Quantitation of Contacts Among Sensory, Motor, and Serotonergic Neurons in the Pedal Ganglion of Aplysia

Abstract

Present models of long-term sensitization in Aplysia californica indicate that the enhanced behavioral response is due, at least in part, to outgrowth of sensory neurons mediating defensive withdrawal reflexes. Presumably, this outgrowth strengthens pre-existing connections by formation of new synapses with follower neurons. However, the relationship between the number of sensorimotor contacts and the physiological strength of the connection has never been examined in intact ganglia. As a first step in addressing this issue, we used confocal microscopy to examine sites of contact between sensory and motor neurons in naive animals. Our results revealed relatively few contacts between physiologically connected cells. In addition, the number of contact sites was proportional to the amplitude of the EPSP elicited in the follower motor neuron by direct stimulation of the sensory neuron. This is the first time such a correlation has been observed in the central nervous system. Serotonin is the neurotransmitter most closely examined for its role in modulating synaptic strength at the sensorimotor synapse. However, the structural relationship of serotonergic processes and sensorimotor synapses has never been examined. Surprisingly, serotonergic processes usually made contact with sensory and motor neurons at sites located relatively distant from the sensorimotor synapse. This result implies that heterosynaptic regulation is due to nondirected release of serotonin into the neuropil.

Synaptic Facilitation and Behavioral Dishabituation in Aplysia: Dependence on Release of Ca2+ from Postsynaptic Intracellular Stores, Postsynaptic Exocytosis, and Modulation of Postsynaptic AMPA Receptor Efficacy

“…Stimuli that produce dishabituation of the withdrawal reflex, such as tail shock, activate facilitatory interneurons, some of which are serotonergic (Glanzman et al, 1989;Mackey et al, 1989;Marinesco and Carew, 2002). Release of 5-HT onto sensory and motor neurons within the CNS of Aplysia (Kistler et al, 1985;Marinesco and Carew, 2002;Zhang et al, 2003) causes rapid-onset presynaptic facilitation via processes that involve PKA and PKC [see Byrne and Kandel (1996) for details]. The strength of the presynaptic facilitatory processes wanes rapidly after washout of 5-HT.…”

“…Furthermore, serotonergic processes have been shown to contact both sensory and motor neurons within the central ganglia of Aplysia (Kistler et al, 1985;Marinesco and Carew, 2002;Zhang et al, 2003). Finally, release of 5-HT is required for dishabituation and sensitization of the defensive withdrawal reflex in Aplysia (Glanzman et al, 1989).…”

Regulation of Behavioral and Synaptic Plasticity by Serotonin Release within Local Modulatory Fields in the CNS ofAplysia

“…Therefore, there is a strong correlation between primary SN activation and amplification of 5-HT release. In addition, using confocal microscopy, Zhang et al (2003) showed contacts between tail SNs and serotonergic fibers. Primary SNs therefore appear to be in an ideal position to modulate or trigger 5-HT release locally through axo-axonic contacts.…”

“…Interestingly, Zhang et al (2003) also determined that serotonergic processes usually make contact with SNs or MNs at sites located at a distance from SN-MN synapses. They proposed that serotonergic transmission in this area is essentially volumic or paracrine (i.e., 5-HT travels over distances of a few hundred micrometers before reaching its postsynaptic targets) [for review on paracrine transmission, see Bunin and Wightman (1999)].…”

“…For example, serotonergic neurons in Aplysia are involved in arousal, feeding, and locomotion (Kupfermann and Weiss, 1982;Mackey and Carew, 1983;Lloyd et al, 1984;Rosen et al, 1989;McPherson and Blankenship, 1992;Marinesco et al, 2004b), like their counterparts in mammals (Veasey et al, 1995(Veasey et al, , 1997) ( for review, see Jacobs and Fornal, 1999;Meguid et al, 2000;Siegel, 2004). Moreover, as in the mammalian CNS, serotonergic neurons in Aplysia are tonically active at rest, fire at relatively low frequencies (usually Ͻ1 Hz) (Kupfermann and Weiss, 1982;Marinesco et al, 2004a), and can release 5-HT in a paracrine or volumic manner (Marinesco and Carew, 2002a;Zhang et al, 2003) (for review, see Bunin and Wightman, 1999).…”