Morphine stimulates nitric oxide release in human mitochondria.

The expression of morphine by plants, invertebrate, and vertebrate cells and organ systems, strongly indicates a high level of evolutionary conservation of morphine and related morphinan alkaloids as required for life. The prototype catecholamine, dopamine, serves as an essential chemical intermediate in morphine biosynthesis, both in plants and animals. We surmise that, before the emergence of specialized plant and animal cells/organ systems, primordial multi-potential cell types required selective mechanisms to limit their responsiveness to environmental cues. Accordingly, cellular systems that emerged with the potential for recruitment of the free radical gas nitric oxide (NO) as a multi-faceted autocrine/paracrine signaling molecule, were provided with extremely positive evolutionary advantages. Endogenous morphinergic signaling, in concert with NO-coupled signaling systems, has evolved as an autocrine/paracrine regulator of metabolic homeostasis, energy metabolism, mitochondrial respiration and energy production. Basic physiological processes involving morphinergic/NO-coupled regulation of mitochondrial function, with special emphasis on the cardiovascular system, are critical to all organismic survival. Key to this concept may be the phenomenon of mitochondrial enslavement in eukaryotic evolution via endogenous morphine.

Convergent dysregulation of frontal cortical cognitive and reward systems in eating disorders.

A substantive literature has drawn a compelling case for the functional involvement of mesolimbic/prefrontal cortical neural reward systems in normative control of eating and in the etiology and persistence of severe eating disorders that affect diverse human populations. Presently, we provide a short review that develops an equally compelling case for the importance of dysregulated frontal cortical cognitive neural networks acting in concert with regional reward systems in the regulation of complex eating behaviors and in the presentation of complex pathophysiological symptoms associated with major eating disorders. Our goal is to highlight working models of major eating disorders that incorporate complementary approaches to elucidate functionally interactive neural circuits defined by their regulatory neurochemical phenotypes. Importantly, we also review evidence-based linkages between widely studied psychiatric and neurodegenerative syndromes (e.g., autism spectrum disorders and Parkinson's disease) and co-morbid eating disorders to elucidate basic mechanisms involving dopaminergic transmission and its regulation by endogenously expressed morphine in these same cortical regions.

Targeting mitochondrial biogenesis for promoting health.

Mitochondrial biogenesis is a key physiological process that is required for normal growth and development and for maintenance of ongoing cellular energy requirements during aging. Of equivalent and or greater importance is the regulated enhancement of mitochondrial biogenesis upon physiological demand coupled to multiple cellular insults. Basically, cellular survival mechanisms following a variety of disease-related pathophysiological insults are entrained by convergent mechanisms designed to regain homeostatic control of mitochondrial biogenesis. Recent molecular studies represent a clearly defined approach to maximize normative cellular expression of mitochondrial biogenesis for maintenance of cellular energy requirements and as an anti-aging strategy in healthy human populations. This report focuses on mitochondrial transcription factor A, peroxisome proliferator-activated receptor gamma coactivator 1-alpha, PINK1 and Parkin. Designing agents to target mitochondrial function represents a compelling therapeutic strategy for enhancement of cellular expression of mitochondrial biogenesis in diverse human populations afflicted with metabolic, degenerative, neurodegenerative, and metastatic diseases.

Parkinson's disease, L-DOPA, and endogenous morphine: a revisit.

Clinical observations stemming from widespread employment of restorative L-3,4-dihydroxyphenylalanine (L-DOPA) therapy for management of dyskinesia in Parkinson's Disease (PD) patients implicate a regulatory role for endogenous morphine in central nervous system dopamine neurotransmission. Reciprocally, it appears that restorative L-DOPA administration has provided us with a compelling in vivo pharmacological model for targeting peripheral sites involved in endogenous morphine expression in human subjects. The biological activities underlying endogenous morphine expression and its interaction with its major precursor dopamine strongly suggest that endogenous morphine systems are reciprocally dysregulated in PD. These critical issues are examined from historical and current perspectives within our short review.

Identification of a µ opiate receptor signaling mechanism in human placenta.

BACKGROUND: Previous studies report that genes in the morphine biosynthetic pathway have been found in placental tissue. Prior researchers have shown that kappa opioid receptors are present in human placenta. We determined if a µ opiate receptor was present and which subtype was expressed in human placenta. We also sought to demonstrate a functional µ opiate receptor in human placenta. MATERIAL/METHODS: Polymerase chain reactions as well as DNA sequencing were performed to identify the µ opiate receptor subtypes present in human placenta. The functionality of the receptor was demonstrated by real time amperometric measurements of morphine induced NO release. RESULTS: The µ4 opiate receptor sequence was present as well as the µ1 opioid receptor transcript. The addition of morphine to placental tissue resulted in immediate nitric oxide release and this effect was blocked by naloxone. CONCLUSIONS: In the present study, an intact morphine signaling system has been demonstrated in human placenta. Morphine signaling in human placenta probably functions to regulate the immune, vascular, and endocrine functions of this organ via NO.

Variations in critical morphine biosynthesis genes and their potential to influence human health.

Endogenous morphine has been detected in human tissues from the vascular, immune and nervous systems. The genes/enzymes (CYP2D6, COMT and PNMT) that are involved in the biosynthesis of morphine have variations that affect their functionality. Some of these variations are the result of single nucleotide polymorphisms of DNA sequences. This review highlights some of the functional differences in the critical enzymes required for the biosynthesis of morphine that may affect human health. These variations have been shown to change the way animals react to stressors, perceive pain and behave. The presence of morphine signaling in almost all organ systems suggests that it is most likely playing a role in maintaining the health and promoting the normal functioning of these physiological systems.

Endogenous morphine/nitric oxide-coupled regulation of cellular physiology and gene expression: implications for cancer biology.

Cancer is a simplistic, yet complicated, process that promotes uncontrolled growth. In this regard, this unconstrained proliferation may represent primitive phenomena whereby cellular regulation is suspended or compromised. Given the new empirical evidence for a morphinergic presence and its profound modulatory actions on several cellular processes it is not an overstatement to hypothesize that morphine may represent a key chemical messenger in the process of modulating proliferation of diverse cells. This has been recently demonstrated by the finding of a novel opiate-alkaloid selective receptor subtype in human multilineage progenitor cells (MLPC). Adding to the significance of morphinergic signaling are the findings of its presence in plant, invertebrate and vertebrate cells, which also have been shown to synthesize this messenger as well. Interestingly, we and others have shown that some cancerous tissues contain morphine. Furthermore, in medullary histolytic reticulosis, which is exemplified by cells having hyperactivity, the mu3 (mu3) opiate select receptor was not present. Thus, it would appear that morphinergic signaling has inserted itself in many processes taking a long time to evolve, including those regulating the proliferation of cells across diverse phyla.

Endogenous morphine signaling via nitric oxide regulates the expression of CYP2D6 and COMT: autocrine/paracrine feedback inhibition.

We determined changes in mRNA expression in specific enzymes involved in the biosynthesis of morphine in human white blood cells via microarray. Leukocyte exposure to morphine down-regulated catechol-O-methyl transferase (COMT) and CYP2D6 by approximately 50% compared with control values. The treatment did not alter DOPA decarboxylase and dopamine beta-hydroxylase expression, demonstrating the specificity of morphine actions. The verification of the microarray data was accomplished via real-time Taqman reverse transcriptase polymerase chain reaction (RT-PCR) focused on CYP2D6 and COMT expression in different blood samples treated with morphine. The analysis showed similar changes in the expression of CYP2D6 and COMT mRNA. The expression was reduced by 47 +/- 7% for CYP2D6, substantiating the microarray finding of a 54% reduction. Furthermore, exposure of white blood cells to 10(-6) M S-nitroso-N-acetyl-DL-penicillamine (SNAP), a nitric oxide (NO) donor, reduced the expression of CYP2D6 and COMT. Prior naloxone (10(-6) M) or N-nitro-L-arginine methyl ester (L-NAME) (10(-4) M) addition abrogated morphine's down-regulating activity, demonstrating morphine was initiating its actions via stimulating constitutive NO synthase derived NO release via the mu3 opiate receptor splice variant. In the past we demonstrated that UDP-glucurosyltransferase is involved in metabolizing morphine to morphine 6-glucuronide in adrenal chromaffin cells. In the present study its expression was not found in controls and morphine-treated cells, suggesting that morphine 6-glucuronide may not be synthesized in white blood cells. Taken together, it appears that morphine has the ability to modulate its own synthesis via autocrine and paracrine signaling.

The American lobster, Homarus americanus, contains morphine that is coupled to nitric oxide release in its nervous and immune tissues: Evidence for neurotransmitter and hormonal signaling.

OBJECTIVES: The study was designed to determine if morphine was present in lobster tissues. It was also important to determine, as in other animals, if its levels would change in response to stress. In this regard, it was also important to determine if lobster immune and neural tissues express the mu opiate receptor subtype, which was coupled to constitutive nitric oxide synthase derived nitric oxide release. METHODS: Homarus americanus were used in these experiments. Morphine was purified in lobster tissues via high pressure liquid chromatography coupled to UV detection. It was quantified via radioimmunoassay (RIA) and was identified via quadruple time of flight - mass spectrometry. Animals were subject to 2 forms of trauma, namely pereiopod-ablation or lipopolysaccaride (LPS) - injection, and morphine levels determined in nerve cord or hemolymph. Real-time nitric oxide production was determined via an amperometric probe. RT-PCR was used to determine the presence of a micro opiate receptor transcript. RESULTS: In Homarus americanus hemolymph and nerve cord morphine was found. RIA revealed morphine levels of 3.36 pg/mg +/ - 0.48 SEM (N=8) in nerve cord and 717.88 pg/ml +/ - 56.77 SEM (N=58) in hemolymph. In stressed (pereiopod-ablated or LPS-injected) animals, the endogenous morphine levels initially increased significantly by 24% for hemolymph and 48% for nerve cord. By day 5, the stressed and control values for endogenous morphine, in both tissues, was lower and non-distinguishable. In both hemocytes and neural cells, morphine, not met-enkephalin, stimulated constitutive nitric oxide release in a naloxone antagonizable manner, demonstrating a mu opiate receptor mediated phenomenon and suggesting the presence of the mu opiate receptor subtype, micro3, since it is opiate alkaloid selective and opioid peptide insensitive. RT-PCR revealed the presence of a micro opiate receptor transcript in Homarus neural and immune tissues, which exhibits a 100% sequence identity with its human counterpart. CONCLUSION: Taken together, after eliminating all sources of contamination, morphine is present in lobster tissues, potentially demonstrating hormonal and neurotransmitter functions that are involved in the animals' stress response.

Estrogen signaling at the cell surface coupled to nitric oxide release in Mytilus edulis nervous system.

In previous studies we have demonstrated release of nitric oxide (NO) in human tissues following exposure to estrogen. We now designed experiments to determine whether estrogen is present in the neural tissue of Mytilus edulis, a marine mollusk, and whether, as in vertebrates, it stimulates constitutive NO synthase activity. After HPLC purification of 17beta-estradiol (17beta-E(2)) from M. edulis ganglionic tissue, we confirmed the presence of 17beta-E(2) by RIA and ES-Q-TOF-MS analysis. We further found that when either exogenous or endogenous (purified HPLC fraction) 17beta-E(2) was added to pedal ganglia, there was immediate concentration-dependent NO release. Furthermore, 17beta-E(2) conjugated to BSA also stimulated NO release, suggesting mediation by a membrane surface receptor. Tamoxifen, an estrogen receptor antagonist, inhibited the action of both 17beta-E(2) and 17beta-E(2) conjugated to BSA, further supporting the presence of an estrogen receptor. In addition, by Western blot analysis with anti-ER-beta antibodies, we observed a 55-kDa protein in both the membrane and cytosolic fractions in pedal ganglia as well as in human leukocytes (that have been previously shown to express ER-beta). In summary, our results suggest that a physiological dose of estrogen acutely stimulates NO release within pedal ganglia via an estrogen cell surface receptor.

Endogenous morphinergic signaling and tumor growth.

The mu3 opiate receptor subtype has been characterized by various binding assays as opiate alkaloid selective (e.g. morphine) and opioid peptide (e.g. methionine enkephalin) insensitive. This opiate receptor subtype has been found on human, including cancer cell lines, and invertebrate tissues, demonstrating that it has been conserved during evolution. Furthermore, in numerous reports, this receptor is coupled to constitutive nitric oxide release. In this regard, for example, morphine immune down regulating activities parallels those actions formerly attributed to nitric oxide. We have now identified the mu3 receptor at the molecular level and sequence analysis of the isolated cDNA suggests that it is a novel, alternatively spliced variant of the mu opiate receptor gene (MOR). Furthermore, using Northern blot, reverse transcription coupled to polymerase chain reaction (RT-PCR) and sequence analysis, we have demonstrated the expression of this new mu variant in human vascular tissue, mononuclear cells, polymorphonuclear cells, and human neuroblastoma cells. The presence of this mu splice variant, adds to the growing body of evidence supporting the hypothesis that morphine is an endogenous signaling molecule in neural, immune and vascular systems. In addition to their use in the treatment of pain, opioid peptides appear to be important in the growth regulation of normal and neoplastic tissue. This review will focus on the influence of opiate alkaloids, e.g., morphine, on tumor growth, with emphasis on immuno-regulatory and antiproliferative mechanisms.

Cold stress alters Mytilus edulis pedal ganglia expression of mu opiate receptor transcripts determined by real-time RT-PCR and morphine levels.

Previous pharmacological, biochemical and molecular evidence prove that mu-subtype opiate receptors and opiate alkaloids, i.e. morphine, are present in the ganglionic nervous system of the mollusk Mytilus edulis (bivalve). We now present molecular evidence on the effect of rapid temperature changes on mu opiate receptor expression and morphine levels. Using primers, a labeled Taq-Man probe derived from the human neuronal mu1 opiate receptor, and real-time RT-PCR to measure the expression of mu transcripts from Mytilus pedal ganglia, we observe, in animals placed in cold water from room temperature, an enhanced morphine and morphine 6 glucuronide level in addition to a decrease in mu opiate receptor gene expression. This study provides further evidence that mu-type opiate receptors and morphine are expressed in mollusk ganglia and appear to be involved in physiological processes responding to thermal stress.

Cyclic exercise induces anti-inflammatory signal molecule increases in the plasma of Parkinson's patients.

It has been known for many years that immune system alterations occur with Parkinson's disease (PD). Changes in lymphocyte populations in cerebrospinal fluid and blood, immunoglobulin synthesis, and cytokine and acute phase protein production have been observed in patients with PD. Hence, there is evidence for inflammation. In this report we demonstrate that cyclic exercise over months results in a significant increase in the rise of plasma anti-inflammatory signal molecules, such as interleukin-10 and adrenocorticotropin. Additionally, endogenous plasma morphine levels increase with the duration of the cyclic exercise protocol. Morphine is identified and quantified by high performance liquid chromatography coupled to electrochemical detection and nano electro-spray ionization double quadrupole orthogonal acceleration time of flight mass spectrometry. Proinflammatory cytokine, i.e., interleukin-1, interleukin-6, plasma levels did not increase. These results matched with those reported previously, demonstrating enhanced motor skills and mood elevation with this cyclic exercise protocol, suggest that this protocol induces the formation of anti-inflammatory signal molecules, which appear to be associated with alleviation of some of the clinical characteristics of PD.

Reticuline exposure to invertebrate ganglia increases endogenous morphine levels.

OBJECTIVES: Given the presence of morphine, its metabolites and precursors in mammalian and invertebrate tissues, it became important to determine if exposing tissues to an opiate alkaloid precursor, reticuline, would result in increasing endogenous morphine levels. METHOD: Endogenous morphine levels were determined by high pressure liquid chromatography coupled to electrochemical detection and radioimmunoassay following incubation of Mytilus edulis pedal ganglia with reticuline. Nitric oxide (NO) release was determined in real-time via an amperometric probe. Mu opiate receptor affinity for opiate alkaloid precursors was determined by a receptor displacement assay. RESULTS: Morphine is present in the pedal ganglia of Mytilus edulis (1.43 +/- 0.41 ng/mg +/- SEM ganglionic wet weight). Ganglia incubated with 50 ng of reticuline, a morphine precursor in plants, for 1 hour exhibited a statistical increase in their endogenous morphine levels (6.7 +/- 0.7 ng/mg tissue wet weight; P<0.01). This phenomenon is concentration dependent. The increase in ganglionic morphine levels occurs gradually over the 60 min incubation period, beginning 10 minutes post reticuline addition. We show that reticuline (10(-6) M) does not stimulate ganglionic NO release in a manner resembling that of morphine (10(-6) M), which releases NO seconds after its exposure to the ganglia and lasts for 5 minutes. With reticuline, there is a 3 minute delay, which is followed by an extended release period. Furthermore, in binding displacement experiments both reticuline and salutaridine (another morphine precursor) exhibit no binding affinity for the pedal ganglia mu opiate receptor subtype. This finding is further substantiated using the positive control of human monocytes where the mu3 opiate receptor subtype has been cloned. CONCLUSION: Taken together, we surmise that the morphine's precursors are being converted to morphine. The experiments strongly indicate that pedal ganglia can synthesize morphine from reticuline.

Morphine down regulates human vascular tissue estrogen receptor expression determined by real-time RT-PCR.

UNLABELLED: Human vascular endothelial cells express the estrogen receptor-beta (ER-beta), which can be modulated by the opiate alkaloid morphine. OBJECTIVES: To determine if morphine is capable of down regulating the ER-beta receptor in a similar fashion as the mu opiate receptor since they are both coupled to constitutive nitric oxide synthase derived nitric oxide release. METHODS AND RESULTS: Endothelial cells obtain from human vascular tissues (saphenous vein, atria and primary saphenous vein cells) were treated with 1 uM morphine plus or minus the mu opiate receptor antagonist naloxone or CTOP (10 uM) for 24 h at 37 degrees C. Total RNA was isolated from treated and untreated primary endothelial cells, and specific primers and a probe were used to determine the ER-beta gene expression by real-time RT-PCR. Cells treated with morphine exhibited a down-regulation of ER-beta, whereas naloxone and CTOP were able to partially block the morphine effect. In addition, the 266 bp fragment generated by RT-PCR using the same primers as in the real-time PCR was sequenced and revealed a 100% sequence identity as the authentic ER-beta gene sequence. CONCLUSIONS: These results indicate that ER-beta is expressed in human vascular endothelial cells, and morphine appears to regulate this receptor in a similar fashion as the mu opiate receptor.

17-beta estradiol down regulates ganglionic microglial cells via nitric oxide release: presence of an estrogen receptor beta transcript.

OBJECTIVES: In earlier studies we have demonstrated that 17-beta-estradiol and an estrogen cell surface receptor can be found on various human cells where they are coupled to nitric oxide release. We also demonstrated the presence of estrogen signaling in Mytilus edulis ganglia. In the present report, we sought to determine a function for these ganglionic estrogen receptors, transcending a reproductive role for estrogen. MATERIAL & METHODS: Ganglionic microglial egress from excised pedal ganglia was examined microscopically following pharmacological treatments designed to determine a role for 17-beta-estradiol in microglial regulation via nitric oxide. Additionally, we examined the tissue by RT-PCR and sequence analysis for the estrogen receptor beta gene. RESULTS: In ganglia incubated with varying concentrations of 17-beta-estradiol-BSA there is a significant drop in microglial egress at the 24 hour observation period (58.7 +/- 7.4 vs. 17-beta-estradiol-BSA exposed = 14.7 +/- 1.5; P<0.01), which can be antagonized by tamoxifen and significantly diminished by L-NAME, a nitric oxide synthase inhibitor. By RT-PCR and sequence analysis Mytilus edulis pedal ganglia was found to express a 266 bp fragment of the estrogen receptor-beta gene, which exhibits 100% sequence identity with the human counterpart. CONCLUSION: These data suggest that 17-beta-estradiol-BSA is working on estrogen cell surface receptors since 17-beta-estradiol-BSA does not enter the cytoplasm and that these receptors are coupled to constitutive nitric oxide release. This study demonstrates that 17-beta-estradiol can down regulate microglial fMLP induced activation and activation following ganglionic excision.

The presence of 17-beta estradiol in Mytilus edulis gonadal tissues: evidence for estradiol isoforms.

OBJECTIVES: In earlier studies, we demonstrate that 17-beta -estradiol and an estrogen cell surface receptor can be found on various human cells, i.e., vascular endothelial, monocytes, and granulocytes, where they are coupled to nitric oxide release. We further demonstrated this phenomenon in the marine mussel Mytilus edulis ganglionic tissues. In the present report we sought to determine if estrogen can be found in M. edulis reproductive tissues. MATERIAL & METHODS: We determined the presence of 17-beta -estradiol via high pressure liquid chromatography (HPLC) and radioimmunoassay (RIA) in the animals gonads. This substance was further identified via nanoelectro-spray ionization quadrupole time of flight mass spectrometry (Q-TOF-MS). RESULTS: 17-beta -estradiol was identified and quantified in Mytilus gonads. Interestingly, we also determined that estradiol isoforms also were present in this tissue. CONCLUSION: These data demonstrate that 17-beta-estradiol and an estradiol isoform is present in M. edulis gonadal tissues, suggesting that they have functions related to reproduction. This further suggests that estrogen's association with reproductive activities has a long evolutionary history and that this association began in invertebrates.

Immunocytes modulate ganglionic nitric oxide release which later affects their activity level.

Pedal ganglia excised and maintained in culture for up to 2 h, release NO at low levels. The range can vary between 0 to 1.1 nM. Non-stimulated immunocytes do not significantly stimulate ganglionic NO release when incubated with pedal ganglia. However, ganglia exposed to immunocytes that had been previously activated by a 30 min incubation with interleukin 1 beta, release NO significantly above basal levels. In these experiments, 91 +/- 2.5% of the non-stimulated immunocytes exhibited form factors in the 0.72 to 0.89 range (sampled prior to ganglionic addition), whereas 62 +/- 10.3% of the interleukin 1 beta stimulated immunocytes had form factors in the 0.39 to 0.49 range, demonstrating activation. Addition of the nitric oxide synthase inhibitor, L-NAME (10(-4) M), inhibited basal ganglionic NO release as well as that initiated by exposing the ganglia to activated immunocytes. Interestingly, non activated immunocytes, following ganglionic exposure, exhibited activity levels in the 13% range, representing a non significant increase. Cells exposed to interleukin 1 beta had a 65% activity level at the beginning of the experiment, followed by a drop of activity to 19 +/- 3.2% after ganglionic exposure. Repeating this last observation in the presence of L-NAME (10(-4) M), brought the activity level of the immunocytes back to the pre-ganglionic exposure level of activity, demonstrating that ganglionic NO was involved in down regulating immunocyte activity.

Effects of cold stress on morphine-induced nitric oxide production and mu-opiate receptor gene expression in Mytilus edulis pedal ganglia.

OBJECTIVES: Subjecting the marine bivalve Mytilus edulis to an immediate temperature change has been shown to rapidly alter the animals' ganglionic monoamine levels, as well as its ciliary activity. Recently, we extended this observation to include the organism's ganglionic mu opiate receptor and morphine levels. In the past, we demonstrated that M. edulis ganglionic mu receptors exposed to morphine was coupled to the immediate release nitric oxide (NO). In this study, we measured morphine-induced NO release in M. edulis subjected to acute cold stress. METHODS: NO release was monitored with an NO-selective microprobe. Temporal changes in mu opiate receptor expression were also examined over 24 hours. RESULTS: In this study, we demonstrate that after 12h cold exposure (4 degrees C from 24 degrees C), the estimated relative mu opiate receptor (MOR) gene expression in M. edulis pedal ganglia, measured by real-time PCR, did not differ significantly from the control group (1.23+/-0.25, p>0.05). However, the measured M. edulis pedal ganglia MOR expression demonstrated that ganglia significantly (0.77+/-0.05, p<0.001) down regulated their mu opiate receptor mRNA expression after 24h exposure to the cold water. The mean value for control animal (24 degrees C, n=14) morphine-stimulated NO release was 36.7 +/- 9.8 nM. Morphine additions to cold-treated tissues (4 degrees C, n=7) produced an average of 6.7 +/- 4.9 nM NO, which was a statistically significant difference between 25 degrees C and 4 degrees C animals (p=0.025). CONCLUSION: The study further demonstrates that mu opiate receptor expression is coupled to NO release.

A hormonal role for endogenous opiate alkaloids: vascular tissues.

The distribution of morphine-containing cells in the central nervous system, adrenal gland, and its presence in blood may serve to demonstrate that this signal molecule can act as a hormone besides its role in cell-to-cell signaling within the brain. This speculative review is the result of a literature evaluation with an emphasis on studies from our laboratory. Opioid peptides and opiate alkaloids have been found to influence cardiac and vascular function. They have also been reported to promote ischemic preconditioning protection in the heart. Given the presence of morphine and the novel mu(3) opiate receptor on vascular endothelial cells, including cardiac and vascular endothelial cells in the median eminence, it would appear that endogenous opiate alkaloids are involved in modulating cardiac function, possible at the hormonal level. This peripheral target tissue, via nitric oxide coupling to mu opiate receptors, may serve to down regulate the excitability of this tissue given the heart's high performance state as compared to that of the saphenous vein, a passive resistance conduit. With this in mind, morphine and other endogenous opiate alkaloids may function as a hormone.