AB0984 CHANGES IN PLASMA VOLUME WHEN MEASURING BIOCHEMICAL MARKERS OF JOINT TISSUE TURNOVER IN RELATION TO ACUTE PHYSICAL ACTIVITY

Background

Physical activity can induce acute changes in plasma volume (PV) and thereby influence concentration of plasma constituents, such as biochemical markers1,2. However, it remains undocumented to what extent PV changes occur during physical exercise in osteoarthritis (OA) patients.

Objectives

Investigating the direct course and magnitude of PV changes during and after weight-bearing (WB) and non-weight-bearing (NWB) exercise, adrenaline-induced cardiovascular stress and resting in an OA population.

Methods

Data originates from the EFEX-OA-02 study (Reg. no.: NCT04542668), which explored biomarker changes in knee OA patients during and after running, cycling, adrenaline infusion and resting on separate days. Blood samples were obtained before, during, at five time points after and again 24 hours post exercise/infusion. Hemoglobin (Hgb) and derived hematocrit (Hct) were measured at all time-points to monitor PV fluctuations.

Main inclusion criteria: Cumulative Kellgren-Lawrence (KL) radiographic grade of the left and right knee of ≥ 2, 40-75 years of age, bodyweight 50-100 kg and BMI 18.5-35.0 kg/m2.

Active interventions consisted of 4x5-minute intervals each progressing from low intensity and peaking at ≥80% of the heart rate reserve (HRR). For the adrenaline infusion, 0.06 mg/kg of adrenalin was prepared in 50 mL saline and administered intravenously. At rest, blood samples were collected at time points similar to the other study interventions, however the 24-hour follow-up sample was omitted.

Hgb was measured on a ABL800 FLEX blood gas analyzer and converted to Hct using the formula: Hct (%) = (0.0485 × ctHb (mmol/L) + 0.0083) × 100. Estimated PV change (% ΔPV) was calculated as: % ΔPV = [100 / (100 – Hctpre)] x [100 (Hctpre – Hctpost) / Hctpost], where: ΔPV % = Percentage change in PV: Hctpre = Hct before exercise, Hctpost = Hct after exercise. A repeated measures linear mixed-model and 1-way ANOVA was used to assess PV changes and compare interventions, respectively.

Results

Forty subjects were included. Mean age was 60.4 years (Standard deviation (SD): 8.7), 16 (40%) were male, mean BMI was 27.0 kg/m2 (SD: 3.5), and mean score in the nine item pain domain of the Knee Injury and Osteoarthritis Outcome Score (KOOS) at baseline was 67.5 (SD: 15.2) out of 100 (0 and 100 corresponding to max pain and no pain, respectively). Baseline Hgb was 9.05 mmol/L (SD: 0.68) and Hct was 44.7% (SD: 2.9%). All subjects were able to reach the defined peak cardiovascular intensity or higher during exercise, while reaching on average 70% (SD: 8.7%) of that during the adrenaline infusion.

Cycling, running and adrenaline infusion led to acute reductions in PV compared to rest: Cycling -14.3% (95%CI: -10.0 to -18.7%), running -13.9% (95%CI: -10.9 to -17.0%), adrenaline -7.8% (95%CI: -4.2 to -11.5%). The reductions in PV induced by both cycling and running were greater compared to adrenaline (p<0.001). There was no difference in PV changes after cycling vs. running (p=0.99). Thirty minutes after both modes of exercise, PV had returned to baseline. After completion of the adrenaline infusion, PV returned to baseline at 30-60 minutes, but was lower than baseline levels at 120 minutes by -3.9% (95%CI: -0.7 to -7.1%), at 240 minutes by -5.3% (95%CI: -2.1 to -8.6%), and at 24-hour by -5.9% (-2.0 to -9.9%). During seated rest, PV initially increased (3.1%, 95%CI: 0.7-5.4%) (Figure 1).

Conclusion

Moderate-high intensity running, cycling and cardiovascular stress led to rapid reductions in PV of knee OA patients. Adjustment for PV changes should be considered when measuring biochemical markers in relation to physical activity.

References

[1]Kaltreider NL, Meneely GR: THE EFFECT OF EXERCISE ON THE VOLUME OF THE BLOOD. J Clin Invest, 1940.

[2]Novosadová J: The changes in hematocrit, hemoglobin, plasma volume and proteins during and after different types of exercise. Eur J Appl Physiol Occup Physiol, 1977.

Disclosure of Interests

Jonathan Bjerre-Bastos: None declared, Casper Sejersen: None declared, Henning Bay Nielsen: None declared, Niels Secher: None declared, Carl-Christian Kitchen: None declared, Claire Prener Miller: None declared, Abigail Mackey: None declared, Asger Reinstrup Bihlet Employee of: Full-time employee of NBCD/Sanos Group A/S

POS1112 A RANDOMIZED, CROSS-OVER STUDY TO INVESTIGATE THE EFFECT OF WEIGHT-BEARING VS NON-WEIGHT-BEARING EXERCISE AND CARDIOVASCULAR STRESS ON TYPE II COLLAGEN TURNOVER IN KNEE OSTEOARTHRITIS PATIENTS – BIOMARKER DATA FROM THE EFEX-OA-02 STUDY

Background

Biomechanical stress is a prerequisite for OA development and studies have shown a difference in the effect of impact- and shear stress2, although studies of the direct impact of exercise on cartilage turnover have not demonstrated clear trends1.

Objectives

Exploring how weight-bearing (WB), non-weight-bearing (NWB) exercise and cardiovascular stress influence circulating biomarkers of cartilage extracellular matrix turnover in OA.

Methods

EFEX-OA-02 was a randomized, cross-over, clinical study investigating the direct effect of exercise on joint biomarkers in knee OA. Subjects were randomized to an order of cycling and running followed by adrenaline infusion and finally resting one week apart. Exercise and infusion sessions were multiphasic, consisting of 4x5-minute intervals. Peak cardiorespiratory stress (PCS) per interval was set to ≥80% of the heart rate reserve during exercise. Blood samples were obtained before, during, at five time points after and 24 h post exercise/infusion. For adrenaline infusion, 0.06 mg/kg of adrenaline was prepared in a 50 mL saline solution and administered intravenously. At rest, samples were collected at corresponding time points, except for the 24 h sample, which was omitted. Levels of serum C2M, T2CM (type II collagen degradation) and PRO-C2 (type II collagen formation) were measured using ELISA-assays (Nordic Bioscience). Coll2-1 and Coll2-1NO2 (type II collagen degradation) were measured using ELISA (Artialis).

Changes in biomarker concentrations after activity were compared to baseline (BL) and the corresponding resting samples. We used ANCOVA and Dunnett’s test with geometric means of change from BL up to 240 min as the dependent variable and subject and activity as covariates. Paired t-test was used to compare values at 24-hour to BL.

Results

Forty subjects were included. Mean age was 60.4 years (SD: 8.7), 24 (60%) were females, mean BMI was 27.0 kg/m2 (SD: 3.5), 18 had cumulated KL grade of 2 or 3 (45%) and 22 (55%) had KL 4, 5 or 6. and mean KOOS pain at BL was 67.5 (SD: 15.2) corresponding to mild-moderate pain. All subjects reached minimum PCS during exercise, but only an average of 70% (SD: 8.7) of that during infusion.

Cycling induced a small reduction in C2M (peak: -5.3%, 95%CI: -7.8 to -2.7%). PRO-C2 increased rapidly in response to cycling (peak: 11.7%, 95%CI: 4.3 to 19.1%) and running (peak: 12.9%, 95%CI: 3.54 to 22.2%). T2CM decreased up to one hour after cycling (peak: -10.8%, 95%CI: -15.1 to -6.5%) and running (peak: -9.5%, 95%CI: -15.5 to -3.6%), similar to adrenaline, then increased. Coll2-1NO2 increased rapidly following cycling (peak: 12.5%, 95%CI: 2.8 to 22.2%) and running (peak: 9.8%, 95%CI: 0.26 to 19.6%). Trends of increase was found in Coll2-1 (21.3%, 95%CI: 2.9 to 39.6) and Coll2-1NO2 (11.6%, 95%CI: -7.9 to 31.1%) in response to running at 240 min (Figure 1 – Error bars: SE, *Change to resting, †Change from BL, */†: P < 0.05, **/††: P < 0.01 ***/†††: P<0.001).

Figure 1.

At 24h PRO-C2 reduced -9.4% (95%CI: -18.2 to -0.5%) after cycling, Coll2-1NO2 reduced -8.33% (95%CI: -17.0 to 0.3%) after running and T2CM elevated by 6.0% (95%CI: -0.8 to 12.8%) after running and 7.1% (95%CI: 0.5 to 13.7%) after cycling.

Conclusion

Running, cycling and adrenaline infusion induced rapid small-to-moderate changes in circulating biomarkers reflecting type II collagen turnover. Changes after adrenaline-infusion suggests a cardiovascular contribution to exercise-induced changes. This model could potentially be used to evaluate treatment effects on collagen turnover.

References

[1]Bjerre-Bastos JJ, Karsdal MA, Boesen M, Bliddal H, Bay-Jensen A, Andersen JR, Bihlet AR: The acute and long-term impact of physical activity on biochemical markers and MRI measures in osteoarthritis—Perspectives for clinical osteoarthritis research. Transl Sport Med, 2020.

[2]Vincent TL: Mechanoflammation in osteoarthritis pathogenesis. Semin Arthritis Rheum, 2019.

Disclosure of Interests

Jonathan Bjerre-Bastos: None declared, Casper Sejersen: None declared, Henning Bay Nielsen: None declared, Mikael Boesen Speakers bureau: Speaker for Novartis and Eli Lilly, Niels Secher: None declared, Gregorio Distajo: None declared, Vincent Flood: None declared, Yves Henrotin Employee of: Founder and President of Artialis SA, Melanie Uebelhoer Employee of: Employee of Artialis, Abigail Mackey: None declared, Peter Krustrup: None declared, Carl-Christian Kitchen: None declared, Ema Petersen: None declared, Christian Thudium Shareholder of: Shareholder Nordic Bioscience A/S, Employee of: Full-time employee at Nordic Bioscience A/S, Jeppe Ragnar Andersen Employee of: Full-time employee of NBCD/Sanos Group A/S, Asger Reinstrup Bihlet Employee of: Full-time employee of NBCD/Sanos Group A/S

Discovery of sterically-hindered phenol compounds with potent cytoprotective activities against ox-LDL-induced retinal pigment epithelial cell death as a potential pharmacotherapy

Late-stage dry age-related macular degeneration (AMD) or geographic atrophy (GA) is an irreversible blinding condition characterized by degeneration of retinal pigment epithelium (RPE) and the associated photoreceptors. Clinical and genetic evidence supports a role for dysfunctional lipid processing and accumulation of harmful oxidized lipids in the pathogenesis of GA. Using an oxidized low-density lipoprotein (ox-LDL)-induced RPE death assay, we screened and identified sterically-hindered phenol compounds with potent protective activities for RPE. The phenol-containing PPARγ agonist, troglitazone, protected against ox-LDL-induced RPE cell death, whereas other more potent PPARγ agonists did not protect RPE cells. Knockdown of PPARγ did not affect the protective activity of troglitazone in RPE, confirming the protective function is not due to the thiazolidine (TZD) group of troglitazone. Prototypical hindered phenol trolox and its analogs potently protected against ox-LDL-induced RPE cell death whereas potent antioxidants without the phenol group failed to protect RPE. Hindered phenols preserved lysosomal integrity against ox-LDL-induced damage and FITC-labeled trolox was localized to the lysosomes in RPE cells. Analogs of trolox inhibited reactive oxygen species (ROS) formation induced by ox-LDL uptake in a dose-dependent fashion and were effective at sub-micromolar concentrations. Treatment with trolox analog 2,2,5,7,8-pentamethyl-6-chromanol (PMC) significantly induced the expression of the lysosomal protein NPC-1 and reduced intracellular cholesterol level upon ox-LDL uptake. Our data indicate that the lysosomal-localized hindered phenols are uniquely potent in protecting the RPE against the toxic effects of ox-LDL, and may represent a novel pharmacotherapy to preserve the vision in patients with GA.

A Proinflammatory Function of Toll-Like Receptor 2 in the Retinal Pigment Epithelium as a Novel Target for Reducing Choroidal Neovascularization in Age-Related Macular Degeneration

Current treatments for choroidal neovascularization, a major cause of blindness for patients with age-related macular degeneration, treat symptoms but not the underlying causes of the disease. Inflammation has been strongly implicated in the pathogenesis of choroidal neovascularization. We examined the inflammatory role of Toll-like receptor 2 (TLR2) in age-related macular degeneration. TLR2 was robustly expressed by the retinal pigment epithelium in mouse and human eyes, both normal and with macular degeneration/choroidal neovascularization. Nuclear localization of NF-κB, a major downstream target of TLR2 signaling, was detected in the retinal pigment epithelium of human eyes, particularly in eyes with advanced stages of age-related macular degeneration. TLR2 antagonism effectively suppressed initiation and growth of spontaneous choroidal neovascularization in a mouse model, and the combination of anti-TLR2 and antivascular endothelial growth factor receptor 2 yielded an additive therapeutic effect on both area and number of spontaneous choroidal neovascularization lesions. Finally, in primary human fetal retinal pigment epithelium cells, ligand binding to TLR2 induced robust expression of proinflammatory cytokines, and end products of lipid oxidation had a synergistic effect on TLR2 activation. Our data illustrate a functional role for TLR2 in the pathogenesis of choroidal neovascularization, likely by promoting inflammation of the retinal pigment epithelium, and validate TLR2 as a novel therapeutic target for reducing choroidal neovascularization.

Hemostatic markers in patients at risk of cerebral ischemia

BACKGROUND

Increased levels of markers of hemostasis may assist in the determination of the extent of carotid occlusive disease and the identification of neurologically intact individuals at increased risk of ischemic events.

METHODS

We conducted a prospective study of 304 subjects, including 82 with a recent (< or =7 days) transient ischemic attack (TIA), 157 asymptomatic individuals with a cervical bruit, and 65 control subjects. Baseline evaluation included a neurological assessment, ECG, cervical ultrasonography, and cerebral CT and/or MRI. Levels of markers of coagulation and fibrinolytic activity were also determined. Results were analyzed in relation to the degree of carotid disease and the subsequent occurrence of cerebral and cardiac ischemic events.

RESULTS

Over a mean follow-up period of 2.8 years (SD, 1.3 years), 114 ischemic events occurred. Survival analyses showed that prothrombin fragment 1.2 (F(1.2)) was a predictor of time to cerebral and cardiac ischemic events in the combined TIA and asymptomatic bruit group (relative risk [RR], 1.46; 95% CI, 1.18 to 1.81) as well as in the asymptomatic bruit group separately (RR, 1.70; 95% CI, 1.14 to 2.53). In the TIA group, both F(1.2) (RR, 2.36; 95% CI, 1.19 to 4.68) and severe (> or =80%) carotid stenosis (RR, 3.53; 95% CI, 1.19 to 10.51) were predictive of time to ischemic stroke, myocardial infarction, or vascular death.

CONCLUSIONS

In patients with TIAs and in asymptomatic individuals with cervical bruits, F(1.2) levels were found to be independent predictors of subsequent cerebral and cardiac ischemic events. Our results are consistent with an active role of the coagulation system through upregulation of thrombin in carotid disease progression and in the pathogenesis of ischemic events in patients at risk.

Oligomerization of the sarcoplasmic reticulum Ca2+-ATPase SERCA2 in cardiac muscle

The slow/cardiac isoform of the sarcoplasmic reticulum Ca2+-ATPase plays an important role in cardiac muscle Ca2+-homeostasis. To determine the native configuration of the SERCA2 ion pump, a chemical cross-linking analysis of heart microsomes was performed. Using one- and two-dimensional immunoblotting following incubation with the hydrophilic probe bis-sulfosuccinimidyl suberate or the hydrophobic crosslinker dithiobis-succinimidyl-propionate, we demonstrate here that SERCA2 forms high-molecular-mass aggregates. In contrast to the Na+/Ca2+-exchanger, Ca2+-ATPase clusters can be stabilized by hydrophilic 1.2 nm crosslinkers and are sensitive to chemical reduction. Hence, the native form of this important Ca2+-regulatory membrane protein involved in cardiac muscle relaxation appears not to exist as a monomeric ion pump unit. Protein-protein interactions might play an important role in the physiological functioning of this Ca2+-ATPase isoform, as has previously been shown for skeletal muscle Ca2+-pumps, Ca2+-binding proteins and Ca2+-channels.

Significance of carotid restenosis following endarterectomy

BACKGROUND AND PURPOSE

The clinical significance of restenosis after carotid endarterectomy as detected by duplex ultrasound has not been clearly established. To address this problem, we retrospectively evaluated the experience at two university-affiliated hospitals.

METHODS

All charts of patients with carotid endarterectomies between June 1987 and April 1995 were reviewed. Inclusion required neurological assessment and postoperative duplex ultrasound. Exclusion was based on a known source of cardioembolic disease, or recent (<6 months) myocardial infarction. Primary clinical endpoints were ipsilateral transient ischemic attack (TIA) or ischemic stroke. Contributing vascular risk factors were also identified. The effect of restenosis on event-free survival was analyzed using life tables and Gehan-Wilcoxon rank sum tet. Logistic regression was used to identify independent risk factors for restenosis and vascular events.

RESULTS

One hundred and eighty-seven patients were identified who underwent a total of 207 endarterectomies. Mean follow-up was 30.4 +/- 20.9 months during which a total of 64 vascular events, including 42 TIAs, 18 strokes, and 4 vascular deaths occurred. Of these 21 TIAs and 8 strokes were ipsilateral to the side of endarterectomy. Event rates were compared for patients with ipsilateral high- (>/=50%) and low-grade (<50%) restenosis. These two groups were comparable in terms of baseline risk factors. There was no significant difference in vascular event rates (for either ipsilateral events or events in any vascular territory) between the group with high- and low-grade restenosis. Nor was any such difference in event rates shown for patients who showed ipsilateral progression of carotid disease on serial ultrasound. However, patients operated for symptomatic carotid disease had a significantly higher risk of neurological events (p = 0.035). Logistic regression failed to disclose any other risk factors that were independently predictive of either restenosis or vascular events during follow-up.

CONCLUSION

This study does not show a difference in vascular event rates for higher grades of carotid restenosis after carotid endarterectomy. Routine surveillance with carotid ultrasound does not appear to identify patients at higher risk for postoperative cerebrovascular events.

Lack of effect of aspirin in asymptomatic patients with carotid bruits and substantial carotid narrowing. The Asymptomatic Cervical Bruit Study Group

OBJECTIVE

To determine the effectiveness of aspirin in preventing ischemic events in patients with asymptomatic carotid stenosis.

DESIGN

Double-blind, placebo-controlled trial.

SETTING

University-affiliated hospitals.

PATIENTS

372 neurologically asymptomatic patients with carotid stenosis of 50% or more in at least one artery as determined by luminal diameter reduction on duplex ultrasonography.

INTERVENTION

Patients were randomly assigned to receive either enteric coated aspirin, 325 mg/d, or identically appearing placebo. Duration of therapy was 2.0 years for the aspirin recipients and 1.9 years for the placebo recipients.

OUTCOME MEASURES

Patients were scheduled for a clinical examination every 6 months for assessment of the occurrence of any clinical event in the composite end point, which consisted of transient ischemic attack, stroke, myocardial infarction, unstable angina, or death.

RESULTS

At baseline, the 188 patients receiving aspirin and the 184 patients receiving placebo had similar demographic, ultrasonographic, and laboratory characteristics. The median duration of follow-up was 2.3 years. The annual rate of all ischemic events and death from any cause was 12.3% for the placebo group and 11.0% for the aspirin group (P = 0.61). The Cox proportional hazards analysis yielded an adjusted hazard ratio (aspirin-placebo) of 0.99 (95% CI, 0.67 to 1.46; P = 0.95). The annual rates for vascular events only were 11% for the placebo group and 10.7% for the aspirin group (P = 0.99). The multivariate analysis yielded a hazard ratio of 1.08 (CI, 0.72 to 1.62; P = 0.71).

CONCLUSION

Aspirin did not have a significant long-term protective effect in asymptomatic patients with high-grade (> or = 50%) carotid stenosis.

Hemostatic markers in acute transient ischemic attacks

BACKGROUND AND PURPOSE

Hemostatic abnormalities have been shown previously in stroke patients. The purpose of this study was to assess the activity of selected parameters of the coagulation system in acute reversible cerebral ischemia.

METHODS

We measured fibrinopeptide A, thrombin-antithrombin III, and D-dimer in 36 patients in both the acute (< 7 days) and postacute stage (1 and 3 months) after a transient ischemic attack (TIA). The results were compared with those of 20 asymptomatic patients with a history of remote TIA and 65 age- and sex-matched controls.

RESULTS

Mean fibrinopeptide A and thrombin-antithrombin III values were elevated in the acute stage after a TIA (P < .02) compared with levels at 1 month. In contrast, D-dimer was significantly increased at all three times points after the event when compared with remote TIA (P < .05) or control subjects (P < .001). No association could be found between marker levels and clinical outcome or the degree of cervical atherosclerosis as assessed by duplex ultrasonography.

CONCLUSIONS

These findings suggest that after acute reversible cerebral ischemia, there is early transient activation of thrombogenesis and ongoing fibrinolysis.

The subcortical afferents to caudate nucleus and putamen in primate: a fluorescence retrograde double labeling study

The cellular origin and degree of collateralization of the subcortical afferents to the caudate nucleus and the putamen in squirrel monkeys (Saimiri sciureus) were studied using the following combinations of fluorescent retrograde tracers: Evans blue and DAPI-Primuline, Fast blue and Nuclear yellow, True blue and Nuclear yellow. After the injections, cells containing the tracer delivered in caudate nucleus (caudate-labeled cells) and others labeled with the complementary tracer injected in putamen (putamen-labeled cells) occur in large number in intralaminar nuclei, substantia nigra pars compacta, midbrain raphe nuclei and central midbrain tegmentum. In addition, a small to moderate number of putamen-labeled cells is found in external pallidum, pulvinar and laterodorsal thalamic nuclei, and basolateral amygdaloid nucleus, whereas some caudate and putamen-labeled cells are scattered in ventral tegmental area and locus coeruleus. However, ver few double-labeled cells are present in all these structures. In rostral intralaminar nuclei, the labeled cells are not confined to the know cytoarchitectonic boundaries of the nuclei but impinge slightly upon ventrolateral and mediodorsal nuclei. At this level, the caudate-labeled cells lie more dorsally and medially relative to putamen-labeled cells, but a high degree of intermingling exists and some double-labeled cells occur particularly in nucleus centralis lateralis. In caudal intralaminar nuclei, caudate-labeled cells are strictly confined to parafascicular nucleus and putamen-labeled cells present only in centre median, without any overlap between the two neuronal populations. In substantia nigra pars compacta, clusters of caudate-labeled cells are closely intermingled with clusters of putamen-labeled cells according to a complex mosaic-like pattern that varies along the rostrocaudal extent of the structure. Overall, however, caudate-labeled cells predominate rostrodorsally and putamen-labeled cells are more abundant caudoventrally in substantia nigra pars compacta, with only a few double-labeled cells. Some caudate and putamen-labeled cells are also scattered in contralateral substantia nigra pars compacta. In dorsal raphe nucleus, putamen labeled cells tend to occupy a more lateral position relative to caudate-labeled cells, with again very few double-labeled neurons. The caudate and putamen-labeled cells are less numerous and more closely intermingled in nucleus centralis superior. Numerous striatal afferent cells are also found bilaterally in the peribrachial region of midbrain tegmentum, comprising the pedunculopontine nucleus area.(ABSTRACT TRUNCATED AT 400 WORDS)

The output organization of the substantia nigra in primate as revealed by a retrograde double labeling method

The cellular origin and degree of collateralization of the efferent projections of the substantia nigra pars reticulata (SNr) in the squirrel monkey (Saimiri sciureus) were studied using the following combinations of fluorescent retrograde tracers: Evans blue and DAPI-Primuline, Fast blue and Nuclear yellow, True blue and Nuclear yellow. In a first series of experiments one tracer was injected in the ventral anterior (VA) and ventral lateral (VL) thalamic nuclei, and the complementary tracer was delivered in the peribrachial area of midbrain tegmentum. After thalamo-tegmental injections numerous nigrothalamic neurons occur in clusters, particularly in rostrolateral part of SNr, whereas the nigrotegmental neurons prevail in caudomedial segment of SNr. However, a significant overlap exists between these two populations. The nigrothalamic and nigrotegmental neurons are present in about equal number in SNr with as much as 60% of these neurons being double-labeled. In a second series of experiments injections were made concomitantly in VA/VL nuclei and in superior colliculus. After thalamo-collicular injections the nigrothalamic neurons are found in larger number than the nigrocollicular neurons which are mostly confined to the middle third of SNr. About 15-20% of all SNr positive neurons are double-labeled, although this proportion climbs to 30-40% in certain sections taken through the middle third of SNr. Finally, injections were made concomittantly in superior colliculus and in midbrain tegmentum. In contrast to the findings obtained after thalamo-tegmental and thalamo-collicular injections, only about 10% of SNr neurons appear to be double-labeled after colliculo-tegmental injections. All injections made in present study have produced retrograde cell labeling in contralateral SNr. However, by far the largest number of contralateral labeled neurons is found after superior colliculus injection. These findings reveal that the SNr neurons in primate, as those in rat and cat, display a high degree of axonal branching. As such, the output organization of SNr appears to differ markedly from that of the substantia nigra pars compacta, but is remarkably similar to that of the internal pallidum which is the other major output structure of the basal ganglia.