Mechanosensory gating of proprioceptor input to modulatory projection neurons

Abstract

Sensorimotor gating commonly occurs at sensory neuron synapses onto motor circuit neurons and motor neurons. Here, using the crab stomatogastric nervous system, we show that sensorimotor gating also occurs at the level of the projection neurons that activate motor circuits. We compared the influence of the gastro-pyloric receptor (GPR) muscle stretch-sensitive neuron on two projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), with and without a preceding activation of the mechanosensory ventral cardiac neurons (VCNs). MCN1 and CPN2 project from the paired commissural ganglia (CoGs) to the stomatogastric ganglion (STG), where they activate the gastric mill (chewing) motor circuit. When stimulated separately, the GPR and VCN neurons each elicit the gastric mill rhythm by coactivating MCN1 and CPN2. When GPR is instead stimulated during the VCN-gastric mill rhythm, it slows this rhythm. This effect results from a second GPR synapse onto MCN1 that presynaptically inhibits its STG terminals. Here, we show that, during the VCN-triggered rhythm, the GPR excitation of MCN1 and CPN2 in the CoGs is gated out, leaving only its influence in the STG. This gating effect appears to occur within the CoG and does not result from a ceiling effect on projection neuron firing frequency. Additionally, this gating action enables GPR to either activate rhythmic motor activity or act as a phasic sensorimotor feedback system. These results also indicate that the site of sensorimotor gating can occur at the level of the projection neurons that activate a motor circuit.

Figure 1.

A, Schematic of the stomatogastric nervous system, including somata location and axonal pathways of the GPR and VCN sensory neurons and the projection neurons MCN1 and CPN2. Each GPR neuron projects to and arborizes within the STG and each CoG. There is a single MCN1 and CPN2 in each CoG. MCN1 projects through the inferior oesophageal nerve (ion) and stn to innervate the STG, whereas CPN2 projects through the superior oesophageal nerve (son) and stn to innervate the STG. dgn, Dorsal gastric nerve; lgn, lateral gastric nerve; lvn, lateral ventricular nerve; mgn, medial gastric nerve; mvn, medial ventricular nerve. B, Proprioceptor (GPR) and mechanoreceptor (VCN) neuron actions on projection neurons (MCN1, CPN2) that activate the gastric mill circuit (e.g., LG, Int1) (Beenhakker and Nusbaum, 2004; Beenhakker et al., 2004; Blitz et al., 2004). T-bars, Synaptic excitation; circles, synaptic inhibition. Line breaks in the sensory and projection neuron axons represent additional distance between the STG and CoG. C, Simultaneous intracellular recordings of the LG protractor and Int1 retractor neurons before and during the VCN-triggered gastric mill rhythm. Most hyperpolarized Vm: LG, −70 mV; Int1, −52 mV. D, Stimulating GPR at the behaviorally appropriate time slows the VCN-elicited gastric mill rhythm by prolonging the retractor phase. Rhythmic stimulation of GPR (bars, 5 Hz) during the retractor phase (DG neuron active) of a VCN-triggered gastric mill rhythm caused a progressively increasing prolongation of the retractor phase (Beenhakker et al., 2005).

Figure 2.

The GPR neuron fails to excite MCN1 and CPN2 during the VCN-triggered gastric mill rhythm. A, Before VCN stimulation, in the absence of a gastric mill rhythm, GPR stimulation (bars, 5 Hz) excited both MCN1 and CPN2. B, During the VCN-triggered gastric mill rhythm, GPR stimulation (bars, 5 Hz) failed to alter the activity of either MCN1 (1) or CPN2 (2). Note that, despite the loss of GPR influence on MCN1 and CPN2 in the CoG during the gastric mill rhythm, GPR stimulation still prolonged the gastric mill retractor phase (LG interburst). The MCN1 recording in A and B are from the same preparation. The CPN2 recordings in both panels are also from the same preparation but a different preparation from those in which MCN1 was recorded.

Figure 3.

GPR stimulation does not alter the firing frequency of MCN1 and CPN2 during the VCN-triggered gastric mill rhythm. A, GPR stimulation during the retractor (Ret.) phase of the VCN-triggered rhythm did not alter the MCN1 firing frequency during either that phase (p = 0.47; n = 5) or the subsequent protractor (Prot.) phase (p = 0.08; n = 5; RM-ANOVA), despite its ability to prolong the gastric mill cycle period (Per.) at these times (*p < 0.05; n = 5). B, GPR stimulation during the retractor phase did not activate CPN2 during that phase (n = 4), nor did it alter the CPN2 firing frequency during the subsequent protractor phase (p = 0.35; n = 4; paired t test). Pre-GPR (white), during-GPR (MCN1, gray), or protraction immediately after-GPR (CPN2, gray), and post-GPR (black) stimulation during the VCN-triggered gastric mill rhythm are shown.

Figure 4.

The loss of GPR excitation of CPN2 does not result from a synaptic action in the STG. A, Left, With transmitter release selectively suppressed in the STG by superfusion of low Ca²⁺ saline (see Materials and Methods), GPR stimulation (Stim.) (5 Hz) excited CPN2 in the CoGs (pre-GPR, 5.3 Hz; post-GPR, 43.5 Hz). Note that neither the LG (lgn) nor DG (dgn) neurons were active, resulting from the suppression of transmitter release in the STG. CPN2 was recorded intra-axonally in the stn nerve (CPN2_stn) near the STG (Beenhakker and Nusbaum, 2004). Right, Under the same conditions, VCN stimulation elicited a long-lasting activation of CPN2. Here, the CPN2 response was tonic activity because of the lack of gastric mill-timed feedback from the STG (note the lack of activity in LG and DG). At this time, GPR stimulation (5 Hz) did not alter the CPN2 firing frequency (pre-GPR, 23 Hz; post-GPR, 22.5 Hz). Both panels show recordings from the same preparation. Most hyperpolarized Vm, CPN2_stn, −57 mV. B, Under conditions where transmitter release was suppressed in the STG with low Ca²⁺ saline, GPR stimulation consistently excited CPN2 when the VCNs had not been recently stimulated (*p < 0.05; n = 6). However, after VCN stimulation, GPR no longer influenced CPN2 activity (p = 0.09; n = 6). Gray bars, Pre-GPR stimulation; black bars, post-GPR stimulation.

Figure 5.

The failure of GPR to excite MCN1 is not a consequence of a ceiling effect in the MCN1 firing frequency. A, In the absence of a gastric mill rhythm, MCN1 was injected with sufficient depolarizing current to enable its firing frequency (29 Hz) to be comparable with that occurring during the VCN-triggered gastric mill rhythm. This stimulation of MCN1 elicited the gastric mill rhythm, monitored here by the coordinated rhythmic bursting of the LG and gastric mill (GM) protractor neurons alternating with the rhythmic bursting of the DG retractor neuron. Despite this elevated level of MCN1 activity, brief GPR stimulation (5 Hz) still excited MCN1 and thereby prolonged the protractor (LG neuron) phase of the rhythm. Most hyperpolarized Vm: MCN1, −47 mV; LG, −64 mV. B, Stimulating GPR during episodes when MCN1 was depolarized to fire at levels comparable with its activity in response to VCN stimulation consistently resulted in a prolongation of the protractor phase (*p < 0.05; n = 3) and no change in the retractor phase duration (p = 0.07; n = 3).

Figure 6.

The failure of GPR to excite CPN2 is not a consequence of a ceiling effect in the CPN2 firing frequency. A, CPN2 was excited by GPR stimulation (Stim.) in the absence of previous activation of the VCN neurons. This GPR stimulation caused CPN2 to depolarize and produce a prolonged action potential burst. B, CPN2 was depolarized by constant amplitude intracellular current injection to produce an activity level (28 Hz) comparable with that observed during the VCN-triggered gastric mill rhythm (Beenhakker and Nusbaum, 2004). Under this condition, GPR stimulation still excited CPN2. A and B are from the same preparation.

Figure 7.

Artificially increasing the MCN1 firing frequency during GPR stimulation eliminates the GPR-mediated prolongation of the gastric mill retractor phase during the VCN-triggered gastric mill rhythm. A, Left, Rhythmic VCN stimulation triggered a long-lasting excitation of CPN2 (data not shown) and MCN1. This MCN1 and CPN2 activity elicited the gastric mill rhythm, represented by the alternating bursting activity of LG and DG. Right, During this VCN-triggered gastric mill rhythm, GPR stimulation (bars, 5 Hz) prolonged the gastric mill cycle period by selectively increasing the retractor (LG interburst) phase. The tonically active unit in the dgn corresponds to the activity of the anterior gastric receptor (AGR) neuron. AGR is a muscle tendon proprioceptor neuron that is spontaneously active in the isolated STNS (Combes et al., 1995; Smarandache and Stein, 2007). Most hyperpolarized Vm: MCN1, −44 mV. B, Left, VCN stimulation triggered the gastric mill rhythm, during which the MCN1 firing frequency was 22.5 Hz. Most hyperpolarized Vm: MCN1, −56 mV. Right, During the ongoing gastric mill rhythm, GPR stimulation (bars, 5 Hz) was paired with intracellular depolarizing current injection in MCN1 (+1 nA), which increased the MCN1 firing frequency to 32.5 Hz (see expanded time scale for MCN1 recording during one protractor phase pre-MCN1 and during-MCN1 depolarization). Most hyperpolarized Vm: MCN1, −31 mV. Note that, unlike in A, there was no change in the gastric mill rhythm cycle period or the duration of either phase. All recordings were from the same preparation.

Figure 8.

Artificially increasing the MCN1 firing frequency during GPR stimulation consistently prevents GPR from prolonging the retractor phase of the VCN-triggered gastric mill rhythm. A, As shown previously, during an ongoing VCN-triggered gastric mill rhythm, stimulating GPR increased the gastric mill cycle period (**p < 0.01; n = 5) by selectively prolonging the retractor (Ret. Dur.) phase (LG interburst) of the VCN-gastric mill rhythm (**p < 0.01; n = 5). The protractor (Prot. Dur.) phase (LG burst) was not altered by these stimulations (p = 0.29; n = 5). Gray bars, Pre-GPR stimulation; black bars, during GPR stimulation. B, During times when the MCN1 firing frequency was artificially increased by intracellular depolarizing current injection during the VCN-triggered gastric mill rhythm, GPR stimulation no longer altered the gastric mill cycle period (p = 0.18; n = 6), nor did it change either the retractor phase (p = 0.23; n = 6) or protractor phase (p = 0.92; n = 6). Gray bars, Pre-MCN1 depolarization; black bars, during MCN1 depolarization.

Figure 9.

Schematic of the GPR proprioceptor influence on the gastric mill system in the absence and presence of the VCN mechanosensory influence on this system. A, When there has not been a recent activation of the VCN neurons (pale labeling) and there is no gastric mill rhythm in progress, GPR stimulation (dark labeling) causes a relatively long-lasting excitation of MCN1 and CPN2 in the CoG, leading to the activation of the gastric mill rhythm in the STG (Blitz et al., 2004). Under this condition, GPR also causes a shorter-lasting presynaptic inhibition of MCN1_STG (Beenhakker et al., 2005). B, During the VCN-triggered gastric mill rhythm, GPR stimulation no longer excites MCN1 and CPN2 (pale labeling). However, GPR inhibition of MCN1_STG persists during this condition (dark labeling), enabling GPR to continue providing rhythmic regulation of the gastric mill rhythm. T-bars, Synaptic excitation; filled circles, synaptic inhibition; sine wave, gastric mill rhythm activity.