A new hypothalamic polypeptide is a regulator of myelopoiesis

Restoration of desmosomal junction protein expression and inhibition of H3K9-specific histone demethylase activity by cytostatic proline-rich polypeptide-1 leads to suppression of tumorigenic potential in human chondrosarcoma cells

Disruption of cell-cell junctions and the concomitant loss of polarity, downregulation of tumor-suppressive adherens junctions and desmosomes represent hallmark phenotypes for several different cancer cells. Moreover, a variety of evidence supports the argument that these two common phenotypes of cancer cells directly contribute to tumorigenesis. In this study, we aimed to determine the status of intercellular junction proteins expression in JJ012 human malignant chondrosarcoma cells and investigate the effect of the antitumorigenic cytokine, proline-rich polypeptide-1 (PRP-1) on their expression. The cell junction pathway array data indicated downregulation of desmosomal proteins, such as desmoglein (1,428-fold), desmoplakin (620-fold) and plakoglobin (442-fold). The tight junction proteins claudin 11 and E-cadherin were also downregulated (399- and 52-fold, respectively). Among the upregulated proteins were the characteristic for tumors gap junction β-5 protein (connexin 31.1) and the pro-inflammatory pathway protein intercellular adhesion molecule (upregulated 129- and 43-fold, respectively). We demonstrated that PRP-1 restored the expression of the abovementioned downregulated in chondrosarcoma desmosomal proteins. PRP-1 inhibited H3K9-specific histone demethylase activity in chondrosarcoma cells in a dose-dependent manner (0.5 µg/ml PRP, 63%; 1 µg/ml PRP, 74%; and 10 µg/ml PRP, 91% inhibition). Members of the H3K9 family were shown to transcriptionally repress tumor suppressor genes and contribute to cancer progression. Our experimental data indicated that PRP-1 restores tumor suppressor desmosomal protein expression in JJ012 human chondrosarcoma cells and inhibits H3K9 demethylase activity, contributing to the suppression of tumorigenic potential in chondrosarcoma cells.

Regulation of onco and tumor suppressor MiRNAs by mTORC1 inhibitor PRP-1 in human chondrosarcoma

Metastatic chondrosarcoma of mesenchymal origin is the second most common bone malignancy and does not respond either to chemotherapy or radiation; therefore, the search for new therapies is relevant and urgent. This study aimed to reveal the comparative analysis of miRNAs and their targets in human JJ012 chondrosarcoma cell line between control and experimental samples, treated with mTORC1 inhibitor, cytostatic antiproliferative proline-rich polypeptide (PRP-1). Examination of tumor-specific microRNA expression profiles has revealed widespread deregulation of these molecules in diverse cancers. It was reported that microRNAs can function as novel biomarkers for disease diagnostics and therapy, as well as a novel class of oncogenes and tumor suppressor genes. mTORC 1 inhibitor PRP-1 caused significant upregulation of tumor suppressors, such as miR20a, miR125b, and miR192; and downregulation of onco miRNAs, miR509-3p, miR589, miR490-3p, miR 550 in human chondrosarcoma JJ012 cell line.

The protective and immunomodulatory effects of hypothalamic proline-rich polypeptide galarmin against methicillin-resistant Staphylococcus aureus infection in mice

The present research summarizes the protective and immunomodulatory activity of hypothalamic proline-rich polypeptide galarmin against methicillin-resistant Staphylococcus aureus (MRSA). The protective effect of galarmin was shown on MRSA-infected animals' survival and weight loss recovery. The immunological impact of galarmin was evaluated in terms of immunocompetent cell recruitment, serum immunoglobulins, complement components C3 and C4, and pro- and anti-inflammatory cytokines (IL-6, IL-8, IL-10, IL-1b, TNFa, and KC) secretion. Galarmin efficiently protects mice against lethal MRSA infection (100% of survival vs. 0% in the untreated group) when intramuscularly injected 24 h before infection and during the 1-h post-infection period at a concentration of 1 μg per mouse, while its higher concentrations (5 and 10 μg) were protective when injected in parallel to the infection process. The protective effect of galarmin was not due to a direct effect on MRSA, but should be attributed to an action on the host response to infection. Galarmin significantly increased and modulated the levels of IL-6, IL-8, IL-1b, IL-10, and KC in both peritoneal lavages and blood, leukocyte and platelet counts, lymphocytes percentage, serum IgM and IgG, and complement C3 and C4 components secretion. The experimental results allow concluding that galarmin is a powerful immunomodulatory and protective agent for the in vivo prophylaxis and treatment of MRSA-induced infection.

Antitumorigenic effect of brain proline rich polypeptide-1 in human chondrosarcoma

Proline rich polypeptide (PRP-1) produced by neurosecretory cells of the hypothalamus is one of the fragments of neurophysin-vasopressin-associated glycoprotein. The primary structure of the neuropeptide PRP-1 isolated from neurosecretory granules of bovine neurohypophysis. We investigated PRP-1 action on chondrosarcoma, the second most common malignancy in bone, which primarily affects the cartilage cells. This deadly disease does not have any effective treatment. Earlier we demonstrated MYC oncogene inactivating effect by 1 lg/ml concentration brain PRP-1 In the present study we observed reduced viable sarcoma JJ012 cell numbers in comparison with control (89% growth inhibition) when treated with low concentrations of PRP-1 (0.5–1 lg/ml). Higher concentrations did not exhibit inhibitory effect. We assume that PRP-1 in low concentration impedes cell cycle progression. The fact that low concentrations of PRP-1 abolished Myc activity prompts to think that the antitumorigenic effect of PRP-1 in low concentrations is mediated through oncogene inactivation.

Cytostatic effect of novel mTOR inhibitor, PRP-1 (galarmin) in MDA 231 (ER-) breast carcinoma cell line. PRP-1 inhibits mesenchymal tumors

Activation of the PI3K-Akt-mTOR pathway is implicated both in the establishment of tumors and as well as a target for therapy in many types of solid malignancy, its blockade represents an opportunity to improve outcomes in patients with tumors that are associated with poor prognosis. Our experimental data indicates that proline-rich polypeptide-1 (PRP-1, galarmin) is immunomodulator cytokine, produced by hypothalamic neurosecretory cells and exerts its antiproliferative effect on the tumor cells of mesenchymal origin via inhibiting mTOR kinase activity and repressing cell cycle progression. The goal of these investigations was to elucidate the antiproliferative action of PRP-1 on the breast carcinoma cell line MDA 231 (ER-) and to compare PRP-1 action previously reported on other mesenchymal tumors. These experiments confirmed maximum inhibition of cell growth at 0.5 and 1 μg/ml PRP-1 (71% and 63%, respectively) and inhibition at 10 μg/ml of 44%. There was no inhibitory effect observed on luminal T47-D (ER+) cells. Videomicroscopy results demonstrated dividing cells in the cytokine-treated MDA 231 (ER-), suggesting that the cells were not in the state of dormancy. The flow cytometry experiments confirmed that PRP-1-treated cells were accumulated in S phase. No apoptosis, caspase activation, or senescence was detected after treatment with this cytokine. Experiments with mTOR with PRP-1 (10 μg/ml) indicated statistically significant 40% inhibition of mTOR kinase activity in immunoprecipitates of the MDA 231 (ER-) cell line. PRP-1 is a novel mTOR inhibitor with strong antiproliferative action in mesenchymal tumors mostly resistant to radiation and chemotherapy.

Cytostatic effect of the hypothalamic cytokine PRP-1 is mediated by mTOR and cMyc inhibition in high grade chondrosarcoma

This study aimed to further elucidate the molecular mechanisms of antiproliferative action of proline rich polypeptide 1 (PRP-1) cytokine, produced by neurosecretory cells of the hypothalamus to be considered as alternative adjuvant therapy for metastatic chondrosarcoma, which does not respond to chemotherapy or radiation and currently without any effective treatment. Rapid cell proliferation assay of human primary cultures from high grade chondrosarcoma patients biopsies and human chondrosarcoma JJ012 cell line indicated 50 and 80% inhibition in PRP-1 treated samples correspondingly. Videomicroscopy detected that despite the treatment there are still dividing cells, meaning that cells are not in the state of dormancy, rather PRP-1 repressed the cell cycle progression, exhibited cytostatic effect. The mammalian target of rapamycin (mTOR) is an intracellular serine/threonine protein kinase that has a crucial role in a nutrient sensitive signaling pathway that regulates cell growth. Experiments with mTOR pathway after PRP-1 (10 μg/ml) treatment indicated statistically significant 30% inhibition of mTOR activity and its 56% inhibition in immunoprecipitates with PRP-1 concentrations effective for cell proliferation inhibition. Treatment with PRP- caused inhibition of mTOR and downstream target cMyc oncogenic transcription factor sufficient to trigger the cytostatic effect in high grade, but not in low grade chondrosarcomas. The fact that lower concentrations than 10 μg/ml peptide with cytostatic effect did not inhibit mTOR, but inhibited cMyc prompted us to assume that PRP-1 binds to two different receptors facilitating the antiproliferative effect.

Proline-rich hypothalamic polypeptide has opposite effects on the proliferation of human normal bone marrow stromal cells and human giant-cell tumour stromal cells

In the present study we carried out experiments in vitro and in vivo and investigated the effect of proline-rich polypeptide (PRP) on the proliferation and effectiveness of colony formation of MMSCs in vitro. Various routes and doses of PRP administration to rats increased the number of MMSCs in bone marrow and spleen. Our research revealed opposite effects of PRP on the proliferation of bone marrow stromal cells obtained from normal humans and stromal cells isolated from a human giant-cell tumour.

Hypothalamic proline rich polypeptide regulates hematopoiesis

The AGAPEPAEPAQPGVY proline-rich polypeptide (PRP-1) was isolated from neurosecretory granules of the bovine neurohypophysis; it is produced by N. supraopticus and N. paraventricularis. It has been shown that PRP-1 has many potentially beneficial biological effects including immunoregulatory, hematopoietic, antimicrobial and anti-neurodegenerative properties. Here we demonstrated that PRP-1 administration influence on redistribution of monocytes, granulocytes and lymphocytes between bone marrow (BM) and peripheral blood and promotes the influx of granulocytes and monocytes/macrophages from BM into peripheral blood and accumulation of immature granulocyte and monocyte in BM and delayed the maturation of T cells in BM. PRP-1 increased colony-forming cell proliferation in rat cells in vivo. In PRP-treated rat BM, the CFU number at day 4, 7 and 14 was considerably increased in comparison with untreated rats BM and no difference was found at day 21 and day 28. We found that PRP-1 enhances erythroid and myeloid colonies formation in human CD34(+) progenitor cell culture in the presence of different growth factors and down-regulates T cells colony formation and specific surface markers expression during induction of human CD34(+) progenitor cells differentiation into T lymphocytes lineage. We suggested that the hypothalamic PRP-1 possibly represents an endogenous peptide whose primary functions are to regulate neuronal survival and differentiation and hematopoiesis within neurosecretory hypothalamus-bone marrow humoral axis.

Proline-rich polypeptide-1 protects the cells in vitro from genotoxic effects of mitomycin C

A new proline-rich polypeptide (PRP-1) has been earlier shown to possess a broad spectrum of biological activities and seems to be a potential medicine. The potential genotoxic properties of PRP-1 and protective effect of PRP-1 against genotoxic action of Mitomycin C (MMC) were analyzed in details in the present work. DNA and chromosome damages were studied in KCL-22 cell line of human myeloid leukemia by the Comet assay and micronucleus induction test, respectively. The results suggest that DNA damages are, at least partly, transient and reparable. PRP-1 at the doses 0.5-2.0 microg/ml does not possess genotoxic activity. Moreover, this peptide expresses both preventive and therapeutic effects against MMC-induced DNA damage. Pre-treatment of cells with PRP-1 also prevents the appearance of daughter cells bearing as heavy MMC-induced DNA/chromosome damages as MNs. Thus, the polypeptide studied is able to protect the cells from genotoxic action of MMC. This defense includes not only DNA but also heritable chromosome damage in post-mitotic cells. Possible mechanisms of PRP-1 protective action are discussed.

Hypothalamic Proline-Rich Polypeptide Enhances Bone Marrow Colony-Forming Cell Proliferation and Stromal Progenitor Cell Differentiation

The AGAPEPAEPAQPGVY proline-rich peptide (PRP-1) was isolated from neurosecretory granules of the bovine neurohypophysis; it is produced by N. supraopticus and N. paraventricularis. It has been shown that PRP-1 has many potentially beneficial biological effects, including immunoregulatory, hematopoietic, antimicrobial, and antineurodegenerative properties. Here we showed that PRP increased colony-forming cell (CFC) proliferation in rat bone marrow (BM) cells in vivo. In PRP-treated rat BM, the CFU number at day 7 and day 14 was considerably increased in comparison with untreated rat BM and no difference was found at day 21 and day 28. The related peptide [arg8]vasopressin did not reveal CFC proliferation. PRP failed to farther increase CFC proliferation in vitro in BM obtained from PRP-treated or untreated rats. After 3–4 days of human BM stromal cell cultivation in the presence of 2–20 μg/ml PRP the appearance of cells expressing CD15, CD10, CD11a, CD11b, CD3, CD4, and CD16 surface antigens did not differ from the untreated cells. PRP increased the appearance of CD14-positive cells upon 3–4-day incubation with both adult and fetal BM stromal cells. Our results suggest a previously undescribed role for the hypothalamic peptide within neurosecretory hypothalamus–bone marrow humoral axis, because PRP enhances BM colony-forming cell proliferation and stromal cell differentiation.

Effect of Hypothalamic Proline-Rich Peptide (PRP-1) on Neuronal and Bone Marrow Cell Apoptosis

The AGAPEPAEPAQPGVY proline-rich peptide (PRP-1) was isolated from neurosecretory granules of the bovine neurohypophysis; it is produced by N. supraopticus and N. paraventricularis. It has been shown that PRP-1 has many potentially beneficial biological effects including immunoregulatory, hematopoietic, antimicrobial and anti-neurodegenerative properties. Here we investigated the influence of PRP-1 on staurosporine-induced apoptosis of postnatal hippocampal cells and on doxorubicin-induced bone marrow granulocyte- and monocyte apoptosis. The intention was to further characterize the effect of PRP-1 on the survival rate of neurons and in context with myelopoiesis. We demonstrate that PRP-1 significantly reduced apoptosis of postnatal hippocampal cells induced by staurosporine. The protective effect of PRP-1 against apoptotic cell death was shown to be both time- and dose-dependent. Neuroprotection was more pronounced after prolonged pretreatment of the cells with PRP-1 before the induction of apoptosis with staurosporine. The related peptide [arg(8)]vasopressin did not reveal neuroprotection. PRP-1 also significantly reduced apoptosis of bone marrow monocytes and granulocytes induced by doxorubicin. This protective effect lasted for 2-4 h and was not detectable anymore after 24 h when PRP-1 and doxorubicin were added simultaneously. Previously obtained data and results of the current studies suggested that the hypothalamic PRP-1 possibly represents an endogenous peptide whose primary functions are to regulate myelopoiesis and neuron survival as we provide evidence that PRP can differentially reduce both staurosporine- and doxorubicin-induced hippocampal and bone marrow cell apoptosis.

Evidence of Haematopoiesis within the Developing Human Diencephalon

AIM

The sites of haematopoiesis during human ontogeny can be correlated to the sites where haematopoiesis occurs in vertebrate phylogeny. As haematopoiesis has been described in the diencephalon and pituitary gland of water-inhabiting vertebrates we wanted to find out whether such a phenomenon also occurs in human embryos.

MATERIAL AND METHODS

Paraffin-embedded specimens from the diencephalon and pituitary gland of human embryos at the 7th to 22nd gestational week and from adults were investigated by conventional histology and immunohistology for the presence of haematopoietic cells.

RESULTS

Cellular accumulations predominantly of erythroid and megakaryocytic lineage were identified in the floor of the developing diencephalon of the 7th/8th gestational week. At the older developmental stages of the 18th to 22nd gestational week loose aggregates of haematopoietic cells within the leptomeningeal spaces adjacent to the hypophyseal infundibulum were detected in 2 out of 7 cases analyzed.

CONCLUSIONS

As it has been proposed that lymphohaematopoietic clusters occasionally occur within the brain in bone marrow-less vertebrates as a response to noxious agents, we speculate that this temporal appearance of haematopoietic cell clusters within the diencephalon floor in early human ontogeny could also be due to fetal immunomodulations.

Treatment and prophylaxis of anthrax by new neurosecretory cytokines

In 1881, Louis Pasteur described the Bacillus anthracis vaccine, which plays an important role for the treatment and prophylaxis of anthrax. Currently, treatment for anthrax infection involves the use of several different antibiotics, used in combination with vaccines, which possess potential virulence in white mice and guinea pigs. We discovered several new immunomodulators cytokines (polypeptides) produced by the neurosecretory cells of hypothalamus, some of which can be used as drugs for the treatment and prophylaxis of the anthrax. The proline-rich polypeptides, which consist from 10 to 15 amino acids and four proline residues, are of the special interest; one of them (PRP-1), which consist of 15 amino acids and has the following primary structure ALa-GLy-ALa-Pro-GLu-Pro-Ala-GLu-Pro-Ala-GLn-Pro-GLy-Val-Tyr (AGAPEPAEPAQPGVY) possesses antibacterial activity, and a new proline-rich peptide described by Galoyan and called Gx-NH2. Both were tested for treatment against the anthrax bacillus or anthrax strain N55 vaccine in guinea pigs and mice in vivo, and in vitro preparations. The results of experiments show that these hypothalamic neurosecretory cytokines have a strong prophylaxis and therapeutic properties towards animals infected by episodic strain of anthrax and anthrax vaccine N55. The conventional concepts concerning the function of hypothalamic neurosecretion and hypothalamic mechanisms of adaptation have to be reconsidered.

Neuroprotective action of hypothalamic peptide PRP-1 at various time survivals following spinal cord hemisection

The purpose of the present study was to evaluate the neuroprotective action of proline-rich peptide-1 (PRP-1) produced by hypothalamic nuclei cells (nuclei paraventricularis and supraopticus) following lateral hemisection of spinal cord (SC). The dynamics of rehabilitative shifts were investigated at various periods of postoperative survival (1-2, 3, and 4 weeks), both with administration of PRP-1 and without it (control). We registered evoked spike flow activity in both interneurons and motoneurons of the same segment of transected and symmetric intact sides of SC and below it on the stimulation of mixed (n. ischiadicus), flexor (n. gastrocnemius) and extensor (n. peroneus communis) nerves. In the control group (administration of 0.9% saline as placebo), no significant decrease of post-stimulus activity of neurons was observed on the transected side by the 2nd week. This activity strongly decreased by week 3 postaxotomy, with some increase on the intact side, possibly of compensatory origin. No shifts occurred by the 4th week. Regardless of the period of administration, PRP-1 increased neuronal activity on the transected side, with the same activation levels on both SC sides. These data were confirmed by histochemical investigation. PRP-1 administration, both daily and every other day, for a period of 2-3 weeks led to prevention of scar formation and promotion of the re-growth of white matter nerve fibers in the damaged area. It also resulted in prevention of neuroglial elements degeneration and reduction in gliosis expression in the lesion supporting neuronal survival. Thus, PRP-1 achieved protection against "tissue stress", which was also confirmed by the registration of activity on the level of transection and restoration of the motor activity on the injured side. The obtained data propose the possibility of PRP-1 application in clinical practice for prevention of neurodegeneration of traumatic origin.

Hypothalamic proline-rich polypeptide is an oxidative burst regulator

The AGAPEPAEPAQPGVY proline-rich polypeptide (PRP) was isolated from neurosecretory granules of the bovine neurohypophysis; it is produced by N. supraopticus and N. paraventricularis. PRP possesses immune-modulating activity, preventing the death of Gram-negative bacteria-infected mice. Here we show that PRP does not affect human peripheral blood neutrophlis and monocytes phagocytosis but dramatically enhances spontaneous or fMLP- and PMA-induced, and also phagocytosis-dependent, oxidative burst. We demonstrated the regulatory role of PRP on the oxidative burst induction of normal and relapsing inflammatory disease (Behcet's disease and familial Mediterranean fever) neutrophils and monocytes. Our results suggest a previously undescribed role for the hypothalamic peptide within primary activated neutrophils and monocytes, since we provide evidence that PRP can differentially regulate both chemotaxis- and phagocytosis-dependent oxidative burst in normal and inflammatory disease effector cells.

Hypothalamic proline-rich polypeptide is a regulator of oxidative burst in human neutrophils and monocytes

OBJECTIVE

The effects of proline-rich polypeptide (PRP) isolated from neurosecretory granules of bovine neurohypophysis produced by nuclei supraopticus and paraventricularis on phagocytosis, bacterial intracellular killing and oxidative burst induction in normal human cells and inflammatory cells from patients with Behçet's disease (BD), i.e. peripheral blood neutrophils and monocytes, were investigated.

METHODS

Intracellular killing of Staphylococcus aureus by neutrophils and monocytes of normal controls and BD patients, phagocytic activity as well as spontaneous and N-formyl-Met-Leu-Phe (fMLP)- or phorbol 12-myristate 13-acetate (PMA)-induced activation of their respiratory burst were determined by quantitative flow cytometry using highly specific fluorescence probes.

RESULTS

PRP does not affect human peripheral blood neutrophil and monocyte phagocytosis but dramatically enhances spontaneous or fMLP- and PMA-induced oxidative burst as well as the intracellular killing of S. aureus. PRP induced the upregulation of the spontaneous or fMLP- and PMA-induced oxidative burst in normal PMNs and monocytes; the number of inflammatory BD cells did neither increase further nor undergo spontaneous or PMA-stimulated oxidative burst. In BD patients, increased spontaneous production of reactive oxygen intermediates (ROIs) by neutrophils and monocytes is characterized by impaired intracellular protein-kinase-C (PKC)-dependent oxidative burst regulation as well as over-regulation of chemotaxis/inflammation-mediated respiratory burst induction. PRP restores rather the impaired intracellular PKC-dependent regulation of ROI production in inflammatory diseased cells than the chemotaxis/induction of the inflammation-mediated respiratory burst.

CONCLUSION

We demonstrated the regulatory role for PRP on oxidative burst in neutrophils and monocytes from normal controls and BD patients. Our results suggest that PRP differentially affects both chemotaxis- and PKC-dependent oxidative burst in normal and inflammatory cells from patients.

Survival of trauma-injured neurons in rat brain by treatment with proline-rich peptide (PRP-1): an immunohistochemical study

The objective of this immunohistochemical research was to reveal the distribution of a proline-rich peptide-1 (PRP-1) in various brain structures of intact and trauma-injured rats and to identify the mechanisms of promotion of neuronal recovery processes following PRP-1 treatment. PRP-1, produced by bovine hypothalamic magnocellular cells and consisting of 15 amino acid residues, is a fragment of neurophysin vasopressin associated glycoprotein isolated from bovine neurohypophysis neurosecretory granules. PRP-1-immunoreactivity (PRP-1-IR) was detected in the brain of intact rats in the neurons of paraventricular (PVN) and supraoptic (SON) nuclei in the hypothalamus, in almost all cell groups in the medulla oblongata, in Purkinje and some cerebellar nuclei cells, and in nerve fibers. At 3 weeks after hemisection of the spinal cord (SC) an asymmetry of PRP-1 localization in the PVN and SON was observed: no PRP-1-IR was exhibited at the affected sides of both nuclei. Daily intramuscular administration of PRP-1 for 3 weeks significantly increased the number of PRP-1-immunoreactive (PRP-1-Ir) varicose nerve fibers, and cells in PVN and SON and in cell groups of the limbic system and brain stem. Tanycytes in the median eminence and covering ependyma also demonstrated strong PRP-1-IR. PRP-1 treatment also activated neuropeptide Y-IR (NPY-IR) in nerve fibers and immunophilin fragment-IR (IphF-IR) in lymphocytes and nerve cells. A strong increase of PRP-1-IR was observed in the PVN and SON of SC-injured rats following the treatment with another PRP (PRP-3). Preliminary physiological data demonstrate that PRP-3 is more "aggressive" in the recovery processes than PRP-1. Based on the findings regarding PRP action on neurons survival, axons regeneration, and the number of IphF-Ir lymphocytes and NPY-Ir nerve fibers, PRP is suggested to act as a neuroprotector, functioning as a putative neurotransmitter and immunomodulator.

Effect of hypothalamic proline-rich-polypeptide on voltage-gated Ca2+ currents in retinal ganglion cells

Neurotrophins are molecules that regulate neuronal survival, nervous system plasticity, and many other physiological functions of neuronal and glial cells. Here we studied the physiological action of a novel neurosecretory polypeptide proline-rich polypeptide (PRP), isolated from bovine neurohypophysis neurosecretory granules, on voltage-gated Ca currents and spike firing activity of retinal ganglion cells. PRP reversibly increased high voltage-activated L-type Ca current, but was without effect on low voltage-activated T-type current. PRP also increased the spike after hyperpolarization and reduced the frequency of spike firing, most likely by affecting a Ca-dependent potassium current.