Effects of electrical stimulation of the vagus nerve on the development of visual habituation in the cat.

The vagus nerve participates in the control and regulation of important autonomous functions, emotional tasks, and neural activity. Electrical vagus nerve stimulation (VNS) is an approved procedure for the treatment of refractory epilepsy in humans. VNS has also been shown to improve mood complaints and cognitive function in both human patients and animals. Thus, the purpose of this study was to analyse and describe the effects of VNS on the development and establishment of sensory habituation and electrographic activity of the visual pathway in freely moving cats. Six cats had implants placed in the optic chiasm (OC), lateral geniculate body (LGB), mesencephalic reticular formation (MRF), primary visual cortex (VC) of the left hemisphere, and left vagus nerve. Immediately after surgery, all cats presented anisocoria and relaxation of the left nictitant membrane. Also showed vegetative-type responses such as myosis, licking, and swallowing during VNS. Animals were then subjected to repeated luminous stimuli at intervals of 1 and 3s to cause habituation. The effect of VNS on the frequency and latency of the habituation episodes and the electrographic changes in the registered brain structures were analysed. Latency analysis showed that VNS delayed the first habituation episode. VNS had transitory effects on the neural activity of the primary visual pathway structures, which caused a small but measurable delay in the establishment of habituation. In conclusion, VNS interferes with the development and establishment of visual habituation, an elementary form of non-associative learning, in freely moving cats.

Retinohypothalamic projections in the common marmoset (Callithrix jacchus): A study using cholera toxin subunit B.

Retinal projections in vertebrates reach the primary visual, accessory optic, and circadian timing structures. The central feature of the circadian timing system is the principal circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The direct projections from the retina to the SCN are considered the entrainment pathway of the circadian timing system. In this study, unilateral intravitreal injections of cholera toxin subunit B were used to trace the retinal projections to the marmoset hypothalamus. The retinohypothalamic tract reaches the ventral suprachiasmatic nucleus bilaterally, as anticipated from previous studies. However, labeled fibers were found in several other hypothalamic regions, such as the medial and lateral preoptic areas, supraoptic nucleus, anterior and lateral hypothalamic areas, retrochiasmatic area, and subparaventricular zone. These results reveal new aspects of retinohypothalamic projection in primates and are discussed in terms of their implications for circadian as well as noncircadian control systems.

Projections of the basal retrochiasmatic area: a neural site involved in the photic control of pineal metabolism.

It has been shown that the basal retrochiasmatic area (RCHb), situated immediately ventral to the third ventricle behind the suprachiasmatic nucleus and in front of the arcuate nucleus, is implicated in the nocturnal inhibitory process of melatonin production induced by short-term retinal photo-stimulation. In the present study, the projections of the RCHb have been examined using the Phaseolus vulgaris leucoagglutinin (PHA-L) method in the rat. Considering the putative role of the RCHb in contributing to the short-term photo-inhibition of the pineal gland during the night, it is reasonable to suppose that the RCHb may ultimately inhibit the sympathetic outflow of the upper thoracic segments, known to be critically involved in the control of melatonin secretion. Of particular interest, the present anterograde tract-tracing study indicates all possible paths from the RCHb which may conceivably be involved in influencing the sympathetic outflow and, therefore, melatonin production. Thus, apart from a direct projection to the intermediolateral column at thoracic levels of the spinal cord, the RCHb is in a position to control the sympathetic outflow through other potential routes, such as the dorsal parvicellular part of the paraventricular nucleus of the hypothalamus, lateral hypothalamic area, ventromedial nucleus of the hypothalamus, lateral part of the periaqueductal gray and Barrington's nucleus.

Accessory optic-pretectal interactions in the pigeon.

This electrophysiological study analyzes the influence of the nucleus of the basal optic root (nBOR) of the avian accessory optic system on units within the lentiform nucleus (LM), which is the avian equivalent of the pretectal nucleus of the optic tract. A prominent depression of the spontaneous firing rate of neurons within the LM occurred following electrical stimulation of the ipsilateral nBOR. A close correlation was also found between the directional selectivity of LM units and the apparent displacements generated by rotations of the head around the horizontal semicircular canal axis. This is consistent with a possible role of the LM in the coordinate transformation from visual inputs to a vestibular reference system.

Evidence for recrossing of the pathway from the retina to the ipsilateral nucleus of the optic tract.

Single-unit recordings of the nucleus of the optic tract (NOT) under visual stimulation were performed in 5 opossums. Most of the units were directionally selective. Receptive fields for the contralateral eye lie mainly in the contralateral field while those for the ipsilateral eye were mainly in the ipsilateral field. As the nasal retina does not project ipsilaterally, recrossing must occur in the pathway from the retina to the ipsilateral NOT. Possible sites for this recrossing are discussed.

Accessory optic-pretectal interactions in the pigeon

This electrophysiological study analyzes the influence of the nucleus of the basal optic root (nBOR) of the avian accessory optic sustem on units within the lentiform nucleus (LM), which is the avian equivalent of the pretectal nucleus of the optic trat. A prominent depression of the spontaneous firing rat of neurons within the LM occurred following electrical stimulation of the ipsilateral nBOR. A close correlation was also found between the directional selectivity of LM units and the apparent displacements generated by rotations of the head around the horizontal semicircular canal axis. This is consistent with a possible rolo of the LM in the coordinate transformation from visual imputs to a vestibular referense system

Evidence for recrossing of the pathway from the retina to the ipsilateral nucleus of the optic tract

Single-unit recordings of the nucleous of the optic tract (NOT) under visual stimulation were performed in 5 opossums. Most of the units were directionally selective. Receptive fields for the contralateral eye lie mainly in the contralateral field while those for the ipsilateral eye were mainly in the ipsilateral field. As the nasal does not project ipsilaterally, recrossing must occur in the pathway from the retina to the ipsilateral NOT. Possible sites for this recrossing are discussed