Thermosensitivity of preoptic and anterior hypothalamic neurons in the capsaicin-desensitized rat

Summary Single-unit studies on preoptic and anterior hypothalamic neurons in the capsaicindesensitized rat having deficient thermoregulation against heat showed that the proportion of warm-sensitive and cold-sensitive neurons in a total population of neurons recorded was reduced to about half of the normal.     

An electron microscopic examination of the corticospinal projection to the cervical spinal cord in the rat: lack of evidence for cortico-motoneuronal synapses

We investigated whether direct, cortico-motoneuronal connections are present in the rat, using both light microscopic and electron microscopic techniques. Corticospinal fibres were labelled using the anterograde tracer, biotinylated dextran-amine (BDA), which was injected into forelimb sensorimotor cortex. Motoneurons were retrogradely labelled after injection of cholera toxin subunit B (CTB) into forelimb muscles, contralateral to the injected hemisphere. Terminals of peripheral afferent fibres, which were also labelled by CTB, were easily distinguishable from, and much larger than, BDA-labelled corticospinal terminals. At the light microscope level, corticospinal terminals were found in all laminae contralateral to the injection site, most extensively in laminae VI and VII of cervical segments C5–C8. Although labelling in the ventral horn (lamina IX) was present, it was extremely sparse. A total of 47 corticospinal synapses were studied at the electron microscope level; most of these were in lamina VII and the majority (35/47; 74%) made axo-dendritic contacts with asymmetrical synapses; one made an axo-somatic synapse, and in the remaining 11 cases no postsynaptic structure could be identified. All corticospinal terminals contained spherical boutons. Serial sectioning of eight BDA-labelled corticospinal boutons in lamina IX revealed that most (seven out of eight) did not make synaptic contacts with any neuronal structure, and none made any contact with adjacent dendrites of CTB-labelled motoneurons. Thus these results provide no positive ultrastructural evidence for direct cortico-motoneuronal synaptic connections within lamina IX between corticospinal axon boutons and the proximal dendrites of forelimb motoneurons. The results confirm other lines of evidence suggesting that such connections are not present in the rat. Electronic Publication     

In vivo GABA release from the medial preoptic area of diestrous and ovariectomized rats

Summary The push-pull cannula technique was used to examine the endogenous release of GABA from the medial preoptic area (MPO) of unanesthetized rats. In diestrous females the mean resting release of GABA was 27.1±2.0 pmol/min. GABA release was significantly elevated by increasing the potassium concentration in the perfusion solution to 50 mM, whereas it was dramatically inhibited by mercaptoproprionic acid (1.0 mM), a glutamic acid decarboxylase inhibitor. A comparison between diestrous females and chronically castrated animals indicated that endogenous GABA release in OVX animals was only 60–70% of that in diestrous animals. A model for the presynaptic inhibition of NE by estrogen receptive GABAergic neurons in the MPO is proposed.Partially supported by the German Research Society (grant No. WU 60/5)Dr. Ondo was supported by a NICHHD Research Career Development Award (HD 00248)     

Estrogen reduces the excitability of the female rat medial amygdala afferents from the medial preoptic area but not those from the lateral septum

Electrical stimulation of the medial amygdala (AMY) elicited antidromic action potentials in neurons in the preoptic area (POA) and the lateral septum (LS) of 36 urethane-anesthetized ovariectomized female rats, which were either treated with estrogen o not treated. The extracellular potentials from the two sites showed similar characteristics, with the exception of the sensitivity to estrogen: they had latencies between 3 and 35 ms. Thresholds were as low as 100 A. The mean relative refractory period was 2.2 ms. The peak-to-peak amplitudes of the positive-negative biphasic potential ranged from 1.0 mV to 12.0 mV. Estrogen had site-specific effects on parameters of antidromic activation in the POA. Estrogen-treated rats had a significantly higher threshold (937 vs 664 A) and a longer refractory period (2.5 vs 2.1 ms) than the ovariectomized rats (P < 0.05 for each). The effects were absent in the LS. Selective cutting of the stria terminalis diminished the AMY-induced antidromic responses in the POA and LS. Electrical stimulation of the stria blocked the AMY-induced antidromic potentials by collision. Thus, estrogen-sensitive POA efferents as well as non-estrogen-sensitive LS efferents project to the AMY via the stria terminalis. Reductions in axonal excitability would inhibit neural conduction and transmission. Estrogen may therefore reduce the AMY inputs from the POA, without affecting those from the LS. Such alterations in the neural impulse flow may underlie estrogen-dependent neuroendocrine or behavioral regulation.     

Effects of air constituents on thermosensitivities of preoptic neurons: hypoxia versus hypercapnia

The effects of hypoxia (10% O₂) on the thermosensitivities of preoptic neurons were studied in urethanized rats and compared to the effects of hypercapnia (10% CO₂). This was examined by regression of neuronal activity on preoptic temperature. During hypoxia, the slope of the regression line increased significantly in 8 (23%) of 35 warmsensitive neurons and decreased in eight other neurons (P<0.05). During hypercapnia, the slope of the regression line decreased significantly in 7 (30%) of the 23 warmsensitive neurons (P<0.05). No neuron was found that significantly increased the slope of the regression line. The effects of hypoxia on thermosensitivities (i.e. the slope of the regression line) of PO neurons differed from those of hypercapnia in chi-square analysis (P<0.05). Responses of the cold-sensitive neurons to hypoxia or hypercapnia did not generally differ from those of the warm-sensitive neurons. During hypoxia and hypercapnia, arterial blood pressure, respiratory frequency, heart rate, and EEG were recorded to examine their relations to neuronal activity. The present results indicate that the thermosensitivities of preoptic neurons are modified by both hypoxia and hypercapnia, but that hypoxic differ from hypercapnic effects.     

Effects of carbon dioxide on preoptic thermosensitive neurons in vitro

Effects of carbon dioxide (CO₂) on the firing rates of preoptic thermosensitive neurons were examined in rat brain slice preparations. The perfusing medium was saturated with gas mixtures consisting of 90% O₂ and one of various concentrations (5%, 6.3%, 7.5%, and 10%) of CO₂ balanced with N₂. The medium containing 5% CO₂ was used as control. Most preoptic neurons were inhibited during application of a high CO₂ medium. An excitatory effect of CO₂ on a small number of neurons was also significant, although this was weak and transient compared to the inhibitory effect. Thermosensitivities of the neurons did not correlate with their CO₂ sensitivities. The influence of CO₂ tended to be more evident at higher temperatures. We conclude that the direct effect of CO₂ on PO thermosensitive neurons as well as on thermally insensitive neurons is mainly inhibitory.     

Somatostatin-mRNA expression in brainstem projections into the medial preoptic nucleus

The medial preoptic nucleus plays an important role in the regulation of neuroendocrine processes, vegetative functions, sexual behaviour and the modulation of the somatomotoric system. The connections of the medial preoptic nucleus to other areas of the central nervous system are very complex, and the area receives afferents using numerous transmitters and neuropeptides. Previous investigations have shown that this nucleus receives afferents from various brainstem nuclei that also contain somatostatinergic neurons. This study was carried out to investigate if somatostatin-projecting neurons of the brainstem are afferents to the medial preoptic nucleus. This was approached by combining somatostatin-mRNA in situ hybridisation with True Blue retrograde tracing. Our results demonstrate somatostinergic brain-stem projections into the medial preoptic nucleus mainly in the pedunculopontine nucleus and in the nucleus of the solitary tract (50% together). Other important somatostatinergic afferents into the medial preoptic nucleus originate in the cuneiform area, the dorsal parabrachial nucleus and in the lateral reticular nucleus (37% together). Less important are the somatostatinergic projections coming from the central grey, the laterodorsal tegmental nucleus, the locus coeruleus and the nucleus raphe magnus. Considering that these areas are involved in diverse functions such as cardiovascular regulation (nucleus of the solitary tract), transmission of visceral sensibility (dorsal parabrachial nucleus), modulation of the somatomotoric system (pedunculopontine nucleus) and in the regulation of neuroendocrine mechanisms (locus coeruleus), it seems tenable that the somatostatin projections demonstrated here also have a diverse functional quality within the medical preoptic nucleus where they terminate.Abbreviations ac anterior commissure - acp anterior commissure, posterior part - AP area postrema - Arc arcuate hypothalamic nucleus - BL basolateral amygdaloid nucleus - BM basomedial amygdaloid nucleus - CA1 CA1 field of Ammon's horn - cc corpus callosum - CG central grey - Cnf cuneiform area - Cu cuneiform nucleus - CPu caudate putamen - DCo dorsal cochlear nucleus - DM dorsomedial hypothalamic nucleus - DPB dorsal parabrachial nucleus - DR dorsal raphe nucleus - DTg dorsal tegmental nucleus - ec external capsule - En endopiriform nucleus - f fornix - FR/Pn pontine reticular formation - Gr gracile nucleus - hipp hippocampus - La lateral amygdaloid nucleus - LC locus coeruleus - LDTg laterodorsal tegmental nucleus - LHRH luteinizing hormone-releasing hormone - Lo lateral olfactory tract - LPO lateral preoptic area - LRt lateral reticular nucleus - LS lateral septum - MdD dorsal reticular nucleus of medulla oblongata - mlf medial longitudinal fasciculus - mfb medial forebrain bundle - mlPP area from peripeduncular nucleus caudally to adjacent parts of medial lemniscus rostrally - MnPN median preoptic nucleus - MPN medial preoptic nucleus - MPO medial preoptic area - MRG median raphe group - OX optic chiasm - PaPC parvocellular paraventricular hypothalamic nucleus - Pe periventricular hypothalamic nucleus - PPTg pedunculopontine tegmental nucleus - RMg raphe magnus nucleus - RPn raphe pontis nucleus - RtTg reticulotegmental nucleus - SC superior colliculus - SCh suprachiasmatic nucleus - Sol nucleus of the solitary tract - Sp5 nucleus of the spinal tract of the trigeminal nerve - SubC subcoeruleus nucleus - VMH ventromedial hypothalamic nucleus - VTA ventral tegmental area - III third ventricle     

Projection of scrotal thermal afferents to the preoptic and hypothalamic neurons in rats

Effects of thermal stimulation (18–41.5°C) of the scrotum were observed on neurons in the preoptic area and hypothalamus (POHY) in anesthetized rats. Changes in firing rate of POHY neurons occurred, with three exceptions, within the scrotal temperature range of 30–41°C. There was no dynamic response and little or no sign of rapid adaptation. Out of 46 warm-sensitive neurons, 34 increased and 9 decreased in firing rate with a rise in scrotal temperature. Out of 26 cold-sensitive neurons, 19 were excited and 4 were suppressed by scrotal cooling. The 3 exceptions were excited by scrotal warming above 36°C, and were also excited by cooling below 30°C. Out of 54 thermally-insensitive neurons, 16 were excited by scrotal warming, 7 by cooling and 2 by warming and cooling. The scrotal temperature change needed to produce full change in POHY neuronal activity varied from less than 1°C to 7°C, and was usually less than 4°C. Thermal stimulation of either half of the scrotum had identical influence on the same POHY neuron.     

Experimental electron microscopic studies on excitatory and inhibitory synapses in the cerebellar cortex of the rat

Summary Excitatory synapses in the rat cerebellar cortex contain large, spheroid vesicles and inhibitory synapses contain small, ovoid vesicles. Presynaptic terminals can be characterized by the ratio of spheroid: ovoid vesicles (Q-value). Neither the inhibition of acetylcholine synthesis by hemicholinium, nor the inhibition of GABA synthesis by thiosemicarbazide, does induce any significant alterations in the Q values. It is suggested that synaptic vesicles bind mediator substances on their surface membranes.     

Role of the preoptic brain in the regulation of preovulatory gonadotropin surges in the hamster

Summary These studies have examined the role of the preoptic-suprachiasmatic area (POA-SC) in brains of cyclic female hamsters in regulating proestrous preovulatory gonadotropin surges. The spontaneous release of LH and FSH which normally occurs on proestrous afternoon was blocked with phenobarbital. Temporal changes in serum LH and FSH were measured in such blocked animals after delivery of direct current (100 A/60 s) to the POA-SC or that portion of the medial basal hypothalamus (MBH) which includes the arcuate nuclei and the median eminence. Bilateral dc treatment of MBH resulted in a 30-fold increase in serum LH and a 4-fold rise in serum FSH over basal concentrations. Unilateral MBH dc treatment produced a 12-fold increase in serum LH but FSH levels remained basal. In contrast, the delivery of dc to the POA-SC did not evoke any increase in serum LH or FSH. Sham electrode placement in the MBH or POA-SC also did not alter basal LH and FSH serum concentrations. These results suggest that, unlike the rat, passage of dc with concomitant production of an irritative lesion and deposition of ferrous ion does not activate structures responsible for preovulatory gonadotropin surges in hamsters.In a second study, discrete electrochemical lesions were produced in the basal anterior portion of the preoptic brain. These lesions did not involve the medial preoptic area or the suprachiasmatic nuclei. Following such brain destruction, spontaneous ovulation, LH surges, and 4-day vaginal cyclicity ceased. When the suprachiasmatic nuclei or extensive regions of the dorsal medial preoptic area, including the anterior commissure, were destroyed, vaginal cyclicity was disrupted for only 8–12 days. Thereafter, these animals had spontaneous preovulatory gonadotropin surges and ovulated. Seemingly, input from the medial basal anterior preoptic region (anterior to the SC) is essential for preovulatory LH and FSH surges to occur.This research was supported by USPHS Grant HD-02138Postdoctoral Fellow, Reproductive Endocrine Training Program supported by USPHS Grant HD-00435.     

Electron microscopic evidence for scorpion toxin binding to synapses of rat brain cortex

The venom from the scorpion Centruroides noxius Hoffman was fractionated by Sephadex G-50 gel filtration. The toxic fraction (n.II) was coupled to Sepharose-4B and used for purification of specific horse immunoglobulins. The purified immunoglobulins were linked to ferritin with glutaraldehyde and repurified in the same affinity column. This complex bound to the postsynaptic membrane of rat brain tissue previously exposed to toxin.     

Direct neural connection from the medial preoptic area to the hypothalamic arcuate nucleus of the rat

Summary Direct neural connections from the medial preoptic area (MPOA) to the arcuate nucleus were studied on light and electron microscope level by a multistep experimental procedure. The hypothalamic deafferentation technique of Halász and Pupp (1965) was used in itself or was combined with electrolytic lesions. In order to eliminate all fibers of more rostral source traversing the medial preoptic area an extended rostral preoptic deafferentation was made, and a survival time of at least 3 weeks was chosen to allow for the complete disappearance of the degenerated fragments from the arcuate nucleus. In the main experimental group the medial preoptic area was destroyed by an electrolytic lesion 3 or more weeks following such rostral preoptic deafferentations. In these animals degenerated fibers and terminals certainly of preoptic origin were found distributed bilaterally, with a predominance on the side of the lesion. — These data suggest that axons originating from medial preoptic neurons terminate in the arcuate nucleus, thus constituting a preoptico-tuberal pathway.     

Intrahypothalamic connections: An electron microscopic study in the rat

Summary Terminal degeneration within the hypothalamus was studied by electron microscopy 1 or 2 days (1) after carefully placed microlesions in the arcuate, anterior periventricular, ventromedial, premammillary and posterior hypothalamic nuclei and (2) after microlesions placed in the hypothalamus deafferented 3 weeks earlier.In the median eminence terminal degeneration was found after each of these lesions. Projections from the ventromedial nucleus reach the arcuate, suprachiasmatic, and anterior periventricular nuclei.Projections from the arcuate nucleus terminate in the medial preoptic, anterior periventricular, and ventromedial nuclei.After lesioning the premammillary nuclei degeneration was found in the supraoptic, arcuate, anterior hypothalamic and ventromedial nuclei.     

The significance of nucleus raphe dorsalis and centralis for thermoafferent signal transmission to the preoptic area of the rat

Summary Neuronal spike-trains were recorded extracellularly within the preoptic area, which is generally considered the center of information processing for thermoregulation. The neuronal responses were tested by thermal stimulation of the scrotal and abdominal skin of the rat. After a neuron had been identified as warm responsive, electrical stimulation and/or lesion were applied in medial midbrain to test the influence on the preoptic neurons. Post-stimulus-histograms were determined by using 600 single shocks. Furthermore a small area of the medial midbrain was electrolytically lesioned and the thermal response of the neuron was tested again. All lesion, stimulation and recording sites were examined histologically. Electrical stimulation of either nucleus raphe dorsalis or centralis influenced the responsiveness of the recorded preoptineurons. The post-stimulus-histograms predominantly reveal polysynaptic ascending pathways. Lesion of either nucleus raphe dorsalis or centralis abolished the responsiveness of the recorded preoptic neurons. Lesions lateral to these nuclei were ineffective. By this it is concluded that both nucleus raphe dorsalis and centralis are essentially involved in the thermal signal transmission to the preoptic area.     

Assessing the role of the medial preoptic area in ethanol-induced hypothermia

Administration of ethanol produces hypothermia. The preoptic area/anterior hypothalamus (POA/AH) contains warm- and cold-sensitive neurons that are important for temperature regulation. The present study evaluated the effect of ethanol on Fos immunoreactivity (Fos-ir) in the medial preoptic area (MPOA) and the effect of lesions to the MPOA on ethanol-induced hypothermia. Rats receiving 1.5-g/kg ethanol showed an increase in Fos-ir in the MPOA. However, lesions to the MPOA did not affect core body temperature. These findings indicate that ethanol increases neural activity in the MPOA, but this increased activity does not influence ethanol-induced changes in core body temperature.     

Medial preoptic neuronal responses to connatural females in sexually inactive male monkeys (Macaca fuscata)

The effect of repeated administration of thyroxine on beta-adrenergic receptor binding was studied in several brain regions in the rat using [3H]dihydroalprenolol as a ligand. Thyroxine treatment resulted in an increased density of beta-adrenergic receptors in the cerebral cortex while a decreased binding was found in subcortical tissue representing the thalamus, striatum and parts of the limbic system. There was no change in binding in the cerebellum or in the brainstem. The results indicate that thyroxine may regulate beta-adrenergic receptors in certain brain areas.     

Cellular cementogenesis in rat molars: the role of cementoblasts in the deposition of intrinsic matrix fibers of cementum proper

The formation of intrinsic fibers was examined in the advanced stage of rat cellular cementogenesis by light and electron microscopy. Using scanning electron microscopy, cementoblasts showed wing-like processes, partly encircling principal fibers. At the cementum-facing side of the cells these processes showed segmentation into finger-like processes, arranged in parallel with the cementum surface. Transmission electron microscopy showed many cytoplasmic fragments around intrinsic fibers at the cementum surface. These fragments contained microtubules and collagenous secretory granules that were arranged in parallel with the cementum surface and the intrinsic fibers. The wing-like processes contained microtubules and secretory granules that were arranged perpendicularly to the cementum surface and in parallel with the principal fibers. These observations suggest that: (1) the cytoplasmic fragments are cross-sectioned finger-like processes; (2) cementoblasts secrete intrinsic fibers from the finger-like processes and additional principal fibers from the wing-like processes; (3) cementoblasts constantly shorten their wing-like processes by forming finger-like processes. This development starts at the side facing the cementum and proceeds towards the periodontal ligament. With the segmentation, the cementoblasts change the arrangement of secretory granules to secrete intrinsic fibers around preformed principal fibers.     

Corticosterone stimulates the development of preoptic catecholamine neurons in tadpoles Bufo bufo japonicus

Summary In Bufo bufo japonicus catecholamine neurons in the preoptic recess organ (PRO) became detectable at the metamorphic climax by formaldehyde-induced fluorescence (Falck-Hillarp technique). In hypophysectomized tadpoles metamorphosis was inhibited and no fluorescent neurons appeared in the PRO. Implantation of a pituitary graft to the hypophysectomized tadpoles induced metamorphosis and development of PRO catecholamine neurons. Administration of corticosterone to hypophysectomized tadpoles resulted in the development of PRO catecholamine neurons in spite of the unmetamorphosed state. On the other hand, prolactin administration had no effect on the PRO neurons of hypophysectomized tadpoles. From these results, in conjunction with our previous results indicating that thyroxine treatment induces development of the PRO catecholamine neurons in thyroidectomized animals but not in hypophysectomized animals, it is concluded that corticosterone is a primary hormone for the development of PRO catecholamine neurons in toad tadpoles.     

Activation of central warm-sensitive neurons and the tail vasomotor response in rats during brain and scrotal thermal stimulation

The effects of preoptic and hypothalamic thermal stimulation on tail skin temperature were observed at different scrotal temperatures. The threshold hypothalamic temperature for tail vasodilation at a scrotal temperature of 40°C was significantly lower than that at a scrotal temperature of either 25°C or 33°C. The effects of scrotal thermal stimulation on tail skin vasodilated by higher hypothalamic temperatures were observed. Cooling the scrotum from 42 to 30°C invariably caused a rapid fall in tail temperature, whereas scrotal cooling from 30 to 25°C did not cause any significant change. Cooling of either the left or right half of the scrotum caused a similar fall in tail temperature. The temperature characteristics of the preoptic hypothalamic thermo-sensitive neurons were determined at scrotal temperatures of 32, 36 and 26°C. The firing rate of warm-sensitive neurons at a given hypothalamic temperature was highest at a scrotal temperature of 36°C, while that of cold-sensitive neurons was lowest at that temperature. The scrotal temperature range over which the number of neurons activated by scrotal warming increased rapidly was between 36 and 39°C when hypothalamic temperature was held at 36–37°C.     

Interaction of hunger and thirst in rats with lesions of the preoptic area

In the rat stimuli which increase thirst decrease feeding. This study investigated the possibility that the suppression of feeding by hypertonic saline is mediated by the preoptic area. Feeding in rats with lesions in the lateral preoptic area was suppressed by increased osmolality even though these rats did not drink to hypertonic saline. Some of these rats with lesions in the preoptic area did not drink to the extracellular thirst stimulus, isoproterenol, but feeding was suppressed by the substance. Therefore the suppression of feeding by hypertonic saline and isoproterenol can be separated from the effect of these substances on water intake. It is unlikely that the interaction between hunger and thirst takes place in the preoptic area. Hypertonic saline and isoproterenol may act on other thirst systems or directly on feeding systems to decrease food intake.