1. Synaptic transmission between olfactory receptor neurons and mitral/tufted cells was examined using a whole-cell recording technique in a hemisected preparation of the turtle olfactory bulb. To determine the olfactory receptor neuron transmitter, we isolated components of the synaptic response of mitral/tufted cells to olfactory nerve stimulation using postsynaptic receptor antagonists. 2. Low-intensity stimulation of the olfactory nerve evoked monosynaptic excitatory postsynaptic potentials in mitral/tufted cells that consisted of a rapid and prolonged depolarization with little contribution from other bulb neurons. The exogenous application of glutamate mimicked the response of mitral/tufted cells to olfactory nerve stimulation. 3. Olfactory nerve stimulation evoked in mitral/tufted cells a two component response that was reversibly blocked by glutamate receptor antagonists. The first, a rapid depolarization of short duration, was sensitive to the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX); the second, a depolarization of slower onset but longer duration, was sensitive to the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP5). When DNQX and AP5 were both present the postsynaptic response was completely abolished. These results strongly support the notion that glutamate is the neurotransmitter at the olfactory nerve to mitral/tufted cell synapse.
1. The olfactory bulb contains high concentrations of zinc and copper. Whole cell recording techniques were used to examine the modulatory actions of zinc and copper on N-methyl-D-aspartate (NMDA), gamma-aminobutyric acid (GABA), and glycine receptors on rat olfactory bulb neurons in culture and acutely isolated from adult animals. 2. Zinc and copper were effective antagonists of both NMDA-and GABA-mediated currents. The median inhibiting concentrations (IC50s) for zinc were 19 microM for NMDA receptors and 17 microM for GABA receptors. The IC50s for copper were 22 microM for NMDA receptors and 18 microM for GABA receptors. 3. Zinc and copper (100 microM) had no effect on the steady-state, desensitized component of currents evoked by high concentrations of glycine (300 microM). In contrast, when low, nondesensitizing concentrations of glycine (30 microM) were used, 100 microM zinc dramatically potentiated the current and 100 microM copper blocked the current. 4. The effects of zinc and copper on NMDA-, GABA-, or glycine-mediated currents were not voltage dependent, irrespective of whether the effect was potentiation or inhibition. 5. These results provide the first evidence for an inhibitory effect of copper on NMDA receptors, and the first evidence that the effects of zinc and copper on glycine receptors are dependent on the state of the receptor. These results suggest that endogenous zinc and copper may act as allosteric neuromodulators of amino acid receptors on olfactory bulb neurons. Furthermore, zinc and copper may provide a mechanism for differential modulation of inhibitory transmission because of their distinct effects on glycine versus GABA receptors.
The ['4C]2-deoxy-D-glucose (2DG) technique was employed to study the ontogeny of functional organization in the olfactory bulbs of rats from birth to 21 days postnatal. These observations were correlated with the histological maturation of the olfactory bulb laminae. In O-day pups (within 12 hr of parturition), foci of increased metabolic activity were elicited in the olfactory bulb by the odor of amyl acetate. The foci, generally poorly defined, were localized over the glomerular layer, which, in histological sections, was poorly differentiated. In suckling pups injected with 2DG, a single focus characteristically was observed in the main olfactory bulb near the medial border of the accessory olfactory bulb. In this region, a well differentiated complex of glomeruli was observed in the histological sections.During the ensuing 21 days postnatal, the focal patterns of 2DG uptake in animals exposed to amyl acetate odor progressed through a series of changes, each of which could be related to parallel stages of development in the histology of the bulb. Particularly striking were the establishment of sharply defined 2DG foci and the formation of distinct individual olfactory glomeruli by the end of the 1st week. Generally, by 15 days postnatal, the adult patterns of 2DG uptake and histological lamination were established.The modified glomerular complex also exhibited focal 2DG uptake beginning on day 0, but these were better defined than those associated with the glomeruli of the main olfactory bulb. This observation correlated with the more advanced histological differentiation of this specialized region. These data suggest that the modified glomerular complex may mature earlier than other regions of the olfactory bulb. This earlier maturation may reflect the importance of the modified glomerular region for processing odor cues relevant to suckling behavior in neonatal rats.Taken together, these results provide evidence for several successive changes in the functional organization of the developing olfactory system of neonatal rats.The neural substrates of olfactory function in the neonatal rat are not well understood. Behavioral studies generally agree upon the critical role of olfaction during the perinatal period of development (Blass and Teicher,
Olfactory transduction is thought to occur by processes that are mainly restricted to the specialized cilia emanating from the distal end of the receptor neuron's single dendrite. The involvement of a cAMP-based second messenger system seems likely, and a cyclic nucleotide-sensitive current has been recorded in patches of membrane from the cilia. However, the small diameter of the cilia and the high density of channels within the membrane limit the application of the patch recording technique in the cilia. We have found that the cAMP-sensitive channels also exist at a much lower density within the far more accessible dendritic membrane. Recording from on-cell patches, we have observed single-channel activity in response to extracellularly applied odor substances. The channels have a single-channel conductance of 40 pS and a reversal potential near 0 mV. These same channels are activated by treatments that elevate intracellular cyclic nucleotide concentrations. The results provide a direct demonstration that the cyclic nucleotide-gated channel is the conductance pathway for the odor-elicited current.
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