Neuropeptide Y (NPY) has robust anxiolytic properties and is reduced in patients with anxiety disorders. However, the mechanisms by which NPY modulates circuit function to reduce anxiety behavior are not known. Anxiolytic effects of NPY are mediated in the CA1 region of hippocampus, and NPY injection into hippocampus alleviates anxiety symptoms in the predator scent stress model of stress-induced anxiety. The mechanisms that regulate NPY release, and its effects on CA1 synaptic function, are not fully understood. Here we show in acute hippocampal slices from mice that endogenous NPY, released in response to optogenetic stimulation or synaptically evoked spiking of NPYϩ cells, suppresses both of the feedforward pathways to CA1. Stimulation of temporoammonic synapses with a physiologically derived spike train causes NPY release that reduces short-term facilitation, whereas the release of NPY that modulates Schaffer collateral synapses requires integration of both the Schaffer collateral and temporoammonic pathways. Pathway specificity of NPY release is conferred by three functionally distinct NPYϩ cell types, with differences in intrinsic excitability and short-term plasticity of their inputs. Predator scent stress abolishes the release of endogenous NPY onto temporoammonic synapses, a stress-sensitive pathway, thereby causing enhanced short-term facilitation. Our results demonstrate how stress alters CA1 circuit function through the impairment of endogenous NPY release, potentially contributing to heightened anxiety.

Although long-term depression (LTD) is a well-studied form of synaptic plasticity, it is clear that multiple cellular mechanisms are involved in its induction. In the leech, LTD is observed in a polysynaptic connection between touch mechanosensory neurons (T cells) and the S interneuron following low frequency stimulation. LTD elicited by 450 s low frequency stimulation was blocked by N-methyl-D-aspartic acid (NMDA) receptor antagonists. However, LTD elicited by 900 s low frequency stimulation was insensitive to NMDA receptor antagonists and was instead dependent on cannabinoid signaling. This LTD was blocked by both a cannabinoid receptor antagonist and by inhibition of diacylglycerol lipase, which is necessary for the synthesis of the cannabinoid transmitter 2-arachidonyl glycerol (2-AG). Bath application of 2-AG or the cannabinoid receptor agonist CP55 940 also induced LTD at this synapse. These results indicate that two forms of LTD coexist at the leech T-to-S polysynaptic pathway: one that is NMDA receptor-dependent and another that is cannabinoid-dependent and that activation of either form of LTD is dependent on the level of activity in this circuit.

Most studies of long-term potentiation (LTP) have focused on potentiation induced by the activation of postsynaptic NMDA receptors (NMDARs). However, it is now apparent that NMDAR-dependent signaling processes are not the only form of LTP operating in the brain (Malenka and Bear, 2004). Previously, we have observed that LTP in leech central synapses made by the touch mechanosensory neurons onto the S interneuron was NMDAR-independent (Burrell and Sahley, 2004). Here we examine the cellular mechanisms mediating T-to-S (T→S) LTP and find that its induction requires activation of metabotropic glutamate receptors (mGluRs), voltage-dependent Ca 2+ channels (VDCCs) and protein kinase C (PKC). Surprisingly, whenever LTP was pharmacologically inhibited, long-term depression (LTD) was observed at the tetanized synapse, indicating that LTP and LTD were activated at the same time in the same synaptic pathway. This co-induction of LTP and LTD likely plays an important role in activity-dependent regulation of synaptic transmission. Keywordsmetabotropic glutamate receptor; voltage-dependent Ca 2+ channel; protein kinase C; neuroplasticity; invertebrate NMDAR-dependent long-term potentiation (LTP) and long-term depression (LTD) are thought to be critical cellular substrates for mediating learning and memory because their initiation requires coincident activity in both the pre-and postsynaptic neurons (activity dependence) and the resulting changes are restricted to the co-activated synapses (synapse specificity). However, it is now clear that other molecules can perform coincidence-detection in place of NMDARs for both LTP and LTD (Malenka and Bear, 2004;Anwyl, 2006). This heterogeneity in cellular mechanisms mediating LTP and LTD, along with the structural complexity of the vertebrate brain, complicates efforts to determine the functional contribution of synaptic changes to learning-related changes in behavior. The medicinal leech (Hirudo medicinalis) has a number of properties that make it a useful model for studies of LTP and LTD. Most neurons in the leech CNS are large and easily visualized and there are far fewer neurons in the leech CNS (~400 neurons/ganglion with 21 body ganglia plus the head and tail ganglia )) compared to a mammalian brain. Therefore, it is possible to record

Persistent, bidirectional changes in synaptic signaling (that is, potentiation and depression of the synapse) can be induced by the precise timing of individual pre-and postsynaptic action potentials. However, far less attention has been paid to the ability of paired trains of action potentials to elicit persistent potentiation or depression. We examined plasticity following the pairing of spike trains in the touch mechanosensory neuron (T cell) and S interneuron (S cell) in the medicinal leech. Long-term potentiation (LTP) of T to S signaling was elicited when the T-cell spike train preceded the S-cell train. An interval 0 to +1 sec between the T-and S-cell spike trains was required to elicit long-term potentiation (LTP), and this potentiation was NMDA receptor (NMDAR)-dependent. Long-term depression (LTD) was elicited when S-cell activity preceded T-cell activity and the interval between the two spike trains was 20.2 sec to 210 sec. This surprisingly broad temporal window involved two distinct cellular mechanisms; an NMDAR-mediated LTD (NMDAR-LTD) when the pairing interval was relatively brief (,21 sec) and an endocannabinoid-mediated LTD (eCB-LTD) when longer pairing intervals were used (21 to 210 sec). This eCB-LTD also required activation of a presynaptic transient receptor potential vanilloid (TRPV)-like receptor, presynaptic Ca 2+ release from intracellular stores and activation of voltage-gated Ca 2+ channels (VGCCs). These findings demonstrate that the pairing of spike trains elicits timing-dependent forms of LTP and LTD that are supported by a complex set of cellular mechanisms involving NMDARs and endocannabinoid activation of TRPV-like receptors.[Supplemental material is available for this article.]Associative forms of long-term neuroplasticity, as first proposed by Hebb (1949), depend on the temporal relationship between pre-and postsynaptic activity. Studies of spike-timing-dependent plasticity (STDP) have provided experimental confirmation of this principle, specifically that in most connections long-term potentiation (LTP) is produced when a presynaptic action potential (or spike) precedes the postsynaptic spike, whereas long-term depression (LTD) is produced when the order is reversed (see review by Caporale and Dan 2008). In addition to the temporal order being important in determining the sign of the synaptic change, temporal proximity is also critical in that the time between the pre-and postsynaptic activity must be sufficiently brief, usually ,40 msec, to support potentiation or depression.It has been noted that the time intervals between pre-and postsynaptic activity that support STDP are not as long as those observed in many associative learning experiments, in which interstimulus intervals on the scale of seconds can be used to elicit successful conditioning (Drew and Abbott 2006). Most studies of STDP involve pairing single pre-and postsynaptic spikes or brief bursts of two to three spikes; however, many neurons communicate using relatively long trains of action potentials that are tens...