Corticofugal effects on the neuronal activity in the emotiogenic/motivational zones of the hypothalamus

Neuronal responses to stimulation of the proreal (field 8) and cingular (field 24) cortices, pyriform lobe (periamygdalar cortex), and hippocampus (CA3) were studied in the lateral (HL) and ventromedial (Hvm) hypothalamus, dorsal hypothalamic region (aHd), and projection region of the medial forelimb bundle (MFB); single and repeated (series of a 6–300 sec⁻¹ frequency) stimuli were used. At single stimulations, the minimum proportion of inhibitory responses with respect to excitatory effects was observed when the neocortex (the proreal gyrus) was stimulated; this proportion became successively greater at stimulations of the intermediate cortex (the cingular gyrus) and paleocortex (the pyriform cortex), while stimulation of the archicortex (the hippocampus) evoked mostly inhibitory responses. At repeated stimulation of the cortical structures, inhibitory responses prevailed in the neurons under study: their total number was nearly four times larger than that of excitatory reactions. The response patterns to single and serial stimulations of the cortical structures allowed us to demonstrate: (i) significant diversity of the influences received by hypothalamic neurons from the cortical structures and (ii) the dependence of the pattern of these influences on the phylogenetic specificity of the above structures.     

Effects of intraoral mechanoreceptor stimulation on reticular formation neurones, in the rabbit

The effects of intraoral mechanoreceptor stimulation on the firing rate of single neurones of the brain stem reticular formation (RF) were investigated in rabbits. 30% of RF neurones responded to periodontal mechanoreceptor stimulation; 16% to mucosal mechanoreceptor stimulation and 6% to both types of stimuli. Periodontal stimulation induced mainly inhibitory effects localized within the mesencephalic and rostral pontine RF. Among periodontal afferents incisors were the most widely represented. The effects of mucosal mechanoreceptor stimulation were predominant in the medullary and pontine RF and they were mainly excitatory. The present results support the hypothesis that brain stem RF neurones can be recruited into regulating mastication and biting also by stimulation of intraoral mechanoreceptors.     

Neurochemical mechanisms in inhibitory effects of stimulation of the periaqueductal gray matter on high-threshold reflex activity of the reticular formation

A study was made of microinjections of antagonists of various neuromodulators on the dynamics of inhibition of the spino-bulbo-spinal reflexes which were evoked by stimulation of the central gray matter (PAG) in rats anesthetized with chloralose. Injections were made into the reticular gigantocellular nucleus (GN), which is the basic supraspinal center of this reflex. Administering methysergide (a blocker of serotonin receptors) was accompanied by significant (two to four times) diminution of inhibition evoked by PAG stimulation with a short, high-frequency series of stimuli. Long inhibition caused by long, rhythmic stimulation of the PAG was diminished less significantly: from 6–10 to 2.5–4 min. When the opiate receptors of the GN neurons were blocked with naloxone, duration of inhibition was reduced by two to five times. The most clearly expressed diminution of both types of inhibitions was noted with injections of haloperidol, an antagonist of catecholamines. Our data indicate that evidently all of these neuromediator (neuromodulator) systems participate in inhibition of high-threshold, reflex activity of the reticular formation evoked by stimulation of the PAG, but their participation in this process is unequal.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 4, pp. 455–463, July–August, 1991.     

Effect of local cooling of the cortex on evoked potentials in the reticular formation in response to somatosensory stimulation

Potentials evoked in nuclei of the reticular formation by electrodermal stimulation of the limbs were investigated in acute experiments on unanesthetized, immobilized rats during cooling of the somatosensory cortex in the area of representation of one forelimb. Evoked potentials in the reticular formation were found to depend on the degree of cold inhibition of the cortical primary response to the same stimulation. The peak time of the main negative wave increased from 40–50 to 60–80 msec with a simultaneous decrease in its amplitude or its total disappearance in the case of deep cooling of the cortex. Cooling of the cortex had a similar although weaker effect on the earlier wave of the evoked potential with a peak time of 14 msec, recorded in the ventral reticular nucleus. In parallel recordings of potentials evoked by stimulation of other limbs they remained unchanged at these same points of the reticular formation or were reduced in amplitude while preserving the same temporal parameters. Cooling of the cortex thus selectively delays the development and reduces the amplitude of the response to stimulation of the limb in whose area of representation transformation of the afferent signal into a corticofugal volley is blocked. Consequently the normal development of both late and early components of the potential evoked in the reticular formation by somatic stimulation requires an additional volley, descending from the cortex, and formed as a result of transformation of the same afferent signal in the corresponding point of the somatosensory cortex.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 1, pp. 32–38, January–February, 1981.     

Effects of stimulating the caudal zones of the mesencephalic reticular formation on the postcruciate cortex of the cat]

The influence of the electrical stimulation of the dorsolateral (DL) and the ventromedial (VM) portions of the caudal area of the mesencephalic reticular formation on the neuronal activity of the lateral zone of the gyrus sygmoideus posterior was investigated in cats. The mesencephalo-cortical influences were mainly excitatory in character; the predominance of the neuronal reactions with a short latency (3--12 msec) during the VM stimulation and reactions with a longer latency (20--25 msec) during the DL stimulation was revealed. The VM and the DL were found to influence both cortical neurones, the ones sensitive to somesthetic stimuli (mono-, di- and polyvalent) and the ones which failed to respond to the electrocutaneous stimulation of the limbs.     

Assessment of the reinforcing properties of self stimulation in rats using the "discordance" index]

Histogramms of the duration of pedal pressing (PDH) as a function of stimulation parameters, were studied in fifteen rats with electrodes inserted in the lateral hypothalamus and with fixed series of brain stimulation. The "discordance" parameter was used to estimate the changes in PDH modes relative to the moment of the end of brain stimulation. It was found that an increase in the stimulation charge brought about a gradual replacement of positive discordance (the duration of pressing exceeds that of stimulation) by a negative one (duration of pressing is shorter than that of stimulation) and a rise of negative discordance. It is assumed that in the case of positive discordance the duration of the total brain stimulation possesses reinforcing properties, while in the case of negative discordance the beginning of the stimulation possesses reinforcing properties, and its continuation becomes negative.     

Effect of phosphocreatine on reflex activity of the brain stem reticular formation

The effects of phosphocreatine (PCr) on startle reflexes, known to be relayed through the brain stem reticular formation (RF), were investigated on chloralose-anesthetized rats. PCr (10^–6 to 10^–3 M) was either applied superficially to the bottom of the fourth ventricle or microinjected into the reticular gigantocellular nucleus. The PCr effect was found to depend on its concentration. At low concentrations (10^–6 to 10^–5 M), PCr markedly facilitated the reflexes; sometimes its application gave rise to additional, later and longer, discharges following startle reflexes, whereas an inhibitory effect predominated at higher PCr concentration (10^–4 to 10^–3 M). Possible mechanisms of PCr action on the studied reflexes are discussed.Neirofiziologiya/Neurophysiology, Vol. 25, No. 3, pp. 272–278, 1993.     

Changes in collagen metabolism during the chronic electrical stimulation of the midbrain reticular formation

The content of 11-hydroxycorticosteroids, free and bound hydroxyproline in the peripheral blood, as well as the level of free and net hydroxyproline in the aortic wall and myocardium of rabbits were studied in chronic prolonged electrical stimulation of the mesencephalon reticular system. The experiments have shown that electrical stimulation leads to the activation of adrenal cortex function and is accompanied by alterations in collagen metabolism, accumulation of collagen in the aorta and decreased collagen level in the myocardium.