PACAP38 modulates activity of NMDA receptors in cultured chick cortical neurons

Comparison of the effects of PACAP-38 and its analog, acetyl-[Ala15, Ala20] PACAP-38-propylamide, on spatial memory, post-learning BDNF expression and oxidative stress in rat

We compared the effects of single intraveinous injection of pituitary adenylate cyclase-activating polypeptide-38 (P38) to those of its analog, acetyl-[Ala¹⁵, Ala²⁰]PACAP-38-propylamide (P38-alg) on spatial memory in the Morris water maze (MWM) using a weak massed-learning procedure, post-training brain derived neurotrophic factor (BDNF) and post-training oxidative stress biomarker assays in male Wistar rats. Acquisition of the MWM task following P38 (30 μg/kg) and P38-alg (30 μg/kg) treatments was similar to control group (Saline: 0.9% NaCl) and there was no interaction between treatments and performance. However, in the probe test, P38-treated group showed a specific interest for the target quadrant whereas the two other groups exhibited less focused place searching behavior. Moreover, P38 had an anxiogenic effect as measured by the distribution of swimming at the periphery of the pool. The swimming test resulted in a decrease in BDNF contents in the hippocampus. P38 but not P38-alg treatment restored BDNF expression. In terms of oxidative stress, both P38 and P38-alg treatments had antioxidative effects. The activity of antioxidative enzymes in the neocortex was increased. However only P38 reduced the levels of carbonylated proteins (CP). These data show that P38 and P38-alg have different behavioral and neurobiological effects. Thus, P38-alg and other analogs with specific functional profiles, inducing beneficial central effects (e.g. neuroprotection) while minimizing undesired peripheral effects may be useful for potential therapeutical use.

Modulation of oligodendrocyte differentiation and maturation by combined biochemical and mechanical cues

Extracellular matrix (ECM) proteins play a key role during oligodendrogenesis. While fibronectin (FN) is involved in the maintenance and proliferation of oligodendrocyte progenitor cells (OPCs), merosin (MN) promotes differentiation into oligodendrocytes (OLs). Mechanical properties of the ECM also seem to affect OL differentiation, hence this study aimed to clarify the impact of combined biophysical and biochemical elements during oligodendrocyte differentiation and maturation using synthetic elastic polymeric ECM-like substrates. CG-4 cells presented OPC- or OL-like morphology in response to brain-compliant substrates functionalised with FN or MN, respectively. The expression of the differentiation and maturation markers myelin basic protein — MBP — and proteolipid protein — PLP — (respectively) by primary rat oligodendrocytes was enhanced in presence of MN, but only on brain-compliant conditions, considering the distribution (MBP) or amount (PLP) of the protein. It was also observed that maturation of OLs was attained earlier (by assessing PLP expression) by cells differentiated on MN-functionalised brain-compliant substrates than on standard culture conditions. Moreover, the combination of MN and substrate compliance enhanced the maturation and morphological complexity of OLs. Considering the distinct degrees of stiffness tested ranging within those of the central nervous system, our results indicate that 6.5 kPa is the most suitable rigidity for oligodendrocyte differentiation.

Glycine transporter-1 blockade leads to persistently reduced relapse-like alcohol drinking in rats

BACKGROUND

Residual dysfunction of multiple neurotransmitter systems due to chronic alcohol use is likely responsible for the occurrence of compulsive alcohol seeking during abstinence and relapse behavior. There is increasing evidence that glycine, which activates both glycine and N-methyl-D-aspartate receptors, contributes to excessive alcohol consumption. We therefore hypothesized that the blockade of glycine transporter 1 might interfere with compulsive alcohol consumption and relapse behavior.

METHODS

We used our animal model of alcoholism--long-term alcohol consumption with repeated deprivation phases in rats--to study the effects of a selective blocker of glycine transporter 1 Org25935. The abstinence-promoting drug acamprosate was used as a reference compound. Subsequently, we examined alterations in dorsal striatal gene expression caused by chronic ethanol (EtOH) consumption, focusing on glycinergic and glutamatergic signaling-related genes. Gene expression profiles of Org25935-treated EtOH-drinking rats were compared with vehicle-treated EtOH-drinking versus age-matched EtOH-naive rats.

RESULTS

We found that repeated treatment with Org25935 reduced compulsive relapse-like drinking without the development of tolerance. Importantly, these antirelapse properties were maintained for at least 6 weeks in a treatment-free period. This persistent effect was paralleled by a reversal of altered expression levels of a set of glycinergic and glutamatergic signaling-related genes to the levels found in EtOH-naive control rats.

CONCLUSIONS

This study shows that treatment of rats with Org25935 leads to a reduction of compulsive alcohol consumption and relapse-like drinking behavior--an effect that persists into treatment-free periods. This long-term antirelapse effect might result from a restoration of normal glycinergic and glutamatergic signaling function.

Neuroprotective effect of PACAP against NMDA-induced retinal damage in the mouse

Retinal excitotoxicity is one of the major causes of retinal ganglion cell (RGC) death in glaucoma. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic peptide with potent neuroprotective activity; however, whether it exerts such an effect in the retina and the mechanism by which RGCs are protected is still not well understood. In this study, we examined the effect of exogenous and endogenous PACAP on RGC death induced by N-methyl-D: -aspartate acid (NMDA). The vitreous body of anesthetized adult male mice (C57/BL6J) was injected with NMDA (40 nmol in a 2 μL saline solution). The number of RGCs decreased from days 1 to 7 after NMDA injection, and the number of dUTP end-labeling (TUNEL)-positive cells, an indicator of cell death, peaked at day 3. However, when PACAP38 (10(-8), 10(-10), 10(-12), 10(-14), or 10(-16)M) was co-administered with NMDA, the 10(-10)M dose resulted in significantly increased RGC survival at day 7, and a decrease in the number of TUNEL-positive RGCs at day 3. We next investigated the neuroprotective effect of endogenous PACAP using PACAP heterozygote(+/-) mice. Under normal circumstances, there was no significant difference in the number of RGCs in the PACAP(+/-) mice compared with their wild-type counterparts. However, the number of RGCs significantly decreased in the PACAP(+/-) mice 7 days after NMDA injection, relative to their wild-type counterparts. The number of TUNEL-positive RGCs peaked at day 1 in the PACAP(+/-) mice. These effects in the PACAP(+/-) mice were reversed by intravitreous injection of 10(-10)M PACAP38. This suggests that exogenous PACAP is able to counteract NMDA-induced toxicity, and that endogenous PACAP exerts a neuroprotective effect in the retina.

Modulation of AMPA receptor-mediated ion current by pituitary adenylate cyclase-activating polypeptide (PACAP) in CA1 pyramidal neurons from rat hippocampus

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neurotrophic and neuromodulatory peptide, was recently shown to enhance NMDA receptor-mediated currents in the hippocampus (Macdonald, et al. 2005. J Neurosci 25:11374-11384). To check if PACAP might also modulate AMPA receptor function, we tested its effects on AMPA receptor-mediated synaptic currents on CA1 pyramidal neurons, using the patch clamp technique on hippocampal slices. In the presence of the NMDA antagonist D-AP5, PACAP (10 nM) reduced the amplitude of excitatory postsynaptic currents (EPSCs) evoked in CA1 pyramidal neurons by stimulation of Schaffer collaterals. Following a paired-pulse stimulation protocol, the paired-pulse ratio was unaffected in most neurons, suggesting that the AMPA-mediated EPSC was modulated by PACAP mainly at a postsynaptic level. PACAP also modulated the currents induced on CA1 pyramidal neurons by applications of either glutamate or AMPA. The effects of PACAP were dose-dependent: at a 0.5 nM dose, PACAP increased AMPA-mediated current; such effect was blocked by PACAP 6-38, a selective antagonist of PAC1 receptors. The enhancement of AMPA-mediated current by PACAP 0.5 nM was abolished when cAMPS-Rp, a PKA inhibitor, was added to the intracellular solution. At a 10 nM concentration, PACAP reduced AMPA-mediated current; such effect was not blocked by PACAP 6-38. The inhibitory effect of 10 nM PACAP was mimicked by Bay 55-9837 (a selective agonist of VPAC2 receptors), persisted in the presence of intracellular BAPTA and was abolished by intracellular cAMPS-Rp. Stimulation-evoked EPSCs in CA1 neurons were significantly reduced following application of the PAC1 antagonist PACAP 6-38; this result indicates that PAC1 receptors in the CA1 region are tonically activated by endogenous PACAP and enhance CA3-CA1 synaptic transmission. Our results show that PACAP differentially modulates AMPA receptor-mediated current in CA1 pyramidal neurons by activation of PAC1 and VPAC2 receptors, both involving the cAMP/PKA pathway; the functional significance will be discussed in light of the multiple effects exerted by PACAP on the CA3-CA1 synapse at different levels.

Pituitary adenylate cyclase-activating polypeptide attenuates streptozotocin-induced apoptotic death of RIN-m5F cells through regulation of Bcl-2 family protein mRNA expression

Oxidative stress, followed by the apoptotic death of pancreatic beta cells, is considered to be one of causative agents in the evolution of the type 2 diabetic state; therefore, the protection of beta cells can comprise an efficacious strategy for preventing type 2 diabetes. In the present study, RIN-m5F cells (i.e. the rat insulinoma beta cell line) were stimulated with streptozotocin, resulting in a time- and concentration-dependent release of lactate dehydrogenase. There appeared to be significant apoptotic cell death after 2 h of treatment with streptozotocin at 10 mM, as demonstrated by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining and 2.6-fold activation of cellular caspase-3, an apoptotic enzyme. By contrast, some neuropeptides of the glucagon-secretin family and coenzyme Q(10), an endogenous mitochondrial antioxidant, could attenuate streptozotocin cytotoxicity, and especially pituitary adenylate cyclase-activating polypeptide (PACAP), at a concentration of 10(-7) M, exhibited 34% attenuation of lactate dehydrogenase release from streptozotocin-treated RIN-m5F cells. Quantitative RT-PCR experiments indicated the inhibitory effect of PACAP on streptozotocin-evoked up-regulation of pro-apoptotic factor (Noxa and Bax) and a 2.3-fold enhancement of Bcl-2 mRNA expression, a pro-survival protein, was also observed after addition of PACAP. The data obtained suggest the anti-apoptotic role of PACAP in streptozotocin-treated RIN-m5F cells through the regulation of pro-apoptotic and pro-survival factors.

Regulation of pituitary adenylyl cyclase-activating polypeptide (PACAP, ADCYAP1: adenylyl cyclase-activating polypeptide 1) in the treatment of schizophrenia

BACKGROUND

Deficiency of pituitary adenylyl cyclase-activating polypeptide (PACAP) and its specific receptor, PAC1, causes a schizophrenia-like phenotype in mice. In addition, the relation of the PACAP and PAC1 genes to schizophrenia has been shown by single-nucleotide polymorphism association studies. Furthermore, PACAP is reported to be involved in the function of disrupted-in-schizophrenia 1.

OBJECTIVE

To summarize briefly the recent evidence relating the PACAP system and schizophrenia and discuss the application of PACAP to the treatment of schizophrenia.

RESULTS/CONCLUSION

The regulation of PACAPergic signals is an interesting potential treatment for schizophrenia. Further studies of PACAP signals and the association of PACAP signals with schizophrenia should shed the light on the utility of this approach in the treatment of schizophrenia.

The role of vasoactive intestinal polypeptide and pituitary adenylate cyclase-activating polypeptide in the neural pathways controlling the lower urinary tract

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are expressed in the neural pathways regulating the lower urinary tract. VIP-immunoreactivity (IR) is present in afferent and autonomic efferent neurons innervating the bladder and urethra, whereas PACAP-IR is present primarily in afferent neurons. Exogenously applied VIP relaxes bladder and urethral smooth muscle and excites parasympathetic neurons in bladder ganglia. PACAP relaxes bladder and urethral smooth muscle in some species (pig) but excites the smooth muscle in other species (mouse). Intrathecal administration of VIP in cats with an intact spinal cord suppresses reflex bladder activity, but intrathecal administration of VIP or PACAP in rats enhances bladder activity and suppresses urethral sphincter activity. PACAP has presynaptic facilitatory effects and direct excitatory effects on lumbosacral parasympathetic preganglionic neurons. Chronic spinal cord transection produces an expansion of VIP-IR (cats) and PACAP-IR (rats) in primary afferent axons in the lumbosacral spinal cord and unmasks spinal excitatory effects of VIP on bladder reflexes in cats. Intrathecal administration of PACAP6-38, a PAC1 receptor antagonist, reduces bladder hyperactivity in chronic spinal-cord-injured rats. These observations raise the possibility that VIP or PACAP have a role in the control of normal or abnormal voiding.

Effects of intrathecal administration of pituitary adenylate cyclase activating polypeptide on lower urinary tract functions in rats with intact or transected spinal cords

Effects of intrathecally administered pituitary adenylate cyclase activating polypeptide-38 (PACAP-38, 0.1-30 microg) on lower urinary tract function were examined in unanesthetized, decerebrate rats with an intact spinal cord and after chronic spinal cord transection (SCT). PACAP-38 was also studied in rats with intact or bilaterally transected hypogastric nerves (HGNs), to determine if sympathetic pathways to the bladder influenced responses. In SCT rats with intact HGNs under isovolumetric conditions, 30 mug of PACAP-38 but not lower doses (0.1-10 microg) increased (mean 194%) bladder contraction amplitude (BCA). In SCT rats with sectioned HGNs, 10 microg and 30 microg of PACAP-38 increased BCA by 62% and 195%, respectively. On the other hand, during continuous infusion cystometrograms (CMGs) in SCT rats with intact or sectioned HGNs, PACAP-38 (10 microg and 30 microg) markedly reduced or completely suppressed BCA (60% and 90%, respectively) and reduced external urethral sphincter (EUS) EMG activity (58% and 91%, respectively). During CMGs in spinal cord intact rats, with intact HGNs PACAP-38 30 microg increased BCA (26%) but after HGN section PACAP-38 10 microg and 30 microg increased BCA by 21% and 35%. These results suggest that after SCT, PACAP-38 activates spinal circuitry to facilitate the parasympathetic outflow to the urinary bladder and that the elimination of sympathetic pathways enhances this effect. The decrease in BCA by PACAP-38 during CMGs in SCT rats is most reasonably attributed to a reduction in urethral outlet resistance due to suppression of excitatory EUS reflexes.

PACAP and PDF signaling in the regulation of mammalian and insect circadian rhythms

Endogenous circadian clocks are inherent to all living organisms. They are needed to guarantee successful life since they regulate very important biological processes such as behavior and reproduction. Secretin-like G-protein coupled receptors are very important factors in the signal transduction pathways of circadian clocks. In this review, we will focus on the role of two secretin-like signaling pathways that play an important role in the regulation of the mammalian and the insect clock, respectively: the pituitary adenylate cyclase-activating polypeptide (PACAP) and pigment dispersing factor (PDF) signaling pathways. Both pathways are most likely related although their function in the biological clock differs.

Psychostimulant-induced attenuation of hyperactivity and prepulse inhibition deficits in Adcyap1-deficient mice

Psychostimulants, including amphetamine, act as antihyperkinetic agents in humans with hyperkinetic disorder such as attention-deficit hyperactivity disorder and are known to be effective in enhancing attention-related processes; however, the underlying mechanisms have not been adequately addressed. Mice lacking the Adcyap1 gene encoding the neuropeptide pituitary adenylate cyclase-activating polypeptide (Adcyap1(-/-)) display psychomotor abnormalities, including increased novelty-seeking behavior and hyperactivity. In this study, Adcyap1(-/-) mice showed sensory-motor gating deficits, measured as deficits in prepulse inhibition (PPI), and showed normal PPI in response to amphetamine. Amphetamine also significantly decreased hyperlocomotion in Adcyap1(-/-) mice, and this paradoxical antihyperkinetic effect depended on serotonin 1A (5-HT(1A)) receptor signaling. c-Fos-positive neurons were increased in the prefrontal cortex in amphetamine-treated Adcyap1(-/-) mice, suggesting increased inhibitory control by prefrontal neurons. Additionally, amphetamine produced an antihyperkinetic effect in wild-type mice that received the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin. These results indicate that Adcyap1(-/-) mice act as a model of hyperlocomotion and PPI deficits and suggest that 5-HT(1A)-mediated pathways are important determinants of the psychostimulant-elicited, rate-dependent effects that are in a negative function of the baseline rate of activity.

Modulation of NMDA receptors by pituitary adenylate cyclase activating peptide in CA1 neurons requires G alpha q, protein kinase C, and activation of Src

At CA1 synapses, activation of NMDA receptors (NMDARs) is required for the induction of both long-term potentiation and depression. The basal level of activity of these receptors is controlled by converging cell signals from G-protein-coupled receptors and receptor tyrosine kinases. Pituitary adenylate cyclase activating peptide (PACAP) is implicated in the regulation of synaptic plasticity because it enhances NMDAR responses by stimulating Galphas-coupled receptors and protein kinase A (Yaka et al., 2003). However, the major hippocampal PACAP1 receptor (PAC1R) also signals via Galphaq subunits and protein kinase C (PKC). In CA1 neurons, we showed that PACAP38 (1 nM) enhanced synaptic NMDA, and evoked NMDAR, currents in isolated CA1 neurons via activation of the PAC1R, Galphaq, and PKC. The signaling was blocked by intracellular applications of the Src inhibitory peptide Src(40-58). Immunoblots confirmed that PACAP38 biochemically activates Src. A Galphaq pathway is responsible for this Src-dependent PACAP enhancement because it was attenuated in mice lacking expression of phospholipase C beta1, it was blocked by preventing elevations in intracellular Ca2+, and it was eliminated by inhibiting either PKC or cell adhesion kinase beta [CAKbeta or Pyk2 (proline rich tyrosine kinase 2)]. Peptides that mimic the binding sites for either Fyn or Src on receptor for activated C kinase-1 (RACK1) also enhanced NMDAR in CA1 neurons, but their effects were blocked by Src(40-58), implying that Src is the ultimate regulator of NMDARs. RACK1 serves as a hub for PKC, Fyn, and Src and facilitates the regulation of basal NMDAR activity in CA1 hippocampal neurons.

Roles of PACAP‐Containing Retinal Ganglion Cells in Circadian Timing

The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms in physiology and behavior. The clock-driven rhythms need daily adjustment (entrainment) to be synchronized with the astronomical day of 24 h. The most important stimulus for entrainment of the clock is the light-dark (LD) cycle. In this review functional elements of the light entrainment pathway will be considered with special focus on the neurotransmitter pituitary adenylate cyclase-activating polypeptide (PACAP), which is found exclusively in the monosynaptic neuronal pathway mediating light information to the SCN, the retinohypothalamic tract (RHT). The retinal ganglion cells of the RHT are intrinsically photosensitive due to the expression of melanopsin and seem to constitute a non-image forming photosensitive system in the mammalian eye regulating circadian timing, masking behavior, light-regulated melatonin secretion, and the pupillary light reflex. Evidence from in vitro and in vivo studies and studies of mice lacking PACAP and the specific PACAP receptor (PAC1) indicate that PACAP and glutamate are neurotransmitters in the RHT which in a clock and concentration-dependent manner interact during light entrainment of the clock.

Class II G protein-coupled receptors and their ligands in neuronal function and protection

G protein-coupled receptors (GPCRs) play pivotal roles in regulating the function and plasticity of neuronal circuits in the nervous system. Among the myriad of GPCRs expressed in neural cells, class II GPCRs which couples predominantly to the Gs-adenylate cyclase-cAMP signaling pathway, have recently received considerable attention for their involvement in regulating neuronal survival. Neuropeptides that activate class II GPCRs include secretin, glucagon-like peptides (GLP-1 and GLP-2), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase activating peptide (PACAP), corticotropin-releasing hormone (CRH), vasoactive intestinal peptide (VIP), parathyroid hormone (PTH), and calcitonin-related peptides. Studies of patients and animal and cell culture models, have revealed possible roles for class II GPCRs signaling in the pathogenesis of several prominent neurodegenerative conditions including stroke, Alzheimer's, Parkinson's, and Huntington's diseases. Many of the peptides that activate class II GPCRs promote neuron survival by increasing the resistance of the cells to oxidative, metabolic, and excitotoxic injury. A better understanding of the cellular and molecular mechanisms by which class II GPCRs signaling modulates neuronal survival and plasticity will likely lead to novel therapeutic interventions for neurodegenerative disorders.

Pituitary adenylate cyclase-activating polypeptide is required for the development of spinal sensitization and induction of neuropathic pain

The prolonged sensitization of pain transmission after nerve injury by increasing excitability of spinal neurons and thereby promoting repair is an adaptive response of the body. The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is widely distributed in the nervous system and implicated in neurotransmission, neural plasticity, and neurotrophic actions. Although PACAP is distributed in the spinal cord and dorsal root ganglia, a role of PACAP in pain responses remains essentially unknown. Here we show that mice lacking the PACAP gene (PACAP-/-) did not exhibit inflammatory pain induced by intraplantar injection of carrageenan or neuropathic pain induced by L5 spinal nerve transection, whereas they did retain normal nociceptive responses. Intrathecal administration of NMDA induced mechanical allodynia in wild-type mice, but not in PACAP-/- mice. The NMDA-induced allodynia in PACAP-/- mice was reproduced by simultaneous intrathecal injection of PACAP with NMDA. Concomitant with the increase in PACAP immunoreactivity after nerve injury, NADPH-dependent nitric oxide synthase (NOS) activity visualized by NADPH diaphorase histochemistry markedly increased in the superficial layer of the spinal cord of wild-type mice, which was not observed in PACAP-/- mice. Simultaneous addition of PACAP and NMDA caused translocation of neuronal NOS from the cytosol to the membrane and stimulated NO production in vitro. These results demonstrate that PACAP might promote the functional coupling of neuronal NOS to NMDA receptors for both inflammatory and neuropathic pain to occur.

Pituitary adenylate cyclase-activating polypeptide (PACAP(1-38)) enhances N-methyl-D-aspartate receptor function and brain-derived neurotrophic factor expression via RACK1

We recently identified a novel mechanism for modulation of the phosphorylation state and function of the N-methyl-d-aspartate (NMDA) receptor via the scaffolding protein RACK1. We found that RACK1 binds both the NR2B subunit of the NMDA receptor and the nonreceptor protein-tyrosine kinase, Fyn. RACK1 inhibits Fyn phosphorylation of NR2B and decreases NMDA receptor-mediated currents in CA1 hippocampal slices (Yaka, R., Thornton, C., Vagts, A. J., Phamluong, K., Bonci, A., and Ron, D. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 5710-5715). Here, we identified the signaling cascade by which RACK1 is released from the NMDA receptor complex and identified the consequences of the dissociation. We found that activation of the cAMP/protein kinase A pathway in hippocampal slices induced the release of RACK1 from NR2B and Fyn. This resulted in the induction of NR2B phosphorylation and the enhancement of NMDA receptor-mediated activity via Fyn. We identified the neuropeptide, pituitary adenylate cyclase activating polypeptide (PACAP(1-38)), as a ligand that induced phosphorylation of NR2B and enhanced NMDA receptor potentials. Finally, we found that activation of the cAMP/protein kinase A pathway induced the movement of RACK1 to the nuclear compartment in dissociated hippocampal neurons. Nuclear RACK1 in turn was found to regulate the expression of brain-derived neurotrophic factor induced by PACAP(1-38). Taken together our results suggest that activation of adenylate cyclase by PACAP(1-38) results in the release of RACK1 from the NMDA receptor and Fyn. This in turn leads to NMDA receptor phosphorylation, enhanced activity mediated by Fyn, and to the induction of brain-derived neurotrophic factor expression by RACK1.

Modulation of nociceptive transmission by pituitary adenylate cyclase activating polypeptide in the spinal cord of the mouse

Superficial layers of the dorsal horn receive a dense plexus of nerve fibers immunoreactive to pituitary adenylate cyclase activating polypeptide (PACAP). In vivo experiments were conducted in the mice to evaluate the effects of PACAP-38, herein referred to as PACAP, PACAP receptor antagonist PACAP(6-38) and PACAP-antiserum on nociceptive behaviors induced by radiant heat, intrathecally administered N-methyl-D-aspartate (NMDA) or intraplantarly administered formalin. PACAP (0.05-0.5 microg) dose-dependently decreased the paw-withdrawal latencies induced by thermal stimulation and enhanced the aversive licking and biting behaviors induced by intrathecally injected NMDA. Pretreatment with the PACAP receptor antagonist PACAP(6-38) (0.5-2 microg) or PACAP-antiserum (1:500-2,000 dilution) dose-dependently attenuated the second phase, but not the first phase, of nociceptive responses to formalin. Next, the effects of PACAP on NMDA- and kainate-induced currents evoked in single dorsal horn neurons were studied. Whole-cell patch recordings were made from superficial dorsal horn neurons of spinal cord slices from 14- to 20-day-old mice. PACAP at the concentrations of 100 and 200 nM, which caused no significant change of holding currents, increased NMDA-but not kainate-induced currents in superficial dorsal horn neurons. Our results suggest that exogenously applied PACAP sensitizes the dorsal horn neurons to formalin stimulation, and facilitates NMDA receptor-mediated nociceptive response. As a corollary, PACAP, which may be released from primary afferent fibers potentiates nociceptive transmission to the dorsal horn by interacting primarily with NMDA receptors.

The pituitary adenylate cyclase-activating polypeptide modulates glutamatergic calcium signalling: investigations on rat suprachiasmatic nucleus neurons

Circadian rhythms generated by the hypothalamic suprachiasmatic nucleus (SCN) are synchronized with the external light/dark cycle by photic information transmitted directly from the retina via the retinohypothalamic tract (RHT). The RHT contains the neurotransmitters glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP), which code chemically for 'light' or 'darkness' information, respectively. We investigated interactions of PACAP and glutamate by analysing effects on the second messenger calcium in individual SCN neurons using the Fura-2 technique. PACAP did not affect NMDA-mediated calcium increases, but influenced signalling cascades of non-NMDA glutamate receptors, which in turn can regulate NMDA receptors. On the one hand, PACAP amplified/induced glutamate-dependent calcium increases by interacting with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate signalling. This was not related to direct PACAPergic effects on the second messengers cAMP and calcium. On the other hand, PACAP reduced/inhibited calcium increases elicited by glutamate acting on metabotropic receptors. cAMP analogues mimicked this inhibition. Most neurons displaying PACAPergic neuromodulation were immunoreactive for vasoactive intestinal polypeptide, which is a marker for retinorecipient SCN neurons. The observed PACAPergic effects provide a broad range of interactions that allow a fine-tuning of the endogenous clock by the integration of 'light' and 'darkness' information on the level of single SCN neurons.

Differential effects of PACAP-38 on synaptic responses in rat hippocampal CA1 region

Pituitary adenylate cyclase-activating polypeptide (PACAP-38) is a member of the vasointestinal polypeptide (VIP)/secretin/glucagon family of neuropeptides for which neuroregulatory functions have been postulated. PACAP-38 receptors are expressed in different brain regions, including hippocampus. In this study, we examined the dose-dependent effects of PACAP-38 on the excitatory postsynaptic field potential (fEPSP) evoked at the Schaffer collateral-CA1 synapse in rat hippocampal slices. Bath application of low dose (0.05 nM) of PACAP-38 induced long-lasting facilitation of the fEPSP. This enhancement was blocked by the cholinergic receptor antagonist atropine and partially by the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV) and therefore, shares a common mechanism with LTP. In contrast, a high dose (1 microM) of PACAP-38 induced a persistent depression of the fEPSP that was not blocked by antagonists of cholinergic receptors (i.e., atropine and mecamylamine), adenosine receptors (i.e., DCPCX), or glutamatergic NMDA receptors (APV). Intermediate doses (0.1-0.5 microM) of PACAP-38 produced an initial decrease of the fEPSP followed by an enhancement. This decrease was not blocked by atropine whereas the facilitation was. These results show that PACAP-38 modulates CA1 synaptic transmission in a dose-dependent manner and that the peptide interacts with cholinergic and glutamatergic systems.

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.

Effects of pituitary adenylate cyclase activating polypeptide on lumbosacral preganglionic neurons in the neonatal rat spinal cord

The effects of PACAP-38 on phasic and tonic preganglionic neurons (PGN) in L6 and S1 spinal cord slices from neonatal rats (5--11 days old) were studied using the whole-cell patch clamp technique. PGN were identified by retrograde axonal transport of a fluorescent dye (Fast Blue, 5 microl of 4% solution) injected into the intraperitoneal space 3--7 days prior to the study. Bath application of pituitary adenylate cyclase activating polypeptide (PACAP) (20 nM) increased the frequency of spontaneous excitatory postsynaptic potentials (EPSPs) and spontaneous firing in both types of PGN. PACAP markedly increased the number (200--800%) and frequency of action potentials elicited by depolarizing current pulses in phasic PGN, but had a smaller effect on tonic PGN. PACAP decreased the threshold for action potential generation by approximately 25% in both types of neurons (e.g. -34.0+/-1.5 to -38.4+/-1.7 mV from a holding potential of -50 mV in phasic PGN, P<0.005). PACAP did not affect the duration of the action potential. The amplitude of the spike after hyperpolarization was not changed but the duration was significantly reduced by PACAP from 204.4+/-12.2 to 106.2+/-8.1 ms in tonic but not in phasic PGN. PACAP suppressed a transient outward current that was also suppressed by 4-aminopyridine (0.5 mM). These results coupled with the immunohistochemical identification of a dense collection of PACAP fibers in the region of the PGN, raises the possibility that PACAP may function as an excitatory transmitter in lumbosacral parasympathetic reflex pathways in the neonatal rat.

Ontogeny of PACAP immunoreactivity in extrinsic and intrinsic innervation of chicken gut

An immunohistochemical study was conducted on the ontogeny of pituitary adenylate cyclase-activating polypeptide-27 (PACAP) immunoreactive elements within the extrinsic and intrinsic nerve supply of the chicken embryo gut. The first PACAP-immunoreactivity was detected in the extrinsic nerve supply at E 4 within the pharyngeal region and the primary sympathetic chain. At E 5.5 it appeared in the vagus nerve, the spinal cord, the secondary sympathetic chain, some perivascular plexuses and the Remak ganglion. In the intrinsic nerve supply, the first PACAP-immunoreactive elements were shown at E 4.5-E 5 in the mesenchymal bud of the proventriculus/gizzard. Then they gradually appeared also cranially and caudally both in myenteric and submucous plexuses.

PACAP-38 enhances excitatory synaptic transmission in the rat hippocampal CA1 region

Specific receptors for pituitary adenylate cyclase-activating polypeptide (PACAP), a novel peptide with neuroregulatory and neurotrophic functions, have been identified recently in different brain regions, including the hippocampus. In this study, we examined the effects of PACAP-38 on the excitatory postsynaptic field potentials (fEPSPs) evoked at the Schaffer collateral-CA1 synapses. Brief bath application of PACAP-38 (0.05 nM) induced a long-lasting facilitation of the basal transmission. Enhancement of this response was occluded in part by previous high-frequency-induced long-term potentiation (LTP). PACAP-38 did not significantly alter the paired-pulse facilitation (PPF). PACAP-38 has been shown to have a presynaptic effect on the septohippocampal cholinergic terminals, which results in an increase in basal acetylcholine (ACh) release. To assess whether the PACAP-38 enhancement of CA1 synapses was related to the activation of the cholinergic system we examined the effect of this peptide in the presence of atropine, a muscarinic receptor antagonist. The enhancement of the fEPSPs by PACAP-38 was blocked by bath application of atropine. These results show that PACAP-38 induces facilitation of hippocampal synaptic transmission through activation of the cholinergic system via the muscarinic receptors.

Structure and distribution of the mRNAs encoding pituitary adenylate cyclase-activating polypeptide and growth hormone-releasing hormone-like peptide in the frog, Rana ridibunda

The structure of the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has been characterized in several species including protochordates, fish, amphibians, birds, and mammals. Although PACAP has been shown to stimulate frog pituitary and adrenal cell activity, the structure of the PACAP precursor and the expression of its gene have not yet been reported in any amphibian species. In this study, we have characterized two cDNA variants encoding PACAP of the frog Rana ridibunda, one of which encodes a second peptide exhibiting strong homologies to growth hormone-releasing hormone (GHRH) of fish and mammals. Northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analyses revealed that PACAP/GHRH-like peptide mRNAs are predominantly expressed in the brain and spinal cord and, to a lesser extent, in the neurointermediate lobe of the pituitary. Other tissues including the testis and the distal lobe of the pituitary do not express the PACAP precursor gene. The distribution of PACAP/GHRH-like peptide mRNAs in the frog brain has been determined by in situ hybridization histochemistry. High levels of expression were found in the accessory olfactory bulb, the distal pallium, the ventral part of the magnocellular preoptic nucleus, the ventral hypothalamic nucleus, the posterior tuberculum, and the ventral habenular nucleus. These data contribute to the understanding of the evolution of the PACAP and GHRH genes in vertebrates and provide the anatomical bases to elucidate the roles of PACAP and the GHRH-like peptide in amphibians.

Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuron-derived peptide regulating glial glutamate transport and metabolism

In the brain, glutamatergic neurotransmission is terminated predominantly by the rapid uptake of synaptically released glutamate into astrocytes through the Na(+)-dependent glutamate transporters GLT-1 and GLAST and its subsequent conversion into glutamine by the enzyme glutamine synthetase (GS). To date, several factors have been identified that rapidly alter glial glutamate uptake by post-translational modification of glutamate transporters. The only condition known to affect the expression of glial glutamate transporters and GS is the coculturing of glia with neurons. We now demonstrate that neurons regulate glial glutamate turnover via pituitary adenylate cyclase-activating polypeptide (PACAP). In the cerebral cortex PACAP is synthesized by neurons and acts on the subpopulation of astroglia involved in glutamate turnover. Exposure of astroglia to PACAP increased the maximal velocity of [(3)H]glutamate uptake by promoting the expression of GLT-1, GLAST, and GS. Moreover, the stimulatory effects of neuron-conditioned medium on glial glutamate transporter expression were attenuated in the presence of PACAP-inactivating antibodies or the PACAP receptor antagonist PACAP 6-38. In contrast to PACAP, vasoactive intestinal peptide promoted glutamate transporter expression only at distinctly higher concentrations, suggesting that PACAP exerts its effects on glial glutamate turnover via PAC1 receptors. Although PAC1 receptor-dependent activation of protein kinase A (PKA) was sufficient to promote the expression of GLAST, the activation of both PKA and protein kinase C (PKC) was required to promote GLT-1 expression optimally. Given the existence of various PAC1 receptor isoforms that activate PKA and PKC to different levels, these findings point to a complex mechanism by which PACAP regulates glial glutamate transport and metabolism. Disturbances of these regulatory mechanisms could represent a major cause for glutamate-associated neurological and psychiatric disorders.

Pituitary adenylate cyclase activating peptide phase shifts circadian rhythms in a manner similar to light

The endogenous circadian pacemaker in mammals is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Various cues can reset circadian rhythm phase, thereby entraining the internal rhythm to the environmental cycle, and these effects can be investigated using an in vitro method to measure phase shifts of the SCN. Although pituitary adenylate cyclase activating peptide (PACAP) is localized in retinal inputs to the SCN, it has been reported to alter clock phase only during the subjective day (Hannibal et al., 1997), whereas light alters phase only in the subjective night. In this study we show that PACAP can reset the clock in the photic pattern during the subjective night when applied in 10 pM to 1 nM doses. This appears to be mediated via a glutamatergic mechanism, possibly by potentiation of NMDA currents as is seen at 10-100 pM. Given at higher doses (>10 nM), PACAP shifts in the subjective day, apparently via activation of adenylate cyclase and increased intracellular cAMP. These results indicate dose and phase specificity of the effects of PACAP, and a new role as a transmitter in the retinohypothalamic tract.

VIP and PACAP: very important in pain?

Neuropathic pain arising from direct trauma to, or compression injury of, peripheral nerves is a common clinical problem. It is characterized by the development of abnormal pain states (spontaneous pain, hyperalgesia, allodynia), which can persist long after the initial injury has resolved. The underlying mechanisms are poorly understood and, as a consequence, treatment is often unsatisfactory. Some of the main contributing factors are thought to be the morphological and phenotypic changes that occur centrally, including alterations in the expression of neurotransmitters and their associated receptors, both in the dorsal root ganglia and in the spinal dorsal horn. This article focuses on the functional role of the two structurally related peptides VIP and PACAP within the spinal cord, and their possible contribution to the altered transmission of sensory information in neuropathic conditions.

Control of voltage-independent zinc inhibition of NMDA receptors by the NR1 subunit

Zinc inhibits NMDA receptor function through both voltage-dependent and voltage-independent mechanisms. In this report we have investigated the role that the NR1 subunit plays in voltage-independent Zn2+ inhibition. Our data show that inclusion of exon 5 into the NR1 subunit increases the IC50 for voltage-independent Zn2+ inhibition from 3-fold to 10-fold when full length exon 22 is also spliced into the mature NR1 transcript and the NMDA receptor complex contains the NR2A or NR2B subunits; exon 5 has little effect on Zn2+ inhibition of receptors that contain NR2C and NR2D. Mutagenesis within exon 5 indicates that the same residues that control proton inhibition, including Lys211, also control the effects of exon 5 on Zn2+ inhibition. Amino acid exchanges within the NR1 subunit but outside exon 5 (E181Q, E339Q, E342Q, N616R, N616Q, D669N, D669E, C744A, and C798A) that are known to decrease the pH sensitivity also decrease the Zn2+ sensitivity, and concentrations of spermine that relieve tonic proton inhibition also relieve Zn2+ inhibition. In summary, our results define the subunit composition of Zn2+-sensitive NMDA receptors and provide evidence for structural convergence of three allosteric regulators of receptor function: protons, polyamines, and Zn2+.