Convergent mechanisms mediate preparatory states and repetition priming in the feeding network of Aplysia

Abstract

Improvement of behavioral responses to expected stimuli has been attributed to a change in the preparatory state. In this study, we take advantage of the accessibility of the nervous system of Aplysia and develop an in vitro analog of the preparatory state for feeding behaviors. We provide evidence that the change in the preparatory state may be elicited initially by activity of identified serotonergic metacerebral cells (MCCs). We demonstrate, however, that the preparatory state is maintained through MCC-independent repetition priming that is embedded in the properties of the behavior-generating network. Both MCC-dependent and MCC-independent processes converge on the same site in the behavior-generating network to mediate the change in the preparatory state. Thus, we propose a model of how multiple neuronal structures interact to elicit and maintain changes in the preparatory states in the CNS.

Figure 1.

Effects of MCC stimulation on the latency of CBI-2-elicited CMPs. A, Paradigm used in the experiment shown in B. CBI-2 was stimulated at 9 Hz for the duration of the protraction phase to elicit one CMP every 4 min. MCC was stimulated at 8 Hz for 1 min. MCC stimulation preceded CBI-2 stimulation by either 1 or 2 min. B, All traces are aligned with respect to the beginning of CBI-2 stimulation, one example of which is shown at the bottom. Gray arrows indicate the onset of CBI-2 stimulation. Black arrows indicate the onset of activity in the I2 nerve (I2N). B1, I2N recording from a control CMP. B2, I2N recording from a CMP elicited 1 min after MCC stimulation. B3, I2N recording from a CMP elicited 2 min after MCC stimulation. C, Grouped data from five preparations. Latency was normalized to the average of the latency to the onset of B61/62 firing of two CMPs preceding the stimulation of the MCC. Data are presented as average ± SEM.

Figure 2.

Effect of MCC stimulation when CMPs are elicited frequently. A, Paradigm used in the experiment shown in B. CBI-2 was stimulated at 9 Hz for the duration of the protraction phase to elicit five CMPs with 30 sec intervals. MCC was stimulated at 8 Hz for 1 min so that the end of the MCC stimulation was 30 sec before the first CBI-2-elicited CMP. At least 20 min of rest were allowed between the control CMP and MCC stimulation. B, All traces are aligned with respect to the beginning of CBI-2 stimulation. Gray arrows indicate the onset of CBI-2 stimulation. Black arrows indicate the onset of activity in the I2 nerve (I2N). I2 recordings are shown for the control CMP. First, second and fifth CMPs elicited subsequent to the end of MCC stimulation. C, Grouped data from five preparations (filled circles). Latency was normalized to the latency of the control CMP. Data are plotted as means ± SEM (n = 5). Data from Figure 1 (the 2 min time point) are plotted for comparison (open circle).

Figure 3.

Latency of frequently elicited CMPs with and without MCC prestimulation. A, All traces are aligned with respect to CBI-2 stimulation. CBI-2 was used to elicit five CMPs at 30 sec intervals. I2 nerve (I2N) activity, Bn2 activity (retraction monitor), and activity of the radula closer motoneuron B8 are shown for CMPs 1, 2, and 5. Gray arrows indicate the beginning of CBI-2 stimulation. Black arrows indicate the onset of activity in the I2 nerve. B, The MCC was stimulated for 1 min, so that the end of the stimulation was 30 sec before the first stimulation of CBI-2. CBI-2 was used to elicit five CMPs at 30 sec intervals. The order of runs (with and without MCC) was randomized (20 min between runs). CBI-2-elicited CMPs 1, 2, and 5 are shown. C, Normalized grouped data from five preparations. The latency of all CMPs after the MCC stimulation was normalized to the first CMP elicited by CBI-2 when CBI-2 stimulation was not preceded by stimulation of the MCC.

Figure 4.

MCC-elicited potentiation of CBI-2 to B61/62 PSPs. CBI-2 was stimulated to fire action potentials at 2.5 Hz, and PSPs were recorded in the soma of ipsilateral B61/62. MCC was then stimulated at 8 Hz for 1 min. A1, PSPs elicited by CBI-2 subsequent to the end of MCC stimulation were binned into 20 sec bins. Dots represent individual measurements, and bars represent the average for each bin. A2, Examples of individual representative PSPs recorded in the same experiment as A1. B, Normalized grouped data from five preparations. Data were binned into 20 sec bins as in A. Data are plotted as the average ± SEM (n = 5) for each bin. C, Results of the variance method analysis of the data in B. Quantal content and quantal number were estimated for the control measurements and the measurements in the 40-60 bin. All measurements were normalized to those estimated in the control group. Data are plotted as means ± SEM (n = 5). D, Diagrammatic representation of connections and actions of MCC, CBI-2, and B61/62. The dotted line represents the transition between the appetitive phase and the consummatory phase. MCC is active primarily during the appetitive phase and facilitates the CBI-2 to B61/62 synapse that becomes activated in the consummatory phase that follows (MCC-dependent preparatory state). As the consummatory responses are generated repetitively, the MCC-dependent facilitation of the CBI-2 to B61/62 synapse is replaced by the homosynaptic PTP at the same synapse (repetition priming). The black triangle indicates a fast synaptic connection; the gray triangle indicates a modulatory connection.