[The association between baseline TPOAb and/or TgAb positivity and thyroid immune-related adverse events in patients with malignancies following treatment with immune checkpoint inhibitors].

Objective: To investigate the association between positive anti-thyroid peroxidase antibody (TPOAb) and/or anti-thyroglobulin antibody (TgAb) and the occurrence of thyroid immune-related adverse events (irAEs) in patients with malignant tumors who treated with immune checkpoint inhibitors (ICIs). Methods: A case-control study. A total of 116 patients with malignant tumor who received ICIs treatment and underwent thyroid function evaluation at Peking Union Medical College Hospital from January 2017 to April 2023 were enrolled retrospectively, including 77 males and 39 females, with a median age of (M(Q1, Q3)) 63.0 (55.0, 70.0) years. The patients were divided into the euthyroid group (n=58) and the thyroid irAEs group (n=58) according to whether thyroid irAEs occurred after ICIs treatment. The clinical characteristics and baseline anti-thyroid antibodies associated with the occurrence of thyroid irAEs after ICIs treatment in patients with malignant tumors were evaluated. Variables with statistical significance in univariate analysis were included in multivariate logistic regression model to analyze the risk factors for thyroid irAEs in patients with malignant tumors who received ICIs treatment. Results: In irAEs group, therewore 4 (3.4%) cases of clinical thyrotoxicosis, 23(19.8%) cases of subclinical thyrotoxicosis, 23 (19.8%) cases of clinical hypothyroidism, and 8(6.9%) cases of subclinical hypothyroidism. The positive rate of anti-thyroid antibodies at baseline in the thyrioid irAEs group was higher than that in the euthyroid group［16/58（27.6%）vs 3/58（5.2%），P=0.001］. After at least one course of ICIs treatment, the incidence of thyroid irAEs in patients with positive anti-thyroid antibodies at baseline was 84.2% (16/19), whereas it was 43.3% (42/97) in patients with negative anti-thyroid antibodies(P=0.001). Univariate logistic regression analysis showed that gender (OR=2.812, 95%CI:1.257-6.293), baseline thyroid autoantibodies were positive (OR=6.984, 95%CI: 1.909-25.547), baseline TgAb positivity (OR=8.909, 95%CI: 1.923-41.280), and baseline TPOAb positivity (OR=7.304, 95%CI: 1.555-34.308) were associated with thyroid irAEs (all P<0.05). Multivariate logistic regression analysis indicated that baseline TgAb positivity (OR=7.637, 95%CI: 1.617-36.072) was a risk factor for thyroid irAEs (P=0.01). Conclusions: The incidence of thyroid irAEs is higher in patients who are positive for baseline TPOAb and/or TgAb compared to those who are negative for TPOAb and TgAb. Patients with positive TgAb at baseline are at high risk of developing thyroid irAEs.

Cell-free DNA from bile outperformed plasma as a potential alternative to tissue biopsy in biliary tract cancer.

BACKGROUND: Biliary tract cancers (BTCs) are rare and highly heterogenous malignant neoplasms. Because obtaining BTC tissues is challenging, the purpose of this study was to explore the potential roles of bile as a liquid biopsy medium in patients with BTC. PATIENTS AND METHODS: Sixty-nine consecutive patients with suspected BTC were prospectively enrolled in this study. Capture-based targeted sequencing was performed on tumor tissues, whole blood cells, plasma, and bile samples using a large panel consisting of 520 cancer-related genes. RESULTS: Of the 28 patients enrolled in this cohort, tumor tissues were available in eight patients, and plasma and bile were available in 28 patients. Somatic mutations were detected in 100% (8/8), 71.4% (20/28), and 53.6% (15/28) of samples comprising tumor tissue DNA, bile cell-free DNA (cfDNA), and plasma cfDNA, respectively. Bile cfDNA showed a significantly higher maximum allele frequency than plasma cfDNA (P = 0.0032). There were 56.2% of somatic single-nucleotide variant (SNVs)/insertions and deletions (indels) shared between bile and plasma cfDNA. When considering the genetic profiles of tumor tissues as the gold standard, the by-variant sensitivity and positive predictive value for SNVs/indels in bile cfDNA positive for somatic mutations were both 95.5%. The overall concordance for SNVs/indels in bile was significantly higher than that in plasma (99.1% versus 78.3%, P < 0.0001). Moreover, the sensitivity of CA 19-9 combined with bile cfDNA achieved 96.4% in BTC diagnosis. CONCLUSION: We demonstrated that bile cfDNA was superior to plasma cfDNA in the detection of tumor-related genomic alterations. Bile cfDNA as a minimally invasive liquid biopsy medium might be a supplemental approach to confirm BTC diagnosis.

Association between maximum mouth opening and area of bony fusion in simulated temporomandibular joint bony ankylosis.

The aim of this study was to investigate the quantitative association between active/passive maximum mouth opening (AMMO/PMMO) and the severity of simulated temporomandibular joint (TMJ) bony ankylosis. Twenty-eight male sheep were divided randomly and equally into surgical and control groups. Surgical group animals underwent bilateral TMJ osteotomy during which left lateral pterygoid muscle function was blocked. Control animals did not undergo surgery. Body weight, AMMO/PMMO, and TMJ morphological features were evaluated preoperatively and at 12 and 24 weeks post-surgery. In the surgical group, only the right TMJ complexes with maintained lateral pterygoid muscle function developed TMJ bony ankylosis. The AMMO/PMMO and end-feel distance in the surgical group were significantly lower than those in the control group (P < 0.001, both) at 12 and 24 weeks post-surgery. Moreover, AMMO (r = -0.940 and -0.952, P < 0.001, both) and PMMO (r = -0.944 and -0.953, P < 0.001, both) were negatively correlated with the area (mm2) of bony fusion post-surgery. These findings may be useful for the clinical treatment of early mandibular condyle fracture, with the use of occlusal pads/open-mouth plates to relax the lateral pterygoid muscle and block its function. When bony ankylosis developed in the TMJ, the greater the area of bony fusion, the more limited were AMMO/PMMO.

[Influence of environmental factors on the two-species biofilm formed by Streptococcus oligofermentans and Streptococcus mutans].

Objective: To study the influence of environmental factors on the two-species biofilm formed by the combinations of Streptococcus oligofermentans (So) with Streptococcus mutans (Sm) and Streptococcus sanguinis (Ss) with Sm so as to evaluate the role of So in maintaining the microecological balance of the oral cavity. Methods: Single-and two-species biofilms were grown on saliva-coated surfaces (glass tube and 96-well plate). Colony-counting method and safranin staining method were used to measure the biofilms formed under various oxygen conditions (aerobic and anaerobic), sucrose conditions (0%, 1% and 5% sucrose concentrations) and pH conditions (5.5, 6.0, 6.5, 7.0, 7.5 and 8.0). Results: Comparing the numbers of Sm in two co-cultures under various conditions, Sm counts in So+Sm group [(7.70±2.46)×10(8) CFU/ml] were significantly lower than those in Ss+Sm group [(9.00±1.13)×10(8) CFU/ml] in aerobic environment (P<0.05). Sm counts in So+Sm group [(2.80±0.52)×10(8) CFU/ml] were also significantly lower than those in the Ss+Sm group [(4.00±1.25)×10(8) CFU/ml] in anaerobic environment (P<0.05). The Sm counts in So+Sm group [(8.90±0.82)×10(8) CFU/ml] were significantly higher than those in Ss+Sm group [(7.50±1.73)×10(8) CFU/ml] in 0% sucrose environment (P<0.05). The Sm counts in So+Sm group [(5.70±2.94)×10(8) CFU/ml] were significantly lower than those in Ss+Sm group [(10.30±3.21) ×10(8) CFU/ml] in 1% sucrose environment (P<0.05). The Sm counts in So+Sm group [(6.10±1.71)×10(8) CFU/ml] were also significantly lower than those in Ss+Sm group [(7.40±1.20)×10(8) CFU/ml] in 5% sucrose environment (P<0.05). The Sm counts in So+Sm group [(3.50±1.50)×10(8) CFU/ml] were significantly lower than those in Ss+Sm group [(10.70±2.80)×10(8) CFU/ml] in pH7.0 environment (P<0.05). Comparing the formation of biofilm after 24 h cultivation, the Sm counts in So+Sm group were significantly lower than those in Ss+Sm group both in aerobic and anaerobic environments (P<0.05). The Sm counts in So+Sm group were significantly higher than those in Ss+Sm group in 0% sucrose environment (P<0.05). The Sm counts in So+Sm group were significantly lower than those in Ss+Sm group in 1% and 5% sucrose and pH 7.0 environments (P<0.05). Both So and Ss had no inhibitory effect on Sm in pH5.5 and pH8.0 environments. Conclusions: In the in vitro two-species co-culture systems, So showed stronger inhibitory effects than Ss on Sm and its inhibitory ability might influenced by various environmental factors.

Adenosine inhibits evoked synaptic transmission primarily by reducing presynaptic calcium influx in area CA1 of hippocampus.

Adenosine is a potent modulator of neuronal activity throughout the nervous system. Previous evidence suggests that adenosine presynaptically inhibits synaptic transmission. The mechanism of presynaptic inhibition is uncertain. Adenosine may inhibit transmitter release by reducing voltage-dependent Ca2+ currents, activating K+ currents, or by mechanisms downstream to Ca2+ influx. By simultaneously recording the presynaptic Ca2+ transient ([Ca]t) and the field excitatory postsynaptic potential (fEPSP) at CA3-CA1 synapses of hippocampal slices, we found that adenosine, through activation of presynaptic A1 receptors, inhibits the fEPSP primarily by reducing the [Ca]t. Reduced [Ca]t was due to inhibition of omega-conotoxin GVIA-sensitive and some unidentified Ca2+ channels, probably including Q-type, but not to omega-agatoxin-IVA-sensitive Ca2+ channels. The adenosine A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine enhanced both the [Ca]t and the fEPSP, suggesting a tonic inhibition of the [Ca]t by endogenous adenosine in the hippocampal slice.

Fast kinetics of exocytosis revealed by simultaneous measurements of presynaptic capacitance and postsynaptic currents at a central synapse.

The rate of release from nerve terminals depends on both the number of release sites and the rate of release at each site. The latter remains largely unknown at central synapses. We addressed this issue by simultaneously measuring the nerve terminal membrane capacitance and the postsynaptic current at single calyceal synapses in rat brainstem. We found that a 10 ms presynaptic step depolarization depleted a releasable pool containing 3300-5200 vesicles. Released vesicles were endocytosed with a time constant of a few seconds to tens of seconds. Release of only one third of this pool saturated both postsynaptic AMPA and NMDA receptors. A release site can release more than three vesicles in 10 ms (>300 vesicles per second). We conclude that both a large number of release sites and a fast release rate at each site enable synapses to release at a high rate.

The reduced release probability of releasable vesicles during recovery from short-term synaptic depression.

Recovery from synaptic depression is believed to depend mainly on replenishment of the releasable pool of vesicles. We observed that during recovery from depression in a calyx-type synapse, part of the releasable pool was replenished rapidly. Half recovery occurred within 1 s, even in the absence of residual calcium. Vesicles that had recently entered the releasable pool had a 7- to 8-fold lower release probability than those that had been in the pool for more than 30 s. These results suggest that the reduction in the release probability of releasable vesicles contributes greatly to the level of depression. How synapses maintain transmission during repetitive firing is in debate. We propose that during repetitive firing, accumulation of intracellular Ca2+ may facilitate release of the rapidly replenished but reluctant vesicles, making them available for sustaining synaptic transmission.

Nerve activity but not intracellular calcium determines the time course of endocytosis at the frog neuromuscular junction.

We used FM1-43 imaging and intracellular recordings of synaptic potentials to measure the time course of endocytosis in frog motor nerve terminals following tetanic nerve stimulation, and we used fura-2 imaging of intraterminal Ca2+ concentration to compare endocytic rate and [Ca2+]i. Following a 30 Hz tetanus, endocytosis declined exponentially with a time constant that depended on the duration of stimulation. The level of [Ca2+]i rose from a resting value of about 100 nM to more than 500 nM during 30 Hz stimulation, and rapidly declined to 200-250 nM after stimulation. [Ca2+]i returned to resting level with a time course that, like endocytosis, depended on the duration of tetanic stimulation. However, the rate of [Ca2+]i recovery was much slower than the rate of endocytosis, leading to the conclusion that endocytic rate is not determined solely by the instantaneous level of [Ca2+]i.

Imaging synaptic activity in intact brain and slices with FM1-43 in C. elegans, lamprey, and rat.

The fluorescent probe FM1-43 has been used extensively for imaging vesicle recycling; however, high nonspecific adsorption resulting in elevated background levels has precluded its use in certain tissues, notably brain slices. We have found that a sulfobutylated derivative of beta-cyclodextrin (ADVASEP-7) has a higher affinity for FM1-43 than the plasma membrane. ADVASEP-7 was used as a carrier to remove FM1-43 nonspecifically bound to the outer leaflet of the plasma membrane or extracellular molecules, significantly reducing background staining. This has enabled us to visualize synaptic vesicle recycling in the nematode C. elegans, intact lamprey spinal cord, and rat brain slices.

MiR-124 regulates osteoblast differentiation through GSK-3ß in ankylosing spondylitis.

OBJECTIVE: Ankylosing spondylitis (AS) is a spastic and spinal joint disease with the characteristic of pathological ossification. Bioinformatics analysis demonstrated that there is a complementary binding site between microRNA-124 (miR-124) and the 3'-UTR of glycogen synthase kinase-3ß (GSK-3ß) mRNA. We aimed to investigate the role of miR-124 in regulating GSK-3ß expression, Wnt/ß-catenin pathway activity, and osteoblast differentiation of spinal ligament fibroblasts. PATIENTS AND METHODS: The ligament tissues of AS and the femoral neck fracture patients were collected. MiR-124 and GSK-3ß mRNA expressions were detected by using quantitative Real-time PCR (qRT-PCR). GSK-3ß and ß-catenin protein expressions were detected by using Western blot. Ligament fibroblasts were isolated and induced to differentiate into osteoblasts. Alizarin red S staining (ARS) was used to identify osteoblast differentiation. Expressions of miR-124, GSK-3ß, ß-catenin, Osterix, and runt-related transcription factor 2 (RUNX2) were detected during differentiation. The cells were divided into two groups as agomiR-normal control (NC) transfection group and agomir miR-124 transfection group. Alkaline phosphatase (ALP) activity and Alizarin Red S staining were detected. RESULTS: MiR-124 and ß-catenin expressions in the ligament of AS patients increased, while GSK-3ß level reduced compared with control. MiR-124, ß-catenin, Osterix, and RUNX2 expressions gradually elevated, whereas GSK-3ß level gradually declined following increased osteoblasts differentiation. Antagomir miR-124 transfection significantly up-regulated the expression of GSK-3ß in osteoblast differentiation, significantly decreased the expression of ß-catenin, Osterix, and RUNX2, and significantly inhibited osteoblast differentiation. CONCLUSIONS: MiR-124 decreased and GSK-3ß elevated in AS ligament tissue. Down-regulation of miR-124 expression enhanced GSK-3ß expression, weakened Wnt/ß-catenin pathway activity, and inhibited the differentiation of ligament fibroblasts into osteoblasts.

Synaptic transmission. Kinetics of synaptic-vesicle recycling.

The kinetics of different steps in synaptic-vesicle recycling, including exocytosis, internalization and repriming, have recently been estimated in various types of living cell.

Presynaptic inhibition of elicited neurotransmitter release.

Activation of presynaptic receptors for a variety of neurotransmitters and neuromodulators inhibits transmitter release at many synapses. Such presynaptic inhibition might serve as a means of adjusting synaptic strength or preventing excessive transmitter release, or both. Previous evidence showed that presynaptic modulators inhibit Ca2+ channels and activate K+ channels at neuronal somata. These modulators also inhibit spontaneous transmitter release by mechanisms downstream of Ca2+ entry. The relative contribution of the above mechanisms to the inhibition of elicited release has been debated for a long time. Recent evidence at synapses where the relationship between transmitter release and presynaptic Ca2+ influx has been well characterized suggests that inhibition of presynaptic voltage-dependent Ca2+ channels plays the major role in presynaptic inhibition of elicited neurotransmitter release. In addition, modulation of the release machinery might contribute to inhibition of elicited release.

[Antibacterial and residual antimicrobial activities of five final irrigants in infected root canal: an in vitro comparative study].

OBJECTIVE: To evaluate antibacterial and residual antimicrobial activities of five root canal irrigants including Qmix, MTAD(mixture of a tetracycline isomer, an acid, and a detergent), 0.2% cetrimide(CTR), 2% chlorhexidine(CHX) and 17% ethylene diaminetetraacetic acid(EDTA) and to find the most optimal final irrigants for using in root canal therapy. METHODS: The standard enterococcus infection models were built up in 100 single rooted incisors with single canal. Totally 30 teeth were selected by using random number tablefor detecting the quality of the bacteria model. Crown-down technique with rotary ProTaper system was used to prepare the root canals. Then the teeth were randomly divided into seven groups of which five groups were irrigated with five different irrigants respectively, one group was irrigated with distilled water(distilled water group) and one group was no-irrigation group. Each tooth was sectioned into three parts: apical 1/3, middle 1/3 and coronal 1/3. After irrigation, specimenswere cultivated from day 0 to day 14. All statistical analyses were performed by means of SPSS 17.0 software. Chi-squared test was used to evaluate antibacterial activities. Generalized estimating equations was used to evaluate residual antimicrobial activities. RESULTS: All samples rinsed with Qmix, MTAD, CTR, CHX were bacteria-free in 0 day. The samples rinsed with EDTA and distilled water had no bacteria in 7 coronal sections, 6 middle sections and 9 apical sections, respectively. The results of Qmix, MTAD, CTR and CHX groups showed significant difference when compared with that of distilled water, EDTA and control groups(P<0.05). Residual antimicrobial resultsin EDTA, distilled water, no-irrigation groups showed significant differences compared with that of Qmix, MTAD, CTR, CHX groups according to pairwise comparison(P<0.05) on day 1, 2 and 3. There was no significant difference between the other two groups(P>0.05). Antimicrobial properties on the coronal 1/3 and apical 1/3, middle 1/3 and apical 1/3 showed significant difference(P<0.05) while middle 1/3 and coronal 1/3 showed no significant difference(P>0.05). CONCLUSIONS: Qmix, MTAD, CTR and CHX had an antimicrobial activity, but could not destroy Enterococcus faecalis completely. Antimicrobial activity in coronal 1/3 was better than in apical 1/3. Qmix, MTAD, CTR and CHX had a residual antimicrobial activity with various lasting times. The lasting time of residual antimicrobial activity was as follow: MTAD> CTR>Qmix>CHX. EDTA had no antibacterial and residual antimicrobial activities.

Protein kinase c increases the apparent affinity of the release machinery to Ca2+ by enhancing the release machinery downstream of the Ca2+ sensor.

Modulation of the release probability of releasable vesicles in response to Ca(2+) influx (Prob(Ca)) is involved in mediating several forms of synaptic plasticity, including short-term depression, short-term augmentation, and potentiation induced by protein kinases. Given such an important role, however, the mechanism underlying modulation of the Prob(Ca) is unclear. We addressed this question by investigating how the activation of protein kinase C modulates the Prob(Ca) at a calyx-type nerve terminal in rat brainstem. Various lengths of step depolarization were applied to the nerve terminal to evoke different amounts of Ca(2+) currents and capacitance jumps, the latter of which reflect vesicle release. The relationship between the capacitance jump and the Ca(2+) current integral was sigmoidal and was fit well with a Hill function. The sigmoidal relationship was shifted significantly to the left during the application of the PKC activator 12-myristate 13-acetate (PMA), suggesting that PMA increases the apparent affinity of the release machinery to Ca(2+). This effect was blocked in large part by the application of the PKC inhibitor bisindolylmaleimide, suggesting that the effect is mediated mainly by the activation of PKC. We also found that PMA increased the rate of miniature EPSCs evoked by the application of hypertonic sucrose solution, which triggers release downstream of the Ca(2+) influx. Taken together, our results suggest that PKC enhances the apparent affinity of the release machinery to Ca(2+) by a mechanism downstream of the binding between Ca(2+) and its sensor. These results have provided the first example of the mechanisms underlying modulation of the Prob(Ca).

[Rapid membrane effects of adrenocorticoids on celiac ganglion cells of guinea pig].

Intracellular recording technique was used to determine the effects of corticosterone, dexamethasone, aldosterone and cholesterol on the resting membrane potentials and membrane resistance of cells in isolated celiac ganglion of guinea pigs. Hyperpolarization was elicited by corticosterone in 15 out of 83 cells, but was not caused by dexamethasone, which depolarized the membrane in 2 out of 18 cells. In addition, depolarization was also elicited by corticosterone in 2 out of 83 cells. Aldosterone and cholesterol caused no detectable changes of membrane potential and membrane resistance. It is suggested that the membrane effects of adrenocorticoids, which obviously could not be explained by traditional genomic mechanism for their short latency, may indicate the existence of membrane receptors for adrenocorticoids in celiac ganglion neurons of guinea pigs.

Pharmacological identification of two types of presynaptic voltage-dependent calcium channels at CA3-CA1 synapses of the hippocampus.

The effects of voltage-dependent Ca channel (VDCC) antagonists on synaptic transmission were investigated at CA3-CA1 synapses of guinea pig hippocampal slices. After selectively loading presynaptic structures in area CA1 with the calcium indicator fura-2, we simultaneously recorded a presynaptic calcium transient ([Ca]t) and the corresponding field excitatory postsynaptic potential (fEPSP) evoked by a single stimulus given to the Schaffer collateral-commissural (SCC) pathway. Application of nifedipine did not reduce either the [Ca]t of the fEPSP, suggesting that nifedipine-sensitive Ca channels do not significantly contribute to evoked synaptic transmission at low stimulation frequency. Application of omega-conotoxin GVIA (omega-CgTX) or omega-agatoxin-IVA (omega-Aga-IVA) dose-dependently blocked both the [Ca]t and the fEPSP. The time course of the block of the [Ca]t was similar to that of the fEPSP. About 40% of the total [Ca]t was omega-CgTX sensitive, and more than 20% was omega-Aga-IVA sensitive. Combined application of these two blockers showed no overlap of the omega-CgTX-sensitive with the omega-Aga-IVA-sensitive [Ca]t. These results suggest that there are at least two types of presynaptic VDCCs at CA3-CA1 synapses of the hippocampus: omega-CgTX-sensitive and omega-Aga-IVA-sensitive Ca channels. Our results also suggest that these two types of Ca channels are colocalized at a single presynaptic terminal. During application of omega-CgTX or omega-Aga-IVA, the initial slope of the fEPSP varied approximately as the fourth power of the amplitude of the [Ca]t, suggesting that omega-CgTX-sensitive and omega-Aga-IVA-sensitive Ca channels have about equal efficacy in triggering transmitter release. These results in combination with similar findings at the squid giant synapse suggest that the nonlinear relationship between transmitter release and the Ca influx is well conserved from the molluscan to the mammalian nervous system.

Kinetics of synaptic depression and vesicle recycling after tetanic stimulation of frog motor nerve terminals.

We measured the time courses of two key components of the synaptic vesicle cycle during recovery from synaptic depression under different conditions, and used this and other information to create a kinetic model of the vesicle cycle. End plate potential (EPP) amplitudes were used to follow recovery from synaptic depression after different amounts of tetanic stimulation. This provided an estimate of the time course of vesicle mobilization from the reserve pool to the docked (readily releasable) pool. In addition, FM1-43 was used to measure the rate of membrane retrieval after tetanic stimulation, and the amount of membrane transferred to the surface membrane. This provided a measure of the rate of refilling of the reserve pool with recycled vesicles. The time courses of both synaptic depression and endocytosis were slowed by prolonged tetanic stimulation. This behavior could be fitted by a simple model, assuming a first-order kinetics for both vesicle endocytosis and mobilization. The results show that a nearly 20-fold decrease in the rate constant of endocytosis greatly delays refilling of the depleted reserve pool. However, to fully account for the slower recovery of depression, a decrease in the rate constant of vesicle mobilization from the reserve pool of about sixfold is also required.

Presynaptic calcium is increased during normal synaptic transmission and paired-pulse facilitation, but not in long-term potentiation in area CA1 of hippocampus.

We examined the relationship between presynaptic calcium levels and postsynaptic potentials during normal synaptic transmission, paired-pulse facilitation (PPF), and long-term potentiation (LTP) in CA3-CA1 synapses of hippocampus. By selectively loading the presynaptic terminals with the calcium indicator fura-2, we simultaneously recorded a presynaptic calcium (Ca) transient and the corresponding field EPSP evoked by a single stimulus given to the Schaffer collateral-commissural pathway in guinea pig hippocampal slices. A volume average presynaptic Ca influx was obtained by taking the first time derivative of the Ca transient. Our data indicate that the synaptic transmission represented by the initial slope of the field EPSP is approximately proportional to the fourth power of the presynaptic Ca influx, the volume average Ca current. Our results in combination with similar findings at the squid giant synapse (Augustine et al., 1985b; Augustine and Charlton, 1986) suggest that the relationship between Ca influx and transmitter release is well conserved from the molluscan to the mammalian nervous system. A transient increase of the residual Ca level ([Ca]res) is generally thought to be the mechanism underlying PPF (Katz and Miledi, 1968; Charlton et al., 1982); however, the relationship between PPF and the presynaptic [Ca]res had not been examined before. Our results demonstrate that PPF is approximately linearly related to the [Ca]res. This finding further supports the residual Ca hypothesis for PPF. Accumulated evidence from other groups suggests that the presynaptic site contributes to the maintenance of LTP in CA3-CA1 synapses (Bekkers and Stevens, 1990; Malinow and Tsien, 1990); however, our data show that neither an increase of the Ca transient nor a sustained increase of the [Ca]res occurs in the presynaptic terminals during maintenance of LTP. This suggests that the presynaptic mechanism underlying LTP must be downstream to Ca influx.