Posterior Hypothalamic Deafferentation Research Articles

Hypothalamic deafferentation in prepuberal beef heifers: Effects of gonadotropin-releasing hormone and estradiol benzoate on luteinizing hormone secretion

Hypothalamic control of luteinizing hormone (LH) secretion was investigated in crossbred beef heifer calves by comparing anterior (AHD), posterior (PHD), and complete (CHD) hypothalamic deafferentation with sham operated controls (SOC). Heifers ( n = 16) were fitted with an indwelling jugular catheter for 6 days before cranial surgery, and assigned randomly to treatments. Blood for radioimmunoassay of LH was collected sequentially at 15-min intervals during an 8-h period on days − 1 before and day 6 after hypothalamic deafferentation or sham operation. On the day of surgery, blood samples were collected sequentially at 15-min intervals 2 h before induction of anesthesia and throughout surgery and early recovery. Seven months after hypothalamic deafferentation, all experimental and sham operated heifers were ovariectomized and treated with vegetable oil (i.m.) plus saline (i.v.), vegetable oil plus gonadotropin releasing hormone (GnRH), estradiol benzoate (EB, 1 mg) in vegetable oil. After ovariectomy basal plasma concentrations of LH increased ( P < 0.01) compared with the low circulating hormone levels before ovariectomy. The amplitude of LH response to GnRH was greater ( P < 0.01) in CHD and PHD when compared with SOC and AHD heifers. Injection of EB failed to induce a LH surge in CHD and PHD 900–1100 min later when compared with the robust response seen in SOC and AHD heifers. Injection of EB plus GnRH elicited LH release in all deafferentated and sham operated heifers. These results indicate a transient change in LH secretion after AHD or CHD in prepuberal heifers with intact ovaries. After OVX, the integrity of the neural connection of the posterior hypothalamus is required for EB-induced LH release in beef heifers.

Prolactin secretion after hypothalamic deafferentation in beef calves: Response to haloperidol, α-methyl-ρ-tyrosine, thyrotropin-releasing hormone and ovariectomy

In ruminant species photoperiod regulates prolactin (PRL) secretion. It is hypothesized that the inhibition of PRL secretion resides in dopaminergic neurons of the medial basal hypothalamus (MBH). To test this hypothesis, anterior (AHD), posterior (PHD) and complete (CHD) hypothalamic deafferentation and sham operation control (SOC) surgeries were carried out during May (long-day photoperiod) in beef heifer calves (6–8 mo old) to measure basal PRL secretion and PRL secretion as affected by intravenous secretagogues. On the day of surgery (day 0), PRL secretion reflected stress of anesthesia and surgery in all groups. Thyrotropin-releasing hormone (TRH), α-methyl-ρ-tyrosine (αMρT), and haloperidol (HAL) was iv injected on days 11, 13 and 15, respectively. AHD, PHD, CHD, and SOC calves responded to TRH (100 μg) with an acute increase in PRL that peaked within 20 min. All heifers responded to αMρT (10 mg/kg BW) with an acute elevation in PRL within 10 min and remaining elevated for 3 h. HAL (0.1 mg/kg BW) induced an acute increase in PRL secretion in all groups, peaking within 15–30 min. Seven months later (December, short-day photoperiod) these heifers were ovariectomized. Basal plasma PRL levels were seasonally low, PRL secretion in AHD, PHD and CHD animals abruptly increased within 15 min to iv injection of 100 μg TRH to a greater amount than seen in SOC heifers. Although a biphasic effect on PRL secretion entrains under long-day and short-day photoperiods, hypothalamic deafferentation in cattle did not affect the pituitary gland's responsiveness to secretagogues.

Posterior hypothalamic deafferentation or 5,7-dihydroxytryptamine inhibit corticotropin-releasing hormone, ACTH and corticosterone responses following photic stimulation

The effect of posterior hypothalamic deafferentation (PHD) or hypothalamic serotonin (5-HT) depletion by 5,7-dihydroxytryptamine on corticotropin-releasing hormone (CRH), ACTH and corticosterone (CS) responses, following photic stimulation, was investigated in male rats. In intact animals, photic stimulation caused median eminence (ME) CRH depletion and a rise in serum ACTH and CS levels. In rats with PHD, in which previous studies have demonstrated 5-HT depletion, or with neurotoxin induced hypothalamic 5-HT reduction, these responses were markedly inhibited. This indicates that the blockage in the release of ME CRH into the portal circulation in rats with hypothalamic 5-HT depletion is responsible for the inhibition of the pituitary-adrenocortical responses following photic stimulation.

Medial posterior hypothalamic input is involved in adrenocortical activation following forebrain limbic stimulation

Previous experiments have demonstrated that posterior hypothalamic deafferentation, which involved also the medial forebrain bundle, has prevented the rise in serum corticosterone following limbic stimuli. Consequently, the effects of a small medial posterior hypothalamic deafferentation, excluding the medial forebrain bundle, on corticosterone responses following electrical stimulation of the hippocampus, amygdala, septum and reticular formation in the rat were studied. Posterior hypothalamic deafferentation did not change basal corticosterone levels but significantly inhibited the adrenocortical responses following stimulation of the above structures when compared to intact or sham-stimulated rats. Posterior hypothalamic deafferentation did not affect median eminence corticotropin releasing factor-41 content. It is concluded that a medial posterior hypothalamic input is involved in adrenocortical activation following limbic stimulation.

Adrenal responses and neurotransmitters in posterior hypothalamic deafferentation

The effects of a small posterior hypothalamic deafferentation (PHD) on adrenocortical responses to peripheral neural stimuli were investigated in rats. PHD inhibited the rise in plasma corticosterone (CS) following photic and acoustic stimulation, but did not affect the adrenocortical response following sciatic nerve stimulation. PHD did not change the content of norepinephrine in the paraventricular nucleus of the hypothalamus, however, it reduced the serotonin content by about 30%. The possible role of serotonin or of another tonic caudal input into the hypothalamus for the activation of the pituitary-adrenocortical axis, following certain neural stimuli, is discussed.

Neuropharmacological Characterization of Serotoninergic Stimulation of Vasopressin Secretion in Conscious Rats

In this study we have evaluated a possible role for brain serotoninergic neurons in the regulation of vasopressin secretion using pharmacological methods. In order to accomplish this, we have developed a specific and sensitive vasopressin radioimmunoassay along with a highly reproducible plasma extraction protocol. These tools were used to evaluate the plasma vasopressin response to several pharmacological challenges in conscious rats. Treatment with the serotonin (5-HT) releaser p-chloroamphetamine caused a significant increase in plasma vasopressin concentration. This effect was blocked by posterior hypothalamic deafferentation which separates serotonin cell bodies in the midbrain from their nerve terminals in the hypothalamus. Administration of graded doses of several 5-HT agonists had no effect. However, treatment with MK212, a serotonin agonist with 5-HT1 + 5-HT2 activity, induced a significant increase in plasma vasopressin concentration. The effect of MK212 on plasma vasopressin was completely abolished by the selective 5-HT2 receptor blocker LY53857. These studies confirm and extend studies by others that provide pharmacological evidence for serotoninergic regulation of vasopressin secretion via a selective 5-HT2 receptor mechanism. The specific neuroanatomical site(s) where serotonin exerts this effect are unknown, and the physiological consequences of these studies remain to be established.

Hypothalamic afferent connections mediating adrenocortical responses that follow hippocampal stimulation

This study identified some neural pathways which mediate the adrenocortical responses that follow hippocampal stimulation. The increase in plasma corticosterone following dorsal hippocampus stimulation, in rats with electrodes chronically implanted under pentobarbital anesthesia, was blocked by dorsal fornix and lateral septal lesions and by small posterior hypothalamic deafferentation. Fimbria transection, lateral septal lesions, and posterior hypothalamic deafferentation, but not midbrain reticular formation lesions, also blocked the adrenocortical responses to ventral hippocampus stimulation. Our present and previous studies indicate that the dorsal and ventral hippocampal effects on the hypothalamus, which increase plasma corticosterone concentrations, are mediated by the dorsal fornix and fimbria, respectively, as well as by the lateral septum. A posterior hypothalamic input, which does not involve the medial forebrain bundle or the midbrain reticular formation is also essential for the activation of this response.

Growth hormone and prolactin secretion after hypothalamic deafferentation in pigs.

Control of growth hormone (GH) and prolactin (PRL) secretion was investigated in ovariectomized, prepuberal Yorkshire gilts by comparing the effects of anterior (AHD), complete (CHD), and posterior (PHD) hypothalamic deafferentation with sham-operated controls (SOC). Blood samples were collected sequentially via an indwelling jugular catheter at 20-min intervals during surgery and recovery from anesthesia (Day 0) and Days 1 and 2 after cranial surgery. Mean serum concentrations of GH after AHD, CHD, and PHD were reduced (P less than 0.01) when compared with SOC gilts. Furthermore, episodic GH release evident in SOC animals was obliterated after hypothalamic deafferentation. PRL concentrations in peripheral serum of hypothalamic deafferentated gilts remained similar (P greater than 0.05) to those of SOC animals. These results indicate that anterior and posterior hypothalamic neural pathways play a minor role in the control of PRL secretion in the pig in as much as PRL levels remained unchanged after hypothalamic deafferentation. These findings may be interpreted to suggest that the hypothalamus by itself seems able to maintain tonic inhibition of PRL release. In contrast, the maintenance of episodic GH secretion depends upon its neural connections traversing the anterior and posterior aspects of the hypothalamus in the pig.

Hypothalamic deafferentation and luteinizing hormone-releasing hormone effects on secretion of luteinizing hormone in prepubertal pigs.

The control of luteinizing hormone (LH) secretion was investigated in ovariectomized, prepubertal Yorkshire pigs by comparing the effects of anterior (AHD), complete (CHD), and posterior (PHD) hypothalamic deafferentation to sham-operated controls (SOC). Gilts (n = 16) were assigned randomly to treatments, fitted with an indwelling jugular catheter, and ovariectomized 2 days before deafferentation or sham-operation (Day 0). Blood for radioimmunoassay (RIA) of LH was collected sequentially at 20-min intervals for a period of 2 h before and 24, 48, 72, and 96 h after hypothalamic deafferentation or SOC. Episodic LH release after AHD or CHD was abolished (p less than 0.01), but not after PHD or SOC. Concentrations of serum LH in AHD and CHD dropped (p less than 0.01) at 24 and 48 h after surgery. Levels of LH before and after surgery in PHD and SOC were similar (p greater than 0.05). Infusion of 25 micrograms LH-releasing hormone (LHRH) i.v. at 72 and 96 h after hypothalamic deafferentation and SOC increased (p less than 0.01) serum LH to peak levels within 15 min. after infusion; LH returned to basal levels 60-80 min later. By 96 h after surgery, LH response to LH-releasing hormone (LHRH) was less in AHD and CHD as compared with the response at 72 h postinjection. Concentrations of LH in PHD and SOC were similar (p greater than 0.05) at 72 and 96 h, respectively. The results from this study clearly indicate that neural stimuli originating or traversing the neural areas rostral to the median eminence are required for secretion of LH in the pig.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of extrahypothalamic structures on the hypothalamic cell nuclear binding of corticosterone.

Hypothalamic or hippocampal slices were taken from adrenolectomized male rats either sham operated or with anterior (AHD) of posterior (PHD) hypothalamic deafferentations and from rats with dorsal hippocampectomy (DHIPP). The slices were incubated in Krebs-Ringer bicarbonate buffer in the presence of 10-60 nM 3H-corticosterone (CS) with or without 500 fold excess of unlabeled CS for 30 min. The cell nuclear binding in the hypothalami from either PHD of DHIPP rats were markedly reduced (by 50-70%) as compared to control or AHD rats. The binding of 3H-CS in hippocampi from AHD or PHD rats was not affected. These results suggest that extrahypothalamic structures can modulate the binding of CS in the hypothalamus by neural inputs entering the medio-basal hypothalamus from the caudal direction; such modulation may explain previously observed influences upon hypothalamic sensitivity to the feedback action of glucocorticoids by extrahypothalamic structures.

Modifications of adrenocortical responses following frontal cortex simulation in rats with hypothalamic deafferentations and medial forebrain bundle lesions

With the purpose of delineating the neural pathways in the rat which mediate adrenocortical responses following frontal cortex stimulation, the effects of partial hypothalamic deafferentations and medial forebrain bundle lesion were studied. In intact and sham-operated animals, cortical stimulation through permanently implanted electrodes caused a significant increase in plasma corticosterone levels. In rats with anterior hypothalamic deafferentation and bilateral medial forebrain bundle lesions the adrenal response to cortical stimulation was blocked completely, while in animals with posterior hypothalamic deafferentation there occurred a normal rise in plasma corticosterone. These studies demonstrate that the frontal cortex effects on adrenocortical secretion are neurally mediated and involve an anterior hypothalamic input, more specifically the medial forebrain bundle.

Factors that influence test session performance measured 0, 3, or 6 h after inhibitory avoidance training

Rats were trained in a step-down inhibitory avoidance task using a 0.3-mA, 60-Hz footshock, and were tested at 0, 3, and 6 h from training. Retrieval scores (test session minus training session step-down latencies) were higher in control groups at 0 than at 3 or 6 h. Test session performance at 0 h was unaffected by the pretraining ip injection of ACTH1-24 (0.2 microgram/kg), epinephrine-HCl (5.0 micrograms/kg), human beta-endorphin (1.0 microgram/kg), or naloxone-HCl (0.4 mg/kg); or by a pretreatment with dexamethasone phosphate (2.0 mg/kg in divided doses 24 and 12 h before training); or by anterior or posterior hypothalamic deafferentation. Test session performance at 0 h was depressed by prior bilateral transection of the fornix, which suggests it depends on hippocampal function. The effect of the fornix lesion on test session performance at 0 h was not counteracted by ACTH, epinephrine, or beta-endorphin administration. When animals were tested 3 h after training, the post-training administration of ACTH and epinephrine caused an enhancement of test session performance; neither post-training beta-endorphin or naloxone, nor pretest ACTH, epinephrine, or beta-endorphin administration, had any effect in these animals. At 6 h from training, the post-training facilitatory action of ACTH and epinephrine was still present and the post-training depressant effect of beta-endorphin and the post-training facilitatory effect of naloxone became manifest, and so did the naloxone-reversible pretest facilitation induced by ACTH, epinephrine, or beta-endorphin. The influence of post-training naloxone or pretest beta-endorphin on retrieval scores at 6 h was not observed in the fornix-lesioned animals. Test session performance of this task at 0 h from training is regulated by different mechanisms than those which regulate test session performance at 3 or 6 h; in particular, it is less susceptible to modulation by the drugs used in the present study and it depends on the fornix; At least two major classes of modulatory factors influence retrieval scores at later times: consolidation-enhancing effects of ACTH and epinephrine, which become manifest at 3 h, and mechanisms related to beta-endorphin, which involve a form of state dependency and only become manifest at 6 h from training.

Posterior hypothalamic deafferentation abolishes the amnestic effect of electroconvulsive shock in rats

The amnestic effect of immediate post-training transcorneal electroconvulsive shock ECS (15.0 mA, 60 Hz, 2 sec) on step-down inhibitory avoidance learning (0.5 mA, 60 Hz training footshock) was studied in intact rats and in rats submitted to bilateral surgical transection of the dorsal fornix, to anterior or posterior hypothalamic deafferentation, and in sham-operated animals. Animals were tested for retention 24 hr after training. The amnestic effect of ECS was observed in all groups except in the one with the posterior hypothalamic lesion. Fornix-lesioned animals showed a moderate retention deficit which was considerably worsened by the ECS treatment. The results indicate that the amnestic effect of ECS requires integrity of posterior hypothalamic pathways. One possibility is that the amnestic effect of ECS may be mediated by posterior afferent fibers to the hypothalamus acting on hypothalamic opioid systems such as have been previously proposed to play a role in ECS-induced amnesia.

Response of the rat brain β-endorphin system to novelty: Importance of the fornix connection

In control rats, a step-down inhibitory avoidance training trial using a 0.8 mA footshock, or simple exposure to the training apparatus without footshock, was followed by a decrease of beta-endorphin-like immunoreactivity measured in the hypothalamus and ventral thalamus. The effect of inhibitory avoidance training was also measured in rats submitted to a brain sham operation, to bilateral transection of the dorsal fornix, to anterior or to posterior hypothalamic deafferentation, to adrenal medullectomy, to an adrenal sham operation, to 16 daily ip injections of 0.2 mg/kg dexamethasone, or to 16 daily ip injections of 1 ml/kg saline. The diencephalic beta-endorphin-like immunoreactivity response to training was abolished by fornix transection and was unaffected by all other treatments. This suggests that the response is not mediated by anterior or posterior neural afferents to the hypothalamus, or by a hypersecretion of epinephrine by the adrenal medullae, or of ACTH by the pituitary gland. The response, instead, appears to require the integrity of the pathway that sends projections from the septo-hippocampal system to the hypothalamus. Previous evidence had suggested that the diencephalic beta-endorphin-like immunoreactivity response to training is a result of novelty, and the septo-hippocampal system has been postulated to play a role in the registration of novelty.

A temporal study of post-adrenalectomy increase in ACTH secretion in the rat: effect of various hypothalamic deafferentations

Adult male rats, intact (N) or with complete (CHD), anterior (AHD), or posterior (PHD) hypothalamic deafferentations were biolaterally adrenalectomized. At 3, 6, 12 and 20 days post-adrenalectomy they were decapitated and trunk blood was collected for ACTH determinations. In N rats, ACTH markedly elevated up to 850 pg/ml. A similar ACTH response was found in PHD rats but the values were lower by approximately 20%. In contrast, in either AHD or CHD rats, ACTH responses were markedly attenuated and reached a plateau of about 350 pg/ml. These data suggest that: (1) neural inputs entering the medio-basal hypothalamus (MBH) from both the caudal and rostral directions are important for obtaining maximal ACTH responses following adrenalectomy; (2) at least part of this ACTH response is mediated by sites inside the MBH or in the pituitary.

ACTH and corticosterone secretion following insulin in intact and in variously hypothalamic deafferented male rats.

Adult male rats, intact (N) or with posterior (PHD), anterior (AHD) or complete (CHD) hypothalamic deafferentations, were injected with either 0.04 or 0.2 mu/100 g b.wt. of insulin. Forty-five minutes later they were decapitated and trunk blood was collected for serum glucose, adrenocorticotropic hormone (ACTH) and corticosterone (CS) determinations. The high insulin dose reduced serum glucose by approximately 50% and elicited a marked increase in serum ACTH and CS levels in all groups of animals as compared to the vehicle-treated group. In contrast, the low insulin dose which reduced serum glucose approximately 30% elicited a significant adrenocortical response only in the intact or PHD groups but failed to stimulate this response in animals with CHD or AHD. These results demonstrate that (1) CNS sites, outside the medial basal hypothalamus (MBH), mediate the adrenocortical response during the initial stages of hypoglycemia by a neural pathway impinging upon the CRF neurons from the rostral direction, and (2) the adrenocortical response during the more enhanced hypoglycemia stages is mediated by a systemic mechanism which acts directly on the MBH.

Adrenocortical responses following limbic stimulation in rats with hypothalamic deafferentations.

In view of the involvement of limbic structures in adrenocortical regulation, their afferent projections to the mediobasal hypothalamus were investigated. Electrical stimulation via chronically implanted electrodes in the dorsal hippocampus, the medial septal nuclei, the basolateral amygdala or the mesencephalic reticular formation all elicited a significant increment in plasma corticosterone levels in adult male rats under pentobarbital anesthesia. Complete or anterior hypothalamic deafferentation blocked these adrenocortical responses completely, and posterior hypothalamic deafferentation attenuated them to a marked extent. In animals with bilateral medial forebrain bundle lesions, hippocampal stimulation had no effect upon plasma corticosterone levels. These studies demonstrate that extrahypothalamic effects upon adrenocortical secretion are neurally mediated, and that the integrity of neural pathways impinging upon the mediobasal hypothalamus from both the rostral and the caudal directions is essential to these effects.

Effects of hypothalamic deafferentations on adrenocortical responses in the rat following hippocampal stimulation.

Adrenocortical responses, as expressed by changes in plasma corticosterone levels, following ether stress and dorsal hippocampal stimulation, were studied in intact rats and in rats with complete, anterior of posterior hypothalamic deafferentations. Ether stress produced normal responses in all experimental groups. In the three groups with hypothalamic deafferentations, the adrenocortical response following hippocampal stimulation was completely blocked, when compared to intact animals. The results suggest that the hippocampal signal enters the hypothalamus anteriorly, but that caudal propagation and posterior re-entry into the hypothalamus are also essential for the adrenocortical activation by the hippocampus.

Neural pathways mediating basal and stress-induced secretion of luteinizing hormone, follicle-stimulating hormone, and testosterone in the rat.

Adult male rats, intact or bearing complete, anterior, or posterior hypothalamic deafferentations (CHD, AHD, or PHD, respectively) or bilateral medial forebrain bundle (MFB) lesions, were acutely exposed to visual, audiogenic, or thermal stress. Two to 30 min after stress onset, the rats were decapitated, and trunk blood was collected from serum LH, FSH, and testosterone (T) determinations. While basal serum LH levels were found to be normal in all experimental groups, FSH levels were reduced in CHD and AHD rats, and serum T concentrations were found to be 3-fold greater than control values in the AHD group. In intact animals, exposure to all modalities caused significant elevations in serum levels of both LH and T, with no effect on FSH secretion. In the CHD and AHD groups, the LH and T responses were eliminated, with the exception of the T response to heat exposure, which persisted in CHD animals. In the PHD group, the LH and T responses persisted and were, in fact, potentiated. Bilateral medial forebrain bundle lesions inhibited the LH and T responses to audiogenic and thermal, but not to visual, stimulation. These data demonstrate that 1) basal FHS, but not LH, secretion is dependent upon extrahypothalamic afferents to the medial basal hypothalamus; 2) acute neurogenic stress stimulates LH and T, but not FSH, secretion; and 3) central nervous system sites, rostral to the medial basal hypothalamus, mediate the stress-induced elevations in LH release.

Effects of sciatic nerve stimulation on ACTH secretion, in intact and in variously hypothalamic deafferentated male rats.

Serum ACTH levels were determined by RIA following sciatic nerve stimulation, in intact rats and in rats bearing anterior (AHD), posterior (PHD) or complete (CHD) hypothalamic deafferentations. In both N and PHD groups, serum ACTH concentrations were markedly elevated following stimulation. In CHD animals this response was completely eliminated, and in the AHD group only a very slight and marginally significant increase in serum ACTH occurred, upon stimulation. These results demonstrate that the increased corticosterone secretion which occurs following sciatic nerve stimulation is due to elevated blood levels of ACTH, and confirm that this somatosensory stimulation impinges upon the hypothalamus from the rostral directions.