Age dependent changes in serotonin and dopamine receptors in Aplysia californica

Altered expression of ionotropic L-Glutamate receptors in aged sensory neurons of Aplysia californica

The simplified nervous system of Aplysia californica (Aplysia) allows for detailed studies of physiological and molecular changes in small sets of neurons. Sensory neurons of the biting and tail withdrawal reflexes are glutamatergic and show reduced L-Glutamate current density in aged animals, making them a good candidate to study age-related changes in glutamatergic responses. To examine if changes in ionotropic L-Glu receptor (iGluR) transcription underlie reduced physiology, mRNA expression of iGluR was quantified in two sensory neuron clusters of two cohorts of Aplysia at both sexual maturity (~8 months) and advanced age (~12 months). Sensory neuron aging resulted in a significant overall decrease in expression of iGluR subunits in both sensory neuron clusters and cohorts. Although the individual subunits differentially expressed varied between sensory neuron clusters and different cohorts of animals, all differentially expressed subunits were downregulated, with no subunits showing significantly increased expression with age. Overall declines in transcript expression suggest that age-related declines in L-Glu responsiveness in Aplysia sensory neurons could be linked to overall declines in iGluR expression, rather than dysregulation of specific subunits. In both sensory neuron clusters tested the N-methyl-D-aspartate receptor subtype was expressed at significantly greater levels than other iGluR subtypes, suggesting an in vivo role for NMDAR-like receptors in Aplysia sensory neurons.

Decreased response to acetylcholine during aging of aplysia neuron R15

How aging affects the communication between neurons is poorly understood. To address this question, we have studied the electrophysiological properties of identified neuron R15 of the marine mollusk Aplysia californica. R15 is a bursting neuron in the abdominal ganglia of the central nervous system and is implicated in reproduction, water balance, and heart function. Exposure to acetylcholine (ACh) causes an increase in R15 burst firing. Whole-cell recordings of R15 in the intact ganglia dissected from mature and old Aplysia showed specific changes in burst firing and properties of action potentials induced by ACh. We found that while there were no significant changes in resting membrane potential and latency in response to ACh, the burst number and burst duration is altered during aging. The action potential waveform analysis showed that unlike mature neurons, the duration of depolarization and the repolarization amplitude and duration did not change in old neurons in response to ACh. Furthermore, single neuron quantitative analysis of acetylcholine receptors (AChRs) suggested alteration of expression of specific AChRs in R15 neurons during aging. These results suggest a defect in cholinergic transmission during aging of the R15 neuron.

Do different neurons age differently? Direct genome-wide analysis of aging in single identified cholinergic neurons

Aplysia californica is a powerful experimental system to study the entire scope of genomic and epigenomic regulation at the resolution of single functionally characterized neurons and is an emerging model in the neurobiology of aging. First, we have identified and cloned a number of evolutionarily conserved genes that are age-related, including components of apoptosis and chromatin remodeling. Second, we performed gene expression profiling of different identified cholinergic neurons between young and aged animals. Our initial analysis indicates that two cholinergic neurons (R2 and LPl1) revealed highly differential genome-wide changes following aging suggesting that on the molecular scale different neurons indeed age differently. Each of the neurons tested has a unique subset of genes differentially expressed in older animals, and the majority of differently expressed genes (including those related to apoptosis and Alzheimer's disease) are found in aging neurons of one but not another type. The performed analysis allows us to implicate (i) cell specific changes in histones, (ii) DNA methylation and (iii) regional relocation of RNAs as key processes underlying age-related changes in neuronal functions and synaptic plasticity. These mechanisms can fine-tune the dynamics of long-term chromatin remodeling, or control weakening and the loss of synaptic connections in aging. At the same time our genomic tests revealed evolutionarily conserved gene clusters associated with aging (e.g., apoptosis-, telomere- and redox-dependent processes, insulin and estrogen signaling and water channels).

Pharmacological characterization of an adenylyl cyclase-coupled 5-HT receptor in aplysia: comparison with mammalian 5-HT receptors

We attempted to identify compounds that are effective in blocking the serotonin (5-hydroxytryptamine, 5-HT) receptor(s) that activate adenylyl cyclase (AC) in Aplysia CNS. We call this class of receptor 5-HT(apAC). Eight of the 14 antagonists tested were effective against 5-HT(apAC) in CNS membranes with the following rank order of potency: methiothepin > metergoline approximately fluphenazine > clozapine > cyproheptadine approximately risperidone approximately ritanserin > NAN-190. GR-113808, olanzapine, Ro-04-6790, RS-102221, SB-204070, and spiperone were inactive. Methiothepin completely blocked 5-HT stimulation of AC with a K(b) of 18 nM. Comparison of the pharmacological profile of the 5-HT(apAC) receptor with those of mammalian 5-HT receptor subtypes suggested it most closely resembles the 5-HT(6) receptor. AC stimulation in Aplysia sensory neuron (SN) membranes was also blocked by methiothepin. Methiothepin substantially inhibited two effects of 5-HT on SN firing properties that are mediated by a cAMP-dependent reduction in S-K(+) current: spike broadening in tetraethylammonium/nifedipine and increased excitability. Consistent with cyproheptadine blocking 5-HT stimulation of AC, cyproheptadine also blocked the 5-HT-induced increase in SN excitability. Methiothepin was less effective in blocking AC-mediated modulatory effects of 5-HT in electrophysiological experiments on SNs than in blocking AC stimulation in CNS or SN membranes. This reduction in potency appears to be due to effects of the high ionic strength of physiological saline on the binding of this antagonist to the receptor. Methiothepin also antagonized AC-coupled dopamine receptors but not AC-coupled small cardioactive peptide receptors. In conjunction with other pharmacological probes, this antagonist should be useful in analyzing the role of 5-HT in various forms of neuromodulation in Aplysia.