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Neural signalling: Does colocalization imply cotransmission?
Dispatch
R809
Neural signalling: Does colocalization imply cotransmission? Eve Marder
Recent results suggest that neurons that contain multiple neurotransmitters may make synaptic connections with different target neurons that are mediated by only a subset of their transmitter complement. Address: Volen Center and Biology Department, Brandeis University, Waltham, Massachusetts 02454, USA. E-mail: [email protected] Current Biology 1999, 9:R809–R811 0960-9822/99/$ – see front matter © 1999 Elsevier Science Ltd. All rights reserved.
Today it is widely accepted that many, perhaps most, neurons contain multiple signaling substances that can function as neurotransmitters [1]. It is, however, far easier to demonstrate the presence of multiple substances — such as amino acids, amines, neuropeptides, ATP and so on — within a neuron than it is to establish their physiological roles, or indeed to show that they have any biological activity at all [2]. It is not an accident that much of what we know about the release of cotransmitters from identified neurons has been discovered using peripheral synapses of invertebrates and vertebrates [1,2], and that we know relatively little about the control of cotransmitter release from most central synapses. Many neurons project to multiple postsynaptic sites, often at quite large distances from each other. In extreme cases, neurons may send projections to a number of different brain regions or ganglia, and synapse onto several different types of target neuron. In such cases, do neurons that contain multiple transmitter substances release all of their cotransmitters at all of their targets, or can they liberate
different subsets of their transmitter complement at different synaptic endings? In some cases, it is clear that several cotransmitters are liberated at the same terminals [1–3], although it has been suggested that some neurons may segregate different bioactive substances to different subsets of their branches [4]. Figure 1 illustrates several possibilities that could occur when a neuron produces multiple neurotransmitters. In case 1, a neuron liberates the same mixture of neurotransmitters at each of its endings, and all of its targets respond to all of its cotransmitters. In case 2, the neuron again liberates the same mixture of neurotransmitters at each of its endings, but here the different target neurons have receptors for different subsets of the cotransmitters, thereby creating the situation in which the different synapses made by the same neuron are mediated by different transmitter substances. In case 3, the neuron liberates different substances from different presynaptic terminals, so that different synapses evoked by the same neuron are mediated by different neurotransmitters. A recent study by Blitz and Nusbaum [5] addressed this issue in the case of the modulatory proctolin neuron (MPN) of the stomatogastric nervous system of the crab Cancer borealis [6,7]. The MPN has its soma in the oesophageal ganglion and projects to the paired commissural ganglia and to the single stomatogastric ganglion (Figure 2). Immunoctyochemical studies showed that MPN contains the neuropeptide proctolin [6,7] and the inhibitory neurotransmitter γ-amino butyric acid (GABA) [8]. Previous work demonstrated that the actions of the MPN in the stomatogastric ganglion could be well-mimicked by proctolin [6,7]. But
Figure 1 Three cases describing the possible postsynaptic actions of neurons that produce multiple cotransmitters. In case 1, the neuron has two different cotransmitters — represented by the dark red and dark blue synaptic vesicles — which are both liberated from all of the presynaptic terminals of the neuron, and all of the postsynaptic sites contain receptors for both transmitters (light red and light blue). In case 2, all of the processes of the neuron release both cotransmitters, but the target neurons have receptors to just one of the two cotransmitters. In case 3, different processes of the neuron liberate different cotransmitters, although the targets may have receptors for both.
Case 1
Case 2
Case 3
Current Biology
R810
Current Biology Vol 9 No 21
Figure 2
MPN
OG CPN2
CoG
STG Current Biology
The stomatogastric nervous system of the crab Cancer borealis, showing the projections of the modulatory proctolin neuron (MPN, red) and those of the commissural ganglion neuron CPN2. As discussed in the text, the MPN releases GABA (green) in the commissural ganglion (CoG) and proctolin (blue) in the stamatogastic ganglion (STG). OG, oesophageal ganglion.
on the basis of their new findings, Blitz and Nusbaum [5] argue that the MPN liberates GABA, but not proctolin, onto its targets in the commissural ganglia. The MPN inhibits two neurons of the commissural ganglion, MCN1 and CPN2 (Figure 2). Because both of these neurons project to the crab’s stomatogastric ganglion, where they activate the gastric mill rhythm [9,10], inhibition of MCN1 and CPN2 by the MPN results in inhibition of the gastric mill rhythm [11]. Blitz and Nusbaum [5] asked whether the inhibitory actions of the MPN on MCN1 and CPN2 could be mimicked by proctolin and/or GABA. When proctolin was applied to the commissural ganglia, strong gastric mill rhythms were evoked, and MCN1 and CPN2 were strongly activated. Moreover, when synaptic transmission in the commissural ganglia was blocked by placing them in low Ca2+/Mn2+-containing saline, proctolin applications depolarized both MCN1 and CPN2. These results argue that, although MCN1 and CPN2 have proctolin receptors, they evoke an excitatory response, similar to that evoked by proctolin on stomatogastric ganglion neurons [12]. The application of GABA, in contrast, inhibited both MCN1 and CPN2, and these GABA responses persisted in low Ca2+/Mn2+ saline. Furthermore, the effects of both GABA and the MPN on MCN1 and CPN2 could be blocked by picrotoxin. Together, the data reported by Blitz and Nusbaum [5] are consistent with the interpretation that MPN liberates
GABA, but not proctolin, from its terminals within the commissural ganglia, but proctolin from its terminals in the stomatogastric ganglion. These data suggest that, of the various possibilities described above, case 3 (Figure 1) most accurately accounts for the functional connections made by MPN. What is still not known for this preparation, is whether the terminals of MPN in the commissural ganglia contain both proctolin and GABA — as do their somata in the oesophageal ganglion — but only release GABA (at least under the physiological conditions studied here), or whether the different neurotransmitters are differentially sorted into its different projections. Resolving this issue will require immunocytochemical studies, at the electron microscope level, on the MPN profiles in the neuropils of the stomatogastric ganglion and commissural ganglia. In the mid 1930s, Sir Henry Dale wrote one of the most misquoted papers in the history of neuroscience [13]. Dale knew that the neurotransmitter acetylcholine inhibited some of its targets and excited others. For many years ‘Dale’s Law’ was that a neuron contains only one neurotransmitter that it releases from all of its terminals. In his classic paper [13], Dale described the findings of the time that, in cross-regeneration experiments, one kind of cholinergic neuron could substitute for another, but adrenergic postganglionic neurons could not substitute for cholinergic neurons. It was observations of this kind that Dale was commenting on when he said “the nature of the chemical function, whether cholinergic or adrenergic, is characteristic for each particular neurone, and unchangeable”. We now know that neurons can release many different neurotransmitters, and that the same neuron can evoke functionally different postsynaptic responses on different targets. The paper by Blitz and Nusbaum [5] argues that the somatic colocalization of neurotransmitters does not imply functional cotransmission at all of a neuron’s terminals. One imagines that Dale would take great pleasure in seeing the rich functional diversity possible with colocalization and cotransmission. Acknowledgements Research supported by the W.M. Keck Foundation and NS17813 from the National Institute of Health.
References 1. Kupfermann I: Functional studies of cotransmission. Physiol Rev 1991, 71:683-732. 2. Lundberg JM: Pharmacology of cotransmission in the autonomic nervous system: integrative aspects on amines, neuropeptides, adenosine triphosphate, amino acids, and nitric oxide. Pharmacological Rev 1996, 48:113-178. 3. Vilim FS, Price DA, Lesser W, Kupfermann I, Weiss KR: Costorage and corelease of modulatory peptide cotransmitters with partially antagonistic actions of the accessory radula closer muscle of Aplysia californica. J Neurosci 1996, 16:8092-8104. 4. Sossin WS, Sweet-Cordero A, Scheller RH: Dale’s hypothesis revisited: different neuropeptides derived from a common prohormone are targeted to different processes. Proc Natl Acad Sci USA 1990, 87:4845-4848. 5. Blitz DM, Nusbaum MP: Distinct functions for cotransmitters mediating motor pattern selection. J Neurosci 1999, 19:6774-6783.
Dispatch
6. Nusbaum MP, Marder E: A modulatory proctolin-containing neuron (MPN). I. Identification and characterization. J Neurosci 1989, 9:1591-1599. 7. Nusbaum MP, Marder E: A modulatory proctolin-containing neuron (MPN). II. State-dependent modulation of rhythmic motor activity. J Neurosci 1989, 9:1600-1607. 8. Blitz DM, Christie AE, Coleman MJ, Norris BJ, Marder E, Nusbaum MP: Different proctolin neurons elicit distinct motor patterns from a multifunctional neuronal network. J Neurosci 1999, 19:5449-5463. 9. Coleman MJ, Meyrand P, Nusbaum MP: A switch between two modes of synaptic transmission mediated by presynaptic inhibition. Nature 1995, 378:502-505. 10. Norris BJ, Coleman MJ, Nusbaum MP: Recruitment of a projection neuron determines gastric mill motor pattern selection in the stomatogastric nervous system of the crab, Cancer borealis. J Neurophysiol 1994, 72:1451-1463. 11. Blitz DM, Nusbaum MP: Motor pattern selection via inhibition of parallel pathways. J Neurosci 1997, 17:4965-4975. 12. Golowasch J, Marder E: Proctolin activates an inward current whose voltage dependence is modified by extracellular Ca2+. J Neurosci 1992, 12:810-817. 13. Dale H: Pharmacology and nerve endings. Proc R Soc Med 1935, 28:319-332.
R811
If you found this dispatch interesting, you might also want to read the June 1999 issue of
Current Opinion in Neurobiology which included the following reviews, edited by Mary B Kennedy and Mu-ming Poo, on Signalling mechanisms : The bacterial K+ channel structure and its implications for neuronal channels Gary Yellen Calcium channelopathies in the central nervous system Joanna Jen Seizure disorders in mutant mice: relevance to human epilepsies Ram S Puranam and James O McNamara Regulation of back-propagating action potentials in hippocampal neurons Daniel Johnston, Dax A Hoffman, Costa M Colbert and Jeffrey C Magee Clearance of glutamate inside the synapse and beyond Dwight E Bergles, Jeffrey S Diamond and Craig E Jahr Roles of metabotropic glutamate receptors in LTP and LTD in the hippocampus Zuner A Bortolotto, Stephen M Fitzjohn and Graham L Collingridge Calcium- and activity-dependent synaptic plasticity Robert S Zucker Optical detection of synaptic vesicle exocytosis and endocytosis Venkatesh N Murthy Synaptic vesicle docking and fusion Sandra M Bajjalieh Assembly of signaling machinery at the postsynaptic membrane Joachim Kirsch Brain protein serine/threonine phosphatases Nancy E Price and Marc C Mumby Structure, development, and plasticity of dendritic spines Kristen M Harris Effects of estrogen in the CNS Catherine S Woolley Signal transduction underlying growth cone guidance by diffusible factors Hong-jun Song and Mu-ming Poo Axonal atrophy: the retraction reaction Michael Bernstein and Jeff W Lichtman The full text of Current Opinion in Neurobiology is in the BioMedNet library at http://BioMedNet.com/cbiology/nrb
R809
Neural signalling: Does colocalization imply cotransmission? Eve Marder
Recent results suggest that neurons that contain multiple neurotransmitters may make synaptic connections with different target neurons that are mediated by only a subset of their transmitter complement. Address: Volen Center and Biology Department, Brandeis University, Waltham, Massachusetts 02454, USA. E-mail: [email protected] Current Biology 1999, 9:R809–R811 0960-9822/99/$ – see front matter © 1999 Elsevier Science Ltd. All rights reserved.
Today it is widely accepted that many, perhaps most, neurons contain multiple signaling substances that can function as neurotransmitters [1]. It is, however, far easier to demonstrate the presence of multiple substances — such as amino acids, amines, neuropeptides, ATP and so on — within a neuron than it is to establish their physiological roles, or indeed to show that they have any biological activity at all [2]. It is not an accident that much of what we know about the release of cotransmitters from identified neurons has been discovered using peripheral synapses of invertebrates and vertebrates [1,2], and that we know relatively little about the control of cotransmitter release from most central synapses. Many neurons project to multiple postsynaptic sites, often at quite large distances from each other. In extreme cases, neurons may send projections to a number of different brain regions or ganglia, and synapse onto several different types of target neuron. In such cases, do neurons that contain multiple transmitter substances release all of their cotransmitters at all of their targets, or can they liberate
different subsets of their transmitter complement at different synaptic endings? In some cases, it is clear that several cotransmitters are liberated at the same terminals [1–3], although it has been suggested that some neurons may segregate different bioactive substances to different subsets of their branches [4]. Figure 1 illustrates several possibilities that could occur when a neuron produces multiple neurotransmitters. In case 1, a neuron liberates the same mixture of neurotransmitters at each of its endings, and all of its targets respond to all of its cotransmitters. In case 2, the neuron again liberates the same mixture of neurotransmitters at each of its endings, but here the different target neurons have receptors for different subsets of the cotransmitters, thereby creating the situation in which the different synapses made by the same neuron are mediated by different transmitter substances. In case 3, the neuron liberates different substances from different presynaptic terminals, so that different synapses evoked by the same neuron are mediated by different neurotransmitters. A recent study by Blitz and Nusbaum [5] addressed this issue in the case of the modulatory proctolin neuron (MPN) of the stomatogastric nervous system of the crab Cancer borealis [6,7]. The MPN has its soma in the oesophageal ganglion and projects to the paired commissural ganglia and to the single stomatogastric ganglion (Figure 2). Immunoctyochemical studies showed that MPN contains the neuropeptide proctolin [6,7] and the inhibitory neurotransmitter γ-amino butyric acid (GABA) [8]. Previous work demonstrated that the actions of the MPN in the stomatogastric ganglion could be well-mimicked by proctolin [6,7]. But
Figure 1 Three cases describing the possible postsynaptic actions of neurons that produce multiple cotransmitters. In case 1, the neuron has two different cotransmitters — represented by the dark red and dark blue synaptic vesicles — which are both liberated from all of the presynaptic terminals of the neuron, and all of the postsynaptic sites contain receptors for both transmitters (light red and light blue). In case 2, all of the processes of the neuron release both cotransmitters, but the target neurons have receptors to just one of the two cotransmitters. In case 3, different processes of the neuron liberate different cotransmitters, although the targets may have receptors for both.
Case 1
Case 2
Case 3
Current Biology
R810
Current Biology Vol 9 No 21
Figure 2
MPN
OG CPN2
CoG
STG Current Biology
The stomatogastric nervous system of the crab Cancer borealis, showing the projections of the modulatory proctolin neuron (MPN, red) and those of the commissural ganglion neuron CPN2. As discussed in the text, the MPN releases GABA (green) in the commissural ganglion (CoG) and proctolin (blue) in the stamatogastic ganglion (STG). OG, oesophageal ganglion.
on the basis of their new findings, Blitz and Nusbaum [5] argue that the MPN liberates GABA, but not proctolin, onto its targets in the commissural ganglia. The MPN inhibits two neurons of the commissural ganglion, MCN1 and CPN2 (Figure 2). Because both of these neurons project to the crab’s stomatogastric ganglion, where they activate the gastric mill rhythm [9,10], inhibition of MCN1 and CPN2 by the MPN results in inhibition of the gastric mill rhythm [11]. Blitz and Nusbaum [5] asked whether the inhibitory actions of the MPN on MCN1 and CPN2 could be mimicked by proctolin and/or GABA. When proctolin was applied to the commissural ganglia, strong gastric mill rhythms were evoked, and MCN1 and CPN2 were strongly activated. Moreover, when synaptic transmission in the commissural ganglia was blocked by placing them in low Ca2+/Mn2+-containing saline, proctolin applications depolarized both MCN1 and CPN2. These results argue that, although MCN1 and CPN2 have proctolin receptors, they evoke an excitatory response, similar to that evoked by proctolin on stomatogastric ganglion neurons [12]. The application of GABA, in contrast, inhibited both MCN1 and CPN2, and these GABA responses persisted in low Ca2+/Mn2+ saline. Furthermore, the effects of both GABA and the MPN on MCN1 and CPN2 could be blocked by picrotoxin. Together, the data reported by Blitz and Nusbaum [5] are consistent with the interpretation that MPN liberates
GABA, but not proctolin, from its terminals within the commissural ganglia, but proctolin from its terminals in the stomatogastric ganglion. These data suggest that, of the various possibilities described above, case 3 (Figure 1) most accurately accounts for the functional connections made by MPN. What is still not known for this preparation, is whether the terminals of MPN in the commissural ganglia contain both proctolin and GABA — as do their somata in the oesophageal ganglion — but only release GABA (at least under the physiological conditions studied here), or whether the different neurotransmitters are differentially sorted into its different projections. Resolving this issue will require immunocytochemical studies, at the electron microscope level, on the MPN profiles in the neuropils of the stomatogastric ganglion and commissural ganglia. In the mid 1930s, Sir Henry Dale wrote one of the most misquoted papers in the history of neuroscience [13]. Dale knew that the neurotransmitter acetylcholine inhibited some of its targets and excited others. For many years ‘Dale’s Law’ was that a neuron contains only one neurotransmitter that it releases from all of its terminals. In his classic paper [13], Dale described the findings of the time that, in cross-regeneration experiments, one kind of cholinergic neuron could substitute for another, but adrenergic postganglionic neurons could not substitute for cholinergic neurons. It was observations of this kind that Dale was commenting on when he said “the nature of the chemical function, whether cholinergic or adrenergic, is characteristic for each particular neurone, and unchangeable”. We now know that neurons can release many different neurotransmitters, and that the same neuron can evoke functionally different postsynaptic responses on different targets. The paper by Blitz and Nusbaum [5] argues that the somatic colocalization of neurotransmitters does not imply functional cotransmission at all of a neuron’s terminals. One imagines that Dale would take great pleasure in seeing the rich functional diversity possible with colocalization and cotransmission. Acknowledgements Research supported by the W.M. Keck Foundation and NS17813 from the National Institute of Health.
References 1. Kupfermann I: Functional studies of cotransmission. Physiol Rev 1991, 71:683-732. 2. Lundberg JM: Pharmacology of cotransmission in the autonomic nervous system: integrative aspects on amines, neuropeptides, adenosine triphosphate, amino acids, and nitric oxide. Pharmacological Rev 1996, 48:113-178. 3. Vilim FS, Price DA, Lesser W, Kupfermann I, Weiss KR: Costorage and corelease of modulatory peptide cotransmitters with partially antagonistic actions of the accessory radula closer muscle of Aplysia californica. J Neurosci 1996, 16:8092-8104. 4. Sossin WS, Sweet-Cordero A, Scheller RH: Dale’s hypothesis revisited: different neuropeptides derived from a common prohormone are targeted to different processes. Proc Natl Acad Sci USA 1990, 87:4845-4848. 5. Blitz DM, Nusbaum MP: Distinct functions for cotransmitters mediating motor pattern selection. J Neurosci 1999, 19:6774-6783.
Dispatch
6. Nusbaum MP, Marder E: A modulatory proctolin-containing neuron (MPN). I. Identification and characterization. J Neurosci 1989, 9:1591-1599. 7. Nusbaum MP, Marder E: A modulatory proctolin-containing neuron (MPN). II. State-dependent modulation of rhythmic motor activity. J Neurosci 1989, 9:1600-1607. 8. Blitz DM, Christie AE, Coleman MJ, Norris BJ, Marder E, Nusbaum MP: Different proctolin neurons elicit distinct motor patterns from a multifunctional neuronal network. J Neurosci 1999, 19:5449-5463. 9. Coleman MJ, Meyrand P, Nusbaum MP: A switch between two modes of synaptic transmission mediated by presynaptic inhibition. Nature 1995, 378:502-505. 10. Norris BJ, Coleman MJ, Nusbaum MP: Recruitment of a projection neuron determines gastric mill motor pattern selection in the stomatogastric nervous system of the crab, Cancer borealis. J Neurophysiol 1994, 72:1451-1463. 11. Blitz DM, Nusbaum MP: Motor pattern selection via inhibition of parallel pathways. J Neurosci 1997, 17:4965-4975. 12. Golowasch J, Marder E: Proctolin activates an inward current whose voltage dependence is modified by extracellular Ca2+. J Neurosci 1992, 12:810-817. 13. Dale H: Pharmacology and nerve endings. Proc R Soc Med 1935, 28:319-332.
R811
If you found this dispatch interesting, you might also want to read the June 1999 issue of
Current Opinion in Neurobiology which included the following reviews, edited by Mary B Kennedy and Mu-ming Poo, on Signalling mechanisms : The bacterial K+ channel structure and its implications for neuronal channels Gary Yellen Calcium channelopathies in the central nervous system Joanna Jen Seizure disorders in mutant mice: relevance to human epilepsies Ram S Puranam and James O McNamara Regulation of back-propagating action potentials in hippocampal neurons Daniel Johnston, Dax A Hoffman, Costa M Colbert and Jeffrey C Magee Clearance of glutamate inside the synapse and beyond Dwight E Bergles, Jeffrey S Diamond and Craig E Jahr Roles of metabotropic glutamate receptors in LTP and LTD in the hippocampus Zuner A Bortolotto, Stephen M Fitzjohn and Graham L Collingridge Calcium- and activity-dependent synaptic plasticity Robert S Zucker Optical detection of synaptic vesicle exocytosis and endocytosis Venkatesh N Murthy Synaptic vesicle docking and fusion Sandra M Bajjalieh Assembly of signaling machinery at the postsynaptic membrane Joachim Kirsch Brain protein serine/threonine phosphatases Nancy E Price and Marc C Mumby Structure, development, and plasticity of dendritic spines Kristen M Harris Effects of estrogen in the CNS Catherine S Woolley Signal transduction underlying growth cone guidance by diffusible factors Hong-jun Song and Mu-ming Poo Axonal atrophy: the retraction reaction Michael Bernstein and Jeff W Lichtman The full text of Current Opinion in Neurobiology is in the BioMedNet library at http://BioMedNet.com/cbiology/nrb