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Modulation of a subthreshold calcium current by the neuropeptide FMRFamide in Aplysia neuron R15
R H Kramer1, E S Levitan, G M Carrow
1Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02254.
Abstract:
1. The effect of the endogenous neuropeptide FMRFamide (Phe-Met-Arg-Phe-amide) on the Aplysia bursting pacemaker neuron R15 was studied. Brief local applications of FMRFamide, both on R15 somata in situ, and on R15 somata that were isolated and maintained in primary cell culture, cause a hyperpolarization of the membrane potential and a suppression of spontaneous bursting or beating pacemaker activity. 2. Two-electrode voltage-clamp experiments revealed that FMRFamide decreases the amplitude of an inward current, which activates with depolarization starting at a membrane potential less depolarized than the threshold for action potentials. Previous studies have established that this subthreshold inward current is carried by calcium and is essential for the generation of bursting pacemaker activity in Aplysia neurons. The effect of FMRFamide on the subthreshold inward current of R15 is blocked by divalent cation calcium channel blockers, such as cobalt and manganese, and is unaffected by changing the external concentration of potassium or chloride ions, or addition of blockers of the calcium-activated potassium current, such as external tetraethylammonium or internal EGTA. 3. The subthreshold calcium current of R15 is also decreased by dopamine and by an unidentified synaptic neurotransmitter. These substances mimic and occlude the action of FMRFamide on the subthreshold calcium current, suggesting that all three transmitters converge to affect the same population of calcium channels in neuron R15. 4. The subthreshold calcium current is enhanced by neurotransmitters that elevate cyclic AMP in R15, including serotonin, and the Aplysia neuropeptide egg-laying hormone (ELH). Likewise, the effect of FMRFamide on the subthreshold calcium current is enhanced by serotonin, ELH, and a cyclic AMP analog, suggesting that FMRFamide and cyclic AMP have antagonistic actions on the same population of calcium channels in neuron R15. 5. We conclude that the suppression of spontaneous bursting or beating pacemaker activity in neuron R15 by FMRFamide is due to a decrease in the subthreshold calcium current. The subthreshold calcium current in R15 is a common target for modulation by many different transmitters, acting via several distinct molecular mechanisms.
Insights
The neuropeptide FMRFamide suppresses Aplysia neuron activity by reducing a key calcium current. This finding reveals a common target for various neurotransmitters modulating neuronal excitability.
Area of Science:
- Neuroscience
- Cellular Electrophysiology
- Neuropeptide Signaling
Background:
- The Aplysia bursting pacemaker neuron R15 generates rhythmic electrical activity essential for behaviors.
- Endogenous neuropeptides play crucial roles in modulating neuronal function and network activity.
Purpose of the Study:
- To investigate the cellular mechanisms by which the neuropeptide FMRFamide (Phe-Met-Arg-Phe-amide) affects the activity of Aplysia neuron R15.
- To determine the specific ion currents targeted by FMRFamide and its interaction with other neurotransmitters.
Main Methods:
- Two-electrode voltage-clamp recordings from Aplysia neuron R15 in situ and in primary cell culture.
- Application of FMRFamide, dopamine, serotonin, egg-laying hormone (ELH), and various channel blockers (cobalt, manganese, tetraethylammonium, EGTA).
Main Results:
- FMRFamide application caused hyperpolarization and suppressed bursting activity in R15.
- FMRFamide reduced the amplitude of a subthreshold inward calcium current, essential for bursting.
- This FMRFamide effect was blocked by calcium channel blockers and mimicked/occluded by dopamine, suggesting convergence on calcium channels.
- FMRFamide's action was antagonized by cyclic AMP-elevating neurotransmitters like serotonin and ELH, indicating opposing modulation of the same calcium channels.
Conclusions:
- FMRFamide suppresses pacemaker activity in R15 by decreasing the subthreshold calcium current.
- The subthreshold calcium current in R15 is a convergence point for diverse neurotransmitter systems acting through distinct molecular pathways.