Expression and physiological actions of neuropeptide Y in guinea pig parasympathetic cardiac ganglia

Comparative specialization of intrinsic cardiac neurons in humans, mice, and pigs

Intrinsic cardiac neurons (ICNs) play a crucial role in the proper functioning of the heart; yet a paucity of data pertaining to human ICNs exists. We took a multidisciplinary approach to complete a detailed cellular comparison of the structure and function of ICNs from mice, pigs, and humans. Immunohistochemistry of whole and sectioned ganglia, transmission electron microscopy, intracellular microelectrode recording and dye filling for quantitative morphometry were used to define the neurophysiology, histochemistry, and ultrastructure of these cells across species. The densely packed, smaller ICNs of mouse lacked dendrites, formed axosomatic connections, and had high synaptic efficacy constituting an obligatory synapse. At Pig ICNs, a convergence of subthreshold cholinergic inputs onto extensive dendritic arbors supported greater summation and integration of synaptic input. Human ICNs were tonically firing, with synaptic stimulation evoking large suprathreshold excitatory postsynaptic potentials like mouse, and subthreshold potentials like pig. Ultrastructural examination of synaptic terminals revealed conserved architecture, yet small clear vesicles (SCVs) were larger in pigs and humans. The presence and localization of ganglionic neuropeptides was distinct, with abundant VIP observed in human but not pig or mouse ganglia, and little SP or CGRP in pig ganglia. Action potential waveforms were similar, but human ICNs had larger after-hyperpolarizations. Intrinsic excitability differed; 93% of human cells were tonic, all pig neurons were phasic, and both phasic and tonic phenotypes were observed in mouse. In combination, this publicly accessible, multimodal atlas of ICNs from mice, pigs, and humans identifies similarities and differences in the evolution of ICNs.

The role of the tripartite synapse in the heart: how glial cells may contribute to the physiology and pathophysiology of the intracardiac nervous system

One of the major roles of the intracardiac nervous system (ICNS) is to act as the final site of signal integration for efferent information destined for the myocardium to enable local control of heart rate and rhythm. Multiple subtypes of neurons exist in the ICNS where they are organized into clusters termed ganglionated plexi (GP). The majority of cells in the ICNS are actually glial cells; however, despite this, ICNS glial cells have received little attention to date. In the central nervous system, where glial cell function has been widely studied, glia are no longer viewed simply as supportive cells but rather have been shown to play an active role in modulating neuronal excitability and synaptic plasticity. Pioneering studies have demonstrated that in addition to glia within the brain stem, glial cells within multiple autonomic ganglia in the peripheral nervous system, including the ICNS, can also act to modulate cardiovascular function. Clinically, patients with atrial fibrillation (AF) undergoing catheter ablation show high plasma levels of S100B, a protein produced by cardiac glial cells, correlated with decreased AF recurrence. Interestingly, S100B also alters GP neuron excitability and neurite outgrowth in the ICNS. These studies highlight the importance of understanding how glial cells can affect the heart by modulating GP neuron activity or synaptic inputs. Here, we review studies investigating glia both in the central and peripheral nervous systems to discuss the potential role of glia in controlling cardiac function in health and disease, paying particular attention to the glial cells of the ICNS.

Angiotensin II potentiates adrenergic and muscarinic modulation of guinea pig intracardiac neurons

The intrinsic cardiac plexus represents a major peripheral integration site for neuronal, hormonal, and locally produced neuromodulators controlling efferent neuronal output to the heart. This study examined the interdependence of norepinephrine, muscarinic agonists, and ANG II, to modulate intrinsic cardiac neuronal activity. Intracellular voltage recordings from whole-mount preparations of the guinea pig cardiac plexus were used to determine changes in active and passive electrical properties of individual intrinsic cardiac neurons. Application of either adrenergic or muscarinic agonists induced changes in neuronal resting membrane potentials, decreased afterhyperpolarization duration of single action potentials, and increased neuronal excitability. Adrenergic responses were inhibited by removal of extracellular calcium ions, while muscarinic responses were inhibited by application of TEA. The adrenergic responses were heterogeneous, responding to a variety of receptor-specific agonists (phenylephrine, clonidine, dobutamine, and terbutaline), although α-receptor agonists produced the most frequent responses. Application of ANG II alone produced a significant increase in excitability, while application of ANG II in combination with either adrenergic or muscarinic agonists produced a much larger potentiation of excitability. The ANG II-induced modulation of firing was blocked by the angiotensin type 2 (AT(2)) receptor inhibitor PD 123319 and was mimicked by the AT(2) receptor agonist CGP-42112A. AT(1) receptor blockade with telmasartin did not alter neuronal responses to ANG II. These data demonstrate that ANG II potentiates both muscarinically and adrenergically mediated activation of intrinsic cardiac neurons, doing so primarily via AT(2) receptor-dependent mechanisms. These neurohumoral interactions may be fundamental to regulation of neuronal excitability within the intrinsic cardiac nervous system.

Regulation of neuronal pituitary adenylate cyclase-activating polypeptide expression during culture of guinea-pig cardiac ganglia

The trophic neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) increases in many different neuron types following injury; a response postulated to support cell survival and regeneration. In acutely isolated cardiac ganglia, approximately 1% of the cardiac neurons exhibited PACAP immunoreactivity whereas after 72 h in culture, approximately 25% of the neurons were PACAP immunoreactive. In contrast, there was no increase in vasoactive intestinal polypeptide (VIP)-immunoreactive (IR) cells. Using a combination of immunocytochemical and molecular techniques, we have quantified PACAP expression, during explant culture of guinea-pig cardiac ganglia. Using real time polymerase chain reaction, PACAP transcript levels increased progressively up to 48 h in culture with no further increase after 72 h. PACAP transcript levels were reduced by neurturin at 48 h in culture but not after 24 or 72 h in culture. In addition, neurturin partially suppressed the percentage of PACAP-IR neurons after 72 h in culture, an effect mediated by activation of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase signaling pathways. The addition of different known regulatory molecules, including ciliary neurotrophic factor (CNTF), interleukin-1 beta (Il-1beta), tumor necrosis factor-alpha (TNFalpha), fibroblast growth factor basic (bFGF), transforming growth factor-beta (TGF-beta) and nerve growth factor (NGF) did not increase the percentage of PACAP-IR neurons after 24 h in culture; a result indicating that the generation and secretion of these factors did not stimulate PACAP expression. The presence of 20 nM PACAP or 10 muM forskolin increased the percentage of PACAP-IR cardiac neurons in 24 h cultures, but not in 72 h cultures. Neither treatment enhanced the number of VIP-IR neurons. The addition of the PACAP selective receptor (PAC(1)) receptor antagonist, M65 (100 nM) suppressed the 20 nM PACAP-induced increase in percentage of PACAP-IR cells in 24 h cultures indicating the effect of PACAP was mediated through the PAC(1) receptor. However, 100 nM M65 had no effect on the percentage of PACAP-IR cells in either 24 or 48 h cultures not treated with exogenous PACAP, suggesting that endogenous release of PACAP likely did not contribute to the enhanced peptide expression. We postulate that the enhanced PACAP expression, which occurs in response to injury is facilitated in the explant cultured cardiac ganglia by the loss of a target-derived inhibitory factor, very likely neurturin. In intact tissues the presence of neurturin would normally suppress PACAP expression. Lastly, our results indicate that many common trophic factors do not enhance PACAP expression in the cultured cardiac neurons. However, the stimulatory role of an, as yet, unidentified factor cannot be excluded.

Activation of the Y1 receptor by neuropeptide Y regulates the growth of prostate cancer cells

This study deals with the role of neuropeptide Y (NPY) in the regulation of cell proliferation. NPY is expressed in the normal and tumoral prostate, but no data on its possible role in prostate cancer (PCa) progression are available. Therefore, we evaluated the direct effect of NPY on the growth of the human PCa cell lines LNCaP (androgen dependent) and DU145 and PC3 (androgen independent). All PCa cell lines expressed Y1-R gene and protein. NPY treatment reduced the proliferation of LNCaP and DU145 cells and increased that of PC3 cells. The Y1-R antagonist BIBP3226 abolished such effects, suggesting a mandatory role of Y1-R in this process. LNCaP cells showed elevated constitutive levels of phosphorylated ERK1/2, which were not affected by NPY. In DU145 cells, NPY stimulated a long-lasting ERK1/2 activation, whereas, in PC3 cells, this effect was rapid and transient and required activation of protein kinase C. Moreover, in both cell lines, pretreatment with BIBP3226 prevented the NPY-induced ERK1/2 phosphorylation, further supporting Y1-R involvement. NPY treatment reduced forskolin-stimulated cAMP accumulation only in PC3 cells and did not change intracellular calcium concentration in any PCa cell line. These data indicate that NPY may directly regulate PCa cell growth via Y1-R. The direction of this effect appears to be related to the time kinetics of MAPK activation, i.e. long-lasting vs. transient, and to the clone-specific involvement of other intracellular signals. These findings suggest that NPY-related mechanisms might play a relevant role in the progression of PCa, at both androgen dependent and independent stages.

Trophic factor modulation of cocaine- and amphetamine-regulated transcript peptide expression in explant cultured guinea-pig cardiac neurons

The present study investigated the influence of trophic factors on the expression of cocaine- and amphetamine-regulated transcript peptide (CARTp) in guinea-pig cardiac ganglia maintained in explant culture. In acutely isolated cardiac ganglia preparations, <1% of the cholinergic cardiac neurons exhibited CARTp immunoreactivity. In contrast, this number increased to >25% of the cardiac neurons after 72 h in explant culture. This increase in the number of CARTp neurons in cultured cardiac ganglia explants was accompanied by an increase in CARTp transcript levels as assessed by real time polymerase chain reaction. Treatment of cardiac ganglia cultures with neurturin or glial-derived trophic factor (both at 10 ng/ml) for 72 h prevented the increase in neurons that exhibited CARTp immunoreactivity. In contrast, treatment with ciliary neurotrophic factor (50 ng/ml) for 72 h produced a small significant increase in the percentage of CARTp-immunoreactive cardiac neurons and treatment with nerve growth factor (100 ng/ml) had no effect. Neurturin treatment also decreased cardiac neuron CARTp levels after 72 h in explant culture. Cardiac neurons exhibited immunoreactivity to the neurturin receptor GFRalpha2 whereas non-neural cells preferentially exhibited immunoreactivity to the glial-derived neurotrophic factor receptor GFRalpha1 and neurturin transcripts were detected in cardiac tissue extracts. We hypothesize that a target-derived inhibitory factor, very likely neurturin, is a critical factor suppressing the expression of CARTp in guinea-pig cardiac neurons. These observations contrast with those reported in sympathetic neurons that suggest up-regulation of trophic factors after axotomy or during explant culture is a key factor contributing to the up-regulation of many neuropeptides.

Protection from neuronal damage evoked by a motivational excitation is a driving force of intentional actions

Motivation may be understood as an organism's subjective attitude to its current physiological state, which somehow modulates generation of actions until the organism attains an optimal state. How does this subjective attitude arise and how does it modulate generation of actions? Diverse lines of evidence suggest that elemental motivational states (hunger, thirst, fear, drug-dependence, etc.) arise as the result of metabolic disturbances and are related to transient injury, while rewards (food, water, avoidance, drugs, etc.) are associated with the recovery of specific neurons. Just as motivation and the very life of an organism depend on homeostasis, i.e., maintenance of optimum performance, so a neuron's behavior depends on neuronal (i.e., ion) homeostasis. During motivational excitation, the conventional properties of a neuron, such as maintenance of membrane potential and spike generation, are disturbed. Instrumental actions may originate as a consequence of the compensational recovery of neuronal excitability after the excitotoxic damage induced by a motivation. When the extent of neuronal actions is proportional to a metabolic disturbance, the neuron theoretically may choose a beneficial behavior even, if at each instant, it acts by chance. Homeostasis supposedly may be directed to anticipating compensation of the factors that lead to a disturbance of the homeostasis and, as a result, participates in the plasticity of motivational behavior. Following this line of thought, I suggest that voluntary actions arise from the interaction between endogenous compensational mechanisms and excitotoxic damage of specific neurons, and thus anticipate the exogenous compensation evoked by a reward.

Presence and co-localization of vasoactive intestinal polypeptide with neuronal nitric oxide synthase in cells and nerve fibers within guinea pig intrinsic cardiac ganglia and cardiac tissue

The presence of vasoactive intestinal polypeptide (VIP) has been analyzed in fibers and neurons within the guinea pig intrinsic cardiac ganglia and in fibers innervating cardiac tissues. In whole-mount preparations, VIP-immunoreactive (IR) fibers were present in about 70% of the cardiac ganglia. VIP was co-localized with neuronal nitric oxide synthase (nNOS) in fibers innervating the intrinsic ganglia but was not present in fibers immunoreactive for pituitary adenylate cyclase-activating polypeptide, choline acetyltransferase (ChAT), tyrosine hydroxylase, or substance P. A small number of the intrinsic ChAT-IR cardiac ganglia neurons (approximately 3%) exhibited VIP immunoreactivity. These few VIP-IR cardiac neurons also exhibited nNOS immunoreactivity. After explant culture for 72 h, the intraganglionic VIP-IR fibers degenerated, indicating that they were axons of neurons located outside the heart. In cardiac tissue sections, VIP-IR fibers were present primarily in the atria and in perivascular connective tissue, with the overall abundance being low. VIP-IR fibers were notably sparse in the sinus node and conducting system and generally absent in the ventricular myocardium. Virtually all VIP-IR fibers in tissue sections exhibited immunoreactivity to nNOS. A few VIP-IR fibers, primarily those located within the atrial myocardium, were immunoreactive for both nNOS and ChAT indicating they were derived from intrinsic cardiac neurons. We suggest that, in the guinea pig, the majority of intraganglionic and cardiac tissue VIP-IR fibers originate outside of the heart. These extrinsic VIP-IR fibers are also immunoreactive for nNOS and therefore most likely are a component of the afferent fibers derived from the vagal sensory ganglia.

Pituitary adenylate cyclase activating polypeptide (PACAP) decreases neuronal somatostatin immunoreactivity in cultured guinea-pig parasympathetic cardiac ganglia

Postganglionic parasympathetic neurons in guinea-pig cardiac ganglia exhibit choline acetyltransferase (ChAT)-immunoreactivity, and a large fraction (60%) of the ChAT-positive cardiac neurons co-express somatostatin-immunoreactivity. This co-expression remained when the cardiac ganglia explants were maintained in culture for 72 h (40% somatostatin-immunoreactive). The guinea-pig cardiac ganglia neurons express the high affinity pituitary adenylate cyclase activating polypeptide (PACAP)-selective PAC1 receptor, and treatment of the ganglia explants with 20 nM PACAP27 for 72 h to evaluate PACAP regulation of somatostatin expression revealed a dramatic 85% decrease in the number of somatostatin-IR neurons (6% somatostatin-IR neurons) compared with untreated control explant preparations. The decrease in percentage of somatostatin-IR neurons by PACAP27 was time- and concentration-dependent, and selective for PACAP27; PACAP38 and vasoactive intestinal polypeptide were less effective. PACAP6-38, a PACAP antagonist, eliminated the PACAP27-induced change in somatostatin positive neurons. The PACAP-mediated decrease in somatostatin-IR neurons was eliminated in calcium-deficient solutions and by the addition of nifedipine, indicating a requirement for calcium influx through L-type calcium channels. The addition of either the calmodulin inhibitor N-(4-aminobutyl)-1-naphthalenesulfonamide or the MEK inhibitor PD98059, also eliminated the PACAP27-induced decrease in somatostatin-IR cells. The PACAP27-mediated effect on somatostatin expression was not affected by inhibitors of protein kinase A or phospholipase C, but was reduced by the adenylyl cyclase inhibitor SQ22356, suggesting cAMP involvement. Semiquantitative and quantitative reverse transcription PCR prosomatostatin transcript measurements showed that cardiac ganglia prosomatostatin mRNA levels were not diminished by chronic PACAP27 exposure despite the dramatic decrement in somatostatin-expressing neurons. Neuronal peptide-IR content represents a balance between production and secretion. These results suggested that one of the primary effects of PACAP exposure may be enhanced levels of neuropeptide release that exceeded production levels, resulting in somatostatin depletion and a decrement in the number of identifiable somatostatin-expressing cardiac neurons.

TRPC6 immunoreactivity is colocalized with neuronal nitric oxide synthase in extrinsic fibers innervating guinea pig intrinsic cardiac ganglia

Tachykinins depolarize guinea pig intracardiac neurons by activating nonselective cationic channels. Recently, members of the transient receptor potential family of membrane channels (TRPC) have been implicated in the generation of G protein-coupled receptor-activated nonselective cationic currents. We have investigated whether guinea pig cardiac neurons exhibit immunoreactivity to TRPC. Our results showed that nerve fibers within guinea pig intrinsic cardiac ganglia exhibited immunoreactivity to TRPC6. After culture of cardiac ganglia whole-mount explants for 72 hours, the TRPC6-IR fiber networks were absent. Therefore, the TRPC6-IR fibers were derived from sources extrinsic to the heart. A small percentage ( approximately 3%) of intracardiac neurons also exhibited TRPC6 immunoreactivity in control preparations, and the percentage of cells exhibiting TRPC6 immunoreactivity was not changed following explant culture for 72 hours. The few intrinsic TRPC6-IR neurons also exhibited nitric oxide synthase (NOS) immunoreactivity, indicating that they were nitrergic as well. We compared the immunohistochemical staining patterns of TRPC6-IR fibers with the staining patterns of a number of other neurotransmitters or neurotransmitter synthetic enzymes that mark specific extrinsic inputs to the intrinsic cardiac ganglia. The TRPC6-IR fibers were not immunoreactive for choline acetyltransferase, tyrosine hydroxylase, or substance P. However, the TRPC6-IR fibers exhibited immunoreactivity to neuronal NOS. Therefore, we propose that the TRPC6-IR fibers within the guinea pig intrinsic cardiac ganglia are vagal sensory fibers that also contain NOS. We found, in support of this conclusion, that TRPC6-IR cells were also present in sections of nodose ganglia.

Sigma receptors inhibit high-voltage-activated calcium channels in rat sympathetic and parasympathetic neurons

Studies on the expression and cellular function of sigma receptors in autonomic neurons were conducted in neonatal rat intracardiac and superior cervical (SCG) ganglia. Individual neurons from SCG and intracardiac ganglia were shown to express transcripts encoding the sigma-1 receptor using single-cell RT-PCR techniques. The relationship between sigma receptors and calcium channels was studied in isolated neurons of these ganglia under voltage-clamp mode using the perforated-patch configuration of the whole cell patch-clamp recording technique. Bath application of sigma receptor agonists was shown to rapidly depress peak calcium channel currents in a reversible manner in both SCG and intracardiac ganglion neurons. The inhibition of barium (I(Ba)) currents was dose-dependent, and half-maximal inhibitory concentration (IC50) values for haloperidol, ibogaine, (+)-pentazocine, and 1,3-Di-O-tolylguanidin (DTG) were 6, 31, 61, and 133 microM, respectively. The rank order potency of haloperidol > ibogaine > (+)-pentazocine > DTG is consistent with the effects on calcium channels being mediated by a sigma-2 receptor. Preincubation of neurons with the irreversible sigma receptor antagonist, metaphit, blocked DTG-mediated inhibition of Ca2+ channel currents. Maximum inhibition of calcium channel currents was > or =95%, suggesting that sigma receptors block all calcium channel subtypes found on the cell body of these neurons, which includes N-, L-, P/Q-, and R-type calcium channels. In addition to depressing peak Ca2+ channel current, sigma receptors altered the biophysical properties of these channels. Following sigma receptor activation, Ca2+ channel inactivation rate was accelerated, and the voltage dependence of both steady-state inactivation and activation shifted toward more negative potentials. Experiments on the signal transduction cascade coupling sigma receptors and Ca2+ channels demonstrated that neither cell dialysis nor intracellular application of 100 microM guanosine 5'-O-(2-thiodiphosphate) trilithium salt (GDP-beta-S) abolished the modulation of I(Ba) by sigma receptor agonists. These data suggest that neither a diffusible cytosolic second messenger nor a G protein is involved in this pathway. Activation of sigma receptors on sympathetic and parasympathetic neurons is likely to modulate cell-to-cell signaling in autonomic ganglia and thus the regulation of cardiac function by the peripheral nervous system.

Expression of neuropeptide Y receptors in human prostate cancer cells

BACKGROUND

Neuroendocrine molecules are now believed to play a significant role in the progression of human prostate cancer (CaP), especially in the androgen-independent stage.

MATERIALS AND METHODS

In the present study, we evaluated the presence and the function of the receptors for neuropeptide Y (NPY) in human CaP cell lines (the androgen-dependent LNCaP, and the androgen-independent DU 145 and PC-3).

RESULTS

The presence of high-affinity binding sites for NPY was shown on PC-3 cells (radioreceptor assay). Reverse transcription-polymerase chain reaction analysis indicated that these sites correspond to the Y1 and Y2 receptor isoforms. A Y1 receptor protein (70 kDa) was also detected in PC-3 cell extracts by Western blot analysis. The activation of these receptors by NPY resulted in a reduction of forskolin-induced cAMP accumulation and an increase of [Ca2+]i. Moreover, a prolonged treatment with NPY induced a dose-related proliferation of PC-3 cells.

CONCLUSIONS

By showing that NPY receptors are expressed in the androgen-independent cell line PC-3 and that their activation results in cell proliferation, the present date suggest that NPY-related mechanisms might be relevant in certain stages of CaP, such as the progression of the disease during the androgen-independent stage.

Distribution of cocaine- and amphetamine-regulated transcript peptide in the guinea pig intrinsic cardiac nervous system and colocalization with neuropeptides or transmitter synthetic enzymes

This study was conducted to establish the presence of cocaine- and amphetamine-regulated transcript peptide (CARTp) immunoreactivity in neurons and fibers within guinea pig atrial whole-mount preparations containing the intrinsic cardiac ganglia. Many cardiac ganglia, but not all, in a given whole-mount preparation, were innervated by CARTp-immunoreactive (IR) fibers. Following explant culture of whole mounts for 72 hours, the CARTp-IR fiber networks were absent, but the number of CARTp-IR neurons was increased markedly. These observations suggested that the majority of the CARTp-IR fibers in the intracardiac ganglia were derived from sources extrinsic to the heart. In control whole-mount preparations, very few CARTp-positive neurons were present. The few intrinsic CARTp-IR neurons also exhibited choline acetyltransferase (ChAT) immunoreactivity, indicating that they make up a small subpopulation of cholinergic postganglionic neurons. Some CARTp-IR neurons also exhibited nitric oxide synthase (NOS) immunoreactivity, indicating that they were nitrergic as well. We compared the immunohistochemical staining patterns of CARTp-IR fibers with the staining patterns of a number of other neurotransmitters or neurotransmitter synthetic enzymes that mark specific extrinsic inputs. The CARTp-IR fibers were not immunoreactive for ChAT, tyrosine hydroxylase, calcitonin gene-related peptide, or substance P. However, virtually all CARTp-IR fibers exhibited immunoreactivity to neuronal NOS (a marker for nitric oxide-producing neurons). CARTp-IR cells and NOS-IR cells were present in the nodose ganglia. In addition, CARTp-IR neurons in the nodose also were stained positively for NADPH-diaphorase. Thus, we propose that most CARTp-IR fibers within the guinea pig intrinsic cardiac ganglia are vagal afferent fibers that also contain NOS.

PACAP peptides modulate guinea pig cardiac neuron membrane excitability and neuropeptide expression

Morphological studies identified PACAP-immunoreactive nerve fibers in dense pericellular arrangements around virtually every cholinergic parasympathetic neuron of guinea pig cardiac ganglia; all postganglionic cardiac neurons expressed membrane-associated PAC1 receptor protein. Characterization of the alternative splice variants established predominant expression of the PAC1(very short) receptor transcript containing neither HIP nor HOP exons. PACAP depolarized cardiac neurons and increased membrane excitability; the excitability resulted from neither altered action potential properties nor inhibition of IM. Treatment of cardiac ganglia explants with PACAP significantly reduced the numbers of cholinergic neurons coexpressing somatostatin immunoreactivity, which did not appear to be correlated with prosomatostatin mRNA expression. The PACAP-mediated decrease in somatostatin immunoreactive neurons required calcium influx through L-type calcium channels and activation of adenylyl cyclase, whereas activation of phospholipase C or protein kinase A was not required. These observations indicate that PACAP through the PAC1 receptors elicits complex actions on guinea pig parasympathetic cardiac ganglia neurons, including modulation of membrane ion conductances and modulation of neuropeptide expression.

Regulation of parasympathetic neurons by mast cells and histamine in the guinea pig heart

The potential interaction between the immune system and the autonomic nervous system was examined in the cardiac ganglia of guinea pigs. Intracellular voltage recordings were used to determine the effects of mast cell degranulation on the membrane properties of parasympathetic neurons in animals actively sensitized to ovalbumin. Stimulation of mast cell degranulation by perfusion with ovalbumin (10 micrograms/ml) produced a depolarization and increase in the excitability of intracardiac neurons. These effects could be mimicked by histamine application, either by perfusion (10 microM) or by local pressure application (100 microM, 1-2 s application). In either case, histamine application resulted in a similar membrane depolarization and increase in excitability. Immunohistochemical experiments demonstrated that histamine-immunoreactive mast cells are located in close proximity to parasympathetic postganglionic neurons. The histamine response was not due to release of other neurotransmitters from adjacent nerve terminals and both the depolarization and increase in excitability were inhibited by the H1 antagonist, pyrilamine (300 nM), and were unaffected by the H2 antagonist cimetidine (5 microM). Incubation of cardiac ganglion preparations from sensitized animals with pyrilamine prior to ovalbumin perfusion resulted in the inhibition of both the depolarization and increase in excitability. These results demonstrate that mast cell degranulation, and the subsequent release of histamine, results in the stimulation of intracardiac neurons via the activation of H1 receptors. Thus, local inflammatory reactions in the cardiac tissue can lead to the rapid activation of parasympathetic neurons, thereby altering cardiac function.

Neuropeptide Y (NPY) expression is increased in explanted guinea pig parasympathetic cardiac ganglia neurons

While expression of neuropeptides by sympathetic neurons is altered by decentralization and axotomy, it is not known whether similar experimental paradigms also modulate the chemical phenotype of parasympathetic cardiac ganglia neurons. The present study tested whether guinea pig parasympathetic neuron neuropeptide Y (NPY) expression was altered when cardiac ganglia preparations were maintained as organ explants in the presence or absence of colchicine. Two experimental approaches were used to examine NPY expression. First, immunocytochemical techniques were used to quantitate numbers of neurons within the cardiac ganglia exhibiting NPY-immunoreactivity; second, reverse transcription PCR was used to examine proNPY mRNA expression. In control cardiac ganglia preparations, approximately 4% of ganglia neurons exhibited NPY-immunoreactivity. The percentage of NPY-immunopositive neurons in 30- and 72-h explanted cardiac ganglia preparations, maintained in the absence of colchicine, increased to 11 and 16%, respectively. Colchicine treatment of explanted preparations further increased the percentage of NPY-positive ganglia cells 24% (30 h) and 32% (72 h). All NPY-immunoreactive neurons from control ganglia and explanted ganglia were choline acetyltransferase(ChAT)-immunoreactive, indicating retention of the cholinergic phenotype. ProNPY mRNA also was increased following ganglia explantation, consistent with the increase in the numbers of NPY-immunoreactive neurons. NPY transcripts were further increased after 30 h, but not after 72 h in colchicine-treated, explanted cardiac ganglia preparations. These results demonstrate that NPY expression is altered in explanted cardiac ganglia preparations, providing evidence that the chemical phenotype of parasympathetic cardiac neurons can be modulated.

Activation of various G-protein coupled receptors modulates Ca2+ channel currents via PTX-sensitive and voltage-dependent pathways in rat intracardiac neurons

In the present study, we examined the ability of several putative neurotransmitters and neuromodulators to modulate voltage-dependent Ca2+ channel currents in adult rat intracardiac neurons. Of 17 compounds tested, acetylcholine (Ach), neuropeptide Y (NPY), norepinephrine (NE), and met-enkephalin (met-Enk) were effective modulators of the Ca2+ currents. The neurotransmitter-induced current inhibition was associated with slow activation kinetics and relief by a strong depolarizing prepulse. Overnight pretreatment of neurons with pertussis toxin (PTX, 500 ng/ml) significantly attenuated the neurotransmitter-induced current inhibition. Heterologous expression of transducin, a known chelator of G-protein betagamma subunits, almost completely abolished the neurotransmitter-induced current inhibition. Taken together, our data suggest that four different neurotransmitters inhibit the Ca2+ channel currents in adult rat intracardiac neurons via a pathway that is voltage-dependent, membrane-delimited, and utilizes betagamma subunits released from PTX-sensitive G-proteins. The Ca2+ channel inhibition by non-cholinergic neurotransmitters may play important roles in regulation of neuronal excitability and Ach release at synapses in intracardiac ganglia, thereby contributing to cholinergic control of cardiac functions.

Pituitary adenylate cyclase-activating polypeptide expression and modulation of neuronal excitability in guinea pig cardiac ganglia

Cardiac output is regulated by the coordinate interactions of stimulatory sympathetic and inhibitory parasympathetic signals. Intracardiac parasympathetic ganglia are integrative centers of cardiac regulation, and modulation of the parasympathetic drive on the heart is accomplished by altering intrinsic cardiac ganglion neuron excitability. The pituitary adenylate cyclase-activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) family of peptides modulates cardiac function, and in guinea pig heart, PACAP appears to act directly on intrinsic parasympathetic cardiac ganglia neurons through PACAP-selective receptors. A multidisciplinary project tested whether cardiac PACAP peptides act through PACAP-selective receptors as excitatory neuromodulators amplifying the parasympathetic inhibition from guinea pig cardiac ganglia. The in vivo sources of regulatory PACAP peptides were localized immunocytochemically to neuronal fibers and a subpopulation of intrinsic postganglionic cardiac neurons. RT-PCR confirmed that cardiac ganglia expressed proPACAP transcripts and have PACAP peptide biosynthetic capabilities. Messenger RNA encoding PACAP-selective PAC1 receptor isoforms were also present in cardiac ganglia. Alternative splicing of PAC1 receptor transcripts produced predominant expression of the very short variant with neither HIP nor HOP cassettes; lower levels of the PAC1HOP2 receptor mRNA were present. Almost all of the parasympathetic neurons expressed membrane-associated PAC1 receptor proteins, localized immunocytochemically, which correlated with the population of cells that responded physiologically to PACAP peptides. PACAP depolarized cardiac ganglia neurons and increased neuronal membrane excitability. The rank order of peptide potency on membrane excitability in response to depolarizing currents was PACAP27>PACAP38>VIP. The PACAP-induced increase in excitability was not a function of membrane depolarization nor was it caused by alterations in action potential configuration. These results support roles for PACAP peptides as integrative modulators amplifying, through PACAP-selective receptors, the parasympathetic cardiac ganglia inhibition of cardiac output.

Neural control of left ventricular contractility in the dog heart: synaptic interactions of negative inotropic vagal preganglionic neurons in the nucleus ambiguus with tyrosine hydroxylase immunoreactive terminals

Recent physiological evidence indicates that vagal postganglionic control of left ventricular contractility is mediated by neurons found in a ventricular epicardial fat pad ganglion. In the dog this region has been referred to as the cranial medial ventricular (CMV) ganglion [J.L. Ardell, Structure and function of mammalian intrinsic cardiac neurons, in: J.A. Armour, J.L. Ardell (Eds.). Neurocardiology, Oxford Univ. Press, New York, 1994, pp. 95-114; B.X. Yuan, J.L. Ardell, D.A. Hopkins, A.M. Losier, J.A. Armour, Gross and microscopic anatomy of the canine intrinsic cardiac nervous system, Anat. Rec., 239 (1994) 75-87]. Since activation of the vagal neuronal input to the CMV ganglion reduces left ventricular contractility without influencing cardiac rate or AV conduction, this ganglion contains a functionally selective pool of negative inotropic parasympathetic postganglionic neurons. In the present report we have defined the light microscopic distribution of preganglionic negative inotropic neurons in the CNS which are retrogradely labeled from the CMV ganglion. Some tissues were also processed for the simultaneous immunocytochemical visualization of tyrosine hydroxylase (TH: a marker for catecholaminergic neurons) and examined with both light microscopic and electron microscopic methods. Histochemically visualized neurons were observed in a long slender column in the ventrolateral nucleus ambiguus (NA-VL). The greatest number of retrogradely labeled neurons were observed just rostral to the level of the area postrema. TH perikarya and dendrites were commonly observed interspersed with vagal motoneurons in the NA-VL. TH nerve terminals formed axo-dendritic synapses upon negative inotropic vagal motoneurons, however the origin of these terminals remains to be determined. We conclude that synaptic interactions exist which would permit the parasympathetic preganglionic vagal control of left ventricular contractility to be modulated monosynaptically by catecholaminergic afferents to the NA-VL.