Evidence for a high density of secretoneurin-like immunoreactivity in the extended amygdala of the rat

Granin-derived peptides

The granin family comprises altogether 7 different proteins originating from the diffuse neuroendocrine system and elements of the central and peripheral nervous systems. The family is dominated by three uniquely acidic members, namely chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). Since the late 1980s it has become evident that these proteins are proteolytically processed, intragranularly and/or extracellularly into a range of biologically active peptides; a number of them with regulatory properties of physiological and/or pathophysiological significance. The aim of this comprehensive overview is to provide an up-to-date insight into the distribution and properties of the well established granin-derived peptides and their putative roles in homeostatic regulations. Hence, focus is directed to peptides derived from the three main granins, e.g. to the chromogranin A derived vasostatins, betagranins, pancreastatin and catestatins, the chromogranin B-derived secretolytin and the secretogranin II-derived secretoneurin (SN). In addition, the distribution and properties of the chromogranin A-derived peptides prochromacin, chromofungin, WE14, parastatin, GE-25 and serpinins, the CgB-peptide PE-11 and the SgII-peptides EM66 and manserin will also be commented on. Finally, the opposing effects of the CgA-derived vasostatin-I and catestatin and the SgII-derived peptide SN on the integrity of the vasculature, myocardial contractility, angiogenesis in wound healing, inflammatory conditions and tumors will be discussed.

Compartmentation of alpha 1 and alpha 2 GABA(A) receptor subunits within rat extended amygdala: implications for benzodiazepine action

The extended amygdala, a morphological and functional entity within the basal forebrain, is a neuronal substrate for emotional states like fear and anxiety. Anxiety disorders are commonly treated by benzodiazepines that mediate their action via GABA(A) receptors. The binding properties and action of benzodiazepines depend on the alpha-subunit profile of the hetero-pentameric receptors: whereas the alpha1 subunit is associated with benzodiazepine type I pharmacology and reportedly mediates sedative as well as amnesic actions of benzodiazepines, the alpha2 subunit confers benzodiazepine type II pharmacology and mediates the anxiolytic actions of benzodiazepines. We determined the localization of alpha1 and alpha2 subunits within the extended amygdala, identified by secretoneurin immunostaining, to define the morphological substrates for the diverse benzodiazepine actions. A moderate expression of the alpha1 subunit could be detected in compartments of the medial subdivision and a strong expression of the alpha2 subunit throughout the central subdivision. It is concluded that the alpha1 and alpha2 subunits are differentially expressed within the extended amygdala, indicating that this structure is compartmentalized with respect to function and benzodiazepine action.

Lack of a distinctive behavioural effect of chromogranin-derived peptides in rodents

Chromogranins are neuropeptide precursors stored in large dense core vesicles in which they are processed to smaller peptides. Although these peptides are widespread in the CNS, it is still unknown if they are behaviourally active. For example, even though secretoneurin, a 33-amino acid peptide derived from secretogranin II, was shown to induce release of dopamine from rat striatal neurons, work on the functional significance of this result is still missing. In order to investigate the behavioural effects of chromogranin-derived peptides, we studied the total locomotor activity and rearing behaviour of male albino Sprague-Dawley rats in the open field experimental paradigm. Measurements were performed every 5 min during half an hour before and 2 h after an intracerebroventricular injection of GE-19, GAIPIRRH, secretoneurin or vehicle. None of the tested chromogranin-derived peptides (at a concentration of 20 microM) affected locomotion and rearing behaviour. However, the administration of secretoneurin and GAIPIRRH increased the thigmotaxis, suggesting a possible anxiolytic action. In male Swiss albino mice, which were tested in the black-and-white box paradigm, only GE-19 produced sedation at a dose of 0.72 nmol in 41% of the mice. Overall, there is only little evidence that any of the examined chromogranin-derived peptides produces a behaviourally significant effect, even when given intracerebroventricularly.

Differential regulation of chromogranin A, chromogranin B and secretogranin II in rat brain by phencyclidine treatment

Chromogranin A, chromogranin B and secretogranin II belong to the chromogranin family which consists of large protein molecules that are found in large dense core vesicles. Chromogranins are endoproteolytically processed to smaller peptides. This study was designed to elucidate the regulation of chromgranin expression by acute and subchronic phencyclidine administration. The behavioral syndrome produced by phencyclidine represents a pharmacological model for some aspects of schizophrenia [Jentsch and Roth (1999) Neuropsychopharmacology 20, 201-225]. Tissue concentrations of chromogranins were measured with specific radioimmunoassays. Alterations in secretogranin II gene expression were investigated by in situ hybridization. A single dose of phencyclidine (10mg/kg) led to a transient decrease in secretoneurin tissue levels in the prefrontal cortex after 4h followed by an increase in secretoneurin tissue levels after 12h. Repeated phencyclidine treatment (10mg/kg/day) for five days resulted in elevated secretoneurin levels in cortical areas whereas chromogranin A and chromogranin B tissue levels were unchanged. After the same treatment, a significant increase in the number of secretoneurin containing neurons was found in cortical layers II-III, and V-VI as revealed by immunocytochemistry. The increases in secretoneurin levels were paralleled by an increased number of secretogranin II messenger RNA containing neurons as well as by an increased expression of secretogranin II by individual neurons. The present study shows that secretoneurin II tissue concentration and secretogranin II messenger RNA expression is distinctly altered after acute and subchronic phencyclidine application. From these results we suggest that phencyclidine may induce synaptic alterations in specific brain areas and may contribute to a better understanding of synaptic dysfunction which may also occur in schizophrenia.

Secretoneurin: a functional neuropeptide in health and disease

Chromogranins belong to an evolutionarily conserved family of proteins that serve as neuropeptide pro-proteins, besides having other functions. The secretogranin-II-derived peptide secretoneurin is a 33-amino-acid polypeptide generated by proteolytic cleavage at paired dibasic sequences that exerts its effect by binding to specific receptors. Secretoneurin receptors have been kinetically and functionally characterized indicating that they are G-protein linked. Localization of secretoneurin and functional studies have helped to elucidate roles for secretoneurin, ranging from effects in the central nervous system to the modulation of the inflammatory response in the periphery. It has been shown that secretoneurin possesses biologic activities such as stimulation of dopamine release from striatal neurons and activation of monocyte migration, suggesting that the peptide may modulate both neurotransmission and inflammatory response. With an array of actions as diverse as that seen with other sensory neuropeptides, there is scope for numerous studies and therapeutic possibilities.

Secretoneurin-like immunoreactivity in rat sympathetic, enteric and sensory ganglia

Distribution of secretoneurin-like immunoreactivity (SN-LI) was studied in the rat sympathetic ganglia/adrenal gland, enteric and sensory ganglia by immunohistochemical methods. SN-LI nerve fibers formed basket-like terminals surrounding many of the postganglionic neurons of the superior cervical, stellate, paravertebral chain ganglia, coeliac/superior mesenteric and inferior mesenteric ganglia. Postganglionic neurons of the superior cervical and other sympathetic ganglia exhibited low-to-moderate levels of SN-LI. In all these sympathetic ganglia, clusters of small diameter (< 10 microm) cells, which may correspond to the small intensely fluorescent (SIF) cells, were found to be intensely labeled. Surgical sectioning or ligation of the cervical sympathetic trunk for 7-10 days resulted in a nearly total loss of SN-LI fibers in the superior cervical ganglia, whereas immunoreactivity in the postganglionic neurons and small diameter cells remained essentially unchanged. In the thoracolumbar and sacral segments of the spinal cord, SN-LI nerve fibers were detected in the superficial layers of the dorsal horn as well as in the intermediolateral cell column (ILp). Occasionally, SN-LI somata were noted in the ILp. SN-LI nerve fibers formed a delicate plexus underneath the capsule of the adrenal gland, some of which traversed the adrenal cortex and reached the adrenal medulla. While heavily invested with SN-LI nerve terminals, chromaffin cells seemed to express a low level of SN-LI. In the enteric plexus, varicose SN-LI nerve fibers and terminals formed a pericellular network around many myenteric and submucous ganglion cells; the ganglionic neurons were lightly to moderately labeled. A population of ganglion cells in the dorsal root, nodose and trigeminal ganglia exhibited moderate-to-strong SN-LI. The detection of SN-LI in nerve fibers and somata of various sympathetic ganglia, enteric plexus and adrenal medulla and in somata of the sensory ganglia implies an extensive involvement of this peptide in sympathetic, enteric and sensory signal processing.

Ontogenic development of secretogranin II and of its processing to secretoneurin in rat brain

The ontogenic development of secretogranin II was studied by immunochemistry and immunohistochemistry. Extracts of brains from various developmental stages were analyzed by a radioimmunoassay for secretoneurin, a peptide derived from secretogranin II. From gestational day 13 to adulthood the levels increased from 0.1 to 94 fmol/mg wet weight. Characterization of the immunoreactivity by molecular sieve chromatography revealed that throughout all developmental stages the proprotein secretogranin II was fully processed to the free peptide secretoneurin. In immunohistochemistry secretoneurin-IR was first detected at embryonic day 13. Between embryonic days 14 and 18 a strong increase in the number of secretoneurin immunopositive cells was observed in many brain areas, notably in the amygdala, hypothalamus, olfactory bulb and several brainstem nuclei. The pattern of staining during development is quite similar to that in the adult. The present paper demonstrates that secretoneurin immunoreactivity appears early in embryonic life. Processing of the proprotein secretogranin II starts when the protein is first synthesized apparently at about the same time when the prohormone convertase PC1 and PC2 can be demonstrated.

Neurochemical compartments in the human forebrain: evidence for a high density of secretoneurin-like immunoreactivity in the extended amygdala

Secretoneurin is a 33-amino acid neuropeptide produced by endoproteolytic processing from secretogranin II, which is a member of the chromogranin/ secretogranin family. In this immunocytochemical study we investigated the localization of secretoneurin-like immunoreactivity in the human substantia innominata in relation to the ventral striatopallidal system, the bed nucleus-amygdala complex and the basal nucleus of Meynert. A high density of secretoneurin immunostaining was found in the medial part of the nucleus accumbens. All subdivisions of the bed nucleus of the stria terminalis displayed a very prominent immunostaining for secretoneurin, whereas substance P and enkephalin showed a more restricted distribution. A high concentration of secretoneurin immunoreactivity was also observed in the central and medial amygdaloid nuclei. In the lateral bed nucleus of the stria terminalis and the sublenticular substantia innominata, the appearance of secretoneurin immunoreactivity was very similar to that of enkephalin-like immunoreactivity, exhibiting mostly peridendritic and perisomatic staining. The ventral pallidum and the inner pallidal segment displayed strong secretoneurin immunostaining. Secretoneurin did not label cholinergic neurons in the basal forebrain. This study demonstrates that secretoneurin-like immunoreactivity is prominent in the bed nucleus-amygdala complex, referred to as extended amygdala. The distribution of secretoneurin-like immunoreactivity in comparison with that of other neuroanatomical markers suggests that this forebrain system is a discret compartment in the human forebrain.

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.

Efferent connections of the caudal part of the globus pallidus in the rat

The efferent connections of the caudal pole of the globus pallidus (GP) were examined in the rat by employing the anterograde axonal transport of Phaseolus vulgaris leucoagglutinin (PHA-L), and the retrograde transport of fluorescent tracers combined with choline acetyltransferase (ChAT) or parvalbumin (PV) immunofluorescence histochemistry. Labeled fibers from the caudal GP distribute to the caudate-putamen, nucleus of the ansa lenticularis, reuniens, reticular thalamic nucleus (mainly its posterior extent), and along a thin strip of the zona incerta adjacent to the cerebral peduncle. The entopeduncular and subthalamic nuclei do not appear to receive input from the caudal GP. Descending fibers from the caudal GP course in the cerebral peduncle and project to posterior thalamic nuclei (the subparafascicular and suprageniculate nuclei, medial division of the medial geniculate nucleus, and posterior intralaminar nucleus/peripeduncular area) and to extensive brainstem territories, including the pars lateralis of the substantia nigra, lateral terminal nucleus of the accessory optic system, nucleus of the brachium of the inferior colliculus, nucleus sagulum, external cortical nucleus of the inferior colliculus, cuneiform nucleus, and periaqueductal gray. In cases with deposits of PHA-L in the ventral part of the caudal GP, labeled fibers in addition distribute to the lateral amygdaloid nucleus, amygdalostriatal transition area, cerebral cortex (mainly perirhinal, temporal, and somatosensory areas) and rostroventral part of the lateral hypothalamus. Following injections of fluorescent tracer centered in the lateral hypothalamus, posterior intralaminar nucleus, substantia nigra, pars lateralis, or lateral terminal nucleus, a substantial number of retrogradely labeled cells is observed in the caudal GP. None of these cells express ChAT immunoreactivity, but, except for the ones projecting to the lateral hypothalamus, a significant proportion is immunoreactive to PV. Our results indicate that caudal GP efferents differ from those of the rostral GP in that they project to extensive brainstem territories and appear to be less intimately related to intrinsic basal ganglia circuits. Moreover, our data suggest a possible participation of the caudal GP in feedback loops involving posterior cortical areas, posterior striatopallidal districts, and posterior thalamic nuclei. Taken as a whole, the projections of the caudal GP suggest a potential role of this pallidal district in visuomotor and auditory processes.

Rat brain: distribution of immunoreactivity of PE-11, a peptide derived from chromogranin B

An antiserum was raised against the peptide PE-11 whose sequence is present in the chromogranin B molecule. The antiserum reacts only with the free C-terminal end of this peptide. PE-11 immunoreactivity in brain was characterized by molecular size exclusion high performance liquid chromatography. Only the free peptide and a N-terminally elongated peptide were detected, indicating that proteolytic processing of chromagranin B in brain is quite extensive. In immunohistochemistry PE-11 immunoreactivity was found in varicosities, fibres and perikarya throughout the brain. Strong staining was detected in the shell sector of the nucleus accumbens, in the lateral septum, in subregions of the extended amygdala, in some areas of the hippocampus and of the hypothalamus, in the locus coeruleus, in the Purkinje cells of the cerebellum and in the dorsal horn of the spinal cord. Our results, which demonstrate significant processing of chromogranin B in brain and its widespread distribution, can be taken as an indication that chromogranin B represents a precursor of peptides with functional relevance for this organ.

Secretogranin II: molecular properties, regulation of biosynthesis and processing to the neuropeptide secretoneurin

Secretogranin II is an acidic secretory protein in large dense core vesicles of endocrine, neuroendocrine and neuronal tissues. It comprises, together with chromogranins A and B, the class of proteins collectively called chromogranins. In this review the physico-chemical properties, genomic organization, tissue distribution, synthesis regulation, ontogeny and physiological function of this protein are discussed. Secretogranin II gained interest recently for mainly three reasons: (1) secretogranin II is an excellent marker for the regulated secretory pathway due to its simple and specific metabolic labeling by inorganic sulfate; (2) secretogranin II occurs in a variety of neoplasms arising from endocrine and neuroendocrine cells and was shown to be a useful histological tumor marker for these cells; (3) secretogranin II is the precursor of the recently discovered neuropeptide secretoneurin which induces dopamine release in the striatum of the rat brain.