Peptidergic modulation of synaptic transmission in the parabrachial nucleus in vitro: importance of degradative enzymes in regulating synaptic efficacy

Opioids Induce Bidirectional Synaptic Plasticity in a Brainstem Pain Center in the Rat

Opioids are powerful analgesics commonly used in pain management. However, opioids can induce complex neuroadaptations, including synaptic plasticity, that ultimately drive severe side effects, such as pain hypersensitivity and strong aversion during prolonged administration or upon drug withdrawal, even following a single, brief administration. The lateral parabrachial nucleus (LPBN) in the brainstem plays a key role in pain and emotional processing; yet, the effects of opioids on synaptic plasticity in this area remain unexplored. Using patch-clamp recordings in acute brainstem slices from male and female Sprague Dawley rats, we demonstrate a concentration-dependent, bimodal effect of opioids on excitatory synaptic transmission in the LPBN. While a lower concentration of DAMGO (0.5 µM) induced a long-term depression of synaptic strength (low-DAMGO LTD), abrupt termination of a higher concentration (10 µM) induced a long-term potentiation (high-DAMGO LTP) in a subpopulation of cells. LTD involved a metabotropic glutamate receptor (mGluR)-dependent mechanism; in contrast, LTP required astrocytes and N-methyl-D-aspartate receptor (NMDAR) activation. Selective optogenetic activation of spinal and periaqueductal gray matter (PAG) inputs to the LPBN revealed that, while LTD was expressed at all parabrachial synapses tested, LTP was restricted to spino-parabrachial synapses. Thus, we uncovered previously unknown forms of opioid-induced long-term plasticity in the parabrachial nucleus that potentially modulate some adverse effects of opioids. PERSPECTIVE: We found a previously unrecognized site of opioid-induced plasticity in the lateral parabrachial nucleus, a key region for pain and emotional processing. Unraveling opioid-induced adaptations in parabrachial function might facilitate the identification of new therapeutic measures for addressing adverse effects of opioid discontinuation such as hyperalgesia and aversion.

Neuropeptide Y modulates excitatory synaptic transmission and promotes social behavior in the mouse nucleus accumbens

Social interactions define the human experience, but these integral behaviors are disrupted in many psychiatric disorders. Social behaviors have evolved over millennia, and neuromodulatory systems that promote social behavior in invertebrates are also present in mammalian brains. One such conserved neuromodulator, neuropeptide Y (NPY), acts through several receptors including the Y1r, Y2r, and Y5r. These receptors are present in brain regions that control social behavior, including the nucleus accumbens (NAc). However, whether NPY modulates NAc neurotransmission is unknown. Using whole-cell patch-clamp electrophysiology of NAc neurons, we find that multiple NPY receptors regulate excitatory synaptic transmission in a cell-type specific manner. At excitatory synapses onto D1+ MSNs, Y1r activity enhances transmission while Y2r suppresses transmission. At excitatory synapses onto D1- MSNs, Y5r activity enhances transmission while Y2r suppresses transmission. Directly infusing NPY or the Y1r agonist [Leu³¹, Pro³⁴]-NPY into the NAc significantly increases social interaction with an unfamiliar conspecific. Inhibition of an enzyme that breaks down NPY, dipeptidyl peptidase IV (DPP-IV), shifts the effect of NPY on D1+ MSNs to a Y1r dominated phenotype. Together, these results increase our understanding of how NPY regulates neurotransmission in the NAc and identify a novel mechanism underlying the control of social behavior. Further, they reveal a potential strategy to shift NPY signaling for therapeutic gain.

Ultra-Low Frequency Transcutaneous Electrical Nerve Stimulation on Pain Modulation in a Rat Model with Myogenous Temporomandibular Dysfunction

Masticatory myofascial pain (MMP) is one of the most common causes of chronic orofacial pain in patients with temporomandibular disorders. To explore the antinociceptive effects of ultra-low frequency transcutaneous electrical nerve stimulation (ULF-TENS) on alterations of pain-related biochemicals, electrophysiology and jaw-opening movement in an animal model with MMP, a total of 40 rats were randomly and equally assigned to four groups; i.e., animals with MMP receiving either ULF-TENS or sham treatment, as well as those with sham-MMP receiving either ULF-TENS or sham treatment. MMP was induced by electrically stimulated repetitive tetanic contraction of masticatory muscle for 14 days. ULF-TENS was then performed at myofascial trigger points of masticatory muscles for seven days. Measurable outcomes included maximum jaw-opening distance, prevalence of endplate noise (EPN), and immunohistochemistry for substance P (SP) and μ-opiate receptors (MOR) in parabrachial nucleus and c-Fos in rostral ventromedial medulla. There were significant improvements in maximum jaw-opening distance and EPN prevalence after ULF-TENS in animals with MMP. ULF-TENS also significantly reduced SP overexpression, increased MOR expression in parabrachial nucleus, and increased c-Fos expression in rostral ventromedial medulla. ULF-TENS may represent a novel and applicable therapeutic approach for improvement of orofacial pain induced by MMP.

Increased substance P-like immunoreactivities in parabrachial and amygdaloid nuclei in a rat model with masticatory myofascial pain

This study explores the involvement of substance P (SP) in the parabrachial nucleus (PBN) and central amygdaloid nucleus (CeA) in the nociception-emotion link and of rats with masticatory myofascial pain (MMP) induced by chronic tetanic eccentric muscle contraction. A total of 18 rats were randomly and equally assigned for MMP (MMP group) and sham-MMP induction (sMMP group). MMP was induced by electrical-stimulated repetitive tetanic eccentric contraction of the masseter muscle for 14 consecutive days. Myofascial trigger points in the masseter muscle were identified by palpable taut bands, increased prevalence of endplate noise (EPN), focal hypoechoic nodules on ultrasound and restricted jaw opening. All animals were killed for morphological and SP immunohistochemical analyses. Chronic tetanic eccentric contraction induced significantly thicker masseter muscle confirmed by hypoechogenicity, increased prevalence and amplitudes of EPN, and limited jaw opening. Immunohistochemically, the SP-like positive neurons increased significantly in PBN and CeA of the MMP group. Our results suggested that MMP increases the SP protein levels in PBN and CeA, which play important roles in MMP-mediated chronic pain processing as well as MMP-related emotional processes.

Adverse effects of erenumab on cerebral proliferative angiopathy: A case report

BACKGROUND

Cerebral proliferative angiopathy is a vascular malformation associated with compromised blood-brain barrier and with migraine-like headache. Treating blood-brain barrier-compromised patients with erenumab, an anti-calcitonin gene-related peptide receptor monoclonal antibody, may be risky.

CASE

We describe a case of a 22-year-old chronic migraine patient with cerebral proliferative angiopathy who presented to our hospital in status epilepticus 2 d after his first dose of erenumab. Serial magnetic resonance imaging (MRI) studies demonstrated progressive areas of diffusion restriction including the brain tissue adjacent to the cerebral proliferative angiopathy, bilateral white matter and hippocampi. His 6-month post-presentation magnetic resonance imaging was notable for white matter injury, encephalomalacia surrounding cerebral proliferative angiopathy and bilateral hippocampal sclerosis. He remains clinically affected with residual symptoms, including refractory epilepsy and cognitive deficits.

CONCLUSION

The evidence presented in this case supports further investigation into potential deleterious side effects of erenumab in patients with compromised blood-brain barrier, such as individuals with intracranial vascular malformations.

Functional consequences of neuropeptide and small-molecule co-transmission

Colocalization of small-molecule and neuropeptide transmitters is common throughout the nervous system of all animals. The resulting co-transmission, which provides conjoint ionotropic ('classical') and metabotropic ('modulatory') actions, includes neuropeptide- specific aspects that are qualitatively different from those that result from metabotropic actions of small-molecule transmitter release. Here, we focus on the flexibility afforded to microcircuits by such co-transmission, using examples from various nervous systems. Insights from such studies indicate that co-transmission mediated even by a single neuron can configure microcircuit activity via an array of contributing mechanisms, operating on multiple timescales, to enhance both behavioural flexibility and robustness.

Interaction of calcitonin gene related peptide (CGRP) and substance P (SP) in human skin

Calcitonin gene related peptide (CGRP) and substance P (SP) are neuropeptides that are simultaneously released from nociceptive C-fibers. CGRP is a potent vasodilator, inducing a long-lasting increase in superficial skin blood flow, whereas SP induces only a brief vasodilation but a significant plasma extravasation. CGRP and SP may play important roles in the pathophysiology of various pain states but little is known about their interaction. Different concentrations of SP (ranging from 10^-5M to 10^-9M) were applied to the volar forearm of 24 healthy subjects via dermal microdialysis. SP was applied either alone or in combination with CGRP10^-9M and CGRP 10^-6M. As expected, SP induced a transient increase in skin blood flow that decayed shortly after application. This transient blood flow peak was blunted with co-application of CGRP 10^-9M and inhibited with co-application of CGRP10^-6M. SP alone induced plasma protein extravasation (PPE). However, when CGRP10^-6M was added, the PPE significantly increased. Our results demonstrate a complex interaction of the neuropeptides CGRP and SP. CGRP10^-6M prevented SP-induced early vasodilation but augmented SP-induced PPE. These interactions might explain why vascular symptoms in chronic pain can differ strikingly between individuals.

Anatomical Organization of Multiple Modulatory Inputs in a Rhythmic Motor System

In rhythmic motor systems, descending projection neuron inputs elicit distinct outputs from their target central pattern generator (CPG) circuits. Projection neuron activity is regulated by sensory inputs and inputs from other regions of the nervous system, relaying information about the current status of an organism. To gain insight into the organization of multiple inputs targeting a projection neuron, we used the identified neuron MCN1 in the stomatogastric nervous system of the crab, Cancer borealis. MCN1 originates in the commissural ganglion and projects to the stomatogastric ganglion (STG). MCN1 activity is differentially regulated by multiple inputs including neuroendocrine (POC) and proprioceptive (GPR) neurons, to elicit distinct outputs from CPG circuits in the STG. We asked whether these defined inputs are compact and spatially segregated or dispersed and overlapping relative to their target projection neuron. Immunocytochemical labeling, intracellular dye injection and three-dimensional (3D) confocal microscopy revealed overlap of MCN1 neurites and POC and GPR terminals. The POC neuron terminals form a defined neuroendocrine organ (anterior commissural organ: ACO) that utilizes peptidergic paracrine signaling to act on MCN1. The MCN1 arborization consistently coincided with the ACO structure, despite morphological variation between preparations. Contrary to a previous 2D study, our 3D analysis revealed that GPR axons did not terminate in a compact bundle, but arborized more extensively near MCN1, arguing against sparse connectivity of GPR onto MCN1. Consistent innervation patterns suggest that integration of the sensory GPR and peptidergic POC inputs occur through more distributed and more tightly constrained anatomical interactions with their common modulatory projection neuron target than anticipated.

Activation of the TRPV1 channel attenuates N-methyl-D-aspartic acid-induced neuronal injury in the rat retina

Capsaicin, a transient receptor potential vanilloid type1 (TRPV1) agonist, has been reported to protect against ischemia-reperfusion injury in various organs, including the brain, heart, and kidney, whereas activation of TRPV1 was also reported to contribute to neurodegeneration, including pressure-induced retinal ganglion cell death in vitro. We histologically investigated the effects of capsaicin and SA13353, TRPV1 agonists, on retinal injury induced by intravitreal N-methyl-d-aspartic acid (NMDA; 200 nmol/eye) in rats in vivo. Under ketamine/xylazine anesthesia, male Sprague-Dawley rats were subjected to intravitreal NMDA injection. Capsaicin (5.0 nmol/eye) was intravitreally admianeously with NMDA injection. SA13353 (10mg/kg) was intraperitoneally administered 15 min before NMDA injection. Morphometric evaluation at 7 days after NMDA injection showed that intravitreal NMDA injection resulted in ganglion cell loss. Capsaicin and SA13353 almost completely prevented this damage. Treatment with capsazepine (TRPV1 antagonist, 0.5 nmol/eye), CGRP (8-37) (calcitonin gene-related peptide (CGRP) receptor antagonist, 0.5 pmol/eye), or RP67580 (tachykinin NK1 receptor antagonist, 0.5 nmol/eye) almost completely negated the protective effect of capsaicin in the NMDA-injected rats. Seven days after intravitreal NMDA injection, the cell number of retinal ganglion cell was significantly smaller than in the eye that had received capsaicin in B6.Cg-TgN(Thy1-CFP)23Jrs/J transgenic mice that express the enhanced cyan fluorescent protein in retinal ganglion cells in the retina. These results suggested that activation of TRPV1 protects retinal neurons from the injury induced by intravitreal NMDA in rats in vivo. Activation of CGRP and tachykinin NK1 receptors is possibly involved in underlying protective mechanisms.

Neuropeptide transmission in brain circuits

Neuropeptides are found in many mammalian CNS neurons where they play key roles in modulating neuronal activity. In contrast to amino acid transmitter release at the synapse, neuropeptide release is not restricted to the synaptic specialization, and after release, a neuropeptide may diffuse some distance to exert its action through a G protein-coupled receptor. Some neuropeptides such as hypocretin/orexin are synthesized only in single regions of the brain, and the neurons releasing these peptides probably have similar functional roles. Other peptides such as neuropeptide Y (NPY) are synthesized throughout the brain, and neurons that synthesize the peptide in one region have no anatomical or functional connection with NPY neurons in other brain regions. Here, I review converging data revealing a complex interaction between slow-acting neuromodulator peptides and fast-acting amino acid transmitters in the control of energy homeostasis, drug addiction, mood and motivation, sleep-wake states, and neuroendocrine regulation.

Neuronal sodium leak channel is responsible for the detection of sodium in the rat median preoptic nucleus

Sodium (Na(+)) ions are of primary importance for hydromineral and cardiovascular homeostasis, and the level of Na(+) in the body fluid compartments [plasma and cerebrospinal fluid (CSF)] is precisely monitored in the hypothalamus. Glial cells seem to play a critical role in the mechanism of Na(+) detection. However, the precise role of neurons in the detection of extracellular Na(+) concentration ([Na(+)](out)) remains unclear. Here we demonstrate that neurons of the median preoptic nucleus (MnPO), a structure in close contact with the CSF, are specific Na(+) sensors. Electrophysiological recordings were performed on dissociated rat MnPO neurons under isotonic [Na(+)] (100 mM NaCl) with local application of hypernatriuric (150, 180 mM NaCl) or hyponatriuric (50 mM NaCl) external solution. The hyper- and hyponatriuric conditions triggered an in- and an outward current, respectively. The reversal potential of the current matched the equilibrium potential of Na(+), indicating that a change in [Na(+)](out) modified the influx of Na(+) in the MnPO neurons. The conductance of the Na(+) current was not affected by either the membrane potential or the [Na(+)](out). Moreover, the channel was highly selective for lithium over guanidinium. Together, these data identified the channel as a Na(+) leak channel. A high correlation between the electrophysiological recordings and immunofluorescent labeling for the Na(X) channel in dissociated MnPO neurons strongly supports this channel as a candidate for the Na(+) leak channel responsible for the Na(+)-sensing ability of rat MnPO neurons. The absence of Na(X) labeling and of a specific current evoked by a change in [Na(+)](out) in mouse MnPO neurons suggests species specificity in the hypothalamus structures participating in central Na(+) detection.

Abnormal response of dopaminergic neurons to nicotine without perturbation of nicotinic receptors in alphaCGRP knock-out mice

Alpha-calcitonin gene-related peptide (alphaCGRP) is a neuropeptide with multiple biological properties, including the regulation of nicotinic acetylcholine receptors (nAChRs). We have previously reported a reduction of somatic withdrawal symptoms in alphaCGRP knock-out mice exposed to chronic nicotine, leading us to investigate the contribution of alphaCGRP to the regulations of ventral tegmental area (VTA) neurons and their response to nicotine. The electrophysiological activity of VTA dopaminergic (DA) neurons was recorded in vivo, under anesthesia. These neurons displayed identical spontaneous electrophysiogical activities in wild-type and alphaCGRP-/- mice. However, we found that intravenous administration of nicotine (30 microg/kg) had no significant effect on the activity of DA neurons in alphaCGRP-/- mice, whereas it induced a doubling of the firing rate in wild-type animals. A higher dose (90 microg/kg) produced a significant excitation in both strains, but this effect remained smaller in the mutants. To investigate this difference, we have studied the functional state of nAChRs in wild-type and alphaCGRP-/- mice. Both strains exhibited identical expression of alpha(7) and alpha(4)beta(2) nAChRs as revealed by autoradiographical studies in the VTA. In addition, focal application of acetylcholine on DA neurons recorded by patch-clamp revealed identical currents mediated by nAChRs in mutant animals, as compared to wild-type mice. These data outline the possibility of a contribution of alphaCGRP to the effects of nicotine on DA neurons, by a physiological pathway independent of VTA nicotinic receptors.

Presynaptic inhibition of glutamate release by neuropeptides: use-dependent synaptic modification

Neuropeptides are signaling molecules that interact with G-protein coupled receptors located both pre- and postsynaptically. Presynaptically, these receptors are localized in axons and terminals away from presynaptic specializations. Neuropeptides are stored in dense core vesicles that are distinct from the clear synaptic vesicles containing classic neurotransmitters such as glutamate and GABA. Because they require a stronger Ca(2+) signal than synaptic vesicles, dense core vesicles do not release neuropeptides with single action potentials but rather require high-frequency trains. Thus, neuropeptides only modulate strongly stimulated synapses, providing negative or positive feedback. Many neuropeptides have been found to inhibit glutamate release from presynaptic terminals, and the major mechanism is likely direct interaction of betagamma G-protein subunits with presynaptic proteins such as SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). The use of mouse genetic models and specific receptor antagonists are beginning to unravel the function of inhibitory neuropeptides. The opioid receptors kappa and mu, which are activated by endogenous opioid peptides such as dynorphin, enkephalin, and possibly the endomorphins, are important in modulating pain transmission. Dynorphin, nociceptin/orphanin FQ, and somatostatin and its related peptide cortistatin appear to play a role in modulation of learning and memory. Neuropeptide Y has important functions in ingestive behavior and also in entraining circadian rhythms. The existence of neuropeptides greatly expands the computational ability of the brain by providing additional levels of modulation.

Dendritic domains of nociceptive-responsive parabrachial neurons match terminal fields of lamina I neurons in the rat

This study investigates, in the anesthetized rat, the dendritic extent of parabrachial (PB) neurons whose nociceptive response to noxious stimuli has been previously recorded with an extracellular micropipette. The PB neurons were then injected with biocytin through the recording micropipette, via a juxtacellular technique. The dendritic arborization of individual PB neurons was carefully compared with the projections of medullary (trigeminal) and spinal lamina I neurons. The latter projections were labeled in separate animals that received injections of Phaseolus vulgaris-leucoagglutinin restricted to the superficial layers of spinal or medullary dorsal horn. We report here that: 1) PB neurons excited chiefly by noxious stimulation of the face have their dendritic tree located primarily within the field of lamina I trigeminal projections, i.e., in the caudal portion of PB area, around the external medial and the caudal part of the external lateral subnuclei; and 2) PB neurons excited chiefly by noxious stimulation of the paw or the tail have their dendritic tree located primarily within the field of lamina I spinal projections, i.e., in PB mid-extent, around the borderline between the external lateral and both the lateral crescent and the superior lateral subnuclei. Our results suggest the presence of an extensive excitatory axodendritic link between lamina I projections and PB nociceptive neurons around the lateral crescent and the external medial subnuclei. These findings strengthen the possibility of involvement of a subgroup of PB neurons in nociceptive processes.

Vasopressin differentially modulates non-NMDA receptors in vasopressin and oxytocin neurons in the supraoptic nucleus

Magnocellular neurons of the supraoptic nucleus release the neuropeptides oxytocin and vasopressin from their dendrites to regulate their synaptic inputs. This study aims to determine the cellular mechanism by which vasopressin modulates excitatory synaptic transmission. Presumably by electroporation through perforated patch, we were able to successfully introduce biocytin into cells in which we performed an electrophysiological study. This method enabled us to determine that roughly half of the recorded neurons were immunoreactive to oxytocin-associated neurophysin and showed two characteristic features: an inward rectification and a sustained outward rectification. The remaining half showed a linear voltage-current relationship and was immunoreactive to vasopressin-associated neurophysin. Using these electrophysiological characteristics and post hoc immunohistochemistry to identify vasopressin or oxytocin neurons, we found that vasopressin decreased evoked EPSCs in vasopressin neurons while increasing EPSCs in oxytocin neurons. In both types of neurons, EPSC decay constants were not affected, indicating that desensitization of non-NMDA receptors did not underlie the EPSC amplitude change. In vasopressin neurons, both vasopressin and a V1a receptor agonist, F-180, decreased AMPA-induced currents, an effect blocked by a V1a receptor antagonist SR49059. In oxytocin neurons, AMPA-induced currents were facilitated by vasopressin, whereas F-180 had no effect. An oxytocin receptor antagonist blocked the facilitatory effect of vasopressin. Thus, we conclude that vasopressin inhibits EPSCs in vasopressin neurons via postsynaptic V1a receptors, whereas it facilitates EPSCs in oxytocin neurons through oxytocin receptors.

Calcitonin gene-related peptide regulates expression of neurokinin1 receptors by rat spinal neurons

Although neurokinin 1 (NK1) receptors contribute to hyperalgesia, and their expression is increased in the spinal cord during peripheral inflammation, little is known regarding the signaling molecules and the second messenger pathways that they activate in regulating the expression of the NK1 receptor gene. Because the promoter region of the NK1 receptor contains a cAMP response element (CRE), we tested the hypothesis that calcitonin gene-related peptide (CGRP) regulates the expression of NK1 receptors via a pathway involving activation of the transcription factor cAMP response element binding protein (CREB). Experiments were conducted on primary cultures of neonatal rat spinal neurons. Treatment of cultures with CGRP for 8-24 hr increased (125)I-substance P binding on spinal neurons; the increase in binding was preceded by an elevation in NK1 receptor mRNA. The CGRP-induced change in (125)I-substance P binding was concentration-dependent and was inhibited by the antagonist CGRP(8-37). CGRP increased phosphorylated CREB immunoreactivity and CRE-dependent transcription in neurons, indicating the involvement of the transcription factor CREB. Evidence that CGRP increased cAMP levels in spinal neurons and that the protein kinase A inhibitor H89 attenuated CGRP-induced CRE-dependent transcription suggests that the intracellular pathway stimulated by CGRP leads to activation of protein kinase A. Collectively these data define a role for CGRP as a signaling molecule that induces expression of NK1 receptors in spinal neurons. The data provide evidence that a neuropeptide receptor controls gene expression in the CNS and add another dimension to understanding the cotransmission of substance P and CGRP by primary afferent neurons.

Cellular basis for the effects of substance P in the periaqueductal gray and dorsal raphe nucleus

Substance P (SP) is known to act at supraspinal sites to influence pain sensitivity as well as to promote anxiety. The effects of SP could be mediated in part by actions in the periaqueductal gray (PAG) and the dorsal raphe nucleus (DRN), adjoining mesencephalic cell groups that are strategically positioned to influence both nociception and mood. Previous studies have indicated that SP regulates both enkephalin and serotonin neurotransmission in these brain regions. To determine the mechanism underlying the effects of SP in the PAG and DRN, the distribution of the principal receptor for SP, the neurokinin 1 (NK1) receptor, was examined with respect to other neurotransmitter markers. PAG neurons that had NK1 receptor immunolabeling were interdigitated with and received contacts from enkephalin-containing neurons. However, only a few (16/144; 11%) neurons with NK1 receptor also contained enkephalin immunoreactivity after colchicine treatment. In the DRN, dendrites containing NK1 receptor were selectively distributed in the dorsomedial subdivision. The majority (132/137; 96%) of these dendrites did not contain immunoreactivity for the serotonin-synthesizing enzyme tryptophan hydroxylase. In contrast, neuronal profiles with NK1 receptor in both the PAG and the DRN often contained immunolabeling for glutamate. Light and electron microscopic examination revealed that 48-65% of cell bodies and dendrites with NK1 receptor were dually immunolabeled for glutamate. These data suggest that SP directly acts primarily on glutamatergic neurons in the PAG and DRN. To a lesser extent, enkephalin-containing neurons may be targeted. Through these actions, it may subsequently influence activity of larger populations of neurons containing enkephalin as well as serotonin. This circuitry could contribute to, as well as coordinate, effects of SP on pain perception and mood.

Decrease of substance P in the parabrachial nucleus of multiple system atrophy

In the parabrachial nucleus (PBN), which has been known to play an important regulating role for various autonomic functions, many projecting nerve fiber terminals containing substance P (SP) from the nucleus of the solitary tract (NTS) and other areas are found and effect a modulatory influence on the transmission in the PBN. Postmortem brains were obtained from four multiple system atrophy (MSA) patients with autonomic failure and four control patients without any nervous disease, and an immunohistochemical staining for SP was performed on serial 10-microm-thick sections from paraffin-embedded pons including the PBN after immersion fixation in 10% formalin. In the PBN of all MSA patients, a marked decrease in SP-like immunoreactive (SPLI) nerve fiber terminals was revealed compared with the controls. In addition, an obvious astrocytosis was found in the PBN by simultaneous histopathological evaluation, for the preservation of neurons themselves. Therefore, the projecting SP pathway to the PBN may also be primarily involved in the pathophysiological mechanism of the autonomic failure of MSA patients.

Projection neurons with shared cotransmitters elicit different motor patterns from the same neural circuit

Specificity in the actions of different modulatory neurons is often attributed to their having distinct cotransmitter complements. We are assessing the validity of this hypothesis with the stomatogastric nervous system of the crab Cancer borealis. In this nervous system, the stomatogastric ganglion (STG) contains a multifunctional network that generates the gastric mill and pyloric rhythms. Two identified projection neurons [modulatory proctolin neuron (MPN) and modulatory commissural neuron 1 (MCN1)] that innervate the STG and modulate these rhythms contain GABA and the pentapeptide proctolin, but only MCN1 contains Cancer borealis tachykinin-related peptide (CabTRP Ia). Selective activation of each projection neuron elicits different rhythms from the STG. MPN elicits only a pyloric rhythm, whereas MCN1 elicits a distinct pyloric rhythm as well as a gastric mill rhythm. We tested the degree to which CabTRP Ia distinguishes the actions of MCN1 and MPN. To this end, we used the tachykinin receptor antagonist Spantide I to eliminate the actions of CabTRP Ia. With Spantide I present, MCN1 no longer elicited the gastric mill rhythm and the resulting pyloric rhythm was changed. Although this rhythm was more similar to the MPN-elicited pyloric rhythm, these rhythms remained different. Thus, CabTRP Ia partially confers the differences in rhythm generation resulting from MPN versus MCN1 activation. This result suggests that different projection neurons may use the same cotransmitters differently to elicit distinct pyloric rhythms. It also supports the hypothesis that different projection neurons use a combination of strategies, including using distinct cotransmitter complements, to elicit different outputs from the same neuronal network.

Vasopressin and amastatin induce V(1)-receptor-mediated suppression of excitatory transmission in the rat parabrachial nucleus

We examined actions of arginine vasopressin (AVP) and amastatin (an inhibitor of the aminopeptidase that cleaves AVP) on synaptic currents in slices of rat parabrachial nucleus using the nystatin-perforated patch recording technique. AVP reversibly decreased the amplitude of the evoked, glutamate-mediated, excitatory postsynaptic current (EPSC) with an increase in paired-pulse ratio. No apparent changes in postsynaptic membrane properties were revealed by ramp protocols, and the inward current induced by a brief application of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid was unchanged after AVP. The reduction induced by 1 microM AVP could be blocked by a V(1) AVP receptor antagonist, [d(CH(2))(5)(1)-O-Me-Tyr(2)-Arg(8)]-vasopressin (Manning compound, 10 microM). Bath application of an aminopeptidase inhibitor, amastatin (10 microM), reduced the evoked EPSC, and AVP induced further synaptic depression in the presence of amastatin. Amastatin's effects also could be antagonized by the Manning compound. Corticotropin-releasing hormone slightly increased the EPSC at 1 microM, and coapplication with AVP attenuated the AVP response. Pretreatment of slices with 1 microg/ml cholera toxin or 0.5 microg/ml pertussis toxin for 20 h did not significantly affect AVP's synaptic action. The results suggest that AVP has suppressant effects on glutamatergic transmission by acting at V(1) AVP receptors, possibly through a presynaptic mechanism involving a pertussis-toxin- and cholera-toxin-resistant pathway.

Different proctolin neurons elicit distinct motor patterns from a multifunctional neuronal network

Distinct motor patterns are selected from a multifunctional neuronal network by activation of different modulatory projection neurons. Subsets of these projection neurons can contain the same neuromodulator(s), yet little is known about the relative influence of such neurons on network activity. We have addressed this issue in the stomatogastric nervous system of the crab Cancer borealis. Within this system, there is a neuronal network in the stomatogastric ganglion (STG) that produces many versions of the pyloric and gastric mill rhythms. These different rhythms result from activation of different projection neurons that innervate the STG from neighboring ganglia and modulate STG network activity. Three pairs of these projection neurons contain the neuropeptide proctolin. These include the previously identified modulatory proctolin neuron and modulatory commissural neuron 1 (MCN1) and the newly identified modulatory commissural neuron 7 (MCN7). We document here that each of these neurons contains a unique complement of cotransmitters and that each of these neurons elicits a distinct version of the pyloric motor pattern. Moreover, only one of them (MCN1) also elicits a gastric mill rhythm. The MCN7-elicited pyloric rhythm includes a pivotal switch by one STG network neuron from playing a minor to a major role in motor pattern generation. Therefore, modulatory neurons that share a peptide transmitter can elicit distinct motor patterns from a common target network.

Cloning and characterization of Aplysia neutral endopeptidase, a metallo-endopeptidase involved in the extracellular metabolism of neuropeptides in Aplysia californica

Cell surface metallo-endopeptidases play important roles in cell communication by controlling the levels of bioactive peptides around peptide receptors. To understand the relative relevance of these enzymes in the CNS, we characterized a metallo-endopeptidase in the CNS of Aplysia californica, whose peptidergic pathways are well described at the molecular, cellular, and physiological levels. The membrane-bound activity cleaved Leu-enkephalin at the Gly3-Phe4 bond with an inhibitor profile similar to that of the mammalian neutral endopeptidase (NEP). This functional homology was supported by the molecular cloning of cDNAs from the CNS, which demonstrated that the Aplysia and mammalian NEPs share all the same amino acids that are essential for the enzymatic activity. The protein is recognized both by specific anti-Aplysia NEP (apNEP) antibodies and by the [125I]-labeled NEP-specific inhibitor RB104, demonstrating that the apNEP gene codes for the RB104-binding protein. In situ hybridization experiments on sections of the ganglia of the CNS revealed that apNEP is expressed in neurons and that the mRNA is present both in the cell bodies and in neurites that travel along the neuropil and peripheral nerves. When incubated in the presence of a specific NEP inhibitor, many neurons of the buccal ganglion showed a greatly prolonged physiological response to stimulation, suggesting that NEP-like metallo-endopeptidases may play a critical role in the regulation of the feeding behavior in Aplysia. One of the putative targets of apNEP in this behavior is the small cardioactive peptide, as suggested by RP-HPLC experiments. More generally, the presence of apNEP in the CNS and periphery may indicate that it could play a major role in the modulation of synaptic transmission in Aplysia and in the metabolism of neuropeptides close to their point of release.

Dopamine depresses glutamatergic synaptic transmission in the rat parabrachial nucleus in vitro

Nystatin-perforated patch recordings were made from rat parabrachial neurons in an in vitro slice preparation to examine the effect of dopamine on parabrachial cells and on excitatory synaptic transmission in this nucleus. In current clamp mode, dopamine reduced the amplitude of the evoked excitatory postsynaptic potential without significant change in membrane potential. In cells voltage-clamped at -65 mV, dopamine dose dependently and reversibly decreased evoked, pharmacologically isolated, excitatory postsynaptic currents with an EC50 of 31 microM. The reduction in excitatory postsynaptic current was accompanied by an increase in paired pulse ratio (a protocol used to detect presynaptic site of action) with no change in the holding current or in the decay of the evoked excitatory postsynaptic currents. In addition, dopamine altered neither postsynaptic (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate-induced currents, nor steady-state current voltage curves. Miniature excitatory postsynaptic current analysis revealed that dopamine caused a rightward shift of the frequency-distribution curve with no change in the amplitude-distribution curve, which is consistent with a presynaptic mechanism. The dopamine-induced attenuation of the excitatory postsynaptic current was almost completely blocked by the D1-like receptor antagonist SCH23390 (10 microM), although the D2-like antagonist sulpiride (10 microM) also partially blocked it. Combined application of both antagonists blocked all dopamine-induced synaptic effects. The synaptic effect of dopamine was mimicked by the D1-like agonist SKF38393 (50 microM), but the D2-1ike agonist quinpirole (50 microM) also had a small effect. Combined application of both agonists did not produce potentiated responses. Dopamine's effect on the excitatory postsynaptic current was independent of serotonin, GABA and adenosine receptors, but may have some interactions with adrenergic receptors. These results suggest that dopamine directly modulates excitatory synaptic events in the parabrachial nucleus predominantly via presynaptic D1-like receptors.

Electrophysiological properties of rat lateral parabrachial neurons in vitro

Anatomical studies have demonstrated that the lateral parabrachial nucleus (LPBN) is composed of at least seven separate subnuclei distinguished by cell morphology, spatial clustering, and afferent and efferent connectivity. We hypothesized that neurons within the subnuclear clusters of the LPBN might have distinct electrophysiological properties that correlate with cellular morphology. An in vitro slice preparation was used to intracellularly record the intrinsic properties of 64 neurons located within the external lateral (EL) and central lateral (CL) subnuclei of the LPBN in adult rats. Analysis of intrinsic properties revealed that neurons in the EL subnucleus had significantly wider action potentials and on the average demonstrated more spike frequency adaptation during 2 s of depolarization compared with CL neurons. The majority of both EL and CL area neurons expressed delayed excitation (DE) after membrane hyperpolarization. DE was eliminated with the A-current blocker 4-aminopyridine (1.5-5 mM). Postinhibitory rebound was also observed in a subpopulation of EL and CL neurons. Morphological analysis of 11 LPBN neurons, which were electrophysiologically characterized and filled with 2% biocytin, failed to demonstrate an association between morphology and the electrophysiological profiles of LPBN neurons. The lack of distinct "type" of neuron within a single subnucleus of the LPBN is in agreement with recent findings reported from the neonatal rat.

CGRP activates renal pelvic substance P receptors by retarding substance P metabolism

Substance P and calcitonin gene-related peptide (CGRP) are colocalized in renal pelvic sensory nerves. Increasing renal pelvic pressure results in an increase in afferent renal nerve activity that is blocked by a substance P receptor antagonist but not by a CGRP receptor antagonist. CGRP potentiates the effects of substance P by preventing the metabolism of substance P. Therefore, we examined whether CGRP enhanced the afferent renal nerve activity responses to substance P and increased renal pelvic pressure, a stimulus known to increase substance P release. Combined administration of substance P and CGRP into the renal pelvis resulted in an increase in afferent renal nerve activity (1392+/-217%. s; area under the curve of afferent renal nerve activity versus time) that was greater (P<0.01) than that produced by substance P (620+/-156%. s) or CGRP (297+/-96%. s) alone. Likewise, CGRP enhanced the afferent renal nerve activity response to increased renal pelvic pressure. During renal pelvic administration of the neutral endopeptidase inhibitor thiorphan, the afferent renal nerve activity response to substance P plus CGRP was similar to that produced by either neuropeptide alone. Because these studies suggested that CGRP potentiated the afferent renal nerve activity responses to substance P, we examined whether the afferent renal nerve activity response to CGRP was blocked by a substance P receptor antagonist, RP67580. RP67580 blocked the afferent renal nerve activity response to CGRP by 85+/-12% (P<0.02). We conclude that CGRP activates renal pelvic sensory nerves by retarding the metabolism of substance P, thereby increasing the amount of substance P available for stimulation of substance P receptors.

Immunocytochemical localization of substance P receptor in rat periaqueductal gray matter: a light and electron microscopic study

The distribution of substance P receptor (SPR) protein in the rat periaqueductal gray matter (PAG) was investigated with a polyclonal antibody in the four subdivisions obtained by cytochrome-oxidase histochemistry (Co-hi). At light microscopic analysis, immunoreactivity appeared particularly dense in the dorsal subdivision of the PAG, was less intense in the other subdivisions, and formed several longitudinally organized columns. SPR-like immunoreactivity (SP(R-i)) was localized mostly to cell bodies and dendrites of small and medium-sized neurons, which constituted about 6% of the total neuronal population of the PAG. At the electron microscopic level, SP(R-i) could be observed on postsynaptic as well as on nonsynaptic regions of both cell bodies and dendrites. A small proportion of axons (4.2%) and axon terminals (5.3%) showed SP(R-i), the majority of labeled axon terminals, amounting to about 70% of synapsing elements, formed asymmetric synapses with dendrites. Rare astroglial processes displaying SP(R-i) were also observed scattered throughout the neuropil of all PAG subdivisions. Our observations suggest that 1) also in the PAG, SP may act in a diffuse, nonsynaptic manner, probably on targets that are distant from its sites of release; and 2) SP may modulate excitatory neurotransmission acting presynaptically on those labeled axons that form asymmetric synapses.

Actions of substance P on membrane potential and ionic currents in guinea pig stellate ganglion neurons

Neuropeptides are known to modulate the excitability of mammalian sympathetic neurons by their actions on various types of K+ and Ca2+ channels. We used whole cell patch-clamp recording methods to study the actions of substance P (SP) on dissociated adult guinea pig stellate ganglion (SG) neurons. Under current-clamp conditions, SG neurons exhibited overshooting action potentials followed by afterhyperpolarizations (AHP). The K+ channel blocker tetraethylammonium (1 mM), the Ca2+ channel blocker Cd2+ (0.1-0.2 mM), and SP (500 nM) depolarized SG neurons, decreased the AHP amplitude, and increased the action potential duration. In the presence of Cd2+, the effect of SP on membrane potential and AHP was reduced. Under voltage-clamp conditions, several different K+ currents were observed, including a transient outward K+ conductance and a delayed rectifier outward K+ current (IK) consisting of Ca(2+)-sensitive [IK(Ca)] and Ca(2+)-insensitive components. SP (500 nM) inhibited IK. Pretreatment with Cd2+ (20-200 microM) or the high-voltage-activated Ca2+ channel blocker omega-conotoxin (10 microM) blocked SP's inhibitory effects on IK. This suggests that SP reduces IK primarily through the inhibition of IK(Ca) and that this may occur, in part, via a reduction of Ca2+ influx through voltage-dependent Ca2+ channels. SP's actions on IK were mediated by a pertussis toxin-insensitive G protein(s) coupled to NK1 tachykinin receptors. Furthermore, we have confirmed that 500 nM SP reduced an inward Cd(2+)- and omega-conotoxin-sensitive Ba2+ current in SG neurons. Thus the actions of SP on IK(Ca) may be due in part to a reduction in Ca2+ influx occurring via N-type Ca2+ channels. This study presents the first description of ionic currents in mammalian SG neurons and demonstrates that SP may modulate excitability in SG neurons via inhibitory actions on K+ and Ca2+ currents.

Activation of presynaptic GABAB receptors inhibits evoked IPSCs in rat magnocellular neurons in vitro

1508-1517, 1998. Whole cell recordings (nystatin-perforated patch) were carried out on magnocellular neurons of the rat supraoptic nucleus (SON) to study the modulation of inhibitory postsynaptic currents (IPSCs) by gamma-aminobutyric acid-B (GABAB) receptors. Field stimulation adjacent to the SON in the presence of kynurenic acid, evoked monosynaptic GABAergic IPSCs. Baclofen reversibly reduced the amplitude of the IPSCs in a dose-dependent manner (EC50: 0.68 microM) without apparent effect on the holding current (Vh = -80 mV) or input resistance and altered neither the kinetic properties, nor the reversal potential of IPSCs. Concomittant to IPSC depression, baclofen enhanced the paired-pulse ratio for two consecutive IPSCs [interstimulus interval (ISI): 50 ms], an effect consistent with a presynaptic locus of action. Both actions of baclofen were abolished by CGP35348 (500 microM), a GABAB receptor antagonist. In testing for involvement of synaptically activated presynaptic GABAB receptors, we only recorded paired-pulse facilitation at most ISIs tested (50-500 ms), suggesting that the classical GABAB autoreceptors may not normally be activated in our conditions. However, enhancement of local GABA concentration by perfusion of a GABA uptake inhibitor (NO-711) revealed an action of endogenous GABA at these presynaptic GABAB receptors. The nonselective K+ channel blocker Ba2+ abolished baclofen's effect and pertussis toxin (PTX) pretreatment (200-500 ng/ml for 18-24 h) was ineffective in blocking the baclofen-induced inhibition, making an involvement of PTX-sensitive G protein unlikely. The present results show that presynaptic GABAB receptors that are coupled to PTX-insensitive G-proteins may be activated by endogenous GABA under conditions of reduced GABA uptake, thus regulating the inhibitory synaptic input to SON.

Visceral afferent stimulation-evoked changes in the release of peptides into the parabrachial nucleus in vivo

Previous investigations have demonstrated that the peptides substance P (SP), calcitonin gene-related peptide (CGRP), cholecystokinin (CCK), neurotensin (NT) and somatostatin (SOM) significantly modulate the glutamate-mediated transmission of visceral information through the parabrachial nucleus (PBN) to the ventrobasal thalamus. In addition, we have shown that the staining intensity of SOM, CCK and NT in the PBN decreases significantly following 2 h of vagal stimulation as visualized using immunohistochemistry. As well, the staining intensity of both SP and CGRP in the PBN were shown to increase under similar conditions. The present investigation was done to determine whether the altered peptide staining intensity of these peptides observed following 2 h of vagal stimulation was the result of an altered peptide release from terminals within the PBN. Male Sprague-Dawley rats were anesthetized with sodium thiobutabarbitol and instrumented to record blood pressure and heart rate and for the stimulation of the cervical vagus nerve. A push-pull perfusion cannula was lowered into the region of the PBN for the continuous sampling of extracellular fluid. Radioenzymatic quantification of the perfusates for peptide content revealed that the extracellular fluid concentration of CGRP and SP increased significantly during the 2 h of vagal stimulation. When the vagal stimulation was terminated, the release of both CGRP and SP decreased significantly below prestimulated values for approximately 30 min before returning to prestimulated levels shortly thereafter. In contrast, there was a significant decrease in the release of CCK, SOM and NT into the PBN during the period of vagal stimulation. Extracellular perfusate levels of these peptides returned to normal upon termination of stimulation. These results demonstrate that terminal release of CGRP and SP is significantly increased and terminal release of CCK, SOM and NT is significantly decreased in the PBN during 2 h of vagal stimulation. These results are consistent with our previous finding that the immunohistochemical staining intensity of CGRP and SP is increased while that of CCK, SOM and NT is decreased following vagal stimulation.

Ibogaine and a total alkaloidal extract of Voacanga africana modulate neuronal excitability and synaptic transmission in the rat parabrachial nucleus in vitro

Ibogaine is a natural alkaloid of Voacanga africana that is effective in the treatment of withdrawal symptoms and craving in drug addicts. As the synaptic and cellular basis of ibogaine's actions are not well understood, this study tested the hypothesis that ibogaine and Voacanga africana extract modulate neuronal excitability and synaptic transmission in the parabrachial nucleus using the nystatin perforated patch-recording technique. Ibogaine and Voacanga africana extract dose dependently, reversibly, and consistently attenuate evoked excitatory synaptic currents recorded in parabrachial neurons. The ED50 of ibogaine's effect is 5 microM, while that of Voacanga africana extract is 170 micrograms/ml. At higher concentrations, ibogaine and Voacanga africana extract induce inward currents or depolarization that are accompanied by increases in evoked and spontaneous firing rate. The depolarization or inward current is also accompanied by an increase in input resistance and reverses polarity around 0 mV. The depolarization and synaptic depression were blocked by the dopamine receptor antagonist haloperidol. These results indicate that ibogaine and Voacanga africana extract 1) depolarize parabrachial neurons with increased excitability and firing rate; 2) depress non-NMDA receptor-mediated fast synaptic transmission; 3) involve dopamine receptor activation in their actions. These results further reveal that the Voacanga africana extract has one-hundredth the activity of ibogaine in depressing synaptic responses. Thus, ibogaine and Voacanga africana extract may produce their central effects by altering dopaminergic and glutamatergic processes.

Dendritically released peptides act as retrograde modulators of afferent excitation in the supraoptic nucleus in vitro

Oxytocin (OXT) and vasopressin (VP) are known to be released from dendrites of magnocellular neurons. Here, we show that these peptides reduced evoked EPSCs by a presynaptic mechanism, an effect blocked by peptide antagonists and mimicked by inhibition of endogenous peptidases. Dendritic release of peptides, elicited with depolarization achieved by high frequency stimulation of afferents or with current injection into an individual neuron, induced short-term synaptic depression similar to that seen following exogenous peptide application and was prevented by peptide antagonists. Thus, dendritically released peptides depress evoked EPSCs in magnocellular neurons by activating presynaptic OXT and/or VP receptors. Such a retrograde modulatory action on afferent excitation may serve as a feedback mechanism to permit peptidergic neurosecretory neurons to autoregulate their own activity.