Projections of the olfactory bulb in an elasmobranch fish,Sphyrna tiburo: segregation of inputs in the telencephalon

We have previously shown that the morphological compartmentalization of the elasmobranch olfactory bulb is accompanied by a topographical arrangement of the primary olfactory projections onto the bulb. If this spatial arrangement is significant for the processing of the information, one would expect it to be preserved in the secondary olfactory centers of the telencephalon. In this paper, we describe the elasmobranch secondary projections from the olfactory bulb to the telencephalon, focusing on their spatial arrangements within the forebrain. Results show that the olfactory input onto the telencephalon are segregated. The medial olfactory tract projects rostrally onto the superficial layer of the dorsal pallium and onto the lateral pallium. The lateral olfactory tract projects caudally onto the lateral pallium, the striatum and the area superficialis basalis. Thus, the secondary olfactory projections are segregated within the telencephalon, with an overlapping of the secondary fibers in the main projection area, the lateral pallium.     

Synaptology of the olfactory bulb of an elasmobranch fish, Sphyrna tiburo

The ultrastructure of the elasmobranch olfactory bulb was examined in order to determine the synaptology of the olfactory circuitry in the bonnethead shark, Sphyrna tiburo. The compartmentalization of the bulb, together with the lack of mitral cell basal dendrites, suggests a different way of performing lateral communication between mitral cells of the olfactory bulb. The results show that granule cells assume an important role by directly interlinking mitral cells. A corollary of this is the segregation of the input onto the mitral cell dendritic arborization: afferent fibers synapse onto the intraglomerular mitral terminals, whereas most local circuit interactions utilize extraglomerular synapses located on the shafts and the somas of the mitral dendrites. Therefore, the elasmobranch synaptic pattern is different from that of higher vertebrates; This might represent the use of a different neural route to achieve the same processing task.     

Synaptology of cultured olfactory bulb cells

Cultured embryonic mouse olfactory bulb cells formed asymmetric, symmetric, axodendritic, and dendrodendritic synapses. These neurons contained electron lucent, dense core, and coated vesicles. Dense core and coated vesicles had an average diameter of 71 and 80 nm, respectively. Two statistically different populations of electron lucent vesicles were found, based on synaptic symmetry: electron lucent vesicles from asymmetric synapses had an average diameter of 46 nm with an estimated volume of 49,000 nm³, whereas those from symmetric synapses had an average diameter of 44 nm and an estimated volume of 42,000 nm³. Because these values are similar to those found for intact olfactory bulb, the synapses of these cultured cells have some of the same morphological characteristics as those in the intact olfactory bulb.     

Projections of the olfactory bulb in an elasmobranch fish, Sphyrna tiburo: segregation of inputs in the telencephalon

We have previously shown that the morphological compartmentalization of the elasmobranch olfactory bulb is accompanied by a topographical arrangement of the primary olfactory projections onto the bulb. If this spatial arrangement is significant for the processing of the information, one would expect it to be preserved in the secondary olfactory centers of the telencephalon. In this paper, we describe the elasmobranch secondary projections from the olfactory bulb to the telencephalon, focusing on their spatial arrangements within the forebrain. Results show that the olfactory input onto the telencephalon are segregated. The medial olfactory tract projects rostrally onto the superficial layer of the dorsal pallium and onto the lateral pallium. The lateral olfactory tract projects caudally onto the lateral pallium, the striatum and the area superficialis basalis. Thus, the secondary olfactory projections are segregated within the telencephalon, with an overlapping of the secondary fibers in the main projection area, the lateral pallium.     

A pilot study on morphological compartmentalization and heterogeneity in the elasmobranch olfactory bulb

The olfactory bulb of many elasmobranch fishes is morphologically subdivided into distinct units or sub-bulbs immediately adjacent to the olfactory epithelium. We investigated this morphological feature in two species of shark and one species of ray in order to understand its impact on the arrangement of the primary olfactory projections onto the bulb. Using anterograde tracing methods in vitro (biocytin) as well as in fixed tissue (DiI), we observed a direct segregated projection of the olfactory afferents onto the bulb. Application of tracers to the lateral part of the olfactory epithelium resulted in staining restricted to the this region of the bulb, whereas the same tracers applied to the medial part of the epithelium resulted in staining of the medial olfactory bulb. The sub-bulbs appear to be individual anatomical units that each receive input from the olfactory lamellae. Nissl and myelin staining as well as the Golgi method show that the cytoarchitecture of the sub-bulbs is not substantially different from that of other anamniotes. However, we did note the existence of two types of mitral cell, based on the morphology of their dendritic arborization. Type L cells exhibit a loose dendritic arborization, whereas type T cells are characterized by a dense, bush-like dendritic arborization. Both types of mitral cells lack basal dendrites.     

Intracellular recording from pectoral fin motoneurons of the stingray Dasyatis sabina, an elasmobranch fish

Intracellular recordings were made from antidromically identified pectoral fin motoneurons in unanesthetized, decerebrate stingrays (Dasyatis sabina). These recordings had the three all-or-none components seen in other vertebrate motoneuron recordings. About 25% of the impalements had resting membrane potentials that were greater than -80 mV, which is larger than those of motoneurons from other vertebrate species. A novel depolarizing afterpotential (DAP) is associated with the isolated action potential occurring at the first node of Ranvier of the axon (M-spike). Occlusion experiments exclude recurrent events as the source of this potential. A capacitive source for the DAP is postulated. Using morphological and passive electrical data on motoneurons from previous studies, calculations of the passive decay of the nodal spike indicate that the membrane resistance of the initial segment is low and nearly equal to that of nodal membrane. The soma-dendritic (SD) spike is followed by a prominent, humped delayed depolarization (DD). The DD is temporally associated with the onset of the action potential produced by the initial segment (IS spike). Sources of the long-lasting period of repolarization recorded with the IS spike, which may underlie the DD, are postulated. The afterhyperpolarization (AHP) of stingray motoneurons tends to be shorter and smaller in amplitude than that of other vertebrate motoneurons. A negligible conductance change was often found during the period following an SD spike. No significant correlation was found between AHP duration and axonal conduction velocity. The input conductance of stingray motoneurons ranged between 1.5 X 10(-7) and 13.3 X 10(-7) S. The relationship between input conductance and axonal conduction velocity was determined from 42 motoneurons. These data were fitted by a power function with an exponent of 1.7, indicating that, in terms of membrane conductance properties, large stingray motoneurons are simply scaled-up versions of the small motoneurons.     

Mechanism of anion permeation through the muscle fibre membrane of an elasmobranch fish, Taeniura lymma

1. Properties of anion permeation through the membrane of skeletal muscle fibres of the stingray, Taeniura lymma, were studied with intracellular recording and polarization techniques.2. The Cl conductance of the resting membrane in the normal stingray saline at pH 7.7 is 8-10 times greater than the K conductance.3. The Cl conductance decreases with decreasing external pH, with an apparent pK of 5.3, whereas the K conductance is independent of pH between 4 and 9.4. The Q(10) of the Cl conductance is about 2.0, compared with a value of 1.2-1.4 for the K conductance.5. The Cl conductance is proportional to the external Cl concentration when observed after the fibre is equilibrated in the test solution.6. The permeability sequence obtained by potential measurement is SCN > NO(3) > Cl = Br > I > ClO(3) and the permeability ratio is independent of the mole fraction of anions.7. The conductance sequence determined by total replacement of the external Cl with other anion species differs from the permeability sequence and the conductance observed for partial replacement deviates significantly from that expected from the independence principle.8. Possible mechanisms of anion permeation are discussed.     

Taming time in the olfactory bulb

The biophysics of an unusual synaptic arrangement within the olfactory bulb suggests a way in which rapidly inactivating potassium channels could modulate the timing of oscillations that underlie odor recognition.     

Self-inhibition of olfactory bulb neurons

The GABA (gamma-aminobutyric-acid)-containing periglomerular (PG) cells provide the first level of inhibition to mitral and tufted (M/T) cells, the output neurons of the olfactory bulb. We find that stimulation of PG cells of the rat olfactory bulb results in self-inhibition: release of GABA from an individual PG cell activates GABA(A) receptors on the same neuron. PG cells normally contain high concentrations of intracellular chloride and consequently are depolarized by GABA. Despite this, GABA inhibits PG cell firing by shunting excitatory signals. Finally, GABA released during self-inhibition may spill over to neighboring PG cells, resulting in a lateral spread of inhibition. Given the gatekeeping role of PG cells in the olfactory network, GABA-mediated self-inhibition will favor M/T cell excitation during intense sensory stimulation.     

Sniffing controls an adaptive filter of sensory input to the olfactory bulb

Most sensory stimuli are actively sampled, yet the role of sampling behavior in shaping sensory codes is poorly understood. Mammals sample odors by sniffing, a complex behavior that controls odorant access to receptor neurons. Whether sniffing shapes the neural code for odors remains unclear. We addressed this question by imaging receptor input to the olfactory bulb of awake rats performing odor discriminations that elicited different sniffing behaviors. High-frequency sniffing of an odorant attenuated inputs encoding that odorant, whereas lower sniff frequencies caused little attenuation. Odorants encountered later in a sniff bout were encoded as the combination of that odorant and the background odorant during low-frequency sniffing, but were encoded as the difference between the two odorants during high-frequency sniffing. Thus, sniffing controls an adaptive filter for detecting changes in the odor landscape. These data suggest an unexpected functional role for sniffing and show that sensory codes can be transformed by sampling behavior alone.     

Centrifugal innervation of the mammalian olfactory bulb

Although it has been known for decades that the mammalian olfactory bulb receives a substantial number of centrifugal inputs from other regions of the brain, relatively few data have been available on the function of the centrifugal olfactory system. Knowing the role of the centrifugal projection and how it works is of critical importance to fully understanding olfaction. The centrifugal fibers can be classified into two groups, a group that release neuromodulators, such as noradrenaiine, serotonin, or acetylcholine, and a group originating in the olfactory cortex. Accumulating evidence suggests that centrifugal neuromodulatory inputs are associated with acquisition of odor memory. Because the distribution of the terminals on these fibers is diffuse and widespread, the neuromodulatory inputs must affect diverse subsets of bulbar neurons at the same time. In contrast, knowledge of the role of centrifugal fibers from the olfactory cortical areas is limited. Judging from recent morphological evidence, these fibers may modify the activity of neurons located in sparse and discrete loci in the olfactory bulb. Given the modular organization of the olfactory bulb, centrifugal fibers from the olfactory cortex may help coordinate the activities of restricted subsets of neurons belonging to distinct functional modules in an odor-specific manner. Because the olfactory cortex receives inputs from limbic and neocortical areas in addition to inputs from the bulb, the centrifugal inputs from the cortex can modulate odor processing in the bulb in response to non-olfactory as well as olfactory cues.     

Expression of connexin 57 in the olfactory epithelium and olfactory bulb

In the visual system, deletion of connexin 57 (Cx57) reduces gap junction coupling among horizontal cells and results in smaller receptive fields. To explore potential functions of Cx57 in olfaction, in situ hybridization and immunohistochemistry methods were used to investigate expression of Cx57 in the olfactory epithelium and olfactory bulb. Hybridization signal was stronger in the olfactory epithelial layer compared to the connective tissue underneath. Within the sensory epithelial layer, hybridization signal was visible in sublayers containing cell bodies of basal cells and olfactory neurons but not evident at the apical sublayer comprising cell bodies of sustentacular cells. These Cx57 positive cells were clustered into small groups to form different patterns in the olfactory epithelium. However, individual patterns did not associate with specific regions of olfactory turbinates or specific olfactory receptor zones. Patched distribution of hybridization positive cells was also observed in the olfactory bulb and accessory olfactory bulb in layers where granule cells, mitral cells, and juxtaglomerular cells reside. Immunostaining was observed in the cell types described above but the intensity was weaker than that in the retina. This study has provided anatomical basis for future studies on the function of Cx57 in the olfactory system.     

Dendritic stability in the adult olfactory bulb

In many regions of the adult mammalian brain, pronounced changes in synaptic input caused by lesions or severe sensory deprivation induce marked sprouting or retraction of neuronal dendrites. In the adult olfactory bulb, adult neurogenesis produces less pronounced, but continuously ongoing synapse turnover. To test the structural stability of adult dendrites in this context, we used two-photon microscopy to image dendrites of mitral and tufted (M/T) cells over prolonged periods in adult mice. Although pharmacologically increased activity could elicit morphological changes, under natural conditions such as ongoing neurogenesis, an odor-enriched environment or olfactory-based learning, M/T cell dendrites remained highly stable. Thus, in a context of ongoing adult synaptogenesis, dendritic stability could serve as a structural scaffold to maintain the organization of local circuits.