The mechanisms underlying short-term presynaptic facilitation, the enhancement of transmitter release from sensory neurons in Aplysia, induced by serotonin (5-HT), can be divided into two categories: (1) changes in ionic conductances leading to spike broadening and enhancement of Ca2+ influx; and (2) actions on the machinery for transmitter release that are independent of spike broadening and the resulting increases in Ca2+ influx. Spike broadening and the associated enhancement of excitability are induced by the modulation of K+ conductances in the sensory neuron. The cellular mechanisms that contribute to the enhancement of release that is independent of spike broadening are not known and may involve vesicle mobilization or other steps in exocytotic release. These two facilitatory actions of 5-HT are mediated by at least two second-messenger-activated protein kinase systems, protein kinase A (PKA) and protein kinase C (PKC). These two second-messenger cascades overlap in their contributions to synaptic facilitation. However, their relative contributions to enhancement of transmitter release are not simply synergistic but are state- and time-dependent. The state dependence is a reflection of the synapse's previous history of activity. When the synapse is rested (and not depressed), a brief pulse of 5-HT (lasting from 10 sec to 5 min) produces its actions primarily through PKA via both spike broadening-dependent and -independent mechanisms. The broadening primarily involves the modulation of a voltage-dependent K+ current, IKV, with a small contribution by a voltage-independent K+ current, IKS. By contrast, the enhancement of excitability is mediated primarily by the modulation of IKS. As the synapse becomes depressed with repeated activity, the contribution of PKC becomes progressively more important. As is the case with PKA, PKC produces its action both by broadening the spike via modulation of IKV and by a spike broadening-independent mechanism. In addition to being state-dependent, the mechanisms of facilitation are time-dependent. There are differences in the response to 5-HT when it is given briefly to produce short-term facilitation or when the exposure is prolonged. When exposure is brief (< or = 5 min), PKA dominates. When exposure is prolonged (10-20 min), PKC becomes dominant as it is with depressed synapses. Thus, synaptic plasticity appears to be expressed in several overlapping time domains, and the transition between very short-term facilitation and various intermediate duration phases seems to involve interactive processes between the kinases.

Operant conditioning is a form of associative learning through which an animal learns about the consequences of its behavior. Here, we report an appetitive operant conditioning procedure in Aplysia that induces long-term memory. Biophysical changes that accompanied the memory were found in an identified neuron (cell B51) that is considered critical for the expression of behavior that was rewarded. Similar cellular changes in B51 were produced by contingent reinforcement of B51 with dopamine in a single-cell analog of the operant procedure. These findings allow for the detailed analysis of the cellular and molecular processes underlying operant conditioning.

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Mechanical, chemical, or electrical stimulation of the tail elicits a short-latency (less than 1 s) tail-withdrawal reflex that is graded with the intensity of the stimulus. The tail-withdrawal reflex is not elicited by stimulation of parts of the body outside of the tail region. Mechanoafferent neurons innervating the tail are located in a small subcluster within a large, homogeneous group of medium-size (40-80 micron) cells on the ventrocaudal (VC) surface of each pleural ganglion. The tail sensory neurons within this large VC cluster are activated by tactile pressure or by electrical stimulation of discrete regions of the tail. They adapt slowly to maintained stimulation and sometimes respond to stimulus offset as well. Both mechanical and electrical stimuli produce responses that are graded with the intensity of the stimulus. Cells in the VC cluster appear to be primary mechanoreceptors because they have axons in peripheral nerves (including nerves innervating the tail), they exhibit action potentials lacking prepotentials in response to tactile stimulation, and these action potentials are still produced by cutaneous stimulation when peripheral and central chemical synaptic transmission is blocked. Stimulation of fields all over the body surface evokes synaptically mediated hyperpolarizing responses in individual mechanoafferent neurons that may represent afferent inhibition. Hyperpolarizing responses lasting many seconds can be produced by brief cutaneous stimuli. The mechanoafferent neurons innervating the tail region make strong monosynaptic connections to tail motor neurons in the ipsilateral pedal ganglion, and through these connections this subpopulation of the VC neurons appears to make a substantial contribution to the short-latency tail-withdrawal reflex. In addition, the combined excitatory receptive fields of these mechanoafferents match the excitatory receptive field of the tail-withdrawal reflex. Mechanoafferent neurons in the VC cluster that have receptive fields on other parts of the body (outside the excitatory receptive field of the tail-withdrawal reflex) have not been observed to make monosynaptic connections to the tail motor neurons. The neurons innervating the tail are reliably found in a discrete region within the larger VC cluster. In addition to this gross somatotopic organization, there is evidence of a finer level of somatotopic organization between the position of the excitatory receptive field on the tail and the position of the cell soma in the tail subcluster.(ABSTRACT TRUNCATED AT 400 WORDS)

Both positive and negative feedback loops of transcriptional regulation have been proposed to be important for the generation of circadian rhythms. To test the sufficiency of the proposed mechanisms, two differential equation-based models were constructed to describe the Neurospora crassa and Drosophila melanogaster circadian oscillators. In the model of the Neurospora oscillator, FRQ suppresses frq transcription by binding to a complex of the transcriptional activators WC-1 and WC-2, thus yielding negative feedback. FRQ also activates synthesis of WC-1, which in turn activates frq transcription, yielding positive feedback. In the model of the Drosophila oscillator, PER and TIM are represented by a "lumped" variable, "PER." PER suppresses its own transcription by binding to the transcriptional regulator dCLOCK, thus yielding negative feedback. PER also binds to dCLOCK to de-repress dclock, and dCLOCK in turn activates per transcription, yielding positive feedback. Both models displayed circadian oscillations that were robust to parameter variations and to noise and that entrained to simulated light/dark cycles. Circadian oscillations were only obtained if time delays were included to represent processes not modeled in detail (e.g., transcription and translation). In both models, oscillations were preserved when positive feedback was removed.