Steroid Hormone Modulation of Hippocampal Dependent Spatial Memory

Recent studies show that the adult CNS is capable of considerable re-structuring and re-growth, a property previously thought limited to the developmental period. Hormones play an important role in many of these plastic processes, and the hippocampus, as a target for all the major classes of steroid hormones, undergoes considerable remodeling. Since it is critical for mediating tasks requiring spatial memory, the hippocampus can serve as an important model for understanding not only the mechanisms underlying plasticity of brain circuits but also how these changes impact on a higher order function, spatial memory. In this review, the effects of steroid hormones on hippocampal remodeling are discussed, and the ability of estradiol to enhance spatial memory as well as the ability of both excessive or diminished corticosteroid levels to impair spatial memory are described. The neural mechanisms for these effects, as well as previous and current contributions of McEwen and colleagues to this expanding area of research, are discussed.     

A single serotonergic modulatory cell can mediate reinforcement in the withdrawal network of the terrestrial snail

A cluster of 40 serotonergic cells in the rostral part of pedal ganglia of the terrestrial snail Helix lucorum was shown previously to participate in the modulation of withdrawal behavior and to be necessary during the acquisition of aversive withdrawal conditioning in intact snails. Local extracellular stimulation of the serotonergic cells paired with a test stimulus elicited a pairing-specific increase (the difference between paired and explicitly unpaired sessions was significant, p <.01) of synaptic responses to test stimulation in the premotor interneurons involved in withdrawal. This result suggested participation of serotonergic cells in mediating the reinforcement in the withdrawal network. Intracellular stimulation of only one identified Pd4 cell from the pedal group of serotonergic neurons paired with a test stimulus also significantly increased (the difference between paired and explicitly unpaired sessions was significant, p <.05) synaptic responses to paired nerve stimulation in same premotor interneurons involved in withdrawal. Morphological investigation of a cluster of pedal serotonergic neurons showed that only the Pd4 cell had branches in the parietal ganglia neuropile where the synapses of premotor withdrawal interneurons and of presynaptic neurons are located. The data suggest that a single serotonergic cell can mediate the reinforcement in the withdrawal network of the terrestrial snail. Patterns of responses of the Pd4 cells to tactile and chemical stimuli conform to the suggestion.