We studied the contribution of the Aplysia peripheral nervous system, in the siphon and gill, to habituation of the gill withdrawal reflex. After removal of one central ganglion, the parietovisceral, repeated stimulation of the siphon caused habituation of the reflex as it had with the ganglion intact, showing that there is a peripheral pathway between the siphon and gill with competence to mediate habituation. Repeated electrical stimulation of two efferent nerves to the gill, after removal of the parietovisceral ganglion, resulted in habituation of withdrawal movements, which shows that the terminals of the ganglion neurons in the gill are a site of habituation. Also, stimulation of one nerve dishabituates the withdrawal movements elicited by the other. These identify two sites of habituation in the gill in addition to sites in the parietovisceral ganglion.

The isolated eye of the sea hare Aplysia califomica shows a circadian rhythm of optic nerve impulses when kept in total darkness. Peak activity on the first day of isolation occurs during the projected " dawn" of the light-dark cycle to which the whole animal had previously been entrained. Eyes from animals previously exposed to constant light show a free-running rhythm. This simple photoreceptor provides a quantized output with an ideal control (the other eye) for studies on rhythms.

Nitric oxide (NO) is an unconventional neurotransmitter and neuromodulator molecule that is increasingly found to have important signaling functions in animals from nematodes to mammals. NO signaling mechanisms in the past were identified largely through experiments on mammals, after the discovery of NO's vasodilatory functions. The use of gene knock out mice has been particularly important in revealing the functions of the several isoforms of nitric oxide synthase (NOS), the enzyme that produces NO. Recent studies have revealed rich diversity in NO signaling. In addition to the well-established pathway in which NO activates guanylyl cyclase and cGMP production, redox mechanisms involving protein nitrosylation are important contributors to modulation of neurotransmitter release and reception. NO signaling studies in invertebrates are now generating a wealth of comparative information. Invertebrate NOS isoforms have been identified in insects and molluscs, and the conserved and variable amino acid sequences evaluated. Calcium-calmodulin dependence and cofactor requirements are conserved. NADPH diaphorase studies show that NOS is found in echinoderms, coelenterates, nematodes, annelids, insects, crustaceans and molluscs. Accumulating evidence reveals that NO is used as an orthograde transmitter and cotransmitter, and as a modulator of conventional transmitter release. NO appears to be used in diverse animals for certain neuronal functions, such as chemosensory signaling, learning, and development, suggesting that these NO functions have been conserved during evolution. The discovery of NO's diverse and unconventional signaling functions has stimulated a plethora of enthusiastic investigations into its uses. We can anticipate the discovery of many more interesting and some surprising NO signaling functions.