Midbrain Circuits Research Articles

Evaluation of extrahypothalamic control of reproductive physiology.

Extrahypothalamic areas such as amygdala, hippocampus, and midbrain are anatomically and functionally closely connected to the mediobasal hypothalamus, which is held responsible for the secretion of releasing factors controlling anterior pituitary-gonadal function. Experimental evidence clearly assigns a role to limbic structures for the onset of puberty. In both immature and adult female and male individuals, limbic and midbrain areas concentrate gonadal steroids which can be related to reproductive physiology. Likewise, it is possible to alter pituitary hormone secretion by changing activities of the limbic midbrain circuit, or associate electrical activity of these brain areas with reproductive patterns. However, we are still confronted with our ignorance on why changes are brought about and how systems like the limbic midbrain circuit are interacting.

Neural systems responsible for the gastric secretion provoked by 2-deoxy-D-glucose cytoglucopoenia.

1. The central structures responsible for the gastrosecretory effect of cytoglucopoenia caused by 2-deoxy-D-glucose (2-DG) were investigated in 105 cats prepared with chronic gastric fistulae and subjected to various experimental procedures. 2. Bilateral electrolytic lesion of the caudalmost two thirds of globus pallidus almost suppressed the secretory response and caused aphagia and adipsia. 3. Secretion in response to 2-DG and feeding behaviour were entirely blocked after making a lesion in a large ventromedial area of the meso diencephalic transition comprising the ventral tegmental area of Tsai, the ventral tegmental decussation, the red nucleus, a ventral portion of the central grey matter, the interstitial nuclei of Darkschewitsch and of Cajal, the pre-rubral fields, the reticular part of substantia nigra, the internal portion of the cerebral peduncle and the ventral part of the mesencephalic reticular formation. 4. Microinjection of 2-DG in the medial forebrain bundle, at the level of the hypothalamus, caused intense gastric secretion, whereas the same procedure was totally ineffective when the caudalmost two thirds of the globus pallidus were stimulated. 5. Increasing doses of 2-DG, systemically injected, restored the secretory response in volume and acid concentration and output after intercollicular transection of the brain stem. After the transection, secretion of pepsin was only slightly increased when large doses of 2-DG were administered, thus suggesting a differential control of water, acid and pepsin secretion in response to cytoglucopoenia. 6. It is concluded that there are at least three reflex systems involved in gastric secretion due to cytoglucopoenia: (a) a reflex consisting of afferent and efferent pathways in the medial forebrain bundle area; (b) a reflex whose afferent side is from the hypothalamus and efferent side is from the globus pallidus; (c) a reflex with the afferent side probably originating in the liver and the efferent side in the lower brain stem. 7. The pathways involved in the first two arcs run along Nauta's limbic mid-brain circuit. The three systems are possibly related to control of secretion and feeding behaviour.

An electrographic study of the hippocampus and neocortex in unrestrained rats following septal lesions

1. 1. Electrograms were recorded from several neocortical areas and from the dorsal hippocampus in 52 unanesthetized, unrestrained rats during various stages of wakefulness and sleep before and after lesions were produced in the septal region. 2. 2. The phase of activated sleep was clearly distinguishable from normal wakefulness and sleep by the animals' behaviour and by the EEG patterns. Regular 6–8/sec waves were recorded in the hippocampus and from electrodes placed on visual cortex and frontal or parietal cortex in the awake, alert animal. During activated sleep episodes, these waves were enhanced in amplitude and regularity. Asynchronous low voltage fast activity recorded from other neocortical areas during activated sleep was easily recornized by the presence of periodic trains of low voltage regular spindle activity. 3. 3. Large septal lesions abolished 6–8/sec rhythms in neocortex and hippocampus during periods of wakefulness and activated sleep. In addition, the hippocampal EEG was often severely attenuated in amplitude. However, the EEG alerting reaction was easily elicited post-operatively from neocortex and hippocampus. Except for loss of 6–8/sec waves, the neocortical EEG patterns recorded during activated sleep post-operatively were distinguishable from those recorded before septal lesions. The hippocampal electrograms recorded during activated sleep, though severely altered post-operatively, were clearly different than those electrograms recorded during normal wakefulness and sleep. 4. 4. It is concluded that although ascending fibers of the “limbic midbrain circuit” which course through the septum contribute significantly to hippocampal and neocortical afferent systems, they do not play a major role in the control of wakefulness and sleep in the rat.