Correlation of 125I-LSD autoradiographic labeling with serotonin voltage clamp responses in Aplysia neurons

Comparative localization of two serotonin receptors and sensorin in the central nervous system of Aplysia californica

Aplysia californica is a powerful model for understanding the cellular and molecular mechanisms underlying modulation of neuronal plasticity and learning. In the central nervous system of Aplysia, serotonin is associated with various behaviors. For example, it induces short-, intermediate-, and long-term synaptic changes in sensory neurons during learning and inhibits the afterdischarge of the bag cells that initiate egg-laying behavior. Little is known about the nature and contribution of serotonin receptors involved in the numerous serotonin-mediated physiological responses in Aplysia. Recently, two G(i)-coupled serotonin receptors (5-HT(ap1) and 5-HT(ap2)) were cloned. We now report that, by using in situ hybridization to express the profile of these receptors, we are able to gain critical insight into their roles in the behavior of Aplysia. We compared their distribution to that of sensorin-A, a peptide specifically found in sensory neurons. We wished to determine their involvement in some simple forms of behavioral modifications. 5-HT(ap1) and 5-HT(ap2) mRNAs are expressed in all ganglia of the Aplysia central nervous system. Stronger signal was observed with the 5-HT(ap2) antisense probe than with the 5-HT(ap1) antisense probe. Notably, mRNA coding for the receptors was found in several identified neurons, in the bag cells, in characterized serotonergic neurons, and in neurons of the mechanosensory clusters that expressed sensorin. We also observed heterogeneity of receptor expression between R2 and LPl1 and among neurons of a single cluster of sensory neurons. These results suggest that 5-HT(ap1) and 5-HT(ap2) receptors may regulate the response to serotonin and/or its release in several neurons.

Multiple serotonergic mechanisms contributing to sensitization in aplysia: evidence of diverse serotonin receptor subtypes

The neurotransmitter serotonin (5-HT) plays an important role in memory encoding in Aplysia. Early evidence showed that during sensitization, 5-HT activates a cyclic AMP-protein kinase A (cAMP-PKA)-dependent pathway within specific sensory neurons (SNs), which increases their excitability and facilitates synaptic transmission onto their follower motor neurons (MNs). However, recent data suggest that serotonergic modulation during sensitization is more complex and diverse. The neuronal circuits mediating defensive reflexes contain a number of interneurons that respond to 5-HT in ways opposite to those of the SNs, showing a decrease in excitability and/or synaptic depression. Moreover, in addition to acting through a cAMP-PKA pathway within SNs, 5-HT is also capable of activating a variety of other protein kinases such as protein kinase C, extracellular signal-regulated kinases, and tyrosine kinases. This diversity of 5-HT responses during sensitization suggests the presence of multiple 5-HT receptor subtypes within the Aplysia central nervous system. Four 5-HT receptors have been cloned and characterized to date. Although several others probably remain to be characterized in molecular terms, especially the Gs-coupled 5-HT receptor capable of activating cAMP-PKA pathways, the multiplicity of serotonergic mechanisms recruited into action during learning in Aplysia can now be addressed from a molecular point of view.

Cerebral-abdominal interganglionic coordinating neurons in Aplysia

Three cerebral-abdominal interneurons (CAIs), CC2, CC3, and CC7, were identified in the cerebral ganglion C cluster. The cells send their axons to the abdominal ganglion via the pleural-abdominal connective. CC2 and CC3 are bilaterally symmetrical cells, whereas CC7 is a unilateral cell. CC3 is immunopositive for serotonin and may be the same cell (CB-1) previously described as located in the B cluster rather than the C cluster. We suggest that the full designation of CC3, be CC3(CB-1). All three cells respond to feeding-related inputs. Each CAI has a monosynaptic connection to at least one abdominal ganglion neuron involved in the control of various nonsomatic organs. The CAIs also exert widespread polysynaptic actions in the abdominal and head ganglia. The results suggest that the CAIs may act as interneurons that coordinate visceral responses mediated by the abdominal ganglion, with behaviors such as feeding and head withdrawal, that are controlled by neurons located in the head ganglia of the animal.

Pharmacological characterization of a serotonin receptor involved in an early embryonic behavior of Helisoma trivolvis

In contrast to the abundance of information on the many physiological and developmental actions of serotonin in molluscan nervous systems, comparatively little is known about the serotonin receptors involved in these responses. Embryos of the pulmonate gastropod, Helisoma trivolvis, display a cilia-driven rotational behavior that is regulated by endogenous serotonin. In the present study, two functional assays were used to determine some of the pharmacological properties of the receptors that mediate the cilio-excitatory action of serotonin. Time-lapse video microscopy was used to measure whole embryo rotation rate and cilia beat frequency in isolated cells. In dose-response experiments, serotonin was approximately 10 times more potent in stimulating cilia beat frequency over embryo rotation. In rotation experiments, 5-carboxyamidotryptamine and methysergide had effective agonist activity in dose ranges similar to that of serotonin (1 to 100 microM). In contrast, 8-hydroxydipropylaminotetralin HBr (8-OH-DPAT) displayed agonist activity of lower potency and effectiveness. Several compounds displayed antagonist activity in the 1 to 100 microM dose range, including mianserin, spiperone, ritanserin, 1-(1-naphthyl)piperazine, and propranolol. alpha-Methylserotonin had mixed agonist-antagonist activity, and metoclopramide, MDL-72222, and ketanserin were inactive. Experiments on isolated cells suggested that the extremely effective antagonism displayed by mianserin in the embryo rotation assay was due to its specific activity at ciliary serotonin receptors. These results implicate the presence of a novel serotonin receptor on embryonic ciliated cells that is pharmacologically distinct from those previously characterized in vertebrate or invertebrate systems.