Biomedical subjects

A M Swensen

Publications and source records attributed to A M Swensen.

4 recordsLinked to original sources

PubMed2001

Modulators with convergent cellular actions elicit distinct circuit outputs.

Six neuromodulators [proctolin, Cancer borealis tachykinin-related peptide Ia, crustacean cardioactive peptide (CCAP), red pigment-concentrating hormone, TNRNFLRFamide, and pilocarpine] converge onto the same voltage-dependent inward current in stomatogastric ganglion (STG) neurons of the crab C. borealis. We show here that each of these modulators acts on a distinct subset of pyloric network neurons in the STG. To ask whether the differences in cell targets could account for their differential effects on the pyloric rhythm, we systematically compared the motor patterns produced by proctolin and CCAP. The motor patterns produced in proctolin and CCAP differed quantitatively in a number of ways. Proctolin and CCAP both act on the lateral pyloric neuron and the inferior cardiac neuron. Proctolin additionally acts on the pyloric dilator (PD) neurons, the pyloric (PY) neurons, and the ventricular dilator neuron. Using the dynamic clamp, we introduced an artificial peptide-elicited current into the PD and PY neurons, in the presence of CCAP, and converted the CCAP rhythm into a rhythm that was statistically similar to that seen in proctolin. This suggests that the differences in the network effects of these two modulators can primarily be attributed to the known differential distributions of their receptors onto distinct subsets of neurons, despite the fact that they activate the same current.

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PubMed2001

The roles of co-transmission in neural network modulation.

Neuromodulation provides considerable flexibility to the output of neural networks. In spite of the extensive literature documenting the presence of modulatory peptide co-transmitters in many neurons, considerably less is known about the specific roles of co-transmission in circuit function. This review describes some of the potential consequences of peptide co-transmission in functional circuits, using specific examples from recent work on the actions of identified peptidergic projection neurons acting on the multifunctional neural network within the crustacean stomatogastric ganglion. This system reveals that co-transmission provides projection neurons with a rich assortment of strategies for eliciting multiple outputs from a multifunctional network.

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PubMed2000

Multiple peptides converge to activate the same voltage-dependent current in a central pattern-generating circuit.

The stomatogastric ganglion of the crab, Cancer borealis, is modulated by >20 different substances, including numerous neuropeptides. One of these peptides, proctolin, activates an inward current that shows strong outward rectification (Golowasch and Marder, 1992). Decreasing the extracellular Ca(2+) concentration linearizes the current-voltage curve of the proctolin-induced current. We used voltage clamp to study the currents evoked by proctolin and five additional modulators [C. borealis tachykinin-related peptide Ia (CabTRP Ia), crustacean cardioactive peptide, red pigment-concentrating hormone, TNRNFLRFamide, and the muscarinic agonist pilocarpine] in stomatogastric ganglion neurons, both in the intact ganglion and in dissociated cell culture. Subtraction currents yielded proctolin-like current-voltage relationships for all six substances, and the current-voltage curves of all six substances showed linearization in low external Ca(2+). The lateral pyloric neuron responded to all six modulators, but the ventricular dilator neuron only responded to a subset of them. Bath application of saturating concentrations of proctolin occluded the response to CabTRP and vice versa. N-(6-Aminohexyl)-5-chloro-1-napthalensulfonamide, a calmodulin inhibitor, increased the amplitude and altered the voltage dependence of the responses elicited by CabTRP and proctolin. Together, these data indicate that all six substances converge onto the same voltage-dependent current, although they activate different receptors. Therefore, differential network responses evoked by these substances may primarily depend on the receptor distribution on network neurons.

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PubMed2000

GABA and responses to GABA in the stomatogastric ganglion of the crab Cancer borealis.

The multifunctional neural circuits in the crustacean stomatogastric ganglion (STG) are influenced by many small-molecule transmitters and neuropeptides that are co-localized in identified projection neurons to the STG. We describe the pattern of gamma-aminobutyric acid (GABA) immunoreactivity in the stomatogastric nervous system of the crab Cancer borealis and demonstrate biochemically the presence of authentic GABA in C. borealis. No STG somata show GABA immunoreactivity but, within the stomatogastric nervous system, GABA immunoreactivity co-localizes with several neuropeptides in two identified projection neurons, the modulatory proctolin neuron (MPN) and modulatory commissural neuron 1 (MCN1). To determine which actions of these neurons are evoked by GABA, it is necessary to determine the physiological actions of GABA on STG neurons. We therefore characterized the response of each type of STG neuron to focally applied GABA. All STG neurons responded to GABA. In some neurons, GABA evoked a picrotoxin-sensitive depolarizing, excitatory response with a reversal potential of approximately -40 mV. This response was also activated by muscimol. In many STG neurons, GABA evoked inhibitory responses with both K(+)- and Cl(-)-dependent components. Muscimol and beta-guanidinopropionic acid weakly activated the inhibitory responses, but many other drugs, including bicuculline and phaclofen, that act on vertebrate GABA receptors were not effective. In summary, GABA is found in projection neurons to the crab STG and can evoke both excitatory and inhibitory actions on STG neurons.

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