Abraham J. Susswein

Nitric Oxide and Memory

Nitric oxide (NO) is widely used in neural circuits giving rise to learning and memory. NO is an unusual neurotransmitter in its modes of release and action. Is its association with learning and memory related to its unusual properties? Reviewing the literature might allow the formulation of a general principle on how NO and memory are related. However, other than confirming that there is indeed a strong association between NO and memory, no simple rules emerge on the role of NO in learning and memory. The effects of NO are not associated with a particular stage or form of memory and are highly dependent on species, strain, and behavior or training paradigm. Nonetheless, a review does provide hints on why NO is associated with learning and memory. Unlike transmitters acting via receptors expressed only in neurons designed to respond to the transmitter, NO is a promiscuous signal that can affect a wide variety of neurons, via many molecular mechanisms. In circuits giving rise to learning and memory, it may be useful to signal some events via a promiscuous messenger having widespread effects. However, each circuit will use the promiscuous signal in a different way, to achieve different ends.

Behavioral patterns of Aplysia fasciata along the Mediterranean Coast of Israel

Aplysia fasciata were observed in a number of environments. They were usually aggregated. Animals were occasionally buried in sand. Aplysia were less mobile and more deeply hidden when waves were strong. Aplysia swam only in a calm environment. A wide variety of seaweeds were eaten, but the most common food was Ulva lactuca. Food arousal and satiation occur in the field. Often the pattern of feeding was a gradual slowing down and eventual cessation of eating; however, many meals deviated from this pattern. Appetitive behaviors preceding mating as a male were similar to those preceding eating. Aroused animals mated as males, while passive animals mated as females; mating as a male produced arousal, expressed as an increased likelihood to respond to food or to mate as a male. Two males could simultaneously impregnate one female. Animals mate in large groups, constantly changing partners. Mating groups occur in linear chain, branched chain, and closed chain configurations. Egg laying and egg masses were observed in areas inhabited by animals. Inking was never observed without exprimenter intervention, even when a crab attacked an Aplysia.

A learned change of response to inedible food in Aplysia.

Aplysia fasciata attempt to bite and swallow food wrapped in a plastic net, tasting food through holes in the net. Net-enclosed food cannot be swallowed, and becomes cyclically lodged and pushed out of the buccal cavity. Aplysia gradually modify their response to this food, and eventually cease to respond. Twenty-four hours following training, memory is maintained, as shown by savings upon retraining. An essential component of the behavioral plasticity is food becoming stuck within the buccal cavity: when the lips are stimulated without allowing food to enter the buccal cavity, animals stop responding, but training takes longer, and memory is not retained. Savings upon retraining are contingent upon temporal pairing of food upon the lips and stimuli from within the buccal cavity: when lip stimuli and the experience of food stuck within the buccal cavity occur sequentially (rather than simultaneously), 24 hr later, animals are not significantly different from naive subjects.

CHARACTERIZATION OF BUCCAL MOTOR PROGRAMS ELICITED BY A CHOLINERGIC AGONIST APPLIED TO THE CEREBRAL GANGLION OF APLYSIA CALIFORNICA

Applying the non-hydrolyzable cholinergic agonist carbachol (CCh) to the cerebral ganglion of Aplysia elicits sustained, regular bursts of activity in the buccal ganglia resembling those seen during biting. The threshold for bursting is ∼ 102−4M. Bursting begins after a 2 to 5 min delay. The burst frequency increases over the first 5 bursts, reaching a plateau value of ∼ 3 per minute. Bursting is maintained for over 10 min. Some of the effects of CCh may be attributed to its ability to depolarize and fire CBI-2, a command-like neuron in the cerebral ganglion that initiates biting. CBI-2 is also depolarized by ACh, and by stimulating peripheral sensory nerves. Excitation of CBI-2 caused by carbachol is partially blocked by the muscarinic antagonist atropine. We examined whether CCh-induced bursting is modified in ganglia taken from Aplysia that previously experienced treatments inhibiting feeding, such as satiation, head shock contingent or non-contingent with food, and training animals with an inedible food. No treatment consistently and repeatedly affected the latency, the peak burst period, the length of time that bursting was maintained, or the threshold CCh concentration for eliciting bursting. However, there was a decrease in the rate of the buildup of the buccal ganglion program in previously satiated animals.

Activity patterns and time budgeting of Aplysia fasciata under field and laboratory conditions

Activity patterns of Aplysia fasciata were observed in a protected port environment and in an aquarium. In both, major activities were feeding and mating, which collectively took up about 45% of the total time of the animals. Active behaviors occurred primarily at night; much of the day was spent in the inactive state. Activities were highly synchronized, with large numbers of animals performing the same behaviors simultaneously at a specific time. Mating and eating occurred primarily at different times; relatively few animals were observed performing these behaviors simultaneously. Many animals laying eggs were simultaneously mating as females.

Autaptic Excitation Elicits Persistent Activity and a Plateau Potential in a Neuron of Known Behavioral Function

Synaptic connections from a neuron onto itself (autapses) are not uncommon, but their contributions to information processing and behavior are not fully understood. Positive feedback mediated by autapses could in principle give rise to persistent activity, a property of some neurons in which a brief stimulus causes a long-lasting response. We have identified an autapse that underlies a plateau potential causing persistent activity in the B31/B32 neurons of Aplysia. The persistent activity is essential to the ability of these neurons to initiate and maintain components of feeding behavior. Persistent activity in B31/B32 arises from a voltage-dependent muscarinic autapse and from pharmacologically identical network-based positive feedback. Depolarization via the autapse begins later than network-driven excitation, and the effect of the autapse is therefore overshadowed by the earlier network-based depolarization. In B31/B32 neurons isolated in culture only the autapse is present, and the autapse functionally replaces the missing network-based feedback. Properties of B31/B32 provide insight into a possible general function of autapses. Autapses might function along with synapses from presynaptic neurons as components of feedback loops.

Parametric features of inhibition of feeding in Aplysia by associative learning, satiation, and sustained lip stimulation

In order to determine whether different classes of behavioral plasticity affect common or unique neural loci, the effects of three types of processes that inhibit feeding in Aplysia were quantified. Changes in feeding behavior due to an associative learning task in which animals learn that food is inedible were compared with behavioral effects caused by satiation and by sustained lip stimulation. The data indicate that each process modifying feeding can be characterized by differences in time to stop responding to food, by differences in specificity of the decrement to a particular food, and by different patterns of motor output before complete cessation of responsiveness. The data suggest each process inhibiting feeding acts at a different neural site. Learning that food is inedible may be due to facilitation of a specific sensory pathway onto pattern generators producing rejection responses. Sustained lip stimulation seems to inhibit feeding by causing a decrement in all outputs of a particular sensory pathway. Finally, satiation appears to represent inhibition of feeding motor elements.

Peptide and protein pheromones in molluscs

Pheromones have been implicated in the control of a number of behaviors in molluscs, but few peptide pheromones have been characterized in these animals. Peptide pheromones include: (1) a family of water-borne peptide pheromonal attractants (attractins) in the gastropod Aplysia that are released during egg laying and attract other Aplysia to form egg-laying and mating aggregations; (2) a tetrapeptide (ILME) in the cephalopod Sepia that elutes from egg masses and is thought to be involved in the transport of oocytes in the genital tract during egg laying; and (3) a Sepia sperm-attracting peptide (SepSAP; PIDPGVamide) that is released from oocytes during egg laying to facilitate external fertilization.

Sexual behavior in Aplysia fasciata induced by homogenates of the distal large hermaphroditic duct

Copulation was induced in pairs of Aplysia fasciata by adding to the environment a homogenate of the distal portion of the large hermaphroditic duct. Similar effects were not observed in response to an abdominal ganglion homogenate. The data suggest that the duct may secrete a pheromone initiating copulation in Aplysia.

Purification and characterization of the gonad lectin of Aplysia depilans.

Extracts of gonads and fertilized eggs of Aplysia depilans contain a D‐galacturonic and D‐galactose‐binding lectin. This lectin reacts strongly with rabbit and human erythrocytes independent of ABO blood groups, weakly with dog, mouse, rat, and chick erythrocytes and not at all or very weakly with sheep erythrocytes. Purification of the gonad lectin was easily achieved, with a high yield, by heating to 70°C, precipitation with ammonium sulfate and affinity chromatography on Sepharose 4B. The purified lectin was found to be a glucoprotein of molecular mass around 55–60 kDa; it stimulates mitogenesis of human peripheral lymphocytes.