Trevor J. Hamilton

The anti-epileptic actions of neuropeptide Y in the hippocampus are mediated by Y2 and not Y5 receptors

Neuropeptide Y (NPY) potently inhibits glutamate release and seizure activity in rodent hippocampus in vitro and in vivo, but the nature of the receptor(s) mediating this action is controversial. In hippocampal slices from rats and several wild‐type mice, a Y2‐preferring agonist mimicked, and the Y2‐specific antagonist BIIE0246 blocked, the NPY‐mediated inhibition both of glutamatergic transmission and of epileptiform discharges in two different slice models of temporal lobe epilepsy, stimulus train‐induced bursting (STIB) and 0‐Mg2+ bursting. Whereas Y5 receptor‐preferring agonists had small but significant effects in vitro, they were blocked by BIIE0246, and a Y5 receptor‐specific antagonist did not affect responses to any agonist tested in any preparation. In slices from mice, NPY was without effect on evoked potentials or in either of the two slice seizure models. In vivo, intrahippocampal injections of Y2‐ or Y5‐preferring agonists inhibited seizures caused by intrahippocampal kainate, but again the Y5 agonist effects were insensitive to a Y5 antagonist. Neither Y2‐ nor Y5‐preferring agonists affected kainate seizures in mice. A Y5‐specific antagonist did not displace the binding of two different NPY ligands in WT or mice, whereas all NPY binding was eliminated in the mouse. Thus, we show that Y2 receptors alone mediate all the anti‐excitatory actions of NPY seen in the hippocampus, whereas our findings do not support a role for Y5 receptors either in vitro or in vivo. The results suggest that agonists targeting the Y2 receptor may be useful anticonvulsants.

CO2-induced ocean acidification increases anxiety in Rockfish via alteration of GABAA receptor functioning

The average surface pH of the ocean is dropping at a rapid rate due to the dissolution of anthropogenic CO2, raising concerns for marine life. Additionally, some coastal areas periodically experience upwelling of CO2-enriched water with reduced pH. Previous research has demonstrated ocean acidification (OA)-induced changes in behavioural and sensory systems including olfaction, which is due to altered function of neural gamma-aminobutyric acid type A (GABAA) receptors. Here, we used a camera-based tracking software system to examine whether OA-dependent changes in GABAA receptors affect anxiety in juvenile Californian rockfish (Sebastes diploproa). Anxiety was estimated using behavioural tests that measure light/dark preference (scototaxis) and proximity to an object. After one week in OA conditions projected for the next century in the California shore (1125 ± 100 µatm, pH 7.75), anxiety was significantly increased relative to controls (483 ± 40 µatm CO2, pH 8.1). The GABAA-receptor agonist muscimol, but not the antagonist gabazine, caused a significant increase in anxiety consistent with altered Cl− flux in OA-exposed fish. OA-exposed fish remained more anxious even after 7 days back in control seawater; however, they resumed their normal behaviour by day 12. These results show that OA could severely alter rockfish behaviour; however, this effect is reversible.

Neuropeptide Y in the dentate gyrus.

Neuropeptide Y (NPY) is contained in at least four types of GABAergic interneurons in the dentate gyrus, many of which also contain somatostatin and give rise to the dense NPY innervation of the dentate outer molecular layer. In humans but not rats, minute amounts of NPY are also normally expressed in dentate granule cells, while seizure activity in rats induces robust NPY expression in granule cells. Y1 and Y2 receptors are the most abundant NPY receptors expressed in the dentate gyrus. Y1 receptors are postsynaptic receptors, primarily located on granule cell dendrites in the molecular layer and some interneurons, while Y2 receptors are presynaptic receptors mediating inhibition of glutamate release, and potentially that of NPY and GABA depending on their presynaptic localization, and may also be expressed on some hilar interneurons. In humans, monkeys and mice, Y2 receptors are also present on mossy fibers, but not in most rat species, though functional evidence suggests their presence. Hilar interneurons containing NPY degenerate in temporal lobe epilepsy and in Alzheimers disease and reduced levels of NPY in dentate hilus are associated with depression. By activating Y1 receptors, NPY also exerts powerful neuroproliferative effects on subgranular zone progenitor cells, increasing the number of newly born granule cells in the adult dentate gyrus. Functionally, NPY exerts anticonvulsive actions mediated by Y2 receptors at mossy fiber terminals, but there are no presynaptic responses to NPY at perforant path inputs to dentate granule cells in rats or mice. NPY also has potentially complicated actions on NPY-containing interneurons. Elevated expression of NPY in mossy fibers of the rat, sprouting of NPY interneurons in the human dentate, and over-expression of Y2 receptors in mossy fibers indicate an anticonvulsive role of endogenous NPY in epilepsy. However, the physiological role of NPY in the healthy dentate gyrus remains unclear.

Dopamine modulates synaptic plasticity in dendrites of rat and human dentate granule cells

The mechanisms underlying memory formation in the hippocampal network remain a major unanswered aspect of neuroscience. Although high-frequency activity appears essential for plasticity, salience for memory formation is also provided by activity in ventral tegmental area (VTA) dopamine projections. Here, we report that activation of dopamine D1 receptors in dentate granule cells (DGCs) can preferentially increase dendritic excitability to both high-frequency afferent activity and high-frequency trains of backpropagating action potentials. Using whole-cell patch clamp recordings, calcium imaging, and neuropeptide Y to inhibit postsynaptic calcium influx, we found that activation of dendritic voltage-dependent calcium channels (VDCCs) is essential for dopamine-induced long-term potentiation (LTP), both in rat and human dentate gyrus (DG). Moreover, we demonstrate previously unreported spike-timing–dependent plasticity in the human hippocampus. These results suggest that when dopamine is released in the dentate gyrus with concurrent high-frequency activity there is an increased probability that synapses will be strengthened and reward-associated spatial memories will be formed.

Object recognition memory in zebrafish.

The novel object recognition, or novel-object preference (NOP) test is employed to assess recognition memory in a variety of organisms. The subject is exposed to two identical objects, then after a delay, it is placed back in the original environment containing one of the original objects and a novel object. If the subject spends more time exploring one object, this can be interpreted as memory retention. To date, this test has not been fully explored in zebrafish (Danio rerio). Zebrafish possess recognition memory for simple 2- and 3-dimensional geometrical shapes, yet it is unknown if this translates to complex 3-dimensional objects. In this study we evaluated recognition memory in zebrafish using complex objects of different sizes. Contrary to rodents, zebrafish preferentially explored familiar over novel objects. Familiarity preference disappeared after delays of 5 mins. Leopard danios, another strain of D. rerio, also preferred the familiar object after a 1 min delay. Object preference could be re-established in zebra danios by administration of nicotine tartrate salt (50mg/L) prior to stimuli presentation, suggesting a memory-enhancing effect of nicotine. Additionally, exploration biases were present only when the objects were of intermediate size (2 × 5 cm). Our results demonstrate zebra and leopard danios have recognition memory, and that low nicotine doses can improve this memory type in zebra danios. However, exploration biases, from which memory is inferred, depend on object size. These findings suggest zebrafish ecology might influence object preference, as zebrafish neophobia could reflect natural anti-predatory behaviour.

Reversed Scototaxis during Withdrawal after Daily-Moderate, but Not Weekly-Binge, Administration of Ethanol in Zebrafish

Alcohol abuse can lead to severe psychological and physiological damage. Little is known, however, about the relative impact of a small, daily dose of alcohol (daily-moderate schedule) versus a large, once per week dose (weekly-binge schedule). In this study, we examined the effect of each of these schedules on behavioural measures of anxiety in zebrafish (Danio rerio). Adult wild-type zebrafish were administered either 0.2% ethanol on a daily-moderate schedule or 1.4% ethanol on a weekly-binge schedule for a period of 21 days, and then tested for scototaxis (preference for darkness) during withdrawal. Compared to a control group with no alcohol exposure, the daily-moderate group spent significantly more time on the light side of the arena (indicative of decreased anxiety) on day two of withdrawal, but not day 9 of withdrawal. The weekly-binge group was not significantly different from the control group on either day of withdrawal and showed no preference for either the light or dark zones. Our results indicate that even a small dose of alcohol on a daily basis can cause significant, though reversible, changes in behaviour.

Exposure to bloom-like concentrations of two marine Synechococcus cyanobacteria (strains CC9311 and CC9902) differentially alters fish behaviour

Coastal California regularly experiences blooms of Synechococcus cyanobacteria. We found that black perch exposed to one strain (CC9311), but not another (CC9902), spent more time in the dark area of a tank, and moved less. Our results demonstrate that blooms of specific strains of marine cyanobacteria can differentially affect fish.

Acid–base physiology, neurobiology and behaviour in relation to CO2-induced ocean acidification

ABSTRACT Experimental exposure to ocean and freshwater acidification affects the behaviour of multiple aquatic organisms in laboratory tests. One proposed cause involves an imbalance in plasma chloride and bicarbonate ion concentrations as a result of acid–base regulation, causing the reversal of ionic fluxes through GABAA receptors, which leads to altered neuronal function. This model is exclusively based on differential effects of the GABAA receptor antagonist gabazine on control animals and those exposed to elevated CO2. However, direct measurements of actual chloride and bicarbonate concentrations in neurons and their extracellular fluids and of GABAA receptor properties in aquatic organisms are largely lacking. Similarly, very little is known about potential compensatory mechanisms, and about alternative mechanisms that might lead to ocean acidification-induced behavioural changes. This article reviews the current knowledge on acid–base physiology, neurobiology, pharmacology and behaviour in relation to marine CO2-induced acidification, and identifies important topics for future research that will help us to understand the potential effects of predicted levels of aquatic acidification on organisms. Summary: This article reviews basic acid–base regulatory and neurobiology mechanisms relevant for behavioural alteration in fish exposed to CO2-induced seawater acidification, and identifies areas for future research.

Episodic-like memory in zebrafish

Episodic-like memory tests often aid in determining an animal’s ability to recall the what, where, and which (context) of an event. To date, this type of memory has been demonstrated in humans, wild chacma baboons, corvids (Scrub jays), humming birds, mice, rats, Yucatan minipigs, and cuttlefish. The potential for this type of memory in zebrafish remains unexplored even though they are quickly becoming an essential model organism for the study of a variety of human cognitive and mental disorders. Here we explore the episodic-like capabilities of zebrafish (Danio rerio) in a previously established mammalian memory paradigm. We demonstrate that when zebrafish were presented with a familiar object in a familiar context but a novel location within that context, they spend more time in the novel quadrant. Thus, zebrafish display episodic-like memory as they remember what object they saw, where they saw it (quadrant location), and on whichoccasion (yellow or blue walls) it was presented.

Acute fluoxetine exposure alters crab anxiety-like behaviour, but not aggressiveness.

Aggression and responsiveness to noxious stimuli are adaptable traits that are ubiquitous throughout the animal kingdom. Like vertebrate animals, some invertebrates have been shown to exhibit anxiety-like behaviour and altered levels of aggression that are modulated by the neurotransmitter serotonin. To investigate whether this influence of serotonin is conserved in crabs and whether these behaviours are sensitive to human antidepressant drugs; the striped shore crab, Pachygrapsus crassipes, was studied using anxiety (light/dark test) and aggression (mirror test) paradigms. Crabs were individually exposed to acute doses of the selective serotonin reuptake inhibitor, fluoxetine (5 or 25 mg/L), commonly known as Prozac®, followed by behavioural testing. The high dose of fluoxetine significantly decreased anxiety-like behaviour but had no impact on mobility or aggression. These results suggest that anxiety-like behaviour is more sensitive to modulation of serotonin than is aggressiveness in the shore crab.