Electrophysiological studies of centrifugal and centripetal connections of the anterior olfactory nucleus

A field guide to the anterior olfactory nucleus (cortex)

While portions of the mammalian olfactory system have been studied extensively, the anterior olfactory nucleus (AON) has been relatively ignored. Furthermore, the existing research is dispersed and obscured by many different nomenclatures and approaches. The present review collects and assembles the relatively sparse literature regarding the portion of the brain situated between the olfactory bulb and primary olfactory (piriform) cortex. Included is an overview of the area's organization, the functional, morphological and neurochemical characteristics of its cells and a comprehensive appraisal of its efferent and afferent fiber systems. Available evidence suggests the existence of subdivisions within the AON and demonstrates that the structure influences ongoing activity in many other olfactory areas. We conclude with a discussion of the AON's mysterious but complex role in olfactory information processing.

Interbulbar reciprocal inhibition in frog olfaction

Frogs were submitted in acute experiments to electrical stimulation of both ipsi- and contralateral homologous olfactory nerve bundles subserving the dorsal olfactory mucosae. Mitral layer responses on one side were recorded. When both stimulations were given at different onset times and equal intensity, the ipsilateral mitral layer response fell to a sharp minimum of 42% when the contralateral stimulation was given 25 ms before the ipsilateral one. When both stimuli were adjusted for maximal temporal inhibition and the intensity of the contralateral stimulation was varied, the amplitude of the ipsilateral response decreased as the intensity of the contralateral stimulation increased until intensity equalization was obtained. This interbulbar inhibition was suppressed either by sagittal transection through the interbulbar adhesion or local infusion of bicuculline in the ipsilateral olfactory bulb. It was unaffected by transection of habenular commissure. These findings suggest first, that the neuroreceptors located in the dorsal olfactory mucosae project to both olfactory bulbs, and second, that it exists at mitral layer level of the frog, a reciprocal inhibition which can subserve frog orientation towards proximal odour source.

Effects of locus ceruleus and olfactory bulb stimulation on rat olfactory tubercle neuron activity

The effects of electrical stimulation of the olfactory bulb and the locus ceruleus on olfactory tubercle neurons were examined in rat models. Ipsilateral stimulation of the olfactory bulb produced excitation in 31% of olfactory tubercle neurons tested and inhibition in 17%. Twenty-two percent of the olfactory tubercle neurons were excited, whereas 9% were inhibited by ipsilateral stimulation of the locus ceruleus. Contralateral stimulation of the locus ceruleus produced similar responses in the same neuron entities. A negative-positive evoked potential was recorded in the olfactory tubercle after ipsilateral and contralateral stimulation of the locus ceruleus. Thirty-three percent of the olfactory tubercle neurons that responded orthodromically or antidromically to stimulation of the olfactory bulb were excited by ipsilateral stimulation of the locus ceruleus. In contrast, only 10% responded with excitation to ipsilateral stimulation of the locus ceruleus among the olfactory tubercle neurons that were unresponsive to stimulation of the olfactory bulb. These findings suggest that olfactory tubercle neurons that receive input from or sending output to the olfactory bulb are influenced by the noradrenergic system of the locus ceruleus. A possible role of the olfactory tubercle in olfactory transduction will also be discussed.

Lesions of the olfactory pathways affecting neophobia and learned aversion differentially

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.

Neurobehavioral evidence for the involvement of the vomeronasal system in mammalian reproduction

Jacobson's organ of the vomeronasal system is found in every order of mammals with the possible exception of Cetacea. The equivocal evidence claiming a vestigial or absent organ in humans is reviewed. Based upon anatomical considerations, the sensory epithelium of Jacobson's organ is one of five possible sensory components within the nasal cavity. Many methods designed to test the role of olfaction (sensu strictu) in physiology and behavior do not discriminate among the possible systems. Therefore, erroneous conclusions may have been drawn from the results of intervention experiments. The central neuroanatomical projections of the vomeronasal and olfactory systems are different and relatively independent of each other. The vomeronasal system reciprocally communicates with central areas concerned with reproductive events. On the other hand, the olfactory system may subserve individual maintenance tasks (e.g., feeding). As a periscope from the diencephalon, the vomeronasal system may monitor exogenous hormones, "pheromones".

Unilateral lesions of the olfactory tubercle modifying general arousal effects in the rat olfactory bulb

The centrifugal control exerted by different arousal states on the rat olfactory bulb was investigated. The olfactory tubercle was unilaterally coagulated with either 1 mA or 3 mA current. The vigilance state parameters and multiunit mitral cell activity were recorded in freely moving rats, stimulated either by their usual food odor or by isoamyl acetate, in a hungry or a satiated state. In each animal, a unilateral lesion affected resting activity and the relative proportion of positive (excitatory) and negative (inhibitory) responses in the same way in both olfactory bulbs; these effects were proportional to the extent of the lesion. In wakefulness, a nutritional modulation of the bulb responses for food odor existed in spite of a slight decrease in the general arousal level. In slow wave sleep (SWS), when compared to a control group, the rats with lesions showed an increase of neocortical desynchronization induced by olfactory stimulation, and a large decrease of mitral cell excitability. Inhibition of the olfactory input, which normally develops during SWS, could involve mainly mesencephalic neurons reaching the olfactory tubercle and the bulb via the ventral part of the medial forebrain bundle.

Qualitative and quantitative odour discrimation by mitral cells as compared to anterior olfactory nucleus cells

The responses to odour stimulation of 61 single mitral cells and of 64 single anterior olfactory nucleus (AON) units were simultaneously recorded from rabbits. The test battery consisted of 6 chemical stimuli delivered at two intensity levels corresponding to a hundred-fold difference in concentration. The odour discrimination reaches its best for the mitral cells stimulated weakly; it decreased somewhere when the same cells are stimulated strongly. At AON level the odour discrimination already limited in the case of weak stimuli, almost disappears for strong stimuli. Multidimensional analysis of the odour similarities, respectively in the olfactory bulb and the AON, reveals an unexpectedly high loading of the first axis with an intensity factor. Our results indicate that the discriminatory ability in the mitral cells and in the AON cells are respectively better and poorer than in the chemoreceptors.

Olfactory relationships of the telencephalon and diencephalon in the rabbit. III. The ipsilateral centrifugal fibers to the olfactory bulbar and retrobulbar formations

The axoplasmic retrograde transport of horseradish peroxidase (HRP) from axon terminals to their parent cell bodies and histochemical fluorescence microscopy have been used to study the ipsilateral centrifugal fibers to the olfactory bulbs and anterior olfactory nucleus in the rabbit. Focal injections of peroxidase were placed unilaterally into the main or accessory olfactory bulb or into the anterior olfactory nucleus. In animals with injected HRP confined within the main bulb, perikarya retrogradely labeled with the protein in the ipsilateral forebrain were observed in the anterior prepyriform cortex horizontal limb of the nucleus of the diagonal band, and far lateral preoptic and rostral lateral hypothalamic areas. Brain stem cell groups that contained HRP-positive somata include the locus coeruleus and midbrain dorsal raphe nucleus. Except for the prepyriform cortex, the basal forebrain structures with labeled perikarya correlate well with locations of cell bodies containing acetylcholinesterase and choline acetyltransferase. These somata may represent a cholinergic afferent system to the main olfactory bulb. Peroxidase-labeled cell bodies in the locus coeruleus and midbrain raphe are indicative of noradrenergic and serotonergic innervations respectively of the olfactory bulb. In rabbits in which peroxidase was injected or diffused into the accessory olfactory bulb and anterior alfactory nucleus, HRP-positive somata were identified in the prepyriform cortex bilaterally, the horizontal limb of the diagonal band nucleus, lateral hypothalamic region, nucleus of the lateral olfactory tract, corticomedial complex of the amygdala, mitral and tufted cell layers of the ipsilateral main olfactory bulb, locus coeruleus, and the midbrain raphe. Evidence for centrifugal fibers to the accessory olfactory bulb from the corticomedial complex of the amygdala, locus coeruleus, and possibly the nucleus of the lateral olfactory tract and midbrain raphe is discussed. A similar distribution of labeled perikarya in the forebrain and brain stem was seen in rats in which peroxidase injected into the main olfactory bulb had spread into the accessory bulb and anterior olfactory nucleus. Histochemical fluorescence microscopy of the main and accessory olfactory bulbs in the rabbit and rat revealed fine caliber, green fluorescent fibers and varicosities predominantly in the granule cell layer and less so among cells in the glomerular layer. In sections through the root of the main olfactory bulb, a similar fluorescence was seen in the deep half of the plexiform layer of the pars externa of the anterior alfactory nucleus. These fluorescent fibers likely represent the noradrenergic innervation of the olfactory bulbar and retrobulbar formations. A fluorescent yellow hue was observed in the glomerular layer of the main bulb and may signify a serotonergic innervation of this lamina...