Neuromodulation independently determines correlated channel expression and conductance levels in motor neurons of the stomatogastric ganglion

Abstract

Neuronal identity depends on the regulated expression of numerous molecular components, especially ionic channels, which determine the electrical signature of a neuron. Such regulation depends on at least two key factors, activity itself and neuromodulatory input. Neuronal electrical activity can modify the expression of ionic currents in homeostatic or nonhomeostatic fashion. Neuromodulators typically modify activity by regulating the properties or expression levels of subsets of ionic channels. In the stomatogastric system of crustaceans, both types of regulation have been demonstrated. Furthermore, the regulation of the coordinated expression of ionic currents and the channels that carry these currents has been recently reported in diverse neuronal systems, with neuromodulators not only controlling the absolute levels of ionic current expression but also, over long periods of time, appearing to modify their correlated expression. We hypothesize that neuromodulators may regulate the correlated expression of ion channels at multiple levels and in a cell-type-dependent fashion. We report that in two identified neuronal types, three ionic currents are linearly correlated in a pairwise manner, suggesting their coexpression or direct interactions, under normal neuromodulatory conditions. In each cell, some currents remain correlated after neuromodulatory input is removed, whereas the correlations between the other pairs are either lost or altered. Interestingly, in each cell, a different suite of currents change their correlation. At the transcript level we observe distinct alterations in correlations between channel mRNA amounts, including one of the cell types lacking a correlation under normal neuromodulatory conditions and then gaining the correlation when neuromodulators are removed. Synaptic activity does not appear to contribute, with one possible exception, to the correlated expression of either ionic currents or of the transcripts that code for the respective channels. We conclude that neuromodulators regulate the correlated expression of ion channels at both the transcript and the protein levels.

Fig. 1.

Stomatogastric nervous system and activity changes under different modulatory conditions. A: the stomatogastric nervous system. OG, esophageal ganglion; CoG, commissural ganglion; STG, stomatogastric ganglion; stn, stomatogastric nerve; lvn, lateral ventricular nerve; pdn, pyloric dilator nerve; mvn, medial ventricular nerve. The circle around stn represents a Vaseline well used to block action potential transmission (methods). B: effect of the removal of neuromodulatory input to the STG. Pyloric activity is temporarily lost after decentralization (4 h). Activity is slower but can be identified as pyloric 24 h after decentralization. C: effect of the removal of glutamatergic inhibitory synapses with picrotoxin (PTX). The lateral pyloric (LP) neuron becomes, and remains, tonic over 24 h.

Fig. 2.

Ionic conductance correlations in pyloric dilator (PD) neurons. Conductances of the high-threshold K⁺ current (g_HTK), transient K⁺ current (g_A), and hyperpolarization-activated inward current (g_H) are graphed in all pairwise combinations. Each point represents a different cell. Regression lines are shown only for significant correlations (Pearson correlation analysis, P < 0.05). Measurements were obtained under the 3 conditions indicated: control (top row), after 24 h in 10⁻⁵ M PTX (middle row), and 24 h after decentralization (bottom row). Ionic conductances are reported in μS.

Fig. 3.

Ionic conductance correlations in LP neurons. Conductances g_HTK, g_A, and g_H are graphed in all pairwise combinations. Each point represents a different cell. Regression lines are shown only for significant correlations (Pearson correlation analysis, P < 0.05). Measurements were obtained under the 3 conditions indicated: control (top row), after 24 h in 10⁻⁵ M PTX (middle row), and 24 h after decentralization (bottom row). Ionic conductances are reported in μS.

Fig. 4.

Channel mRNA correlations in PD neurons. mRNA levels of BK-KCa, Shal, and H are graphed in all pairwise combinations. Each point represents a different cell. Regression lines are shown only for significant correlations (Pearson correlation analysis, P < 0.05). Measurements were obtained under the 3 conditions indicated: control (top row), after 24 h in 10⁻⁵ M PTX (middle row), and 24 h after decentralization (bottom row). Numbers indicate mRNA copy numbers.

Fig. 5.

Channel mRNA correlations in LP neurons. mRNA levels of BK-KCa, Shal, and H are graphed in all pairwise combinations. Each point represents a different cell. Regression lines are shown only for significant correlations (Pearson correlation analysis, P < 0.05). Measurements were obtained under the three conditions indicated: control (top row), after 24 h in 10⁻⁵ M PTX (middle row), and 24 h after decentralization (bottom row). Numbers indicate mRNA copy numbers.