Ionic Current Variability and Functional Stability in the Nervous System

Abstract

Identified neurons in different animals express ionic currents at highly variable levels (population variability). If neuronal identity is associated with stereotypical function, as is the case in genetically identical neurons or in unambiguously identified individual neurons, this variability poses a conundrum: How is activity the same if the components that generate it—ionic current levels—are different? In some cases, ionic current variability across similar neurons generates an output gradient. However, many neurons produce very similar output activity, despite substantial variability in ionic conductances. It appears that, in many such cells, conductance levels of one ionic current vary in proportion to the conductance levels of another current. As a result, in a population of neurons, these conductances appear to be correlated. Here, I review theoretical and experimental work that suggests that neuronal ionic current correlation can reduce the global ionic current variability and can contribute to functional stability.

Neuropeptide Receptor Transcript Expression Levels and Magnitude of Ionic Current Responses Show Cell Type-Specific Differences in a Small Motor Circuit

“…It mainly acts as a neurohormone in arthropods and plays important roles in the regulation of heart contractions (Stangier et al, 1987;Tublitz, 1989 , 1997;Donini et al, 2002;Sakai et al, 2004;Sakai et al, 2006); and oviduct contractions (Donini et al, 2001). In the STG, it acts on a number of neurons and alters rhythmic motor activity (Weimann et al, 1997;Richards and Marder, 2000;Marder, 2000, 2001;DeLong et al, 2009), as well as contraction properties of target muscles (JorgeRivera et al, 1998). GPCRs specific for CCAP have been identified in a number of insect species (Cazzamali et al, 2003;Belmont et al, 2006;Arakane et al, 2008;Li et al, 2011;Lee et al, 2013), but so far not in crustaceans.…”

“…I MI cannot be measured in voltage clamp from the anterior burster (AB) neuron, likely due to insufficient space clamp in soma recordings, but CCAP depolarizes synaptically isolated AB neurons in current-clamp recordings ). Among gastric neurons, only LG has been shown to activate I MI in response to CCAP application (DeLong et al, 2009). However, all other gastric mill neurons have been tested for depolarizing responses in current clamp .…”

“…We did not measure the AB to VD connection, the other synapse onto VD, but both LG DeLong et al, 2009) and AB ) respond to CCAP in synaptic isolation, and both express CbCCAPr mRNA (Fig. 4).…”

“…The CbCCAPr transcript was differentially expressed across different neuron types and found in all neurons previously shown to respond to CCAP Marder, 2000, 2001;DeLong et al, 2009). In most cases, neurons lacking CCAP modulation of excitability did not express CbCCAPr.…”

“…In contrast, neuropeptides appear to have only a limited number of subcellular targets, and each neuropeptide acts on only a specific subset of circuit neurons. A number of neuropeptides converge onto the same voltage-gated modulator-activated inward current, I MI (Golowasch and Marder, 1992;Swensen and Marder, 2000;DeLong et al, 2009), and there are only a few reports of neuropeptide actions on synapses (Thirumalai et al, 2006;Zhao et al, 2011) or other voltage-gated ion channels (Rodriguez et al, 2013).…”