Multiple modes of activity in a model neuron suggest a novel mechanism for the effects of neuromodulators

1. Previous examination of the phase space of a mathematical model of a bursting molluscan neuron has demonstrated the existence of multiple stable oscillatory modes. The present study examined the extent to which multistability could be regulated by known modulatory agents, the consequences of that regulation on the response of the neuron to synaptic inputs, the effects of noise, and the potential of multistability to enrich the repertoire of neuromodulatory effects. 2. Coexisting stable attractors may appear when a change is made in a voltage-dependent conductance in a manner that simulates the application of a neuromodulator. A small transient perturbation can shift the model neuron between stable modes, greatly amplifying the original perturbation. Thus the model becomes more sensitive to conventional synaptic inputs. These mode shifts are robust in the presence of low-amplitude synaptic noise. 3. In response to random high-amplitude synaptic noise, a model neuron rendered multistable by a simulated application of a neuromodulator produces apparently random activity, whereas in response to the same synaptic noise, a monostable model neuron produces barely perturbed regular activity. Thus an increase in the number of attractors enhances sensitivity to both conventional synaptic inputs and noise. Conversely, a decrease is associated with a reduction in sensitivity. 4. The response of a neuron to a subsequent transient perturbation in the level of neuromodulator depends on the steady-state level of the neuromodulator. For example, if the steady-state level is associated with a multistable neuron, a mode shift produced by such a transient change in the level of neuromodulator (manifested in our model as a conductance change) can persist after the conductance is returned gradually to its original value. Thus multistable dynamic activity permits the effects of a neuromodulator to persist when the neuromodulator is no longer present. 5. The mechanism of mode shifting between coexisting stable oscillatory modes introduces a number of novel possibilities with potentially profound implications for information processing and storage in a single neuron.

Nonlinear dynamics in a model neuron provide a novel mechanism for transient synaptic inputs to produce long-term alterations of postsynaptic activity

1. A mathematical model of a bursting molluscan neuron has been found to possess multiple modes of electrical activity, such as periodic beating (tonic firing), periodic bursting (bursts of action potentials separated by quiescent periods), and potentially chaotic bursting, all at a single set of parameters. The multiple modes correspond to multiple stable attractors, whose existence is an emergent property of the nonlinear dynamics of the system. 2. Transient synaptic inputs can switch the activity of the neuron between different modes. These mode transitions, which do not require any changes in the biochemical or biophysical parameters of the neuron, provide an enduring response to a transient input, as well as a mechanism for phasic sensitivity (i.e., temporal specificity). 3. These results provide new insights into the role of nonlinear dynamics in information processing and storage at the level of the single neuron.

An identifiable molluscan neuron responds to changes in earth-strength magnetic fields

Diverse animals can orient using geomagnetic cues, but little is known about the neurophysiological mechanisms that underlie magnetic field detection. The marine mollusc Tritonia diomedea (Bergh) has a magnetic sense and its nervous system is amenable to cellular-level electrophysiological analysis. In a semi-intact whole-animal preparation, intracellular recordings from the large, visually identifiable neurons left pedal 5 (LPe5) and right pedal 5 (RPe5) in the brain of Tritonia revealed enhanced electrical activity in response to changes in ambient earth-strength magnetic fields. No such changes in activity were observed in approximately 50 other neurons subjected to identical magnetic stimuli. The responses of LPe5 were characterized by increases in spiking frequency occurring about 6–16 min after the ambient magnetic field had been rotated to a new position. The response was abolished when the brain had been isolated from the periphery of the animal by severing nerves, a procedure that also transected prominent neurites of LPe5. We hypothesize that LPe5 is one component of a neural circuit mediating detection of the earth's magnetic field or orientation to it.