Calcium signaling in mitral cell dendrites of olfactory bulbs of neonatal rats and mice during olfactory nerve Stimulation and beta-adrenoceptor activation

Dopamine Suppresses Synaptic Responses of Fan Cells in the Lateral Entorhinal Cortex to Olfactory Bulb Input in Mice

The lateral entorhinal cortex (LEC) is involved in odor discrimination, odor-associative multimodal memory, and neurological or neuropsychiatric disorders. It receives direct axonal projections from both olfactory bulb (OB) output neurons and midbrain dopaminergic neurons. However, the cellular targets in LEC receiving direct synaptic input from OB output neuron, the functional characteristics of these synapses, and whether or how dopamine (DA) modulates the OB-LEC pathway remain undetermined. We addressed these questions in the present study by combing optogenetic and electrophysiological approaches with four major findings: (1) selective activation of OB input elicited glutamate-mediated monosynaptic responses in all fan cells, the major output neurons in layer II of the LEC; (2) this excitatory synaptic transmission exhibited robust paired-pulse facilitation (PPF), a presynaptically derived short-term synaptic plasticity; (3) DA dramatically attenuated the strength of the OB input-fan cell synaptic transmission via activation of D1 receptors; and (4) DA altered the PPF of this transmission but neither intrinsic properties of postsynaptic neurons nor the kinetic profile of postsynaptic responses, suggesting that presynaptic mechanisms underlie the DA inhibitory actions. This study for the first time demonstrates the FCs in the LEC layer II as the postsynaptic target of direct OB input and characterizes DA modulation of the OB input-fan cell pathway. These findings set the foundation for future studies to examine the synaptic transmission from the OB output neuron axon terminals to other potential cell types in the LEC and to pinpoint the pathophysiological mechanisms underlying olfactory deficits associated with DA-relevant neurological and neuropsychiatric disorders.

Age‑dependent decreases in insulin‑like growth factor‑I and its receptor expressions in the gerbil olfactory bulb

Insulin-like growth factor-I (IGF-I) is a multifunctional protein present in the central nervous system. A number of previous studies have revealed alterations in IGF-I and its receptor (IGF-IR) expression in various regions of the brain. However, there are few reports on age-dependent alterations in IGF-I and IGF-IR expressions in the olfactory bulb, which contains the secondary neurons of the olfactory system. The present study examined the cellular morphology in the olfactory bulb by using cresyl violet (CV) staining at postnatal month (PM) 3 in the young group, PM 6 in the adult group and PM 24 in the aged group in gerbils. In addition, detailed examinations were performed of the protein levels and immunoreactivities of IGF-I and IGF-IR in the olfactory bulb in each group. There were no significant changes in the cellular morphology between the three groups. The protein levels and immunoreactivities of the IGF-I and IGF-IR were the highest in the young group and they decreased with age. He protein levels and immunoreactivities of the IGF-I and IGF-IR were the lowest in the aged group. In brief, our results indicate that IGF-I and IGF-IR expressions are strong in young olfactory bulbs and significantly reduced in aged olfactory bulbs. In conclusion, subsequent decreases in IGF-I and IGF-IR expression with age may be associated with olfactory decline. Further studies are required to investigate the roles of IFG-I and IGF-IR in disorders of the olfactory system.

Adenosine A1 Receptor-Mediated Attenuation of Reciprocal Dendro-Dendritic Inhibition in the Mouse Olfactory Bulb

It is well described that A₁ adenosine receptors inhibit synaptic transmission at excitatory synapses in the brain, but the effect of adenosine on reciprocal synapses has not been studied so far. In the olfactory bulb, the majority of synapses are reciprocal dendro-dendritic synapses mediating recurrent inhibition. We studied the effect of A₁ receptor activation on recurrent dendro-dendritic inhibition in mitral cells using whole-cell patch-clamp recordings. Adenosine reduced dendro-dendritic inhibition in wild-type, but not in A₁ receptor knock-out mice. Both NMDA receptor-mediated and AMPA receptor-mediated dendro-dendritic inhibition were attenuated by adenosine, indicating that reciprocal synapses between mitral cells and granule cells as well as parvalbumin interneurons were targeted by A₁ receptors. Adenosine reduced glutamatergic self-excitation and inhibited N-type and P/Q-type calcium currents, but not L-type calcium currents in mitral cells. Attenuated glutamate release, due to A₁ receptor-mediated calcium channel inhibition, resulted in impaired dendro-dendritic inhibition. In behavioral tests we tested the ability of wild-type and A₁ receptor knock-out mice to find a hidden piece of food. Knock-out mice were significantly faster in locating the food. Our results indicate that A₁ adenosine receptors attenuates dendro-dendritic reciprocal inhibition and suggest that they affect odor information processing.

Using Intrinsic Flavoprotein and NAD(P)H Imaging to Map Functional Circuitry in the Main Olfactory Bulb

Neurons exhibit strong coupling of electrochemical and metabolic activity. Increases in intrinsic fluorescence from either oxidized flavoproteins or reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] in the mitochondria have been used as an indicator of neuronal activity for the functional mapping of neural circuits. However, this technique has not been used to investigate the flow of olfactory information within the circuitry of the main olfactory bulb (MOB). We found that intrinsic flavoprotein fluorescence signals induced by electrical stimulation of single glomeruli displayed biphasic responses within both the glomerular (GL) and external plexiform layers (EPL) of the MOB. Pharmacological blockers of mitochondrial activity, voltage-gated Na⁺ channels, or ionotropic glutamate receptors abolished stimulus-dependent flavoprotein responses. Blockade of GABA_A receptors enhanced the amplitude and spatiotemporal spread of the flavoprotein signals, indicating an important role for inhibitory neurotransmission in shaping the spread of neural activity in the MOB. Stimulus-dependent spread of fluorescence across the GL and EPL displayed a spatial distribution consistent with that of individual glomerular microcircuits mapped by neuroanatomic tract tracing. These findings demonstrated the feasibility of intrinsic fluorescence imaging in the olfactory systems and provided a new tool to examine the functional circuitry of the MOB.

Altered synaptic transmission at olfactory and vomeronasal nerve terminals in mice lacking N-type calcium channel Cav2.2

We investigated the role of voltage-activated calcium (Cav) channels for synaptic transmission at mouse olfactory and vomeronasal nerve terminals at the first synapse of the main and accessory olfactory pathways, respectively. We provided evidence for a central role of the N-type Cav channel subunit Cav2.2 in presynaptic transmitter release at these synapses. Striking Cav2.2 immunoreactivity was localised to the glomerular neuropil of the main olfactory bulb (MOB) and accessory olfactory bulb (AOB), and co-localised with presynaptic molecules such as bassoon. Voltage-clamp recordings of sensory nerve-evoked, excitatory postsynaptic currents (EPSCs) in mitral/tufted (M/T) and superficial tufted cells of the MOB and mitral cells of the AOB, in combination with established subtype-specific Cav channel toxins, indicated a predominant role of N-type channels in transmitter release at these synapses, whereas L-type, P/Q-type, and R-type channels had either no or only relatively minor contributions. In Cacna1b mutant mice lacking the Cav2.2 (α1B) subunit of N-type channels, olfactory nerve-evoked M/T cell EPSCs were not reduced but became blocker-resistant, thus indicating a major reorganisation and compensation of Cav channel subunits as a result of the Cav2.2 deletion at this synapse. Cav2.2-deficient mice also revealed that Cav2.2 was critically required for paired-pulse depression of olfactory nerve-evoked EPSCs in M/T cells of the MOB, and they demonstrated an essential requirement for Cav2.2 in vomeronasal nerve-evoked EPSCs of AOB mitral cells. Thus, Cacna1b loss-of-function mutations are unlikely to cause general anosmia but Cacna1b emerges as a strong candidate in the search for mutations causing altered olfactory perception, such as changes in general olfactory sensitivity and altered social responses to chemostimuli.

Early life trauma and attachment: immediate and enduring effects on neurobehavioral and stress axis development

Over half a century of converging clinical and animal research indicates that early life experiences induce enduring neuroplasticity of the HPA-axis and the developing brain. This experience-induced neuroplasticity is due to alterations in the frequency and intensity of stimulation of pups' sensory systems (i.e., olfactory, somatosensory, gustatory) embedded in mother-infant interactions. This stimulation provides "hidden regulators" of pups' behavioral, physiological, and neural responses that have both immediate and enduring consequences, including those involving the stress response. While variation in stimulation can produce individual differences and adaptive behaviors, pathological early life experiences can induce maladaptive behaviors, initiate a pathway to pathology, and increase risk for later-life psychopathologies, such as mood and affective disorders, suggesting that infant-attachment relationships program later-life neurobehavioral function. Recent evidence suggests that the effects of maternal presence or absence during this sensory stimulation provide a major modulatory role in neural and endocrine system responses, which have minimal impact on pups' immediate neurobehavior but a robust impact on neurobehavioral development. This concept is reviewed here using two complementary rodent models of infant trauma within attachment: infant paired-odor-shock conditioning (mimicking maternal odor attachment learning) and rearing with an abusive mother that converge in producing a similar behavioral phenotype in later-life including depressive-like behavior as well as disrupted HPA-axis and amygdala function. The importance of maternal social presence on pups' immediate and enduring brain and behavior suggests unique processing of sensory stimuli in early life that could provide insight into the development of novel strategies for prevention and therapeutic interventions for trauma experienced with the abusive caregiver.

α(1A)-Adrenergic regulation of inhibition in the olfactory bulb

By regulating inhibition at dendrodendritic synapses between mitral and granule cells (GCs), noradrenergic neurons extending from the brainstem provide an input essential for odour processing in the olfactory bulb (OB). In the accessory OB (AOB), we have recently shown that noradrenaline (NA) increases GABA inhibitory input on to mitral cells (MCs) by exciting GCs. Here, we show that GCs in the main OB (MOB) exhibit a similar response to NA, indicating a common mechanism for noradrenergic regulation of GCMC inhibition throughout the OB. In GCs of the MOB, NA (10 μM) produced a robust excitatory effect that included a slow afterdepolarization that followed a train of action potentials evoked by a current stimulus. The depolarization and slow afterdepolarization in GCs were blocked by the α1A-adrenergic receptor (AR) selective antagonist WB 4101 (30 nm) and mimicked by the α(1A)-AR selective agonist A 61603 (1 μM). In recordings from MCs, A 61603 (30 nm-1 μM) produced a sizeable increase in the frequency of spontaneous and miniature IPSCs, an effect completely abolished by the GABAA receptor antagonist gabazine (5 μM). Likewise, activation of β-ARs increased the frequency of spontaneous IPSCs; however, this effect was smaller and confined to the first postnatal weeks. NA enhanced inhibition in MCs across a broad concentration range (0.1-30 μM) and its effects were completely abolished by a mixture of α1- and β-AR antagonists (1 μM prazosin and 10 μM propranolol). Furthermore, the general α2-AR agonist clonidine (10 μM) failed to affect sIPSC frequency. Thus, the NA-mediated increase in GCMC inhibition in the OB results mostly from activation of the α1A-AR subtype.

Olfactory aversive conditioning alters olfactory bulb mitral/tufted cell glomerular odor responses

The anatomical organization of receptor neuron input into the olfactory bulb (OB) allows odor information to be transformed into an odorant-specific spatial map of mitral/tufted (M/T) cell glomerular activity at the upper level of the OB. In other sensory systems, neuronal representations of stimuli can be reorganized or enhanced following learning. While the mammalian OB has been shown to undergo experience-dependent plasticity at the glomerular level, it is still unclear if similar representational change occurs within (M/T) cell glomerular odor representations following learning. To address this, odorant-evoked glomerular activity patterns were imaged in mice expressing a GFP-based calcium indicator (GCaMP2) in OB (M/T) cells. Glomerular odor responses were imaged before and after olfactory associative conditioning to aversive foot shock. Following conditioning, we found no overall reorganization of the glomerular representation. Training, however, did significantly alter the amplitudes of individual glomeruli within the representation in mice in which the odor was presented together with foot shock. Further, the specific pairing of foot shock with odor presentations lead to increased responses primarily in initially weakly activated glomeruli. Overall, these results suggest that associative conditioning can enhance the initial representation of odors within the OB by enhancing responses to the learned odor in some glomeruli.

Synaptic activation of T-type Ca2+ channels via mGluR activation in the primary dendrite of mitral cells

Mitral cells are the primary output of the olfactory bulb, projecting to many higher brain areas. Understanding how mitral cells process and transmit information is key to understanding olfactory perception. Mitral dendrites possess high densities of voltage-gated channels, are able to initiate and propagate orthodromic and antidromic action potentials, and release neurotransmitter. We show that mitral cells also possess a low-voltage-activated T-type Ca(2+) current. Immunohistochemistry shows strong Cav3.3 labeling in the primary dendrite and apical tuft with weaker staining in basal dendrites and no staining in somata. A low-voltage-activated Ca(2+) current activates from -68 mV, is blocked by 500 microM Ni(2+) and 50 microM NNC 55-0396, but is insensitive to 50 microM Ni(2+) and 500 microM isradipine. 2-photon Ca(2+) imaging shows that T channels are functionally expressed in the primary dendrite where their activity determines the resting [Ca(2+)] and are responsible for subthreshold voltage-dependent Ca(2+) changes previously observed in vivo. Application of the group 1 mGluR agonist dihydroxyphenylglycine (DHPG) (50 microM) robustly upregulates T-channel current in the primary and apical tuft dendrite. Olfactory nerve stimulation generates a long-lasting depolarization, and we show that mGluRs recruit T channels to contribute approximately 36% of the voltage integral of this depolarization. The long-lasting depolarization results in sustained firing and block of T channels decreased action potential firing by 84.1 +/- 4.6%. Therefore upregulation of T channels by mGluRs is required for prolonged firing in response to olfactory nerve input.

Dendritic branching of olfactory bulb mitral and tufted cells: regulation by TrkB

BACKGROUND

Projection neurons of mammalian olfactory bulb (OB), mitral and tufted cells, have dendrites whose morphologies are specifically differentiated for efficient odor information processing. The apical dendrite extends radially and arborizes in single glomerulus where it receives primary input from olfactory sensory neurons that express the same odor receptor. The lateral dendrites extend horizontally in the external plexiform layer and make reciprocal dendrodendritic synapses with granule cells, which moderate mitral/tufted cell activity. The molecular mechanisms regulating dendritic development of mitral/tufted cells is one of the unsolved important problems in the olfactory system. Here, we focused on TrkB receptors to test the hypothesis that neurotrophin-mediate mechanisms contributed to dendritic differentiation of OB mitral/tufted cells.

PRINCIPAL FINDINGS

With immunohistochemical analysis, we found that the TrkB neurotrophin receptor is expressed by both apical and lateral dendrites of mitral/tufted cells and that expression is evident during the early postnatal days when these dendrites exhibit their most robust growth and differentiation. To examine the effect of TrkB activation on mitral/tufted cell dendritic development, we cultured OB neurons. When BDNF or NT4 were introduced into the cultures, there was a significant increase in the number of primary neurites and branching points among the mitral/tufted cells. Moreover, BDNF facilitated filopodial extension along the neurites of mitral/tufted cells.

SIGNIFICANCE

In this report, we show for the first time that TrkB activation stimulates the dendritic branching of mitral/tufted cells in developing OB. This suggests that arborization of the apical dendrite in a glomerulus is under the tight regulation of TrkB activation.

A temporal-specific and transient cAMP increase characterizes odorant classical conditioning

Increases in cyclic adenosine monophosphate (cAMP) are proposed to initiate learning in a wide variety of species. Here, we measure changes in cAMP in the olfactory bulb prior to, during, and following a classically conditioned odor preference trial in rat pups. Measurements were taken up to the point of maximal CREB phosphorylation in olfactory bulb mitral cells. Using both drug and natural unconditioned stimuli we found effective learning was associated with an increase in cAMP at the end of the conditioning trial, followed by a decrease 5 min later. This early timing of a transient cAMP increase occurred only when the odor was paired with an effective drug or natural unconditioned stimulus (US). The data support the hypothesis that the rate of adenylate cyclase activation is enhanced by pairing calcium and G-protein activation and that the timing of transient cAMP signaling is critical to the initiation of classical conditioning.

Neurobiology of infant attachment

A strong attachment to the caregiver is critical for survival in altricial species, including humans. While some behavioral aspects of attachment have been characterized, its neurobiology has only recently received attention. Using a mammalian imprinting model, we are assessing the neural circuitry that enables infant rats to attach quickly to a caregiver, thus enhancing survival in the nest. Specifically, the hyper-functioning noradrenergic locus coeruleus (LC) enables pups to learn rapid, robust preference for the caregiver. Conversely, a hypo-functional amygdala appears to prevent the infant from learning aversions to the caregiver. Adult LC and amygdala functional emergence correlates with sensitive period termination. This study suggests the neonatal brain is not an immature version of the adult brain but is uniquely designed to optimize attachment to the caregiver. Although human attachment may not rely on identical circuitry, the work reviewed here suggests a new conceptual framework in which to explore human attachments, particularly attachments to abusive caregivers.

Olfactory nerve stimulation-induced calcium signaling in the mitral cell distal dendritic tuft

Olfactory receptor neuron axons form the olfactory nerve (ON) and project to the glomerular layer of the olfactory bulb, where they form excitatory synapses with terminal arborizations of the mitral cell (MC) tufted primary dendrite. Clusters of MC dendritic tufts define olfactory glomeruli, where they involve in complex synaptic interactions. The computational function of these cellular interactions is not clear. We used patch-clamp electrophysiology combined with whole field or two-photon Ca2+ imaging to study ON stimulation-induced Ca2+ signaling at the level of individual terminal branches of the MC primary dendrite in mice. ON-evoked subthreshold excitatory postsnaptic potentials induced Ca2+ transients in the MC tuft dendrites that were spatially inhomogeneous, exhibiting discrete "hot spots." In contrast, Ca2+ transients induced by backpropagating action potentials occurred throughout the dendritic tuft, being larger in the thin terminal dendrites than in the base of the tuft. Single ON stimulation-induced Ca2+ transients were depressed by the NMDA receptor antagonist D-aminophosphonovaleric acid (D-APV), increased with increasing stimulation intensity, and typically showed a prolonged rising phase. The synaptically induced Ca2+ signals reflect, at least in part, dendrodendritic interactions that support intraglomerular coupling of MCs and generation of an output that is common to all MCs associated with one glomerulus.

Memory of early maltreatment: neonatal behavioral and neural correlates of maternal maltreatment within the context of classical conditioning

BACKGROUND

While children form an attachment to their abusive caregiver, they are susceptible to mental illness and brain abnormalities. To understand this important clinical issue, we have developed a rat animal model of abusive attachment where odor paired with shock paradoxically produces an odor preference. Here, we extend this model to a seminaturalistic paradigm using a stressed, "abusive" mother during an odor presentation and assess the underlying learning neural circuit.

METHODS

We used a classical conditioning paradigm pairing a novel odor with a stressed mother that predominantly abused pups to assess olfactory learning in a seminaturalistic environment. Additionally, we used Fos protein immunohistochemistry to assess brain areas involved in learning this pain-induced odor preference within a more controlled maltreatment environment (odor-shock conditioning).

RESULTS

Odor-maternal maltreatment pairings within a seminatural setting and odor-shock pairings both resulted in paradoxical odor preferences. Learning-induced gene expression was altered in the olfactory bulb and anterior piriform cortex (part of olfactory cortex) but not the amygdala.

CONCLUSIONS

Infants appear to use a unique brain circuit that optimizes learned odor preferences necessary for attachment. A fuller understanding of infant brain function may provide insight into why early maltreatment affects psychiatric well-being.