Synaptic facilitation by ectopic octopamine and 5-HT receptors in Aplysia

Exogenous expression of an allatotropin-related peptide receptor increased the membrane excitability in Aplysia neurons

Neuropeptides act mostly on a class of G-protein coupled receptors, and play a fundamental role in the functions of neural circuits underlying behaviors. However, physiological functions of some neuropeptide receptors are poorly understood. Here, we used the molluscan model system Aplysia and microinjected the exogenous neuropeptide receptor apATRPR (Aplysia allatotropin-related peptide receptor) with an expression vector (pNEX3) into Aplysia neurons that did not express the receptor endogenously. Physiological experiments demonstrated that apATRPR could mediate the excitability increase induced by its ligand, apATRP (Aplysia allatotropin-related peptide), in the Aplysia neurons that now express the receptor. This study provides a definitive evidence for a physiological function of a neuropeptide receptor in molluscan animals.

Inhibitory responses in Aplysia pleural sensory neurons act to block excitability, transmitter release, and PKC Apl II activation

Expression of the 5-HT(1Apl(a)) receptor in Aplysia pleural sensory neurons inhibited 5-HT-mediated translocation of the novel PKC Apl II in sensory neurons and prevented PKC-dependent synaptic facilitation at sensory to motoneuron synapses (Nagakura et al. 2010). We now demonstrate that the ability of inhibitory receptors to block PKC activation is a general feature of inhibitory receptors and is found after expression of the 5-HT(1Apl(b)) receptor and with activation of endogenous dopamine and FMRFamide receptors in sensory neurons. Pleural sensory neurons are heterogeneous for their inhibitory response to endogenous transmitters, with dopamine being the most prevalent, followed by FMRFamide, and only a small number of neurons with inhibitory responses to 5-HT. The inhibitory response is dominant, reduces membrane excitability and synaptic efficacy, and can reverse 5-HT facilitation at both naive and depressed synapses. Indeed, dopamine can reverse PKC translocation during the continued application of 5-HT. Reversal of translocation can also be seen after translocation mediated by an analog of diacylglycerol, suggesting inhibition is not through blockade of diacylglycerol production. The effects of inhibition on PKC translocation can be rescued by phosphatidic acid, consistent with the inhibitory response involving a reduction or block of production of this lipid. However, phosphatidic acid could not recover PKC-dependent synaptic facilitation due to an additional inhibitory effect on the non-L-type calcium flux linked to synaptic transmission. In summary, we find a novel mechanism downstream of inhibitory receptors linked to inhibition of PKC activation in Aplysia sensory neurons.

Evolutionary conservation of the signaling proteins upstream of cyclic AMP-dependent kinase and protein kinase C in gastropod mollusks

The protein kinase C (PKC) and the cAMP-dependent kinase (protein kinase A; PKA) pathways are known to play important roles in behavioral plasticity and learning in the nervous systems of a wide variety of species across phyla. We briefly review the members of the PKC and PKA family and focus on the evolution of the immediate upstream activators of PKC and PKA i.e., phospholipase C (PLC) and adenylyl cyclase (AC), and their conservation in gastropod mollusks, taking advantage of the recent assembly of the Aplysiacalifornica and Lottia gigantea genomes. The diversity of PLC and AC family members present in mollusks suggests a multitude of possible mechanisms to activate PKA and PKC; we briefly discuss the relevance of these pathways to the known physiological activation of these kinases in Aplysia neurons during plasticity and learning. These multiple mechanisms of activation provide the gastropod nervous system with tremendous flexibility for implementing neuromodulatory responses to both neuronal activity and extracellular signals.

Identification of a serotonin receptor coupled to adenylyl cyclase involved in learning-related heterosynaptic facilitation in Aplysia

Serotonin (5-HT) plays a critical role in modulating synaptic plasticity in the marine mollusc Aplysia and in the mammalian nervous system. In Aplysia sensory neurons, 5-HT can activate several signal cascades, including PKA and PKC, presumably via distinct types of G protein-coupled receptors. However, the molecular identities of these receptors have not yet been identified. We here report the cloning and functional characterization of a 5-HT receptor that is positively coupled to adenylyl cyclase in Aplysia neurons. The cloned receptor, 5-HT(apAC1), stimulates the production of cAMP in HEK293T cells and in Xenopus oocytes. Moreover, the knockdown of 5-HT(apAC1) expression by RNA interference blocked 5-HT-induced cAMP production in Aplysia sensory neurons and blocked synaptic facilitation in nondepressed or partially depressed sensory-to-motor neuron synapses. These data implicate 5-HT(apAC1) as a major modulator of learning related synaptic facilitation in the direct sensory to motor neuron pathway of the gill withdrawal reflex.

Enhancement of presynaptic glutamate release and persistent inflammatory pain by increasing neuronal cAMP in the anterior cingulate cortex

Both presynaptic and postsynaptic alterations are associated with plastic changes of brain circuits, such as learning and memory, drug addiction and chronic pain. However, the dissection of the relative contributions of pre- and postsynaptic components to brain functions is difficult. We have previously shown peripheral inflammation caused both presynaptic and postsynaptic changes and calcium-stimulated cyclic AMP (cAMP) pathway in the anterior cingulate cortex (ACC) is critical in the synaptic plasticity and behavioral sensitization to pain. It remains to be elucidated whether presynaptic or postsynaptic modulation by cAMP in the ACC could be sufficient for enhancing inflammatory pain. In order to address this question, we took advantage of a novel transgenic mouse model, heterologously expressing an Aplysia octopamine receptor (Ap oa₁). This receptor is G protein-coupled and selectively activates the cAMP pathway. We found that activation of Ap oa₁ by octopamine enhanced glutamatergic synaptic transmission in the ACC by increasing presynaptic glutamate release in vitro. Bilateral microinjection of octopamine into the ACC significantly facilitated behavioral responses to inflammatory pain but not acute pain. The present study provides the first evidence linking enhanced presynaptic glutamate release in the ACC to behavioral sensitization caused by peripheral inflammation.

Regulation of synaptic vesicles pools within motor nerve terminals during short-term facilitation and neuromodulation

The reserve pool (RP) and readily releasable pool (RRP) of synaptic vesicles within presynaptic nerve terminals were physiologically differentiated into distinctly separate functional groups. This was accomplished in glutamatergic nerve terminals by blocking the glutamate transporter with dl-threo-beta-benzyloxyaspartate (TBOA; 10 microM) during electrical stimulation with either 40 Hz of 10 pulses within a train or 20- or 50-Hz continuous stimulation. The 50-Hz continuous stimulation decreased the excitatory postsynaptic potential amplitude 60 min faster than for the 20-Hz continuous stimulation in the presence of TBOA (P < 0.05). There was no significant difference between the train stimulation and 20-Hz continuous stimulation in the run-down time in the presence of TBOA. After TBOA-induced synaptic depression, the excitatory postsynaptic potentials were rapidly (<1 min) revitalized by exposure to serotonin (5-HT, 1 microM) in every preparation tested (P < 0.05). At this glutamatergic nerve terminal, 5-HT promotes an increase probability of vesicular docking and fusion. Quantal recordings made directly at nerve terminals revealed smaller quantal sizes with TBOA exposure with a marked increase in quantal size as well as a continual appearance of smaller quanta upon 5-HT treatment after TBOA-induced depression. Thus 5-HT was able to recruit vesicles from the RP that were not rapidly depleted by acute TBOA treatment and electrical stimulation. The results support the notion that the RRP is selectively activated during rapid electrical stimulation sparing the RP; however, the RP can be recruited by the neuromodulator 5-HT. This suggests at least two separate kinetic and distinct regulatory paths for vesicle recycling within the presynaptic nerve terminal.