Neural Circuit of Tail-Elicited Siphon Withdrawal in Aplysia. I. Differential Lateralization of Sensitization and Dishabituation

The tail-elicited siphon withdrawal reflex (TSW) has been a useful preparation in which to study learning and memory in Aplysia. However, comparatively little is known about the neural circuitry that translates tail sensory input (via the P9 nerves to the pleural ganglion) to final reflex output by siphon motor neurons (MNs) in the abdominal ganglion. To address this question, we examined the functional architecture of the TSW circuit by selectively severing nerves of semi-intact preparations and recording either tail-evoked responses in the siphon MNs or measuring siphon withdrawal responses directly. We found that the neural circuit underlying TSW is functionally lateralized. We next tested whether the expression of learning in the TSW reflects the underlying circuit architecture and shows side-specificity. We tested behavioral and physiological correlates of three forms of learning: sensitization, habituation, and dishabituation. Consistent with the circuit architecture, we found that sensitization and habituation of TSW are expressed in a side-specific manner. Unexpectedly, we found that dishabituation was expressed bilaterally, suggesting that a modulatory pathway bridges the two (ipsilateral) input pathways of the circuit, but this path is only revealed for a specific form of learning, dishabituation. These results suggest that the effects of a descending modulatory signal are differentially “gated” during sensitization and dishabituation.

Identification of Mechanoafferent Neurons in Terrestrial Snail: Response Properties and Synaptic Connections

In this study, we describe the putative mechanosensory neurons, which are involved in the control of avoidance behavior of the terrestrial snail Helix lucorum. These neurons, which were termed pleural ventrolateral (PlVL) neurons, mediated part of the withdrawal response of the animal via activation of the withdrawal interneurons. Between 15 and 30 pleural mechanosensory neurons were located on the ventrolateral side of each pleural ganglion. Intracellular injection of neurobiotin revealed that all PlVL neurons sent their axons into the skin nerves. The PlVL neurons had no spontaneous spike activity or fast synaptic potentials. In the reduced “CNS-foot” preparations, mechanical stimulation of the skin covering the dorsal surface of the foot elicited spikes in the PlVL neurons without any noticeable prepotential activity. Mechanical stimulus-induced action potentials in these cells persisted in the presence of high-Mg2+/zero-Ca2+ saline. Each neuron had oval-shaped receptive field 5–20 mm in length located on the dorsal surface of the foot. Partial overlapping of the receptive fields of different neurons was observed. Intracellular stimulation of the PlVL neurons produced excitatory inputs to the parietal and pleural withdrawal interneurons, which are known to control avoidance behavior. The excitatory postsynaptic potentials (EPSPs) in the withdrawal interneurons were induced in 1:1 ratio to the PlVL neuron spikes, and spike-EPSP latency was short and highly stable. These EPSPs also persisted in the high-Mg2+/high-Ca2+ saline, suggesting monosynaptic connections. All these data suggest that PlVL cells were the primary mechanosensory neurons.

Serotonergic modulation of two potassium currents in the pleural sensory neurons of Aplysia

1. The properties of membrane currents that were modulated by serotonin (5-HT) were investigated with two-electrode voltage-clamp techniques in sensory neuron somata isolated from the pleural ganglion of Aplysia californica. The modulatory effects of 5-HT were revealed by computer subtraction of current responses elicited in the presence of 5-HT from current responses elicited prior to the application of 5-HT. The complexities of the resulting 5-HT difference currents (I5-HT) suggested that 5-HT modulated more than one component of membrane current. 2. The 5-HT difference currents appeared to have at least two distinct components. One component was clearly evident at membrane potentials more negative than -10 mV was relatively voltage independent and did not inactivate. A second component was activated at membrane potentials more positive than -10 mV, had complex kinetics, and was highly voltage dependent. In an attempt to identify the membrane currents that were modulated by 5-HT, we compared the pharmacologic sensitivity of I5-HT to that of previously described K+ currents. 3. The two components of I5-HT had different sensitivities to agents that block K+ currents. The relatively voltage-independent component of I5-HT was not blocked by 2 mM 4-aminopyridine (4-AP) and was relatively insensitive to tetraethylammonium (TEA) (estimated Kd of 92 mM). In contrast, the voltage-dependent component of I5-HT was blocked by 4-AP (2 mM) and moderate concentrations of TEA (estimated Kd of 5 mM). 4. The K+ current blockers that were used to examine I5-HT were also used to examine voltage-activated membrane currents. Externally applied TEA blocked the delayed or voltage-dependent K+ current (IK.V) with an estimated dissociation constant (Kd) of 8 mM and a membrane current similar to the Ca2+-activated K+ current (IK.Ca) with an estimated Kd of 0.4 mM. In addition, externally applied 4-AP (2 mM) blocked IK.V. Thus TEA and 4-AP were equipotent in blocking both IK.V and the voltage-dependent component of I5-HT. 5. The suggestion that I5-HT contained multiple components was supported further by examining the modulatory effects of adenosine 3',5'-cyclic monophosphate (cAMP) that mediates some actions of 5-HT on membrane currents in these cells. cAMP difference currents (IcAMP) were similar to the relatively voltage-independent component of I5-HT. The subsequent addition of 5-HT to solutions already containing cAMP resulted in 5-HT difference currents similar to the voltage-dependent component of I5-HT.(ABSTRACT TRUNCATED AT 400 WORDS)

NEUROMOTOR BASES OF THE ESCAPE BEHAVIOUR OF NASSA MUTABILIS

The complex sequence of movements in the escape behaviour of the snail Nassa mutabilis (L.) was described in detail and the neuromotor activity underlying the behaviour was investigated by extra- and intracellular recording. The escape reaction is triggered by a chemical stimulus to the animal's foot, in these experiments either application of KCl solution or contact with a starfish. It consists of a preliminary phase in which the shell tilts to its side, the actual locomotor phase, and a final righting movement. The snail performs leaps, in which the foot and the shell are repeatedly rotated with respect to one another. EMGs recorded from the columellar muscle during the escape reaction showed that bursts of potentials are coupled to the shell rotations. In the intact animal this burst activity ordinarily began 0.6 ± 0.3 s after stimulation with KCl. In an animal dissected for recording from the columellar nerve (which supplies the columellar muscle), KCl stimulation of the dorsum of the foot induced burstlike neuronal activity with a latency of 0.5 ± 0.3 s. The dorsal foot region, the site at which the escape reaction can be triggered, was found to be supplied by the posterior pedal nerves; electrical stimulation of these nerves elicited bursts in the columellar nerve. The left pleural ganglion, which is known to contain neurones that project into the columellar nerve, was also found to contain neurones responsive to KCl stimulation of the foot. These findings suggest that the left pleural ganglion contains a motor centre which is involved in control of activity of the columellar nerve, and is also active during the escape reaction.