticosubthalamic input during periods of synchronized bursting activity.

Surgical management of large rotator cuff tears combined with instability in elite rugby football players

Large rotator cuff tears are extremely uncommon in young people and when they occur they may be associated with shoulder instability. This paper reports on a series of six elite rugby union and rugby league footballers who presented with shoulder instability and large rotator cuff tears. They were treated with a two stage procedure: an open rotator cuff repair followed by an open shoulder stabilisation some 10 weeks later. All had successful outcomes. The paper also highlights the risk of tearing the rotator cuff when a patient continues to play contact sport with an untreated unstable shoulder.

Salvaging unstable or recurrent dislocating total hip arthroplasty with the constrained acetabular component

PURPOSE

To review cases of implantation of constraining acetabular components for unstable or recurrent dislocating total hip arthroplasty at the Department of Orthopaedics, Concord Hospital, Sydney.

METHODS

A retrospective analysis was performed on prospectively collected data of 13 consecutively enrolled patients.

RESULTS

From 1989 to 2000, 13 constraining acetabular components were implanted into 13 patients as a revision procedure. The surgical approach for the implantation of the constrained liner was posterolateral in 11 cases; a modified Hardinge approach was applied in 2 cases. The mean clinical follow-up duration was 43 months (range, 14-121 months) and the mean age at the time of surgery was 73 years (range, 52-84 years). No patients were lost to follow-up. Indications for using the constrained acetabular component were recurrent dislocation in revision hip replacements (n=8), and intra-operative instability (n=5). There were no episodes of dislocation of the constrained arthroplasty. In 7 cases, the constrained component was implanted into a previously well-fixed shell.

CONCLUSION

We recommend the judicious use of the constrained component in cases of hip instability during or after total hip arthroplasty when other methods are not successful.

Dopamine replacement therapy reverses abnormal synchronization of pallidal neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine primate model of parkinsonism

Previous physiological studies have revealed changes in firing rates and synchronization of pallidal neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate model of Parkinson's disease. Several primate and human studies have demonstrated that dopamine replacement therapy (DRT) reverses the changes in the pallidal firing rates; however, the effects of DRT on pallidal synchronization have never been explored. To do so, we recorded the simultaneous activity of pallidal neurons of a vervet monkey before and after induction of severe parkinsonism by systemic MPTP treatment. We subsequently recorded the pallidal activity before and after daily administration of oral DRT. We extended the time scale of our correlation studies to +/-5 sec to allow detection of long-duration synchronized neuronal activity. After MPTP treatment, firing rates decreased in the external segment of the globus pallidus (GP(e)) and increased in the internal segment (GP(i)). A reversal of these rate changes occurred during the "on" periods of DRT. The percentage of correlated pairs increased from 16.7% in the normal state to 46.9% after MPTP treatment and was restored to nearly normal values (25% correlated pairs) under the influence of DRT. These changes in rate and correlation were observed at both the population level and at the level of units recorded continuously before, during, and after the clinical transition from "off" to "on" periods. We conclude that changes in both pallidal discharge rates and synchronization are correlated with the clinical manifestations of parkinsonism and its pharmacological treatment.

Total hip arthroplasty for arthrodesed hips

The benefits of converting an ankylosed or arthrodesed hip to total hip arthroplasty have been reported in the literature as have the technical difficulties associated with this procedure. This review, however, outlines the experience of a single surgeon (WJMB) at a single institution using uncemented prostheses. Between November 1991 and June 1996, 5 arthrodesed hips underwent uncemented total hip arthroplasty in 4 males and 1 female. Clinical and radiological follow-up review was for at least three years in all patients. In general, patients were satisfied with the outcome of their surgery with Harris Hip scores improving from an average of 62 preoperatively to an average of 72 postoperatively. The surgical outcome in these difficult cases was not as satisfactory as for routine total hip arthroplasty. Meticulous preoperative planning is required to aim toward leg length restoration and restoration of the abductor moment arm. A modular prosthesis allows versatility at surgery.

Anatomic variance of the coracoclavicular ligaments

The coracoclavicular ligaments vary widely in morphology and anatomic descriptions. Few authors have adequately described the coracoclavicular ligaments' anatomy, and a number of discrepancies exist in the anatomy literature. This study examines the complex anatomy of the coracoclavicular ligaments and their relationships to clinically important bony landmarks. The geometric dimensions of 24 coracoclavicular ligament specimens from fresh human cadaveric shoulders were examined and quantified with 13 different measurements. Particular attention was given to any inter-specimen anatomic variance. The coracoid insertions of the conoid ligaments displayed high variance, with 33% (8/24) being confluent with the lateral fibers of the superior transverse scapular ligament. A further 15% (3/24) presented an additional lateral fascicle. The distance from the lateral trapezoid ligament to the distal clavicle averaged 15.3 mm. Three distinct and previously unreported conoid ligament variants lend themselves to an anatomic classification (types I, II, and III). A safety margin of 15 mm is suggested for distal clavicle resection in incomplete acromioclavicular joint injuries to preserve the intact coracoclavicular ligament.

Synchrony of rest tremor in multiple limbs in parkinson's disease: evidence for multiple oscillators

Recent evidence points to involvement of central nervous system oscillators in Parkinson's disease (PD) rest tremor. It remains unknown whether one or multiple oscillators cause tremor in multiple limbs. Based on the prediction that multiple oscillators would cause low coherence even with similar average frequency, we studied 22 PD patients using accelerometers on multiple limbs. Records were digitized and spectral analysis was performed. Peak frequencies in the arms, legs, and chin were similar, indicating that biomechanical factors did not determine the frequency. Coherence between different axes of individual accelerometers and between different segments of the same limb was high. However, coherence between tremor in different limbs was low. There was no consistent pattern across patients of ipsi- vs. contralateral predominance of coherence. These data suggest that tremor in PD is generated by multiple oscillatory circuits, which operate on similar frequencies.

Reinforcement-driven dimensionality reduction--a model for information processing in the basal ganglia

Although anatomical studies of the basal ganglia show the existence of extensive convergence and lateral inhibitory connections, physiological studies failed to show correlated neural activity or lateral interaction in these nuclei. These seemingly contradictory results could be explained with a model in which the basal ganglia reduce the dimensionality of cortical information using optimal extraction methods. Simulations of this model predict a transient change in the efficacy of the feed-forward and lateral synapses following changes in reinforcement signal, causing an increase in correlated firing rates. This process ultimately restores the steady-state situation with diminished efficacy of lateral inhibition and no correlation of firing. Our experimental results confirm the model's predictions: rate correlations show a drastic decrease between the input stage (cortex) and output stage (pallidum). Moreover, preliminary analysis revealed that pallidal correlations show a transient increase following discrepancies between the animal's predictions and reality. We therefore propose that by using a reinforcement-driven dimensionality reduction process the basal ganglia achieve efficient extraction of cortical salient information that may then be used by the frontal cortex for execution and planning of forthcoming actions.

A new technique of subtrochanteric shortening in total hip arthroplasty: surgical technique and results of 9 cases

Total hip arthroplasty for severe chronic proximal femoral migration, most commonly seen in congenital dislocation of the hip, has been associated with high rates of complications. A new technique of femoral subtrochanteric shortening osteotomy with the prosthesis in situ is described. This technique minimizes the potential complications, allows for correction of severe femoral neck anteversion, and gives excellent rotational stability, while preserving the proximal femur for better press-fit cementless fixation. In this series, there were 9 cases: 6 women and 2 men with a mean age of 53 years (range, 26-77 years). The average follow-up period was 56 months (range, 6-86 months). The mean preoperative Harris Hip Score was 31 (range, 20-35), and the mean postoperative score was 81 (range, 60-98). At follow-up, all patients reported significant pain relief and functional improvement. All osteotomies appeared to be healed on radiographs by 12 weeks. There were 3 complications. The first complication was a recurrent dislocation resulting from muscle incompetence, which was revised using a constrained liner and a 32-mm head with no further dislocations. The second complication was a breach of the femoral shaft, which was treated operatively using a longer stem. The third complication was a proximal femoral shaft split, which was treated by leaving the cerclage wire in situ. This technique should be considered in cases of congenital dislocation of the hip and when femoral shortening is needed.

Exposure in difficult total knee arthroplasty using coronal tibial tubercle osteotomy

Exposure in a total knee arthroplasty can be challenging regardless of whether it is a difficult primary or a revision. Various techniques both proximal and distal to the patella have been described and implemented to gain exposure and improve knee flexion. When patella eversion is not possible due to previous surgery or severe preoperative knee flexion contracture, a coronal tibial tubercle osteotomy may be utilized. We present successful results utilizing the coronal tibial tubercle osteotomy procedure. The technique involved in this series is based on that described by Whiteside. It involves the development of a long lateral musculoperiosteal flap incorporating the tibial tubercle and anterior tibia, and leaving the proximal tibial cortex intact. This is extended along the tibia distally for 10 cm. It finishes by gradually osteotomising the anterior surface of the tibial crest. The tubercle is reattached with wires at the end of the procedure. This technique minimizes complications that have been associated with the tibial tubercle osteotomy. The 10 knees in 9 patients, who had total knee arthroplasty with a coronal tibial tubercle osteotomy, were reviewed pre and postoperatively. All knees were assessed using the Hospital for Special Surgery knee score (HSS). The scores averaged 43.6 preoperatively (range, 29 57) and 79.2 postoperatively (range, 67 90), and the mean range of motion was 59.5 degrees preoperatively and 78.0 degrees postoperatively. There were no cases of extension lag. Fixed flexion deformity was present in 3 cases postoperatively. Average time to union at the proximal and distal ends of the osteotomy was 8 and 24 weeks respectively. There was no evidence of nonunion and no other significant complications occurred.

A novel approach for assessing the quality and effectiveness of IACUC oversight in investigator compliance

In research facilities that are registered with the U.S. Department of Agriculture (USDA), funded by the Public Health Service, or accredited by the Association for the Assessment and Accreditation of Laboratory Animal Care (AAALAC) International, the Institutional Animal Care and Use Committee (IACUC) is charged with oversight and evaluation of animal care and use under the terms of the Animal Welfare Act and the Guide for the Care and Use of Laboratory Animals. Although the committee's oversight of investigator compliance may be evaluated annually during USDA inspections and triennially during AAALAC International site visits, routinely assessing the quality and effectiveness of the IACUC's performance is difficult. To measure the successfulness of IACUC oversight, our committee retained a management consultant to objectively design and conduct a confidential survey that could be used to determine how the IACUC could improve the process of facilitating researcher compliance with federal regulations and accreditation standards. The consultant based the content of the survey on confidential interviews with all IACUC members, the IACUC administrator, and a cross sectional representation of the key animal-user population at the facility. The survey was then distributed to the entire animal-user population. Vice-presidents, directors, principal investigators, and technicians were included in the distribution. With a response rate of 34%, the survey results indicated that the facilitation process warranted refinements. The consultant provided the IACUC with its recommendations, which were based on the discernible trending information indicated in the survey responses. The IACUC developed a specific plan of action to address the consultant's recommendations and intends to re-survey the animal-user population once the action plan has been fully implemented. In summary, the survey is an excellent way to assess the quality and effectiveness of IACUC oversight in investigator compliance by determining the level of researcher satisfaction. The evaluation, review, and follow-up process using a confidential interview and questionnaire technique can enhance the performance and effectiveness of IACUC oversight.

Potential role of doppler perfusion index in selection of patients with colorectal cancer for adjuvant chemotherapy

BACKGROUND

As yet there is no established method of accurately identifying patients with colorectal cancer who, despite undergoing apparently curative resection, are at high risk of recurrence. We assessed whether the doppler perfusion index (DPI; ratio of hepatic arterial to total liver blood flow) could be used to select patients who should receive adjuvant chemotherapy.

METHODS

We studied 120 patients undergoing curative surgery for colorectal cancer. DPI was measured before surgery with colour duplex doppler ultrasonography. A DPI value of at least 0.3 was defined as abnormal. All patients were followed up until death or for at least 5 years.

RESULTS

At 5 years, patients with Dukes' stage A or B tumours (n=61) had recurrence-free survival of 57% and overall survival of 64%, compared with 39% and 42% for patients with Dukes' stage C tumours (n=59; p=0.016 and p=0.008, respectively). 47 patients had normal DPI values and 73 patients had abnormal values. Patients with normal DPI had recurrence-free survival of 89% and overall survival of 91%, compared with 22% and 29% for those with abnormal DPI values (both p<0.0001).

CONCLUSIONS

DPI can be used to identify patients with colorectal cancer at high risk of recurrence who are in need of adjuvant treatment. However, further studies with larger numbers of patients are needed to confirm these findings.

Structural properties of the intact and the reconstructed coracoclavicular ligament complex

Numerous procedures have been described for the operative management of acromioclavicular joint injuries, but surprisingly little information is available on the ultimate mechanical behavior of the native coracoclavicular ligament complex or on the various methods of reconstruction. We tested 19 fresh-frozen cadaveric bone-ligament-bone preparations of the coracoclavicular ligament in uniaxial tension at 25 mm/min until failure. Seven specimens were left intact, six had the trapezoid ligament sectioned, and six had the conoid ligament sectioned. Reconstruction of the coracoclavicular ligament was achieved using coracoacromial ligament transfers, woven polyester slings, suture anchors, and Bosworth screws; all reconstructions were also tested to failure. The intact coracoclavicular ligament failed by avulsion or midsubstance tear at 500 (+/-134) N, with a stiffness of 103 (+/-30) N/mm and elongation to failure of 7.7 (+/-1.9) mm. There was no significant difference between the contributions of the conoid or trapezoid ligaments in this loading configuration. Coracoclavicular slings and suture anchors provided strength similar to that of the coracoclavicular ligament, but with significantly greater deformations (14 to 26 mm). Screw fixation resulted in comparable stiffness and superior strength to the coracoclavicular ligament, but only if bicortical purchase was obtained. Coracoacromial ligament transfers were the weakest and least stiff, and augmentation with another form of coracoclavicular fixation is recommended. These results provide a useful baseline for comparison of the initial performance of reconstructive techniques with the performance of the native coracoclavicular ligament.

Flexor tendon lengthening in zone II injuries

Primary repair of acute severed flexor tendons in zone II has replaced the "no-man's-land" concept, which advocates secondary procedures. Primary repair can be difficult when there are severe soft-tissue and bony injuries, and may lead to unfavorable results. The authors describe a technique that is simple, quick, and safe, and eliminates the need for tendon harvesting or other secondary procedures. It can also be used when a flexor profundus tendon gap occurs due to the injury.

Between passion and policy: litigating the Guckenberger case

This article describes the Guckenberger lawsuit from the perspective of the attorneys who litigated the case on behalf of Boston University. It first discusses the events leading to the lawsuit, including then-Provost Jon Westling's speeches, his articulation of new policies regarding documentation for accommodations requests, and the university's refusal to allow course substitutions for required foreign language courses. The article then describes the main events in the lawsuit that resulted from those policy changes, focusing first on pretrial matters, such as the university's search for experts, the various motions filed prior to trial, and tactical issues. Moving to the trial itself, the article discusses the nature of the witness and expert testimony presented to the court and touches on the differences of opinion expressed at trial by experts in the field of learning disabilities. Finally, the article sets forth the court's ruling on the major issues in the case and reflects on the case's impact on institutions of higher education.

Biomechanical performance of Bankart repairs in a human cadaveric shoulder model

The objective of this study was to develop a method to evaluate the biomechanical performance of Bankart repairs in a human cadaveric shoulder in a clinically relevant orientation. Twenty fresh-frozen human cadaveric shoulder girdles were used to compare the biomechanical performance of intact anteroinferior capsulolabral complexes with the biomechanical performance of three Bankart lesion reconstruction techniques. Repairs were performed on surgically created Bankart lesions. Evaluations were performed with the shoulders in glenohumeral abduction and external rotation. The repair techniques employed interosseous sutures, Mitek GII suture anchors, or Acufex T-Fix devices. The suture material used in all repairs was No. 2 Ti-Cron. The biomechanical performance of the three reconstruction techniques did not differ, but each was significantly inferior compared with that of the intact shoulder samples. The interosseous repairs failed by suture pullout through soft tissue. Repairs in the Mitek GII group failed by pullout of the suture anchors, suture breakage, or pullout of the suture through soft tissue. Repairs in the T-Fix group failed by pullout of the suture through soft tissue or failure of the polymer portion of the T-Fix suture.

Type 2 fractures of the distal clavicle: a new surgical technique

Neer type 2 fractures of the distal clavicle have a high rate of nonunion and delayed union. In this series nine cases of coracoclavicular ligament reconstruction with Dacron graft material led to union at the fracture site. All patients had no symptoms and returned to full activity. This technique allows for stable fixation with early mobilization and return to work and sports.

Restoration of sensation in paraplegia by a sensory innervated plantar fillet free flap. Case report

Sensory denervation most likely is the key factor to the multiple physiological derangements that predispose paraplegic patients to recurrent decubitus ulceration. A sensory innervated plantar free flap offers the ability to provide soft tissue coverage and to regain sensory innervation of the ulcer prone area in patients with recurrent ulceration. We present a patient in which an innervated plantar free flap was used to restore sensation to the sacral area in a patient with recurrent ulceration.

Sensory reinnervation of muscle free flaps for foot reconstruction

A retrospective study was performed to measure sensibility in sensory-to-motor innervated microvascular muscle transplants to the foot. Seven transplants were on the weight-bearing surface of the foot. Five of seven flaps had light-touch sensation. Mean vibrometer readings of the innervated transplants were 15.7 (range: 5 to 40) vs. 5.3 (range 2 to 9) on the contralateral foot. Semmes-Weinstein scores of the innervated flaps were 3.84 (one), 4.74 (one), 5.46 (two), and 6.45 (three) vs. 3.22 (one), 3.84 (one), 4.08 (one), and 4.17 (four) on the contralateral foot. This study documents that sensory-to-motor innervated free muscle flaps may regain measurable sensibility.

Laparoscopic cholecystectomy. Postoperative sonographic findings

Seventeen consecutive patients undergoing elective laparoscopic cholecystectomy (LC) were serially evaluated with transabdominal ultrasound before, one day after, and six days after LC to document what, if any, changes occur in the surgical bed and surrounding parenchyma. The most common postoperative finding was focal sonolucency in the hepatic parenchyma adjacent to the gallbladder fossa in six (35%) of 17 patients. Five patients (29%) had postoperative fluid collections in the gallbladder fossa; in four of these five, it was technically difficult to dissect the gallbladder from the liver at the time of original surgery. In one patient the fluid resolved by the sixth postoperative day. It persisted in the remaining four. Two patients had transient ductal dilation and one had pneumobilia. Shadowing and ring-down artifact was identified in 12 patients due to surgical clips in the triangle of Calot. Because gallbladder fossa fluid may persist up to six days after uncomplicated laparoscopic cholecystectomy, caution should be used before attaching significance to isolated imaging findings. Clinical judgement remains the best means of selecting which patients need additional evaluation.