Neural nicotinic acetylcholine responses in sensory neurons from postnatal rat

The whole-cell configuration of the patch-clamp technique was used to study nicotinic acetylcholine (ACh) responses in freshly dissociated dorsal root ganglion (DRG) cells from postnatal rat. At negative holding potentials with physiological solutions in the bath and the pipette, ACh (20 microM), nicotine (5 microM) or DMPP (20 microM) activated inward currents in 51% of the cells. Average current density was higher in 1-month-old compared to newborn animals. Nicotinic agonist-induced currents were unaffected by atropine (10 microM) but reversibly blocked by hexamethonium (20 microM). Although labeling with fluorescent alpha-bungarotoxin (BGT) demonstrated the presence of toxin binding sites on DRG cells, DMPP-induced inward currents were unaffected by micromolar BGT. Neuronal bungarotoxin (100 nM), in contrast, led to a largely irreversible block of the nicotinic responses. These results show that postnatal DRG cells express functional nicotinic acetylcholine receptors (nAChR) of a neuronal type.     

GABA-activated Chloride Currents of Postnatal Mouse Retinal Ganglion Cells are Blocked by Acetylcholine and Acetylcarnitine: How Specific are Ion Channels in Immature Neurons?

The goal of this study was to clarify pharmacological properties of GABA_A receptors in cells of the mouse retinal ganglion cell layer in situ. Spontaneous synaptic currents and responses to exogenous GABA were recorded from individual neurons in retinal whole mounts (postnatal days 1–3) or retinal stripe preparations (postnatal days 4–6). Drugs were applied by a fast local superfusion system. Current responses were measured with the patch-clamp technique in the whole-cell configuration. All cells responded to exogenous GABA (average EC₅₀ and Hill coefficient: 16.7 μM and 0.95 respectively) and generated GABAergic synaptic currents in response to elevated KCI. GABA-induced currents of retinal ganglion cells were blocked by bicuculline, picrotoxin and Zn²⁺, as well as strychnine, and increased by pentobarbital, clonazepam and 3α-hydroxy-5α-pregnan-20-one. In some retinal ganglion cells GABA caused an increase in the frequency of spontaneous synaptic currents, which points to a partially depolarizing action of this traditionally inhibitory neurotransmitter in the neural retina. Our major observation is that acetylcholine and acetylcarnitine blocked or reduced GABAergic inhibitory postsynaptic currents and responses to exogenous GABA. This effect was seen in only a fraction of retinal ganglion cells and occurred in both the undesensitized and the desensitized state of the GABA_A receptor. The block was voltage-independent and persisted during coapplication with the nicotinic and muscarinic acetylcholine receptor antagonists D-tubocurarine and atropine. In contrast to GABA-activated Cl⁻ currents, glycine-activated Ch currents remained unaffected by acetylcholine and acetylcarnitine. Acetylcarnitine had no effect on voltage-activated Ca²⁺ channel currents and glutamate-activated currents. Similar results were obtained in a dissociated cell culture preparation from the neonatal rat superior colliculus. In these cells acetylcholine induced a rightward shift in the dose - response curve for GABA. Taken together, these results indicate that acetylcholine and acetylcarnitine can act directly at the GABA_A binding site and thereby reduce the action of GABA in the immature retina.     

Characterization of nicotinic receptors in chick retina using a snake venom neurotoxin that blocks neuronal nicotinic receptor function

Nicotinic receptor function has been described in the retinas of a variety of vertebrate species. Neuronal bungarotoxin (NBT, also known as bungarotoxin 3.1, toxin F, or kappa-bungarotoxin) blocks nicotinic receptors in several neuronal preparations, while the neuromuscular antagonist alpha-bungarotoxin (BGT) fails to block most of these receptors. NBT (100 nM), but not BGT (10 microM), substantially blocks nicotinic function on ganglion cells in intact chick retina. 125I-NBT binds to 2 sites in homogenates of chick retina; one site that is shared with BGT (Kd = 5-7 nM, Bmax approximately 500 fmol/retina) and one which is not (Kd = 2-3 nM, Bmax approximately 100 fmol/retina). 125I-NBT binding to the NBT-specific site (binding in the presence of 1 microM unlabeled BGT) is localized to 2 bands in the inner plexiform layer, corresponding to regions richly innervated by neurons containing immunoreactivity for choline acetyltransferase. Furthermore, this binding is blocked by competitive nicotinic agonists and antagonists, but nicotine or other nicotinic agonists do not displace 125I-NBT binding with very high affinity relative to the displacement of 3H-nicotine reported by others in brain. Thus, of the 2 NBT binding sites, the site not recognized by BGT most likely represents functional nicotinic receptors in the chick retina, but these receptors have relatively low affinity for nicotinic agonists, similar to nicotinic receptors found in autonomic ganglia.     

Kappa-Bungarotoxin: a probe for the neuronal nicotinic receptor in the avian ciliary ganglion

The interaction of snake alpha-neurotoxins with neuronal membranes has been examined in the chick ciliary ganglion. Some, but not all, alpha-neurotoxins block nicotinic transmission in this ganglion. alpha-Bungarotoxin (ABgT), the major alpha-neurotoxin in the venom of Bungarus multicinctus, does not block transmission at high concentrations (1.2 microM) although it binds (Kd = 1 nM) to a pharmacologically nicotinic site in the ganglion. A toxin (kappa-bungarotoxin, KBgT) has been purified from the venom of Bungarus multicinctus. KBgT has a molecular weight of 6500 daltons and a pI of 9.1. KBgT is a potent inhibitor of nicotinic transmission in the ciliary ganglion, producing a reversible (overal several hours) blockade at 75 nM. Pre-exposure of ganglia to 1.2 microM ABgT does not prevent the effects of KBgT, indicating that the blockade occurs at a site distinct from that recognized by ABgT. Binding of [125I]KBgT to ciliary ganglia reveals two binding sites: one which has previously been characterized by [125I]ABgT and one which is not identified by [125I]ABgT. Both of these [125I]KBgT binding sites are blocked following pre-treatment of ganglia with the irreversible nicotinic affinity agent bromoacetylcholine. A two-site model is proposed to account for these observations. One site (the ABgT binding site) is seen by both ABgT and KBgT, and has as yet no physiological function associated with it. The second site is recognized only by the physiologically active KBgT, and may represent binding of the toxin to the physiologically detected nicotinic receptor.     

The interaction of kappa-bungarotoxin with the nicotinic receptor of bovine chromaffin cells.

Whole-cell recording techniques were used to examine acetylcholine-induced nicotinic currents in isolated bovine chromaffin cells. The effects on these currents of kappa-bungarotoxin, a snake venom kappa-neurotoxin, were tested. Exposure of cells to kappa-bungarotoxin (600 nM for 40 min) produced a prolonged blockade of nicotinic currents. The mechanism of this blockade was examined in several ways. Firstly, the pre-exposure of cells to trimetaphan, a competitive nicotinic antagonist, protected against the action of subsequent additions of kappa-bungarotoxin. Secondly, voltage-clamp measurements indicated that the degree of blockade produced by kappa-bungarotoxin was independent of cell membrane potential. Unlike (+)-tubocurarine, kappa-bungarotoxin had no direct agonist effects on nicotinic receptors. It is concluded from the present functional studies and from previously reported binding studies that kappa-bungarotoxin blocks nicotinic responses in bovine chromaffin cells by binding to regions overlying acetylcholine sites on nicotinic receptors.     

Functional properties of neuronal nicotinic acetylcholine receptors in the chick retina during development

Acetylcholine (ACh) has been recognized for a long time as a major neurotransmitter in the retina, however, little is known about the contribution of acetylcholine receptors in synaptic processing. Moreover, even less information is available concerning their role during development. To address this question further, we examined the physiological and pharmacological properties of neuronal nicotinic acetylcholine receptors (nAChRs) in retinal ganglion cells from embryonic (E) 12-18-day-old Leghorn chicks. Patch-clamp recordings in whole-cell configuration revealed that at E12 approximately 21% of the ganglion cells responded to acetylcholine pulses with inward currents. The number of responsive cells progressively increased to 57% at E15 to reach up to 15 positive cells out of 15 cells tested at E18. Acetylcholine-evoked responses could be subdivided, according to their time course, into fast and slowly desensitizing. Taking advantage of the selectivity of the frog toxin epibatidine (Epi), that preferentially activates heteromeric neuronal nicotinic acetylcholine receptors, we compared the currents evoked by this toxin vs. the effects of acetylcholine. A further characterization of the receptor diversity during development was to assess their sensitivity to the alpha-conotoxin MII (alpha-CTX-MII), which has been shown to preferentially block alpha6- and alpha3beta2-containing receptors. These data demonstrate that ganglion cells of the chick retina express multiple receptor subtypes that progressively develop as a function of retina maturation.     

Spontaneous release of acetylcholine affects the physiological nicotinic responses of rat retinal ganglion cells in culture

Nictonic cholinergic responses have been previously documented in mammalian retinal ganglion cells. In the present study, dissociated retinal cells were densely plated and grown in a specific batch of rat serum. When held at negative potentials during whole-cell recording with a patch electrode, the retinal ganglion cells located close to presumptive cholinergic amacrine cells in these cultures were found to respond to acetylcholine (ACh; 20-200 microM) with an apparent outward current in the presence of physiological salines on both sides of the membrane. Other nicotinic agonists (nicotine, carbachol) produced the same effect. Conductance measurements revealed that this apparent outward current was actually a decrease in a tonic inward current. Nicotinic antagonists such as d-turbocurarine (10 microM) and dihydro-beta-erythroidine (20 microM), when applied in dishes that had never been exposed to exogenous ACh, produced a similar decrease in a tonic inward current. The reversal potential of the tonic current suppressed by ACh was similar to the nicotinic current previously studied in these central neurons. Furthermore, purified acetylcholinesterase was capable of modulating the tonic inward current of the retinal ganglion cells. Lowering an excised patch of muscle into dense areas of the retinal cultures activated nicotinic channels in the muscle membrane, indicating the presence of endogenous ACh in the culture fluid. Divalent cations such as Co2+ blocked the tonic inward current of retinal ganglion cells in these cultures. Finally, direct biochemical measurements indicated that low levels of endogenous ACh (on the order of 0.5 microM near putative cholinergic amacrine cells) were present in the retinal cultures. Taken together, these results show that ACh was being spontaneously released into these cultures, resembling at least to some degree the tonic leakage of ACh found in the intact retina. This concentration of ACh was capable of tonically depolarizing the membrane potential of retinal ganglion cells exposed to this dosage. This culture system allows the study of trophic effects of ACh on a central mammalian neuron in a precisely controlled extracellular environment.     

The interaction of κ-bungarotoxin with the nicotinic receptor of bovine chromaffin cells

Whole-cell recording techniques were used to examine acetylcholine-induced nicotinic currents in isolated bovinei chromaffin cells. The effects on these currents of κ-bungarotoxin, a snake venon κ-neurotoxin, were tested. Exposure of cells to κ-bungarotoxin (600 nM for 40 min) produced a prolonged blockade of nicotinic currents. The mechanism of this blockade was examined in several ways. Firstly, the pre-exposure of cells to trimetaphan, a competitive nicotinic antagonist, protected against the action of subsequent additions of κ-bungarotoxin. Secondly, voltage-clamp measurements indicated that the degree of blockade produced by κ-bungarotoxin was independent of cell membrane potential. Unlike (+)-tubocurarine, κ-bungarotoxin had no direct agonist effects on nicotinic receptors. It is concluded from the present functional studies and from previously reported binding studies that κ-bungarotoxin blocks nicotinic responses in bovine chromaffin cells by binding to regions overlying acetylcholine sites on nicotinic receptors.     

Steroids inhibit nicotinic acetylcholine receptors.

Application of progesterone to Xenopus oocytes expressing a cloned neuronal nicotinic acetylcholine (nAChR) revealed two effects. The first effect was a fully reversible reduction of the current induced by acetylcholine (ACh), its onset being nearly instantaneous. The second effect, which developed in a few hours, was an irreversible suppression of ACh-evoked currents. The transient inhibition had an apparent Ki of 7 microM when tested with 50 nM ACh, but the percentage of inhibition was positively correlated to the ACh concentration. A reduction of ACh-induced currents which appeared immediately upon progesterone application was also observed with muscle nAChR expressed in oocytes and with nAChR on membrane patches isolated from ciliary ganglion neurons. Thus nAChRs are modulated by progesterone and steroids may play an important role in nicotinic cholinoception.     

Electrophysiological characterization of nicotinic acetylcholine receptors in cat petrosal ganglion neurons in culture: effects of cytisine and its bromo derivatives

Petrosal ganglion neurons are depolarized and fire action potentials in response to acetylcholine and nicotine. However, little is known about the subtype(s) of nicotinic acetylcholine receptors involved, although alpha4 and alpha7 subunits have been identified in petrosal ganglion neurons. Cytisine, an alkaloid unrelated to nicotine, and its bromo derivatives are agonists exhibiting different affinities, potencies and efficacies at nicotinic acetylcholine receptors containing alpha4 or alpha7 subunits. To characterize the receptors involved, we studied the effects of these agonists and the nicotinic acetylcholine receptor antagonists hexamethonium and alpha-bungarotoxin in isolated petrosal ganglion neurons. Petrosal ganglia were excised from anesthetized cats and cultured for up to 16 days. Using patch-clamp technique, we recorded whole-cell currents evoked by 5-10 s applications of acetylcholine, cytisine or its bromo derivatives. Agonists and antagonists were applied by gravity from a pipette near the neuron surface. Neurons responded to acetylcholine, cytisine, 3-bromocytisine and 5-bromocytisine with fast inward currents that desensitized during application of the stimuli and were reversibly blocked by 1 microM hexamethonium or 10 nM alpha-bungarotoxin. The order of potency of the agonists was 3-bromocytisine > acetylcholine approximately = cytisine > 5-bromocytisine, suggesting that homomeric alpha7 neuronal nicotinic receptors predominate in cat petrosal ganglion neurons in culture.     

Pharmacological and Biochemical Properties of Nicotinic Receptors from Chick Retina

Previous work has established that functional nicotinic receptors in the chick retina are blocked by neuronal bungarotoxin (NBT), and that the binding of radio-iodinated NBT to retinal homogenates is displaced by nicotinic ligands. In the present study, we examined the desensitizing effects of agonists on nicotinically-mediated depolarizations recorded from chick retina. The concentrations of five agonists necessary to reduce the amplitude of these depolarizations by 50% were found to correlate closely with the concentrations of these same agonists previously found necessary to displace 50% of NBT binding. In addition, bromoacetylcholine (BAC), a selective affinity alkylating agent for the agonist binding site, irreversibly inactivated the functional responses of intact chick retina with an inhibiting concentration for 50% block (IC50) near 10-6 M, the same concentration of BAC that displaced 50% of labelled NBT binding from alkylated retinal homogenates. These data suggest that NBT acts at the receptor agonist binding site. Furthermore, this binding site has a relatively low affinity for agonists, in the micromolar range, even in the desensitized state. Multiple subtypes of nicotinic receptors are known to exist in neuronal tissue, and receptors that bind agonists in the nanomolar range have been detergent-solubilized and purified using monoclonal antibodies. Under similar conditions, detergent-solubilization of chick retinal homogenates interfere with the interaction between NBT and the low-affinity neuronal nicotinic receptors. These data suggest that the conditions used to purify high-affinity neuronal nicotinic receptors may denature the subtype(s) of neuronal receptors recognized by NBT.     

Amino acid sequence of toxin F, a snake venom toxin that blocks neuronal nicotinic receptors

The amino acid sequence was determined for toxin F, a component of Bungarus multicinctus venom that blocks nicotinic transmission in the chick ciliary ganglion and the rat superior cervical ganglion. Toxin F was purified by a procedure that includes preparative isoelectric focusing and ion exchange chromatography. Seventy nanomolar toxin F blocks nicotinic transmission in the chick ciliary ganglion; however, the toxin only weakly blocks the binding of 125I-alpha-bungarotoxin to membranes derived from Torpedo californica electroplax (IC50 = 1 microM). These data raise the possibility that toxin F may preferentially recognize neuronal nicotinic receptors. Toxin F focused identically on an isoelectric focusing gel with samples of two similar toxins, bungarotoxin 3.1 and kappa-bungarotoxin. The sequence of toxin F is identical with that recently reported for kappa-bungarotoxin. When the N-terminal portion of bungarotoxin 3.1 was sequenced, it was found to be identical to the other two toxins. These and other data suggest that the 3 toxins are, in fact, the same.     

N-methyl-D-aspartate antagonists prevent kainate neurotoxicity in rat retinal ganglion cells in vitro.

Under defined culture conditions, exogenous glutamate (Glu), NMDA, or an endogenous Glu-related toxin is lethal to rat retinal ganglion cells; these detrimental effects are NMDA receptor mediated because specific NMDA antagonists can prevent cellular injury. In the presence of an endogenous Glu-like toxin, 125 microM kainate (KA) increases the proportion of retinal ganglion cells that die, but the toxicity (due to both KA and the endogenous toxin) is totally prevented by 2-amino-5-phosphonovalerate (APV), a specific NMDA receptor antagonist. These findings indicate that the KA-induced portion of retinal ganglion cell death also appears to be mediated via NMDA receptors. There are at least 2 possible mechanisms for this lethal effect. In addition to KA receptors, KA could directly stimulate NMDA receptors. Alternatively, KA might activate its own specific receptor, which in turn leads to a net increase in the release of an endogenous Glu-related toxin; this endogenous substance would then activate NMDA receptors. Patch-clamp electrophysiology experiments have helped to distinguish between these possibilities. Concentrations of APV that completely block the current elicited by maximal nondesensitizing doses of NMDA exert no detectable inhibition of KA-evoked currents. Hence, at the concentrations used, it appears unlikely that KA directly activates NMDA receptors in this preparation. Furthermore, the fraction of toxicity attributed to the addition of KA can be blocked by the relatively specific non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). This finding is consistent with the hypothesis that KA adds an increment of toxicity in this system by directly interacting with KA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

The mechanism of action of pachycarpine on the nicotinic cholinoreceptors of sympathetic ganglion neurons in the rat

The mechanism of the pachycarpine action was studied in isolated superior cervical ganglion of rat using the patch-clamp method in the whole-cell recording mode. Action of the pachycarpine is not potential-dependent in the membrane potential range from +30 to -50 mV, the blockade increases during hyperpolarization of the membrane (-50-90 mV). Changes in the "dose-effect" dependence show that the blocking of nicotinic acetylcholine receptors is competitive both within the potential-dependent interval and within the potential-independent one.     

Activated retinal glia mediated axon regeneration in experimental glaucoma

Glaucoma, a leading cause of blindness, is a neurodegenerative disease characterized by progressive loss of retinal ganglion cell axons in the optic nerve and their cell bodies in the retina. Reactive retinal glial changes have been observed in glaucoma but the role of such glial changes in the pathogenesis of the condition remains unclear.In the present study we found that retinal ganglion cells in an experimental animal model of glaucoma have an increased axon regenerative potential. Regeneration of adult rat retinal ganglion cell axons after optic nerve crush was significantly increased in vivo when combined with intraocular pressure-induced experimental glaucoma. This enhanced axon regeneration response was correlated with a significant increase in activation of glial fibrillary acidic protein + retinal glia. Using a dissociated retinal ganglion cell culture model we showed that reducing the number of activated retinal glia with a glial specific toxin, α-Aminoadipic acid, significantly reduced the growth potential of retinal ganglion cells from glaucomatous rat eyes, suggesting that activated retinal glia mediate, at least in part, the growth promoting effect. This was shown to be mediated by both membrane-bound and soluble glial-derived factors. Neurotrophin and ciliary neurotrophic/leukemia inhibitory factor blockers did not affect the regenerative potential, excluding these growth factors as principal mediators of the enhanced growth response occurring in glaucomatous retinal cultures.These observations are the first to reveal that retinal ganglion cells from glaucomatous rat eyes have an enhanced regenerative capacity. Furthermore, our results suggest that activated retinal glia mediate at least part of this response. Further work to understand and enhance the regeneration-promoting effect of activated retinal glia is required to determine if this approach could be useful as part of a therapeutic strategy to encourage optic nerve regeneration in glaucoma.     

Denervation does not alter the number of neuronal bungarotoxin binding sites on autonomic neurons in the frog cardiac ganglion

The binding of neuronal bungarotoxin (n-BuTX; also known as bungarotoxin 3.1, kappa-bungarotoxin, and toxin F) was analyzed in normal and denervated parasympathetic cardiac ganglia of the frog Rana pipiens, n-BuTX blocks both EPSPs and ACh potentials at 5-20 nM, as determined by intracellular recording techniques. Scatchard analysis on homogenates indicates that cardiac ganglia have two classes of binding sites for 125I-n-BuTX: a high-affinity site with an apparent dissociation constant (Kd,app) of 1.7 nM and a Bmax (number of binding sites) of 3.8 fmol/ganglion and a low-affinity site with a Kd,app of 12 microM and a Bmax of 14 pmol/ganglion. alpha-Bungarotoxin does not appear to interfere with the binding of 125I-n-BuTX to either site. The high-affinity binding site is likely to be the functional nicotinic ACh receptor (AChR), given the similarity between its affinity for 125I-n-BuTX and the concentration of n-BuTX required to block AChR function. Light microscopic autoradiographic analysis of 125I-n-BuTX binding to the ganglion cell surface reveals that toxin binding is concentrated at synaptic sites, which were identified using a synaptic vesicle-specific antibody. Scatchard analysis of autoradiographic data reveals that 125I-n-BuTX binding to the neuronal surface is saturable and has a Kd,app similar to that of the high-affinity binding site characterized in homogenates. Surface binding of 125I-n-BuTX is blocked by nicotine, carbachol, and d-tubocurarine (IC50 less than 20 microM), but not by atropine (IC50 greater than 10 mM). Denervation of the heart increases the ACh sensitivity of cardiac ganglion cells but has no effect upon the number of high-affinity binding sites for 125I-n-BuTX in tissue homogenates. Moreover, autoradiographic analysis indicates that denervation does not alter the number of 125I-n-BuTX binding sites on the ganglion cell surface. n-BuTX is as effective in reducing ganglion cell responses to ACh in denervated ganglia as it is in normally innervated ganglia. These results suggest that denervation alters neither the total number of nicotinic AChRs in the cardiac ganglion nor the number found on the surface of ganglion cells. These autonomic neurons thus respond differently to denervation than do skeletal myofibers. The increase in ACh sensitivity displayed by cardiac ganglion cells upon denervation cannot be explained by changes in AChR number.     

VIP and PACAP potentiation of nicotinic ACh-evoked currents in rat parasympathetic neurons is mediated by G-protein activation

The effects of vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP27 and PACAP38) on isolated parasympathetic neurons of rat intracardiac and submandibular ganglia were examined under voltage clamp using whole-cell patch-clamp recording techniques. VIP and PACAP (相似文献   

Ouabain induces an increase of retinal ganglion cell survival in vitro: the involvement of protein kinase C

Ouabain is a steroid derivative that can regulate many cellular events such as growth and proliferation. It modulates Na+,K+-ATPase activity leading to the activation of different intracellular pathways through protein-protein interactions that have been characterized during the last few years. The aim of this work was to study the role of ouabain in rat retinal ganglion cell survival after 48 h in culture. Our results demonstrated that ouabain significantly induced an increase in retinal ganglion cell survival. The effect was dose-dependent and was maximal with 3.0 nM. The blockade of protein kinase C activity by 1.25 microM chelerythrine chloride abolished the ouabain effect, indicating an involvement of this intracellular pathway. None of the protein kinase inhibitors usually employed in the study of ouabain-driven intracellular pathways (PD98059, Ly294002, herbimycin, and genistein) was able to influence neuronal survival induced by ouabain. The data presented suggest that ouabain may be the trigger of an intracellular pathway responsible for neuronal survival.     

Localization and function of cat carotid body nicotinic receptors

Acetylcholine and nicotinic agents excite cat carotid body chemoreceptors and modify their response to natural stimuli. The present experiments utilized [125I]alpha-bungarotoxin [( 125I]alpha-BGT) to localize within the chemosensory tissue the possible sites of action of exogenous and endogenous nicotinic cholinergic substances. In vitro equilibrium binding studies of intact carotid bodies determined a Kd of 5.57 nM and a Bmax of 9.21 pmol/g of tissue. Chronic section (12-15 days) of the carotid sinus nerve (CSN) did not change the amount of displaceable toxin binding. In contrast, the specific binding was reduced by 46% following removal of the superior cervical ganglion. Light microscope autoradiography of normal, CSN-denervated and sympathectomized carotid bodies revealed displaceable binding sites concentrated in lobules of type I and type II cells. Treatment of carotid bodies with 50 nM alpha-BGT in vitro reduced by 50% the release of [3H]dopamine (synthesized from [3H]tyrosine) caused by hypoxia or nicotine, and also significantly reduced the stimulus-evoked discharges recorded from the CSN. The data suggest an absence of alpha-BGT binding sites on the afferent terminals of the CSN and that nicotinic receptors located with parenchymal cell lobules may modulate the release of catecholamines from these cells.     

Chromogranin A inhibits retinal dopamine release

Chromogranin A (CGA), a peripherally active prohormone, is a soluble component in the secretory granules of many endocrine tissues and is cosecreted with their peptide or amine hormones. Using an antibody prepared against purified rat adrenal CGA, immunostaining was localized to the inner and outer plexiform layers of the rat retina and to selected ganglion cells. Exogenous CGA (purified from human adrenals) when applied to perfused rat retina potently inhibited the potassium-induced release of endogenous dopamine (DA). This action was dose-dependent, with an IC₅₀ of 3 nM; at 100 nM CGA retinal DA release was completely abolished.