Biochemical mapping of cholecystokinin-, substance P-, [Met]enkephalin-, [Leu]enkephalin- and dynorphin A (1-8)-like immunoreactivities in the human cerebral cortex

The neuropeptide landscape of human prefrontal cortex

The neuropeptide system encompasses the most diverse family of neurotransmitters, but their expression, cellular localization, and functional role in the human brain have received limited attention. Here, we study human postmortem samples from prefrontal cortex (PFC), a key brain region, and employ RNA sequencing and RNAscope methods integrated with published single-cell data. Our aim is to characterize the distribution of peptides and their receptors in 17 PFC subregions and to explore their role in chemical signaling. The results suggest that the well-established anatomical and functional heterogeneity of human PFC is also reflected in the expression pattern of the neuropeptides. Our findings support ongoing efforts from academia and pharmaceutical companies to explore the potential of neuropeptide receptors as targets for drug development.

Human prefrontal cortex (hPFC) is a complex brain region involved in cognitive and emotional processes and several psychiatric disorders. Here, we present an overview of the distribution of the peptidergic systems in 17 subregions of hPFC and three reference cortices obtained by microdissection and based on RNA sequencing and RNAscope methods integrated with published single-cell transcriptomics data. We detected expression of 60 neuropeptides and 60 neuropeptide receptors in at least one of the hPFC subregions. The results reveal that the peptidergic landscape in PFC consists of closely located and functionally different subregions with unique peptide/transmitter–related profiles. Neuropeptide-rich PFC subregions were identified, encompassing regions from anterior cingulate cortex/orbitofrontal gyrus. Furthermore, marked differences in gene expression exist between different PFC regions (>5-fold; cocaine and amphetamine–regulated transcript peptide) as well as between PFC regions and reference regions, for example, for somatostatin and several receptors. We suggest that the present approach allows definition of, still hypothetical, microcircuits exemplified by glutamatergic neurons expressing a peptide cotransmitter either as an agonist (hypocretin/orexin) or antagonist (galanin). Specific neuropeptide receptors have been identified as possible targets for neuronal afferents and, interestingly, peripheral blood-borne peptide hormones (leptin, adiponectin, gastric inhibitory peptide, glucagon-like peptides, and peptide YY). Together with other recent publications, our results support the view that neuropeptide systems may play an important role in hPFC and underpin the concept that neuropeptide signaling helps stabilize circuit connectivity and fine-tune/modulate PFC functions executed during health and disease.

Opioid signal transduction regulates the dendritic morphology of somatostatin and parvalbumin interneurons in the medial prefrontal cortex

The endogenous opioid system is of great importance to normal brain functions. Opiate acts on GABAergic cells in both the ventral tegmental area and the nucleus accumbens to exert psychological effects. However, the effects of opioid signal transduction on the morphology of GABAergic interneurons (INs) of the medial prefrontal cortex (mPFC), a brain region critical for motivational and addictive behaviors, are unclear. By fluorescent dye injection and morphological reconstruction, we found that the total dendrite length and dendritic complexity of both parvalbumin (PV) INs and somatostatin (SST) INs in mPFC were significantly increased after chronic morphine administration, and such changes lasted 7 days after morphine abstinence. We then downregulated the endogenous μ-opioid and δ-opioid receptors (ORs) in the mPFC by adeno-associated virus-mediated shRNA expression. Results showed that downregulating either μ-OR or δ-OR decreased the total dendrite length and dendritic complexity of SST-INs, whereas downregulating neither μ-OR nor δ-OR affected the morphology of PV-INs. Furthermore, δ-OR but not μ-OR knockdown impaired the dendritic structure of SST-INs in the mice upon single morphine administration. Our findings indicate the differential roles of endogenous ORs in the dendritic remodeling of SST-INs and PV-INs in mPFC.

Neurotransmitters, peptides, and neural cell adhesion molecules in the cortices of normal elderly humans and Alzheimer patients: a comparison

Immunocytochemical techniques was used to compare the proportion of neurons expressing various neurotransmitters (tyrosine hydroxylase, choline acetyltransferase and gamma-aminobutyric acid), neuropeptides (Leu-enkephalin and substance P) and neural cell adhesion molecules (NCAM) in the hippocampus, frontal (area 10) and occipital (area 17) cortices of neurologically normal elderly humans to that of age-matched Alzheimer disease (AD) patients. There was no difference in the proportion of GABAergic and cholinergic cells between the normal and AD groups in all three brain regions studied. However, the catecholaminergic cells in the frontal cortex of the AD patients revealed a significant decrease. The catecholaminergic cells present in the cortex were both neurons and astrocytes, as revealed by a double immunostaining of tyrosine hydroxylase and glial fibrillary acid protein (GFAP). Furthermore, the difference in the proportion of cells expressing Substance P and Leu-enkephalin was minimal between the two groups studied. Although there was little difference in the levels of NCAM in the occipital cortex and hippocampus of the two groups, there were significantly fewer positive NCAM neurons in the frontal cortex of AD than normal aging individuals.

Dynorphin A(1-8): stability and implications for in vitro opioid activity

The opioid binding profile and in vitro activity of the endogenous opioid peptide dynorphin A(1-8) have been studied. At opioid receptors in guinea-pig brain dynorphin A(1-8) was nonselective, although with some preference for the delta receptor (Ki 4.6 nM) over µ (Ki 18 nM) and kappa (Ki 40 nM) receptors. However, a high degree of metabolism was observed, with less than 10% of added dynorphin A(1-8) remaining at the end of the binding assay. In the presence of peptidase inhibitors to prevent breakdown of the N- and C-termini and the Gly3-Phe4 bond the major metabolite was [Leu5]enkephalin (representing 49% recovered material). This was reduced by inclusion of an inhibitor of endopeptidase EC 3.4.24.15. In the presence of all the peptidase inhibitors the affinity for kappa receptors (Ki 0.5 nM) relative to µ and delta receptors increased, but no selectivity of binding was observed. This lack of selectivity was confirmed using membranes from C6 glioma cells expressing rat opioid receptors. The agonist effect of dynorphin A(1-8) in the mouse vas deferens (EC50 116 nM) and guinea-pig ileum (EC50 38 nM) was mediated through the kappa receptor as evidenced by the rightward shifts afforded by the kappa -selective antagonist norbinaltorphimine. In the presence of peptidase inhibition potency was improved 2-fold in the mouse vas deferens and 20-fold in the guinea-pig ileum, but this agonist activity was mediated through delta receptors in the vas deferens and µ receptors in the ileum, as a result of the formation and stabilization of [Leu5]enkephalin. The results confirm the absence of receptor selectivity of dynorphin A(1-8) in binding assays but show that its agonist effects, at least in vitro, are mediated exclusively through the kappa opioid receptor.Key words: dynorphin A(1-8), opioid receptors, peptide metabolism, mouse vas deferens, guinea-pig ileum.

Modulation of acetylcholine release in human cortical slices: possible implications for Alzheimer's disease

Superfused slices of human neocortex, prepared from surgically removed tissue (to gain access to subcortical tumors) and prelabelled with [3H]choline, were stimulated electrically to evoke action potential-induced, exocytotic [3H]acetylcholine release. For comparison, rat cortex slices were also used. [3H]ACh release decreased with the age of the patients and was modulated by muscarinic autoreceptors and by 5-hydroxytryptamine1F, neurokinin1, and kappa-opioid receptors located on cholinergic terminals. In addition, 5-hydroxytryptamine2 and delta-opioid receptors located on interneurons were also involved in the modulation of [3H]ACh release. The present findings might help to explain pathological conditions in Alzheimer's disease.

Cellular distribution of the mRNA for the kappa-opioid receptor in the human neocortex: a non-isotopic in situ hybridization study

Opioid receptors (OR) provide primary interaction sites of the human brain with opiates. Presently kappa-OR mRNA expression was studied in different cortical areas (A4, A10, A17) by in situ hybridization using digoxigenin-labeled oligonucleotides and an alkaline phosphatase-mediated color reaction. kappa-OR mRNA was expressed mainly in layers II/III and V pyramidal and layer VI multiform neurons. A4 giant pyramidal and A17 giant stellate neurons stood out labeled. These findings fit in with our data on kappa-OR protein distribution. Combined cellular assessment of protein and mRNA will enable the study kappa-OR expression under physiological and pathological conditions.

A study of tachykinin-immunoreactivity in the cat visual cortex

The localization of tachykinin-immunoreactivity in the cat visual cortex (area 17) was investigated using immunohistochemical methods. Strong laminar specificity was observed, with immunoreactivity highest in layer V, followed by layers I, VI, II and III, and the lowest density in layer IV. Most of the immunoreactive product was localized in neuronal processes. A few immunopositive cell bodies were also present. The immunopositive neurons were non-pyramidal, multipolar, or bipolar in shape, and mostly found in layer V. There were particularly dense immunopositive fibers and varicosities around somata in layer V. These may represent tachykinin-containing presynaptic terminals (boutons). The results provide anatomical evidence that tachykinins may primarily affect layer V neurons in the cat visual cortex.

Evidence for mnemotropic action of cholecystokinin fragments Boc-CCK-4 and CCK-8S

Memory-modulating and reinforcing effects of the cholecystokinin (CCK) fragments, CCK-8S and Boc-CCK-4, after systemic application in rats were investigated. Habituation to the novelty of environmental stimuli was used to test for mnemonic effects using two different tasks (rearing behavior in an open field; head-dips in a hole-board). Immediate posttrial administration of CCK-8S and Boc-CCK-4 resulted in a reduction of rearing and head-dip behavior during testing, indicative of enhanced habituation and, thus, facilitation of memory. In contrast, administration of CCK-8S and Boc-CCK-4 with a delay of 2.5 or 5 h after training or pretrial injection of CCK-8S did not enhance habituation. No evidence for reinforcing or aversive properties of CCK-8S and Boc-CCK-4 was observed in a conditioned place preference task. In summary, the results indicate memory-enhancing effects of peripherally, posttrial-administered CCK-8S and Boc-CCK-4.

Postnatal development of the cholecystokinin innervation of monkey prefrontal cortex

Although the structure and function of primate prefrontal cortex undergo substantial modifications during postnatal development, relatively little is known about the maturation of neurotransmitter systems in these cortical regions. In the primate brain, cholecystokinin is present in the greatest concentrations in prefrontal regions. Thus, in this study, we used immunohistochemical techniques to investigate the postnatal development of the cholecystokinin innervation of monkey prefrontal cortex. In animals aged 4 days through adult, cholecystokinin immunoreactivity was present in nonpyramidal neurons that appeared to represent at least two distinct cell types. The most common type was a vertically oval bitufted neuron, located in layers II-superficial III, which typically had a radially descending axon that gave rise to short collaterals in layer IV. Another frequently observed cell type was a larger multipolar neuron located in the superficial half of layer III. The axon of these neurons branched locally in the vicinity of the cell body. The greatest density of cholecystokinin-containing neurons and processes was present in monkeys less than 1 month of age. The density of immunoreactive structures in every prefrontal region then progressively declined with increasing age, with the most marked changes occurring during the first postnatal year. As a result, the density of labeled neurons in adult monkeys was less than one-third of that in neonatal monkeys. However, labeled structures were significantly more dense in some ventromedial and orbital regions than in dorsal regions of the prefrontal cortex in neonatal, but not in older animals. In all animals, cholecystokinin-containing neurons were present in highest density in layers II-superficial III, and labeled terminal fields were observed in layers II, IV, and VI. In animals less than 1 month of age, fascicles of radial fibers traversed through layers III and V, whereas in animals 1 to 3 months of age, individual radial fibers rather than fiber bundles were present in layers III and V. In addition, immunoreactive pericellular arrays, which appeared to surround unlabeled nonpyramidal cells, were present in layers V and VI and the subcortical white matter in the youngest monkeys. Although many aspects of the cholecystokinin innervation of monkey prefrontal cortex remain constant during postnatal life, the distinct developmental changes in the cholecystokinin innervation of these regions suggest that it may play an important role in the maturation of the cortical circuitry that mediates the acquisition of certain cognitive abilities.

Immunocytochemical localization of enkephalin in the cat visual cortex

The localization of enkephalin-immunoreactivity in the cat visual cortex (area 17) was analyzed by using immunohistochemical methods with a monoclonal antibody directed against enkephalin. The majority of the immunoreactive product was localized in neuronal processes. The density of immunopositive fibers was greatest in layer VI, with moderate staining in layers I, II, III and V, and the least dense staining in layer IV. Layer IVab neurons showed a striking concentration of immunopositive puncta around their cell bodies. Immunopositive neurons were scarcely present in the visual cortex. They were found in all cortical layers, but mostly in layer VI. The immunopositive neurons were non-pyramidal, mostly multipolar in shape and occasionally bipolar. The results provide anatomical evidence that enkephalin may have modulatory effects on visual cortical neurons.

Expression of prodynorphin-derived peptides and mRNA in guinea-pig cortex

The distributions and extent of processing of four prodynorphin-derived peptides (dynorphin A (1-17), dynorphin A (1-8), dynorphin B, and alpha-neoendorphin) were determined in ten regions of the cortex as well as in the striatum of the guinea-pig. There were significant differences between concentrations of these peptides in most cortical regions, with alpha-neoendorphin being several times more abundant than the other peptides, and dynorphin A (1-17) being present in the least amount. There were significant between-region differences in concentration for each peptide, although most regions had concentrations similar to those seen in the striatum. Concentrations of each peptide tended to be higher in piriform, entorhinal, motor, and auditory cortex than in other cortical regions. The extent of processing of prodynorphin varied across cortical regions as well, primarily due to the extent of processing to alpha-neoendorphin. Prodynorphin mRNA levels were not significantly different between cortical regions or from the amount observed in the striatum. Although specific regional variation exists, it appears that in general prodynorphin is expressed and processed in a similar manner in the cortex as in the striatum.

Cholecystokinin- and dopamine-containing mesencephalic neurons provide distinct projections to monkey prefrontal cortex

Retrograde transport and immunohistochemical techniques were utilized to determine if cholecystokinin (CCK)-containing neurons of the primate ventral mesencephalon project to prefrontal cortex, and to examine what relation the CCK innervation of prefrontal cortex bears to the dopaminergic projection to this region. Following injections of Fast blue into monkey prefrontal cortex, retrogradely labeled, CCK-positive neurons were observed predominantly in rostromedial portions of the ventral mesencephalon; these CCK-containing projection neurons were not immunoreactive for tyrosine hydroxylase. Furthermore, dual-labeling studies in the prefrontal cortex revealed that CCK and tyrosine hydroxylase were present in separate populations of axons. These results demonstrate that the CCK innervation of monkey prefrontal cortex arises from both intrinsic and extrinsic sources; in contrast to the rat, the extrinsic CCK innervation of monkey prefrontal cortex is distinct from the dopaminergic mesocortical projection.

Morphology and distribution of neuropeptide-containing neurons in human cerebral cortex

Biopsies of human cerebral cortex were fixed by immersion and immunostained for the detection of neuropeptides in neuronal cell bodies and axons. Four neuropeptides (neuropeptide Y, somatostatin, , substance P and cholecystokinin) were visualized in a series of adjacent sections. All populations of immunoreactive neurons had a morphology characteristic of interneurons, with variations in dendritic arborizations and laminar distribution. The cholecystokinin-immunoreactive neurons were most numerous in the supragranular layers, whereas neurons containing the other three peptides occurred mainly in infragranular layers, or even in neurons populating the subcortical white matter. Quantitatively, each population of neuropeptide-containing neurons accounted for 1.4-2.5% of the total neuronal population. The distribution of these neurons varied slightly between cytoarchitectonic divisions, with substance P- and somatostatin-immunoreactive neurons dominating in the temporal lobe and cholecystokinin-immunoreactive neurons in the frontal lobe. Neuropeptide Y-immunoreactive neurons dominated in the gray matter of the frontal half of the hemisphere and in the subcortical white matter of the caudal half of the hemisphere. Furthermore, co-existence of neuropeptide Y or substance P immunoreactivity within somatostatin-immunoreactive neurons could be demonstrated using double labeling immunofluorescence techniques. The axonal plexuses immunoreactive for neuropeptide Y, somatostatin, or substance P were distributed in all layers, with a strong predominance of horizontally oriented fibers in layer I, a moderate plexus of randomly oriented fibers in the supra- and infragranular layers, and a slightly weaker innervation of layer IV. Immunoreactive axons formed, in addition, complex terminal arbors, mostly in older subjects, suggesting that they resulted from an as yet undefined aging process. The present study underlines several aspects of the organization of the neuropeptide-containing neurons of the human cerebral cortex, which are of particular interest in the light of the involvement of these neurons in several neurodegenerative diseases.

Ontogeny of cholecystokinin-immunoreactive structures in the primate cerebral neocortex

Distribution of cholecystokinin (CCK)-immunoreactive structures was studied in various neocortical areas of macaque monkeys during prenatal and postnatal development. The largest number of CCK-immunoreactive cells was observed at embryonic day 140, and subsequently they decreased in number until postnatal day 60. A few cells which were presumably degenerated neurons were observed during postnatal development. A higher density of CCK-immunoreactive cells was observed in the supragranular layers (layers II and III) than in the infragranular layers (layers V and VI). The number of CCK-immunoreactive cells was larger and changed more conspicuously in the association areas than in the other areas during development. In contrast, in the occipital area, the number of such cells was small and changed only a little. These findings suggest that CCK may be involved in the development and special function of each neocortical areas of the primate.

Distribution of cholecystokinin mRNA and peptides in the human brain

Expression of preprocholecystokinin mRNA was studied in regions of post mortem human brain using RNA blot analysis (Northern blot) and in situ hybridization. Northern blot analysis using a cDNA probe showed high levels of an approximately 0.8 kb preprocholecystokinin mRNA in all regions of neocortex examined. Lower levels of preprocholecystokinin mRNA were detected in amygdaloid body and thalamus. In situ hybridization analysis using the same cDNA probe revealed numerous weakly labelled neurons in different areas of human neocortex and less numerous neurons in hippocampus and amygdaloid body. High-performance liquid-chromatography and gel-chromatography combined with radioimmunoassay of cholecystokinin-like immunoreactivity from human cerebral cortex and caudate nucleus revealed two major forms, one coeluting with sulphated cholecystokinin-8 and the other coeluting with sulphated cholecystokinin-58. Two minor components coeluting with cholecystokinin-4 and cholecystokinin-5 were also detected. The finding of cholecystokinin-like immunoreactivity corresponding to cholecystokinin-8 and cholecystokinin-58 in caudate nucleus where no preprocholecystokinin mRNA was found, indicates the presence of these peptides in afferent nerve terminals.

Cholecystokinin innervation of monkey prefrontal cortex: an immunohistochemical study

Knowledge of the circuitry of chemically identified systems in primate prefrontal cortex is limited. Although cholecystokinin is very abundant in prefrontal cortex (Geola et al.: Journal of Clinical Endocrinology and Metabolism 53(2):270-275, 1981; Taquet et al.: Neuroscience 27(3):871-883, 1988), the organization of cholecystokinin-containing structures in primate prefrontal cortex has not been investigated. Using immunohistochemical and retrograde transport techniques, we characterized the cholecystokinin innervation of prefrontal cortex in macaque monkeys. The use of two antibodies directed against different portions of the cholecystokinin molecule revealed that distinct forms of the molecule were differentially localized in the same cortical neurons. These small, nonpyramidal cholecystokinin-positive neurons had a variety of somal morphologies and the density of labeled cells did not differ among cytoarchitectonic regions. Labeled neurons had a distinctive laminar distribution with the greatest density of cells present in layers II-superficial III. Labeled fibers also had a distinctive laminar pattern of distribution that differed from that of the immunoreactive neurons. In granular prefrontal cortex, terminal fields were evident in layers II, IV, and VI, with the greatest density in layer VI. Agranular area 24 exhibited a bilaminar pattern of immunoreactivity with a band in layer II and a very dense terminal field in layers V-VI. A high density of cholecystokinin-binding sites has been found in layers III-IV of prefrontal cortex and other association areas in the monkey; this finding has been attributed to possible cholecystokinin-containing afferents from the thalamus (Kritzer et al.: Journal of Comparative Neurology 263:418-435, 1987). The mediodorsal nucleus of the thalamus is known to be a source of afferents which terminate in layer IV of prefrontal cortex. However, combined retrograde transport and immunohistochemical techniques failed to reveal the presence of cholecystokinin-positive neurons in the mediodorsal nucleus of the thalamus that project to prefrontal cortex. These findings, and other observations, suggest that the terminal field in layer IV is formed by descending axons that arise from cholecystokinin-containing neurons in layers II and superficial III. This study demonstrates that the cholecystokinin innervation of prefrontal cortex has a laminar specific organization that is preserved across cytoarchitectonic regions. This distribution of immunoreactive structures suggests a distinctive role of cholecystokinin in cortical circuitry that is common to every region of prefrontal cortex.

Regional distribution of the messenger RNA coding for the neuropeptide cholecystokinin in the human brain examined by in situ hybridization

The regional localization of mRNA coding for the neuropeptide cholecystokinin (CCK) has been studied in the human brain by in situ hybridization using a 32P-labelled synthetic oligonucleotide. Autoradiograms were quantified using computer-assisted microdensitometry. Positive hybridizing cells were seen in the neocortex, the claustrum, the hippocampus and the amygdala with the highest densities observed in the claustrum, some cortical layers and the CA2 and CA3 regions of the hippocampus. No significant hybridization signal was observed in the substantia nigra, caudate nucleus, putamen, globus pallidus, nucleus accumbens, thalamus, hypothalamus, medulla oblongata and cerebellum. The topographic distribution of neurons expressing CCK mRNA correlates well with that previously reported by immunocytochemistry or radioimmunoassay in brain areas such as the neocortex, the amygdala and the hippocampus. However, some discrepancies were also found, particularly in the basal ganglia, the midbrain, the thalamus and the hypothalamus. These results show that in situ hybridization with oligonucleotide probes together with a semiquantitative analysis can be used to map the distribution of cells expressing CCK mRNA in human postmortem materials.