In vivo vomeronasal stimulation reveals sensory encoding of conspecific and allospecific cues by the mouse accessory olfactory bulb.

The rodent vomeronasal system plays a critical role in mediating pheromone-evoked social and sexual behaviors. Recent studies of the anatomical and molecular architecture of the vomeronasal organ (VNO) and of its synaptic target, the accessory olfactory bulb (AOB), have suggested that unique features underlie vomeronasal sensory processing. However, the neuronal representation of pheromonal information leading to specific behavioral and endocrine responses has remained largely unexplored due to the experimental difficulty of precise stimulus delivery to the VNO. To determine the basic rules of information processing in the vomeronasal system, we developed a unique preparation that allows controlled and repeated stimulus delivery to the VNO and combined this approach with multisite recordings of neuronal activity in the AOB. We found that urine, a well-characterized pheromone source in mammals, as well as saliva, activates AOB neurons in a manner that reliably encodes the donor animal's sexual and genetic status. We also identified a significant fraction of AOB neurons that respond robustly and selectively to predator cues, suggesting an expanded role for the vomeronasal system in both conspecific and interspecific recognition. Further analysis reveals that mixed stimuli from distinct sources evoke synergistic responses in AOB neurons, thereby supporting the notion of integrative processing of chemosensory information.

Electrophysiology of interneurons in the glomerular layer of the rat olfactory bulb.

In the mammalian olfactory bulb, glomeruli are surrounded by a heterogeneous population of interneurons called juxtaglomerular neurons. As they receive direct input from olfactory receptor neurons and connect with mitral cells, they are involved in the initial stages of olfactory information processing, but little is known about their detailed physiological properties. Using whole cell patch-clamp techniques, we recorded from juxtaglomerular neurons in rat olfactory bulb slices. Based on their response to depolarizing pulses, juxtaglomerular neurons could be divided into two physiological classes: bursting and standard firing. When depolarized, the standard firing neurons exhibited a range of responses: accommodating, nonaccommodating, irregular firing, and delayed to firing patterns of action potentials. Although the firing pattern was not rigorously predictive of a particular neuronal morphology, most short axon cells fired accommodating trains of action potentials, while most delayed to firing cells were external tufted cells. In contrast to the standard firing neurons, bursting neurons produced a calcium-channel-dependent low-threshold spike when depolarized either by current injection or by spontaneous or evoked postsynaptic potentials. Bursting neurons also could oscillate spontaneously. Most bursting cells were either periglomerular cells or external tufted cells. Based on their mode of firing and placement in the bulb circuit, these bursting cells are well situated to drive synchronous oscillations in the olfactory bulb.

Spatial coding of enantiomers in the rat olfactory bulb.

Because of their unique properties, enantiomers (pairs of mirror-symmetric, nonsuperimposable molecules that differ only in optical activity and their interaction with other chiral molecules) have been instrumental in demonstrating that olfactory perception relies on molecular shape. To investigate how molecular structure is encoded by the olfactory system, we combined behavioral discrimination tasks with optical imaging of intrinsic signals. We found that rats can behaviorally discriminate members of a wide range of enantiomer pairs, and imaging revealed enantiomer-selective glomeruli in the olfactory bulb, indicating that the spatial pattern of glomerular activity provides sufficient information to discriminate molecular shape.

Symmetry, stereotypy, and topography of odorant representations in mouse olfactory bulbs.

The molecular basis of vertebrate odorant representations has been derived extensively from mice. The functional correlates of these molecular features were visualized using optical imaging of intrinsic signals in mouse olfactory bulbs. Single odorants activated clusters of glomeruli in consistent, restricted portions of the bulb. Patterns of activated glomeruli were clearly bilaterally symmetric and consistent in different individual mice, but the precise number, position, and intensity of activated glomeruli in the two bulbs of the same individual and between individuals varied considerably. Representations of aliphatic aldehydes of different carbon chain length shifted systematically along a rostral-caudal strip of the dorsal bulb, indicating a functional topography of odorant representations. Binary mixtures of individual aldehydes elicited patterns of glomerular activation that were topographic combinations of the maps for each individual odor. Thus the principles derived from the molecular organization of a small subset of murine olfactory receptor neuron projection patterns-bilateral symmetry, local clustering, and local variability-are reliable guides to the initial functional representation of odorant molecules.

Response correlation maps of neurons in the mammalian olfactory bulb.

To define the relationship between glomerular activation patterns and neuronal olfactory responses in the main olfactory bulb, intracellular recordings were combined with optical imaging of intrinsic signals. Response correlation maps (RCMs) were constructed by correlating the fluctuations in membrane potential and firing rate during odorant presentations with patterns of glomerular activation. The RCMs indicated that mitral/tufted cells were excited by activation of a focal region surrounding their principal glomerulus and generally inhibited by activation of more distant regions. However, the structure of the RCMs and the relative contribution of excitatory and inhibitory glomerular input evolved and even changed sign during and after odorant application. These data suggest a dynamic center-surround organization of mitral/tufted cell receptive fields.

Early development of ocular dominance columns.

The segregation of lateral geniculate nucleus (LGN) axons into ocular dominance columns is believed to involve a prolonged, activity-dependent sorting process. However, visualization of early postnatal ferret LGN axons by direct LGN tracer injections revealed segregated ocular dominance columns <7 days after innervation of layer 4. These early columns were unaffected by experimentally induced imbalances in retinal activity, implying that different mechanisms govern initial column formation and their modification during the subsequent critical period. Instead of activity-dependent plasticity, we propose that ocular dominance column formation relies on the targeting of distinct axonal populations to defined locales in cortical layer 4.

Development of ocular dominance columns in the absence of retinal input.

The initial establishment of ocular dominance columns in visual cortex is believed to involve the segregation of overlapping geniculocortical axons into eye-specific patches based on patterns of correlated activity. However, we found that total removal of retinal influence early in visual development did not prevent segregation of geniculocortical axons into alternating stripes with periodicity normal for ocular dominance columns. Because the patterning of geniculocortical afferents resists this dramatic change in the level, source and pattern of spontaneous activity, we propose that formation of ocular dominance columns relies on molecular cues present on thalamic axons, cortical cells or both.

Optical imaging of odorant representations in the mammalian olfactory bulb.

We adapted the technique of intrinsic signal imaging to visualize how odorant concentration and structure are represented spatially in the rat olfactory bulb. Most odorants activated one or more glomeruli in the imaged region of the bulb; these optically imaged responses reflected the excitation of underlying neurons. Odorant-evoked patterns were similar across animals and symmetrical in the two bulbs of the same animal. The variable sensitivity of individual glomeruli produced distinct maps for different odorant concentrations. Using a series of homologous aldehydes, we found that glomeruli were tuned to detect particular molecular features and that maps of similar molecules were highly correlated. These characteristics suggest that odorants and their concentrations can be encoded by distinct spatial patterns of glomerular activation.

Correlational structure of spontaneous neuronal activity in the developing lateral geniculate nucleus in vivo.

The properties of spontaneous activity in the developing visual pathway beyond the retina are unknown. Multielectrode recordings in the lateral geniculate nucleus (LGN) of awake behaving ferrets, before eye opening, revealed patterns of spontaneous activity that reflect a reshaping of retinal drive within higher visual stages. Significant binocular correlations were present only when cortico-thalamic feedback was intact. In the absence of retinal drive, cortico-thalamic feedback was required to sustain correlated LGN bursting. Activity originating from the contralateral eye drove thalamic activity far more strongly than that originating from the ipsilateral eye. Thus, in vivo patterns of LGN spontaneous activity emerge from interactions between retina, thalamus, and cortex.

Destabilization of cortical dendrites and spines by BDNF.

Particle-mediated gene transfer and two-photon microscopy were used to monitor the behavior of dendrites of individual cortical pyramidal neurons coexpressing green fluorescent protein (GFP) and brain-derived neurotrophic factor (BDNF). While the dendrites and spines of neurons expressing GFP alone grew modestly over 24-48 hr, coexpressing BDNF elicited dramatic sprouting of basal dendrites, accompanied by a regression of dendritic spines. Compared to GFP-transfected controls, the newly formed dendrites and spines were highly unstable. Experiments utilizing Trk receptor bodies, K252a, and overexpression of nerve growth factor (NGF) demonstrated that these effects were mediated by secreted BDNF interacting with extracellular TrkB receptors. Thus, BDNF induces structural instability in dendrites and spines, which, when restricted to particular portions of a dendritic arbor, may help translate activity patterns into specific morphological changes.

Neurotrophins and synaptic plasticity.

Despite considerable evidence that neuronal activity influences the organization and function of circuits in the developing and adult brain, the molecular signals that translate activity into structural and functional changes in connections remain largely obscure. This review discusses the evidence implicating neurotrophins as molecular mediators of synaptic and morphological plasticity. Neurotrophins are attractive candidates for these roles because they and their receptors are expressed in areas of the brain that undergo plasticity, activity can regulate their levels and secretion, and they regulate both synaptic transmission and neuronal growth. Although numerous experiments show demonstrable effects of neurotrophins on synaptic plasticity, the rules and mechanisms by which they exert their effects remain intriguingly elusive.

Relationship between dye coupling and spontaneous activity in developing ferret visual cortex.

Neuronal coupling by gap junctions is common during early development of the brain. Coupling is thought to create functional cell assemblies which may be involved in the functional specification of brain areas and the formation of synaptic circuits. In the present study we used slices from the visual cortex of postnatal ferrets to investigate the temporal relationship of gap junction coupling and formation of functional synapses. Individual neurons were filled with the gap-junction-permeable dye biotin ethylenediamine while spontaneous synaptic currents were recorded using whole-cell patch clamp recording techniques. We found that dye coupling increased during the first 2 postnatal weeks resulting at a peak around P14, after which coupling steadily decreased until adult levels were reached in animals older than P30. Spontaneous synaptic activity increased 30-fold between birth and maturity (from 10.8 +/- 2.4 to 318 +/- 54 events/min). The sharpest rise in synaptic activity, an over 5-fold increase, occurred between P15 and P19, shortly after the invasion of thalamocortical fibers.

Coordination of neuronal activity in developing visual cortex by gap junction-mediated biochemical communication.

During brain development, endogenously generated coordinated neuronal activity regulates the precision of developing synaptic circuits (Shatz and Stryker, 1988; Weliky and Katz, 1997). In the neonatal neocortex, a form of endogenous coordinated activity is present as locally restricted intercellular calcium waves that are mediated by gap junctions (Yuste et al., 1992). As in other neuronal and non-neuronal systems, these coordinated calcium fluctuations may form the basis of functional cell assemblies (for review, seeWarner, 1992; Peinado et al., 1993b). In the present study, we investigated the cellular mechanisms that mediate the activation of neuronal domains and the propagation of intercellular calcium waves in slices from neonatal rat neocortex. The occurrence of neuronal domains did not depend on intercellular propagation of regenerative electrical signals because domains persisted after blockade of sodium and calcium-dependent action potentials. Neuronal domains were elicited by intracellular infusion of inositol trisphosphate (IP3) but not of calcium, indicating the involvement of IP3-related second-messenger systems. Pharmacological stimulation of metabotropic glutamate receptors, which are linked to the production of IP3, elicited similarly coordinated calcium increases, whereas pharmacological blockade of metabotropic glutamate receptors dramatically reduced the number of neuronal domains. Therefore, the propagating cellular signal that causes the occurrence of neuronal domains seems to be inositol trisphosphate but not calcium. Because coordination of neuronal calcium changes by gap junctions is independent of electrical signals, the function of gap junctions between neocortical neurons is probably to synchronize biochemical rather than electrical activity.

Focal photolysis of caged glutamate produces long-term depression of hippocampal glutamate receptors.

Separating contributions of pre- and postsynaptic factors to the maintenance of long-term potentiation (LTP) and long-term depression (LTD) has been confounded by their experimental interdependence. To isolate the postsynaptic contribution, glutamate-receptor-mediated currents were elicited by localized photolysis of caged glutamate in small spots along the dendrites of CA1 hippocampal pyramidal cells. With synaptic transmission blocked, pairing depolarization of pyramidal cells with repeated photolysis of caged glutamate at one site markedly and persistently depressed subsequent responses to glutamate; responses at a second, unpaired site were unchanged. Like synaptically induced LTD at the CA3-CA1 synapse, this depression was site specific, NMDA-receptor dependent and blocked by protein-phosphatase inhibitors. Thus, robust, persistent alterations of postsynaptic glutamate receptor efficacy can occur without presynaptic neurotransmitter release.

Relationships between local synaptic connections and orientation domains in primary visual cortex.

Combined optical imaging of ferret primary visual cortex in vivo and scanning laser photostimulation in brain slices were used to determine the spatial relationships between synaptic inputs onto individual neurons and the pattern of orientation columns. In the upper cortical layers, both excitatory and inhibitory inputs originated primarily from regions with orientation tuning similar to that of the recorded neurons; the shapes of the input tuning curves were indistinguishable. The orientation distributions of both types of inputs centered around the orientation of the recorded neurons, and no evidence for preferential cross-orientation inputs, either excitatory or inhibitory, was observed. These patterns of synaptic connectivity are most consistent with feedforward models for generation of orientation selectivity and are inconsistent with the patterns required by models based on cross-orientation inhibition.

Modulation of intrinsic circuits by serotonin 5-HT3 receptors in developing ferret visual cortex.

Serotonergic projections are widespread in the developing neocortex, but their functions are obscure. The effects of 5-HT3 receptor agonists on cortical circuit response properties were studied in slices of ferret primary visual cortex using high-speed optical imaging of voltage-sensitive dye signals and whole-cell patch-clamp recording. Activation of the 5-HT3 receptor decreased the amplitude and lateral extent of excitation throughout postnatal development. This effect peaks after eye opening, which indicates a function for serotonergic modulation of circuit responses during the period of refinement of cortical connections. Whole-cell patch-clamp recordings from single neurons revealed that synaptic responses evoked by white matter stimulation were reduced by 5-HT3 receptor agonists, whereas the frequency of spontaneous GABAergic synaptic currents was enhanced dramatically. This indicates that the modulation of spontaneous synaptic activity by fast-acting serotonin receptors is reflected in an inhibition of the circuit response, in line with the notion of background synaptic activity altering the spatiotemporal integration properties of cortical cells by changing their membrane potential and their electrotonic structure. These mechanisms may regulate the response properties of intrinsic circuits in both the adult and developing neocortex.

Fast synaptic signaling by nicotinic acetylcholine and serotonin 5-HT3 receptors in developing visual cortex.

Cholinergic and serotonergic fiber systems invade the developing visual cortex several weeks before eye opening; both transmitters have been implicated in plasticity of neocortical circuits. These transmitters have been presumed to act predominantly through second messenger-coupled receptors, because fast cholinergic or serotonergic neurotransmission has never been observed in neocortex. However, acetylcholine and serotonin also act on ligand-gated ion channels; the nicotinic acetylcholine receptor and the serotonin 5-HT3 receptor, respectively. Here, using whole-cell patch-clamp techniques in developing ferret visual cortex, we pharmacologically isolated fast, spontaneous, and evoked cholinergic and serotonergic synaptic events in pyramidal cells and interneurons of all cortical layers. The number of cells receiving such inputs increased with the ingrowth of thalamic afferents, and the frequencies of the spontaneous events increased at eye opening. Thus, both acetylcholine and serotonin can mediate fast synaptic transmission in the visual cortex; the early onset of these mechanisms suggests a role during initial stages of circuit formation and during subsequent experience-dependent remodeling of cortical connections.