Responses of rat lateral hypothalamic neuronal activity to fastigial nucleus stimulation

1. The aim of this study was investigation of neuronal mechanisms underlying inputs from the fastigial nucleus (FN) to the lateral hypothalamic area (LHA). 2. In male anesthetized rats, 295 extracellular and 82 intracellular recordings of LHA responses to electrical stimulation of the FN, which elicited stimulus-locked pressor responses, were examined. 3. Contralateral FN stimulation evoked three types of responses in 48% of spontaneously firing LHA neurons: inhibition with 11 +/- 6 (SD) ms latency followed by excitation (30%), excitation with 15 +/- 12.5 ms latency (14%), and excitation followed by inhibition with 6 +/- 4 ms latency (4%). 4. Contralateral FN stimulation after transection of the inferior cerebellar peduncle (ICP), which resulted in a substantial fall of the fastigial pressor response, also evoked the three types of responses. These responses were unaffected by transection of the ICP. 5. Neuronal activity was recorded intracellularly from 82 LHA neurons, of which 36 (44%) responded to FN stimulation. Of the 36 neurons, 24 showed inhibitory postsynaptic potentials (IPSPs) with a mean latency of 7.5 +/- 2 ms. Of the 24 neurons, 16 were checked for change in IPSP latency with stimulus intensity, and 11 were considered to be monosynaptically connected since their latencies were constant when FN stimulation intensity was changed. The remaining 12 exhibited excitatory postsynaptic potentials (EPSPs) with a longer latency of 10.5 +/- 3 ms, which indicated polysynaptic conduction. The reversal potentials of the IPSP and EPSP were estimated to be about -77 mV and -13 mV, respectively. 6. Most glucose-sensitive neurons (78%), which were identified by their inhibition in response to electrophoretically applied glucose, were inhibited by FN stimulation, whereas only 7% of the glucose-insensitive neurons responded to such stimulation. 7. From the results, it was concluded that LHA neurons receive inhibitory monosynaptic and excitatory polysynaptic inputs from the FN via the superior cerebellar peduncle. These connections may contribute to hypothalamic modulation of feeding behavior.     

Lemniscal recurrent and transcortical influences on cuneate neurons

Intracellular recordings were obtained from cuneate neurons of chloralose-anesthetized, paralysed cats to study the synaptic responses induced by electrical stimulation of the contralateral medial lemniscus. From a total of 178 cells sampled, 109 were antidromically fired from the medial lemniscus, 82 of which showed spontaneous bursting activity. In contrast, the great majority (58/69) of the non-lemniscal neurons presented spontaneous single spike activity. Medial lemniscus stimulation induced recurrent excitation and inhibition on cuneolemniscal and non-lemniscal cells. Some non-lemniscal neurons were activated by somatosensory cortex and inhibited by motor cortex stimulation. Some other non-lemniscal cells that did not respond to medial lemniscus stimulation in control conditions were transcortically affected by stimulating the medial lemniscus after inducing paroxysmal activity in the sensorimotor cortex. These findings indicate that different sites in the sensorimotor cortex can differentially influence the sensory transmission through the cuneate, and that the distinct available corticocuneate routes are selected within the cerebral cortex. From a total of 92 cells tested, the initial effect induced by low-frequency stimulation of the sensorimotor cortex was inhibition on most of the cuneolemniscal neurons (32/52) and excitation on the majority of the non-lemniscal cells (25/40). The fact that a substantial proportion of cuneolemniscal and non-lemniscal cells was excited and inhibited, respectively, suggests that the cerebral cortex may potentiate certain inputs by exciting and disinhibiting selected groups of cuneolemniscal cells. Finally, evidence is presented demonstrating that the tendency of the cuneolemniscal neurons to fire in high-frequency spike bursts is due to different mechanisms, including excitatory synaptic potentials, recurrent activation through lemniscal axonal collaterals, and via the lemnisco-thalamo-cortico-cuneate loop.A corticocuneate network circuit to explain the results is proposed.     

Lateral hypothalamic area and paraventricular nucleus connections with subfornical organ neurons: an electrophysiological study in the rat

The neural pathways from the lateral hypothalamic area (LHA) to the hypothalamic paraventricular nucleus (PVN) mediated by subfornical organ (SFO) neurons were examined in urethane-anesthetized male rats in order to determine the excitability of vasopressin (VP)-secreting neurons in the PVN. Microinjection of angiotensin II (AII) into the LHA excited the activity of nearly half (n = 8) of the SFO neurons (n = 18) activated antidromically by electrical stimulation of the PVN. Microinjection of AII into the LHA also caused excitation of approximately one-quarter (n = 11) of putative VP-secreting neurons (n = 45) in the PVN while the excitatory responses of the putative VP-secreting neurons were blocked or attenuated by pretreatment with the AII antagonist, saralasin (Sar), in the SFO. Electrical stimulation of the LHA, on the other hand, produced excitation (n = 17) or inhibition (n = 14) of the putative VP-secreting neurons (n = 52) in the PVN. About half of the excitatory responses to LHA stimulation were blocked or attenuated by pretreatment with Sar in the SFO, whereas the inhibitory responses were not affected. These results show some possible connections between the LHA and PVN, and suggest that AII-sensitive LHA neurons with efferent projections to the SFO may act to enhance the excitability of putative VP-secreting neurons in the PVN via an excitatory influence on the AII-sensitive SFO neurons.     

幼龄厌食大鼠食欲中枢神经元兴奋性的改变

观察幼龄厌食大鼠摄食中枢(下丘脑外侧区lateral hypothalamic area,LHA)和饱中枢(下丘脑腹内侧核ventromedialhy-pothalamic nuclear,VMN)神经元兴奋性的改变。用特制饲料喂养幼龄大鼠3周制备厌食模型,然后用细胞外记录法,记录大鼠LHA和VMN神经元的自发放电,观察其对电刺激胃迷走神经和静脉注射葡萄糖的反应,比较对照组和模型组间的差异。结果显示:模型组大鼠LHA神经元对胃迷走神经刺激的反应特征与对照组无显著性差异,而VMN神经元对胃迷走神经刺激的兴奋性反应时程延长(P<0.01),刺激强度降低(P<0.01)。模型大鼠中对胃迷走神经刺激有反应的LHA神经元的血糖敏感率降低,而那些对胃迷走神经刺激有反应的VMN神经元的血糖敏感率增加(P<0.01)。以上结果表明:特制饲料改变LHA和VMN神经元对外周传入的摄食负反馈信号的敏感性,从而使LHA和VMN神经元共同作用,发出抑制摄食信号,从而导致了厌食的发生。     

The Effects of Activation of Glutamate Ionotropic Connections of Neurons in the Sensorimotor Cortex in a Conditioned Reflex

Changes in conditioned reflex spike activity of neurons in the sensorimotor cortex were studied during microiontophoretic application of agonists and antagonists of glutamate and GABAergic transmission. The results of these experiments showed that the glutamate ionotropic receptors (AMPA and NMDA) of neurons in the sensorimotor cortex were intensely activated by the arrival of a conditioned signal in the cortex. This response included not only large pyramidal neurons of the deep cortical layers, but also the surrounding inhibitory interneurons. The existence of constant tonic inhibitory regulation of the activity of large pyramidal neurons by the surrounding inhibitory cells was demonstrated, along with the active involvement of this inhibition in organizing the excitatory responses of neurons in the sensorimotor cortex during a conditioned reflex.     

Responses of rat lateral hypothalamic neuron activity to dorsal raphe nuclei stimulation

1. The effects of dorsal raphe (DR) stimulation on neural activity in the rat lateral hypothalamic area (LHA), including specific glucose-sensitive neurons, were investigated by extracellular and intracellular recording in vivo, and the neurotransmitters involved were determined. 2. In 67 adult male anesthetized rats, 287 extracellular and 49 intracellular recordings of LHA responses to electrical stimulation of the DR were examined. 3. To determine neurotransmitter candidates, the effects of serotonin and the serotonin antagonists methysergide, lisuride, and (-)-propranolol were investigated by systemic administration and microelectrophoresis. 4. Of 287 spontaneously firing LHA neurons tested by DR stimulation, 157 (55%) were inhibited. Among these, 51% were glucose sensitive. The serotonin 1 receptor antagonists, lisuride and (-)-propranolol, attenuated the inhibitory responses to both DR stimulation and electrophoretic serotonin application. 5. Seventy-three (25%) were excited by DR stimulation, and 71% of these were glucose insensitive. Methysergide attenuated the excitatory responses to DR stimulation and the inhibitory response to electrophoretic serotonin application, but (-)-propranolol did not attenuate the excitation. 6. Intracellular recordings of LHA neurons during DR stimulation showed monosynaptic excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs) with 3.8 and 3.0 ms latency, respectively. The reversal potential for the former was approximately -17 and for the latter, -94 mV. 7. From the results we concluded that 75% of LHA glucose-sensitive neurons receive inhibitory serotonin inputs from the DR through serotonin 1 receptors, and 20% of glucose-insensitive neurons receive excitatory inputs from the DR through serotonin 2 receptors though 41% of these receive inhibitory inputs through serotonin 1 receptor.     

Neural activities in the substantia nigra modulated by stimulation of the orofacial motor cortex and rhythmical jaw movements in the rat

Neurons related to jaw movements in the substantia nigra pars reticulata were explored by examining changes in their neural activities in response to electrical stimulation of the orofacial sensorimotor cortex and during rhythmical jaw movements induced by mechanical stimulation applied to the oral cavity in the rat. Out of 80 neurons tested, 59 showed changes in their firing patterns of activities in response to the electrical stimulation of the cortex. The responding neurons were mainly located in the dorsolateral part of the substantia nigra pars reticulata. The substantia nigra pars reticulata neurons showing responses were classified into the following five types according to their response patterns: (1) an inhibition preceded by an early excitation and followed by a late excitation (n = 26), (2) an inhibition preceded by an early excitation but not followed by a late excitation (n = 7), (3) an inhibition not preceded by an early excitation but followed by a late excitation (n = 2), (4) an inhibition without early or late excitations (n = 7) and (5) an excitation without an inhibition (n = 17). Out of 18 neurons responding to the cortical stimulation, 11 (61.1%) increased or decreased their neural activities during rhythmical jaw movements. Some of these neurons had a projection to the lateral part of the superior colliculus (n = 5) and/or to the parvicellular reticular formation (n = 2). These results provide first neurophysiological evidence for neurons in the dorsolateral part of the substantia nigra pars reticulata with inputs from and outputs to the areas related to jaw movements. These neurons may participate in the control of jaw movements in the rat.     

CCK as a central satiety factor: behavioral and electrophysiological evidence

CCK and its derivatives potently inhibit feeding, even after vagotomy. This effect is thus considered to be peripheral. Recently, however, the vagal gastric branch was reported to essentially bring feeding inhibition into full play. In the present study, it was found that CCK-8, administered into the third cerebroventricle (III-cv), or into the lateral hypothalamus (LHA), significantly and dose-dependently inhibited feeding induced by electrical stimulation of the contralateral LHA (LHA-ESIF) in the chronic rat. This inhibition by CCK-8 was not affected by systemic pretreatment with proglumide (1 mg), a selective antagonist, while CCK (250 ng) simultaneously microinjected into the III-cv with 5 micrograms proglumide almost completely eliminated the CCK effect on LHA-ESIF. Neuronal activity of the ventromedial hypothalamus (VMH) was enhanced, and that of the LHA was suppressed by electrophoretic direct application of CCK on neurons in urethane-chloralose-anesthetized rats. CCK also markedly decreased the threshold of VMH glucose responding neurons. These results indicate that the satiety effect is not only peripheral, but might also be central, especially through feeding-related hypothalamic neurons, which are probably important in feeding inhibition.     

Paraventricular neurosecretory neurons: synaptic inputs from the ventrolateral medulla in rats

Effects of electrical stimulation of the ventrolateral medulla on discharge activity of neurosecretory neurons in the paraventricular nucleus (PVN) were studied in male rats anesthetized with urethane-chloralose. Among 35 phasically firing neurosecretory neurons, stimulation of the lateral reticular nucleus and its vicinity produced excitation in 10 and inhibition in 2. The stimulation also enhanced the activity of 40% of the PVN neurosecretory neurons that fired continuously (n = 81); of these responsive neurons, half of the neurons tested (n = 12) were inhibited by i.v. administration of phenylephrine. The result suggests that both vasopressin- and oxytocin-secreting neurons in the PVN receive mainly excitatory synaptic inputs from the ventrolateral medulla.     

Influences of cortico-tectal and intertectal connections on visual responses in the cat's superior colliculus

Summary Our experiments, utilizing electrical shocks applied to the lateral- or supra-sylvian gyrus of the cortex, demonstrate an initially excitatory (latency 2–10 msec) but predominantly inhibitory influence of cortico-tectal afferents on the discharge of tectal neurons. Primary or secondary inhibition in tectal cells after cortical stimulation suppressed spontaneous or visually driven activity and limited the frequency of stimulation which tectal neurons could follow.The main influence of the contralateral colliculus on visual responses of tectal cells is inhibitory but again some principally monosynaptic intertectal connections evoked initial excitation (latency 3–10 msec) after electrical stimulation of the contralateral optic tract.Removal of the visual areas 17, 18 and 19 did not cause a loss of movement- or direction-selectivity in neurons of the superior colliculus. Cooling of the occipital cortex, while recording from direction-selective tectal neurons did not alter their essential response characteristics. The response to cortical shocks disappeared in tectal neurons during cooling but could be restored by rewarming of the cortex.It could not be confirmed in our experiments that excitation and movement- or direction-selectivity of neurons in the superior colliculus depend on a specific input from areas 17, 18 and 19 of the cortex.     

Inhibitory effects of ventral tegmental area stimulation on the activity of prefrontal cortical neurons: evidence for the involvement of both dopaminergic and GABAergic components.

The medial prefrontal cortex of the rat receives dopamine and non-dopaminergic projections from the ventral tegmental area. Both electrical stimulation of the ventral tegmental area and local application of dopamine induce an inhibition of the spontaneous activity of most prefrontal cortical neurons, including efferent neurons. In the present study, the techniques of extracellular recording and microiontophoresis were used in anesthetized rats in order to determine whether these dopamine- and ventral tegmental area-induced inhibitory responses involve GABAergic components. Prefrontal cortex output neurons were identified by antidromic activation from subcortical structures. The inhibitory responses evoked by the local application of dopamine were blocked by the iontophoretic application of the D2 antagonist sulpiride, and the GABAA antagonist bicuculline in 89 and 57% of the cases, respectively. In addition, sulpiride and bicuculline abolished the inhibition induced by ventral tegmental area stimulation in 54 and 51% of the prefrontal cortical cells tested, respectively. The implication of a non-dopaminergic mesocortical system in the ventral tegmental area-induced inhibition was further analysed using rats pre-treated with alpha-methylparatyrosine to deplete dopamine stores. The proportion of prefrontal cortical cells inhibited by ventral tegmental area stimulation was markedly reduced (39%) in alpha-methylparatyrosine-treated rats, when compared to controls (86%). Remaining ventral tegmental area-induced inhibition was no longer affected by sulpiride, but in all cases blocked by the local microiontophoretic application of bicuculline. The present results suggest that: (1) the dopamine-induced inhibition of prefrontal cortex neurons could involve cortical GABAergic interneurones; (2) the non-dopaminergic mesocortical system exerts also an inhibitory influence on prefrontal cortical cells and appears to be GABAergic.     

Stress-induced changes in cellular responses in hypothalamic structures to administration of an antigen (lipopolysaccharide) (in terms of c-Fos protein expression)

Stress is known to affect the intensity of the immune response. The involvement of central regulatory structures in mediating these changes was addressed by analyzing the extent of activation of neurons in the hypothalamus (in terms of the number of c-Fos-positive cells) in rats 2 h after i.v. administration of lipopolysaccharide alone and on the background of electrical pain stimulation. Studies were performed using 52 male Wistar rats weighing 200–250 g. c-Fos protein expression was studied by immunohistochemical analysis. Increases in the quantity of c-Fos-positive cells 2 h after administration of lipopolysaccharide were seen in the following hypothalamic structures: AHN, PVH, LHA, VMH, DMH, and PH. After electrical pain stimulation, the number of c-Fos-positive cells increased in these same hypothalamic structures (AHN, PVH, LHA, VMH, DMH, and PH). The combination of electrical pain stimulation and lipopolysaccharide administration led to a decrease in the extent of activation in hypothalamic structures AHN, PVH, LHA, and VMH as compared with the characteristic reaction to lipopolysaccharide without electrical pain stimulation. Electrical pain stimulation suppressed the intensity of the immune response induced by lipopolysaccharide (as assessed by local hemolysis and counts of the numbers of spleen antibody-forming cells). Thus, changes in the extent of activation of hypothalamic structures (AHN, PVH, LHA, VMH) correlated with the development of stress-induced immunosuppression, i.e., morphofunctional mapping of the extent of activation of hypothalamic structures allowed identification of which changes in hypothalamic cell activity occurred with stress-induced changes in immune system responses to antigen administration. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 92, No. 11, pp. 1296–1304. November, 2006.     

Neurochemical mechanisms of the involvement of cortical sensorimotor neurons in alimentary and orientational behavior

The object of these experiments was the study of the features of the neurochemical mechanisms of the involvement of individual neurons of the sensorimotor cortex of the rabbit brain the orienting reaction and the goal-oriented alimentary behavior elicited by stimulation of the hunger center of the lateral hypothalamus using electrical current of varying intensity. It is demonstrated that the neurotransmitters acetylcholine and norepinephrine, in approximately equal percentages, reorganize the reaction of neurons at the subthreshold as much as at the threshold level of stimulation. The microiontophoretic application to cortical sensorimotor neurons of the protein synthesis blocker, cycloheximide, also elicits alterations in their reactions to threshold and subthreshold stimulation of the lateral hypothalamus which are apparently linked to the suppression of synthesis of neuropeptides specific to both behaviors. It is proposed that the orienting, and especially the alimentary, motivational reactions are achieved by the activation of the synthesis of specific peptide molecules in the cortical sensorimotor neurons, which in fact may induce a change in their sensitivity to neurotransmitters.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 70, No. 9, pp. 1311–1315, September, 1984.     

Do non-dopaminergic neurons in the ventral tegmental area play a role in the responses elicited in A10 dopaminergic neurons by electrical stimulation of the prefrontal cortex?

It is rapidly becoming apparent that the prefrontal cortex (PFC) plays a major role in controlling the activity of midbrain dopaminergic (DA) neurons. We have previously demonstrated that electrical stimulation of the PFC elicits inhibition-excitation (IE) and excitation (E) activity patterns in DA neurons in the ventral tegmental area (VTA; A10 cell group). Since non-DA neurons in the VTA are cortically innervated, synapse upon DA neurons and appear to have an inhibitory impact, we determined the extent to which the responses of these neurons to stimulation of the PFC could account for the responses seen in DA neurons upon cortical stimulation. Stimulation of the PFC (0.25 mA and 1.0 mA) elicited three categories of response in the majority of VTA non-DA neurons. Types I and II were characterised by a short-to-moderate latency excitation (referred to as “early excitations”), in the latter case preceded by inhibition. Type III responses consisted of inhibition in the absence of an early excitation. Elements of these responses were compared with the temporal characteristics of key elements of responses elicited in DA neurons by PFC stimulation. Although the early excitations in non-DA neurons preceded the inhibitions in DA neurons exhibiting IE responses, the early excitations began approximately 100 ms before the inhibitions in DA neurons and often ended several tens of milliseconds before the inhibitions began, making a causal relationship between these events unlikely. The inhibitions in Type III responses, combined with the inhibitions which followed the early excitations in many Type I and II responses, showed temporal characteristics that suggested a possible causal relationship with the excitations in DA neurons exhibiting E responses, but not those exhibiting IE responses. However, since the excitatory phases of E and IE responses appear to be homologous, the lack of involvement of non-DA neurons in the excitatory phase of IE responses tends to cast doubt on the involvement of non-DA neurons in the excitation during E responses. In fact, the most coherent impression that emerges is that non-DA neurons in the VTA do not influence the activity of A10 DA neurons on a short time-scale (i.e. phasically), but instead may influence activity on a longer time-scale (i.e. tonically). Received: 3 March 1997 / Accepted: 18 August 1997     

Selective inhibition of glucose-sensitive neurons in rat lateral hypothalamus by noxious stimuli and morphine

On the basis of their responsiveness to electrophoretically applied glucose, neurons in the lateral hypothalamic area (LHA) have been characterized as either glucose sensitive or glucose nonsensitive. Glucose-sensitive neurons are important in feeding control (4, 36-38, 44, 54). The aim of this study was to increase understanding of the neurophysiological mechanisms involved in the disturbance of feeding by pain. Radiant heating of the scrotum, strong tail pinch, and immersion of the tail in hot water were used as noxious stimuli. In order to correlate the responses of LHA neurons to noxious inputs with possible local release of endogenous opiates, effects of electrophoretically applied morphine and naloxone were also tested. The effects of glucose, morphine, and noxious stimulation were studied in a total of 165 neurons recorded from 75 adult male urethane-chloralose-anesthetized rats. Of 52 neurons determined to be glucose sensitive, 36 (69%) were inhibited by both noxious stimulation and morphine. A majority of the glucose-nonsensitive neurons did not respond to either morphine or noxious stimulation (87/113, 74%). The relation of glucose sensitivity to inhibition by pain and/or morphine was statistically significant (Fisher's exact probability test, P less than 0.01). Naloxone attenuated the inhibitory effects of both pain and morphine, thus suggesting mediation of both by the same neuronal mechanism. From this evidence we conclude that LHA glucose-sensitive neurons are involved in the suppression of feeding by noxious stimulation.     

Feeding related lateral hypothalamic neuron responses to odors depend on food deprivation in rats

It has been investigated feeding related LHA neuronal activity and responses to odor stimulation in rats at various levels of satiation. Extracellular responses of 168 neurons to three odors, isoamylacetate (AA), cineole (CL), and isovaleric acid (VA), were recorded from 168 LHA neurons of Wistar-SPF male rats. Of 168 units, 107 (63.7%) responded to from one to three odors, but not to light or phonic stimulation. Of the responding units, 94.4% (101/107) were excited, and 5.6% were inhibited. In response to a single electrical stimulation (0.5 msec, 1-10 V) of the OB, 61 units were excited with latencies of 6-43 msec (19.8 +/- 12.0 msec, mean +/- S.D.) indicating compound OB-LHA relations--mono- and polysynaptic through myelinated and nonmyelinated fibers. The results suggest predominantly excitatory effects of both electrical stimulation of the OB and odor stimulation on the LHA. Firing frequency in response to AA or VA was significantly (p less than 0.05) greater for the long fasting group (38 hr, LF, n = 8) than for the NF (nonfasting, n = 12) group; differences between the LF and MF (24 hr, n = 6) groups were not significant. Glucose-sensitive neurons (GSN, n = 19) responded more to odors than non-GSNs (n = 86), and discharge frequency increase depended markedly on food deprivation. Food deprivation results suggest that responsiveness of feeding related LHA neurons to odors depends on the degree of satiation. In conclusion, it was confirmed that olfactory functions are important in the responses of hypothalamic feeding related neurons.     

Structural and Functional Characteristics of Neurons in the Sensorimotor Cortex of Rats with Different Resistance to Emotional Stress

Wistar rats behaviorally active in the open field test (resistant to emotional stress) are characterized by polymorphism of neurons in layer V of the sensorimotor cortex and the presence of hyperchromatic cells, which probably determines resistance to emotional stress in these rats. Atrophy of hyperchromatic neurons reflecting transient inhibition of cell activity was noted in Wistar rats subjected to stress. In the sensorimotor cortex of behaviorally passive animals (predisposed to emotional stress) groups of densely packed hyperchromatic cells and pronounced pericellular edema were revealed. In these rats stress caused irreversible changes in cortical neurons and death of some cells. The presence of ischemic cortical neurons in rats subjected to emotional stress suggests that cerebral hypoxia plays a role in structural and functional disorganization of the sensorimotor cortex during emotional stress.     

Viscerosomatic neurons in the lower thoracic spinal cord of the cat: excitations and inhibitions evoked by splanchnic and somatic nerve volleys and by stimulation of brain stem nuclei

Single-unit electrical activity has been recorded from 95 viscerosomatic neurons in the T9 and T11 segments of the cat's spinal cord. These neurons were excited by electrical and/or natural stimulation of visceral and somatic afferent fibers. The excitatory and inhibitory effects on these neurons of volleys in somatic and visceral afferent fibers and of electrical and chemical stimulation of the nucleus raphe magnus (NRM) and adjacent areas of the reticular formation (Ret. F.) have been studied. Electrical stimulation of the splanchnic nerve produced, after the initial excitation of the neurons, a period of inhibition lasting for up to 1 s. This inhibition reduced the responsiveness of the neurons to all inputs, somatic and visceral, and was still present after spinalization of the animals with cold block, which indicates a segmental organization of the inhibition. Electrical stimulation of afferent fibers within the somatic receptive field of the neurons produced, after the initial excitation, a period of inhibition similar to that induced by visceral afferent volleys. During this period of inhibition all inputs to the neurons were reduced. Reversible spinalization of the animals with cold block did not abolish this inhibition. On the basis of the effects of reversible spinalization on the visceral input to viscerosomatic neurons, two types of neurons were distinguished: 1) neurons whose visceral responses increased in the spinal state (neurons under tonic descending inhibition) and 2) neurons whose visceral responses were decreased or abolished in the spinal state (neurons subject to descending excitation). Neurons under tonic descending inhibition were inhibited by electrical stimulation of locations within the NRM and Ret. F. This inhibition lasted for less than 100 ms and could be evoked at intensities of stimulation of 100 microA or less. Neurons under descending excitation were also inhibited by electrical stimulation in the NRM and Ret. F. but, in addition, the inhibition was preceded by an excitation in 75% of these neurons. Chemical stimulation with DL-homocysteic acid (DLH) of locations within the NRM and Ret. F. was used to activate cell bodies, but not axons, located in these brain stem sites. The only effect observed following injections of DLH into the NRM and Ret. F. was inhibition of viscerosomatic neurons including those with descending excitation as well as those with descending inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of hypothalamic stimulation on activity of dorsomedial medulla neurons that respond to subdiaphragmatic vagal stimulation

1. Effects of hypothalamic stimulation on activity of dorsomedial medulla neurons that responded to subdiaphragmatic vagal stimulation were investigated in urethan-anesthetized rats. 2. Extracellular recordings were made from 231 neurons in the nucleus of the tractus solitarius (NTS) that fired repetitively in response to single-pulse subdiaphragmatic vagal stimulation and from 320 neurons in the dorsal motor nucleus of the vagal nerve (DMV) that responded antidromically to subdiaphragmatic vagal stimulation. The mean latencies of responses to subdiaphragmatic vagal stimulation were 90.3 +/- 17.1 ms (mean +/- SD) for NTS neurons, and 90.8 +/- 11.2 ms for DMV neurons. This indicated that both afferent and efferent subdiaphragmatic vagal fibers were thin and unmyelinated and had a conduction velocity of approximately 1 m/s. 3. In extracellular recordings from 320 DMV neurons, marked inhibition preceded the antidromic response and subdiaphragmatic vagal stimulation evoked orthodromic spikes in only a few neurons. 4. Intracellular recordings from 66 DMV neurons revealed inhibitory postsynaptic potentials (IPSPs) before the antidromic responses. These IPSPs suppressed spontaneous firing and prevented excitatory postsynaptic potentials (EPSPs) from generating action potentials. 5. Stimulation in all hypothalamic loci studied, the ventromedial hypothalamic nucleus (VMH), the lateral hypothalamic area (LHA), and the paraventricular nucleus (PVN), induced responses with similar characteristics of excitation alone or excitation followed by inhibition in most NTS and DMV neurons. 6. No reciprocal effect of VMH and LHA stimulation was observed on NTS and DMV neurons. 7. Intracellular recordings from DMV neurons revealed monosynaptic EPSPs in response to stimulation of the VMH, the LHA, and the PVN. 8. PVN stimulation evoked significantly more responses in NTS and DMV neurons than VMH stimulation and more responses in DMV neurons than LHA stimulation. This suggests a difference in the number of connections between each hypothalamic site and the dorsomedial medulla. 9. The same dorsomedial medulla neurons were tested with VMH and LHA stimulation. The respective mean latencies of the antidromic and the orthodromic NTS neuron responses were 37.3 +/- 3.2 and 39.6 +/- 12.9 ms for VMH stimulation and 29.8 +/- 5.3 and 31.8 +/- 8.7 ms for LHA stimulation. The mean latencies of the orthodromic DMV neuron responses were 39.4 +/- 8.3 ms for VMH stimulation and 31.1 +/- 5.2 ms for LHA stimulation. The estimated conduction velocity from the VMH to the dorsomedial medulla was approximately 0.25 m/s and from the LHA it was approximately 0.33 m/s, which was significantly faster.(ABSTRACT TRUNCATED AT 400 WORDS)     

RNA content of neurons in the ventromedial nuclei and lateral hypothalamic area relative to feeding status

The total RNA content of hypothalamic and cortex neurons in relation to the feeding status of adult male Wistar rats was studied. Experimental conditions including food deprivation (12 and 24 hours) and relative satiation (short-term refeeding, glucose or glycerol administration) changed in different ways the total RNA content of the neurons in the ventromedial hypothalamic nuclei (VMH) and in the lateral hypothalamic area (LHA) with respect to fasting or satiety. Only the long-term absence of food (24 hours) significantly increased the total RNA content of the VMH cells, while the RNA content of the LHA neurons significantly decreased in both the 12 and 24 hr fasted rats compared with those fed ad lib. The sixty minute free access to food after 12 or 24 hours of fasting fully reversed these changes. The short-term food intake significantly increased the RNA content of the LHA cells of the 12 and 24 hr fasted animals while the total RNA content of the VMH neurons significantly decreased only in the 24 hr fasted rats. The effect of glucose and glycerol administration on the RNA content of the LHA neurons (in 12 hr fasted rats) was similar to the effect of refeeding. One hour after giving glucose (1 g/kg b.wt.) or glycerol (300 mg/kg b.wt.) the total RNA content in the LHA neurons significantly increased. No changes in RNA content were observed in the neurons of the cortex when comparing the experimental and control rats. The results demonstrated the close relationship between the RNA content of the hypothalamic neurons and the feeding status.(ABSTRACT TRUNCATED AT 250 WORDS)