The termination of centrifugal fibres in the olfactory bulb

The mitral cells (MCs) of the olfactory bulb (OB) are relay neurons between the periphery and the central nervous structures. MCs receive in turn a centrifugal control from several higher brain centers that depends on the nutritional state. In this study, we investigated the effects of orexin A (ORX), a novel molecule known to regulate food intake and whose receptors are present in the OB, on the electrophysiological activity of single MCs. Using icv-injections and direct applications on the OB, we determined the respective central and local effects of this molecule on the MCs' spontaneous firing activity and responsiveness to different odors. Icv-injections and local OB-applications were found to induce a significant decrease in spontaneous firing activity in 14% and 50% of the recorded MCs, respectively. In one case, ORX application on the OB caused a significant firing increase. Effects of OB-applications had shorter delays. The responsiveness of some MCs to food and non-food odors was also changed, but the proportion of changes was not statistically significant. Icv-injection effects likely resulted from a local action of ORX on the OB. Changes of spontaneous firing activity and odor responsiveness are discussed in terms of regulation of the functioning of the olfactory system.

The effects of centrifugal afferents on membrane potentials of identified granule cells in the main olfactory bulb were studied in anaesthetized rats. Cells were located in the granule cell layer using evoked field potential profiles, and trans-synaptic activation via antidromic stimulation of output cell axon collaterals. Intracellular recordings maintained for 4–30 min showed complex spontaneous spike dis-charges and allowed characterization of the cell's input resistance, and on some occasions its morphology following intracellular injection of Lucifer Yellow. Stimulation in the nucleus of the horizontal limb of the diagonal band, but not surrounding regions, produced hyperpolarizing responses in 13 of 27 cells in the granule cell layer; four of these were morphologically identified as granule cells of two types, in five the responses had reversal potentials more negative than the resting potential, and six were identified as granule cells by monosynaptic activation from output axon collaterals. A different set of three cells in the granule cell layer responded with depolarization.

The results are consistent with the inhibition of tonic activity of granule cells by the nucleus of the horizontal limb of the diagonal band, leading to disinhibition of mitral and tufted cells via dendrodendritic synapses of granule cells on mitral/tufted cell secondary dendrites.

We present intracellular data which demonstrates a unique facilitatory centrifugal influence on the output cells of the olfactory bulb; the source being the lateral component of the nucleus of the horizontal limb of the diagonal band (HDB), part of the basal forebrain magnocellular complex. Damage to this facilitatory HDB influence may explain the loss of olfactory sensitivity seen early in Alzheimer's disease in which pathological changes occur in the basal forebrain.

The branching pattern of axons arising from cells in the basal forebrain and laterodorsal tegmental nucleus of the pontine grey, was examined using the double retrograde transport of rhodamine- and coumarin-labelled latex microspheres injected into the olfactory bulb and the mediodorsal thalamic nucleus. About 30–50% of basal forebrain neurones were double-labelled, and approximately 30–40% of laterodorsal tegmental neurones were double-labelled ipsilateral to the injections. No contralateral single or double label was observed in the basal forebrain, but approximately 30% of the contralateral laterodorsal tegmental projection was also double-labelled.

The autoradiographic distributions of μ opiate receptors, labeled in vitro by [¹²⁵I]d-Ala²-MePhe⁴-Met(o)⁵-ol-enkephalin (FK), and δ-opiate receptors, labeled by [³H]d-Ala²-d-Leu⁵-enkephalin (DADLE) in the presence of oxymorphone to block high affinity binding to the μ site, were examined and compared in the forebrain of the rat. The μ- and δ-receptors were differentially distributed in most structures. μ Binding sites were found in nearly all gray matter structures and showed heterogeneous patterns of density that were correlated with cytoarchitecture and neuronal connections. Laminar density profiles were seen in laminated structures such as olfactory bulb, cerebral cortex and hippocampus. Highest μ binding densities were in striatal patches and the habenular streak. δ Sites had distinct laminar patterns in the main olfactory bulb and cortex which differed from the μ patterns. The external plexiform layer of the main olfactory bulb had the greatest density of δ binding sites; cortex and striatum were also densely labeled. The septum, globus pallidus, preoptic area and hypothalamus were lightly labeled by both ligands. The magnocellular hypothalamic nuclei had negligible μ and δ labeling. The thalamus had dense μ but sparse δ sites. μ And δ binding sites were both present in the amygdala but had different distributions. Two fibers tracts — optic tract and fasciculus retroflexus — had FK labeling. In contrast, a portion of the corpus callosum was labeled by DADLE and not by FK. The results suggest an association of μ-opiate receptors with sensory, especially olfactory, and limbic projections in the forebrain, and δ-opiate receptors with intrinsic and commissural forebrain pathways.

Choline acetyltransferase (ChAT) activity, acetylcholinesterase (AChE) activity and muscarinic cholinergic receptor binding were determined in homogenates of olfactory bulbs from rats killed at intervals from 4 days before through 60 days after birth. In addition, the localization of muscarinic receptors was determined using an in vitro autoradiographic technique in 6-μm thick coronal sections of olfactory bulbs from rats killed at similar intervals after birth. All 3 cholinergic parameters were present in measurable quantities at birth and showed substantial increases between 1 and 20 days after birth. The most rapid increase in cholinergic parameters occurred between days 10 and 20 after birth. ChAT activity and muscarinic receptor binding decreased between days 20 and 35 and increased again between postnatal days 35 and 60. A similar developmental pattern was observed for autoradiographic grain density overlying the granule cell layer of the neonatal bulb. These data suggest that (1) centrifugal cholinergic afferents are present in the rat olfactory bulb at birth, (2) during the early postnatal period (between 10 and 20 days) synaptogenesis occurs resulting in an overproduction of cholinergic synapses and (3) between postnatal days 20 and 35, a period of synaptic reorganization occurs characterized by substantial regression.