Olfactory pathway evoked potentials in response to hypothalamic stimulation

Centrifugal projections to the main olfactory bulb revealed by transsynaptic retrograde tracing in mice

A wide range of evidence indicates that olfactory perception is strongly involved in food intake. However, the polysynaptic circuitry linking the brain areas involved in feeding behavior to the olfactory regions is not well known. The aim of this article was to examine such circuits. Thus, we described, using hodological tools such as transsynaptic viruses (PRV152) transported in a retrograde manner, the long-distance indirect projections (two to three synapses) onto the main olfactory bulb (MOB). The ß-subunit of the cholera toxin which is a monosynaptic retrograde tracer was used as a control to be able to differentiate between direct and indirect projections. Our tracing experiments showed that the arcuate nucleus of the hypothalamus, as a major site for regulation of food intake, sends only very indirect projections onto the MOB. Indirect projections to MOB also originate from the solitary nucleus which is involved in energy homeostasis. Other indirect projections have been evidenced in areas of the reward circuit such as VTA and accumbens nucleus. In contrast, direct projections to the MOB arise from melanin-concentrating hormone and orexin neurons in the lateral hypothalamus. Functional significances of these projections are discussed in relation to the role of food odors in feeding and reward-related behavior.

Nucleus of the tractus solitarius projections to the olfactory tubercle: an HRP study

Labelled cells were found in the nucleus of the tractus solitarius (NTS) after horseradish peroxidase injections in the olfactory tubercle (OT) of the rat. These results suggest a direct pathway from the NTS to the OT. The importance of this pathway in a neural circuit related to autonomic functions is discussed.

Vagus nerve stimulation modifies the electrical activity of the olfactory bulb

Evoked potential and unit activity recording techniques were used to study the effects of the vagus nerve stimulation on the olfactory bulb. A biphasic potential was evoked in the olfactory bulb by a single pulse delivered to the vagus nerve. Half of the neurons studied decreased discharge frequency after single pulse or train stimulation. The interval during which neurons ceased activity corresponded to the duration of the negative wave of the evoked potential. Responsive neurons were marked with horseradish peroxidase applied iontophoretically. Responsive neurons were located in the periglomerular layer of the olfactory bulb. These results suggest the existence of a vagus nerve-olfactory bulb pathway. The functional significance of this pathway is discussed.

Differential projections from locus coeruleus to olfactory bulb and olfactory tubercle: an HRP study

The microiontophoretic administration of horseradish peroxidase (HRP) to the olfactory bulb (OB) or olfactory tubercle (OT) in cats and rats yielded similar results in both species. After an OB HRP-injection ipsilateral and contralateral labelled neurons were seen in the piriform cortex, polymorphic layer of OT, magnocellular preoptic region, lateral hypothalamus, ventromedial hypothalamic nucleus and locus coeruleus (LC). In both species more labelled structures were found after an OT HRP-injection than after an OB HRP-injection. The substantia nigra in rats was more abundantly labelled after an OT injection than after an OB one. In cats the dorsal and the ventral raphe were also labelled. In either species, OT HRP-injections resulted in a higher frequency of LC labelled neurons than after OB injections. These results favor the hypothesis that the OT plays an important role as a relay station for efferent inflow from the brain stem en route to the OB.

Locus coeruleus influences upon the olfactory tubercle

Locus coeruleus (LC) influences upon the olfactory tubercle (OT) were studied. Evoked potentials and unit activity recording techniques were used as well as the horseradish peroxidase (HRP) labelling method. A biphasic potential was recorded in cat's OT following a single pulse delivered to the LC. Latencies were 35 msec and 80 msec for the first positive and second negative component respectively. The largest potentials were recorded between the superficial part of the pyramidal layer and the polymorphic layer. This potential was tested in order to determine whether it was an OT generated potential caused by LC stimulation. Contralateral recordings were made. Extracellularly recorded OT neurons decreased their discharge frequency upon repetitive LC stimulation in rats. The decreases endured longer when the stimulus frequency or train duration were increased. Neurons from which unit data were used in the analysis were verified to be in the OT polymorphic layer. Labelled neurons were seen in the LC after HRP application to the OT in rats and cats. Results support the presence of LC fibers which influence OT neurons. The relation between the LC as a general arousal system and the OT as an important olfactory information processing structure is discussed.