Age-, region-, and day/night-related variation of the chloride reversal potential in the rat suprachiasmatic nucleus.

The master control of mammalian circadian rhythms is the suprachiasmatic nucleus (SCN), which is formed by the ventral and dorsal regions. In SCN neurons, GABA has an important function and even excitatory actions in adulthood. However, the physiological role of this neurotransmitter in the developing SCN is unknown. Here, we recorded GABAergic postsynaptic currents (in the perforated-patch configuration using gramicidin) to determine the chloride reversal potential (ECl) and also assessed the immunological expression of the Na-K-Cl cotransporter 1 (NKCC1) at early ages of the rat (postnatal days (P) 3 to 25), during the day and night, in the two SCN regions. We detected that ECl greatly varied with age and depending on the SCN region and time of day. Broadly speaking, ECl was more hyperpolarized with age, except for the oldest age studied (P20-25) in both day and night in the ventral SCN, where it was less negative. Likewise, ECl was more hyperpolarized in the dorsal SCN both during the day and at night; while ECl was more negative at night both in the ventral and the dorsal SCN. Moreover, the total NKCC1 fluorescent expression was higher during the day than at night. These results imply that NKCC1 regulates the circadian and developmental fluctuations in the [Cl-]i to fine-tune ECl, which is crucial for either excitatory or inhibitory GABAergic actions to occur in the SCN.

Increased glutamatergic neurotransmission between the retinohypothalamic tract and the suprachiasmatic nucleus of old mice.

Circadian rhythms synchronize to light through the retinohypothalamic tract (RHT), which is a bundle of axons coming from melanopsin retinal ganglion cells, whose synaptic terminals release glutamate to the ventral suprachiasmatic nucleus (SCN). Activation of AMPA-kainate and NMDA postsynaptic receptors elicits the increase in intracellular calcium required for triggering the signaling cascade that ends in phase shifts. During aging, there is a decline in the synchronization of circadian rhythms to light. With electrophysiological (whole-cell patch-clamp) and immunohistochemical assays, in this work, we studied pre- and postsynaptic properties between the RHT and ventral SCN neurons in young adult (P90-120) and old (P540-650) C57BL/6J mice. Incremental stimulation intensities (applied on the optic chiasm) induced much lesser AMPA-kainate postsynaptic responses in old animals, implying a lower recruitment of RHT fibers. Conversely, a higher proportion of old SCN neurons exhibited synaptic facilitation, and variance-mean analysis indicated an increase in the probability of release in RHT terminals. Moreover, both spontaneous and miniature postsynaptic events displayed larger amplitudes in neurons from aged mice, whereas analysis of the NMDA and AMPA-kainate components (evoked by RHT electrical stimulation) disclosed no difference between the two ages studied. Immunohistochemistry revealed a bigger size in the puncta of vGluT2, GluN2B, and GluN2A of elderly animals, and the number of immunopositive particles was increased, but that of PSD-95 was reduced. All these synaptic adaptations could be part of compensatory mechanisms in the glutamatergic signaling to ameliorate the loss of RHT terminals in old animals.

GABA Neurotransmission of the Suprachiasmatic Nucleus Is Modified During Rat Postnatal Development.

The suprachiasmatic nucleus (SCN) of the hypothalamus is the brain structure that controls circadian rhythms in mammals. The SCN is formed by two neuroanatomical regions: the ventral and dorsal. Gamma-aminobutyric acid (GABA) neurotransmission is important for the regulation of circadian rhythms. Excitatory GABA effects have been described in both SCN regions displaying a circadian variation. Moreover, the GABAergic system transfers photic information from the ventral to the dorsal SCN. However, there is almost no knowledge about GABA neurotransmission during the prenatal or postnatal development of the SCN. Here, we used whole-cell patch-clamp recordings to study spontaneous inhibitory postsynaptic currents (IPSCs) in the two SCN regions, at two zeitgeber times (day or night), and at four postnatal (P) ages: P3-5, P7-9, P12-15, and P20-25. The results herein show that the three analyzed parameters of the IPSCs, frequency, amplitude, and decay time, were significantly affected by the postnatal age: mostly, the IPSC frequency increased with age, principally in the ventral SCN in both day and night recordings; similarly, the amplitude of IPSCs augmented with age, especially at night, whereas the IPSC decay time was reduced (it was faster) with postnatal age, mainly during the day. Our findings first reveal that parameters of GABA neurotransmission are modified by postnatal development, implying that synaptic adjustments are required for an appropriate maturation of the GABAergic system in the SCN.

Correction to: Stingray spear injury to the pediatric spinal cord: case report and review of the literature.

The original version of this article unfortunately contained an error in the spelling of the last name of one of the co-authors. The corresponding author did not notice that the last name of one of the co-authors, Nathan Shlobin, was misspelled as "Schlobin". The correct spelling of his last name is "Shlobin". Given in this article is the corrected author name.

Synergistic antidepressant-like effect of capsaicin and citalopram reduces the side effects of citalopram on anxiety and working memory in rats.

RATIONALE: We have previously shown that in rats, capsaicin (Cap) has antidepressant-like properties when assessed using the forced swimming test (FST) and that a sub-threshold dose of amitriptyline potentiates the effects of Cap. However, synergistic antidepressant-like effects of the joint administration of Cap and the selective serotonin reuptake inhibitor citalopram (Cit) have not been reported. OBJECTIVES: To assess whether combined administration of Cap and Cit has synergistic effects in the FST and to determine whether this combination prevents the side effects of Cit. METHODS: Cap, Cit, and the co-administration of both substances were evaluated in a modified version of the FST (30-cm water depth) conducted in rats, as well as in the open field test (OFT), elevated plus maze (EPM), and Morris water maze (MWM). RESULTS: In line with previous studies, independent administration of Cap and Cit displayed antidepressant-like properties in the FST, while the combined injection had synergistic effects. In the OFT, neither treatment caused significant increments in locomotion. In the EPM, the time spent in the closed arms was lower in groups administered either only Cap or a combination of Cap and Cit than in groups treated with Cit alone. In the MWM, both Cap and the joint treatment (Cap and Cit) improved the working memory of rats in comparison with animals treated only with Cit. CONCLUSION: Combined administration of Cap and Cit produces a synergistic antidepressant-like effect in the FST and reduces the detrimental effects of Cit on anxiety and working memory.

Stingray spear injury to the pediatric spinal cord: case report and review of the literature.

Stingray injuries are rare, mostly causing injuries to the lower extremities but occasionally fatal if there is direct puncture of the thorax, abdomen, or neck. Direct combined stingray injury to the central nervous system has not been reported in the literature. Herein we present the case of a 12-year-old boy who, while wading at the seashore of the Costa Rica's Pacific Ocean, sustained a combined oblique penetrating injury to the C6 vertebra caused by a Stingray. He initially presented to the hospital with a complete asymmetric right C6/left T1 ASIA A examination, priapism, and loss of anal sphincter tone. Imaging revealed fracture of the posterior elements of C6 with an oblique trajectory into the left radicular foramen. T2W images did not reveal anatomical section but rather edema and minor bleeding in the epidural space. The patient underwent medical management and serial imaging. During the next 3 months, there was recovery of sensation on the right hemi body, bilateral paresthesias and asymmetric progressive improvement in strength on both legs. Acute care management and midterm term follow up are provided, along with a review of the literature for salient management considerations when evaluating and treating combined penetrating and envenomation injuries caused by stingrays. To our knowledge, this is the first report of such injury to the spine.

Genotoxicity in fishes environmentally exposed to As, Se, Hg, Pb, Cr and toxaphene in the lower Colorado River basin, at Mexicali valley, Baja California, México.

The environmental exposure to As, Se, Hg, Pb, Cr and toxaphene was assessed for 11 freshwater fish species in irrigation channels, agricultural return flow drains, a drain collecting lagoon and sections of the Colorado River at the Mexicali valley in Baja California, México, during August 2015-April 2016. Arsenic (2.90 ng ml-1) and Se (1.41 ng ml-1) in water had the highest concentrations in the return flow drains (Hardy River and Xochimilco Lagoon, respectively). However, fish axial muscle tissue had the highest concentration of Se (8.3 µg g-1) and Hg (0.36 µg g-1) in Colorado River fresh water, while As (1.7 µg g-1) in Hardy River fish was highest. Selenium concentrations in all fishes and toxaphene in Cyprinus carpio and Ameiurus natalis are above the safe levels for human consumption (0.3 µg g-1 and 180 ng g-1 respectively). Toxaphene was detected in the fish axial tissue, having the highest concentrations in Poecilia latipinna (690 ng g-1) in the Colorado River. The low proportion of the 8-Cl toxaphene congeners in fish suggests degradation of this pollutant. Tilapia. sp. cf. zillii had the most genotoxic damage with 7.4 micronucleated erythrocytes per 10,000 erythrocytes in Xochimilco Lagoon and 2 in Hardy River. The genotoxicity in all the fish species studied was significantly correlated to the concentrations of As and Se in water.

Functional Pre- and Postsynaptic Changes between the Retinohypothalamic Tract and Suprachiasmatic Nucleus during Rat Postnatal Development.

The suprachiasmatic nucleus (SCN) is the main brain clock in mammals. The SCN synchronizes to the light-dark cycle through the retinohypothalamic tract (RHT). RHT axons release glutamate to activate AMPA-kainate and N-methyl-D-aspartate (NMDA) postsynaptic receptors in ventral SCN neurons. Stimulation of SCN NMDA receptors is necessary for the activation of the signaling cascades that govern the advances and delays of phase. To our knowledge, no research has been performed to analyze the functional synaptic modifications occurring during postnatal development that prepare the circadian system for a proper synchronization to light at adult ages. Here, we studied the pre- and postsynaptic developmental changes between the unmyelinated RHT-SCN connections. Spontaneous NMDA excitatory postsynaptic currents (EPSCs) were greater in amplitude and frequency at postnatal day 34 (P34) than at P8. Similarly, both quantal EPSCs (miniature NMDA and evoked quantal AMPA-kainate) showed a development-dependent increase at analyzed stages, P3-5, P7-9, and P13-18. Moreover, the electrically evoked NMDA and AMPA-kainate components were augmented with age, although the increment was larger for the latter, and the membrane resting potential was more depolarized at early postnatal ages. Finally, the short-term synaptic plasticity was significantly modified during postnatal development as was the estimated number of quanta released and the initial release probability. All of these synaptic modifications in the unmyelinated RHT-SCN synapses suggest that synchronization to light at adult ages requires developmental changes similar to those that occur in myelinated fast communication systems.

Development-Dependent Changes in the NR2 Subtype of the N-Methyl-D-Aspartate Receptor in the Suprachiasmatic Nucleus of the Rat.

The suprachiasmatic nucleus (SCN) is the main brain clock that regulates circadian rhythms in mammals. The SCN synchronizes to the LD cycle through the retinohypothalamic tract (RHT), which projects to ventral SCN neurons via glutamatergic synapses. Released glutamate activates N-methyl-D-aspartate (NMDA) receptors, which play a critical role in the activation of signaling cascades to enable phase shifts. Previous evidence indicates that presynaptic changes during postnatal development consist of an increase in RHT fibers impinging on SCN neurons between postnatal day (P) 1 to 4 and P15. The aim of this study was to evaluate postsynaptic developmental changes in the NR2 subunits that determine the pharmacological and biophysical properties of the neuronal NMDA receptors in the ventral SCN. To identify the expression of NR2 subtypes, we utilized RT-PCR, immunohistochemical fluorescence, and electrophysiological recordings of synaptic activity. We identified development-dependent changes in NR2A, C, and D subtypes in mRNA and protein expression, whereas NR2B protein was equally present at all analyzed postnatal ages. The NR2A antagonist PEAQX (100 nM) reduced the frequency of NMDA excitatory postsynaptic currents (EPSCs) at P8 significantly more than at P34, but the antagonists for NR2B (3 µM Ro 25-6981) and NR2C/D (150 nM PPDA) did not influence NMDA EPSCs differently at the 2 analyzed postnatal ages. Our results point to P8 as the earliest analyzed postnatal age that shows mRNA and protein expression similar to those found at the juvenile stage P34. Taken together, our findings indicate that postsynaptic development-dependent modifications in the NR2 subtypes of the NMDA receptor could be important for the synchronization of ventral SCN neurons to the LD cycle at adult stages.

On the sources of PBDEs in coastal marine sediments off Baja California, Mexico.

Polybrominated diphenyl ethers (PBDEs) are widely distributed compounds in all types of matrices. In the northern portion of the Southern California Bight (SCB), there were reports of some of the largest PBDE concentrations in marine mammals and mussels. Because of this, we decided to analyze the status of PBDEs in the southern part of the SCB. An analysis of 91 samples of marine surface sediment was carried out. All of the 91 samples contained measurable amounts of PBDEs, which is a manifestation of the widespread distribution of these chemical substances. However, the levels detected are between one and two orders of magnitude smaller than those reported in southern California. Currents appear to control the distribution of PBDEs along the coast and the sedimentation sites with largest concentrations are favored by local bathymetry. Maximum concentrations were located in the middle and deeper platforms ranging from 0.02 to 5.90 (with a median 0.71) ng·g(-1) d.w. Deca-BDE mixture is largely predominant in the sediments followed by the penta-BDE mixture. The mass balance for the latitudinal strata shows the largest concentrations in the north where the largest population centers are present and with a very clear southward gradient. The mass balance calculation values showed about 36kg of PBDEs for the north, 22kg for the center, and 10kg for the south strata. In terms of depth, the PBDEs are mainly located on the middle and deep platforms rather than near point discharges, which is different than that reported by other authors.

Are the clinical guideline recommendations on gastroprotection being followed? A review in patients taking nonsteroidal anti-inflammatory drugs. / ¿Qué tanto se siguen las recomendaciones de las guías clínicas sobre gastroprotección? Una revisión en enfermos que consumen antiinflamatorios no esteroideos.

INTRODUCTION AND AIMS: The chronic use of nonsteroidal anti-inflammatory drugs (NSAIDs) can cause complications in the gastrointestinal tract. The use of proton pump inhibitors (PPIs) is recommended in high-risk patients to prevent them. OBJECTIVE: The aim of this article was to evaluate the gastroprotection measures taken in persons with chronic NSAID use. MATERIALS AND METHODS: A descriptive cross-sectional study was conducted. The clinical records were reviewed of patients seen as outpatients at the Rheumatology Department over a 4-month period, choosing those with chronic NSAID use, and intentionally looking for gastroprotection measures according to the recommendations published by the American College of Gastroenterology. RESULTS: A total of 417 patients (347 women; mean age: 48.12±14.2 years) were included. The most frequent diagnosis was rheumatoid arthritis (65%). Nine patients (2.1%) had a history of peptic ulcer, 48 (11.5%) patients were 65 years of age or older, 26 (6.2%) patients took NSAIDs and aspirin, and 130 (31.2%) took NSAIDs with steroids. Tests for Helicobacter pylori infection were done in just 53 cases, and there were positive results in only 9 (16%). Some risk for gastrointestinal toxicity was established in 211 cases and only 65 (30.8%) received gastroprotection. In contrast, 31 (15%) patients received gastroprotection when there was no indication for it. CONCLUSION: Prophylaxis with PPIs in chronic NSAID users was inadequately employed. It was not prescribed in the majority of patients (69.2%) and it was used with no justification in others (15%).

Overlap syndromes of autoimmune hepatitis: diagnosis and treatment.

Some patients with autoimmune liver disease have characteristics of cholestasis, as well as of autoimmune hepatitis. Despite the fact that this is a relatively frequent clinical condition seen in referral centers for liver diseases, there is little evidence as regards the clinical management of these syndromes due to their low prevalence and the lack of standardized definitions and diagnostic criteria. This is relevant, given that published studies report that there is a lower therapeutic response and poorer outcome in patients with overlap syndrome than in those presenting solely with autoimmune hepatitis. Whether overlap syndromes are distinct entities or the presence of 2 concurrent diseases is still a subject of debate. They should be suspected in autoimmune hepatitis patients that present with signs of cholestasis, as it is known that overlap behavior tends to be more aggressive, with higher rates of cirrhosis and the need for liver transplantation. Treatment response is also poorer and should be directed at the predominant component. Standardized definitions are necessary so that these syndromes can be studied in controlled clinical trials.

Work instability in rheumatoid arthritis patients from Argentina: prevalence and associated factors.

To determine the prevalence of and associated factors to work instability (WI) in rheumatoid arthritis (RA) Argentinean patients. Observational cross-sectional study that assessing employment status in currently working RA patients. They answered the validated version of RA work instability scale (RA-WIS). High-risk WI was considered when RA-WIS was ≥17. Factors associated with high-risk WI were examined by univariable and multivariable analysis. Four-hundred and fifty RA patients were enrolled; of these, 205 patients were currently employed, but only 172 have completed questionnaires required [RA-WIS and health assessment questionnaire (HAQ-A)]. Their mean age was 49.3 ± 10.8 years; 81.3 % were female; and their mean disease duration was 8.1 ± 7.2 years. Fifty-two percent of patients were doing manual work. The mean RA-WIS score was 11.4 ± 6.8, and 41 % of patients had a high-risk WI. High-risk WI was associated with radiographic erosions (p < 0.001) and HAQ-A >0.87 (p < 0.001) in the univariable analysis, whereas in the multivariable logistic regression analysis the variables associated with a high-risk WI were as follows: HAQ-A >0.87 [odds ratio (OR) 12.31; 95 % CI 5.38-28.18] and the presence of radiographic erosions (OR 4.848; 95 % CI 2.22-10.5). In this model, having a higher monthly income (OR 0.301; 95 % CI 0.096-0.943) and a better functional class (OR 0.151; 95 % CI 0.036-0.632) were protective. Forty-one percent of RA working patients had high-risk WI. The predictors of high RA-WIS were HAQ-A ≥0.87 and radiographic erosions, whereas having a better functional class and have higher incomes were protective.

Clinical and biological acceptance of a fibrocollagen-coated mersylene patch for tracheal repair in growing dogs.

BACKGROUND: Collagen-covered prostheses can be used as a non-circumferential segmental tracheal replacement. However, the applicability of these implants in young subjects has not yet been reported. METHODS: In this experimental, longitudinal study, dogs aged 29-32 days underwent limited segmental tracheal replacement with a polyester prosthesis or were allocated to a control, untreated group. The dogs were evaluated clinically, endoscopically and tomographically for up to one year. RESULTS: Although there was evidence of tracheal growth in the experimental group, tomographic measurements were significantly smaller in this group than in the control group throughout the observation period. At the end of the study, there was no evidence of implant rejection, stenosis or collapse. Normal respiratory epithelium had grown across the implanted membrane in the experimental group. CONCLUSION: The homologous collagen mersylene membrane allowed for limited structural tracheal growth and was functionally integrated into the segmented tracheal wall in growing dogs.

Patient satisfaction and quality of life following laparoscopic Nissen fundoplication.

BACKGROUND: Today, antireflux surgery has an established position in the management of gastroesophageal reflux disease. Some case series have shown good short-term results, but there is still little information regarding long-term results. Studies have recently focused on evaluating residual symptomatology and its impact on quality of life. OBJECTIVES: To determine the postoperative quality of life and degree of satisfaction in patients that underwent laparoscopic Nissen fundoplication. PATIENTS AND METHODS: A total of 100 patients (59 women and 41 men) were studied after having undergone laparoscopic Nissen fundoplication. The variables analyzed were level of satisfaction, gastrointestinal quality of life index (GIQLI), residual symptoms, and the Visick scale. RESULTS: No variation was found in relation to sex; 49 men and 51 women participated in the study. The mean age was 49 years. The degree of satisfaction encountered was: satisfactory in 81 patients, moderate in 3, and bad in 2 patients. More than 90% of the patients would undergo the surgery again or recommend it. The Carlsson score showed improvement at the end of the study (p<0.05). In relation to the GIQLI, a median of 100.61 points±21.624 was obtained. Abdominal bloating, regurgitation, and early satiety were the most frequent residual symptoms. The effect on lifestyle measured by the Visick scale was excellent. CONCLUSIONS: The level of satisfaction and quality of life obtained were comparable with reported standards; and the residual symptoms after antireflux surgery were easily controlled.

Persistent organic pollutants associated to water fluxes and sedimentary processes in the Colorado River delta, Baja California, México.

Polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) were studied in sediment cores from two distinctive modern channels of the Colorado River (CR) delta. Their abundance and temporal changes are associated with flood-flows from the CR across the USA-Mexico border. The CR channel is directly exposed to river flood-flows while the Hardy River (HR) is a local channel derived mainly from agricultural runoff, geothermal effluents, and treated urban wastewater. Different headwater compositions and degrees of exposure to flood-flows appear to be the factors controlling the composition of persistent organic pollutants (POPs). Enrichment of OCPs (46 ng g(-1) dwt in HR and 4.37 ng g(-1) dwt in CR) occurred during or a few years after flooding. PCB-138 (4.2 ng g(-1)dwt) is enriched in HR suggesting its origin in dielectric oils from the geothermal power plant. PCB-28 (2.1 ng g(-1)dwt) in CR may be related with atmospheric input and/or re-deposition of upstream sediments. In surficial sediments (0-3 cm), only HR exceeds international sediment quality guidelines (4,4'-DDE=8.16 ng g(-1)dwt and ΣDDT=8.34 ng g(-1)dwt).

Historical trends of polychlorinated dibenzo-p-dioxins and dibenzofurans in three dated sediment cores from Mexico.

This paper describes the first report of dioxins and furans (PCDDs/Fs) in sediment cores from Mexico. Sedimentation rates and vertical fluxes were estimated using (210)Pb dating. Two cores correspond to marine sediments and one to an endorheic lake. Concentrations of PCDDs/Fs found in the three sites are typical of non-impacted areas with low concentrations when compared to reference values. However the PCDDs/Fs sediment profiles show an increasing concentration trend in the upper core sections. This behavior is different from that found at many sites around the globe where diminishing concentrations have been reported. A strong predominance of OCDD was observed, and a comparison to typical composition profiles of industrial and other sources did not result in clear origin assignments for these measured compounds. We suggest that local sources may be responsible for the increase in concentration and, because these undetermined sources have not been curtailed, their importance is still growing.

Anhydrobiosis quotient: a novel approach to evaluate stability in desiccated bacterial cells.

AIM: The major objective of this study was the development of a methodology to quantify the anhydrobiotic ability of bacteria and its application to evaluate the stability of desiccated bacterial cells using the biocontrol agent Tsukamurella paurometabola C-924 as a model of anhydrobiote. METHODS AND RESULTS: Tsukamurella paurometabola C-924 was desiccated by spray-drying. Samples of desiccated cells were stored at several temperatures and viability and residual moisture were measured at different intervals of time. The term anhydrobiosis quotient (epsilon) was defined, and a scale of anhydrobiotic ability for classifying micro-organisms in terms of tolerance to desiccation was established (1 < or = epsilon < or = 15). The anhydrobiosis quotient was used to evaluate the stability of the anhydrobiotic cells. As a main result, changes in the anhydrobiosis quotient at several temperatures were fitted using a reparameterized Weibull model, which was found to be robust for the prediction of the stability at 4 degrees C. CONCLUSIONS: A novel methodology was developed to evaluate the desiccated state in bacteria. The anhydrobiosis quotient allows the quantitative estimation of the anhydrobiotic ability, and the mathematical model developed allows the prediction of the desiccated state of bacterial populations. SIGNIFICANCE AND IMPACT OF THE STUDY: The new methodology could be applied in studying the anhydrobiosis state of bacterial populations as a predictive tool for industrial and environmental microbiology.

Trace metals in sediments and Zostera marina of San Ignacio and Ojo de Liebre lagoons in the central pacific coast of Baja California, Mexico.

San Ignacio and Ojo de Liebre lagoons in central Baja California, Mexico are nursery and grazing grounds for whales and turtles. Ojo de Liebre Lagoon also supports a salt mine operation. By concentrating trace metals via evaporation, this activity might harm biota. Consequently, salt mining might be incompatible with the lagoon's ecological role. Eelgrass can incorporate these elements and reroute them to other organisms. Trace metals in sediments (Cd, Co, Cu, Mn, Ni, Pb, Zn, and Fe) were measured at both lagoons. Some (Cu, Mn, Pb, and Zn) were also measured in Zostera marina patches at both lagoons. The results did not show elevated metal concentration at any lagoon, either for sediments or eelgrass. No statistically significant differences between lagoons were found. However, eelgrass at both lagoons showed larger concentration ranges than in sediments. Also, a correlation exists between sediment metal concentration and its concentration in eelgrass. Surprisingly, several sediment metal concentrations are higher than those considered as elevated for the Southern California Bight.

Distribution of mercury in surficial sediments from Todos Santos Bay, Baja California, México.

During 2004 the spatial distribution of total Hg in sediments from Todos Santos Bay, Baja California, México was studied to evaluate the degree of environmental impact in this bay. The results showed low concentrations and no Hg enrichment at any site. These findings suggest natural levels of Hg in the water of Todos Santos Bay. The regional distribution of Hg/Fe shows lower values in the East and higher in the West of the bay. No significant correlations (p<0.05) were found between Hg and organic matter or particle size, suggesting that the distribution of Hg is not controlled by these variables.