Octopamine promotes rhythmicity but not synchrony in a bilateral pair of bursting motor neurons in the feeding circuit of Aplysia.

Octopamine-like immunoreactivity was localized to a limited number (<40) of neurons in the Aplysia central nervous system, including three neurons in the paired buccal ganglia (BG) that control feeding movements. Application of octopamine (OA) to the BG circuit produced concentration-dependent (10(-8)-10(-4) mol l(-1)) modulatory actions on the spontaneous burst activity of the bilaterally paired B67 pharyngeal motor neurons (MNs). OA increased B67's burst duration and the number of impulses per burst. These effects reflected actions of OA on the intrinsic tetrodotoxin-resistant driver potential (DP) that underlies B67 bursting. In addition to its effects on B67's burst parameters, OA also increased the rate and regularity of burst timing. Although the bilaterally paired B67 MNs both exhibited rhythmic bursting in the presence of OA, they did not become synchronized. In this respect, the response to OA differed from that of dopamine, another modulator of the feeding motor network, which produces both rhythmicity and synchrony of bursting in the paired B67 neurons. It is proposed that modulators can regulate burst synchrony of MNs by exerting a dual control over their intrinsic rhythmicity and their reciprocal capacity to generate membrane potential perturbations. In this simple system, dopaminergic and octopaminergic modulation could influence whether pharyngeal contractions occur in a bilaterally synchronous or asynchronous fashion.

Serotonergic immunoreactivity in the pedal ganglia of the pulmonate snail Megalobulimus abbreviatus after thermal stimulus: A semi-quantitative analysis.

Using an immunohistochemical procedure and optical densitometry, the distribution of neurons containing serotonin (5-HT) was investigated in the pedal ganglia of Megalobulimus abbreviatus after thermal "non-functional stimulus" (22 degrees C) and stressful thermal conditions (50 degrees C). The animals were sacrificed at different times (3 h, 6 h and 24 h) following these stimuli. In control animals, the results showed the location of these serotonergic immunoreactive elements (5HT-ir) in this ganglion to be similar to those shown in other studies, where the anterior region of ventral sections showed the largest number of 5HT-ir neurons. In the anterior neurons, significant differences (p < 0.01) were observed between the groups of animals stimulated at 50 degrees C and 22 degrees C and sacrificed after 6 h. In the medial neurons, significant differences (p < 0.05) were observed between the control group and the groups of animals stimulated at 50 degrees C and sacrificed after 6 and 24 h. Neuropilar area 1 showed differences (p < 0.01) in 5HT-ir between the control group and the groups of animals stimulated at 50 degrees C and sacrificed after 3 and 24 h. Neuropilar area 2 showed a significant difference (p < 0.05) between the groups of animals stimulated at 22 degrees C and sacrificed after 3 and 24 h. These results suggest the involvement of 5-HT in the nociceptive circuit of M. abbreviatus, mainly that of the medial neurons and neuropilar area 1, which showed increases in 5HT-ir after thermal aversive stimuli. These results could be helpful in drawing cellular homologies with other gastropods.