A phase 2 study of immunogenic cell death inducer PT-112 in patients with metastatic castration-resistant prostate cancer

TPS292

Background: PT-112 is a novel small I-O molecule currently in development for several cancers. Preclinical studies have shown the robust induction of immunogenic cell death by PT-112, leading to an anti-cancer adaptive immune response. Biodistribution experiments in mice showed PT-112’s partial bone affinity. In two prior phase I studies, late-line metastatic castration-resistant prostate cancer patients (mCRPC pts) were treated with evidence of anti-cancer activity, including tumor volume reductions, improvements in PET and bone scans, PSA and alkaline phosphatase (ALP) reductions, and anecdotal cases of improvement in disease-related pain. This, taken together with the immunogenic and osteotropic properties observed in preclinical work, provided a strong rationale to further explore PT-112 in mCRPC, an immunologically “cold” disease with a high prevalence of bone metastases. Moreover, in an analysis of >6,000 mCRPC pts, circulating tumor cell (CTC) declines (CTC0 and CTC conversions) were correlated with improvements in survival to a greater extent than PSA declines (PSA50) (Heller et al., JCO, 36:6 2017), prompting incorporation of CTC changes as a secondary endpoint. Methods: The primary objective of the study is to define the dose regimen of PT-112 for pivotal study. Pts are randomized to one of three arms, receiving IV PT-112 on 28-day cycles via one hour infusions on (1) Days 1 and 15 at 250 mg/m2, (2) Days 1 and 15 at 360 mg/m2, or (3) Days 1 and 15 of cycle 1 at 360 mg/m2 followed by 250 mg/m2 on Day 15 of subsequent cycles. Key eligibility criteria include radiographically progressive disease at study entry; ≥3 life-prolonging therapies for metastatic disease including 1-2 taxanes, ≥1 new generation anti-androgen therapies, and other drugs FDA approved on the basis of survival; and allowing bone-only metastatic disease. Efficacy assessments comprise disease control rate at 4 months (DCR4) using RECIST and PCWG3 criteria, objective response rate, CTC0 and CTC conversion, PSA50 and ALP reductions. Additionally, T cell receptor sequencing is conducted to characterize treatment-induced changes in T cell fraction and clonal expansions, as a means of generating meaningful supportive data on the immune mechanism of PT-112 monotherapy. A Fleming two-stage design will be used to assess each arm, with a 20% DCR4 as the null hypotheses, requiring ≥6 of 25 pts responding in stage one and ≥14 of 45 in total to reject the null. In addition, other efficacy measures, such as CTC responses, as well as exposure/response and exposure/safety analyses will be applied to characterize the risk/benefit ratio and select the optimal dose regimen for PT-112. As of October 11th, 2022, 38 pts have been enrolled out of a planned maximum of 135. Clinical trial information: NCT02266745 .

Abstract 1115: PT-112 induces potent mitochondrial stress and immunogenic cell death in human prostate cancer cell lines

Purpose: To investigate PT-112 in a human prostate cancer (PC) cell panel and assess differential sensitivity, cell death mechanism, induction of mitochondrial stress, and release of damage-associated molecular patterns (DAMPs).

Background: PT-112 is a novel pyrophosphate-platinum conjugate with clinical activity in advanced solid tumors including lung, thymoma and castration-resistant PC, and in multiple myeloma. PT-112’s cancer cell death was shown previously to be independent of DNA damage. In vitro and in vivo mouse experiments have shown that PT-112 causes mitochondrial reactive oxygen species (mtROS) accumulation, DAMP release, immunogenic cell death (ICD), and T cell infiltration.

Methods: Sensitivity to PT-112 was assessed in human PC cell lines (LNCap, LNCap-C4, LNCap-C4-2, DU-145, 22Rv1, VCap and PC-3) and the non-tumorigenic prostate cell line RWPE-1. We analyzed parameters involved in the cell death process such as apoptotic and necroptotic markers, mitochondrial membrane potential, and mtROS and autophagy by flow cytometry. We also evaluated PT-112’s induction of ICD markers calreticulin (CRT) cell surface exposure and ATP secretion. Finally, a possible link between HIF-1alpha expression and PT-112 sensitivity was investigated.

Results: PT-112 caused growth inhibition and cancer cell death without affecting healthy RWPE-1 cells. The pan-caspase inhibitor Z-VAD-fmk significantly reduced cell death, with more mild effects seen with the RIPK1/2 inhibitor necrostatin-1 in certain cell lines. PT-112-induced cell death was accompanied by a prominent increase of mtROS and decrease in mitochondrial membrane potential, as well as by DAMP emission (ATP release and CRT exposure). PT-112 activated markers of autophagy, and there was a positive relationship between HIF-1alpha expression and the sensitivity to PT-112 in this panel.

Conclusions: PT-112 was broadly active in the PC cell lines tested, while sparing benign prostate cells, indicative of PT-112 cancer cell selectivity and of activity that crosses the varied malignant prostate phenotypes, including androgen receptor positive and negative cell lines. Cell death was primarily apoptotic, as shown by the inhibitory effects of Z-VAD-fmk. Consistent with prior work reported in glycolytic murine cells, in this PC cell panel PT-112 induced mtROS accumulation and mitochondrial membrane depolarization, as well as DAMP release, demonstrating that these may be fundamental and linked responses of cancer cells to PT-112. The apparent induction of autophagy by PT-112 may be a cellular defense mechanism to drug-induced stress. The association between PT-112 sensitivity and HIF-alpha expression should be studied further, as validation of this finding could have clinical applications. Future studies will explore relationships across mitochondrial stress, ICD and HIF-1alpha in PT-112-treated cancer cells

Citation Format: Ruth Soler-Agesta, Tyler D. Ames, Matthew Price, José Jimeno, Christina Y. Yim, Raquel Moreno-Loshuertos, Alberto Anel. PT-112 induces potent mitochondrial stress and immunogenic cell death in human prostate cancer cell lines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1115.

MEF2C Phosphorylation Is Required for Chemotherapy Resistance in Acute Myeloid Leukemia

In acute myeloid leukemia (AML), chemotherapy resistance remains prevalent and poorly understood. Using functional proteomics of patient AML specimens, we identified MEF2C S222 phosphorylation as a specific marker of primary chemoresistance. We found that Mef2cS222A/S222A knock-in mutant mice engineered to block MEF2C phosphorylation exhibited normal hematopoiesis, but were resistant to leukemogenesis induced by MLL-AF9 MEF2C phosphorylation was required for leukemia stem cell maintenance and induced by MARK kinases in cells. Treatment with the selective MARK/SIK inhibitor MRT199665 caused apoptosis and conferred chemosensitivity in MEF2C-activated human AML cell lines and primary patient specimens, but not those lacking MEF2C phosphorylation. These findings identify kinase-dependent dysregulation of transcription factor control as a determinant of therapy response in AML, with immediate potential for improved diagnosis and therapy for this disease.Significance: Functional proteomics identifies phosphorylation of MEF2C in the majority of primary chemotherapy-resistant AML. Kinase-dependent dysregulation of this transcription factor confers susceptibility to MARK/SIK kinase inhibition in preclinical models, substantiating its clinical investigation for improved diagnosis and therapy of AML. Cancer Discov; 8(4); 478-97. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 371.

G0S2 represses PI3K/mTOR signaling and increases sensitivity to PI3K/mTOR pathway inhibitors in breast cancer

G0/G1 switch gene 2 (G0S2) is a direct retinoic acid target implicated in cancer biology and therapy based on frequent methylation-mediated silencing in diverse solid tumors. We recently reported that low G0S2 expression in breast cancer, particularly estrogen receptor-positive (ER+) breast cancer, correlates with increased rates of recurrence, indicating that G0S2 plays a role in breast cancer progression. However, the function(s) and mechanism(s) of G0S2 tumor suppression remain unclear. In order to determine potential mechanisms of G0S2 anti-oncogenic activity, we performed genome-wide expression analysis that revealed an enrichment of gene signatures related to PI3K/mTOR pathway activation in G0S2 null cells as compared to G0S2 wild-type cells. G0S2 null cells also exhibited a dramatic decreased sensitivity to PI3K/mTOR pathway inhibitors. Conversely, restoring G0S2 expression in human ER+ breast cancer cells decreased basal mTOR signaling and sensitized the cells to pharmacologic mTOR pathway inhibitors. Notably, we provide evidence here that the increase in recurrence seen with low G0S2 expression is especially prominent in patients who have undergone antiestrogen therapy. Further, ER+ breast cancer cells with restored G0S2 expression had a relative increased sensitivity to tamoxifen. These findings reveal that in breast cancer G0S2 functions as a tumor suppressor in part by repressing PI3K/mTOR activity, and that G0S2 enhances therapeutic responses to PI3K/mTOR inhibitors. Recent studies implicate hyperactivation of PI3K/mTOR signaling as promoting resistance to antiestrogen therapies in ER+ breast cancer. Our data establishes G0S2 as opposing this form of antiestrogen resistance. This promotes further investigation of the role of G0S2 as an antineoplastic breast cancer target and a biomarker for recurrence and therapy response.

Refractory testicular germ cell tumors are highly sensitive to the second generation DNA methylation inhibitor guadecitabine

Testicular germ cell tumors (TGCTs) are the most common cancers of young males. A substantial portion of TGCT patients are refractory to cisplatin. There are no effective therapies for these patients, many of whom die from progressive disease. Embryonal carcinoma (EC) are the stem cells of TGCTs. In prior in vitro studies we found that EC cells were highly sensitive to the DNA methyltransferase inhibitor, 5-aza deoxycytidine (5-aza). Here, as an initial step in bringing demethylation therapy to the clinic for TGCT patients, we evaluated the effects of the clinically optimized, second generation demethylating agent guadecitabine (SGI-110) on EC cells in an animal model of cisplatin refractory testicular cancer. EC cells were exquisitely sensitive to guadecitabine and the hypersensitivity was dependent on high levels of DNA methyltransferase 3B. Guadecitabine mediated transcriptional reprogramming of EC cells included induction of p53 targets and repression of pluripotency genes. As a single agent, guadecitabine completely abolished progression and induced complete regression of cisplatin resistant EC xenografts even at doses well below those required to impact somatic solid tumors. Low dose guadecitabine also sensitized refractory EC cells to cisplatin in vivo. Genome-wide analysis indicated that in vivo antitumor activity was associated with activation of p53 and immune-related pathways and the antitumor effects of guadecitabine were dependent on p53, a gene rarely mutated in TGCTs. These preclinical findings suggest that guadecitabine alone or in combination with cisplatin is a promising strategy to treat refractory TGCT patients.

G0S2 Suppresses Oncogenic Transformation by Repressing a MYC-Regulated Transcriptional Program

Methylation-mediated silencing of G0-G1 switch gene 2 (G0S2) has been detected in a variety of solid tumors, whereas G0S2 induction is associated with remissions in patients with acute promyelocytic leukemia, implying that G0S2 may possess tumor suppressor activity. In this study, we clearly demonstrate that G0S2 opposes oncogene-induced transformation using G0s2-null immortalized mouse embryonic fibroblasts (MEF). G0s2-null MEFs were readily transformed with HRAS or EGFR treatment compared with wild-type MEFs. Importantly, restoration of G0S2 reversed HRAS-driven transformation. G0S2 is known to regulate fat metabolism by attenuating adipose triglyceride lipase (ATGL), but repression of oncogene-induced transformation by G0S2 was independent of ATGL inhibition. Gene expression analysis revealed an upregulation of gene signatures associated with transformation, proliferation, and MYC targets in G0s2-null MEFs. RNAi-mediated ablation and pharmacologic inhibition of MYC abrogated oncogene-induced transformation of G0s2-null MEFs. Furthermore, we found that G0S2 was highly expressed in normal breast tissues compared with malignant tissue. Intriguingly, high levels of G0S2 were also associated with a decrease in breast cancer recurrence rates, especially in estrogen receptor-positive subtypes, and overexpression of G0S2 repressed the proliferation of breast cancer cells in vitro. Taken together, these findings indicate that G0S2 functions as a tumor suppressor in part by opposing MYC activity, prompting further investigation of the mechanisms by which G0S2 silencing mediates MYC-induced oncogenesis in other malignancies.

Abstract 2026: G0S2 functions as a tumor suppressor gene through inhibition of c-Myc

G0/G1 switch gene 2 (G0S2) is a direct retinoic acid target gene that is widely expressed in diverse organs and has been reported to have a role in adipose lipolysis by negatively regulating adipose triglyceride lipase (ATGL). G0S2 is also commonly silenced by DNA methylation in a variety of solid tumors including glioma, head and neck and lung cancers, and G0S2 induction is associated with retinoic acid-mediated clinical remissions in acute promyelocytic leukemia. This evidence suggests that G0S2 may possess tumor suppressor activity although definitive proof and mechanistic details are lacking. We now clearly demonstrate using G0S2 null immortalized mouse embryonic fibroblasts (MEFs) that G0S2 can function to oppose oncogene-induced transformation. G0S2 null MEFs were readily transformed with H-RAS or EGFR while wild-type MEFs were not. Importantly, re-introduction of G0S2 reversed H-RAS transformation of G0S2 null MEFs. Although G0S2 is a known regulator of fat metabolism through attenuating ATGL activity, we found that G0S2 repression of oncogene-induced transformation was independent of ATGL inhibition. Detailed microarray analysis demonstrated that G0S2 knockout alone upregulated gene signatures associated with transformation, proliferation and Myc transcriptional responses. G0S2 knockout also induced c-Myc expression at the transcript and protein level. Finally, transient and stable shRNA-mediated repression of c-Myc completely abrogated oncogene-induced transformation of G0S2 null MEFs. Taken together, these findings indicate that G0S2 can function as a tumor suppressor by opposing Myc. Studies to elucidate the mechanism underlying G0S2 regulation of Myc are ongoing. Understanding the functions and mechanisms of this novel tumor suppressor pathway may lead to new pharmacologic strategies for many cancers where G0S2 is silenced.

Citation Format: Christina Y. Yim, David J. Sekula, Sarah J. Freemantle, Ethan Dmitrovsky, Michael J. Spinella. G0S2 functions as a tumor suppressor gene through inhibition of c-Myc. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2026. doi:10.1158/1538-7445.AM2015-2026

Headway and hurdles in the clinical development of dietary phytochemicals for cancer therapy and prevention: lessons learned from vitamin A derivatives

Accumulating epidemiologic and preclinical evidence support the pharmacologic use of a variety of dietary chemicals for the prevention and treatment of cancer. However, it will be challenging to translate these findings into routine clinical practice since phytochemicals have pleiotropic biological activities that have to be balanced for optimal efficacy without unacceptable and potentially unanticipated toxicities. Correctly matching patient populations and settings with optimal, natural product-based phytochemical therapies will require a greater understanding of the specific mechanisms underlying the efficacy, toxicity, and resistance of each agent in a variety of normal, premalignant, and malignant settings. This, in turn, necessitates continued commitment from the basic research community to guide carefully designed and informed clinical trials. The most developed class of anticancer phytochemicals consists of the derivatives of vitamin A called retinoids. Unlike other natural product chemicals currently under study, the retinoids have been extensively tested in humans. Over 30 years of clinical investigation has resulted in several disappointments, but there were some spectacular successes where certain retinoid-based protocols are now FDA-approved standard of care therapies to treat specific malignancies. Furthermore, retinoids are one of the most evaluated pharmacologic agents in the ultra-challenging setting of interventional cancer prevention. This review will summarize the development of retinoids in cancer therapy and prevention with an emphasis on currently proposed mechanisms mediating their efficacy, toxicity, and resistance.

Response to high-dose intravenous immune globulin as a valuable factor predicting the effect of splenectomy in chronic idiopathic thrombocytopenic purpura patients

This study was conducted to verify whether the response to high-dose intravenous immune globulin (IVIG) was related to the effect of splenectomy in chronic idiopathic thrombocytopenic purpura (ITP) patients. A total of 79 patients over 16 years of age were enrolled in this study. The response to the treatment was classified on the basis of the platelet count as no response (NR, <50 x 10(9)/l), incomplete response (IR, (50-150) x 10(9)/l), and complete response (CR, >150 x 10(9)/l). The response was evaluated after the infusion of high-dose IVIG, within 2 weeks after splenectomy (immediate response), and during a follow-up period of more than 6 months after splenectomy (sustained response), respectively. 58 patients (73.4%) showed responses (CR or IR) to high-dose IVIG. After splenectomy, immediate responses were observed in 73 patients (92%). The response to high-dose IVIG had no relationship with the immediate response to splenectomy (P = 0.333). A follow-up evaluation was possible with 58 patients; 6 patients with NR in immediate responses did not show any response during the follow-up period, and 17 patients relapsed within 6 months after immediate responses, so 35 patients (60.3%) had sustained responses. Responders to IVIG had significantly higher sustained response rates to splenectomy than non-responders (62% vs. 38%, P = 0.001). These results indicate that the response to high-dose IVIG could be a valuable factor predicting the sustained response to splenectomy in chronic ITP patients.

Technetium-99m MDP bone scintigraphic findings of hypercalcemia in accelerated phase of chronic myelogenous leukemia

Hypercalcemia in accelerated phase of chronic myelogenous leukemia (CML) is very rare. Its pathogenesis is considered humoral hypercalcemia of malignancies mediated by parathyroid hormone-related protein (PTHrP). In severe hypercalcemia, calcifications in kidneys, skin, vessels, heart, and stomach may occur. Our two cases were admitted because of severe hypercalcemia in accelerated phase of CML. On Tc-99m methylene diphosphonate (MDP) bone scintigraphies, a marked tracer accumulation was seen in the lung, heart, stomach and kidney. We report increased tracer accumulation of multiple organs on Tc-99m MDP bone scintigraphy in two rare hypercalcemic patients with CML.

Cytokines secreted by lymphokine-activated killer cells induce endogenous nitric oxide synthesis and apoptosis in DLD-1 colon cancer cells

IL-2-activated killer lymphocytes (LAK cells) secrete inflammatory cytokines such as interferon-gamma (IFN-gamma) and tumor necrosis factor alpha (TNFalpha) that can induce nitric oxide (NO) synthesis. We evaluated whether LAK cells could activate NO synthesis in human cancer cells. LAK cells and their culture supernatants induced NO synthesis in DLD-1 colon cancer cells in a dose-dependent manner. NO synthesis was inhibited completely by blocking antibodies to IFN-gamma, demonstrating a key role for this LAK cell cytokine in regulating NO synthesis. The addition of TNFalpha antibodies resulted in partial inhibition. Induction of iNOS mRNA and protein expression in DLD-1 cells was detected. Endogenous NO production inhibited DLD-1 cell proliferation and induced apoptosis, processes that were inhibitable by the NO synthase inhibitor N(G)-monomethyl-l-arginine. Our study has identified a novel, non-contact-dependent LAK cell cytotoxic mechanism: induction of growth inhibition and programmed cell death due to endogenous NO synthesis in susceptible human cancer cells.

Interaction of two classes of ADP-ribose transfer reactions in immune signaling

CD38 is a bifunctional ectoenzyme predominantly expressed on hematopoietic cells where its expression correlates with differentiation and proliferation. The two enzyme activities displayed by CD38 are an ADP-ribosyl cyclase and a cyclic adenosine diphosphate ribose (cADPR) hydrolase that catalyzes the synthesis and hydrolysis of cADPR. T lymphocytes can be induced to express CD38 when activated with antibodies against specific antigen receptors. If the activated T cells are then exposed with NAD, cell death by apoptosis occurs. During the exposure of activated T cells to NAD, the CD38 is modified by ecto-mono-ADP-ribosyltransferases (ecto-mono-ADPRTs) specific for cysteine and arginine residues. Arginine-ADP-ribosylation results in inactivation of both cyclase and hydrolase activities of CD38, whereas cysteine-ADP-ribosylation results only in the inhibition of the hydrolase activity. The arginine-ADP-ribosylation causes a decrease in intracellular cADPR and a subsequent decrease in Ca(2+) influx, resulting in apoptosis of the activated T cells. Our results suggest that the interaction of two classes of ecto-ADP-ribose transfer enzymes plays an important role in immune regulation by the selective induction of apoptosis in activated T cells and that cADPR mediated signaling is essential for the survival of activated T cells.

Nitric oxide exposure inhibits induction of lymphokine-activated killer cells by inducing precursor apoptosis

Nitric oxide synthesis is strongly induced during IL-2 treatment of mice and humans. While this free radical can act as an antitumor mechanism by inhibiting cellular respiration and DNA synthesis in cancer cells, immunosuppressive effects have also been suggested. We evaluated the effects of NO exposure on the induction of murine lymphokine-activated killer (LAK) cells from splenocytes by IL-2 (6000 IU/ml). When splenocytes were exposed to pure NO gas for 30 min prior to the addition of IL-2, complete abrogation of LAK cell cytotoxicity was observed. In contrast, cytolytic activity of already activated LAK cells was only minimally affected by NO exposure. NO exposure markedly depressed cellular proliferation in response to concanavalin A or IL-2. Immunostaining of LAK cell cultures following NO exposure revealed a marked decrease in CD8+, and peanut lectin (PNA+)/CD56+ subsets (48 and 69%). Dual staining of LAK cells for DNA strand breaks and either PNA or CD8+ identified the induction of programmed cell death in these subsets 12-24 h following NO exposure. These experiments demonstrate that NO has the capacity to inhibit LAK cell induction by inducing apoptosis of cytolytic lymphocyte precursors.

Receptor-mediated activation of murine peritoneal macrophages by antithrombin III acts as a costimulatory signal for nitric oxide synthesis

We evaluated the effect of antithrombin III (ATIII), a serine protease inhibitor (SERPIN), on induction of nitric oxide (NO) synthesis in murine peritoneal macrophages. Incubation of macrophages with ATIII plus interferon-gamma (IFN-gamma) but not ATIII alone induced nitrite accumulation (a metabolite of NO) in a dose-dependent manner. Expression of the inducible nitric oxide synthase isoform was confirmed by Western blot. NO synthesis was inhibited by NG-monomethyl-l-arginine, by complexing ATIII with thrombin or by rabbit anti-human ATIII antiserum. Addition of polymyxin B to macrophage cultures failed to inhibit ATIII/IFN-gamma-induced NO synthesis, excluding lipopolysaccharide contamination. 125I-ATIII bound to macrophages in a dose-dependent, specific, and saturable manner, with a Km of approximately 7.1 nM. Our results demonstrate that ATIII, but not ATIII/thrombin complex, acts to costimulate macrophage activation and NO synthesis via a novel receptor mediated mechanism, which may indicate a role for SERPINs in macrophage activation.

Differences in immunophenotyping of mucosal lymphocytes between ulcerative colitis and Crohn's disease

OBJECTIVES

Immunologic studies have characterized the numbers and types of inflammatory cells in diseased inflammatory bowel disease (IBD) mucosa but have yielded conflicting results regarding intestinal lymphocytes activation in IBD. We investigated the levels of lymphocytes subsets, interleukin-2 receptor, transferrin receptor, and T cell receptors in mainly isolated lamina propria lymphocytes. Including intraepithelial lymphocytes of normal colonic mucosa or IBD (ulcerative colitis and Crohn's disease) mucosa to understand the pathogenesis of IBD. We have results from this study.

RESULTS

1) In comparing ulcerative colitis with control, IL-2R (p < 0.05), TR (p < 0.01), and CD3/HLA-DR (< 0.05) showed a significant increase. 2) In comparing Crohn's disease with control, CD3 (P < 0.05), TCR alpha/beta (p < 0.01) and TCR gamma/delta (p < 0.05) showed a significant decrease. 3) In comparing Crohn's disease with ulcerative colitis, CD19 (p < 0.01), TR (p < 0.01), TCR alpha/beta (p < 0.01) and TCR gamma/delta (p < 0.05) showed a significant decrease.

CONCLUSION

From these results, there are increased T cell markers, IL-2R, TR, and CD3/HLA-DR in UC, but differently, decreased CD3, TCR alpha/beta and TCR gamma/delta in CD compared with control. In addition, definitive differences in lymphocytes markers, CD19, TR, TCR alpha/beta and TCR gamma/delta, which are higher in UC than in CD, may elucidate the different immunopathogenesis between UC and CD.

Effects of nitric oxide (NO) synthesis inhibition on antitumor responses during interleukin-2 (IL-2) treatment of mice

OBJECTIVES

To evaluate if L-arginine: NO pathway is activated in tumor tissues during IL-2 therapy and to evaluate whether IL-2 induced NO synthesis represents an antitumor effector mechanism or an inhibitory factor against therapeutic effects of IL-2.

METHODS

Four groups [untreated control, NG-monomethyl-L-arginine (MLA) therapy only, IL-2 therapy only, IL-2/MLA therapy groups] of BALB/c mice were injected intraperitoneally with 2 x 10(8) Meth A tumor cells on day 0. MLA was administered subcutaneously with Alzet continuous infusion pumps on day 2. IL-2 therapy (180,000 IU s.c. every 12h for 5 days) was started on day 3. NO production within ascites tumors was assessed by measuring nitrite concentrations in cultures of ascites cells harvested on day 8. Survival and the rate of body weight increment of the mice were measured to evaluate therapeutic responses. Daily urinary nitrate excretion was monitored to demonstrate the effectiveness of MLA in inhibiting NO synthesis.

RESULTS

Nitrite production in supernatants of Meth A ascites cell cultures was 63 +/- 14 microM in IL-2 treated mice and 3.2 +/- 1.5 microM in untreated controls (p < 0.001). MLA prevented the IL-2 therapy induced increase in nitrite production. IL-2 therapy did not decrease the rate of body weight increment and marginally prolonged mean survival to 18.2 days, compared to 16.6 days in control mice (p = 0.255). MLA administration decreased the rate of body weight increment and prolonged mean survival of IL-2 treated mice (21.8 days, p = 0.001 versus IL-2 alone). Interestingly, the MLA treatment increased the rate of body weight increment and diminished the survival of control mice to 11.6 days (p = 0.003). MLA administration via Alzet continuous infusion pumps achieved approximately 60% suppression of urinary nitrate excretion by control mice. Subcutaneous IL-2 treatment strongly induced nitric oxide synthesis (up to 3.5 mumoles of urinary nitrate/ mouse/day). MLA also effectively suppressed IL-2 induced NO production.

CONCLUSION

L-arginine: NO pathway can be activated in malignant ascites by IL-2 therapy and NO synthesis functions as an inhibitory mechanism against IL-2 induced anti-tumor effects.

Nitric oxide synthesis contributes to IL-2-induced antitumor responses against intraperitoneal Meth A tumor

IL-2 therapy is a potent inductive stimulus for nitric oxide (NO.) synthesis in mice and humans. It is not yet clear whether NO. can contribute to IL-2-induced therapeutic responses. The murine skin cancer Meth A is relatively resistant to lymphokine-activated killer (LAK) cell killing, allowing evaluation of the role of IL-2-induced NO. synthesis in vivo, without contribution by LAK cells. Subcutaneous IL-2 treatment of mice bearing i.p. Meth A tumor increased nitrite production by cells derived from ascites (63 +/- 14 microM vs 3.2 +/- 1.5 microM in untreated controls). N omega-monomethyl-L-arginine (MLA), NO. synthase inhibitor, prevented this increase. NO. production correlated in an inverse fashion with tumor cell proliferation in vitro. Evidence for IL-2-induced heme nitrosylation was demonstrated in tumor cells by electron paramagnetic resonance spectroscopy. By immunomagnetic depletion experiments, macrophages were implicated as a major source of NO. synthesis. Cytologic and flow-cytometric evaluation revealed that IL-2 treatment resulted in enhanced lymphocyte and macrophage recruitment into malignant ascites, and decreases in tumor cell recovery. MLA administration further increased host cell recovery. Subcutaneous IL-2 therapy increased urinary nitrate excretion up to eightfold in mice, and appeared to produce a significant survival advantage that was prevented by MLA administration.

Nitric oxide synthesis contributes to IL-2-induced antitumor responses against intraperitoneal Meth A tumor

IL-2 therapy is a potent inductive stimulus for nitric oxide (NO.) synthesis in mice and humans. It is not yet clear whether NO. can contribute to IL-2-induced therapeutic responses. The murine skin cancer Meth A is relatively resistant to lymphokine-activated killer (LAK) cell killing, allowing evaluation of the role of IL-2-induced NO. synthesis in vivo, without contribution by LAK cells. Subcutaneous IL-2 treatment of mice bearing i.p. Meth A tumor increased nitrite production by cells derived from ascites (63 +/- 14 microM vs 3.2 +/- 1.5 microM in untreated controls). N omega-monomethyl-L-arginine (MLA), NO. synthase inhibitor, prevented this increase. NO. production correlated in an inverse fashion with tumor cell proliferation in vitro. Evidence for IL-2-induced heme nitrosylation was demonstrated in tumor cells by electron paramagnetic resonance spectroscopy. By immunomagnetic depletion experiments, macrophages were implicated as a major source of NO. synthesis. Cytologic and flow-cytometric evaluation revealed that IL-2 treatment resulted in enhanced lymphocyte and macrophage recruitment into malignant ascites, and decreases in tumor cell recovery. MLA administration further increased host cell recovery. Subcutaneous IL-2 therapy increased urinary nitrate excretion up to eightfold in mice, and appeared to produce a significant survival advantage that was prevented by MLA administration.

Effectiveness and toxicity of protracted nitric oxide synthesis inhibition during IL-2 treatment of mice

The current study was designed to characterize nitric oxide (NO.) synthesis during interleukin-2 (IL-2) treatment of mice, and to determine whether NO. mediated IL-2-induced "vascular leak." We developed a technique for chronic subcutaneous infusion of the NO. synthase inhibitor N omega monomethyl-L-arginine (MLA) via osmotic minipump to aid in further study of these processes. After IL-2 administration to C3H/HeN mice (180,000 IU i.p. b.i.d. for 5 days), NO. synthesis increased two-to-three fold, peaking on days 5-8. Administration of MLA reduced NO. synthesis in both IL-2-treated mice (from 2.7 to 1 microM/mouse/day), and normal mice (from 1 to 0.5 microM/mouse/day). This agent decreased IL-2-induced radiolabeled albumin accumulation in the liver after i.p. IL-2 administration (p < 0.02). MLA infusions resulted in minimal systemic toxicity in mice, as reflected by complete blood counts or serum chemistries. MLA also did not impair lymphokine-activated killer cell induction in vitro or in vivo, or alter IL-2-induced tumor responses in a 3-day pulmonary metastasis model. These experiments demonstrated that NO. is a mediator involved in the genesis of vascular permeability induced by IL-2 treatment. Studies designed to further evaluate the toxicity and usefulness of MLA infusions to modify this IL-2 induced toxicity appear to be warranted.

Glycosylphosphatidylinositol-anchored NAD glycohydrolase is released from peritoneal macrophages activated by interferon-gamma and lipopolysaccharide

We have previously shown that an ectoenzyme, NAD glycohydrolase (NADase) could be solubilized by treatment with bacterial phosphatidylinositol phospholipase C (PIPLC). However, it is unknown whether endogenous PIPLC can cleave this ectoenzyme. In this study, we used mouse peritoneal exudate macrophages which have been known to have relatively high activity of NADase. The results show that release of ecto-NADase was markedly increased when mouse peritoneal macrophages were costimulated with interferon-gamma (IFN-gamma) and bacterial lipopolysaccharide (LPS), compared to unstimulated cells. This increase was preceded by markedly enhanced activity of endogenous glycosylphosphatidylinositol phospholipase C (GPIPLC). The cross-reacting determinant (CRD) of the glycosylphosphatidylinositol anchor in released NADase from activated macrophages was detected by immunoblotting with anti-CRD antibody. Taken together, ecto-NADase is release from peritoneal exudate macrophages during IFN-gamma/LPS-induced activation and endogenous GPIPLC is involved in the NADase release from the activated macrophages.

Use of N-acetyl cysteine to increase intracellular glutathione during the induction of antitumor responses by IL-2

IL-2 therapy can induce marked oxidative stress via reactive oxygen and nitrogen intermediates. Glutathione, the major intracellular reductant, may become rate limiting to cytotoxic lymphocyte activation and proliferation under these circumstances. N-Acetyl cysteine (NAc-cys) was used to increase intracellular glutathione levels during lymphokine-activated killer (LAK) cell activation by IL-2. Incubation of splenocytes with NAc-cys (0.6 to 1.0 mM) resulted in significant changes in intracellular reduced and total glutathione (92% and 58% increase, respectively) at 96 h. These levels correlated with markedly enhanced cell proliferation (threefold) and cytolytic effector cell generation (&gt; fivefold increase in LU/10(6) cells) induced by the combination of NAc-cys with IL-2. IL-2 exposure by itself unexpectedly increased intracellular reduced glutathione by 43%. IL-2 and NAc-cys were synergistic in increasing glutathione levels (reduced glutathione: 292% increase; total: 251% increase). Inhibition of glutathione synthesis, using L-buthionine-(S,R)-sulfoximine reversed the effects of NAc-cys on intracellular glutathione, as well as cellular proliferation and cytotoxicity. This experiment established that the effects of NAc-cys required de novo glutathione synthesis. In conjunction with IL-2/LAK treatment, oral NAc-cys administration (260 to 900 mg/kg/day for 7 days) significantly decreased tumor progression in a refractory s.c. tumor model. A small fraction of mice (11 to 17%) had complete tumor regressions. NAc-cys may be useful as an adjunct to increase the antitumor activity of IL-2/LAK therapy.

Use of N-acetyl cysteine to increase intracellular glutathione during the induction of antitumor responses by IL-2

IL-2 therapy can induce marked oxidative stress via reactive oxygen and nitrogen intermediates. Glutathione, the major intracellular reductant, may become rate limiting to cytotoxic lymphocyte activation and proliferation under these circumstances. N-Acetyl cysteine (NAc-cys) was used to increase intracellular glutathione levels during lymphokine-activated killer (LAK) cell activation by IL-2. Incubation of splenocytes with NAc-cys (0.6 to 1.0 mM) resulted in significant changes in intracellular reduced and total glutathione (92% and 58% increase, respectively) at 96 h. These levels correlated with markedly enhanced cell proliferation (threefold) and cytolytic effector cell generation (> fivefold increase in LU/10(6) cells) induced by the combination of NAc-cys with IL-2. IL-2 exposure by itself unexpectedly increased intracellular reduced glutathione by 43%. IL-2 and NAc-cys were synergistic in increasing glutathione levels (reduced glutathione: 292% increase; total: 251% increase). Inhibition of glutathione synthesis, using L-buthionine-(S,R)-sulfoximine reversed the effects of NAc-cys on intracellular glutathione, as well as cellular proliferation and cytotoxicity. This experiment established that the effects of NAc-cys required de novo glutathione synthesis. In conjunction with IL-2/LAK treatment, oral NAc-cys administration (260 to 900 mg/kg/day for 7 days) significantly decreased tumor progression in a refractory s.c. tumor model. A small fraction of mice (11 to 17%) had complete tumor regressions. NAc-cys may be useful as an adjunct to increase the antitumor activity of IL-2/LAK therapy.

Induction of iron-derived EPR signals in murine cancers by nitric oxide. Evidence for multiple intracellular targets

The cell-mediated immune response to syngeneic tumors activates the cytokine-inducible nitric oxide synthase. We observed that syngeneic murine tumors exhibited EPR signals related to iron-nitrosyl complex formation. Three different EPR active iron-nitrosyl species were observed, an Fe(RS)2(NO)2 signal and two differentiable heme-nitrosyl complexes. Hemoglobin assays showed that the heme-nitrosyl signals were not derived from contaminating hemoglobin. Signal amplitudes were attenuated in mice treated with N omega-mono-methyl-L-arginine (MLA), an inhibitor of nitric oxide synthase. Tumors grown in vivo contained EPR signals while those grown in culture without continuing cytokine stimulation lost the signals after a few days. Cultured cells that were treated with cytokines, or that were cocultivated with cytokine-activated macrophages, regained EPR active complexes. These results show that the cell-mediated immune response to syngeneic tumors involves the induction of nitric oxide synthase. While nitric oxide synthesis is induced in both tumor infiltrating macrophages and in the tumor cells themselves, only tumor cells contributed to formation of heme-nitrosyl complexes. This result indicates the presence of a novel intracellular target for NO within tumor cells.

Macrophage nitric oxide synthesis delays progression of ultraviolet light-induced murine skin cancers

The role of macrophages in the host immune response against cancers remains uncertain. Since nitric oxide synthesis represents a significant macrophage antitumor mechanism in vitro, we evaluated whether NO was synthesized during the immune response to growing murine skin cancers. NO synthesis was readily detectable in enzymatically dissociated tumors (RD-995 and LR-298) and was inhibited by N omega-monomethyl-L-arginine (MLA) and by macrophage depletion. Nitrosylation of iron-sulfur and heme complexes was observed in these tumors using electron paramagnetic resonance spectroscopy. NO production in the presence of increasing concentrations of MLA correlated inversely with tumor cell proliferation in vitro. To elucidate the role of NO during in vivo tumor progression, tumor-bearing mice were treated with continuous infusions of the nitric oxide synthase inhibitor MLA. MLA-treated mice demonstrated increased growth and delayed rejection of the highly antigenic UV radiation-induced regressor tumor LR-298. These experiments demonstrate that macrophage-derived NO synthesis can contribute to the antitumor immune response in vivo.

Rhythmic delta-frequency activities in the nucleus accumbens of anesthetized and freely moving rats

In urethane-anesthetized rats, a membrane potential oscillation (MPO) of up to 30 mV and 0.5-2 Hz (delta frequency range) was found in neurons of the nucleus accumbens. The membrane potential oscillations were of similar frequency and reversed in phase to the extracellular EEG of about 0.5 mV. In freely moving rats, a rhythmic delta EEG of 0.5-3 Hz was found in the nucleus accumbens, and it was of highest amplitude and regularity during awake immobility and face washing, less regular during slow wave sleep, and of the lowest amplitude during body and head movements and rapid eye movement sleep. The behavioural relation of the accumbens EEG was not critically affected after amphetamine, haloperidol, and parachlorophenylalanine, which depleted serotonin, although the accumbens EEG during awake immobility was less regular after blocking muscarinic cholinergic receptors by atropine sulfate. However, stimulation of the ventral tegmental area suppressed the accumbens delta membrane potential oscillations and EEG, and this effect was antagonized by haloperidol, suggesting that endogenous dopamine release may suppress the accumbens delta rhythm. It was concluded that the delta rhythm in the nucleus accumbens may represent a state of bilateral synchrony among accumbens neurons that is perhaps characteristic of an idling system, while desynchronization of the delta rhythm may occur closely with motor action.

Intrinsic membrane potential oscillations in hippocampal neurons in vitro

Membrane potential oscillations (MPOs) of 2-10 Hz and up to 6 mV were found in almost all stable hippocampal CA1 and CA3 neurons in the in vitro slice preparation. MPOs were prominent for pyramidal cells but less pronounced in putative interneurons. MPOs were activated at threshold depolarizations that evoked a spike and the frequency of the MPOs increased with the level of depolarization. MPOs were distinct from and seemed to regulate spiking, with a spike often riding near the top of a depolarizing MPO wave. Analysis of the periodicity of the oscillations indicate that the period of MPOs did not depend on the afterhyperpolarization (AHP) following a single spike. MPOs persisted in low (0-0.1 mM) Ca2+ medium, with or without Cd2+ (0.2 mM), when synaptic transmission was blocked. Choline-substituted low-Na+ (0-26 mM) medium, 3 microM tetrodotoxin (TTX) or intracellular injection of QX-314 reduced or abolished the fast Na(+)-spike and reduced inward anomalous rectification. About 40% of CA1 neurons had no MPOs after Na+ currents were blocked, suggesting that these MPOs were Na(+)-dependent. In about 60% of the cells, a large depolarization activated Ca(2+)-dependent MPOs and slow spikes. MPOs were not critically affected by extracellular Ba2+ or Cs2+, or by 0.2 mM 4-aminopyridine, with or without 2 mM tetraethylammonium (TEA). However, in 5-10 mM TEA medium, MPOs were mostly replaced by 0.2-3 Hz spontaneous bursts of wide-duration spikes followed by large AHPs. Low Ca2+, Cd2+ medium greatly reduced the spike width but not the spike-bursts. In conclusion, each cycle of an MPO in normal medium probably consists of a depolarization phase mediated by Na+ currents, possibly mixed with Ca2+ currents activated at a higher depolarization. The repolarization/hyperpolarization phase may be mediated by Na+/Ca2+ current inactivation and partly by TEA-sensitive, possibly the delayed rectifier, K+ currents. The presence of prominent intrinsic, low-threshold MPOs in all hippocampal pyramidal neurons suggests that MPOs may play an important role in information processing in the hippocampus.

Low doses of accumbens dopamine modulate amygdala suppression of spontaneous exploratory activity in rats

The effect of pharmacological stimulation of the amygdala on spontaneous locomotor activity in the rat and its modulation by accumbens dopamine were investigated. Bilateral injection of N-methyl-D-aspartic acid into the basolateral nucleus of the amygdala produced a dose-dependent suppression of spontaneous locomotor activity in the rat. The suppression of locomotor activity was reversed completely by injection of L-glutamic acid diethyl ester, a putative glutamatergic antagonist, into the nucleus accumbens but partially enhanced by injection of nipecotic acid, a GABA uptake inhibitor, into the ventral pallidum. Furthermore, low doses of dopamine injected into the accumbens, which by itself did not elicit hyperactivity in the animals, completely reversed the suppression of locomotor activity following amygdala stimulation. These results show that the projection from the amygdala to nucleus accumbens has an inhibitory effect on spontaneous locomotor activity in rats and that dopamine in the accumbens attenuated this suppression effect possibly due to its neuromodulatory action as demonstrated in previous electrophysiological experiments.

Impaired interleukin-2 receptor expression on lymphocytes from patients with chronic active hepatitis type B

To evaluate cell mediated immunopathogenic mechanisms in chronic hepatitis B virus (HBV) infection, we investigated the changes of T4/T8 ratios from the peripheral blood, the percentages of IL-2 receptor expression after stimulation of mitogens (Con A, PHA) and a specific antigen (Hepatitis type B surface antigen, HBs), and the proliferative response mediated by IL-2 receptors after rIL-2 stimulation on mixed mononuclear cell. These experiments were performed comparatively in 5 groups which consisted of serologically negative normal subjects, chronic HBV carriers, patients with chronic active hepatitis (CAH) type B, patients with acute hepatitis (AH) type B, and the antibody positive healthy subjects. There were significant decreases of T4/T8 ratios in chronic HBV carriers, in patients with CAH type B, and in patients with AH type B, compared with negative normal controls. There were no significant differences between patients with CAH type B and the HBs negative normal controls in the percentage of IL-2 receptor positive cells after in-vitro HBs-stimulation and the proliferative response assessed by the incorporation of 3H-thymidine, whereas in patients with AH type B there were significant increases in both. Thus, in addition to a relatively decreased T4/T8 ratio, the impairment of IL-2 receptor expression on the lymphocytes after HBs-stimulation caused a defective response of cellular proliferation, and this might be one of the leading immunopathogenic roles in chronic HBV infection.

Neuromodulatory action of dopamine in the nucleus accumbens: an in vivo intracellular study

Intracellular recordings were made from neurons in the nucleus accumbens in situ to determine how dopamine produces the selective neuromodulatory action in the accumbens observed in previous studies. Electrical stimulation of the basolateral nucleus of the amygdala was found to produce monosynaptically evoked depolarizing and hyperpolarizing postsynaptic potential sequences in a large proportion of the accumbens neurons sampled. Dopamine applied iontophoretically or released endogenously by stimulation of the ventral tegmental area produced consistent membrane depolarization and an increase in membrane conductance but not an increase in spontaneous activity of the accumbens neurons. Stimulation of the ventral tegmental area with trains of 10 pulses at 10 Hz prior to stimulation of the amygdala produced 8-58% reduction in the amplitude of the depolarizing postsynaptic potential but no change in the late hyperpolarizing postsynaptic potential. Although attenuation of the depolarizing postsynaptic potential amplitude from ventral tegmental area stimulation was often accompanied by membrane depolarization, it appeared that the two responses were not causally related. The effect of ventral tegmental area stimulation on the evoked depolarizing postsynaptic potential and the membrane potential were blocked by haloperidol indicating the involvement of dopamine. Iontophoretically applied dopamine produced responses similar to ventral tegmental area stimulation with two exceptions: (i) iontophoretically applied dopamine produced consistently stronger maximal attenuation of the depolarizing postsynaptic potential than did ventral tegmental area stimulation; and (ii) iontophoretically applied dopamine always attenuated both the depolarizing postsynaptic potential and hyperpolarizing postsynaptic potential whereas ventral tegmental area stimulation produced selective attenuation of the depolarizing postsynaptic potential only. These electrophysiological results are complementary to those from pharmacological experiments and suggest that one of several physiological functions of dopamine in the nucleus accumbens is a neuromodulatory one involving presynaptic action on non-dopaminergic terminals.

Mesolimbic dopamine projection modulates amygdala-evoked EPSP in nucleus accumbens neurons: an in vivo study

In urethane anesthetized rats, excitatory postsynaptic potential (EPSP) recorded intracellularly from nucleus accumbens neurons following stimulation of the amygdala was attenuated by repetitive stimulation of the ventral tegmental area (VTA). VTA stimulation also depolarized the resting membrane potential of accumbens neurons. Attenuation of the EPSP and membrane depolarization were frequently dissociated but both were blocked by haloperidol, a dopamine antagonist.

Intracellular records of theta rhythm in hippocampal CA1 cells of the rat

In the urethane-anesthetized rat, intracellular recordings from hippocampal CA1 cells, some of them identified as projection (probably pyramidal) cells, showed oscillations of the resting membrane potential in the theta frequency range ('intracellular theta rhythm') which is phase-locked to the extracellularly recorded theta rhythm. Current injection or acetate ion diffusion, which reversed an inhibitory postsynaptic potential (IPSP) evoked by alvear stimulation, inverted the phase relationship between intracellular and extracellular theta rhythms. A high correlation was also found between amplitudes of the intracellular theta and the evoked IPSP at different membrane potentials. These results indicate that hippocampal theta rhythm in the urethane-anesthetized rat is predominantly caused by a rhythmic modulation of impinging IPSPs on pyramidal cells.

Chemical modulation of ephaptic activation of CA3 hippocampal pyramids

In rats under urethane anaesthesia, antidromic population spikes were evoked in CA3 pyramidal layer by fimbrial/commissural stimulation at a very low frequency (approximately 0.5 Hz). Submaximal population spikes--between 20 and 90% of maximum--were enhanced by 8-38% by applications of acetylcholine and bicuculline, or by medial septal stimulation. Noradrenaline had a less pronounced and regular facilitatory action, whereas gamma-aminobutyrate and glutamate only depressed population spikes. Maximal enhancement by acetylcholine or bicuculline was observed when the antidromic population spike was initially at 38-53% of maximum amplitude. A simple explanation of these results is that acetylcholine and bicuculline, by raising their excitability, facilitate the excitation of non-invaded pyramidal cells by antidromic field potentials. They are fully in keeping with previous intracellular observations on ephaptic interactions between CA3 neurons, and provide a further illustration, in situ, of the importance of increased excitability and disinhibition--whether caused by drugs or synaptic action--in promoting synchronized excitation by ephaptic currents.

Excitatory input from sensory motor cortex to neostriatum and its modification by conditioning stimulation of the substantia nigra

Extracellular single unit responses were recorded from 381 neurons in the neostriatum of urethane-anesthetized rats. Single pulse stimulation of the sensory motor cortex produced strong excitatory responses in neostriatal neurons with a mean onset latency of 7.9 ms. A train of 10 pulses (0.15 ms, 300-600 microA) at 10 Hz delivered to the substantia nigra 100 ms before cortical stimulation attenuated the original excitatory response of 22 of 53 striatal neurons tested and enhanced the excitatory response of 12 of these neurons. These attenuating and enhancing effects were reduced or abolished by the systemic administration of haloperidol, a dopamine antagonist. Single pulse nigral stimulation 100 ms before the cortical stimulation had little or no effect on the excitatory response of neostriatal neurons. The iontophoretic application of dopamine also had attenuation and enhancement effects on the excitatory response of striatal neurons to cortical stimulation. The results suggest that the attenuation and enhancement of the excitatory responses of striatal neurons to cortical stimulation which resulted from nigral conditioning stimulation is mediated by nigrostriatal dopaminergic neurons.

Response of ventral pallidal neurons to amygdala stimulation and its modulation by dopamine projections to nucleus accumbens

It has been shown that the nucleus accumbens receives input from the amygdala and that mesolimbic dopaminergic projection from the ventral tegmental area (VTA) modulates the response of accumbens neurons to amygdala input. Since the nucleus accumbens projects to the ventral pallidum, the purpose of this study was to investigate, using electrophysiological techniques, whether or not the nucleus accumbens relays the projection from the amygdala to the ventral pallidum and whether or not the mesolimbic dopamine projection interacts with this pathway. Extracellular single-unit recordings were obtained from the ventral pallidum of urethan-anesthetized rats, and the responses of these neurons to electrical stimulation of the amygdala were investigated. Of 392 neurons tested, 36% were inhibited and 11% were excited following amygdala stimulation. Latency of onset of inhibitory responses showed a bimodal distribution with peaks in the ranges of 4-6 ms and 16-18 ms, respectively. Fifty-four percent of inhibitory responses with latencies greater than 12 ms were attenuated by 1) injection of procaine hydrochloride into the nucleus accumbens, or 2) injection of d-amphetamine into the nucleus accumbens, or 3) stimulation of VTA with a train of 10 pulses (10 Hz) prior to stimulation of amygdala. Acute administration of haloperidol intraperitoneally or injection of 6-hydroxydopamine into the ipsilateral VTA, 2 days prior to the recording experiment, reduced the attenuating effects of intra-accumbens injection of d-amphetamine and VTA conditioning stimulations on the inhibitory response of ventral pallidal neurons to amygdala stimulation. These results support the hypothesis that the nucleus accumbens provides a link between the amygdala and the ventral pallidum. Since the amygdala is a limbic structure and the ventral pallidum has possible connections with the extrapyramidal motor system, it is suggested that the amygdala to nucleus accumbens to ventral pallidum projection may be a bridge between the limbic and motor systems. We also suggest that this relay of output from the amygdala to the ventral pallidum via the nucleus accumbens is under the modulating influence of the mesolimbic dopamine projection from the ventral tegmental area.

Response of nucleus accumbens neurons to amygdala stimulation and its modification by dopamine

Extracellular single unit recordings were obtained from the nucleus accumbens of urethane anesthetized rats. It was found that electrical stimulation of the basal lateral and basal medial nuclei of the amygdala produced strong excitatory responses in neurons of the nucleus accumbens, in particular the medial region. Latencies of activation were relatively short with a mean of 10.7 ms. Dopamine applied iontophoretically had a marked attenuating effect on the excitatory response of nucleus accumbens neurons to amygdala stimulation. The spontaneous activity of all neurons recorded from the nucleus accumbens was also suppressed by dopamine, but the excitatory response was more sensitive to dopamine inhibition than the spontaneous activity. Neurons in the nucleus accumbens showed a variety of responses to single-pulse electrical stimulation of the ventral tegmental area (VTA). Some units in the nucleus accumbens received convergent inputs from both the amygdala and the VTA. Stimulation of the VTA also attenuated the response of nucleus accumbens neurons to excitatory inputs from the amygdala. A train of 10 pulses (0.15 ms, 200--600 microA) at 10 Hz delivered to the VTA at 100 ms before stimulation of the amygdala caused attenuation of the original excitatory response. The attenuating effect could be observed irrespective of whether individual single-pulse stimulation of the VTA elicited a response in that particular accumbens neuron or not. 6-Hydroxydopamine injected into the VTA 2 days prior to the recording experiment, or haloperidol injected intraperitoneally 1 h before the recording session, abolished this attenuating effect. However, responses to single-pulse stimulations of the VTA were not abolished. The results suggest that the attenuation of the excitatory response to amygdala stimulation was due to the release of dopamine from mesolimbic dopaminergic neurons. Responses to single-pulse stimulations of the VTA were probably due to activation of non-dopaminergic neurons projecting from the same area. It is suggested as a working hypothesis that this inhibitory effect of dopamine may be an important function of the mesolimbic dopamine pathway in modulating the extent to which limbic structures can exert an influence on the motor system through the accumbens.

Effect of picrotoxin and nipecotic acid on inhibitory response of dopaminergic neurons in the ventral tegmental area to stimulation of the nucleus accumbens

Electrical stimulation of nucleus accumbens inhibited the activity of neurons in the ventral tegmental area (VTA). Inhibition was potentiated by nipecotic acid, a GABA uptake inhibitor, and was attenuated by picrotoxin, a GABA antagonist. Four units inhibited by nucleus accumbens stimulation were also antidromically activated. These observations suggest that neurons in the VTA, including specifically those projecting to the nucleus accumbens, receive a descending GABAergic input from that area.

Electrophysiological studies of neurons in the ventral tegmental area of Tsai

Extracellular recordings were obtained from single neurons in the ventral tegmental area of rats anesthetized with urethane. It was found that the area appeared to contain two groups of neurons with distinctly different spike durations, firing rates and firing patterns. One group (group A) had properties similar to those of nigral dopaminergic neurons: slow random firing rates, unusually long spike durations and slow conduction velocities. The discharge rate of the majority of these neurons was reduced by iontophoretically applied dopamine. It was concluded that neurons of this group were probably A10 dopaminergic neurons. The other group (group B) had relatively faster and rhythmical firing rates, short spike durations and faster conduction velocities and were considered to be non-dopaminergic. Forty-nine units in the ventral tegmental area were antidromically activated by electrical stimulation of the nucleus accumbens. Units antidromically activated included neurons of group A and group B, suggesting that the nucleus accumbens received dural projections of dopaminergic and non-dopaminergic fibres from the ventral tegmental area. The discharge rate of 141 out of 142 neurons tested in the ventral tegmental area (group A: 66/66, group B: 75/76) was found to be reduced by GABA. The inhibition was blocked by the simultaneous application of picrotoxin. Picrotoxin alone activated 47.7% of 155 units tested. These results provide further evidence of a GABAergic input to dopaminergic and non-dopaminergic neurons projecting to the limbic forebrain structures.