Peptidergic neurons: facts and trends

Brain peptides: what, where, and why?

Within the past decade, a large number of peptides have been described within the vertebrate central nervous system. Some of these peptides were previously known to be present in nonneural vertebrate tissues, as well as in lower species, in which they may serve as primitive elements of intercellular communication prior to the development of neuronal or endocrine systems. In vertebrates, these peptides are thought to have neurotransmitter or neuromodulatory roles and appear to be involved in the regulation of a number of homeostatic systems, although the mechanisms of their actions are still unclear.

Microheterogeneity ("neurotypy") of neurofilament proteins

Neurofilaments purified from adult rat brainstem by two methods were electrophoresed on NaDodSO4/polyacrylamide gels to separate the triplet proteins (approximate Mrs of 200,000, 155,000, and 68,000) which, in turn, were electroblotted onto nitrocellulose paper. On Coomassie blue-stained gels that were not electroblotted, the same banding pattern was seen with both methods of preparation. Immunocytochemical staining of the electroblots with each of five monoclonal antibodies revealed that three of the monoclonal antibodies were specific for the Mr 200,000 neurofilament protein and two, for both the Mrs 200,000 and 155,000 neurofilament proteins. None of the antibodies reacted with the Mr 68,000 band. The Mr 200,000 band could be resolved into doublet bands. Individual monoclonal antibodies reacted with either one or both of the Mr 200,000 doublets. The immunocytochemical staining of the neurofilament triplets on electroblots was compared to that of adult rat cerebellar paraffin sections. Each monoclonal antibody had a unique pattern of staining, reacting only with certain subpopulations of neurons or their processes. Correlation of the staining patterns in cerebellar tissue sections with those of neurofilament polypeptides on electroblots suggested that different neurofilament polypeptides can be localized to different structures and subpopulations of neurons and that molecular heterogeneity ("neurotypy") may be revealed within the Mrs 200,000 and 155,000 neurofilament polypeptides.

Adipokinetic hormone and AKH-like peptide demonstrated in the corpora cardiaca and nervous system of Locusta migratoria by immunocytochemistry

An antiserum was raised against tyrosine-adipokinetic hormone ([Tyr1]-AKH). In immunohistochemical procedures, it revealed the AKH cells in the glandular lobes of the corpora cardiaca (CC) of Locusta migratoria with high specificity. In addition, an immunologically related peptide was detected in certain neurons of the central nervous system which suggests that this AKH-like peptide may have a neurotransmitter function. The glandular lobes contain immunoreactive AKH cells in all post-embryonic stages, and no essential differences in morphology and distribution of the cells in nymphs and adults were seen. The amount of AKH, stored predominantly in the cell projections, differ widely among cells and individuals. The brain of adults and nymphs contains several small populations of intensely stained neurons. In last-instar and adult specimens, each half contains 10-12 "normal"-sized neurons in the protocerebrum (including the optic lobe) and deutocerebrum, and in addition 15-18 small reactive neurons. Their axons and numerous branchings traverse the neuropile of proto-, deuto-, and tritocerebrum, except for the pedunculate bodies and antennal lobes. Some of the axons run into the storage lobe of the CC; it is unknown if their content is released into the haemolymph. Other axons run into the ganglia of the stomatogastric nervous system and into the circumoesophageal connectives. The suboesophageal ganglion also contains 8 immunoreactive neurons. It is unknown to which extent the immunoreactive substances in glandular and nervous tissue are chemically and physiologically related.

Immunocytochemistry of brain-reactive monoclonal antibodies in peripheral tissues

Among a total of 135 tissue-reactive monoclonal antibodies previously prepared, 81 were brain-selective and were classified into neuronal and non-neuronal categories. The neuronal antibodies were again subdivided into antineurofibrillar, antiperikaryonal-neurofibrillar, and antisynapse-associated groups. On the basis of morphologic, developmental, biochemical, and pathologic criteria, the antibodies in at least two of these groups were found to detect heterogeneous antigens (called "neurotypes") rather than different antigenic determinants in single antigens. On examining the distribution in peripheral organs of staining patterns of 11 antineuronal brain-reactive antibodies, we now confirm that these antibodies are, indeed, largely brain-specific. In general, non-neuronal elements in liver, lung, heart, thymus, intestine, adrenal, and spleen remained unstained. However, most of the antibodies stained peripheral neural elements. Occasional antibodies did stain selected, non-neuronal structures. Four out of five antineurofibrillar antibodies stained nerve fibers in adrenal medulla, intestine and thymus. All of three antiperikaryonal-neurofibrillar antibodies also stained nerve fibers in the adrenal medulla, but not in other organs. Two out of three antisynapse-associated antibodies stained what appear to be nerve contacts on adrenal medullary cells, but not on any other peripheral cells examined. The non-neuronal peripheral staining patterns were restricted to selective nuclear staining exhibited by two out of five antineurofibrillar antibodies and the staining of macrophage and selected cardiac muscle nuclei by two of three antisynapse-associated antibodies. However, one antineurofibrillar antibody also stained the cytoplasm of selected liver cells. Among non-neuronally reacting antibodies, two antibodies stained nuclei of all cells except neurons in brain as well as peripheral organs. An antibody staining the ciliary epithelium of choroid plexus also stained basal bodies of ciliated bronchial epithelium. The overall data suggest that the specificity of brain-reactive antibodies is high and that their cross-reactivity with epitopes in non-nervous tissue is rare. In these cases, the antibodies seem to provide specific reagents for these additional structures as well as for their specific brain antigens.

Isolation of molluscan opioid peptides

An acid extract of neural tissues of the mollusc, Mytilus edulis, was fractionated by high-pressure chromatography. Peak fractions with retention times of that of Met-enkephalin, Leu-enkephalin and Met-enkephalin-Arg-Phe were subjected to competitive displacement assays in the same neural tissues. The results showed that these fractions have the same binding activities as the authentic neuropeptides. The peptides from these fractions were purified by high-pressure liquid chromatography under isocractic conditions. Sequential amino acid analyses showed these peptides to have the same primary structures as Met-enkephalin, Leu-enkephalin and Met-enkephalin-Arg-Phe.

Sensory input to growth stimulating neuroendocrine cells of Lymnaea stagnalis

Several environmental factors influence the growth of the basommatophoran freshwater snail Lymnaea stagnalis. Growth is hormonally controlled by 4 cerebral clusters of ca 50-75 peptidergic, neuroendocrine Light Green Cells (LGC). The present light, transmission, and scanning electron microscopic study shows that the LGC are synaptically contacted by a tentacle sensory system (TSS). The TSS consists of 2 types of primary sensory neurone, viz. ca 150 S1-cells and ca 50-100 S2-cells. A S1-cell has a non-ciliated dendrite and an axon branch that synaptically contacts the soma of a S2-cell. A S2-cell has a branching, ciliated dendrite. Probably, S1- and S2-cells have different sensory modalities and can integrate sensory information by intersensory interaction. The S2-axons run through the tentacular nerves, the cerebral ganglia, and the intercerebral commissure. In each ganglion S2-axons branch and form synaptic contacts on the axons and somata of the LGC and on glial cells that surround the LGC. In an LGC-cluster, 1-3 LGC-somata are particularly strongly innervated. Probably, the TSS is involved in the environmental control of growth in L. stagnalis.

Localization of pancreatic polypeptide (PP)-like immunoreactivity in the central and visceral nervous systems of the cockroach Periplaneta

The central and visceral nervous systems of the cockroach Periplaneta americana were studied by means of the peroxidase-antiperoxidase immunocytochemical method, with the use of antibody to bovine pancreatic polypeptide (PP). PP-like immunoreactive neuron somata are most numerous in the brain; at least 6 pairs of cell groups occur in clearly defined regions. Three pairs of cells each are also present in the suboesophageal ganglion and the thoracic ganglia, one pair of single cell each in the first abdominal and the frontal ganglia, and 4 to 6 pairs of single cells in the terminal ganglion. No reactive cells were found in the retrocerebral complex and the second to the fifth abdominal ganglia. The axons containing PP-like immunoreactivity issue many branches that are distributed in the entire brain-retrocerebral complex, ventral cord, and visceral nervous system. PP-like immunoreactive material produced in the brain seems to be transported by three routes: protocerebrum to corpora cardiaca (-allata) through the nervi corporis cardiaci, tritocerebrum to visceral nervous system through frontal commissures, and to ventral cord through circumoesophageal connectives. A possible homology between the mammalian brain-GEP (gastro-entero-pancreatic) system and the brain-midgut system of this insect is discussed.

Immunocytochemical demonstration of peptidergic cells in the pond snail Lymnaea stagnalis with an antiserum to the molluscan cardioactive tetrapeptide FMRF-amide

With an antiserum to the molluscan cardioactive tetrapeptide FMRF-amide immunoreactive perikarya and nerve fibers were identified in the central and peripheral nervous system of the pond snail Lymnaea stagnalis. Their localization is described. The same antiserum yielded reactive product in particular cells of the epithelium of the alimentary tract. The use of two different fixatives, glutaraldehyde, and a mixture of glutaraldehyde, picric acid, and acetic acid (GPA) showed that certain nerve cells can be identified only in material fixed with either the one or the other of these two fixatives, a result which indicates that in Lymnaea more than one FMRF-amide-like substances may occur. "Positive" axon endings were found in the periphery of various nerves, i.e., in places where neurohormones are released into the blood. Other fibers were found to end, probably synaptically, on other neurons, on epithelial cells in the stomach, and between muscle cells in various parts of the body, e.g., in the heart. In these cases the FMRF-amide-like substance may function as a neurotransmitter or a neuromodulator.

Immunoreactive material resembling vertebrate neuropeptides in the corpus cardiacum and corpus allatum of the insect Leucophaea maderae

The presence and differential distribution of substances antigenically related to known vertebrate neuropeptides demonstrated within the corpus cardiacum of the insect Leucophaea are as follows: Of ten mammalian antisera tested, six yielded substantial immunoreactive deposits resembling oxytocin, somatostatin, Substance P, met-enkephalin, bombesin, and neurotensin, respectively. In the remaining four, the reaction was moderate (vasopressin, beta-endorphin) or marginal (LH-RF, calcitonin). With regard to their regional distribution, these biochemically distinct reaction products seem to fall into two groups: (1) Materials resembling oxytocin, vasopressin, met-enkephalin, beta-endorphin (and presumably also neurotensin and LH-RF) predominate in the central release area of the organ and are considered to be of extrinsic (cerebral) origin. (2) Substances localized primarily in areas rich in intrinsic glandular cells of the corpus cardiacum, and revealed by antisera raised against somatostatin, Substance P, and bombesin, are judge to be synthesized and stored within this organ. In peptidergic fibers entering ther adjacent corpora allata, thus far Substance P-, beta-endorphin-, and LH-RF-like immunoreactivities have been demonstrated. Some of these "new" neuropeptides may be contained in classical neurosecretory neurons, formerly identified by less specific methods, others must be assigned to additional peptidergic neurons heretofore unknown.

Interanimal transferability of taste aversion learning for 0.1% saccharin

Three experiments were performed to assess the interanimal transferability of conditioned taste aversion to 0.1% saccharin. Two experiments used an intracerebrospinal fluid (subdural) route for administering brain extracts and a third used an intraperitoneal (IP) route. As assessed by repeated measurements ANOVA, saccharin consumption was significantly lower during extinction of conditioned aversion for experimental recipients (ER) receiving extracts from aversively conditioned donors, than that of control recipients (CR), receiving extracts from unconditioned donors in one subdural experiment, F(21, 189) = 1.61, p less than 0.05. In the IP experiment the results were in the same direction, though not significant, F(34, 238) = 1.39, p less than 0.1. Results of the other subdural experiment are discussed. It is concluded that these experiments with the conditioned taste aversion paradigm have potential as a model for investigations of behavioral interanimal transfer (BIT) and for neuromolecular research aimed at identification of associated putative neurochemical(s) and the elaboration of their mechanism of action.

Evidence for peptide-mediated neurotransmission in a molluskan brain

The previous report (Snow, 1982) characterized the monosynaptic actions of an identified cerebral interneuron (C2) in the marine mollusk Tritonia. The C2 neurons produce four types of postsynaptic potentials in an identified pedal neuron (Pd5). A high-molecular-weight (approximately 1400 daltons by Sephadex G-15 gel filtration) compound, which could mimic the four postsynaptic responses in Pd5, was isolated from C2 somata. The C2 somata had the ultrastructural characteristics of peptide-secreting cells, including profuse rough endoplasmic reticulum and large (170 nm average diameter) dense secretory vesicles. These data are consistent with the hypothesis that the synaptic transmitter of C2 neurons is a peptide(s).

Immunoreactive neuropeptide systems in avian embryos (domestic mallard, domestic fowl, Japanese quail)

In embryos of the domestic mallard, domestic fowl, and Japanese quail vasotocin-, mesotocin-, luliberin (LHRH)-, met-enkephalin-, corticotropin-, and somatostatin-immunoreactive perikarya and fiber formations were visualized at different incubation stages by means of the PAP technique (Sternberger 1979). The most striking results were: (1) Vasotocin-, mesotocin-, and luliberin-immunoreactive systems display, up to the late embryonic period, morphological features most probably related to a neurohormonal function. (2) Met-enkephalin immunoreactivity appears very late during embryonic life; it is restricted to fiber networks and not found in perikarya. (3) Corticotropin immunoreactivity is observed in the tuberal region temporarily at the end of the second and the beginning of the last third of the incubation period. (4) Somatostatin-immunoreactive material is present (i) at the end of the first third of incubation, in association with the olfactory system; (ii) during the same period, adjacent to thin-layered portions of the roof of the brain; (iii) shortly thereafter, in cells of both pancreatic primordia and thyroid gland; and (iv) onward from the middle of the incubation period, in a mesencephalic cell group. The striking difference, in the early embryo, between the mature somatostatin plays a role in the development of the brain, as well as the pancreas, and the thyroid gland.

Gastro-intestinal and neurohormonal peptides in the alimentary tract and cerebral complex of Ciona intestinalis (Ascidiaceae)

Polypeptide-hormone producing cells were localized in the alimentary tract and cerebral ganglion of Ciona intestinalis using cytochemical, immunocytochemical and electron-microscopical methods. Antisera to the following peptides of vertebrate type were employed: bombesin, human prolactin (hPRL), bovine pancreatic polypeptide (PP), porcine secretin, motilin, vasoactive intestinal polypeptide (VIP), beta-endorphin, leu-enkephalin, met-enkephalin, neurotensin, 5-hydroxytryptamin (5-HT), cholecystokinin (CCK), human growth (GH), ACTH, corticotropin-like intermediate lobe peptide (CLIP) and gastric inhibitory peptide (GIP). Immunoreactive cells were found both in the alimentary tract epithelium and in the cerebral ganglion for bombesin, PP, substance P, somatostatin, secretin and neurotensin. Additionally, in the cerebral ganglion only, there were cells immunoreactive for beta-endorphin, VIP, motilin and human prolactin. 5-HT positive cells, however, were restricted to the alimentary tract. No immunoreactivity was obtained either in the cerebral ganglion or in the alimentary tract with antibodies to leu-enkephalin, met-enkephalin, CCK, growth hormone, ACTH, CLIP and GIP. Prolactin-immunoreactive and pancreatic polypeptide-immunoreactive cells were argyrophilic with the Grimelius' stain and were found in neighbouring positions in the cerebral ganglion. At the ultrastructural level five differently granulated cell types were distinguished in the cerebral ganglion. Granules were present in the perikarya as well as in axons. The possible functions of the peptides as neurohormones, neuroregulators and neuromodulators are discussed.

Neurotypy: regional individuality in rat brain detected by immunocytochemistry with monoclonal antibodies

Hybridomas from spleen cell fusions of six BALB/c mice immunized with hypothalamus were analyzed by immunocytochemistry for antibodies reactive with paraffin sections of fixed rat brain. In a total of 135 antibody producers, 60% were brain specific. Among these, 54% reacted with glial elements, pituitary cells, or basal lamina of intracerebral capillaries, with little variation among individual hybridomas in each of these groups. Forty-six percent of brain-specific antibodies reacted with neuronal structures, localizing on nerve fibers, neurofibrils, or perikarya. Neuron-specific hybridomas could be classified into groups that localized in anatomically defineable overall patterns. Within these patterns individual hybridomas exhibited extensive qualitative localization diversity ("neurotypy"). Conceivably, the genetic message for a common "proantigen" within an overall pattern may be slightly modified during differentiation of a neuron, thus leading to minor variability in antigenic expression. During antibody formation, similar minor changes occur in the differentiation of the genetic message for the antibody variable region. Apparently, minor changes in the antibody combining site among groups of hybridomas is reflected in the detectability of minor neurotypic changes among differentiated neuronal proantigens. If neurotypy proves to be the result of single-base substitutions or of variability in alignment of peptide-coding exons, the Scharrer concept of the fundamental significance of neurosecretion could also become applicable to neuronal specialization.

High affinity binding of an enkephalin analog in the cerebral ganglion of the insect Leucophaea maderae (Blattaria)

Specific high affinity binding sites for a synthetic enkephalin analog, DALA (D-Ala2-Met-enkephalinamide) were demonstrated in the cerebral ganglia of adults and nymphs of the insect Leucophaea. In both sexes binding was monophasic and saturable with respect to the concentration of the radioligand used. The same amount of brain tissue from adult females had 30% more binding sites for DALA than that from adult males. By contrast, no sex-related difference in binding-site density per mg protein was observed in the brains from immature (nymphal) specimens. The results strongly suggest the presence, in this invertebrate, of opiate receptors that appear to be confined to certain areas of the nervous tissue.

A new hypothalamic substance, and not luteinizing hormone-releasing hormone, is detected immunocytochemically by antibody to luteinizing hormone-releasing hormone

Adjacent paraffin sections of rat hypothalami fixed in Bouin's fluid were treated either with buffer or with luteinizing hormone-releasing hormone (LHRH) before immunocytochemical staining with anti-LHRH. Upon buffer pretreatment, pituitary gonadotrophs were unstained and hypothalamic fibers were stained. Upon LHRH pretreatment, pituitary gonadotrophs were stained (receptor reaction) and hypothalamic fibers were unstained. Extension of washes and use of series of neutralizing antisera between LHRH application and immunocytochemical staining, as well as the absence of inhibiting concentrations of LHRH in the later washes and neutralizing antisera removed from the sections, excluded the possibility that the disappearance of visualization of hypothalamic fibers was due to blockage of anti-LHRH in immunocytochemical staining. The results suggested that LHRH removed from the sections an immunocytochemically stainable but as yet unknown analog of LHRH and replaced it with LHRH, which in turn became lost during subsequent immunocytochemical processing. This idea was confirmed by the isolation by high-pressure liquid chromatography of a peak, distinct from LHRH, upon treatment of hypothalami with LHRH. It is suggested that the new substance may be carrier-held and that this substance, rather than LHRH, is normally detected by immunocytochemistry with anti-LHRH. Added LHRH binds not only to high-affinity pituitary receptors but also to low-affinity hypothalamic carriers.

Immunocytochemical identification of neural elements in the central nervous systems of a snail, some insects, a fish, and a mammal with an antiserum to the molluscan cardio-excitatory tetrapeptide FMRF-amide

With an antiserum to the molluscan cardio-excitatory tetrapeptide FMRF-amide neurons and/or nerve fibers were immunocytochemically identified in the central nervous systems of a snail (Lymnaea stagnalis), some insects (Leptinotarsa decemlineata, Periplaneta americana, Locusta migratoria, Pieris brassicae), a fish (Poecilia latipinna) and a mammal (mouse). The fact that immunoreactive material was observed in neurohaemal organs (corpora cardiaca of the insects) as well as in axon terminals ending on other neurons, seems to indicate that this peptide can function as a neurohormone and/or as a neurotransmitter. The results sustain the hypothesis that biologically active peptides have a wide distribution in the animal kingdom.

Localization of somatostatin-, substance P- and calcitonin-like immunoreactivity in the neural ganglion of Ciona intestinalis L. (Ascidiaceae)

Indirect immunofluorescence studies using antisera to synthetic somatostatin, human calcitonin and substance P indicate, in the neural complex of the sea-squirt, Ciona intestinalis L., that these polypeptides are present in large perikarya situated at the periphery of the cerebral ganglion as well as in some smaller perikarya in the medulla. In the medullary and transitional zone, there are nerve fibres that cross-react positively with anti-calcitonin and anti-substance P.

Secretion of axonally transported neural peptides from the nervous system of Aplysia

The possibility that proteins reaching the abdominal ganglion of Aplysia by axonal transport from the circumesophageal ganglia might be subject to secretion in that structure was examined. Transported labeled protein was found to be released from the abdominal ganglion; such release was enhanced by exposure to a high K+ medium and by electrical stimulation of the transporting axons. Stimulation of release was inhibited by lowering the Ca2+/Mg2+ ratio of the medium. The released material is predominantly of 1--2000 daltons in molecular weight and appears to have been derived from a group of transported peptides of about the same size. The possibility is raised that these data may reflect the existence of a peptidergic second-order neurosecretory pathway in this nervous system.

Pituitary hormones in brain: where, how, and why?

Peptide and protein hormones usually considered as being of pituitary origin have been detected within the central nervous system by means of radioimmunoassay, bioassay, and immunocytochemical techniques. Intracerebral administration of some of these hormones or fragments thereof elicit behavioral responses, suggesting that they may have a physiological role similar to that described for other peptidergic neurotransmitter or neuromodulator substances. Evidence available for some of these hormones indicates that they are synthesized within the central nervous system and that their regulation may differ from that of their pituitary counterparts.