Temporal covariance of postsynaptic membrane potential and synaptic input--role in synaptic efficacy in visual cortex

Intrinsic connections in tree shrew V1 imply a global to local mapping

The local-global map hypothesis states that locally organized response properties--such as orientation preference--result from visuotopically organized local maps of non-retinotopic response properties. In the tree shrew, the lateral extent of horizontal patchy connections is as much as 80-100% of V1 and is consistent with the length summation property. We argue that neural signals can be transmitted across the entire extent of V1 and this allows the formation of maps at the local scale that are visuotopically organized. We describe mechanisms relevant to the formation of local maps and report modeling results showing the same patterns of horizontal connectivity, and relationships to orientation preference, seen in vivo. The structure of the connectivity that emerges in the simulations reveals a 'hub and spoke' organization. Singularities form the centers of local maps, and linear zones and saddle-points arise as smooth border transitions between maps. These findings are used to present the case for the local-global map hypothesis for tree shrew V1.

A spike based learning rule for generation of invariant representations

For biological realism, models of learning in neuronal networks often assume that synaptic plasticity solely depends on locally available signals, in particular only on the activity of the pre- and post-synaptic cells. As a consequence, synapses influence the plasticity of other synapses exclusively via the post-synaptic activity. Inspired by recent research on the properties of apical dendrites it has been suggested, that a second integration site in the apical dendrite may mediate specific global information. Here we explore this issue considering the example of learning invariant responses by examining a network of spiking neurones with two sites of synaptic integration. We demonstrate that results obtained in networks of units with continuous outputs transfer to the more realistic neuronal model. This allows a number of more specific experimental predictions, and is a necessary step to unified description of learning rules exploiting timing of action potentials.

Dynamic modulation of cerebral cortex synaptic function by nitric oxide

Our experiments demonstrate that NO exerts several actions in the cerebral cortex (see Fig. 4). Its production is mediated by neuronal activity through at least two pathways, NMDA receptors and AMPA receptors. By virtue of its diffusion in extracellular space, NO can interact with synapses that are near the production site but not necessarily anatomically connected to the NO source by a conventional synaptic linkage. NO's primary action is amplification of the release of the excitatory neurotransmitter, L-glutamate, thus effectively creating a positive feed-forward gain system. However, a number of effective brakes, presumably activated under physiological conditions, serve to limit the cascade. These include NO's ability to inhibit NMDA receptors, its negative feedback on the rate limiting enzyme, NOS (Rengasamy and Johns, 1993; Park et al., 1994; Ravichandran et al., 1995) and other inhibitory actions (Figs. 3H and L). Under conditions of extremely strong activation or curtailment of the inhibitory feedback mechanisms, as might occur with a change in the local redox milieu (see Lipton, this volume), the amplification cascade may proceed unchecked leading to neurotoxicity (see Dawson, this volume). NO's ability to modulate synaptic function is indicated by both its positive and negative modulatory role in a form of activity-dependent synaptic plasticity, covariance-induced synaptic potentiation. These opposing effects may be due to NO's ability to amplify glutamate release and inhibit NMDA receptors, respectively. The actions of endogenous NO in vivo are primarily facilitatory in visual cortex (Fig. 4). However, inhibitory actions also occur in vivo. The targets for NO in vivo, are potentially more diverse including the neurotransmitter release process, NMDA receptors, other receptors and ion channels and the cerebral vasculature. However, regardless of the signaling pathways, the net result of endogenous NO production in the intact visual cortex is a potent modulation of cells' responses to visual stimulation. Thus, it is likely that this signal plays an important role in ongoing information processing in the mature cerebral cortex, dynamically altering the effective strength of cortical networks.

Transient synaptic potentiation in the visual cortex. I. Cellular mechanisms

The cellular mechanisms that underlie transient synaptic potentiation were studied in visual cortical slices of adult guinea pigs (> or = age 5 wk postnatal). Postsynaptic potentials (PSPs) elicited by stimulation of the white matter/layer VI border were recorded with conventional intracellular techniques from layer II/III neurons. Transient potentiation (average duration 23 +/- 3 min, mean +/- SE) was evoked by 60 low-frequency (0.1 Hz) pairings of weak afferent stimulation with coincident intracellular depolarizing pulses (80 ms) of the postsynaptic cell. Fifty-one percent (47 of 92) of the pairing protocols led to significant enhancement (+26 +/- 3%) of the PSP peak amplitude. Blockade of action potential output from the recorded neuron during pairing with Lidocaine, N-ethyl bromide quaternary salt in the recording micropipette did not reduce the likelihood of potentiation (7 of 14 protocols = 50%). Thus transient synaptic potentiation does not require action potential output from the paired cell or recurrent synaptic activation in the local cortical circuit. Rather, the modification occurs at synaptic sites that directly impinge onto the activated neuron. Intracellular postsynaptic blockade of inhibitory PSPs only onto the paired cell with the chloride channel blocker 4,4'-dinitro-stilbene-2,2'-disulfonic acid and the potassium channel blocker cesium in he micropipette also did not reduce the likelihood of induction of potentiation (6 of 9 protocols = 67%). These results suggest that the potentiation is due to a true upregulation of excitatory synaptic transmission and that it does not require a reduction of inhibitory components of the compound PSP for induction. Chelation of postsynaptic intracellular calcium with 1,2-bis-2-aminophenoxy ethane-N,N,N',N'-tetraacetic acid (BAPTA) in all cases effectively blocked the induction of potentiation (no change in the PSP, 9 of 13 protocols; induction of synaptic depression, 4 of 13 protocols), suggesting that a rise in the intracellular postsynaptic calcium level is critical for the pairing-induced synaptic potentiation to occur. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV) reversibly blocked potentiation of the PSP peak amplitude in most cells (14 of 16) that were capable of significant potentiation of control solution. Blockade of nitric oxide production with bath application of the competitive inhibitor of nitric oxide synthase, L-nitro-arginine (LNA), did not significantly affect the likelihood of synaptic potentiation (11 of 20 cells). It did, however, block subsequent enhancement for several cells (2 of 4) that had previously had their inputs potentiated. Moreover, LNA increased the overall average magnitude of synaptic potentiation (with an additional +28%) when induction was successful. These results suggest that endogenous cortical nitric oxide production can both positively and negatively modulate this NMDA receptor-mediated type of synaptic plasticity.

Mechanisms for regulating synaptic efficiency in the visual cortex

Brief epochs of pairing of low frequency synaptic activation and postsynaptic depolarization, in vitro, in supragranular neurons of nature guinea-pig visual cortex lead to a transient (20-60 min) synaptic potentiation. The process is due to a true up-regulation of excitatory synapse efficiency onto the activated neuron. The potentiation requires NMDA receptor activation and a postsynaptic calcium signal for induction and it is modifiable by endogenous nitric oxide (NO) production in the mature cortex. In the cortex of young animals (< PND 21), the pairing-induced potentiation is robust and depends on a postsynaptic calcium signal but it is independent of NMDA receptor activation and NO production. The ability of cortical synaptosomes to release endogenous glutamate is enhanced by NMDA receptor activation and this enhancement is NO-dependent. The NO signal, however, does not amplify the glutamate release of all synapses but only those that have activated voltage-gated calcium channels and were presumably more active at the time of the NO signal. Electrophysiological recordings from visual cortical neurons in anesthetized cats with local iontophoresis of compounds that inhibit or facilitate endogenous cortical NO production reveal the capacity for NO to modulate visual responses in vivo. NO appears to act in the intact cortex by amplifying signals of visual inputs that were co-active at the time of the NO production. The adult visual cortex is capable of dramatic alterations in synaptic efficiency over brief periods suggesting a dynamic cortical network. NMDA receptors and nitric oxide contribute to these processes.

Role of NO production in NMDA receptor-mediated neurotransmitter release in cerebral cortex

L-Glutamate and norepinephrine are examples of a major excitatory neurotransmitter and a neuromodulator in the cerebral cortex, respectively. Little is known of how chemical signaling between the anatomically distinct chemical pathways occurs. Specific activation of the N-methyl-D-aspartate (NMDA) class of glutamate receptor in synaptosomal preparations from guinea pig cerebral cortex caused release of both of these chemicals, and this release was blocked by agents that inhibit nitric oxide (NO) production or remove NO from the extracellular space. Furthermore, neurotransmitter release correlated with cortical NO production after NMDA receptor stimulation. These results suggest that NO production and its extracellular movement may be links in the pathway from NMDA receptor activation to changes in chemical signaling in surrounding synaptic terminals in the cerebral cortex.