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While there is evidence that feedback projections from cortical and neuromodulatory structures to the olfactory bulb are crucial for maintaining the oscillatory dynamics of olfactory bulb processing, it is not clear how changes in dynamics are related to odor perception. Using electrical lesions of the olfactory peduncle, sparing output from the olfactory bulb while decreasing feedback inputs to the olfactory bulb, we demonstrate here a role for feedback inputs to the olfactory bulb in the formation of odor–reward associations, but not for maintaining primary bulbar odor representations, as reflected by spontaneous odor discrimination.

The intrinsic organization of the olfactory bulb (OB) was studied in the red stingray using the rapid Golgi method. The OB is horse shoe-shaped, surrounding the equator region of the nasal capsule. As seen in the sagittal sections, the OB is round with the long olfactory peduncle extending from the dorsocaudal region and the olfactory fibers in a thick bundle entering from the rostroventral aspect. Although not so distinct, the following areas are distinguished. A rostroventral ovoid area adjacent to the entrance of the olfactory fibers consists exclusively of the olfactory fibers running in various directions. Dorsocaudal to the olfactory fiber area is a wide crescent region containing thin bundles of olfactory fibers, olfactory glomeruli, mitral cells and a few disseminated granule cells. A narrow crescent area made up of scattered granule cells is located dorsocaudally to the above wide crescent area. The outermost region consists of a fiber layer encapsulating the dorsal to caudal aspect of the OB. Thus, while the major constituents of the vertebrate OB are recognized, the lamination is very obscure.

A nutritional modulation of electrical mitral cell multiunit responses was studied in three groups of rats, with either a unilateral section of the olfactory peduncle (OP) or of the lateral olfactory tract (LOT), or with unilaterally damaged olfactory mucosa (zinc sulphate treatment). The control and lesion-side activity was chronically recorded in the bulbs of hungry and satiated animals receiving series of 10 identical stimulations with a food odor and odors non-associated to nutrition. Patterns of positive, negative and null responses were thus collected. In the control-side bulbs of the LOT and OP groups, as in intact animals, the percentage of positive responses was maximal in hungry rats receiving food odor, and minimal in satiated rats receiving isoamyl acetate, as a result of the nutritional modulation of responses. In the lesion-side bulbs of the LOT group, activation increased irrespective of the nutritional state of the animal and the nutritional meaning of the odorant. This shift was reproduced in the lesion-side bulbs of the OP group. In these animals, no nutritional modulation was observed, an effect ascribed to the disruption of activating fibers in the anterior limb of the anterior commissure (AAC). The zinc sulphate treated animals displayed a lack of olfactory responses in the treated-side bulbs, and no response modulation in the control-side bulbs, where the percentage of positive responses for non-alimentary odors was slightly increased, presumably as a consequence of the suppression of interbulbar inhibition. The results are discussed in terms of the quantity of olfactory information necessary for the centrifugal nutritional modulation to be released. The neuronal circuits subserving these various effects in the bulb are considered.

Rats with restricted or large bilateral ablations of the olfactory bulb were subjected to a schedule of daily electrical stimulation of the amygdala (kindling). Seizure development progressed at the same rate in animals with damage restricted to the main olfactory bulbs and in sham-operated controls. Animals with large ablations, including the olfactory peduncle, developed seizures at a significantly faster rate. This result is interpreted as a disinhibition of limbic seizure-generating mechanisms by removal of neuroanatomically related olfactory areas.

The structure of the claustrum was studied in the dog brain using Weigert's, Klüver-Barrera's, Manns' and Nissl's methods. It consists of two main parts arranged one above the other. The dorsal part is situated in the depth of the neocortex and extends from the gyrus orbitalis to the gyrus compositus posterior. The ventral part of the claustrum underlying the olfactory cortex continues from the cuadal fragments of the olfactory peduncle to the entorhinal area, where it fuses with its deep layers. The claustrum can be regarded as a fragment of a bigger cellular formation present in different parts of the hemisphere.

Summary. 1 The external and internal anatomy of the brain have been described. 2 Characters suggesting Ungulate affinities have been found in the gyrus ambiens, in the fissuration of the cortex and in the distribution of the cortical areas. 3 Despite the high degree of osmatism, the brain as shown by the amount of cortical differentiation is by no means a very primitive one. On the other hand, it is much nearer the generalized type of Proto-Oarnivore or Ungulate brain than is Sus communis, for instance. 4 The brain of Orycteropus has advanced sufficiently far from the generalized type to suggest that it was advancing along the line of the Ungulates. 5 The anterior olfactory nucleus appears to be the same structure which previous writers called the olfactory peduncle with its prolongation into the bulb. Since its cell type differs from the other secondary olfactory centres, and it is continuous with the cellular laminae of the bulb, it more properly belongs to this structure than to the olfactory cortex. 6 It is thought that the wide extent and pattern of the secondary olfactory cortex needs no other explanation than the response to the macrosmatism. 7 The constitution of the amygdaloid nucleus conforms to the account given by Johnstone for other mammals.

The olfactory epithelium and the afferent nerve fibers to the olfactory lobes were destroyed in one group of rats, while in another the olfactory peduncle was destroyed bilaterally. Following either o

Summary Five nurse sharks were killed between 6 and 43 days after the olfactory tract had been cut unilaterally. Whereas the axon degeneration was demonstrated best with the Nauta-Laidlaw technique after 3–4 weeks survival time, the terminal degeneration, as identified in Fink-Heimer preparations, was best seen in a 13-day specimen. The distribution of olfactory tract fibers is entirely ipsilateral to the lesion, the majority terminating near the attachment of the olfactory peduncle to the hemisphere. Terminal degeneration is restricted caudally to a tongue-like extension of the lateral olfactory area on the ventral surface of the hemisphere with a spill-over into the area superficialis basalis of Johnston. The results indicate that the distribution of olfactory bulb fibers in the shark is much more restricted than had been suggested by most earlier descriptions based on studies of normal material.

A technique of cryogenic blockade was used in waking rabbits to produce complete and reversible isolation of the olfactory bulb from the rest of the brain. During cooling of the olfactory peduncle epileptiform activity occurred spontaneously in the pyriform cortex in 3 out of 20 sessions, but never in the bulb. Following removal of the cryoblockade, during the seizure state, epileptiform discharges appeared simultaneously in the bulb and pyriform cortex. In the control state, without cooling of the peduncle, epileptiform activity could be evoked in the bulb and cortex by intense electrical stimulation of either the bulb or the lateral olfactory tract. During the cryoblockade, however, intense stimulation of the bulb failed to evoke seizure-like discharges. The results demonstrate a dependency on more central olfactory structures for the induction and maintenance of epileptiform activity in the olfactory bulb.

The number and size of the axons in the later olfactory tract of the rat have been measured at several rostro-caudal levels, from material prepared for electron microscopy. Immediately caudal to the olfactory peduncle, an average of 42,000 +/- 3,000 axons were counted in the tract, while near the caudal limit of the tract an average of 32,000 +/- 2,800 axons were counted. The average internal cross-sectional area of axons measured at two levels of the tract was 1.6 +/- 1.3 mum2 and 1.1 +/- 0,9 mum2, corresponding to average internal diameters 1.4 +/- 1.3 mum and 1.2 +/- 1.1 mum, respectively. The axons in the lateral part of the tract were found to be significantly larger than those in the medial part of the tract; for one level the average cross-sectional area of axons in the lateral part of the tract was 1.6 +/- 1.0 mum2 (equivalent diameter 1.4 +/- 1,1 mum) while only 0.7 +/- 0.6 mum2 (equivalent diameter 0.9 +/- 0.9 mum) in the medial part of the tract. The thickness of the myelin sheath of the axons is generally related to axon diameter, increasing from 0.1-0.2 mum for axons 0.4 to 0.8 mum in diameter to 0.3-0.4 mum for axons greater than 2.0 mum in diameter. The ratio of the inside diameter to the outside diameter of the fiber (ratio "g") is between 0.7 and 0.8 for most axons in the lateral olfactory tract. The axons which leave the tract laterally and medially are substantially smaller than the axons within the tract (average cross-sectional area 0.55 +/- 0.35 mum2 on the lateral side) and probably are collaterals of the axons within the tract. Unmyelinated nerve processes, probably axons, were also found in the tract. Qualitative observations from light and electron microscopical material agree well with the quantitative data, and further suggest that mixing of axons from different parts of the olfactory bulb occurs in the rostral part of the tract. This is supported by experiments with injections of 3H-amino acids into localized portions of the olfactory bulb. These indicate that there is some degree of point-to-point organization in the most rostral part of the tract, but that this is lost in the caudal part of the olfactory peduncle.

The contribution of ascending olfactory pathways in neophobia and learned aversion to the same food was investigated in male rats bearing lesions of both olfactory peduncles, or one olfactory peduncle and the opposite lateral olfactory tract or anterior limb of the anterior commissure. The animals were fed on usual stock diet (S) offered as a choice with novel vanilla food (V) on test days: during neophobia (N), then before and after aversive conditioning (Aa, At). Daily food intake was measured, and the preference was expressed as V ( V + S ) . Experiment 1 included a neophobia test, before aversive conditioning (3 mEq/kg LiCl, i.p.). In Experiment 2, aversion only was studied (0.9 mEq/kg). In the neophobia test, the preference ratio was 7% in unoperated controls, and 43–52% in the 3 lesioned groups. The same controls had preference ratios equal to 64% and 22%, before and after aversive learning. Similar drops were observed for any lesioned group in Expt. 1. The decrease was less obvious, although significant, in rats of Expt. 2 with asymmetric lesions; those with both olfactory peduncles cut through maintained the same preference ratio (48%) before and after LiCl treatment. The data are interpreted assuming that: (1) lateral olfactory tract and anterior commissure both contribute to information processing in neophobia and aversion; (2) olfactory cues subserve neophobia prepotently; and (3) one cannot account for the sensory determinism of neophobia and aversion calling for a single mechanism.

TRITIATED thymidine ([ 3 H]thymidine) autoradiography at different times after experiments showed that postnatally generated neurones in the adult lizard olfactory bulbs are not generated in a local germinative zone but in the ventricular zone (VZ) of the cerebral hemispheres, which is a long distance from the olfactory bulbs. The new cells originating in the VZ of rostral telencephalon migrate to both main and accessory olfactory bulbs through the olfactory peduncle, along a highly restricted pathway orthogonal to the orientation of radial glial fibres. Our results suggest that the olfactory bulb interneurones of adult lizards may follow a tangential migration route from their germinative zone, as is the case in mammals.