Zeta (zeta), the opioid growth factor receptor: identification and characterization of binding subunits

The Opioid Growth Factor in Growth Regulation and Immune Responses in Cancer

It has become apparent that endogenous opioids act not only as neurotransmitters and neuromodulators, but have multiple functions in the body. Activation of the opioid system by opiate drugs is associated with a risk of cancer development through direct stimulation of tumor cell proliferation and through immunosuppression. In contrast, the endogenous peptide opioid [Met5]-enkephalin, now commonly referred to as Opioid Growth Factor (OGF), negatively regulates cell proliferation in a wide number of cells during development, homeostasis, and neoplasia. This action is mediated through the opioid growth factor receptor, originally designated the zeta (ζ) opioid receptor. Further, contrary to the traditional notion of opiates as immunosuppressive, endogenous OGF has been shown to possess a number of positive immunomodulatory properties and may provide a beneficial effect in cancer by augmenting the activity of cells involved in both innate and acquired immunity. Taken together, the evidence supports consideration of opioid peptides such as OGF as new strategies for cancer therapy.

Addiction and the cerebellum with a focus on actions of opioid receptors

Increasing evidence suggests that the cerebellum could play a role in the higher cognitive processes involved in addiction as the cerebellum contains anatomical and functional pathways to circuitry controlling motivation and saliency. In addition, the cerebellum exhibits a widespread presence of receptors, including opioid receptors which are known to play a prominent role in synaptic and circuit mechanisms of plasticity associated with drug use and development of addiction to opioids and other drugs of abuse. Further, the presence of perineural nets (PNNs) in the cerebellum which contain proteins known to alter synaptic plasticity could contribute to addiction. The role the cerebellum plays in processes of addiction is likely complex, and could depend on the particular drug of abuse, the pattern of use, and the stage of the user within the addiction cycle. In this review, we discuss functional and structural modifications shown to be produced in the cerebellum by opioids that exhibit dependency-inducing properties which provide support for the conclusion that the cerebellum plays a role in addiction.

Opioid Growth Factor and its Derivatives as Potential Non-toxic Multifunctional Anticancer and Analgesic Compounds

Despite significant research progress on the pathogenesis, molecular biology, diagnosis, treatment, and prevention of cancer, its morbidity and mortality are still high around the world. The emerging resistance of cancer cells to anticancer drugs remains still a significant problem in oncology today. Furthermore, an important challenge is the inability of anticancer drugs to selectively target tumor cells thus sparing healthy cells. One of the new potential options for efficient and safe therapy can be provided by opioid growth factor (OGF), chemically termed Met-enkephalin. It is an endogenous pentapeptide (Tyr-Gly-Gly-Phe-Met) with antitumor, analgesic, and immune-boosting properties. Clinical trials have demonstrated that OGF therapy alone, as well as in combination with standard chemotherapies, is a safe, non-toxic anticancer agent that reduces tumor size. In this paper, we review the structure-activity relationship of OGF and its analogues. We highlight also OGF derivatives with analgesic, immunomodulatory activity and the ability to penetrate the blood-brain barrier and may be used as safe agents enhancing chemotherapy efficacy and improving quality of life in cancer patients. The reviewed papers indicate that Met-enkephalin and its analogues are interesting candidates for the development of novel, non-toxic, and endowed with an analgesic activity anticancer drugs. More preclinical and clinical studies are needed to explore these opportunities.

The effect of low-dose naltrexone on solid Ehrlich carcinoma in mice: The role of OGFr, BCL2, and immune response

AIMS

Cancer is a major worldwide health problem. Cancer cells express opioid growth factor (OGF) which controls their growth. Naltrexone in low dose (LDN) blocks opioid receptors intermittently and controls the replication of cancer cells. The aim of this study was to investigate the effect of LDN and its chemotherapeutic additive effect on the growth of solid Ehrlich carcinoma in mice with focus on the OGFr and immune responses.

MAIN METHODS

Sixty female Swiss albino mice were assigned into 5 groups (n: 12 mice each): (i): normal control, (ii): Solid Ehrlich carcinoma (SEC), (iii): SEC treated with LDN, (iv): SEC treated with 5-fluorouracil (5-FU), (v): SEC treated with LDN + 5-FU. All drugs were started when the tumor became palpable on 9th day. At the end of the study animals were sacrificed, blood and tissue samples were collected. Tumor weight and volume were measured. Splenocytes and myeloid derived suppressor cells (MDSC) were counted. Tumor expression of opioid growth factor receptors (OGFr), serum level of IFN-γ, tumor histopathology (H&E) and immunohistochemistry staining of p21, p53, Bcl2 were assessed.

KEY FINDINGS

All drug-treated groups showed reduction in tumor weight and volume, significant increase of splenocyte with tendency to reduce MDSC cell counts. LDN led to significant increase in OGFr both in solo and in combination with 5FU. Serum IFN-γ is significantly increased by LDN but decreased by 5-FU. Also, LDN and 5FU increased immunehistochemical staining of p21 while decreased immunostaining of Bcl2. In animals treated with a combination of LDN and 5FU a maximal downregulation of the antiapoptotic mediator BCL2 was observed.

SIGNIFICANCE

The current study suggested that LDN may play a role in inhibiting cancer cell growth and highlights the possibility of promising combination with cancer chemotherapeutics, which guarantee further clinical studies for approval.

Intermittent blockade of OGFr and treatment of autoimmune disorders

IMPACT STATEMENT

This mini-review presents information on the intermittent blockade of the opioid growth factor (OGF)-OGF receptor (OGFr) axis by low-dose naltrexone (LDN), and the role of enkephalin (i.e. OGF) in autoimmune disorders, specifically multiple sclerosis, Crohn's, and fibromyalgia. Clinical reports on subjects taking LDN have documented reduced fatigue, few side-effects, and improved overall health. Preclinical studies on mice with experimental autoimmune encephalomyelitis (EAE), the animal model of multiple sclerosis, revealed that immunization for EAE reduces serum OGF. Intermittent OGFr blockade with LDN restores serum enkephalin levels that correlate with reduced behavioral and pathological signs of EAE; LDN also increases enkephalin levels in naïve mice. The interplay between LDN, and the onset and treatment of autoimmune diseases, chronic pain, and other addictive behaviors requires further investigation, but highlights a central role for enkephalins and intermittent blockade of the OGF-OGFr pathway in pathogenesis and treatment of these disorders.

Featured Article: Selective blockade of the OGF–OGFr pathway by naltrexone accelerates fibroblast proliferation and wound healing

Naltrexone (NTX) is an opioid receptor antagonist that acts at classical and non-classical opioid receptors including the opioid growth factor receptor (OGFr). Animal models of type 1 and type 2 diabetes, as well as normal rodents, have shown that topical NTX enhances the healing rates of corneal epithelium and full-thickness cutaneous wounds. The mechanism of this general opioid antagonist on growth, and in particular the specific receptor pathway involved, is not understood. Tissue culture studies using NIH 3T3 fibroblasts and primary rat auricular fibroblasts were established to evaluate growth following opioid receptor antagonist treatment. Treatment of cells with CTOP, naltrindole, or nalmefene, selective antagonists for mu, delta, and kappa opioid receptors, respectively, did not accelerate cell replication. Addition of the classical opioid receptor peptides DAMGO, DPDPE, or EKC did not alter cell growth, suggesting that the classical opioid receptors were not involved in cutaneous wound healing. However, NTX (10−6 M) increased the growth of NIH 3T3 fibroblasts in culture over a 96-h period, and the specific ligand OGF decreased cell growth, supporting that the OGF-OGFr axis is tonically active and constitutively expressed in fibroblasts, the primary cell type in granulation tissue of the skin. Transfection of NIH 3T3 cells with OGFr siRNA reduced receptor protein; subsequent treatment with NTX did not accelerate cell proliferation. These data indicate that blockade of the OGFr pathway enhances proliferation of fibroblasts in vitro, and in a primary culture of auricular fibroblasts, suggesting that the effect of NTX on growth is mediated through the OGF-OGFr axis. Finally, antagonists for classical opioid receptors as well as NTX were topically applied to cutaneous wounds in type 1 diabetic rats; only NTX accelerated wound closure. These studies indicate that the mechanistic pathway underlying the effects of NTX to enhance cutaneous wound closure in diabetic and nondiabetic subjects is specific blockade of the OGF–OGFr regulatory axis.

Targeting the opioid growth factor: Opioid growth factor receptor axis for treatment of human ovarian cancer

The opioid growth factor (OGF) – opioid growth factor receptor (OGFr) axis is a biological pathway that is present in human ovarian cancer cells and tissues. OGF, chemically termed [Met5]-enkephalin, is an endogenous opioid peptide that interfaces with OGFr to delay cells moving through the cell cycle by upregulation of cyclin-dependent inhibitory kinase pathways. OGF inhibitory activity is dose dependent, receptor mediated, reversible, protein and RNA dependent, but not related to apoptosis or necrosis. The OGF-OGFr axis can be targeted for treatment of human ovarian cancer by (i) administration of exogenous OGF, (ii) genetic manipulation to over-express OGFr and (iii) use of low dosages of naltrexone, an opioid antagonist, which stimulates production of OGF and OGFr for subsequent interaction following blockade of the receptor. The OGF-OGFr axis may be a feasible target for treatment of cancer of the ovary (i) in a prophylactic fashion, (ii) following cytoreduction or (iii) in conjunction with standard chemotherapy for additive effectiveness. In summary, preclinical data support the transition of these novel therapies for treatment of human ovarian cancer from the bench to bedside to provide additional targets for treatment of this devastating disease.

The opioid growth factor-opioid growth factor receptor axis: homeostatic regulator of cell proliferation and its implications for health and disease

The opioid growth factor (OGF), chemically termed [Met(5)]-enkephalin, is an endogenous opioid peptide that interacts with the OGF receptor (OGFr) to delay the G(1)/S interface of the cell cycle by modulating cyclin-dependent inhibitory kinase (CKI) pathways. The OGF-OGFr axis is a tonically active, inhibitory pathway that is an important regulator during homeostasis and re-epithelialization, and plays a role in the onset and progression of autoimmune diseases and cancer. Modulation of the OGF-OGFr axis can be accomplished by a variety of pharmacological and molecular approaches including use of intermittent or continuous exposure to the opioid antagonist naltrexone, genetic manipulation of OGFr expression, and antibody neutralization of OGF. Clinically, OGF is a biological therapy that has potential application for treatment of cancer. Currently, naltrexone at low dosages is being evaluated for treatment of autoimmune diseases such as Crohn's and multiple sclerosis. High dosages of naltrexone are effective in reversing dry eye and accelerating the repair of corneal abrasions in normal and diabetic rats; these studies are under investigation in the clinical setting. Naltrexone also enhances full-thickness wound closure in animal models of Type 1 or Type 2 diabetes, and translation of this knowledge to the clinic is planned. In summary, understanding the OGF-OGFr axis as a homeostatic regulator of proliferation has substantial implications for maintaining human health and treatment of disease.

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

Gene expression of OGFr in the developing and adult rat brain and cerebellum

The native opioid peptide, [Met5]-enkephalin (termed opioid growth factor (OGF)), is a tonically active negative growth factor targeted to cell proliferation in the developing nervous system. OGF action is mediated by the OGF receptor (OGFr). The present study investigates gene expression of OGFr in the developing and adult brain and cerebellum of the rat using Northern blot analysis and normalization to GAPDH. OGFr was detected in whole brain at embryonic day 20 and birth, and was at least twofold greater than neonatal levels during the first week of life. From postnatal day 15 onwards to adulthood, levels of OGFr mRNA in the whole brain were detectable but less than those at birth. OGFr mRNA in cerebellum was found on embryonic day 20, and remained relatively constant until postnatal day 12 when a sharp increase was recorded. In the third week of life and continuing into adulthood, cerebellar OGFr mRNA was detected at levels comparable to those in postnatal week 1. These results show that message for OGFr is developmentally regulated prior to and after birth, is ubiquitously expressed during development, and is present in the adult brain and cerebellum even though OGF receptor binding is not recorded.

The expression and function of the OGF-OGFr axis - a tonically active negative regulator of growth - in COS cells

This study was designed to examine the presence and role of the opioid growth factor (OGF, [Met(5)]-enkephalin) and the OGF receptor (OGFr) in COS-7 cells; these cells lack classical opioid receptors. Preproenkephalin mRNA, which encodes OGF, was detected by Northern blot analysis, and OGFr mRNA was recorded by RT-PCR. Receptor binding analysis showed specific and saturable binding (K(d)=3.5nm, B(max)=44fmol/mg protein) for OGFr in the nuclear fraction. Both OGF and OGFr were recorded in COS-7 cells by immunocytochemistry. Addition of OGF to log-phase COS-7 cultures depressed growth by 41.6% from control levels, whereas opioid-receptor blockade by the opioid antagonist, naltrexone, increased the number of cells by 29.8% from control values. The effect of OGF was receptor mediated. Exposure to a wide variety of synthetic and natural opioid peptides, including those selective for micro, delta, and kappa opioid receptors, showed that only OGF had an effect. Treatment with antisense OGFr oligonucleotides increased the number of cells by over 2-fold compared to wild-type cultures of COS-7 cells and preparations receiving scrambled oligonucleotides. These results indicate that the OGF-OGFr axis is present and functions in COS-7 cells, and in the absence of classical opioid receptors.

Synthetic beta-endorphin-like peptide immunorphin binds to non-opioid receptors for beta-endorphin on T lymphocytes

The synthetic decapeptide H-SLTCLVKGFY-OH (termed immunorphin) corresponding to the sequence 364-373 of the CH3 domain of human immunoglobulin G heavy chain was found to compete with [125I]beta-endorphin for high-affinity receptors on T lymphocytes from the blood of healthy donors (K(i) = 0.6 nM). Besides immunorphin, its synthetic fragments H-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 15 nM), H-Leu-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 8.0 nM), H-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 3.4 nM), H-Thr-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 2.2 nM), H-Leu-Thr-Cys-Leu-Val-Lys-Gly-Phe-Tyr-OH (K(i) = 1.0 nM) possessed the ability to inhibit specific binding of [125I]beta-endorphin to T lymphocytes. Tests of the specificity of the receptors revealed that they are not sensitive to naloxone and Met-enkephalin, i.e. they are not opioid receptors. K(d) values characterizing the specific binding of 125I- labeled immunorphin and its fragment H-Val-Lys-Gly-Phe-Tyr-OH to the receptors have been determined to be 7.4 nM and 36.3 nM, respectively.

Inhibition of passive-avoidance memory formation in the day-old chick by the opioid cytochrophin-4

Cytochrophin-4 (cyt-4), a tetrapeptide with opioid-like activity, caused amnesia when injected into chick forebrain 5 hr after passive-avoidance training. Bilateral injections of cyt-4 directly into the lobus parolfactorius (LPO) resulted in the chicks being amnesic for the training task 24 hr later, whereas unilateral injections of cyt-4 were effective only when injected into the right LPO. Cyt-4-induced amnesia was reversed by the general opioid antagonist, naloxone, indicating that cyt-4 was acting via an opioid receptor. The mu- and delta-opioid receptors (but not kappa-opioid or ORL(1)-receptors) have been shown to be involved in memory formation 5 hr after training (). Because an antagonist of the mu-opioid receptor inhibited memory, we attempted to reverse the effect of cyt-4 using mu-opioid receptor agonists. Met[enk] was unable to reverse the inhibition of memory formation by cyt-4 suggesting that the mu-opioid receptor is not involved in this effect. However endomorphin-2 (endo-2) reversed the effect of cyt-4. We further investigated the action of endo-2 using an irreversible antagonist of the mu-receptor, beta-funaltrexamine (beta-FAN), and found that endo-2 reversed beta-FAN-induced amnesia indicating that endo-2 was not acting on the mu-opioid receptor in the chick. Because unilateral injections of beta-FAN were not amnesic (bilateral injections were amnesic) this provided further evidence that the effect of cyt-4 was not mediated via the mu-opioid receptor. Coinjection of the delta-receptor agonist, (D-Pen(2), L-Pen(5))enkephalin (DPLPE), reversed the disruptive effect of cyt-4 on memory. However, memory modulation via the delta-opioid receptor was not lateralized to the right hemisphere suggesting that cyt-4 does not act via this receptor either. It was shown that an antagonist of the epsilon-opioid receptor inhibited memory at the 5 hr time point. We conclude that the epsilon-opioid receptor or an unidentified opioid receptor subtype could be involved in the action of cyt-4.

Cloning, sequencing, expression and function of a cDNA encoding a receptor for the opioid growth factor, [Met(5)]enkephalin

The native opioid growth factor (OGF), [Met(5)]enkephalin, is a tonic inhibitory peptide that modulates cell proliferation and tissue organization during development, cancer, cellular renewal, wound healing and angiogenesis. OGF action is mediated by a receptor mechanism. We have cloned and sequenced a 2.1-kilobase (kb) cDNA for a receptor to OGF (OGFr). The open reading frame was found to encode a protein of 580 amino acids, and eight imperfect repeats of nine amino acids each were a prominent feature. The protein encoded by this cDNA exhibited the pharmacological, temporal and spatial characteristics of the OGFr. Functional studies using antisense technology demonstrated an enhancement in cell growth. The molecular organization of the OGFr has no homology to classical opioid receptors. These results provide molecular validity for the interaction of OGF and OGFr in the regulation of growth processes.

Methionine-enkephalin in bone and joint tissues

Methionine-enkephalin (met-enk), an endogenous opiate, mimics many of the effects of morphine by binding to opiate receptors, thereby eliciting similar cellular and behavioral effects. Using biochemical and immunohistochemical techniques, several peptides have been identified in bone and joint tissues. Here we report, for the first time, the presence as well as concentration of met-enk in bone and joint tissues. Immunohistochemistry using electron and immunofluorescence microscopy showed cellular and neuronal distribution of met-enk in bone and joint tissues. The concentration of met-enk analyzed by high performance liquid chromatography electrochemical detection or radioimmunoassay was high in bone marrow, periosteum, ankle joint tissue, and cortical bone. Analysis by fast atom bombardment mass spectrometry suggested that the recovered fragment was met-enk Administration of met-enk inhibits osteoblast cell growth in culture, which is reversible by naltrexone. In arthritic rats, the concentration of met-enk was significantly decreased in ankle joints compared with controls, suggesting a role for met-enk in the pathophysiology of adjuvant arthritis.

Agonist-induced desensitization and down-regulation of delta opioid receptors alter the levels of their 125I-beta-endorphin cross-linked products in subcellular fractions from NG108-15 cells

The delta opioid binding sites in subcellular fractions from NG108-15 cells were characterized with respect to their relative molecular size and levels under conditions of receptor adaptation. 125I-beta-Endorphin was cross-linked to preparations enriched in plasma membranes (P20), nuclear membranes or nuclear matrices. Five cross-linked bands appear in all subcellular fractions. The largest molecular size reaction product in nuclear matrix preparations (approximately 72 kDa) differed from that in the other two fractions-(approximately 83 kDa). Immunoblot analyses with an antibody to the delta opioid receptor gave a P20 band pattern similar to that for the corresponding cross-linked products. To determine which cross-linked products in P20 are glycoproteins, labeled membranes were solubilized and purified by wheat germ agglutinin chromatography. The absence of a approximately 36 kDa band after purification suggests that this product is not a glycoprotein. The remaining four bands were present in N-acetyl-D-glucosamine eluates, although their % distribution changes in favor of the largest molecular size band (approximately 83 kDa). Immunoblotting of the eluate gave a single diffuse band at approximately 73 kDa, suggesting the native glycoprotein has a molecular size in the 70-80 kDa range. Etorphine-induced desensitization of cell surface receptors increased the amount of some cross-linked products associated with nuclear membranes. The same treatment did not affect the relative density of the four larger molecular size bands in P20, but increased the density of the approximately 26 kDa product two fold. Etorphine-induced down-regulation evoked an elevation of cross-linked products in nuclear matrix preparations, while all band densities of P20 were diminished. These results suggest that nuclear matrix associated opioid binding sites represent internalized, truncated forms of the glycosylated delta opioid receptor found in P20.

kappa-opioid receptor expression defines a phenotypically distinct subpopulation of astroglia: relationship to Ca2+ mobilization, development, and the antiproliferative effect of opioids

To assess the role of kappa-opioid receptors in astrocyte development, the effect of kappa-agonists on the growth of astroglia derived from 1-2-day-old mouse cerebra was examined in vitro. kappa-Opioid receptor expression was assessed immunocytochemically (using KA8 and KOR1 antibodies), as well as functionally by examining the effect of kappa-receptor activation on intracellular calcium ([Ca2+]i) homeostasis and DNA synthesis. On days 6-7, as many as 50% of the astrocytes displayed kappa-receptor (KA8) immunoreactivity or exhibited increases in [Ca2+]i in response to kappa-agonist treatment (U69,593 or U50,488H). Exposure to U69,593 (100 nM) for 72 h caused a significant reduction in number and proportion of glial fibrillary acidic protein-immunoreactive astrocytes incorporating bromodeoxyuridine (BrdU) that could be prevented by co-administering the kappa-antagonist, nor-binaltorphimine (300 nM). In contrast, on day 14, only 5 or 14%, respectively, of the astrocytes were kappa-opioid receptor (KA8) immunoreactive or displayed functional increases in [Ca2+]i. Furthermore, U69,593 (100 nM) treatment failed to inhibit BrdU incorporation at 9 days in vitro. Experimental manipulations showed that kappa-receptor activation increases astroglial [Ca2+]i both through influx via L-type channels and through mobilization of intracellular stores (which is an important Ca2+ signaling pathway in cell division). Collectively, these results indicate that a subpopulation of developing astrocytes express kappa-opioid receptors in vitro, and suggest that the activation of kappa-receptors mobilizes [Ca2+]i and inhibits cell proliferation. Moreover, the proportion of astrocytes expressing kappa-receptors was greatest during a period of rapid cell growth suggesting that they are preferentially expressed by proliferating astrocytes.

Peptide quantification by tandem mass spectrometry

This manuscript reviews the literature on the mass spectrometry (MS) and tandem mass spectrometry (MS/MS) quantification of biologically important peptides that have been extracted from tissues. The most important aspect of this quantification process is the use of MS/MS to link the protonated molecule ion, (M + H)(+) , of the peptide with one or more of its amino acid sequence-determining fragment ions. The actual name of a peptide cannot be used in any study until the amino acid sequence of that peptide has been firmly established. This article reviews the analytical data obtained from the measurement of opioid peptides in human pituitary tissues. For example, the proopiomelanocortin (POMC)-derived beta-endorphin (BE) and the proenkephalin-derived methionine enkephalin (ME) opioid peptides have been quantified. The biogenesis of opioid neuropeptides is briefly reviewed; critical aspects of pituitary neuropeptides are discussed, including their localization and regulation, and their role in tumor formation; other analytical methods used to detect and measure neuropeptides are mentioned, including radioimmunoassay (RIA), radioreceptorassay (RRA), in situ hybridization, mRNA, and cDNA methods; and the MS and MS/MS methods are described. The use of stable isotope-incorporated synthetic peptide internal standards is described. Data are presented on the measurement of BE and ME in control pituitaries and in pituitary tumors (PRL-secreting and nonsecreting tumors). A significant alteration in the POMC peptide BE was found between the control and tumor tissues. That difference suggests that the POMC neuropeptidergic system had been down-regulated in those tumors. © 1997 John Wiley & Sons, Inc.

Production and characterization of polyclonal and monoclonal antibodies to the zeta (zeta) opioid receptor

The opioid growth factor, [Met5]-enkephalin, is an inhibitory agent of cell proliferation and maturation that interacts with the zeta (zeta) opioid receptor to modulate growth. In order to learn more about this receptor, polyclonal and monoclonal antibodies were raised against binding subunits identified on two-dimensional gels by ligand blotting. Using Western blotting, the polyclonal antibodies and some of the monoclonal antibodies recognized all 4 binding polypeptides (32, 30, 17, and 16 kDa) in developing rat cerebellum; no reaction was recorded in adult cerebellum. In addition, other monoclonals were able to distinguish only certain subunits (e.g. 17 kDa). The monoclonal antibodies and their F(ab')2 fragments, as well as the polyclonal antibodies, blocked the binding of [3H][Met5]-enkephalin to preparations of developing cerebellum. Both the polyclonal and monoclonal antibodies immunoprecipitated zeta opioid binding polypeptides from 6-day-old cerebellar homogenates solubilized by the zwitterionic detergent, CHAPS. Immunocytochemistry performed with polyclonal antibodies showed immunoreactivity associated with proliferating and differentiating cerebellar cells, but no specific staining was detected in the adult cerebellum. These results have identified and characterized antibodies to the zeta opioid receptor, and the antibodies were used to localize this receptor; these antibodies will be valuable to further cellular and molecular studies.