H.U. Aguilar-Baturoni

Afferent connections to the lateral hypothalamus: A horseradish peroxidase study in the rat ☆

Horseradish peroxidase, 13% Sigma Type VI, was administered iontophoretically to the mid lateral hypothalamus (LH) of male hooded rats. Animals were perfused intracardially on the following day and brains were removed and sliced in the coronal or sagittal planes into 30-50 micrometer sections. Sections were processed with DAB and BDH for the brown and blue reaction products and later examined by bright and dark field microscopy for the presence and location of retrogradely labeled neurons. Results indicate that a significant number of afferent connections to the LH originate in the olfactory and accumbens nuclei, pyriform cortex, olfactory tracts, magnocellular and medial preoptic and anterior hypothalamic regions, stria terminalis, stria hypothalamic tract, diagonal tract of Broca, caudate-putamen and globus pallidus, internal capsule, lateral septal nuclei, lateral preoptic area and anterior medial forebrain bundle, the various amygdaloid nuclei, zona incerta, perifornical region, dorsal and ventral medial hypothalamic areas, supraoptic, paraventricular and periventricular nuclei, posterior hypothalamus and medial forebrain bundle, ventral thalamic nuclei, the fields of Forel, arcuate and mammillary nuclei, adjacent to the fasciculus retroflexus, in the ventral tegmental area of Tsai, interpeduncular nucleus, substantia nigra, mesencephalic reticular formation, periaqueductal gray, locus coeruleus and parabrachial region. Results are discussed in terms of previous anatomical and neurophysiological data, probable pathways, and the function of LH neurons.

Effects of cervical vagus nerve stimulation on hypothalamic neuronal activity

The effects of cervical vagus nerve stimulation on the activity of 56 neurons recorded in various parts of the rat brain were determined. Recordings were made from neurons in both the ipsilateral and contralateral hemispheres during vagus nerve stimulation. Both frequency, 10 to 100 Hz, and voltage, 1 to 20 V, of 0.5 msec pulses were applied to the nerve in a random manner over a 4 sec period while monitoring ongoing single neuronal activity. Frequency response relationships were established for 64% of the lateral preoptic-lateral hypothalamic-medial forebrain bundle (LPA-LH-MFB) neurons which were tested. Four types of LPA-LH-MFB neuronal responses were observed. Twenty percent of the neurons increased in discharge frequency as stimulation frequency was increased, 9% increased in discharge frequency as stimulation frequency was decreased, 23% decreased in discharge frequency as stimulation frequency was increased, and 14% decreased in discharge frequency as stimulation frequency was decreased. Increasing the stimulation voltage always enhanced the magnitude of the effects observed due to changing the stimulation frequency. Neurons observed in some other parts of the brain were not affected by the same stimulation. In addition, cells tested in the LPA-LH-MFB area which were previously tested and affected by gastric distension were also affected similarly by vagus nerve stimulation. Results are discussed in terms of peripheral afferent control over LPA-LH-MFB neuronal activity related to ingestive behavior.

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.

A bipolar electrode for peripheral nerve stimulation.

The construction of a bipolar electrode for the stimulation of peripheral nerves is described. The electrode was designed for acute stimued in which the effects of peripheral nerve stimulation on central neural activity in rats were determined. Since the electrode can be secured into a fixed position, neural recordings can be made during the stimulation of various visceral nerves and the electrode can be easily adapted for chronic behavioral studies.

Lateral preoptic area and lateral hypothalamic interconnections demonstrated by horseradish peroxidase

Abstract Horseradish peroxidase, 30% Sigma Type VI, was administered iontophoretically to the lateral preoptic area (LPA) and lateral hypothalamus (LH) of male hooded rats. Animals were perfused intracardially on the following day, brains were removed and later sliced into 30–50 μm sections and processed with DAB and BDH for the brown and blue reaction products. Both LPA and LH ejections of horseradish peroxidase (HRP) resulted in the extensive labeling of soma and axons along the entire extent of the LPA, LH and the medial forebrain bundle (MFB). In addition, ejections in both areas resulted in similar labeling in the nucleus accumbens, stria hypothalamic tract, stria terminalis, medial hypothalamic area, amygdaloid nuclei, and the zona incerta. These results demonstrate a considerable degree of interconnectivity within the LPA and LH along the entire extent of the MFB and that presynaptic inputs from other adjacent hypothalamic and limbic areas terminate on neurons in the LPA-LH-MFB neuropil.

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.

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 cats 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.

Olfactory bulb neurons respond to gastric distension

Single unit discharges in the olfactory bulb (OB) were recorded extracellularly in anesthetized rats during controlled gastric distension. The activity of 37 neurons in the periglomerular layer of the OB modulated by gastric distension was analyzed. Twelve of the 37 cells decreased, eight decreased and then increased in frequency, and 17 displayed more complex effects due to gastric distension. Generally, the latency of the responses to gastric distension was less than one second. All those cells which responded to gastric distension also responded to vagus nerve stimulation. This effect was abolished or attenuated by cutting the vagus nerve at the cervical level. The threshold volume by which the stomach was infused for the initial decrease in OB neuronal discharge frequency was 3.0 ml. When more than 8 ml were infused, increases in firing frequencies were always observed. The results of the present experiments indicate that the discharge frequency of the periglomerular OB neurons are modulated by visceral afferents.

Efferent evoked responses in the olfactory pathway of the cat

Abstract Evoked potentials were recorded in the olfactory bulb, lateral olfactory tract, olfactory tubercle, prepyriform cortex and anterior commissure of curarized cats, after stimulation of various areas of bulbo-mesencephalic reticular formation. The potentials in all the recorded olfactory stations were bilateral, with identical peak latencies for homologous structures, but with shorter latencies for the olfactory tubercle (4.9 msec) than for the olfactory bulb (5.5 msec). The latency for the prepyriform cortex was 8.5 msec. The evoked potentials in the olfactory bulb were abolished by section of the lateral olfactory tract, and by electrolytic lesion or transient blockage of the olfactory tubercle by topoligically applied procaine. Section of the anterior commissure or prepyriform cortex did not abolish the evoked potentials in the olfactory structures; on the contrary, a slight enhancement of amplitude was usually found. The destruction of the olfactory tubercle abolished the potentials evoked in the prepyriform cortex. The conduction velocity of the efferent fibers in lateral olfactory tract ranged between 5 and 10 m/sec. These data point to the existence of a pathway from the bulbo-mesencephalic areas to the olfactory tract, the olfactory tubercle being the common structure influencing the olfactory bulb and olfactory cortex.

Transient cessation of female rat sexual cycle after electrolytic damage of locus coeruleus

In order to evaluate the possible participation of the locus coeruleus (LC) in the regulation of estrous cyclicity, bilateral LC electrolytic lesions were made in intact female rats. Vaginal smears were sampled daily and the estrous cyclicity was established. LC lesions resulted in a transient loss of cyclicity characterized by a sustained diestrous period. The delay in recovery was dependent on the percentage of LC destroyed. The cyclicity of sham operated rats and of nonoperated rats showed no change. Results are discussed within the context of the LC as a generalized influencing system.