Intercircuit control of motor pattern modulation by presynaptic inhibition

Abstract

Rhythmically active neural networks can control the modulatory input that they receive via their synaptic effects onto modulatory neurons. This synaptic control of network modulation can occur presynaptically, at the axon terminals of the modulatory neuron. For example, in the crab stomatogastric ganglion (STG), a gastric mill network neuron presynaptically inhibits transmitter release from a modulatory projection neuron called modulatory commissural neuron 1. We showed previously that the gastric mill rhythm-timed presynaptic inhibition of the STG terminals of MCN1 is pivotal for enabling MCN1 to activate this rhythm. We also showed that MCN1 excites the pyloric rhythm within the STG. Here we show that, because MCN1 stimulation conjointly excites the gastric mill and pyloric rhythms, the gastric mill rhythm-timed presynaptic inhibition of MCN1 causes a rhythmic interruption in the MCN1-mediated excitation of the pyloric rhythm. Consequently, during each protraction phase of the gastric mill rhythm, presynaptic inhibition suppresses MCN1 excitation of the pyloric rhythm, thereby weakening the pyloric rhythm. During the retraction phase, presynaptic inhibition is absent and MCN1 elicits a faster, stronger, and modified pyloric rhythm. Thus, in addition to its role in enabling a neural circuit to regulate the modulatory transmission that it receives, presynaptic inhibition is also used effectively to rhythmically control the activity level of a distinct, but behaviorally related, neural circuit.

Fig. 1.

The stomatogastric nervous system (STNS) of the crab Cancer borealis. A, Schematic illustration of the STNS, including the soma location and branching pattern of MCN1.Stippled areas indicate neuropil regions.B, Combined intracellular and extracellular recordings of an ongoing pyloric rhythm. This is a three-phase motor pattern with consecutive impulse bursts in (1) AB, PD, andLPG; (2) IC and LP; and (3) PY and VD. The LPG action potentials are recorded in the dvn, but they are obscured by the larger PD spikes, with which they are coactive. Most hyperpolarized membrane potential: AB, −58 mV. Abbreviations: (ganglia) CoG, commissural ganglion;OG, esophageal ganglion; STG, stomatogastric ganglion; (nerves) dgn, dorsal gastric nerve; dvn, dorsal ventricular nerve;ion, inferior esophageal nerve; lgn, lateral gastric nerve; lpn, lateral pyloric nerve;lvn, lateral ventricular nerve; mvn, medial ventricular nerve; pdn, pyloric dilator nerve;pyn, pyloric nerve; son, superior esophageal nerve; stn, stomatogastric nerve; (interneurons) AB, anterior burster;MCN1, modulatory commissural neuron 1; (motor neurons)IC, inferior cardiac constrictor; LP, lateral pyloric constrictor; LPG, lateral posterior gastric; PD, pyloric dilator; PY, pyloric constrictor; VD, ventricular dilator. C, Schematic circuit diagram of the MCN1 influence on the gastric mill and pyloric systems. Symbols: t-bars indicate both fast and slow transmitter-mediated synaptic excitation; filled circles indicate transmitter-mediated synaptic inhibition;resistor symbol indicates electrical coupling. Based on data from Nusbaum et al. (1992), Coleman and Nusbaum (1994), andColeman et al. (1995).

Fig. 2.

Excitation of the pyloric rhythm by tonic stimulation of MCN1. A, Excitation of an ongoing pyloric rhythm by MCN1 stimulation. MCN1 was activated, selectively, by tonic stimulation of the ion. Before MCN1 stimulation, there was a relatively weak, but regular, pyloric rhythm evident in thelvn, but little or no activity in VD andIC (mvn). Activation of MCN1 enhanced the pyloric rhythm, and this excitation persisted for >10 cycles after termination of MCN1 stimulation. Note that LG was maintained subthreshold for transmitter release by constant-amplitude hyperpolarizing current injection. Preparation had bothsons transected to reduce the background excitation to the pyloric rhythm from other spontaneously active projection neurons.V_holding: LG, −88 mV.B, Evidence that the excitation of the pyloric rhythm resulting from ion stimulation is attributable solely to activation of MCN1. The stomatogastric nerve axon of MCN1 (MCN1_SNAX) was recorded intra-axonally near the entrance to the STG (Coleman and Nusbaum, 1994). During excitation of the pyloric rhythm that resulted from ion stimulation, each ion stimulus elicited an action potential in MCN1 that propagated to the STG and was recorded in MCN1_SNAX. Six seconds after the start of ion stimulation, MCN1_SNAX was hyperpolarized by current injection to −100 mV (between the arrowheads), causing the MCN1 action potentials to fail to propagate actively past the SNAX recording site. Consequently, the pyloric rhythm returned to its prestimulus level of activity. The peak of each MCN1_SNAXaction potential reached +20 mV when they occurred atV_rest (−52 mV), but they only reached −70 mV when MCN1_SNAX was held at −100 mV. Note that the pyloric rhythm was again excited when the hyperpolarizing current was removed from MCN1_SNAX. Tonic, small-amplitude units inmvn and lvn are artifacts fromion stimulation.

Fig. 3.

Initiation of the pyloric rhythm by MCN1 stimulation. Stimulation of MCN1 elicited the pyloric rhythm in this preparation, which had both sons transected and the LG neuron hyperpolarized (not shown). Note that the pyloric rhythm stopped as soon as stimulation was terminated. Most hyperpolarized membrane potential during pyloric rhythm: AB, −64 mV;PD, −60 mV.

Fig. 4.

Phase relationships of the pyloric motor neurons before, during, and after MCN1 stimulation. The beginning and end of each box represent the mean ± SD onset and offset times of the impulse burst in the indicated neuron, expressed as a fraction of the pyloric cycle period. The pyloric cycle extends from the onset of a PD neuron burst to the onset of its next burst. Two consecutive normalized pyloric cycles are shown. Results are pooled from six preparations, all of which had both ions and sonstransected, and the LG neuron hyperpolarized by constant amplitude current injection. For each neuron, the onset and offset of activity inB and C were compared with the equivalent point in A, using the paired Student’s ttest (*p < 0.05; **p < 0.01).

Fig. 5.

During conjoint activation of the gastric mill- and pyloric rhythms by MCN1 stimulation, there are gastric mill rhythm-timed reductions in pyloric rhythm activity. A, Influence of the gastric mill rhythm-timed LG burst when there was a weak pyloric rhythm before MCN1 stimulation. Left, Before MCN1 stimulation, the pyloric rhythm was weak and intermittent and there was no gastric mill rhythm (note the lack of LG activity). The DG neuron was bursting intermittently. Right, During MCN1 stimulation, the gastric mill rhythm was activated, as evident by the alternating bursts in DG and LG. When LG was not active, MCN1 activity excited the pyloric rhythm. However, this rhythm was terminated during each LG burst. Note that, during each LG burst, there was no activity in PD (pdn) and only tonic activity in PY (pyn). Tonically active unit indgn that is nearly the same amplitude as the DG neuron is the anterior gastric receptor (AGR) sensory neuron.B1, Influence of the gastric mill rhythm-timed LG burst when there was a strong pyloric rhythm before MCN1 stimulation. Here, MCN1 stimulation activated the gastric mill rhythm and excited the pyloric rhythm. The pyloric rhythm slowed, but did not terminate, during each LG burst. MCN1 was activated via constant-amplitude intrasomatic depolarizing current injection beginning before the recording shown. B2, Expanded time scale. The effect of the gastric mill rhythm-timed presynaptic inhibition on the pyloric rhythm is evident in the reduced frequency of membrane potential oscillations in the pyloric pacemaker interneuron, AB, during each LG burst (single star) when compared with its frequency during the LG interburst interval (double star). The amplitude of the AB oscillations was also smaller during each LG burst. MCN1 firing frequency was ∼25 Hz. Most hyperpolarized membrane potential: MCN1, −40 mV;LG, −64 mV; AB, −58 mV.

Fig. 6.

Percentage increase in pyloric cycle frequency elicited by MCN1 activity during the protraction (LG) and retraction (DG) phases of the gastric mill rhythm. These were compared with the pyloric cycle frequency during the first three cycles immediately after termination of MCN1 stimulation. The first three cycles poststimulation were selected for comparison because there are usually three pyloric rhythm cycles during each LG burst, and MCN1 transmitter release is either suppressed or reduced during the LG bursts. Data are plotted as a function of the mean pyloric cycle frequency for 10 cycles before each MCN1 stimulation. Each vertical set of three data points (DG-timed, LG-timed,post-MCN1-timed) for a given control cycle frequency is from the same MCN1 stimulation. Results are shown from 10 of 14 preparations. The remaining four experiments involved control cycle frequencies already represented in this figure and gave results comparable to those shown.

Fig. 7.

Elimination of LG neuron activity removed the rhythmic weakening of the pyloric rhythm that occurs during MCN1 stimulation. During MCN1 stimulation, the pyloric rhythm cycle frequency slowed during each gastric mill rhythm-timed LG burst (first two bars). Suppression of the next anticipated LG burst by injection of constant amplitude hyperpolarizing current into LG (between arrows) eliminated the next anticipated reduction in the pyloric rhythm (third bar). Removal of hyperpolarizing current enabled LG to resume bursting, resulting again in reduced pyloric rhythm activity time-locked to each LG burst (fourth bar). Most hyperpolarized membrane potentials: LG, −64 mV; PD, −54 mV.

Fig. 8.

Imposed gastric mill rhythm-like bursts in LG do not mimic the LG-mediated reduction in pyloric rhythm activity that occurs during MCN1 stimulation. Left, Ongoing pyloric rhythm with no gastric mill rhythm and no activity in MCN1 (not shown).Right, Stimulating LG in gastric mill rhythm-like bursts did not interfere with the pyloric rhythm. The pyloric cycle frequency before, during, and in between LG stimulations is not significantly different (one-way ANOVA, p > 0.05;n = 9 preparations). Most hyperpolarized membrane potential: LG, −64 mV.

Fig. 9.

Schematic circuit diagram illustrating the relative influence of MCN1 on the pyloric rhythm (represented by thepdn) during the protraction and retraction phases of the gastric mill rhythm. A, During the retraction phase of the gastric mill rhythm, LG is synaptically inhibited by Int1, enabling MCN1 to have transmitter-mediated influences on the STG circuits (Coleman et al., 1995) (this article). B, During the protraction phase, LG fires a burst of action potentials that presynaptically inhibits the STG terminals of MCN1 (Coleman and Nusbaum, 1994; Coleman et al., 1995). This reduces or suppresses the transmitter-mediated effects of MCN1 on the gastric mill (Coleman et al., 1995)- and pyloric rhythms (this article). Active neurons are indicated by black shading; inactive neurons are indicated by gray shading. Symbols are the same as Figure 1C.