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Biomedical subjects

R G Dyer

Publications and source records attributed to R G Dyer.

At least 55 records · Page 3Linked to original sources
PubMed1983

Hypophysectomy does not prevent increased cerebral dopamine turnover following sulpiride administration.

Hypophysectomy is claimed to prevent increased forebrain dopamine turnover produced by administration of sulpiride. We have measured the increase in dopamine metabolite concentrations caused by sulpiride following surgical removal of the pituitary. In saline-treated control animals and in hypophysectomized rats 1 week or 1 month following surgery, administration of sulpiride caused marked elevations of striatal, nucleus accumbens and tuberculum olfactorium, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations which were maximal 4-8 h following drug administration. The maximal increases in nucleus accumbens and tuberculum olfactorium were generally comparable in control and hypophysectomized animals, except for a greater increase in HVA levels in the nucleus accumbens 1 month following hypophysectomy. However, maximal increases in HVA and DOPAC in striatum were more pronounced in hypophysectomized rats 1 week or 1 month following surgery compared with control animals. At 30 min following sulpiride administration only inconsistent changes in dopamine turnover were observed in both control and hypophysectomized rats. Hypophysectomy does not prevent sulpiride from increasing forebrain dopamine turnover suggesting this is due to a direct interaction with cerebral dopamine receptors.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1982

Stimulation of single unit activity in the preoptic area and anterior hypothalamus, and secretion of luteinizing hormone, by ovarian steroids in ovariectomized rats with diencephalic islands.

The spontaneous activity of 454 single hypothalamic neurons was recorded in 42 chronically ovariectomized rats after severance of all neural connections with the diencephalon. In 15 of these diencephalic island preparations progesterone was administered immediately before the recording session (and just after deafferentation of the diencephalon) and oestrogen 72 h beforehand. Thirteen rats were given two injections of oestrogen at these times and the remaining 14 rats were similarly treated with equal volumes of oil. Blood samples were obtained from all rats just before each hormone or oil injection, and 4, 5, 6 and 7 h after the second one, for subsequent measurement of plasma luteinizing hormone (LH) concentration. Only the group of rats given progesterone at the time of the second injection showed a significant increase in plasma LH concentration during the recording period. There was however some individual variation and the greatest LH surge was obtained from a rat given two injections of oestrogen. For steroid-treated rats the size of the LH surge was significantly correlated (P less than 0.01) with the mean firing rate of the neurons recorded in the preoptic and anterior hypothalamic areas (p.o.--a.h.). No similar correlation could be established for p.o.--a.h. cells recorded in oil-treated rats or for cells recorded in other parts of the hypothalamus in steroid-treated rats. The mean firing rate of all p.o.--a.h. cells recorded from rats treated with oestrogen followed by progesterone was significantly higher (P less than 0.05) than in either of the other two groups of animals. The oestrogen--progesterone treatment also significantly changed the regularity of discharge of the slow firing (less than 2 Hz) p.o.--a.h. cells, but this phenomenon could not be related to any alteration in plasma LH concentration. The experiments have demonstrated for the first time that the magnitude of the steroid-stimulated LH surge in ovariectomized rats is significantly correlated with the increase in the electrical activity of p.o.--a.h. neurons.

Action Potentials↗
PubMed1982

The supraoptic nucleus as an osmoreceptor.

The "Progress' review is written to challenge the prevalent belief that neurones of the supraoptic nucleus are not involved in the reception of osmotic stimuli. Evidence is presented that these neurones are depolarised by physiological increases in the osmotic pressure of the extracellular fluid. However, generation of action potentials in these depolarised cells still requires the presence of synaptic input from currently unidentified neurones. We suggest that the osmoreceptor regulating vasopressin secretion is probably composed of more than one type of neurone and discuss this suggestion in the context of selected experiments previously cited as evidence that the supraoptic nucleus is not involved in osmoreception.

Animals↗
PubMed1981

Electrical activity of antidromically identified tuberoinfundibular neurones during stimulated release of luteinizing hormone and prolactin in pro-oestrous rats.

Extracellular activity was recorded from tuberoinfundibular neurones in 28 urethane-anaesthetized pro-oestrous rats. In each rat the electrical activity of one antidromically identified neurone was analysed before and during electrical stimulation in the preoptic/anterior hypothalamic areas (PO/AH). This stimulation (50 Hz, 30 s on and 30 s off for 15 min) resulted in significant increases in plasma concentrations of prolactin (from 46 +/- 11 (S.E.M.) ng/ml before stimulation to 86 +/- 17 ng/ml 20 min later; P less than 0.02) and LH (27 +/- 6 to 48 +/- 11 ng/ml; P less than 0.01). A substantial proportion (11 out of 28) of the tuberoinfundibular neurones was inhibited by PO/AH stimulation. If such cells are directly involved with the secretion of anterior pituitary hormones it is probable that they synthesize and secrete inhibitory factors. We have suggested that cells inhibited by stimulation of the PO/AH are possibly the neurones which secrete prolactin inhibitory factor. The excited cells (eight out of 28) gave complex and variable responses to hypothalamic stimulation and were unable to follow applied stimuli at a ratio of more than one extra action potential for every ten stimulus pulses. We propose that this may explain why it is necessary to stimulate the PO/AH at ten times the frequency required for ovulation to occur following stimulation in the median eminence in anaesthetized pro-oestrous rats.

Animals↗
PubMed1981

Effects of neonatal exposure to monosodium glutamate on the electrical activity of neurones in the mediobasal hypothalamus, and on the plasma concentrations of thyroid-stimulating hormone and prolactin, following stimulation of the rostral hypothalamus in adult female rats.

Action potentials were recorded from 174 neurones in the mediobasal hypothalamus of ovariectomized adult female rats exposed neonatally to monosodium glutamate (MSG) and from 145 neurones in control rats. All of the animals, which were anaesthetized with urethane, had been ovariectomized for at least 3 weeks and received two injections of oestradiol benzoate (20 microgram/100 g body weight, i.m.) 72 h and immediately before the recording experiments. The response of each neurone to electrical stimulation of the median eminence and rostral hypothalamus (preoptic and anterior hypothalamus areas; PO/AH) was analysed. The most striking feature of the results obtained was the significant (P less than 0.001) loss of inhibitory responses in those neurones remaining in the adult rats after neonatal treatment with MSG. The loss of inhibitory responses applied to both stimulation sites. In each rat the response of one neurone, which was antidromically identified as projecting to the median eminence, was recorded before and during stimulation of the PO/AH at 50 Hz for 30s in every min for 15 min. Before and after this stimulation blood was collected from a jugular vein for estimation by radioimmunoassay of concentrations of prolactin and TSH. In the MSG-treated rats significantly (P less than 0.05) fewer neurones were inhibited by the 50 Hz stimulation than in control rats. In control rats the plasma concentrations of prolactin nearly quadrupled as an immediate consequence of this treatment, whereas in MSG-treated rats plasma concentrations barely doubled. However, in the MSG-treated rats plasma concentrations of prolactin continued to rise after stimulation ceased, possibly as a consequence of enhanced secretion of thyrotrophin releasing hormone.

Action Potentials↗
PubMed1981

Secretion of luteinizing hormone in ovariectomized adult rats treated neonatally with monosodium glutamate.

We have studied the possible effects of monosodium glutamate (MSG) on LH secretion in ovariectomized rats. In experiment 1 MSG-treated and control rats were given oestradiol benzoate at noon and 72 h later half the rats in each group were given a second injection of oestradiol benzoate or progesterone. Blood samples were taken immediately before and 6 h after these i.m. injections. At 78 h there were no significant differences in plasma LH concentration measured in the two groups of rats given progesterone or in the two groups given a second injection of oestradiol benzoate although for both MSG-treated and control rats progesterone produced a significantly (P less than 0.01) greater LH surge than did oestradiol benzoate. In experiment 2 100 ul blood samples were collected at 5-min intervals for up to 3 h from MSG-treated and control rats. For rats showing more than one pulsatile discharge of LH, peak and trough values for plasma LH concentrations were no significantly influenced by MSG treatment. However the mean pulse height was significantly (P less than 0.001) greater in the MSG-treated group than in control rats. Pulsatile release stopped more quickly in the MSG-treated rats and their mean plasma LH concentration after 120 min of blood sampling was significantly (P less than 0.05) lower than that obtained in the control animals. Thus, although some aspects of LH secretion seem to be significantly different in MSG-treated rats, these effects may result from the greater sensitivity of the MSG-treated animals to experimental manipulation.

Animals↗
PubMed1980

Bursting activity in tuberoinfundibular neurones during electrical stimulation of the rostral hypothalamus.

The firing of tuberoinfundibular neurones antidromically identified as projecting to the median eminence was recorded during electrical stimulation of the preoptic and/or anterior hypothalamic areas (PO/AH) of the rostral hypothalamus. Parameters of stimulation were chosen which are known to cause increased secretion of most adenohypophyseal hormones. Twenty-three of 58 neurones were excited during the periods of stimulation and approximately half of these excited cells fired in a bursting fashion reminiscent of that previously observed in other endocrine neurones.

Animals↗
PubMed1980

Discharge of gonadotrophin-releasing hormone from the mediobasal part of the hypothalamus: effect of stimulation frequency and gonadal steroids.

The release of gonadotrophin-releasing hormone (Gn-RH), in response to electrical stimulation of the mediobasal part of the hypothalamus incubated in vitro, was studied in both male and female rats. In male rats significant release of Gn-RH occurred during the 10-min experimental period only when the incubated tissue was stimulated at frequencies of 10 Hz or greater. There was no release when stimulated at 5 Hz. There was also no release of hormone when the mediobasal hypothalami were incubated in a Ca2+ free medium. The amount of Gn-RH released during a 10-min incubation period increased progressively as the frequency of stimulation was raised from 10--100 Hz. During short (4-min) incubation periods the effectiveness of each stimulus pulse for Gn-RH release also increased with the frequency of stimulation. However, when stimulated for 10 min there was no increase in hormone released per stimulus pulse when frequency of stimulation was raised above 10 Hz. The amount of Gn-RH released in response to stimulation at 50 Hz was greater in male rats than in females. For the females, there was no significant difference between the amounts of Gn-RH released at dioestrus and pro-oestrus. In both male and female rats gonadectomised 4 weeks prior to hypothalamic incubation, the response to electrical stimulation at 50 Hz was reduced when compared with intact controls. Indeed, for the females there was no longer a statistically significant increase in the amount of Gn-RH in the incubation medium after 50 Hz stimulation. Ovariectomised female rats, injected twice daily for 3.5 days with 20 microgram of oestradiol benzoate released Gn-RH in response to 50 Hz stimulation in the same amounts as intact control animals. By contrast, there was no recovery of Gn-RH release to normal levels in castrated male rats similarly treated with 1.25 mg testosterone propionate.

Animals↗
PubMed1980

Relationship between duration of urethane or pentobarbitone anaesthesia in male and female rats and adenohypophyseal response to luteinising hormone-releasing hormone.

1 Luteinizing hormone releasing hormone (50 ng/100 g body weight) was injected into male and female rats which had been anaesthetized with either urethane (1.2 g/kg) or pentobarbitone (45 g/kg) for 15, 100 or 240 min and serial blood samples were taken for estimation of plasma luteinizing hormone concentrations. 2 The rats anaesthetized with urethane tended to show a greater response to the releasing hormone than those given pentobarbitone. 3 The magnitude of the responses observed in male rats and in female rats anaesthetized with pentobarbitone did not change as the period of anaesthesia prior to injection of releasing hormone was lengthened from 15 min to 4 h. By contrast, in the groups of female rats anaesthetized with urethane the magnitude of the response to luteinizing hormone releasing hormone was related to the length of pretreatment anaesthesia. Thus both dioestrous and pro-oestrous rats given releasing hormone only 15 min after the onset of urethane anaesthesia had significantly (P less than 0.001 and less than 0.05 respectively) higher concentrations of luteinizing hormone (LH) in the plasma than rats treated with the releasing hormone after 240 min of anaesthesia. 4 These effects were not due to a differential action of the anaesthetics on the mechanism for clearing LH from the plasma.

Anesthesia↗
PubMed1979

A method for recording single unit activity from the brains of foetal pigs in utero.

A technique is described for recording single unit activity from foetal miniature pigs in utero. The method involved anaesthetizing the sows with halothane and exteriorizing a sufficient length of the uterus to contain a single foetus. Both foetus and uterus were placed in a stereotaxic frame which was held independent of the mothers' support. Preliminary analysis of recordings obtained from spontaneously firing single nerve cells in the post-central cerebral cortex indicate that the patterns of spike activity in prenatal pigs are very similar to those observed in prepubertal animals.

Animals↗
PubMed1979

Inhibition by pentobarbitone and urethane of the in vitro response of the adenohypophysis to luteinising hormone-releasing hormone in male rats.

1 The effect of urethane (ethyl carbamate) and sodium pentobarbitone on the luteinising hormone-releasing hormone (LH-RH)-stimulated secretion of luteinising hormone (LH) was investigated with hemipituitaries obtained from male rats and incubated in vitro. 2 Urethane and pentobarbitone were added to the incubation medium to provide final concentrations of 2.2 mg or 4 micrograms/ml and 100 micrograms or 0.1 microgram/ml respectively. The high doses of these anaesthetics blocked the LH-RH stimulated secretion of LH. The low doses significantly reduced the amounts of LH released in response to LH-RH but did not block the response completely. 3 Both concentrations of urethane reduced the basal release of LH. 4 The inhibitory action of the anaesthetics was reversible. 5 The results indicate that the two anaesthetics most commonly used in neuroendocrine experiments have a significant inhibitory action on the release of LH from the adenohypophysis.

Animals↗
PubMed1979

Electrochemical deposition of iron onto rat oxytocinergic neurones does not result in milk-ejection.

Exposure of the preoptic-tuberoinfundibular system to iron ions causes ovulation in the pro-oestrous rat. This electrochemical treatment has been used frequently to study the way that the hypothalamus triggers secretion of gonadotrophic hormone and most investigators have assumed that the effect is mediated by the direct excitation of those neurones exposed to the cation. The present paper reports experiments designed to confirm that iron directly excites hypothalamic neurones to the firing frequencies essential for ovulation. Because of the difficulty of measuring input-output relationships in the diffuse preoptic-tuberoinfundibular system the experiments were performed on the anatomically distinct oxytocinergic neurones of the paraventricular-hypophysial tract in lactating rats. These neurones were simulated electrically (from 10 to 50 Hz for between 1 and 300 s) and subjected to the electrochemical treatment by depositing iron ions from the tip of the stimulating electrode (up to 250 muA anodal current for between 60 and 180 s). Changes in intramammary pressure were used to indicate oxytocin release and mammary glands were calibrated for sensitivity by intravenous injection of oxytocin. Electrical stimulation in excess of about 15 Hz invariably caused release of sufficient oxytocin to cause a rise in intramammary pressure. In contrast, no changes in pressure were observed during or after the electrochemical deposition of iron. The results suggest that the response of hypothalamic neurones to electrochemical treatment is not the same as their response to electrical stimulation.

Animals↗
PubMed1978

Electrical stimulation of the hypothalamus: new observations on the parameters necessary for ovulation in rats anaesthetised with pentobarbitone during the pro-oestrous "critical period".

The experiments were designed to specifiy optimal parameters of hypothalamic electrical stimulation to induce ovulation in rats anaesthetised with pentobarbitone during the "critical period" on the afternoon of pro-oestrous. We have established that duration of stimulation in the arcuate region of the hypothalamus is of greater importance than total number of pulses delivered. The minimum period of stimulation to ensure that all rats ovulated was 45 min. However, stimulation in the arcuate region did not have to be continuous throughout this period. Indeed, three 5-min bursts at 100 Hz distributed equally through 45 min always caused ovulation whereas a continuous 15-min stimulation at the same frequency was without effect. With 30 min of continuous stimulation in the arcuate region, at frequencies ranging from 10 to 100 Hz, approximately half of the rats ovulated.

Anesthesia, General↗
PubMed1978

Preovulatory secretion of gonadotrophins and TSH in cyclic rats with small lesions in the preoptic area.

Animals with unilateral or bilateral lesions, with respectively 85 and 70% of intact preoptic area tissue remaining, returned to regular oestrous cycles. By comparison with sham-operated controls, rats with lesions had more days with a leucocytic smear in each of the 1st, 2nd or 3rd post-operative cycle, before returning to a 4-day cycle. Hormone measurements of blood collected during the afternoon of pro-oestrus showed that, in rats with bilateral lesions, the duration of the preovulatory LH secretion was significantly shortened, although peak levels were unchanged; the initial phase of the preovulatory FSH secretion was lost; and much of the prolactin surge was also inhibited. The TSH surge was unaffected.

Animals↗