Presynaptic Regulation Research Articles

Tyramine Reveals Failing α2-Adrenoceptor Control of Catecholamine Release and Total Peripheral Vascular Resistance in Hypertensive Rats

α2-Adrenoceptor-activation lowers central sympathetic output, peripheral, vesicular norepinephrine release, epinephrine secretion, and modulates vascular tension. We previously demonstrated that α2-adrenoceptor-mediated inhibition of basal norepinephrine release was not reflected in plasma unless re-uptake through the norepinephrine transporter (NET) was blocked. Tyramine activates reverse norepinephrine transport through NET. Here we tested the hypothesis that tyramine, by engaging NET in release, also blocks re-uptake, and therefore allows manipulation of pre-junctional α2-adrenoceptors to directly regulate norepinephrine overflow to plasma. We compared in anesthetized spontaneously hypertensive rats (SHRs) and normotensive controls (WKYs), the effect of α2-adrenoreceptor antagonist (L-659,066) and/or agonist (clonidine) on norepinephrine overflow and increase in total peripheral vascular resistance (TPR) evoked by tyramine-infusion (1.26 μmol/min/kg, 15 min) and epinephrine secretion activated by the surgical stress. TPR was computed as blood pressure divided by cardiac output, recorded as ascending aortic flow. Plasma catecholamine concentrations after tyramine were higher in SHRs than WKYs. Pre-treatment with L-659,066 increased the catecholamine concentrations in WKYs, but only if combined with clonidine in SHRs. Clonidine alone reduced tyramine-induced norepinephrine overflow in SHRs, and epinephrine in both strains. Tyramine-induced increase in TPR was not different after clonidine, eliminated after L-659,066 and L-659,066 + clonidine in WKYs, but only after L-659,066 + clonidine in SHRs. We conclude that tyramine-infusion does allow presynaptic regulation of vesicular release to be accurately assessed by measuring differences in plasma norepinephrine concentration. Our results indicate that presynaptic α2-adrenoceptor regulation of norepinephrine release from nerve vesicles and epinephrine secretion is dysfunctional in SHRs, but can be restored by clonidine.

Synaptic transmission at parasympathetic neurons of the major pelvic ganglion from normal and diabetic male mice

Bladder and erectile dysfunction are common urologic complications of diabetes and are associated with reduced parasympathetic autonomic control. To determine whether disruption of ganglionic neurotransmission contributes to the loss of function, we investigated synaptic transmission at parasympathetic, major pelvic ganglion (MPG) neurons in control and chronically (20 wk) diabetic mice. In contrast to what has been reported for sympathetic neurons, diabetes did not cause an interruption of synaptic transmission at parasympathetic MPG neurons from streptozotocin-treated C57BL/6J (STZ) or db/db mice. Cholinergically mediated excitatory postsynaptic potentials (EPSPs) were suprathreshold during 5-s trains of 5-, 10-, and 20-Hz stimuli. Asynchronous neurotransmitter release, observed as miniature EPSPs (mEPSPs) during and after stimulation, permitted quantitative assessment of postganglionic, cholinergic receptor sensitivity. mEPSP amplitude following tetanic stimulation (recorded at -60 mV) was reduced in STZ (4.95 ± 0.4 vs. 3.71 ± 0.3 mV, P = 0.03), but not db/db mice. The number of posttetanic mEPSPs was significantly greater in db/db mice at all frequencies tested. Assessment of basic electrophysiological properties revealed that parasympathetic MPG neurons from db/db mice had less negative membrane potentials, lower input resistances, and shorter afterhyperpolarizations relative to their control. MPG neurons from STZ had longer afterhyperpolarizations but were otherwise similar to controls. Membrane excitability, measured by the membrane responsiveness to long-duration (1 s), suprathreshold depolarizing pulses, was unchanged in either model. The present study indicates that, while parasympathetic neurotransmission at the MPG is intact in chronically diabetic mice, obese, type 2 diabetic animals exhibit an altered presynaptic regulation of neurotransmitter release.

Thalamic glutamatergic afferents into the rat basolateral amygdala exhibit increased presynaptic glutamate function following withdrawal from chronic intermittent ethanol

Amygdala glutamatergic neurotransmission regulates withdrawal induced anxiety-like behaviors following chronic ethanol exposure. The lateral/basolateral amygdala receives multiple glutamatergic projections that contribute to overall amygdala function. Our lab has previously shown that rat cortical (external capsule) afferents express postsynaptic alterations during chronic intermittent ethanol exposure and withdrawal. However, thalamic (internal capsule) afferents also provide crucial glutamatergic input during behavioral conditioning, and they have not been studied in the context of chronic drug exposure. We report here that these thalamic inputs express altered presynaptic function during withdrawal from chronic ethanol exposure. This is characterized by enhanced release probability, as exemplified by altered paired-pulse ratios and decreased failure rates of unitary events, and increased concentrations of synaptic glutamate. Quantal analysis further implicates a withdrawal-dependent enhancement of the readily releasable pool of vesicles as a probable mechanism. These functional alterations are accompanied by increased expression of vesicle associated protein markers. These data demonstrate that chronic ethanol modulation of glutamate neurotransmission in the rat lateral/basolateral amygdala is afferent-specific. Further, presynaptic regulation of lateral/basolateral amygdala thalamic inputs by chronic ethanol may be a novel neurobiological mechanism contributing to the increased anxiety-like behaviors that characterize withdrawal.

Reduced anticipatory dopamine responses to food in rats exposed to high fat during early development

We have previously demonstrated that exposure to high fat (HF) during early development alters the presynaptic regulation of mesolimbic dopamine (DA), and increases incentive motivation for HF food rewards. The goal of the present experiments was to examine the long-term consequences of early exposure to HF on anticipatory and consumatory nucleus accumbens (NAc) DA responses to HF food rewards. Mothers were maintained on a HF (30% fat) or control diet (CD; 5% fat) from gestation day 13 to postnatal day 22 when offspring from both diet groups were weaned and maintained on the CD until adulthood. In vivo NAc DA responses to food anticipation and consumption were measured in a Pavlovian conditioning paradigm using voltammetry in freely moving rats. HF-exposed offspring displayed reduced NAc DA responses to a tone previously paired with the delivery of HF food rewards. In an unconditioned protocol, consumatory NAc DA responses could be isolated, and were similar in HF and control offspring. These data demonstrate that exposure to HF through maternal diet during early development might program behavioral and functional responses associated with mesolimbic DA neurotransmission, thus leading to an increased HF feeding and obesity.

Combinatorial use of translational co-factors for cell type-specific regulation during neuronal morphogenesis in Drosophila

The translational regulators Nanos (Nos) and Pumilio (Pum) work together to regulate the morphogenesis of dendritic arborization (da) neurons of the Drosophila larval peripheral nervous system. In contrast, Nos and Pum function in opposition to one another in the neuromuscular junction to regulate the morphogenesis and the electrophysiological properties of synaptic boutons. Neither the cellular functions of Nos and Pum nor their regulatory targets in neuronal morphogenesis are known. Here we show that Nos and Pum are required to maintain the dendritic complexity of da neurons during larval growth by promoting the outgrowth of new dendritic branches and the stabilization of existing dendritic branches, in part by regulating the expression of cut and head involution defective. Through an RNA interference screen we uncover a role for the translational co-factor Brain Tumor (Brat) in dendrite morphogenesis of da neurons and demonstrate that Nos, Pum, and Brat interact genetically to regulate dendrite morphogenesis. In the neuromuscular junction, Brat function is most likely specific for Pum in the presynaptic regulation of bouton morphogenesis. Our results reveal how the combinatorial use of co-regulators like Nos, Pum and Brat can diversify their roles in post-transcriptional regulation of gene expression for neuronal morphogenesis.

CaV2.1 voltage activated calcium channels and synaptic transmission in familial hemiplegic migraine pathogenesis

Studies on the genetic forms of epilepsy, chronic pain, and migraine caused by mutations in ion channels have given crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological disorders. In this review we focus on the role of mutated CaV2.1 (i.e., P/Q-type) voltage-activated Ca2+ channels, and on the ultimate consequences that mutations causing familial hemiplegic migraine type-1 (FHM1) have in neurotransmitter release. Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of CaV2.1 channels. Mutated CaV2.1 channels activate at more hyperpolarizing potentials and lead to a gain-of-function in synaptic transmission. This gain-of-function might underlie alterations in the excitatory/ inhibitory balance of synaptic transmission, favoring a persistent state of hyperexcitability in cortical neurons that would increase the susceptibility for cortical spreading depression (CSD), a mechanism believed to initiate the attacks of migraine with aura.

Role of Key Aromatic Residues in the Ligand-binding Domain of α7 Nicotinic Receptors in the Agonist Action of β-Amyloid

Soluble β-amyloid (Aβ) resides in certain regions of the brain at or near picomolar concentration, rising in level during the prodromic stage of Alzheimer disease. Recently, we identified the homomeric α7 nicotinic acetylcholine receptor (α7-nAChR) as one possible functional target for picomolar Aβ. This study was aimed at addressing which residues in α7-nAChRs potentially interact with Aβ to regulate the presynaptic function of this receptor. Site-directed mutagenesis was carried out to study the key aromatic residues in the mouse α7-nAChR agonist-binding pocket. Mutations of tyrosine188 resulted in a decrease in activation of presynaptic α7-nAChRs by ACh and Aβ but with no change in response to nicotine, indicating the critical role of Tyr-188 in presynaptic regulation by Aβ. Coimmunoprecipitation additionally revealed direct binding of Aβ to α7-nAChRs and to the Tyr-188 mutant receptor. In contrast, mutations of Tyr-195 in α7-nAChR led to decreased activation by nicotine without apparent effects on ACh- or Aβ-induced responses. Agonist-induced responses of Tyr-93 mutant α7-nAChRs indicated possible interactions of nicotine and Aβ with its hydroxyl group, but there was no change in presynaptic responses after mutation of Trp-149. All of the mutants were shown to be expressed on the plasma membrane using cell surface labeling. Together, these results directly demonstrate an essential role for the aromatic residue Tyr-188 as a key component in the agonist binding domain for the activation of α7-nAChRs by Aβ.

Presynaptic regulation of quantal size: K+/H+ exchange stimulates vesicular glutamate transport

The amount of neurotransmitter stored in a single synaptic vesicle can determine the size of the postsynaptic response, but the factors that regulate vesicle filling remain poorly understood. A proton electrochemical gradient (ΔμH+) generated by the vacuolar H+-ATPase drives the accumulation of classical transmitters into synaptic vesicles. The chemical component of ΔμH+ (ΔpH) has received particular attention for its role in the vesicular transport of cationic transmitters as well as protein sorting and degradation. Thus, considerable work has addressed the factors that promote ΔpH. However, synaptic vesicle uptake of the principal excitatory transmitter glutamate depends on the electrical component of ΔμH+ (Δψ). We now find that rat brain synaptic vesicles express monovalent cation/H+ exchange activity that converts ΔpH into Δψ, and this promotes synaptic vesicle filling with glutamate. Manipulating presynaptic K+ at a glutamatergic synapse influences quantal size, demonstrating that synaptic vesicle K+/H+ exchange regulates glutamate release and synaptic transmission.

Dopamine Signaling in Dorsal Versus Ventral Striatum: The Dynamic Role of Cholinergic Interneurons

Mesostriatal dopaminergic neurons and striatal cholinergic interneurons participate in signaling the motivational significance of environmental stimuli and regulate striatal plasticity. Dopamine (DA) and acetylcholine (ACh) have potent interactions within the striatum at multiple levels that include presynaptic regulation of neurotransmitter release and postsynaptic effects in target cells (including ACh neurons). These interactions may be highly variable given the dynamic changes in the firing activities of parent DA and ACh neurons. Here, we consider how striatal ACh released from cholinergic interneurons acting at both nicotinic and muscarinic ACh receptors powerfully modulates DA transmission. This ACh–DA interaction varies in a manner that depends on the frequency of presynaptic activation, and will thus strongly influence how DA synapses convey discrete changes in DA neuron activity that are known to signal events of motivational salience. Furthermore, this ACh modulation of DA transmission within striatum occurs via different profiles of nicotinic and muscarinic receptors in caudate–putamen compared to nucleus accumbens, which may ultimately enable region-specific targeting of striatal function.

Mechanisms of bi-directional modulation of thalamocortical transmission in barrel cortex by presynaptic kainate receptors

Presynaptic kainate receptors play an important role in synaptic transmission and short-term plasticity to profoundly regulate network activity in many parts of the mammalian brain. In primary sensory neocortex, where short-term synaptic plasticity is important for receptive field structure and information processing, kainate receptors are highly expressed and regulate thalamocortical inputs, particularly during development. However, the mechanisms of the kainate receptor-dependent presynaptic regulation of thalamocortical transmission are unclear. We therefore investigated this issue using electrophysiology in neonatal thalamocortical slices of barrel cortex combined with pharmacology and biochemical analyses. We show that presynaptic kainate receptors can both facilitate or depress synaptic transmission depending on the extent of their activation. This bi-directional regulation is mediated in part by kainate receptors that directly influence thalamocortical axonal excitability, but also likely involves receptors acting at thalamocortical terminals to regulate transmitter release. The efficacy of kainate in regulating thalamocortical transmission is low compared to that reported for other inputs. Consistent with this low efficacy, our biochemical analyses indicate that the presynaptic kainate receptors regulating neonatal thalamocortical inputs likely lack the high kainate affinity GluK4 and 5 subunits. Thus thalamocortical transmission can be bi-directionally regulated by low affinity kainate receptors through two mechanisms. Such presynaptic regulation provides a potentially powerful mechanism to influence sensory processing during development of barrel cortex.

Maternal high-fat intake alters presynaptic regulation of dopamine in the nucleus accumbens and increases motivation for fat rewards in the offspring

High caloric intake during early postnatal development can have long term consequences for the offspring. We previously reported that the adult offspring of dams fed a high-fat diet during the last week of gestation and throughout lactation display blunted locomotor response to amphetamine (AMP) and reduced sensitization to the drug compared to offspring of control diet dams. Here, we report that the subsensitivity of high-fat offspring to AMP's locomotor stimulant action reflects, at least in part, altered regulation of nucleus accumbens (NAc) dopamine (DA) transmission. When compared to controls, the DA response of high-fat animals to AMP, as measured with microdialysis, was attenuated in the NAc, but unaffected in the prefrontal cortex (PFC). A relatively higher activity of NAc synaptosomal DA transporter sites without changes in vesicular monoamine transporter (VMAT) uptake capacity was also observed in high-fat offspring. Moreover, ventral tegmental area (VTA) D2 receptor mRNA levels were decreased in high-fat offspring, suggesting a reduction in DA release-regulating D2 autoreceptors in terminal regions such as the NAc. The magnitude of locomotor response to D2/3 receptor activation (with quinpirole) was greater in high-fat than in control animals despite having comparable postsynaptic D2 mRNA levels in the NAc. Finally, while operant responding for a sugar-enriched food reward did not differ between diet groups, high-fat offspring displayed increased operant responding for a fat-enriched reward compared to controls. These findings add to mounting evidence that early life exposure to elevated dietary maternal fat can lead to long lasting changes in DA-mediated behavioral responses to stimulant drugs and fat-enriched foods.

Coordinating Different Homeostatic Processes

In this issue of Neuron, Bergquist and colleagues study a rapid form of presynaptic homeostatic regulation at the Drosophila neuromuscular junction. They show that the K(+) channel genes shal and shaker are reciprocally regulated in the central nervous system and suggest a hierarchical organization of intrinsic and synaptic homeostatic regulatory processes.

Striatal Muscarinic Receptors Promote Activity Dependence of Dopamine Transmission via Distinct Receptor Subtypes on Cholinergic Interneurons in Ventral versus Dorsal Striatum

Striatal dopamine (DA) and acetylcholine (ACh) regulate motivated behaviors and striatal plasticity. Interactions between these neurotransmitters may be important, through synchronous changes in parent neuron activities and reciprocal presynaptic regulation of release. How DA signaling is regulated by striatal muscarinic receptors (mAChRs) is unresolved; contradictory reports indicate suppression or facilitation, implicating several mAChR subtypes on various neurons. We investigated whether mAChR regulation of DA signaling varies with presynaptic activity and identified the mAChRs responsible in sensorimotor- versus limbic-associated striatum. We detected DA in real time at carbon fiber microelectrodes in mouse striatal slices. Broad-spectrum mAChR agonists [oxotremorine-M, APET (arecaidine propargyl ester tosylate)] decreased DA release evoked by low-frequency stimuli (1-10 Hz, four pulses) but increased the sensitivity of DA release to presynaptic activity, even enhancing release by high frequencies (e.g., >25 Hz for four pulses). These bidirectional effects depended on ACh input to striatal nicotinic receptors (nAChRs) on DA axons but not GABA or glutamate input. In caudate-putamen (CPu), knock-out of M(2)- or M(4)-mAChRs (not M(5)) prevented mAChR control of DA, indicating that M(2)- and M(4)-mAChRs are required. In nucleus accumbens (NAc) core or shell, mAChR function was prevented in M(4)-knock-outs, but not M(2)- or M(5)-knock-outs. These data indicate that striatal mAChRs, by inhibiting ACh release from cholinergic interneurons and thus modifying nAChR activity, offer variable control of DA release probability that promotes how DA release reflects activation of dopaminergic axons. Furthermore, different coupling of striatal M(2)/M(4)-mAChRs to the control of DA release in CPu versus NAc suggests targets to influence DA/ACh function differentially between striatal domains.

Presynaptic Inhibition Selectively Weakens Peptidergic Cotransmission in a Small Motor System

The presence and influence of neurons containing multiple neurotransmitters is well established, including the ability of coreleased transmitters to influence the same or different postsynaptic targets. Little is known, however, regarding whether presynaptic regulation of multitransmitter neurons influences all transmission from these neurons. Using the identified neurons and motor networks in the crab stomatogastric ganglion, we document the ability of presynaptic inhibition to selectively inhibit peptidergic cotransmission. Specifically, we determine that the gastropyloric receptor (GPR) proprioceptor neuron uses presynaptic inhibition to selectively regulate peptidergic cotransmission from the axon terminals of MCN1, a projection neuron that drives the biphasic (retraction, protraction) gastric mill (chewing) rhythm. MCN1 drives this rhythm via fast GABAergic excitation of the retraction neuron Int1 and slow peptidergic excitation of the lateral gastric (LG) protraction neuron. We first demonstrate that GPR inhibition of the MCN1 axon terminals is serotonergic and then establish that this serotonergic inhibition weakens MCN1 peptidergic excitation of LG without altering MCN1 GABAergic excitation of Int1. At the circuit level, we show that this selective regulation of MCN1 peptidergic cotransmission is necessary for the normal GPR regulation of the gastric mill rhythm. This is the first demonstration, at the level of individual identified neurons, that a presynaptic input can selectively regulate a subset of coreleased transmitters. This selective regulation changes the balance of cotransmitter actions by the target multitransmitter neuron, thereby enabling this neuron to have state-dependent actions on its target network. This finding reveals additional flexibility afforded by the ability of neurons to corelease multiple neurotransmitters.

The mechanisms of neuronal interaction regulation mechanisms

339 It is well known that synaptic transmission is the main neuronal interaction mechanism representing a specialized chemical signaling. The interaction of two neurons can be regulated at the preand postsynaptic levels. Using the electron microscopic immunocytochemistry it was shown that the axons making synapses on the vertebrate motoneurons are contacted not only by GABAergic, but also by glutamateand glycine-containing terminals. It testifies that multiple neurotransmitters can be used for presynaptic regulation of synaptic transmission. This statement is confirmed by postsynaptic potential and current modifications after membrane receptor agonist and antagonisat application. The co-release of different transmitters and postsynaptic effect interactions are considered as possible mechanisms of synaptic transmission regulation.

Presynaptic regulation of isolated neonatal rat carotid body type I cells by histamine

The type I cells are the chemoreceptive elements of the carotid bodies and are critical in defining the ventilatory response to hypoxia and hypercapnia. Recent evidence has suggested that histamine is released by the carotid body in response to hypoxia and acts as an excitatory neurotransmitter. Here we use isolated neonatal rat type I cells to assess the presynaptic actions of histamine and define the receptor subtypes that mediate them. All four histamine receptor subtypes are expressed on the type I cells, however activation of these receptors with histamine or selective agonists caused no rise in intracellular calcium ([Ca(2+)](i)) and histamine did not augment calcium entry or modulate macroscopic currents evoked in type I cells. Thus activation of histamine receptors on type I cells is unlikely to provide a presynaptic positive feedback mechanism during chemotransduction and any excitatory role attributed to the actions of histamine is likely to come from a postsynaptic effect on the carotid sinus nerve (CSN).

Trace amine-associated receptor 1 as a monoaminergic modulator in brain

Brain monoaminergic systems play critical roles in mood, cognition, emotion, reward, learning and attention, and aberrance in brain monoaminergic activity is associated with a variety of neuropsychiatric disorders/diseases. The present commentary focuses on trace amine-associated receptor 1 (TAAR1) and its potential regulatory roles in brain monoaminergic systems. TAAR1 was discovered in 2001 and has been established to be a G-protein-coupled receptor signaling through the cAMP pathway. This receptor is activated by a broad spectrum of agonists, although there are notable species differences in ligand efficacy and potency. TAAR1 is expressed and widely distributed in brain monoaminergic systems and co-localized with the dopamine transporter in a subset of dopaminergic neurons in rhesus monkey and mouse brain substantia nigra. TAAR1 activation by the common biogenic amines, the trace amine β-phenylethylamine and methamphetamine alters the monoamine transporter function in both mouse and rhesus monkey brain synaptosomes, suggesting a modulatory role for this receptor in the presynaptic regulation of monoaminergic activity. However, little is known about other functional roles of TAAR1 in the brain. With a purpose to promote further studies on this receptor, we herein discuss the recent findings that provide insights into the functional significance and biological relevance of this receptor as a modulator in brain monoaminergic systems.

α 2-Adrenoceptor subtypes-mediated physiological, pharmacological actions

α 2-Adrenoceptors are involved in various physiological functions, particularly in the cardiovascular system and the central nervous system. Different adrenoceptor subtypes (α 2A, α 2B and α 2C) have been recognised and the different subtypes may have role in activation of distinct physiological and pharmacological pathways. Some of the actions of α 2-adrenoceptor stimulants are likely to be mediated exclusively by α 2A-adrenoceptor subtype, like antihypertensive and bradycardic effects. α 2A-Adrenoceptor may have dominant role in sedative and hypothermic actions, in inhibition of gastric acid secretion and gastric motor activity, as well as in stabilisation of thrombus. Besides α 2A-adrenergic receptors α 2B and α 2C-adrenoceptor subtypes may also be involved in some of the classical effects of α 2-adrenoceptor stimulants, like in presynaptic regulation of transmitter release and antinociceptive action. α 2B-Adrenoceptor has dominant role in the vasoconstrictor effect of α 2-adrenoceptor agonists, and its activation induces contraction of rat uterus in late pregnancy. α 2B-Adrenoceptor mediates gastric mucosal protective action initiated centrally in the rat, as well as it may have role also in developmental or reproductive processes. α 2C-Adrenoceptor subtypes may be involved in stress-dependent depression and other psychiatric illnesses like attention deficit hyperactivity disorder—together with α 2A-adrenoceptor. α 2C-Adrenergic receptors seem to mediate peripheral actions as well, like venous vasoconstriction. Identification of separate physiological roles for different α 2-adrenergic receptor subtypes could improve design of novel compounds for specific therapeutic goals.

Neuropeptide S selectively inhibits the release of 5‐HT and noradrenaline from mouse frontal cortex nerve endings

Neuropeptide S (NPS) is a recently identified neurotransmitter/neuromodulator able to increase arousal and wakefulness while decreasing anxiety-like behaviour. As several classical transmitters play a role in arousal and anxiety, we here investigated the possible presynaptic regulation of transmitter release by NPS. Synaptosomes purified from mouse frontal cortex were prelabelled with [(3)H]5-hydroxytryptamine (5-HT), noradrenaline, dopamine, choline, D-aspartate or GABA and depolarized in superfusion with 12-15 mmol.L(-1) KCl to evoke [(3)H]neurotransmitter exocytosis. NPS was added at different concentrations (0.001 to 100 nmol.L(-1)). NPS behaved as an extremely potent inhibitor of the evoked overflow of [(3)H]5-HT and [(3)H]noradrenaline exhibiting EC50 values in the low picomolar range. The inhibitory action of NPS on [(3)H]5-HT release was mimicked by [Ala(2)]NPS that was, however, about 100-fold less potent than the natural peptide. NPS (up to 100 nmol.L(-1)) was unable to affect the depolarization-evoked overflow of [(3)H]D-aspartate and [(3)H]GABA. The neuropeptide only weakly reduced the overflow of [(3)H]dopamine and [(3)H]ACh when added at relatively high concentrations. NPS, at low picomolar concentrations, can selectively inhibit the evoked release of 5-HT and noradrenaline in the frontal cortex by acting directly on 5-hydroxytryptaminergic and noradrenergic nerve terminals. These direct effects may explain only in part the unique behavioural activities of NPS, while an indirect involvement of other transmitters, especially of glutamate, must be considered.

Phr1 regulates retinogeniculate targeting independent of activity and ephrin-A signalling

Proper functioning of the mammalian visual system requires that connections between the eyes and their central targets develop precisely. At birth, axons from the two eyes project to broad, overlapping regions of the dorsal–lateral geniculate nucleus (dLGN). In the adult, retinal axons segregate into distinct monocular regions at stereotyped locations within the dLGN. This process is driven by both molecular cues and activity-dependent synaptic competition. Here we demonstrate that Phr1, an evolutionarily conserved regulator of synapse formation and axon guidance, defines a novel molecular pathway required for proper localization of retinogeniculate projections. Following conditional excision of Phr1 in the retina, eye-specific domains within the dLGN are severely disturbed, despite normal spontaneous retinal wave activity and monocular segregation. Although layer placement is dramatically altered, Phr1 mutant retinal axons respond to ephrin-A in vitro. These findings indicate that Phr1 is a key presynaptic regulator of retinogeniculate layer placement independent of activity, segregation, or ephrin-A signaling.