Different sensory systems share projection neurons but elicit distinct motor patterns

Abstract

Considerable research has focused on issues pertaining to sensorimotor integration, but in most systems precise information remains unavailable regarding the specific pathways by which different sensory systems regulate any single central pattern-generating circuit. We address this issue by determining how two muscle stretch-sensitive neurons, the gastropyloric receptor neurons (GPRs), influence identified projection neurons that regulate the gastric mill circuit in the stomatogastric nervous system of the crab and then comparing these actions with those of the ventral cardiac neuron (VCN) mechanosensory system. Here, we show that the GPR neurons activate the gastric mill rhythm in the stomatogastric ganglion (STG) via their excitation of two identified projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), in the commissural ganglion. Support for this conclusion comes from the ability of the modulatory proctolin neuron (MPN), a projection neuron that suppresses the gastric mill rhythm via its inhibitory actions on MCN1 and CPN2, to inhibit the GPR-elicited gastric mill rhythm. Selective elimination of MCN1 and CPN2 access to the STG also prevents GPR activation of this rhythm. The VCN neurons also elicit the gastric mill rhythm by coactivating MCN1 and CPN2, but the GPR-elicited gastric mill rhythm is distinct. These distinct rhythms are likely to result partly from different MCN1 activity levels under these two conditions and partly from the presence of additional GPR actions in the STG. These results support the hypothesis that different sensory systems differentially regulate neuronal circuit activity despite their convergent actions on a single subpopulation of projection neurons.

Figure 1.

Schematic of the stomatogastric nervous system, including somata location and axonal pathways of the bilateral pair of GPR sensory neurons and the identified projection neurons that innervate the STG. Each GPR projects to and arborizes within the STG and each CoG (Katz et al., 1989). There is a single MCN1, CPN2, MCN5, and MCN7 in each CoG and two MPNs in the OG. For clarity, the complete projection of only one copy of each projection neuron is shown. dpon, Dorsal posterior oesophageal nerve; lvn, lateral ventricular nerve; mvn, medial ventricular nerve; vcn, ventral cardiac nerve.

Figure 2.

Rhythmic GPR stimulation elicits the gastric mill rhythm. Extracellular recordings of STG nerves were used to monitor the pyloric (mvn) and gastric mill rhythms (lgn, dgn, mvn). A, Left, Before rhythmic GPR stimulation, there was no gastric mill rhythm, but there was a weak pyloric rhythm. A, Right, During rhythmic GPR stimulation (black bars), a gastric mill rhythm was elicited, as is evident from the rhythmic alternating bursting in the DG and LG/GM neurons and the gastric mill-timed inhibition of the IC and VD neurons. Note that this gastric mill rhythm was entrained to the GPR stimulations. In both panels, the tonically active unit (dgn) represents the activity of the AGR neuron, a muscle tendon proprioceptor neuron that fires tonically in the isolated STNS (Combes et al., 1995). B, Rhythmic GPR stimulation elicited, but did not entrain, the gastric mill rhythm. A and B are from different preparations.

Figure 3.

Rhythmic GPR stimulation entrains the gastric mill rhythm over a limited range of stimulation frequencies. The cycle frequency of each GPR-elicited gastric mill rhythm is plotted as a function of the frequency of the corresponding rhythmic GPR stimulation. The straight line represents perfect entrainment of the gastric mill rhythm to the stimulation frequency (n = 3-11 per stimulation frequency). Error bars represent SD.

Figure 4.

GPR excites all four identified CoG projection neurons. A brief GPR stimulation (black bar) excited all four of these projection neurons, albeit to different extents. Note that in each case, the increased firing of the projection neuron persisted for at least several seconds after the period of stimulation. Each recording was from a different preparation. Most hyperpolarized V_m: MCN1, -62 mV; CPN2, -56 mV; MCN5, -60 mV; MCN7, -70 mV.

Figure 5.

MCN1 and CPN2 are excited and exhibit gastric mill-timed activity during the GPR-elicited gastric mill rhythm.A, Left, Before GPR stimulation, there was no gastric mill rhythm, but MCN1 was weakly active. A, Right, During GPR stimulation (black bars), the gastric mill rhythm occurred, and MCN1 activity was strengthened and became gastric mill timed. Most hyperpolarized V_m: MCN1, -60 mV. B, In a different preparation, GPR stimulation elicited the gastric mill rhythm (dgn, LG, GM) and coordinately activated CPN2 to burst in time with this rhythm. Most hyperpolarized V_m: CPN2, -68 mV; GM, -64 mV; LG, -78 mV.

Figure 6.

Stimulation of the MPN projection neuron reversibly suppressed the GPR-elicited gastric mill rhythm as well as MCN1 activity. MCN1 activity was monitored with an extracellular recording (ion) (Coleman and Nusbaum, 1994). The rhythmic GPR stimulation excited MCN1 and CPN2 (data not shown) and elicited the gastric mill rhythm (dgn, lgn). Brief (2 sec) MPN stimulation reversibly inhibited MCN1 and CPN2 and suppressed the gastric mill rhythm. MCN1 and CPN2 activity as well as the gastric mill rhythm resumed ∼10 sec after the end of the MPN stimulation. Rhythmic GPR stimulation began before the start of the panel.

Figure 7.

The actions of MCN1 and CPN2 in the STG are necessary for GPR activation of the gastric mill rhythm. MCN1 activity was monitored with an extracellular recording (ion), and CPN2 activity was monitored and manipulated with an intracellular recording of its stn axon (CPN2_SNAX) near the STG. A, Left, Rhythmic GPR stimulation (black bars) activated MCN1 and CPN2 and elicited the gastric mill rhythm (dgn, lgn, GM). The large-amplitude rhythmic bursts in the ion represent the activity of an oesophageal motor neuron (Oesoph MN) during an ongoing oesophageal rhythm, which is generated in the CoGs. A, Right, Schematic of the experimental setup. Most hyperpolarized V_m: CPN2_SNAX, -49 mV; GM, -73 mV. B, Selectively eliminating access to the STG for MCN1 and CPN2 prevented GPR activation of the gastric mill rhythm (dgn, lgn, GM). Rhythmic DG bursting persisted because of the direct excitatory action of GPR on the DG neuron (Katz and Harris-Warrick, 1989). Most hyperpolarized V_m: GM, -73 mV. CPN2_SNAX was hyperpolarized via an explicitly unbalanced bridge circuit.

Figure 8.

Comparison of the GPR-elicited and VCN-elicited gastric mill rhythms. The burst duration, number of action potentials, intraburst firing frequencies, and phase relationships of gastric mill circuit neurons are plotted for GPR-elicited (white bars) and VCN-elicited (black bars) rhythms. Non-parametric Mann-Whitney rank sum test was performed to compare these data; ^*p < 0.05; ^**p < 0.01. VCN data from Beenhakker and Nusbaum (2004). Error bars represent SD.

Figure 9.

Schematic of the CoG projection neuron targets of the VCN mechanosensory system and the GPR proprioceptor system that enable each sensory system to activate the gastric mill rhythm. Note that, for each sensory system, only the MCN1 and CPN2 projection neurons are necessary to drive the gastric mill rhythm despite the ability of each sensory system to activate at least some of the other CoG projection neurons that influence the gastric mill circuit. Despite sharing the coactivation of MCN1 and CPN2, the VCN and GPR systems elicit different versions of the gastric mill rhythm. For example, as is evident in the gastric mill recordings, the GPR-elicited rhythm is slower than the VCN-elicited rhythm.