An aplysia dopamine1-like receptor: molecular and functional characterization.

In Aplysia, the neurotransmitter dopamine is involved in the regulation of various physiological processes and motor functions, like feeding behaviour, and in the siphon-gill withdrawal reflex. In this paper, we report the characterization of the first Aplysia D1-like dopamine receptor (Apdop1) mainly expressed in the CNS, heart and buccal mass. Following expression of the Apdop1 receptor in HEK293 cells, a higher level of cAMP was observed in the absence of the receptor ligand, showing that Apdop1 is constitutively active. This activity was blocked by the inverse agonist flupentixol. Application of dopamine (EC50 of 35 nm) or serotonin (EC50 of 36 microm) to Apdop1-transfected HEK293 cells further increased the level of cAMP, suggesting that the receptor is linked to the stimulatory Gs protein pathway. When expressed in cultured sensory neurons, Apdop1 immunoreactivity was observed in the cell body and neurites. Control sensory neurons responded to dopamine with a decrease in excitability mediated by a pertusis toxin-sensitive G protein. Expression of Apdop1 produced an increase in hyperpolarization in the absence of agonist and an increase in membrane excitability following stimulation by dopamine. In the presence of pertussis toxin to inhibit the Gi protein inhibitory pathway responsible for decrease in excitability mechanism, Stimulation of membrane excitability was observed. Apdop1 sensitivity to dopamine makes it a potential modulator of operant conditioning procedure.

cAMP-response elements in Aplysia creb1, creb2, and Ap-uch promoters: implications for feedback loops modulating long term memory.

The Aplysia genes encoding for cAMP-response element-binding protein 1 (CREB1), CREB2, and ubiquitin C-terminal hydrolase (Ap-uch) have been implicated in the formation of long term memory. However, nothing is known about the promoter regions of these genes or the transcription factors that regulate them. We cloned the promoter regions of creb1, creb2, and Ap-uch and identified a canonical cAMP-response element (CRE) in the promoter region of creb1. Variants of the canonical CRE were identified in all three promoters. TATA boxes and C/EBP-binding motifs are also present in the promoter regions of these genes. Promoter immunoprecipitation assays and chromatin immunoprecipitation assays indicated that CREB1 and CREB2 bind to the promoter regions of creb1 and creb2, suggesting that feedback loops modulate the formation of long term memory. In a positive feedback loop, phosphorylated CREB1 might induce its own gene via CREs. In support of this suggestion, treatment with serotonin enhanced binding of CREB1 to its promoter region and increased mRNA levels of creb1. Levels of Ap-uch mRNA also increased in response to serotonin; however, binding of CREB1 or CREB2 to the promoter region of Ap-uch was not detected. The finding that the promoter region of creb2 has a CRE raises the intriguing possibility that its expression is regulated by CREB1 and/or CREB2. CREB2 may repress its own gene, forming a negative feedback loop, and CREB2 up-regulation via CREB1 may limit the activity of the CREB1-mediated positive feedback loop.

Immunoglobulin Fc gamma receptor promotes immunoglobulin uptake, immunoglobulin-mediated calcium increase, and neurotransmitter release in motor neurons.

Receptors for the Fc portion of immunoglobulin G (IgG; FcgammaRs) facilitate IgG uptake by effector cells as well as cellular responses initiated by IgG binding. In earlier studies, we demonstrated that amyotrophic lateral sclerosis (ALS) patient IgG can be taken up by motor neuron terminals and transported retrogradely to the cell body and can alter the function of neuromuscular synapses, such as increasing intracellular calcium and spontaneous transmitter release from motor axon terminals after passive transfer. In the present study, we examined whether FcgammaR-mediated processes can contribute to these effects of ALS patient immunoglobulins. F(ab')(2) fragments (which lack the Fc portion) of ALS patient IgG were not taken up by motor axon terminals and were not retrogradely transported. Furthermore, in a genetically modified mouse lacking the gamma subunit of the FcR, the uptake of whole ALS IgG and its ability to enhance intracellular calcium and acetylcholine release were markedly attenuated. These data suggest that FcgammaRs appear to participate in IgG uptake into motor neurons as well as IgG-mediated increases in intracellular calcium and acetylcholine release from motor axon terminals.