Spider toxin and pertussis toxin differentiate post- and presynaptic glutamate receptors

Enhanced incentive motivation in obesity-prone rats is mediated by NAc core CP-AMPARs

Studies in humans suggest that stronger incentive motivational responses to Pavlovian food cues may drive over-consumption leading to and maintaining obesity, particularly in susceptible individuals. However, whether this enhanced incentive motivation emerges as a consequence of obesity or rather precedes obesity is unknown. Moreover, while human imaging studies have provided important information about differences in striatal responsiveness between susceptible and non-susceptible individuals, the neural mechanisms mediating these behavioral differences are unknown. The Nucleus Accumbens (NAc) mediates cue-triggered reward seeking and activity in the NAc is enhanced in obesity-susceptible populations. Therefore here, we used selectively-bred obesity-prone and obesity-resistant rats to examine intrinsic differences in incentive motivation, and the role of NAc AMPARs in the expression of these behaviors prior to obesity. We found that obesity-prone rats exhibit robust cue-triggered food-seeking (Pavlovian-to-instrumental transfer, PIT). Using intra-NAc infusion of AMPAR antagonists, we show that this behavior is selectively mediated by CP-AMPARs in the NAc core. Additionally, biochemical data suggest that this is due in part to experience-induced increases in CP-AMPAR surface expression in the NAc of obesity-prone rats. In contrast, in obesity-resistant rats PIT was weak and unreliable and training did not increase NAc AMPAR surface expression. Collectively, these data show that food cues acquire greater incentive motivational control in obesity-susceptible populations prior to the development of obesity. This provides support to the idea that enhanced intrinsic incentive motivation may be a contributing factor, rather than a consequence of obesity. In addition, these data demonstrate a novel role for experience-induced up-regulation of NAc CP-AMPARs in PIT, pointing to potential mechanistic parallels between the processes leading to addiction and to obesity.

Spider venoms: a rich source of acylpolyamines and peptides as new leads for CNS drugs

Advances in NMR and mass spectrometry as well as in peptide biochemistry coupled to modern methods in electrophysiology have permitted the isolation and identification of numerous products from spider venoms, previously explored due to technical limitations. The chemical composition of spider venoms is diverse, ranging from low molecular weight organic compounds such as acylpolyamines to complex peptides. First, acylpolyamines (< 1000 Da) have an aromatic moiety linked to a hydrophilic lateral chain. They were characterized for the first time in spider venoms and are ligand-gated ion channel antagonists, which block mainly postsynaptic glutamate receptors in invertebrate and vertebrate nervous systems. Acylpolyamines represent the vast majority of organic components from the spider venom. Acylpolyamine analogues have proven to suppress hippocampal epileptic discharges. Moreover, acylpolyamines could suppress excitatory postsynaptic currents inducing Ca+ accumulation in neurons leading to protection against a brain ischemic insult. Second, short spider peptides (< 6000 Da) modulate ionic currents in Ca2+, Na+, or K+ voltage-gated ion channels. Such peptides may contain from three to four disulfide bridges. Some spider peptides act specifically to discriminate among Ca2+, Na+, or K+ ion channel subtypes. Their selective affinities for ion channel subfamilies are functional for mapping excitable cells. Furthermore, several of these peptides have proven to hyperpolarize peripheral neurons, which are associated with supplying sensation to the skin and skeletal muscles. Some spider N-type calcium ion channel blockers may be important for the treatment of chronic pain. A special group of spider peptides are the amphipathic and positively charged peptides. Their secondary structure is alpha-helical and they insert into the lipid cell membrane of eukaryotic or prokaryotic cells leading to the formation of pores and subsequently depolarizing the cell membrane. Acylpolyamines and peptides from spider venoms represent an interesting source of molecules for the design of novel pharmaceutical drugs.

The effects of D-alpha-aminoadipic acid on long-term potentiation in the hippocampus of the rat in vitro

Many studies on long-term potentiation (LTP) in hippocampal region CA1 focus on receptor-mediated events that are often presumed to be linked to postsynaptic processes. Whereas it is now well-known that LTP consists of multiple components involving increases in postsynaptic responsiveness as well as enhanced presynaptic release of transmitter, little specific information has accrued on the nature of the presynaptic receptor-linked events. In the course of a series of experiments examining the actions of several antagonists of N-methyl-D-aspartate (NMDA) receptors on LTP, we made certain observations that suggested the role of a novel type of amino acid receptor which possibly was located presynaptically and that seemed to contribute to the induction of LTP. LTP evoked in region CA1 following high frequency stimulation (HFS) of the Schaffer collateral-commissural pathway measured 20-30 min after HFS always was attenuated incompletely when induced during administration of DalphaAA at doses ranging from 50 mu M to as high as 1000 mu M, whereas 2-amino-5-phosphonopropionate (AP5), at a concentration of 30 mu M, always abolished the process completely. 6,7-Dinitroquinoxaline-2,3-dione (DNQX) (10 mu M) administered alone also did not block LTP completely unless delivered in combination with DalphaAA. These non-AP5-like effects of DalphaAA could not be attributed to incomplete antagonism of postsynaptic NMDA receptors, since DalphaAA (200 mu M) completely and reversibly blocked the membrane depolarising effects of NMDA, as assessed through intracellular recording. Furthermore, the pharmacologically isolated NMDA-receptor-mediated component of the low-frequency, stimulus-evoked synaptic response was always abolished reversibly by DalphaAA (200 mu M). The most parsimonious explanation of these data is that a receptor which is only activated during HFS, is sensitive to the antagonising actions of AP5 and possibly also to DNQX but not to DalphaAA, and which could conceivably exist on terminals of the Schaffer collateral-commissural fibres, makes a significant contribution to LTP.

The effect of L-glutamate on the afferent resting activity in the cephalopod statocyst

The effects of bath application of L-glutamate and of excitatory amino acid agonists and antagonists on the resting activity of afferent crista fibers were studied in isolated preparations of the statocyst of the cuttlefish, Sepia officinalis. L-Glutamate (threshold 10(-5) M) and its agonists quisqualate and kainate (thresholds 10(-6) M) increased the resting activity in a dose-dependent manner. Glutamine (threshold 10(-5) M) was also excitatory, while D-glutamate had no effect. Also, no obvious excitatory effects were seen for NMDA and L-aspartate, nor was any antagonistic effect seen for the selective NMDA-receptor antagonist D-2-amino-5-phosphonovaleric acid (D-AP-5). The spider toxin Argiotoxin636 (threshold 10(-11) M), 2-amino-4-phosphonobutyric acid (AP-4), glutamic acid diethyl ester (GDEE), gamma-D-glutamylaminomethyl-sulfonic acid (GAMS), and kynurenic acid decreased the resting activity and effectively blocked or reversed the effect of L-glutamate and its non-NMDA agonists. Preliminary experiments with statocysts from the squid Sepioteuthis lessoniana and the octopod Octopus bimaculoides gave comparable results. All data show that in cephalopod statocysts L-glutamate, via non-NMDA receptors, has an excitatory effect on the activity of afferent fibers, an effect consistent with its possible function as a hair cell transmitter.