An age-related decline in the cholinergic synaptic response may cause the firing pattern in the jaw-closing motor neurons, which resembles the aversive taste response in the feeding behavior of old Aplysia kurodai.

Anorexia due to aging is recognized as a syndrome of animal feeding behavior. Age-related functional disorders of the brain often cause behavioral changes. We used Aplysia kurodai to study this neural mechanism, following our previous study on food preference behaviors. The age of each wild animal was defined by a previously described method, and a significant age-related decline in food intake was observed. In this study, we explored the effects of aging on a specific inhibitory synaptic response in jaw-closing (JC) motor neurons produced by cholinergic multiaction (MA) neurons, the size of which determines the delay between MA and JC firings and this delay is reduced during aversive taste responses; in our analyses, we found a significant age-related decline in the synaptic response. Thereafter, we further explored whether such functional decline affects the JC firing pattern during the normal feeding response. During the feeding-like rhythmic responses induced by electrical nerve stimulation, the firing of the JC motor neurons advanced toward that of the MA burst, which typically happens during aversive taste responses. These results suggest that the age-related decline in the cholinergic synaptic response may partly cause the JC firing patterns that resemble the aversive taste response in old animals.

Administration of amyloid-ß oligomer to the buccal ganglia may reduce food intake and cholinergic synaptic responses within the feeding neural circuit in Aplysia kurodai.

Anorexia is a behavioral change caused by functional brain disorders in patients with Alzheimer's disease (AD). Amyloid-ß (1-42) oligomers (o-Aß) are possible causative agents of AD that impair signaling via synaptic dysfunction. In this study, we used Aplysia kurodai to study functional disorders of the brain through o-Aß. Administration of o-Aß to the buccal ganglia (feeding brain for oral movements) by surgical treatment significantly reduced food intake for at least five days. Furthermore, we explored the effects of o-Aß on the synaptic function in the feeding neural circuit, focusing on a specific inhibitory synaptic response in jaw-closing motor neurons produced by cholinergic buccal multi-action neurons because we recently found that this cholinergic response decreases with aging, which is consistent with the cholinergic hypothesis for aging. Administration of o-Aß to the buccal ganglia significantly reduced the synaptic response within minutes, whereas administration of amyloid-ß (1-42) monomers did not. These results suggest that o-Aß may impair the cholinergic synapses, even in Aplysia, which is consistent with the cholinergic hypothesis for AD.

A CPG component LE generates depolarization of buccal neurons by producing constant plateau potentials during feeding responses of Aplysia kurodai.

In the buccal ganglia of Aplysia kurodai we have identified neurons (here termed LE neurons, or LE) producing plateau potentials lasting several seconds by application of short depolarizing currents. Results obtained from experiments using various bath solutions suggest that generation of these plateau potentials may be an endogenous property of LE. Application of various intensities or lengths of depolarizing currents induced in LE almost constant plateau potentials with fixed duration and depolarizing size. LE spikes produced monosynaptic EPSPs in the ipsilateral multi-action neuron (MA) and the jaw-closing motor neuron (JC) in the buccal ganglia. Conversely, MA spikes produced monosynaptic IPSPs in LE. There was electrical coupling between LE and both MA and JC. During the feeding-like response elicited by electrical stimulation of the nerve, LE showed rhythmic depolarization almost simultaneously with MA and JC, and firing on the plateau potentials occurred during the period of JC firing, the later phase of radula retraction. Hyperpolarization of LE during the feeding-like response suppressed generation of plateau potentials, though rhythmic small depolarization was still induced. During LE hyperpolarization, the duration of the depolarization of MA and JC was shortened. These results suggest that LE may be an element of the feeding CPG circuit and may contribute to part of the depolarization of MA and JC by generating constant plateau potentials during the feeding response, though LE may not have rhythm-generating ability.

Modulation of a feeding neural circuit by microinjection of K+ channel expression genes into a single identified neuron in Aplysia kurodai.

In Aplysia buccal ganglion expression genes for voltage-dependent K(+) channels (AKv1.1a) were injected into one of four electrically coupled multi-action (MA) neurons that directly inhibit jaw-closing (JC) motor neurons and may cooperatively generate their firing pattern during the feeding response. Following the DNA injection, the firing threshold increased and the spike frequency at the same current decreased in the current-induced excitation of the MA neuron; indicating a decrease in excitability of the MA neuron. This procedure also reduced the firing activity of MA neurons during the feeding-like rhythmic responses induced by the electrical nerve stimulation. Moreover, the firing pattern in JC motor neurons was remarkably changed, suggesting the effective contribution of a single MA neuron or electrically coupled MA neurons to the generation of the firing pattern in the JC motor neurons. This method appears useful for exploring the functional roles of specific neurons in complex neural circuits.

Activity changes in jaw motor neurons induced by egg-laying hormone contribute to the feeding suppression during egg-laying behavior in Aplysia kurodai.

Egg-laying behavior in Aplysia is accompanied by behavioral changes such as feeding suppression. We investigated the effects of the egg-laying hormone (ELH) on food intake, the activity patterns of jaw muscles, and the activity of buccal neurons (multi-action neuron [MA1] and jaw-closing motor neuron [JC2]), which are elements of the feeding neural circuits controlling jaw movements in Aplysia kurodai. Injection of ELH into the body cavity inhibited the intake of seaweed. After ELH application, the rhythmic activity of jaw muscles that was induced by preferred taste stimulation elicited fewer ingestion-like responses and increased the number of rejection-like responses. ELH applied to the buccal ganglia increased the firing activity of JC2 during spontaneous rhythmic responses and during the rhythmic feeding-like responses that were evoked by electrical stimulation of the esophageal nerves. In the 2 types of rhythmic responses, the Dn (normalized value of the delay time of JC2 firing onset) decreased after ELH application as compared with the control. Furthermore, ELH decreased the size of MA1-induced inhibitory postsynaptic currents in JC2. These results suggest that ELH changes the buccal motor program from ingestion to rejection on the basis of our previous results, and may contribute to a decrease in food intake during egg laying.

Effective amino acid composition of seaweeds inducing food preference behaviors in Aplysia kurodai.

Aplysia kurodai feeds on Ulva but rejects Gelidium and Pachydictyon with distinct patterned jaw movements. We previously demonstrated that these movements are induced by taste alone. Thus some chemicals may contribute to induction of these responses. We explored the amino acids composition of Ulva, Gelidium and Pachydictyon extracts used during our taste-induced physiological experiments. These solutions contained many constituents. The concentrations of six amino acids (Asp, Asn, Glu, Gln, Phe, Tau) were obviously different in the three extract solutions. We explored patterned jaw movements following application of solutions containing a pure amino acid. We statistically compared the occurrence numbers of ingestion-like and rejection-like patterned jaw movements (positive and negative values, respectively) for each amino acid. Our results suggested that L-Asn tends to induce ingestion-like responses, likely resulting in a preference of Ulva. In contrast, L-Asp tends to induce rejection-like responses, likely resulting in aversion towards Pachydictyon. In addition, we demonstrated that L-Asn and L-Asp solutions were sufficient to induce muscle activity associated with ingestion-like or rejection-like responses in the jaw muscles of a semi-intact preparation.

Cerebral CBM1 neuron contributes to synaptic modulation appearing during rejection of seaweed in Aplysia kurodai.

The Japanese species Aplysia kurodai feeds well on Ulva but rejects Gelidium with distinctive rhythmic patterned movements of the jaws and radula. We have previously shown that the patterned jaw movements during the rejection of Gelidium might be caused by long-lasting suppression of the monosynaptic transmission from the multiaction MA neurons to the jaw-closing (JC) motor neurons in the buccal ganglia and that the modulation might be directly produced by some cerebral neurons. In the present paper, we have identified a pair of catecholaminergic neurons (CBM1) in bilateral cerebral M clusters. The CBM1, probably equivalent to CBI-1 in A. californica, simultaneously produced monosynaptic excitatory postsynaptic potentials (EPSPs) in the MA and JC neurons. Firing of the CBM1 reduced the size of the inhibitory postsynaptic currents (IPSCs) in the JC neuron, evoked by the MA spikes, for >100 s. Moreover, the application of dopamine mimicked the CBM1 modulatory effects and pretreatment with a D1 antagonist, SCH23390, blocked the modulatory effects induced by dopamine. It could also largely block the modulatory effects induced by the CBM1 firing. These results suggest that the CBM1 may directly modulate the synaptic transmission by releasing dopamine. Moreover, we explored the CBM1 spike activity induced by taste stimulation of the animal lips with seaweed extracts by the use of calcium imaging. The calcium-sensitive dye, Calcium Green-1, was iontophoretically loaded into a cell body of the CBM1 using a microelectrode. Application of either Ulva or Gelidium extract to the lips increased the fluorescence intensity, but the Gelidium extract always induced a larger change in fluorescence compared with the Ulva extract, although the solution used induced the maximum spike responses of the CBM1 for each of the seaweed extracts. When the firing frequency of the CBM1 activity after taste stimulation was estimated, the Gelidium extract induced a spike activity of ~30 spikes/s while the Ulva extract induced an activity of ~20 spikes/s, consistent with the effective firing frequency (>25 spikes/s) for the synaptic modulation. These results suggest that the CBM1 may be one of the cerebral neurons contributing to the modulation of the basic feeding circuits for rejection induced by the taste of seaweeds such as Gelidium.