Functional and molecular diversity of PACAP/VIP receptors in cortical neurons and type I astrocytes

Pituitary adenylyl cyclase-activating polypeptide stimulates DNA synthesis but delays maturation of oligodendrocyte progenitors

The neuropeptide pituitary adenylyl cyclase-activating peptide (PACAP) and one of its receptors (PAC(1)) are expressed in embryonic neural tube, where they appear to regulate neurogenesis and patterning. We now show that PAC(1) gene expression is also present in neonatal rats in the ventricular and subventricular zones and in the optic chiasm, areas that are rich in oligodendrocyte (OL) progenitors (OLP). Because actions of PACAP on OLP have not been reported, we examined the effects of PACAP on the proliferation of purified OLP in culture and on myelinogenesis in cerebellar slices. Northern analyses on total RNA from purified glial cell subtypes revealed an abundant 7 kb hybridizing transcript in OLP, which was confirmed to correspond to the PAC(1) receptor by reverse transcription-PCR. The presence of this receptor was also corroborated by radioligand binding and cAMP assay. In cultured OL, receptor density decreased during maturation but was partially counterbalanced by the appearance of sites that bound both PACAP and the related peptide vasoactive intestinal peptide. PACAP increased DNA synthesis in OLP cultures almost twofold and increased the bromodeoxyuridine-labeling index in O4-positive OLP. PACAP treatment also resulted in decreased sulfate incorporation into sulfatide in cultures of differentiating OL. The PACAP effect on sulfatide synthesis was fully reproduced in a cerebellar explant model. These findings indicate that PACAP may act at two stages during OL development to (1) stimulate proliferation and (2) delay maturation and/or myelinogenesis.

Mobilization of calcium from intracellular stores, potentiation of neurotransmitter-induced calcium transients, and capacitative calcium entry by 4-aminopyridine

In this study we analyzed the effect of 4-aminopyridine (4-AP) on free cytosolic calcium concentration ([Ca(2+)](i)) in basal conditions, after stimulation with neurotransmitters, and during capacitative calcium entry. Using fura-2 ratiometric calcium imaging, we found that 4-AP increased [Ca(2+)](i) in type I astrocytes, neurons, and in skeletal muscle cells. The [Ca(2+)](i) elevation induced by 4-AP was concentration-dependent and consisted of two phases: the first was dependent on intracellular calcium mobilization, and the second was dependent on extracellular calcium influx. 4-AP also increased the second messenger inositol trisphosphate in both neurons and astrocytes. In astrocytes, 4-AP treatment potentiated the sustained phase of the [Ca(2+)](i) elevation induced by ATP and bradykinin. In addition, capacitative calcium entry was potentiated severalfold by 4-AP, in astrocytes and muscle cells but not in neurons. These effects of 4-AP were completely and promptly reversible. 4-AP blocked voltage-sensitive K(+) currents in astrocytes. However, voltage-sensitive K(+) channel blockers inhibiting these currents did not affect agonist-induced calcium transients or capacitative calcium entry, indicating that 4-AP effects on [Ca(2+)](i) were not caused by the blockade of voltage-gated K(+) channels. We conclude that 4-AP is able to affect calcium homeostasis at multiple levels, from increasing basal [Ca(2+)](i) to potentiating capacitative calcium entry. The potentiation of capacitative calcium entry in astrocytes or muscle cells may explain some of the therapeutic activities of 4-AP as a neurotransmission enhancer.

BDNF mediates the neuroprotective effect of PACAP-38 on rat cortical neurons

In primary cultures from rat cerebral cortex, pituitary adenylate cyclase-activating polypeptide (PACAP-38) exerted a protective effect on cell death induced by the excitotoxin NMDA in neuron-enriched cultures and also on apoptotic cell death induced by serum deprivation in mixed neuronal-glial cultures. The neuroprotective effect was already observed at subnanomolar concentrations of PACAP and was slightly more pronounced against excitotoxic cell death. BDNF protein expression was reduced by NMDA and much more markedly by serum deprivation (approximately 28 and 93% reduction respectively). In both cellular injury conditions, the diminished BDNF expression was significantly prevented by PACAP. When purified neuronal cultures were preincubated with an antiserum anti-BDNF, at a concentration without any intrinsic effect on cell viability, the neuprotective effect of PACAP was no longer observed. The results suggest that the neuroprotective effect of PACAP-38 is mediated, at least in part, by preventing the suppressed expression of a neurotrophin essential for cortical neuron survival.

Regulation of neuroblast mitosis is determined by PACAP receptor isoform expression

Although neurogenesis in the embryo proceeds in a region- or lineage-specific fashion coincident with neuropeptide expression, a regulatory role for G protein-coupled receptors (GPCR) remains undefined. Pituitary adenylate cyclase activating polypeptide (PACAP) stimulates sympathetic neuroblast proliferation, whereas the peptide inhibits embryonic cortical precursor mitosis. Here, by using ectopic expression strategies, we show that the opposing mitogenic effects of PACAP are determined by expression of PACAP receptor splice isoforms and differential coupling to the phospholipase C (PLC) pathway, as opposed to differences in cellular context. In embryonic day 14 (E14) cortical precursors transfected with the hop receptor variant, but not cells transfected with the short variant, PACAP activates the PLC pathway, increasing intracellular calcium and eliciting translocation of protein kinase C. Ectopic expression of the hop variant in cortical neuroblasts transforms the antimitotic effect of PACAP into a promitogenic signal. Furthermore, PACAP promitogenic effects required PLC pathway function indicated by antagonist U-73122 studies in hop-transfected cortical cells and native sympathetic neuroblasts. These observations highlight the critical role of lineage-specific expression of GPCR variants in determining mitogenic signaling in neural precursors.

Cannabinoid receptor activation and elevated cyclic AMP reduce glutamate neurotoxicity

Cannabinoid receptor activation in vivo reduces ischemic injury, a phenomenon that has not been successfully reproduced in vitro. Because cyclic adenosine monophosphate (cAMP) levels are radically elevated during ischemic reperfusion, but cannabinoid receptor activation reduces cAMP levels, we hypothesized that cannabinoids might prevent in vitro glutamate toxicity if reperfusion was simulated by cAMP supplementation after glutamate removal. Although neuronal cultures were unaffected by the single addition of either cannabinoid or dibutyryl cAMP (dbcAMP), glutamate toxicity was reduced by 20% when cannabinoid was present during glutamate exposure and either dbcAMP or forskolin was added after glutamate removal. Further studies revealed that cannabinoid receptor activation reduces glutamate toxicity by attenuating calcium influx through N- and P/Q-type calcium channels. The effect of glutamate exposure on neuronal cAMP levels was also examined. Glutamate exposure significantly reduced neuronal cAMP levels, although suppression was even greater when cannabinoid was present. Because neurological outcome after ischemia is poor when cAMP levels during reperfusion are low, it is hypothesized that cAMP elevation after glutamate exposure may offset excitotoxic and/or cannabinoid receptor-induced cAMP depletion. Cannabinoids protect against ischemic injury in vivo, but only reduce toxicity in vitro when cAMP levels are elevated, possibly suggesting that cAMP elevation during reperfusion reduces brain injury by off-setting the effect of Gi/o protein-coupled systems on adenylate cyclase.

Regional and cellular expression of the parkin gene in the rat cerebral cortex

A mutation in the parkin gene has been identified as the cause for an autosomal recessively inherited form of early onset Parkinson's disease. We have recently isolated the mRNA coding for the rat homologue of parkin and showed its widespread expression in the central nervous system (CNS) by in situ hybridization. In the present study we investigated the distribution of parkin in the rat cerebral cortex with a polyclonal antibody that reacts with a single approximately 52-kDa protein, corresponding to the predicted molecular mass of parkin. Conventional light microscopic studies revealed intense parkin immunoreactivity (IR) throughout the cortex. Examination of mixed cortical neuro-glial cultures by indirect immunofluorescence technique coupled to traditional epifluorescence and confocal microscopy analysis demonstrated the expression of parkin in the cytoplasm and neurites of neurons, and its absence in glial fibrillary acidic protein (GFAP)-positive astrocytes. The predominant neuronal parkin-IR and -mRNA expression was confirmed by Western blot analysis and reverse transcription-polymerase chain reaction (RT-PCR), respectively, performed on highly enriched neuronal and type I astrocytes cultures. The information gathered in our study about the cellular and subcellular distribution of parkin should facilitate further research on its physiological role in the nervous system.

cAMP-induced cytoskeleton rearrangement increases calcium transients through the enhancement of capacitative calcium entry

In this report we investigated the correlation between cell morphology and regulation of cytosolic calcium homeostasis. Type I astrocytes were differentiated to stellate process-bearing cells by a 100-min exposure to cAMP. Differentiation of cortical astrocytes increased the magnitude and duration of calcium transients elicited by phospholipase C-activating agents as measured by single cell Fura-2-based imaging. Calcium imaging showed differences in the spatial pattern of the response. In both differentiated and the control cells, the response originated in the periphery and gradually extended into the center of the cell. However, the elevation of cytosolic calcium concentration ([Ca(2+)](i)) was particularly evident within the processes and adjacent to the inner cell membrane of the differentiated astrocytes. In addition, differentiation significantly prolonged the duration of the [Ca(2+)](i) elevation. Potentiation of the calcium transients was mimicked by forskolin-induced differentiation and abolished by a specific protein kinase-A blocker. Conversely, the enhancement of the calcium transients was not mimicked by brief exposure to cAMP not causing morphological differentiation, and in PC12 cells that did not undergo morphological changes after 100 min of cAMP treatment. Impairing cAMP-induced cytoskeleton re-organization, by means of cytochalasin D and nocodazole, prevented the potentiation of the calcium transients in cAMP-treated astrocytes. Phospholipase C activity and sensitivity to inositol (1,4,5)-trisphosphate were not involved in the enhancement of the calcium responses. Also, potentiation of the calcium transients was dependent on extracellular calcium. Calcium storage and thapsigargin-depletable intracellular calcium reservoirs were analogously not increased in differentiated astrocytes. Rearrangement of the cell shape also caused a condensation of the endoplasmic reticulum and altered the spatial relationship between the endoplasmic reticulum and the cell membrane. In conclusion, morphological rearrangements of type I astrocytes increase the magnitude and the duration of agonist-induced calcium transients via enhancement of capacitative calcium entry and is associated with a spatial reorganization of the relationship between cell membrane and the endoplasmic reticulum structures.