Detailed immunoautoradiographic mapping of enkephalinase (EC 3.4.24.11) in rat central nervous system: comparison with enkephalins and substance P

Opioid Withdrawal Abruptly Disrupts Amygdala Circuit Function by Reducing Peptide Actions

While the physical signs of opioid withdrawal are most readily observable, withdrawal insidiously drives relapse and contributes to compulsive drug use, by disrupting emotional learning circuits. How these circuits become disrupted during withdrawal is poorly understood. Because amygdala neurons mediate relapse, and are highly opioid sensitive, we hypothesized that opioid withdrawal would induce adaptations in these neurons, opening a window of disrupted emotional learning circuit function. Under normal physiological conditions, synaptic transmission between the basolateral amygdala (BLA) and the neighboring main island (Im) of GABAergic intercalated cells (ITCs) is strongly inhibited by endogenous opioids. Using patch-clamp electrophysiology in brain slices prepared from male rats, we reveal that opioid withdrawal abruptly reduces the ability of these peptides to inhibit neurotransmission, a direct consequence of a protein kinase A (PKA)-driven increase in the synaptic activity of peptidases. Reduced peptide control of neurotransmission in the amygdala shifts the excitatory/inhibitory balance of inputs onto accumbens-projecting amygdala cells involved in relapse. These findings provide novel insights into how peptidases control synaptic activity within the amygdala and presents restoration of endogenous peptide activity during withdrawal as a viable option to mitigate withdrawal-induced disruptions in emotional learning circuits and rescue the relapse behaviors exhibited during opioid withdrawal and beyond into abstinence.SIGNIFICANCE STATEMENT We find that opioid withdrawal dials down inhibitory neuropeptide activity in the amygdala. This disrupts both GABAergic and glutamatergic transmission through amygdala circuits, including reward-related outputs to the nucleus accumbens. This likely disrupts peptide-dependent emotional learning processes in the amygdala during withdrawal and may direct behavior toward compulsive drug use.

Broadening COVID-19 Interventions to Drug Innovation: Neprilysin Pathway as a Friend, Foe, or Promising Molecular Target?

The coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus is anticipated to transition to an endemic state as vaccines are providing relief in some, but not all, countries. Drug discovery for COVID-19 can offer another tool in the fight against the pandemic. Additionally, COVID-19 impacts multiple organs that call for a systems medicine approach to planetary health and therapeutics innovation. In this context, innovation for drugs that prevent and treat COVID-19 is timely and much needed. As the virus variants emerge under different ecological conditions and contexts in the long haul, a broad array of vaccine and drug options will be necessary. This expert review article argues for a need to expand the COVID-19 interventions, including and beyond vaccines, to stimulate discovery and development of novel medicines against SARS-CoV-2 infection. The Renin-Angiotensin-Aldosterone System (RAAS) is known to play a major role in SARS-CoV-2 infection. Neprilysin (NEP) and angiotensin-converting enzyme (ACE) have emerged as the pharmaceutical targets of interest in the search for therapeutic interventions against COVID-19. While the NEP/ACE inhibitors offer promise for repurposing against COVID-19, they may display a multitude of effects in different organ systems, some beneficial, and others adverse, in modulating the inflammation responses in the course of COVID-19. This expert review offers an analysis and discussion to deepen our present understanding of the pathophysiological function of neprilysin in multiple organs, and the possible effects of NEP inhibitor-induced inflammatory responses in COVID-19-infected patients.

Developmental Profile of Brain Neprilysin Expression Correlates with Olfactory Behaviour of Rats

A neuropeptidase, neprilysin (NEP), is a major amyloid (Aβ)-degrading enzyme involved in the pathogenesis of Alzheimer's disease (AD). The olfactory system is affected early in AD with characteristic Aβ accumulation, but data on the dynamics of NEP expression in the olfactory system are absent. Our study demonstrates that NEP mRNA expression in rat olfactory bulbs (OB), entorhinal cortex (ECx), hippocampus (Hip), parietal cortex (PCx) and striatum (Str) increases during the first postnatal month being the highest in the OB and Str. By 3 months, NEP mRNA levels sharply decrease in the ECx, Hip and PCx and by 9 months in the OB, but not in the Str, which correlates with declining olfaction in aged rats tested in the food search paradigm. One-month-old rats subjected to prenatal hypoxia on E14 had lower NEP mRNA levels in the ECx, Hip and PCx (but not in the OB and Str) compared with the control offspring and demonstrated impaired olfaction in the odour preference and food search paradigms. Administration to these rats of a histone deacetylase inhibitor, sodium valproate, restored NEP expression in the ECx, Hip and PCx and improved olfaction. Our data support NEP involvement in olfactory function.

Neprilysin Controls the Synaptic Activity of Neuropeptides in the Intercalated Cells of the Amygdala

Endogenous opioid peptides in the amygdala regulate many of our behaviors and emotional responses. In particular, the endogenous opioid enkephalin plays a significant role in regulating amygdala activity, but its action is strongly limited by peptidases, which degrade enkephalin into inactive fragments. Inhibiting peptidases may be an attractive method to enhance endogenous opioid signaling; however, we do not know which specific peptidase(s) to target. Using inhibition of glutamate release onto the intercalated cells of the amygdala as an assay for enkephalin activity, we applied specific peptidase inhibitors to determine which peptidase(s) regulate enkephalin signaling in this region. Thiorphan (10 μM), captopril (1 μM), or bestatin (10 μM) were used to inhibit the activity of neprilysin, angiotensin-converting enzyme, or aminopeptidase N, respectively. In rat brain slices containing the intercalated cells, we found that inhibition of glutamate release by a submaximal concentration of enkephalin was doubled by application of all three peptidase inhibitors combined. Then, we tested inhibitors individually and found that inhibition of neprilysin alone could enhance enkephalin responses to the same extent as inhibitors of all three peptidases combined. This indicates neprilysin is the predominant peptidase responsible for degrading enkephalins in the intercalated cells of the amygdala. This differs from the striatum, locus coeruleus, and spinal cord, where multiple peptidases metabolize enkephalin. These data highlight the importance of knowing which specific peptidase(s) control opioid actions in the relevant neural circuit and how they change in disease states to allow rational choices of drugs targeting the specific peptidase of interest. SIGNIFICANCE STATEMENT: Endogenous opioids modulate many of our emotional and behavioral responses. In the amygdala, they modulate our pain, fear, and addictive behaviors. Their actions are terminated when they are catabolized into inactive fragments by at least three different peptidases. In this study, we found that neprilysin selectively controls endogenous opioid concentrations at synapses in the intercalated cells of the amygdala. This peptidase may be a target for regulation of endogenous opioid modulation of amygdala-mediated emotional and behavioral responses.

Intrathecal substance P augments morphine-induced antinociception: possible relevance in the production of substance P N-terminal fragments

The present study sought to examine the mechanism of substance P to modulate the antinociceptive action of intrathecal (i.t.) morphine in paw-licking/biting response evoked by subcutaneous injection of capsaicin into the plantar surface of the hindpaw in mice. The i.t. injection of morphine inhibited capsaicin-induced licking/biting response in a dose-dependent manner. Substance P (25 and 50 pmol) injected i.t. alone did not alter capsaicin-induced nociception, whereas substance P at a higher dose of 100 pmol significantly reduced the capsaicin response. Western blots showed the constitutive expression of endopeptidase-24.11 in the dorsal and ventral parts of lumbar spinal cord of mice. The N-terminal fragment of substance P (1-7), which is known as a major product of substance P by endopeptidase-24.11, was more effective than substance P on capsaicin-induced nociception. Combination treatment with substance P (50 pmol) and morphine at a subthreshold dose enhanced the antinociceptive effect of morphine. The enhanced effect of the combination of substance P with morphine was reduced significantly by co-administration of phosphoramidon, an inhibitor of endopeptidase-24.11. Administration of D-isomer of substance P (1-7), [D-Pro(2), D-Phe(7)]substance P (1-7), an inhibitor of [(3)H] substance P (1-7) binding, or antisera against substance P (1-7) reversed the enhanced antinociceptive effect by co-administration of substance P and morphine. Taken together these data suggest that morphine-induced antinociception may be enhanced through substance P (1-7) formed by the enzymatic degradation of i.t. injected substance P in the spinal cord.

Thiorphan-induced survival and proliferation of rat thymocytes by activation of Akt/survivin pathway and inhibition of caspase-3 activity

The activity of substance P (SP) in the rat thymus seems to be tightly controlled by its bioavailability. In this study, we provide evidence for the expression of the SP-degrading enzyme, neutral endopeptidase (NEP)/CD10, by rat thymocyte subsets, and we illustrate its involvement in the in vivo SP/neurokinin-1 receptor (NK(1)R)-mediated regulation of thymocyte survival and proliferation. NEP/CD10 was expressed at both mRNA and protein levels on a substantial portion (45.5%) of CD5(+) thymocytes, namely on the CD4(+)CD8(+) (double positive; DP) and CD4(+) subsets. Continuous administration of thiorphan, a specific NEP/CD10 inhibitor, by means of miniosmotic pumps, enhanced rat thymocyte preprotachykinin-A (PPT-A) and NK(1)R mRNA expression as well as SP and NK(1)R protein levels in an NK(1)R-dependent manner. Thiorphan increased CD10(+)CD4(+) and CD10(+)DP thymocyte numbers, and an NK(1)R antagonist, (S)1-{2-[3(3-4-dichlorophenyl)-1-(3-iso-propoxyphenylacetyl)-piperidine-3-yl]ethyl}-4-pheny-1-azoniabicyclo[2.2.2]octane, chloride (SR140333), abrogated these stimulatory effects. In addition, the NEP/CD10 inhibitor stimulated interleukin (IL)-2 production, IL-2 receptor alpha chain expression, and concanavalin A-induced proliferation of CD5(+) thymocytes, and it inhibited spontaneous and NK(1)R-dependent thymocyte apoptosis. The thiorphan-protective antiapoptotic and proliferative effects involved the activation of Akt serine-threonine kinase, subsequent up-regulation of survivin mRNA, down-regulation of procaspase-3 mRNA levels, and suppression of caspase-3 activity, which were inhibited by SR140333 and mimicked by exogenous SP administration. Overall, our findings suggest that by controlling SP availability, NEP/CD10 negatively regulates thymocyte homeostasis and development.

Microarray analysis reveals distinctive signaling between the bed nucleus of the stria terminalis, nucleus accumbens, and dorsal striatum

To identify distinct transcriptional patterns between the major subcortical dopamine targets commonly studied in addiction we studied differences in gene expression between the bed nucleus of the stria terminalis (BNST), nucleus accumbens (NAc), and dorsal striatum (dStr) using microarray analysis. We first tested for differences in expression of genes encoding transcripts for common neurotransmitter systems as well as calcium binding proteins routinely used in neuroanatomical delineation of brain regions. This a priori method revealed differential expression of corticotropin releasing hormone ( Crh), the GABA transporter ( Slc6a1), and prodynorphin ( Pdyn) mRNAs as well as several others. Using a gene ontology tool, functional scoring analysis, and Ingenuity Pathway Analysis, we further identified several physiological pathways that were distinct among these brain regions. These two different analyses both identified calcium signaling, G-coupled protein receptor signaling, and adenylate cyclase-related signaling as significantly different among the BNST, NAc, and dStr. These types of signaling pathways play important roles in, amongst other things, synaptic plasticity. Investigation of differential gene expression revealed several instances that may provide insight into reported differences in synaptic plasticity between these brain regions. The results support other studies suggesting that crucial pathways involved in neurotransmission are distinct among the BNST, NAc, and dStr and provide insight into the potential use of pharmacological agents that may target region-specific signaling pathways. Furthermore, these studies provide a framework for future mouse-mouse comparisons of transcriptional profiles after behavioral/pharmacological manipulation.

Metabolism of amyloid-beta peptide and Alzheimer's disease

The accumulation of amyloid-beta peptide (Abeta), a physiological peptide, in the brain is a triggering event leading to the pathological cascade of Alzheimer's disease (AD) and appears to be caused by an increase in the anabolic activity, as seen in familial AD cases or by a decrease in catabolic activity. Neprilysin is a rate-limiting peptidase involved in the physiological degradation of Abeta in the brain. As demonstrated by reverse genetics studies, disruption of the neprilysin gene causes elevation of endogenous Abeta levels in mouse brain in a gene-dose-dependent manner. Thus, the reduction of neprilysin activity will contribute to Abeta accumulation and consequently to AD development. Evidence that neprilysin in the hippocampus and cerebral cortex is down-regulated with aging and from an early stage of AD development supports a close association of neprilysin with the etiology and pathogenesis of AD. Therefore, the up-regulation of neprilysin represents a promising strategy for therapy and prevention. Recently, somatostatin, which acts via a G-protein-coupled receptor (GPCR), has been identified as a modulator that increases brain neprilysin activity, resulting in a decrease of Abeta levels. Thus, it may be possible to pharmacologically control brain Abeta levels with somatostatin receptor agonists.

Ontogeny, regional and cellular distribution of the novel metalloprotease neprilysin 2 in the rat: a comparison with neprilysin and endothelin-converting enzyme-1

The localisation of the gene transcripts of a recently discovered peptidase, neprilysin 2 (NEP2), was established by in situ hybridisation in rat tissues during development and adulthood. It was compared with those of neprilysin (NEP), a closely related enzyme in terms of sequence homology or substrate specificity, and of endothelin-converting enzyme 1 (ECE-1) which, like the other two, belongs to the M-13 sub-family of zinc-dependent metallopeptidases. The ontogeny of the three enzymes differed markedly, the expression of NEP2 being restricted to developing and differentiating fields of the CNS, whereas NEP and ECE-1 genes were broadly expressed early on in the CNS and periphery. In contrast to the wide expression of NEP and ECE-1 in peripheral adult tissues and in CNS, NEP2 was almost exclusively expressed in selected neuronal populations of the brain and spinal cord. The only exceptions were the intermediate and anterior lobes of the pituitary as well as the choroid plexuses, where NEP2 was also strongly expressed. These localisations as well as those in the hypothalamic nuclei, together with the previously established pattern of cleaved peptides, suggest the involvement of NEP2 in the metabolism of neurohormones of the hypothalamo-pituitary axis.Complementary distributions of NEP and NEP2 mRNAs were observed in a large number of brain areas with, for instance the former being highly expressed in the striatum in which NEP2 transcripts were almost undetectable. In contrast, NEP2 was highly expressed in numerous thalamic, hypothalamic and brainstem nuclei from which NEP was absent. Since both peptidases are able to cleave the same neuropeptides, this pattern may suggest a complementary role in their peptide inactivation functions in the CNS. Finally, ECE-1 mRNAs were generally observed in neuronal populations known to express the pre-proendothelin-1 gene, confirming the function of the metallopeptidase in endothelin-1 generation.

Direct interstitial infusion of NK1-targeted neurotoxin into the spinal cord: a computational model

Convection-enhanced delivery of substance P (SP) nocitoxins to the spinal cord interstitium is under consideration for the treatment of chronic pain. To characterize treatment protocols, a three-dimensional finite-element model of infusion into the human dorsal column was developed to predict the distribution of SP-diphtheria toxin fusion protein (SP-DT') within normal and target tissue. The model incorporated anisotropic convective and diffusive transport through the interstitial space, hydrolysis by peptidases, and intracellular trafficking. For constant SP-DT' infusion (0.1 microl/min), the distribution of cytotoxicity in NK1 receptor-expressing neurons was predicted to reach an asymptotic limit at 6-8 h in the transverse direction at the level of the infusion cannula tip ( approximately 60% ablation of target neurons in lamina I/II). Computations revealed that SP-DT' treatment was favored by a stable SP analog (half-life approximately 60 min), high infusate concentration (385 nM), and careful catheter placement (adjacent to target lamina I/II). Sensitivity of cytotoxic regions to NK1 receptor density and white matter protease activity was also established. These data suggest that intraparenchymal infusions can be useful for treatment of localized chronic pain.

Region-specific reduction of A beta-degrading endopeptidase, neprilysin, in mouse hippocampus upon aging

Metabolism of amyloid-beta peptide (A beta) is closely associated with the pathology and etiology of Alzheimer's disease (AD). Neprilysin is the only rate-limiting catabolic peptidase proven by means of reverse genetics to participate in A beta metabolism in vivo. The aim of the present study is to assess whether possible spatial changes in neprilysin level in the brain with aging correlate to AD-vulnerable regions. When neprilysin levels in various brain regions of 10-, 80- and 132-week-old mice were evaluated by neprilysin-dependent endopeptidase activity assay and Western blot-based quantitative analysis, a clear change in neprilysin level with aging was observed in the hippocampal formation, in which the level was reduced by 20% at 132 weeks, compared to the 10-week group. Quantitative immunohistochemical analysis confirmed a marked local reduction of neprilysin levels with aging at the outer molecular layer and polymorphic layer of the dentate gyrus, and the stratum lucidum of the hippocampus, where the densities were reduced by 56%, 82% and 83%, respectively, at 132 weeks compared to the 10-week group. Thus, neprilysin was decreased selectively at the terminal zones and on axons of the lateral perforant path and the mossy fibers. These are the sites that show A beta pathology in mutant amyloid precursor protein (APP) transgenic mice, and that show synaptic loss in AD. The immunoreactivities to synaptic vesicle protein-2 and synaptophysin in the stratum lucidum and the dentate gyrus were unchanged, suggesting that a loss or decrease of synapses was not responsible for the decrease in the neprilysin levels. These observations suggest that downregulation of neprilysin is likely to be related to AD pathology and to the A beta deposition associated with normal aging in humans.

Degradation of nociceptin (orphanin FQ) by mouse spinal cord synaptic membranes is triggered by endopeptidase-24.11: an in vitro and in vivo study

We analyzed spinal metabolic pathway of nociceptin/orphanin FQ related to pain-transmission or modulation in the both in vitro and in vivo experiments. Nociceptin was degraded by spinal synaptic membranes. Major metabolites of nociceptin were free phenylalanine, nociceptin (1-13) and nociceptin (14-17). Both the degradation of nociceptin and the accumulation of the major cleavage metabolites, nociceptin (1-13) and nociceptin (14-17), were strongly inhibited by a metal chelator and also by specific inhibitors of endopeptidase-24.11, thiorphan and phosphoramidon. Furthermore, purified endopeptidase-24.11 hydrolyzed nociceptin at the cleavage site (Lys(13)-Leu(14) bond) identical to that by spinal synaptic membranes. Recently, we have found that nociceptin, injected intrathecally at small doses (fmol order) elicits a behavioral response consisting of scratching, biting and licking in mice. In the present study, we have examined the effect of peptidase inhibitors on the behavioral response elicited by intrathecal injection of nociceptin in mice. Phosphoramidon simultaneously injected with nociceptin additively enhanced nociceptin-induced behavioral response, whereas the nociceptin-induced behavioral response was unaffected by either bestatin, an aminopeptidase inhibitor or captopril, an angiotensin-converting enzyme inhibitor. However, the nociceptin effect was potentiated by combined injection of phosphoramidon and bestatin, indicating that inhibition of aminopeptidase may also contribute to inducing the behavioral response to nociceptin. These data suggest that endopeptidase-24.11 plays a major role in initial stage of nociceptin metabolism at the spinal cord level in mice.

Neurons with mu opioid receptors interact indirectly with enkephalin-containing neurons in the rat dentate gyrus

In the dentate gyrus, mu opioid receptors (MORs) and their enkephalin agonists have overlapping distributions and influence excitability and plasticity. Released endogenous enkephalins can activate at least some of these MORs; however, whether these interactions involve synaptically associated profiles or more distant associations and whether some subcellular compartments (e.g., terminals or dendrites) are more likely to be targeted than others are not known. To elucidate the relationships between potential sites of enkephalin release and MORs, MOR1 and leucine-enkephalin (LE) immunoreactivities were localized in the hilus by electron microscopy, using immunoperoxidase and immunogold markers. Of the 573 MOR-immunoreactive (ir) profiles analyzed, most were axons and terminals (51 and 30%, respectively), and fewer were dendrites (12%), glia (3%), or unclassifiable (4%). Most MOR-ir profiles resembled interneuron processes, while most LE-ir terminals resembled mossy fibers. One third of MOR-ir profiles were within 3 microm and approximately half were within 4 microm of the nearest LE-ir profile. In contrast, few (3%) MOR-ir profiles contacted LE-ir profiles; only 16% of these contacts included observable synapses, and very few profiles (0.5%) colocalized MOR and LE immunoreactivity. MOR-ir axons, terminals, and dendrites were not distributed differently relative to LE-ir profiles. These results suggest that activation of hilar MORs by LE usually involves short-range volume transmission and that dendritic MORs are as likely as axonal and terminal MORs to be activated by released LE. However, the greater abundance of MOR-ir axons and terminals compared to dendrites indicates that presynaptic profiles are a more prominent target for enkephalins and exogenous MOR agonists such as morphine.

Depressant effect on a C-fibre reflex in the rat, of RB101, a dual inhibitor of enkephalin-degrading enzymes

The effect of N-[(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithiol]-1-oxopropyl]-L-phenylalanine benzyl ester (RB101), a dual inhibitor of the enkephalin-degrading enzymes, neutral endopeptidase and aminopeptidase N, was assessed in anaesthetised rats on the C-fibre reflex elicited by electrical stimulation within the sural nerve territory and recorded from the ipsilateral biceps femoris muscle. The temporal evolution of the pharmacological response was monitored by the repeated application of a constant stimulus intensity, namely three times threshold (3 T). In addition, recruitment curves were built by varying the stimulus intensity from 0 to 7 T. RB101 (7.5, 15 and 30 mg kg(-1), i.v.) induced a dose-dependent, naloxone-reversible depression of the reflex, which lasted around 60 min with the highest dose. The ED(50) was calculated as 16.9 mg kg(-1). Analyses of the recruitment curves revealed: (1) a significant increase of threshold; (2) a significant depression of the reflex in the ascending part of the curve; and (3) a lack of major depressive effects on the responses elicited by the strongest stimuli (corresponding to the plateau of the curve). The increase in the nociceptive threshold by enkephalin-degrading enzyme inhibitors, confirms previous data obtained from behavioural tests. In addition, the present study revealed an efficacy of these compounds over a wide range of stimulus intensities, albeit excluding the highest.

Abeta-degrading endopeptidase, neprilysin, in mouse brain: synaptic and axonal localization inversely correlating with Abeta pathology

Metabolism of amyloid-beta peptide (Abeta) is closely associated with the pathology and etiology of Alzheimer's disease (AD). Since neprilysin is the only rate-limiting catabolic peptidase proven by reverse genetics to participate in Abeta metabolism in vivo, we performed detailed immunohistochemical analysis of neprilysin in mouse brain using neprilysin-deficient mice as a negative control. The aim was to assess, at both the cellular and subcellular levels, where Abeta undergoes neprilysin-dependent degradation in the brain and how neprilysin localization relates to Abeta pathology in amyloid precursor protein (APP)-transgenic mice. In hippocampus, neprilysin was present in the stratum pyramidale and stratum lacunosum-moleculare of the CA1-3 fields and the molecular layer of the dentate gyrus. Confocal double immunofluorescence analyses revealed the subcellular localization of neprilysin along axons and at synapses. This observation suggests that after synthesis in the soma, neprilysin, a type II membrane-associated protein, is axonally transported to the terminals, where Abeta degradation is likely to take place. Among various cell types, GABAergic and metabotropic glutamate 2/3 receptor-positive neurons but not catecholaminergic or cholinergic neurons, expressed neprilysin in hippocampus and neocortex, implying the presence of a cell type-specific mechanism that regulates neprilysin gene expression. As expected, Abeta deposition correlated inversely with neprilysin expression in TgCRND8 APP-transgenic mice. These observations not only support the notion that neprilysin functions as a major Abeta-degrading enzyme in the brain but also suggest that down-regulation of neprilysin activity, which may be caused by aging, is likely to elevate local concentrations of Abeta at and around neuronal synapses.

Zinc-enriched boutons in rat spinal cord

The rat spinal cord reveals a complex pattern of zinc-enriched (ZEN) boutons. As a result of in vivo exposure to selenide ions, nanosized clusters of zinc selenide are created in places where zinc ions are present, including the zinc-containing synaptic vesicles of ZEN boutons. The clusters can be silver enhanced by autometallographic (AMG) development. A description of the ZEN bouton patterns is presented and discussed. The distribution of ZEN boutons could indicate that these terminal systems have a differentiated influence on sensory and motor systems.

Afferent connections of the interstitial nucleus of the posterior limb of the anterior commissure and adjacent amygdalostriatal transition area in the rat

The interstitial nucleus of the posterior limb of the anterior commissure is, like the striatum, very rich in tyrosine hydroxylase and acetylcholinesterase, but on the basis of most other neurochemical criteria displays features that are typical of the extended amygdala (Alheid, de Olmos and Beltramino, 1995). Its afferent connections were examined in the rat with retrograde (cholera toxin B subunit) and anterograde (Phaseolus vulgaris leucoagglutinin) tracers and compared to those of the neighboring amygdalostriatal transition area and central amygdaloid nucleus. Deposits of cholera toxin B subunit in the interstitial nucleus of the posterior limb of the anterior commissure result in retrograde labeling that is similar to that seen after cholera toxin B subunit injections in the central amygdaloid nucleus. Retrogradely labeled cells are found in insular, infralimbic, prelimbic, piriform, amygdalopiriform transition, entorhinal and perirhinal cortices, as well as in temporal field CA1 of Ammon horn and ventral subiculum, amygdala (nucleus of the lateral olfactory tract, anterior amygdaloid area, anterior cortical, posterolateral cortical, anterior and posterior basomedial, intercalated cells, basolateral and lateral nuclei), and extended amygdala, primarily in its central division. The latter includes the lateral bed nucleus of the stria terminalis, dorsal portions of the sublenticular region, the lateral pocket of the supracapsular bed nucleus of the stria terminalis and the central amygdaloid nucleus. Retrogradely labeled cells are also seen in midline thalamic nuclei, lateral hypothalamus, ventral tegmental area, retrorubral field, dorsal raphe nucleus, pedunculopontine and dorsolateral tegmental nuclei, locus coeruleus and parabrachial area. The central extended amygdala, lateral hypothalamus and parabrachial area display a substantial retrograde labeling only when the injection involves districts of the interstitial nucleus of the posterior limb of the anterior commissure apposed to the pallidum, i.e. its medial part. Our anterograde results confirm that projections from the lateral bed nucleus of the stria terminalis and central amygdaloid nucleus to the interstitial nucleus of the posterior limb of the anterior commissure target its medial part. They also indicate that structures which provide major afferents to the central extended amygdala (the lateral and posterior basolateral amygdaloid nuclei and the amygdalopiriform transition area) innervate chiefly the medial part of the interstitial nucleus of the posterior limb of the anterior commissure and, to a much lesser degree, its lateral part. The piriform cortex, which has well-acknowledged projections to the ventral striatum, innervates only the rostral sector of the interstitial nucleus of the posterior limb of the anterior commissure. Taken together, these data indicate that the medial part of the interstitial nucleus of the posterior limb of the anterior commissure is closely related to the central extended amygdala. Rostral and lateral parts of the interstitial nucleus of the posterior limb of the anterior commissure, on the other hand, appear as transitional territories between the central extended amygdala and ventral striatum. The afferent connections of the zone traditionally termed amygdalostriatal transition area are in general similar to those of the caudate-putamen, which does not receive projections from the central extended amygdala. After cholera toxin B subunit injections in the caudoventral globus pallidus, a dense retrograde labeling is observed in the amygdalostriatal transition area and overlying striatum, but not in the interstitial nucleus of the posterior limb of the anterior commissure. Our results suggest that the interstitial nucleus of the posterior limb of the anterior commissure and the amygdalostriatal transition area are engaged in distinct forebrain circuits; the former is a dopamine-rich territory intimately related to the central ext

Immunolocalization of tripeptidyl peptidase II, a cholecystokinin-inactivating enzyme, in rat brain

Tripeptidyl peptidase II (EC 3.4.14.10) is a serine peptidase apparently involved in the inactivation of cholecystokinin octapeptide [Rose C. et al. (1996) Nature 380, 403-409]. We have compared its distribution with that of cholecystokinin in rat brain, using a polyclonal antibody raised against a highly purified preparation for immunohistochemistry at the photon and electron microscope levels. Tripeptidyl peptidase II-like immunoreactivity was mostly detected in neurons, and also in ependymal cells and choroid plexuses, localizations consistent with a possible participation of the peptidase in the inactivation of cholecystokinin circulating in the cerebrospinal fluid. Immunoreactivity was mostly detected in cell bodies, large processes and, to a lesser extent, axons of various neuronal populations. Their localization, relative to that of cholecystokinin terminals, appears to define three distinct situations. The first corresponds to neurons with high immunoreactivity in areas containing cholecystokinin terminals, as in the cerebral cortex or hippocampal formation, where pyramidal cell bodies and processes surrounded by cholecystokinin axons were immunoreactive. A similar situation was encountered in many other areas, namely along the pathways through which cholecystokinin controls satiety, i.e. in sensory vagal neurons, the nucleus tractus solitarius and hypothalamic nuclei. The second situation corresponds to cholecystokinin neuronal populations containing tripeptidyl peptidase II-like immunoreactivity, as in neurons of the supraoptic or paraventricular nuclei, axons in the median eminence or nigral neurons. In both situations, localization of tripeptidyl peptidase II-like immunoreactivity is consistent with a role in cholecystokinin inactivation. The third situation corresponds to areas with mismatches, such as the cerebellum, a region devoid of cholecystokinin, but in which Purkinje cells displayed high tripeptidyl peptidase II-like immunoreactivity, possibly related to a role in the inactivation of neuropeptides other than cholecystokinin. Also, some areas with cholecystokinin terminals, e.g., the molecular layer of the cerebral cortex, were devoid of tripeptidyl peptidase II-like immunoreactivity, suggesting that processes other than cleavage by tripeptidyl peptidase II may be involved in cholecystokinin inactivation. Tripeptidyl peptidase II-like immunoreactivity was also detected at the ultrastructural level in the cerebral cortex and hypothalamus using either immunoperoxidase or silver-enhanced immunogold detection. It was mainly associated with the cytoplasm of neuronal somata and dendrites, often in the vicinity of reticulum cisternae, Golgi apparatus or vesicles, and with the inner side of the dendritic plasma membrane. Hence, whereas a fraction of tripeptidyl peptidase II-like immunoreactivity localization at the cellular level is consistent with its alleged function in cholecystokinin octapeptide inactivation, its association with the outside plasma membrane of neurons remains to be confirmed.

Vulnerability to stressor-induced disturbances in self-stimulation from the dorsal and ventral A10 area: differential effects of intraventricular D-Ala2-Met5-enkephalinamide, D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin, and D-Pen2, D-Pen5-enkephalin administration

D-Ala2-Met5-enkephalinamide (DALA) (1.0 microg/microl) was administered intraventricularly to mice responding for electrical stimulation from the dorsal or ventral aspects of the VTA immediately prior to footshock (Experiment 1). Predictably, footshock reduced self-stimulation from the dorsal but not the ventral VTA immediately, 24, and 168 h following the stressor. Intraventricular DALA administration effected a partial attenuation of stressor-induced self-stimulation reductions from the dorsal VTA immediately and 24 h poststressor. Deficits appeared among DALA-Shocked mice responding for brain stimulation from the ventral VTA during comparable test intervals. The long-term depressant influence of footshock on self-stimulation from the dorsal VTA was abolished among DALA-treated mice and DALA-associated reductions in self-stimulation from the ventral A10 region among stressed mice were not evident 1 week later. Administration of D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin (DAGO) (0.01 microg/microl) or D-Pen2, D-Pen5-enkephalin (DPDPE) (1.0 microg/microl) intraventricularly prior to footshock effected an immediate and a delayed antagonism, respectively, of the stressor on self-stimulation from the dorsal VTA, which persisted for 1 week. Prophylactic administration of 0.001 microg/microl DAGO or 0.01 microg/microl DPDPE prior to the stressor failed to influence self-stimulation from the ventral VTA (Experiment 2). Administration of 0.01 microg/microl DAGO or 1.0 microg/microl DPDPE among mice responding for brain stimulation from the dorsal VTA following footshock produced a weak therapeutic effect immediately poststressor, but effected protracted amelioration of footshock-induced reductions of self-stimulation from the dorsal VTA (Experiment 3). Taken together, mu, delta, and mu-delta activation influenced self-stimulation differentially from the dorsal and ventral VTA according to the temporal order of opioid peptide challenge relative to stressor imposition. These data are discussed with respect to stressors, motivational alterations, and the putative modulating influence of endogenous enkephalin activity in subareas of the VTA.

Effects of SR140333, a selective non-peptide NK1 receptor antagonist, on trigemino-thalamic nociceptive pathways in the rat

Trigeminal stimulation of C-fibers increased c-fos expression within the trigeminal nucleus caudalis (NtV) and thalamic neuronal activity which both reflect the transmission of a nociceptive message. We examined the effects on both these phenomena of the selective NK1 and NK2 receptor antagonists, SR140333 and SR48968. SR140333 (0.3, 1 and 3 micrograms/kg intravenously [i.v.]) dose-dependently, reversibly and stereoselectively antagonized the increase of contralateral thalamic activity. This compound, when given i.v. (30 micrograms/kg) or orally (10 mg/kg), also reduced the number of Fos-like immunoreactive cells particularly at the medial and caudal level of the NtV. In contrast, SR48968 did not exert any antagonistic effect either on thalamic activity or on Fos-like immunoreactivity. The data strongly suggest a preferential involvement of NK1 vs NK2 receptors in nociceptive transmission following trigeminal ganglion stimulation. Taken together, our results indicate that SR140333 could provide a potent drug for the relief of pain occurring under excessive activity of sensory trigeminal fibers.

Posttraining administration of substance P and its N-terminal fragment block the amnestic effects of diazepam

This study examined the effects of posttraining administration of substance P (SP) and of certain N- or C-terminal SP-fragments on retention performance of rats treated with diazepam (DZP). Twenty minutes before the training on an inhibitory avoidance task rats were given intraperitoneal injections of either DZP (2 mg/kg) or vehicle. Immediately after they were injected with SP (50 micrograms/kg), SPN 1-7 (167 micrograms/kg), SPC 6-11 (134 micrograms/kg), or vehicle. The posttrial administration of SP and SPN, but not SPC, facilitated avoidance behavior. Animals that received DZP before training and vehicle after the conditioning trial showed impaired retention. In contrast, in animals injected with SP and SPN after the training trial, DZP did not affect retention. These findings suggest that the amnestic effects of DZP can be blocked by the administration of SP and that the amino acid sequence responsible for this effect may be encoded by its N-terminal part.

Visceral afferent stimulation-evoked changes in the release of peptides into the parabrachial nucleus in vivo

Previous investigations have demonstrated that the peptides substance P (SP), calcitonin gene-related peptide (CGRP), cholecystokinin (CCK), neurotensin (NT) and somatostatin (SOM) significantly modulate the glutamate-mediated transmission of visceral information through the parabrachial nucleus (PBN) to the ventrobasal thalamus. In addition, we have shown that the staining intensity of SOM, CCK and NT in the PBN decreases significantly following 2 h of vagal stimulation as visualized using immunohistochemistry. As well, the staining intensity of both SP and CGRP in the PBN were shown to increase under similar conditions. The present investigation was done to determine whether the altered peptide staining intensity of these peptides observed following 2 h of vagal stimulation was the result of an altered peptide release from terminals within the PBN. Male Sprague-Dawley rats were anesthetized with sodium thiobutabarbitol and instrumented to record blood pressure and heart rate and for the stimulation of the cervical vagus nerve. A push-pull perfusion cannula was lowered into the region of the PBN for the continuous sampling of extracellular fluid. Radioenzymatic quantification of the perfusates for peptide content revealed that the extracellular fluid concentration of CGRP and SP increased significantly during the 2 h of vagal stimulation. When the vagal stimulation was terminated, the release of both CGRP and SP decreased significantly below prestimulated values for approximately 30 min before returning to prestimulated levels shortly thereafter. In contrast, there was a significant decrease in the release of CCK, SOM and NT into the PBN during the period of vagal stimulation. Extracellular perfusate levels of these peptides returned to normal upon termination of stimulation. These results demonstrate that terminal release of CGRP and SP is significantly increased and terminal release of CCK, SOM and NT is significantly decreased in the PBN during 2 h of vagal stimulation. These results are consistent with our previous finding that the immunohistochemical staining intensity of CGRP and SP is increased while that of CCK, SOM and NT is decreased following vagal stimulation.

Opioid-related changes in nociceptive threshold and in tissue levels of enkephalins after target disruption of the gene for neutral endopeptidase (EC 3.4.24.11) in mice

Neutral endopeptidase EC 3.4.24.11 (NEP) is localized in peptidergic neurons and various colocalized peptides or other humoral mediators may serve as substrates. Target disruption of the NEP gene was reported to enhance the lethal response to endotoxin shock in mice. We examined thermonociceptive thresholds and enkephalin (ENK) tissue levels in transgenic NEP (-/-) and control wild type NEP (+/+) mice. Hot plate (52 degrees C) latency was 13.1 +/- 1.4 s in NEP (+/+) mice (n = 16) while latency increased significantly (P = 0.031) to 17.7 +/- 1.6 s in NEP (-/-) mice. Naloxone (10 mg/kg) had no effect on hot plate latency in NEP (+/+) mice (12.5 s, n = 8), but significantly decreased the latency in NEP (-/-) mice compared to untreated NEP (-/-) deficient mice (10.5 s, n = 8). Morphine (3 or 10 mg/kg) analgesic response was similar in knockout mice and wild type mice. Methionine-ENK (MET-ENK) and leucine-ENK (LEU-ENK) levels were determined in extracts from cortex, brain stem, hypothalamus, striatum, spinal cord, trigeminal ganglion and heart in treated and untreated mice. ENK-levels varied in a regionally-dependent manner and were significantly decreased in hypothalamus and spinal cord. We conclude that deletion of the NEP gene results in an opioid-related increase in thermonociceptive threshold. Regional differences in opioid metabolism indicate that NEP evokes tissue-specific patterns of ENK-regulation. NEP selectively controls opioid biosynthesis in hypothalamus and spinal cord presumably by feedback regulation.

Physiological role of brain endothelin in the central autonomic control: from neuron to knockout mouse

Although endothelin (ET) was discovered as a potent vascular endothelium-derived constricting peptide, its presumed physiological and pathophysiological roles are now considered much more diverse than originally though. Endothelin in the brain is thought to be deeply involved in the central autonomic control and consequent cardiorespiratory homeostasis, possibly as a neuromodulator or a hormone that functions locally in an autocrine/paracrine manner or widely through delivery by the cerebrospinal fluid (CSF). This notion is based on the following lines of evidence. (1) Mature ET, its precursors, converting enzymes, and receptors all are detected at strategic sites in the central nervous system (CNS), especially those controlling the autonomic functions. (2) The ET is present in the CSF at concentrations higher than in the plasma. (3) There is a topographical correspondence of ET and its receptors in the CNS. (4) The ET is released by primary cultures of hypothalamic neurons. (5) When ET binds to its receptors, intracellular calcium channels. (6) An intracerebroventricular or topical application of ET to CNS sites elicits a pattern of cardiorespiratory changes accompanied by responses of vasomotor and respiratory neurons. (7) Recently generated knockout mice with disrupted genes encoding ET-1 exhibited, along with malformations in a subset of the tissues of neural crest cell lineage, cardiorespiratory abnormalities including elevation of arterial pressure, sympathetic overactivity, and impairment of the respiratory reflex. Definitive evidence is expected from thorough analyses of knockout mice by applying conventional experimental methods.

Substance P and enkephalin immunoreactivities in axonal boutons presynaptic to physiologically identified dorsal horn neurons. An ultrastructural multiple-labelling study in the cat

A combination of intracellular electrophysiological recording and injection of horseradish peroxidase with ultrastructural immunocytochemistry was used to investigate the synaptic interplay between substance P- and enkephalin-immunoreactive axonal boutons and three types of functionally characterized dorsal horn neurons in the cat spinal cord. The dorsal horn neurons were classified as nociceptive specific, wide dynamic range and non-nociceptive based on their responses to innocuous and noxious stimuli. Most of the nociceptive neurons (either nociceptive specific or wide dynamic range) contained enkephalin immunoreactivity, but none of the non-nociceptive neurons were positive for enkephalin. Three types of immunoreactive boutons were found in contact with the functionally characterized dorsal horn neurons. These boutons were positive for either substance P, enkephalin, or substance P+enkephalin. Quantitative analysis revealed that the percentages of substance P-immunoreactive boutons apposed to the cell bodies, proximal dendrites and distal dendrites of nociceptive neurons were significantly higher than those of non-nociceptive neurons. Furthermore, the percentages of substance P+enkephalin-immunoreactive axonal boutons apposed to the distal dendrites of nociceptive neurons were significantly higher than those of non-nociceptive neurons and the percentages of enkephalin-immunoreactive boutons apposed to the cell bodies and proximal dendrites of nociceptive neurons were significantly higher than in non-nociceptive neurons. Finally, neither enkephalin-immunoreactive nor substance P+enkephalin-immunoreactive boutons were ever seen presynaptic to substance P-immunoreactive boutons. These results provide evidence of an anatomical substrate within the dorsal horn for the interaction of substance P-mediated with enkephalin-mediated mechanisms. The data support the idea that the modulation of nociceptive input in the dorsal horn by enkephalinergic neurons occurs mainly via a postsynaptic mechanism, and thus suggest that dorsal horn enkephalinergic neurons participate in a local inhibitory feedback loop in a distinct pathway from the previously postulated opioid-mediated depression of substance P release from primary afferent terminals.

Regulatory mechanisms that modulate signalling by G-protein-coupled receptors

The large and functionally diverse group of G-protein-coupled receptors includes receptors for many different signalling molecules, including peptide and non-peptide hormones and neuro-transmitters, chemokines, prostanoids and proteinases. Their principal function is to transmit information about the extracellular environment to the interior of the cell by interacting with the heterotrimeric G-proteins, and they thereby participate in many aspects of regulation. Cellular responses to agonists of these receptors are usually rapidly attenuated. Mechanisms of signal attenuation include removal of agonists from the extracellular fluid, receptor desensitization, endocytosis and down-regulation. Agonists are removed by dilution, uptake by transporters and enzymic degradation. Receptor desensitization is mediated by receptor phosphorylation by G-protein receptor kinases and second-messenger kinases, interaction of phosphorylated receptors with arrestins and receptor uncoupling from G-proteins. Agonist-induced receptor endocytosis also contributes to desensitization by depleting the cell surface of high-affinity receptors, and recycling of internalized receptors contributes to resensitization of cellular responses. Receptor down-regulation is a form of desensitization that occurs during continuous, long-term exposure of cells to receptor agonists. Down-regulation, which may occur during the development of drug tolerance, is characterized by depletion of the cellular receptor content, and is probably mediated by alterations in the rates of receptor degradation and synthesis. These regulatory mechanisms are important, as they govern the ability of cells to respond to agonists. A greater understanding of the mechanisms that modulate signalling may lead to the development of new therapies and may help to explain the mechanism of drug tolerance.

Substantia innominata: a notion which impedes clinical-anatomical correlations in neuropsychiatric disorders

Comparative neuroanatomical investigations in primates and non-primates have helped disentangle the anatomy of the basal forebrain region known as the substantia innominata. The most striking aspect of this region is its subdivision into two major parts. This reflects the fundamental organizational scheme for this portion of the forebrain. According to this scheme, two major subcortical telencephalic structures, i.e. the striatopallidal complex and extended amygdala, form large diagonally oriented bands. The rostroventral extension of the pallidum accounts for a large part of the rostral subcommissural substantia innominata, while the sublenticular substantia innominata is primarily occupied by elements of the extended amygdala. Also dispersed across this region is the basal nucleus of Meynert, which is part of a more or less continuous collection of cholinergic and non-cholinergic corticopetal and thalamopetal cells, which stretches from the septum diagonal band rostrally to the caudal globus pallidus. The basal nucleus of Meynert is especially prominent in the primate, where it is sometimes inappropriately applied as a synonym for the substantia innominata, thereby tacitly ignoring the remaining components. In most mammals, the extended amygdala presents itself as a ring of neurons encircling the internal capsule and basal ganglia. The extended amygdala may be further subdivided, i.e. into the central extended amygdala (related to the central amygdaloid nucleus) and the medial extended amygdala (related to the medial amygdaloid nucleus), which generally form separate corridors both in the sublenticular region and along the supracapsular course of the stria terminalis. The extended amygdala is directly continuous with the caudomedial shell of the accumbens, and to some extent appears to merge with it. Together the accumbens shell and extended amygdala form an extensive forebrain continuum, which establishes specific neuronal circuits with the medial prefrontal-orbitofrontal cortex and medial temporal lobe. This continuum is particularly characterized by a prominent system of long intrinsic association fibers, and a variety of highly differentiated downstream projections to the hypothalamus and brainstem. The various components of the extended amygdala, together with the shell of the accumbens, are ideally structured to generate endocrine, autonomic and somatomotor aspects of emotional and motivational states. Behavioral observations support this proposition and demonstrate the relevance of these structures to a variety of functions, ranging from the various elements of the reproductive cycle to drug-seeking behavior. The neurochemical and connectional features common to the accumbens shell and the extended amygdala are especially relevant to understanding the etiology and treatment of neuropsychiatric disorders. This is discussed in general terms, and also in specific relation to the neurodevelopmental theory of schizophrenia and to the neurosurgical treatment of neuropsychiatric disorders.

Neutral endopeptidase (EC 3.4.24.11) in labial salivary glands in healthy controls and in patients with Sjögren's syndrome

OBJECTIVE

Neuropeptides from nerve fibres can cause neurogenic inflammation. The potency of these peptides in vitro has led to the hypothesis that enzyme degradative systems are operative in vivo to limit their action. To consider this question neutral endopeptidase (NEP) in labial salivary glands in patients with Sjögren's syndrome was studied.

METHODS

Synthesis of NEP mRNA in situ in labial salivary glands was studied using the reverse transcriptase polymerase chain reaction (RT-PCR). Immunohistochemical staining was used to localise the NEP enzyme protein and its neuropeptide substrates and fluorophotometry to measure the corresponding enzyme activities in saliva.

RESULTS

NEP was found in nerve fibres and in perivascular, periductal, and periacinar axon terminal varicosities. Double labelling of PGP 9.5 and NEP confirmed this neuronal localisation of NEP. Although some fibroblast-like cells and occasional intravascular neutrophils were NEP positive, NEP mRNA was not found in labial salivary glands. Patients with Sjögren's syndrome and healthy controls did not have nerves containing NEP or neuropeptides (vasoactive intestinal peptide, substance P, or calcitonin gene related peptide (CGRP)) in lymphocyte foci. Salivary NEP activity was not decreased in patients compared with controls.

CONCLUSION

NEP in labial salivary glands is almost totally of neuronal origin and plays a part in proteolytic modulation of neuropeptides in salivary glands and saliva. These regulatory interactions seem to be altered in focal lymphocyte accumulations in Sjögren's syndrome.

Attenuation of the exercise pressor reflex. Effect of opioid agonist on substance P release in L-7 dorsal horn of cats

Using alpha-chloralose-anesthetized cats, we studied blood pressure and heart rate responses to static contraction and passive stretch of the triceps surae muscle before and after microdialyzing the mu-opioid agonist [D-Ala2]-methionine enkephalinamide (DAME, 200 mumol/L) into the L-7 dorsal horn of the spinal cord. In addition, we measured contraction-induced substance P release in the dorsal horn before and after drug delivery. After 92 +/- 3 minutes of dialyzing the opioid agonist, contraction-induced increases in mean arterial pressure and heart rate were attenuated from control values of 58 +/- 7 mm Hg and 17 +/- 3 beats per minute to postdrug values of 27 +/- 7 mm Hg and 10 +/- 2 beats per minute, respectively. A similar attenuation was observed for the passive muscle stretches after 97 +/- 5 minutes of dialysis (control, 38 +/- 4 mm Hg and 8 +/- 2 beats per minute; after drug, 23 +/- 4 mm Hg and 5 +/- 1 beats per minute). Prior microdialysis of naloxone (300 mumol/L), a mu-antagonist, blocked this effect, suggesting that the opioid agonist has a specific receptor action. Naloxone alone had no effect on the pressor or tachycardiac responses. The contraction-induced increase in substance P-like immunoreactivity was reduced from a control value of 0.119 +/- 0.024 to 0.047 +/- 0.010 fmol/100 microL by DAME. Time-control experiments revealed no decrease in the release of substance P-like immunoreactivity. Thus, activation of opioid receptors modulates the transmission of group III and IV muscle afferent nerve activity through the L-7 dorsal horn.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of endopeptidase-24.11 (common acute lymphoblastic leukaemia antigen CD10) in the sciatic nerve of the adult rat after lesion and during regeneration

Endopeptidase-24.11, which is identical with the common acute lymphoblastic leukaemia antigen CD10 (CALLA), is a cell surface Zn2+ metalloprotease that regulates peptide-induced responses in different tissues, including the nervous and immune systems. In the peripheral nervous system, high levels of the enzyme are present in all neonatal and early postnatal Schwann cells, while as myelination proceeds it is gradually suppressed in the majority of cells that form myelin but retained in non-myelin-forming cells in the adult animal. In the present study we have investigated the effects of transection, crush and regeneration of the adult rat sciatic nerve on the expression of the endopeptidase by Schwann cells in situ. Endopeptidase-24.11 was monitored by immunocytochemistry using the monoclonal anti-endopeptidase antibody 23B11. For comparison, a parallel study was carried out with a monoclonal antibody directed against the rat nerve growth factor receptor. We found that (i) all Schwann cells of the distal segment re-expressed endopeptidase-24.11 as early as 4 days after axotomy, the level of immunostaining reaching a maximum after 2 weeks, (ii) axonal regeneration repressed Schwann cell expression of endopeptidase-24.11, and (iii) the induction of the nerve growth factor receptor followed a similar pattern to that of endopeptidase-24.11 in the transected and crushed nerve. Enzymatic amplification of endopeptidase-24.11 cDNA from normal and axotomized adult rat sciatic nerve confirmed the expression of endopeptidase-24.11 in these tissues. Our results show that the expression of endopeptidase-24.11 in Schwann cells, as is the case with the nerve growth factor receptor, is induced by the loss of the normal axon-Schwann cell contact. The significant increase in the expression of endopeptidase-24.11 by Schwann cells after axonal damage suggests that the enzyme could play a role in axonal regeneration.

Concomitant variation in immunoreactivity for enkephalins and neutral endopeptidase in the median eminence of the lactating rat

In the brain, neutral endopeptidase (NEP) is implicated in the biological inactivation of enkephalins and substance P. NEP, enkephalins and neurokinins were detected by immunocytochemistry in the median eminence (ME) of lactating female rats either freely nursing or separated from their pups for 48 h. A significant decrease in enkephalin and NEP immunoreactivity was observed in the external zone of the ME after pup removal. In both animal groups, neurokinin immunoreactivity was absent from this region. These results indicate that: (1) a concomitant variation in immunoreactivity for enkephalins and NEP occur in the ME of the lactating rat; (2) in the ME during lactation, NEP may essentially be implicated in the biological degradation of enkephalins.

The role of the periaqueductal grey in vocal behaviour

This is a review of our current knowledge about the role of the periaqueductal grey (PAG) in vocal control. It shows that electrical stimulation of the PAG can evoke species-specific calls with short latency and low habituation in many mammals. The vocalization-eliciting region contains neurones the activity of which is correlated with the activity of specific laryngeal muscles. Lesioning studies show that destruction of the PAG and laterally bordering tegmentum can cause mutism without akinesia. Neuroanatomical studies reveal that the PAG lacks direct connections with the majority of phonatory motoneurone pools but is connected with the periambigual reticular formation, an area which does have direct connections with all phonatory motor nuclei. The PAG receives a glutamatergic input from several sensory areas, such as the superior and inferior colliculi, solitary tract nucleus and spinal trigeminal nucleus. Glutamatergic input, in addition, reaches it from numerous limbic structures the stimulation of which also produces vocalization, such as the anterior cingulate cortex, septum, amygdala, hypothalamus and midline thalamus. Pharmacological blocking of this glutamatergic input causes mutism. The glutamatceptive vocalization-controlling neurones are under a tonic inhibitory control from GABAergic neurones. Removal of this inhibitory input lowers the threshold for the elicitation of vocalization by external stimuli. A modulatory control on vocalization threshold is also exerted by glycinergic, opioidergic, cholinergic, histaminergic and, possibly, noradrenergic and dopaminergic afferents. It is proposed that the PAG serves as a link between sensory and motivation-controlling structures on the one hand and the periambigual reticular formation coordinating the activity of the different phonatory muscles on the other.

Interactions between neutral endopeptidase (EC 3.4.24.11) and the substance P (NK1) receptor expressed in mammalian cells

Interactions between neutral endopeptidase-24.11 (NEP) and the substance P receptor (SPR; NK1) were investigated by examining substance P (SP) degradation, SP binding and SP-induced Ca2+ mobilization in epithelial cells transfected with cDNA encoding the rat SPR and rat NEP. Expression of NEP accelerated the degradation of SP by intact epithelial cells and by membrane preparations, and degradation was reduced by the NEP inhibitor thiorphan. In cells expressing SPR alone, specific 125I-SP binding after 20 min incubation at 37 degrees C was 92.2 +/- 3.1% of maximal binding and was unaffected by thiorphan. Coexpression of NEP in the same cells as the SPR markedly reduced SP binding to 13.9 +/- 0.5% of maximal, and binding was increased to 82.7 +/- 2.4% of maximal with thiorphan. Coexpression of NEP in the same cells as the SPR also reduced to undetectable the increase in intracellular Ca2+ in response to low concentrations of SP (0.3 and 0.5 nM), and significantly reduced the response to higher concentrations (1 and 3 nM). The Ca2+ response was restored to control values by inhibition of NEP with thiorphan. In contrast, SP binding and SP-induced Ca2+ mobilization were only slightly reduced when cells expressing SPR alone were mixed with a 3- to 24-fold excess of cells expressing NEP alone. Therefore, in this system, NEP markedly down-regulates SP binding and SP-induced Ca2+ mobilization only when coexpressed in the same cells as the SPR.

Differential response of neutral endopeptidase 24.11 ("enkephalinase"), and cholinergic and opioidergic markers to hypoglossal axotomy

Neutral endopeptidase 24.11 (NEP; "enkephalinase") may inactivate a number of centrally active neuropeptides including the enkephalins and substance P. In most areas of the central nervous system, the cell types which express NEP activity are not known. The hypoglossal nucleus (N.XII) was selected as a model system to characterize the cytochemical localization of NEP. The effect of hypoglossal nerve axotomy upon the distribution of NEP activity in the hypoglossal nucleus was compared to the effect upon cholinergic markers, the mu opiate receptor, and the enkephalins. By use of a fluorescence histochemical method, NEP was localized at all levels of N.XII to the soma and proximal processes of the majority of the apparent motor neurons in the nucleus. Fluorescent double-labeling studies revealed the presence of numerous enkephalinergic varicosities which localized to the neuropil surrounding NEP-stained motor neurons. To determine whether NEP was synthesized by these motor neurons, 18 rats received a unilateral transection of the hypoglossal nerve. A pronounced decrease in NEP staining in N.XII was observed on the operated side as early as 3 days following axotomy. This decrease persisted at all levels of the nucleus for about 5 weeks. By 7 weeks, the staining between the control and operated sides was indistinguishable. By contrast, there was no apparent change in the density or distribution of enkephalin-immunoreactive varicosities in five animals examined 6 to 32 days following axotomy. Radioligand binding of [3H]DAMGO to the mu-opiate receptor in N.XII was studied in 20 animals by quantitative autoradiography at 2, 6, and 11 days after axotomy. No significant changes in the level of radioligand binding to the mu-receptor were detected in response to axotomy. In contrast to the opiate system, the cholinergic enzymes choline acetyltransferase, acetylcholinesterase, and pseudocholinesterase showed a coordinate decrease in motor neuron-associated staining on the operated side of N.XII at 3, 6, and 11 days following axotomy which paralleled the decrease in NEP staining. By contrast, the lysosomal enzyme marker, acid phosphatase, showed a pronounced increase in staining on the operated side. The results of this study are consistent with the synthesis of NEP by cholinergic N.XII motor neurons and indicates that the enkephalins and NEP in N.XII are closely associated, but derive from separate neuronal populations. The widespread overlap in the distribution of NEP-stained motor neurons and enkephalinergic varicosities in N.XII provides additional anatomical support for a potential role for NEP in the inactivation of centrally active enkephalins.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracisternal neutral endopeptidase-24.11 inhibitors produce inhibition in gastric acid output: independence from opiate, bombesin, or neurotensin-mediated mechanisms

Intracisternal (ic) injection of the neutral endopeptidase-24.11 inhibitor phosphoramidon (1-100 nmol) produced a dose-dependent inhibition of gastric acid secretion in 2-h pylorus-ligated rats. The response resulted from a reduction in acid concentration and volume. Likewise, ic injection of another neutral endopeptidase-24.11 inhibitor Zincov (200 nmol) produced a 63% inhibition in gastric acid output. In contrast, neither intravenous injection of phosphoramidon (100 nmol) nor ic injection of the aminopeptidase inhibitor amastatin (100 nmol) produced any change in gastric acid secretion. The inhibitory effect of ic phosphoramidon (10 nmol) was not reversed by a dose of naloxone sufficient to antagonize the acid inhibitory effects of ic [D-Ala2-D-met5]enkephalinamide (8.5 nmol). Moreover, phosphoramidon-induced inhibition of acid was not reduced by the centrally effective bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 or by reserpine pretreatment at a dose effective to antagonize ic neurotensin-induced inhibition in acid secretion. These results suggest that an endogenous neutral endopeptidase-24.11 sensitive substrate may act in the brain to inhibit gastric acid output by mechanisms independent of CNS opiate, bombesin or neurotensin activity.

Ventral mesencephalic and cortical transplants into the rat striatum display enhanced activity for neutral endopeptidase 24.11 ('enkephalinase'; CALLA)

A role for neutral endopeptidase 24.11 (NEP) in growth and development is supported by the demonstration that NEP hydrolyses and inactivates a number of peptide growth factors including atrial natriuretic peptide, endothelins, bombesin-like peptides, and opioid peptides, including the enkephalins. In the present study, suspensions of cells obtained from the ventral mesencephalon or cortex of rat embryos (ED14) were implanted into the striatum of the adult rat brain. Three to 15 weeks after transplantation the relative distribution of NEP-positive cellular elements was visualized histochemically. NEP staining in the transplants consistently appeared before NEP staining in the surrounding host striatum supporting a relative increase in NEP activity in the transplants. The NEP staining richly visualized cells of varying size and morphology which lacked the normal organization of the host striatum. The histochemical staining in the transplants and the surrounding host tissue was completely blocked by a 100 nM concentration of the selective NEP inhibitors phosphoramidon or JHF-26, supporting the exclusive localization of NEP by this method. NEP localization in the embryonic (ED14) cortex and ventral mesencephalon was also confirmed, suggesting one possible origin for the NEP-positive cells visualized in the transplants. Fluorescent double-labeling studies for NEP and glial fibrillary acidic protein (GFAP) or transforming growth factor alpha precursor (TGF alpha p) revealed the presence of rich glial labeling within the transplants for both GFAP and TGFap. NEP-labeled cells in the transplants were closely associated with glial elements, however, only occasional glial elements in the transplants stained for NEP; supporting a non-astrocytic localization for the NEP in the transplants. The marked enhancement of NEP staining in the transplants may have significance for controlling the rate or pattern of growth of the transplanted cells through inactivation of peptide growth factors produced by, or in response to, the transplants.

Radioimmunocytochemical distribution of neutral endopeptidase (enkephalinase E.C.3.4.24.11) at the ultrastructural level in the rat median eminence

Neutral endopeptidase (E.C.3.4.24.11) was visualized at the ultrastructural level in the external zone of the rat median eminence by using 125I-labelled IgG of a monoclonal serum. A precise analysis of the localization of the immunolabelling, which appears in the form of individual stray silver grains, was undertaken. Among the 1,045 grains counted, 82% were localized over membrane appositions involving nerve endings only and nerve endings plus tanycytes. The difference between the real and a randomly generated population of grains was statistically significant. Our results provide morphological arguments in support of the view of a paracrine action of neuropeptides present in the median eminence especially enkephalins but possibly, substance P, angiotensin, cholecystokinin and neurotensin. These neuropeptides are known to be inactivated by neutral endopeptidase. The action of these peptides may be exerted on nerve endings (autocrine or paracrine) but an intervention on tanycytes cannot be excluded.

Atriopeptin III degradation by endopeptidase 24.11: the Cys-Phe bond is not the preferential cleavage site

Incubation of rANP(5-28)--also called atriopeptin III (AP III)--with purified endopeptidase 24.11 led preferentially to the production of Phe-Arg-Tyr, while other products of minor importance were detected. One of these was identified as rANP(5-25) (atriopeptin I) (AP I). This hydrolysis pattern of endopeptidase 24.11 towards AP III differs from the known favored site of cleavage at the Cys7-Phe8 bond of rANP(1-28). Moreover, by comparison with rANP(1-28), the degradation rate of AP III was slower. These data suggest that N-terminal peptide truncation results in conformational and/or charge modifications leading to a different positioning of the peptide in the endopeptidase 24.11 active site. In most hypothalamic nuclei of the rat brain known to contain AP III and endopeptidase 24.11, the preferential Ser25-Phe26 bond hydrolysis, although supposed to be responsible for a reduced degradation rate, might represent an effective enzymatic pathway of catabolism for AP III.

Enhanced learning produced by injection of neurokinin substance P into the region of the nucleus basalis magnocellularis: mediation by the N-terminal sequence

The effect of unilateral injection of the neurokinin substance P (SP) and of certain N- or C-terminal SP-fragments into the region of the nucleus basalis magnocellularis (NBM) on inhibitory avoidance learning was investigated. Rats with chronically implanted cannulae were tested on a one-trial uphill avoidance task. Immediately after the training trial, rats were injected with 0.74 pmol SP or equimolar dosed SP(1-7), DIME-C7, or SP(7-11). Control groups included vehicle-injected rats and a group given an injection of SP(1-7) 5-h after the trial. When tested 24 h later, rats treated with SP or SP(1-7), but not with DIME-C7 or SP(7-11), exhibited longer step-up latencies than vehicle-treated controls. The retention latencies for rats in the SP(1-7) 5-h delay group did not differ from those of vehicle-injected animals, ruling out proactive effects of SP(1-7) on performance. The results show that SP facilitates retention of an inhibitory avoidance response when injected into the NBM. Furthermore, the amino acid sequence that encodes this effect may be located in the N-terminal part of the SP-molecule.

Endopeptidase-24.11 is suppressed in myelin-forming but not in non-myelin-forming Schwann cells during development of the rat sciatic nerve

Endopeptidase-24.11, which is identical with the common acute lymphoblastic leukemia antigen (CALLA), is a cell surface zinc metalloprotease that has the ability to hydrolyse a variety of physiologically active peptides. Interest in this enzyme is based on the view that it may play a role in the regulation of peptide signals in different tissues, including the nervous and immune systems. We have previously shown that endopeptidase-24.11 is present in Schwann cells in the peripheral nervous system of newborn pigs [Kioussi C. and Matsas R. (1991) J. Neurochem. 57, 431-440]. In the present study we have investigated the developmental expression of the endopeptidase by Schwann cells in the rat sciatic nerve, from embryonic day 16 to maturity. Endopeptidase-24.11 was monitored enzymatically as well as by immunoblotting and immunocytochemistry using the monoclonal anti-endopeptidase antibody 23B11. We found an age-dependent decline in both the enzyme activity and the levels of immunoreactive protein. Endopeptidase-24.11 was first detected at embryonic day 18 and was present in all neonatal and early postnatal Schwann cells. However, as myelination proceeded the endopeptidase was gradually suppressed in the majority of cells that form myelin but retained in non-myelin-forming cells in the adult animal. At this stage, only very few large diameter myelinated fibers expressed weakly endopeptidase-24.11. Schwann cells dissociated from postnatal day 5 nerves and cultured up to one week in the absence of axons expressed endopeptidase-24.11. These results show that the endopeptidase has a distinct developmental profile in the rat sciatic nerve, similar to that of a group of other Schwann cell surface antigens, including the cell adhesion molecules N-CAM and L1 and the nerve growth factor receptor. We suggest that, as is the case with these antigens, endopeptidase-24.11 may play a role in nerve development and/or regeneration. In addition, persistence of endopeptidase-24.11 in a minority of adult myelin-forming Schwann cells suggests a possible role for the enzyme in axon-myelin apposition and maintenance, especially of larger diameter axons.

Membrane localization of endopeptidase-24.11 and peptidyl dipeptidase A (angiotensin converting enzyme) in the pig brain: a study using subcellular fractionation and electron microscopic immunocytochemistry

Brains from piglets were dissected and a block of tissue including the substantia nigra, globus pallidus, and entopeduncular nucleus was homogenized and then fractionated on discontinuous Percoll gradients. Ligand-binding assays using (-)-[3H]nicotine and [3H]quinuclidinyl benzilate served to delineate fractions containing nicotinic and muscarinic acetylcholine receptors. In this system endopeptidase-24.11 exhibited a biphasic distribution, consistent with its presence on both pre- and postsynaptic membranes. Peptidyl dipeptidase A (angiotensin converting enzyme; ACE) was associated with membrane fractions containing muscarinic receptors. An immunoblot of these fractions with an affinity-purified polyclonal antibody to ACE revealed only the neuronal form of ACE (Mr 170,000), the endothelial form (Mr 180,000) being undetectable. Electron microscopic immunoperoxidase staining of the substantia nigra, with an affinity-purified antibody to endopeptidase-24.11 at the preembedding stage, showed this antigen to be confined to the plasma membranes of boutons, axons, and some dendrites. Both pre- and postsynaptic membranes were stained, and occasionally other regions of the dendritic membrane were positive. No staining of synaptic vesicles within the boutons was observed. Thus, two independent approaches indicate that endopeptidase-24.11 is present on both pre- and postsynaptic membranes in the pig substantia nigra. The subcellular fractionation suggests that neuronal ACE is confined to dendritic membranes.

Expression of cALLa/NEP on gliomas: a possible marker of malignancy

First described on pre-B leukemia cells, the common acute lymphoblastic leukemia antigen (cALLa) is also expressed on glioma cells in vitro. Its identity to neutral endopeptidase (NEP) (E.C.3.24.11) was corroborated by our finding that cALLa positive glioma cells had NEP activity. To study cALLa/NEP distribution on glial tumours in vivo, we examined 76 brain tumour biopsies by immunostaining techniques on frozen tissue sections using anti-cALLa (FAH99) and anti-NEP (135 A 3) monoclonal antibodies. We found that 96% of grade 4 gliomas (25/26) expressed NEP. Whereas only 45% (4/9) of grade 3 or anaplastic astrocytomas did. In low grade gliomas, we found 2 positive tumours out of 21 tested (10%). Double immunostaining procedures revealed that NEP was co-expressed with GFAP. However no NEP could be detected on non-glial brain tumours nor on reactive astrocytes. These results suggest that cALLa/NEP expression could be linked to malignant progression of gliomas.

Intraventricular administration of D-Ala2-Met5-enkephalinamide induces rapid recovery of responding for electrical brain stimulation from the ventral tegmental area following uncontrollable footshock

The effects of uncontrollable footshock were assessed in CD-1 mice responding for electrical brain stimulation (ICSS) from the dorsal aspects of the ventral tegmental area (VTA). Uncontrollable footshock provoked a marked reduction of responding for ICSS immediately, 24 h and 168 h following initial stressor exposure. Intraventricular administration of the neuropeptide, D-Ala2-Met5-enkephalinamide (DALA) (1 microgram/microliter delivered in a 1-microliter volume) following induction of reward alterations from the dorsal VTA provoked a rapid recovery of responding for ICSS in the immediate poststressor interval. This ameliorative effect of the neuropeptide deteriorated 24 h after initial exposure to uncontrollable footshock and was absent 1 week later. These data are discussed with respect to stressor-induced anhedonia in the mesocorticolimbic system.