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Effects of Narcotic Analgesics on Hypothalamo-Pituitary-Thyroid Function
Effects of Narcotic Analgesics on Hypo thalamo-Pituit ary-Thyroid Function ROBERT GEORGE Department of Pharmacology and Brain Research Institute, Center for the Health Sciences, University of California, Los Angeles, Calif. 90024 (U.S.A.)
Most of the early publications regarding the interrelationship of morphine and the thyroid dealt chiefly with the effects of administration of thyroid preparations or thyroidectomy on morphine toxicity. Data from studies in mice, rats, guinea pigs, dogs and rabbits have been conflicting, although in man the consensus is that hypothyroidism may lead to increased sensitivity to morphine (Krueger et al., 1941). In view of our present knowledge about thyroid physiology and neuroendocrine control mechanisms it is rather surprising that very few studies have been done to determine the effects of narcotic analgesics, agents which are widely used clinically, on thyroid activity. The first studies in which 1 3 1 1 was used for assessing pituitary-thyroid activity during morphine administration were reported by Ssimel (1958). He injected rats subcutaneously for 5 days with morphine (10 mg/kg) and then measured 13'T uptake by the thyroid and the level of protein-bound 1 3 1 1 in blood. Both were reduced significantlywhen compared with uninjected controls, as were the pituitary and thyroid weights. In another experiment %me1 determined the effect of morphine on the rate of release of thyroid hormone by measuring the radioactivity of the neck region of 13'I-labeled thyroids. Injection of morphine was found to inhibit significantly the normal release rate. Redding et al. (1966) have reported similar findings in mice following the administration of morphine, codeine, dihydromorphinone, levorphan, dextrorphan or meperidine. The injection of 500 pg daily of each drug for 5 days significantlyinhibited thyroidal 1 3 1 1 uptake. On the other hand, when each drug was administered acutely as a single injection of 500 pg per mouse, there was a marked increase in release of 1311 from the thyroid and this effect was abolished by hypophysectomy. The inhibitory effect of chronic codeine administration on thyroid activity has been confirmed by Schreiber et al. (1968). They found that daily feeding of codeine (approximately 5 mg/kg) for a period of 14 days significantly reduced the uptake of 1311 by rat thyroids. However, hypertrophy of the pituitary and thyroid glands produced by methylthiouracil was not altered by chronic codeine administration. Thus, it would seem that References p . 344-345
340
R. GEORGE
codeine can lower the basal rate of thyroid function but does not interfere with hypersecretion of thyrotrophin (TSH) where there is thyroid hormone deficiency. Histological findings also have indicated that chronic morphine administration inhibits pituitary-thyroid function. Hohlweg et al. (1961) observed that injection of morphine twice daily (30 and 60 mg/kg) decreased significantly thyroid weights and pituitary TSH content, and prevented the appearance of thyroidectomy cells in the pituitaries of rats. Although all of the data described above show that narcotic analgesics can modify thyroid gland activity, none show that their action is mediated via the hypothalamicpituitary-thyroid axes. Thus, in collaboration with Doctors Peter Lomax and Norio Kokka, we have attempted to determine the site(s) and the mechanism(s) by which morphine alters thyroid function. In all of our studies we have measured thyroid activity by studying the rate of loss of organically bound 13'1from the thyroid gland of rats. This is a more reliable method for measuring thyroid activity than the 13'1 uptake method and provides a continuous measure of thyroid activity (Brown-Grant et al., 1954a). The procedure consisted of injecting each rat with carrier-free radioiodine as [' 311] Na (5-20 pCi intraperitoneally for 3 days prior to the experiments. The radioactivity of the thyroid gland was measured then by placing the animal's neck over a flat field, 5-cm diameter scintillation detector. Prior conditioning of the animals ensured fairly constant positioning without struggling and the mean of three 10-sec counts was recorded. Decay curves of thyroid radioactivity, after correction for isotopic decay, against time and the release rate was calculated from the graph. In an early study (George and Lomax, 1965) we noted that repeated administration of small doses of morphine sulfate (5-10 mg/kg) inhibited thyroid hormone release within 24 h following the first injection. This inhibition persisted throughout the injection period of 66 h and for 24-30 h following the last injection; the release rate then returned to normal (Table I). The injection of chlorpromazine (5 mg/kg) also promptly inhibited thyroid hormone release whereas reserpine, even in near lethal doses, was ineffective. Since morphine and chlorpromazine are known to increase TABLE I SUMMARY TABLE OF THE EFFECTS OF SYSTEMIC ADMINISTRATION OF MORPHINE ON ~ Y R O I D1311 RELEASE RATES IN INTACT, ADRENALECTOMIZED AND HYPOTHALAMIC LESIONED RATS
Number of
animals -
Intact Adrexa Rostra1 hypothalamic lesions Caudal hypothalamic lesions
10 11 9 9
Mean initial release rate f S.E.M. (%per 2 4 h) _ _ - 16.9 0.7 19.5 1.7 14.8 i 0.6 13.4 5 0.8
Mean release rate after morphine
P
(%)
- .
- -
-
3.0 & 1.5 5 3.0 13.0 &
0.8 0.6 1.1 1.0
-
tO.O1
tO.O1 <0.01
>0.5
-
MORPHINE AND THYROID FUNCTION
34 1
adrenal corticosteroid secretion, and since a variety of stresses and corticosteroid administration (Brown-Grant et al., 1954b, c) inhibit thyroid activity, the effects of morphine and chlorpromazine were investigated in adrenalectomized animals. It was found that adrenalectomy did not interfere with the thyroid-inhibiting effect of both morphine and chlorpromazine; in fact, the effect was slightly enhanced (Table I). In addition, experiments were done to determine whether morphine inhibited thyroid activity by interfering with the action of TSH on the thyroid. It was noted that administration of TSH stimulated release of thyroid hormone in morphinized rats to the same degree as that found in the controls. 'Thus, on the basis of these findings and those reported by others, it appeared that the inhibitory action of morphine on thyroid activity is mediated at the level of the pituitary gland or the czntral nervous system (CNS). In order to further localize this inhibitory site of morphine action, studies were designed to investigate the effects of systemic morphine administration on thyroid release rates in rats with hypothalamic lesions (Lomax and George, 1966). Bilateral electrolytic lesions were made in two groups of rats: one in the region of the anterior hypothalamic/ventromedial nuclei and the other in the medial mammillary nuclei. Repeated injections of morphine (5-10 mg/kg) produced a marked inhibition in thyroid activity in rats with anterior lesions. The inhibition was comparable to that found in intact animals. On the other hand, lesions in the mammillary nuclei completely blocked the thyroid-inhibitory effect of morphine and the release curves were virtually identical with those of untreated controls (Table I). The lesion sites, on histological examination were found to involve, rostrally, the paraventricular and anterior hypothalamic nuclei and, caudally, all or part of the medial mammillary nuclei. Most importantly, the median eminence and the infundibular tract were left intact. From these data it would appear that the thyroid inhibitory effect of morphine is mediated via the caudal hypothalamus in the region ofthe medial mammillary nuclei. It is well established that the anterior region of the median eminence of the hypothalamus is concerned with the regulation of pituitary TSH secretion. Electrolytic lesions in this area reduce the basal level of thyroid secretion and prevent propylthiouracil-induced goiter formation while electrical stimulation of this region increases thyroid activity (Harris and George, 1969), and recently it has been shown by Martin and Reichlin (1970) that such stimulation increases plasma TSH levels as measured by radioimmunoassay in rats. And finally, the presence of thyrotrophinreleasing factor (TRF) in this region has been clearly documented. Also, it is apparent that many regions of the CNS which relay to the hypothalamus participate in the regulation of TSH secretion, and the anterior median eminence region probably represents the final pathway through which reflex inhibition or activation of TSH secretion takes place. Supporting evidence for such a concept comes from the following reports: lesions in the region of the habenular nuclei abolish iodine-deficiency goiter and reduce the inhibitory effect of thyroxine on TSH secretion (Szendgothai and Mess, 1958; Bogdanove, 1962); midbrain transection in the dog inhibits thyroidal 13'1 release (Anderson et al., 1957); electrical stimulation of the hippocampus elicited an increase in release of 13*1-labeledthyroid hormone in References p . 344-345
342
R. GEORGE
dogs (Shizume et al., 1962). Inhibition of thyroid hormone release has been noted also following electrical stimulation of the mesencephalic reticular formation in rats (Kovacs et al., 1965) and caudal hypothalamus in rats (Vertes et al., 1965) and rabbits (Harris and Woods, 1958). Morphine is a compound which has a dual action on the CNS, i.e. it may act as a stimulant or a depressant. The dose of morphine used in these studies (5 mg/kg) was found to cause an elevation in body temperature in the rat in contrast to higher doses which produced a marked hypothermia. These temperature changes are due to a direct action of morphine on the thermoregulatory centers in the anterior hypothalamus (Lotti et a/., 1965a). Also, injection of morphine into rats pretreated with nalorphine, or into rats made tolerant to morphine, frequently produced a rise in their body temperature (Lotti et al., 1965b; 1966). If one assumes that tolerance develops to the depressant and not the stimulant effects of morphine (Seevers and Deneau, 1963) and that nalorphine antagonizes, primarily, the depressant effects of morphine (Woods, 1956), then the increase in core temperature in the rats studied by Lotti et al. (1965a and b) must be due to a stimulant effect of morphine on the thermoregulatory centers in the hypothalamus. Thus, if we can extrapolate from these studies to the effect of morphine on thyroid activity, it would seem possible that the dose of morphine used (5 mg/kg) could have had an excitatory effect on the hypothalamic neurons resulting in a reduction in thyroid activity by activation of inhibitory areas in the posterior hypothalamus. To test this hypothesis, studies were undertaken to determine the effects of microinjection of morphine into various regions of the hypothalamus on thyroid function in the rat (Lomax et al., 1970). Guides for injection cannulae were implanted symmetrically on each side of the midline. The guides were positioned so as to allow injection at various sites, within 1 mm of the mid-sagittal plane, extending from the preoptic to the supramammillary nuclei. The position of the injection sites were verified on completion of the experiments by injecting 1 pl of methyl red at the same point as the drug and sectioning the brains on a freezing microtome. For intracerebral injection, morphine sulfate was dissolved in 0.9 % NaCl. The concentration was adjusted so as to allow a constant injection volume of 1 pl. Doses of 5 or 10 pg of morphine were injected. A total of 36 rats received bilateral injections of morphine sulfate into various regions of the hypothalamus. They were injected with 5 pg in each site at 48 and 56 h, then 10pg at 72 and 80 h after the onset of thyroid counting. Of the animals so treated, 14 exhibited marked inhibition of thyroid l3'I release during the period of injections. The injection sites in these 14 rats were found to lie in two principal areas: between the preoptic nuclei and the chiasma or in the region of the posterior and supramammillary nuclei. In the 6 rats with rostra1 injection sites, morphine cimpletely arrested the release of 13'1, but the release returned to normal after the 72nd h, before the final injection of morphine. In contrast to these animals, the release of 13'1 was abolished throughout the period of morphine administration in the 8 animals injected into the caudal hypothalamic regions. Injection of morphine into other hypothalamic sites failed to alter normal thyroid activity.
MORPHINE AND THYROID FUNCTION
343
The inhibition of thyroid secretion following morphine injection into the rostral hypothalamus was a rather unexpected finding and indicates that the drug does not have a single site of action. These data pointed out that morphine has a more complex action on pituitary-thyroid function than we had visualized from our earlier experiments. As mentioned previously, morphine has a dual action on the CNS, both stimulation and depression of neurons can occur. Tolerance develops to the depressant but not to the stimulant action of morphine. Therefore, it is interesting to note that, in the case of injections into the rostral hypothalamic sites, there was a reversion to the normal thyroid release rate even during the period of morphine administration. This suggests the development of tolerance to morphine. A similar type of tolerance was seen to the hypothermic effect of morphine by Lotti et al. (1966). In contrast to this, tolerance was not detected when morphine was injected into the caudal hypothalamus. It would seem then that there are two target sites for morphine in the hypothalamus: both activation of a caudal site and depression of a rostral site leading to decreased pituitarythyroid activity. This possibility is further supported by the studies of Lotti et al. (1965a) who found that injection of microquantities of morphine into the rostral hypothalamus produced hypothermia, catatonia, respiratory depression and an increase in pain threshold, effects which are commonly noted after a large systemic dose of morphine. Conversely, injection of morphine into the caudal hypothalamus produced hyperthermia and marked excitation, associated with increased motor activity and agressive behavior.
SUMMARY
It appears that narcotic analgesics have a dual action on the secretion of thyrotrophin (TSH) secretion, however, the predominant effect appears to be one which produces inhibition of pituitary-thyroid function. The inhibitory effect of morphine on TSH secretion is mediated via the hypothalamic-pituitary axis and probably by an excitation of inhibitory neurons that lie in the caudal hypothalamus. Although the mechanism(s) by which narcotic analgesics influence hypothalamicpituitary activity has not been clearly elucidated, two possibilities arise. The first is that analgesics exert their effect on the pituitary through a direct action on the hypothalamic neurosecretory cells that contain releasing factors. The second possibility focusses on the presence of the “neurotransmitters”, acetylcholine, dopamine, noradrenaline and serotonin which are found in the hypothalamus. If these transmitters lie in a presynaptic position to neurosecretory cells, it is conceivable that narcotic analgesics can exert their action on the pituitary by interfering with the synthesis and/or release of the neurotransmitters at the presynaptic site, thereby altering the activity of the neurosecretory cells. Of these two possibilities, the latter seems more likely since there is evidence that narcotic analgesics may alter release and turnover of the neurotransmitters (Smith References p . 344-345
344
R. GEORGE
'
1972) and that the neurotransmitters have been implicated in the control of pituitary function (George, 1971). ACKNOWLEDGEMENT
The preparation and part of the work reported in this manuscript were supported by research grants from USPHS, MH-17691, and MH-20787. REFERENCES ANDERSON, E., BATES,R. W., HAWTHORNE, E., HAYMAKER, W., KNOWLTON, K., RIOCH,D. McK., SPENCE,W. R. AND WILSON,H. (1957) The effects of midbrain and spinal cord transection on endocrine function with postulation of a midbrain hypothalamico-pituitary activating system. Recent Progr. Hormone Res., 13, 21-66. BOGDANOVE, E. M. (1962) Regulation of TSH secretion. Fed. Proc., 21, 623-627. BROWN-GRANT, K., VONEULER,C., HARRIS,G. W. AND REICHLIN, S . (1954a) The measurement and experimental modification of thyroid activity in the rabbit. J. Physiol. (Lond.), 126, 1-28. BROWN-GRANT, K., HARRIS, G. W. AND REICHLIN, S. (1954b) The effect of emotional and physical stress on thyroid activity in the rabbit. J. Physiol. (Lond.), 126, 2940. BROWN-GRANT, K., HARRIS,G. W. AND REICHLIN, S. (1954~)The influence of the adrenal cortex on thyroid activity in the rabbit. J. Physiol. (Lond.), 126, 41-51. GEORGE, R. (1971) Hypothalamus: anterior pituitary gland. In Narcotic Drugs Biochemical Pharmacology, D. H. CLOUET (Ed.), Plenum, New York, pp. 283-299. GEORGE,R. AND LOMAX,P. (1965) The effects of morphine, chlorpromazine and reserpine on pituitary-thyroid activity in rats. J. Pharmacol., 150, 129-134. HARRIS,G. W. AND GEORGE,R. (1969) Neurohumoral control of the adenohypophysis and the regulation of the secretion of TSH, ACTH and growth hormone. In The Hypothalamus. W. HAYMAKER, E. ANDERSON AND W. J. H. NAUTA (Eds.), Thomas, Springfield, Ill., pp. 326-388. HARRIS,G. W. AND WOODS,J. W. (1958) The effect of electrical stimulation of the hypothalamus or pituitary gland on thyroid activity. J . Physiol. (Lond.), 143, 246-274. HOHLWEG, V. W., KNAPPE,G. AND DORMER, G. (1961) Tierexperimentelle Untersuchungen iiber den Einfluss von Morphin auf die Gonadotrope und thyrotrope Hypophysenfunktion. Endokr. Bd., 40, 152-159. KRUEGER, H., EDDY,N. B. AND SUMWALT, M. (1941) Pharmacology of the opium alkaloids. U.S. Public Health Service, Public Health Report Suppl. 165, parts 1 and 2. KOVACS, S., VERTES, Z., SANDOR,A. AND VERTES, M. (1965) The effect of mesencephalic lesions and stimulation on pituitary-thyroid function. Actu physiol. acad. sci. hung., 26,227-233. LOMAX, P. AND GEORGE, R. (1966) Thyroid activity following administration of morphine in rats with hypothalamic lesions. Bruin Res., 2, 361-367. LOMAX, P., KOKKA, N . AND GEORGE, R. (1970) Thyroid activity following intracerebral injection of morphine in the rat. Neuroendocrinology, 6, 146-152. LOTTI,V. J., LOMAX, P. AND GEORGE, R. (1965a) Temperature responses in the rat following intracerebral microinjection of morphine. J. Phurmacol., 156, 135-139. LOTTI,V. J., LOMAX, P. AND GEORGE, R. (1965b) N-allylnonnorphine antagonism of the hypothermic effect of morphine in the rat following intracerebral and systematic administration. J. Pharmacol., 156,420-425. LOTTI,V. J., LOMAX, P. AND GEORGE, R. (1966) Acute tolerance to morphine following systemic and intracerebral injection in the rat. Int. J. Neuropharmacol., 5, 3542. MARTIN,J. B. AND REICHLIN, S. (1970) Thyrotropin secretion in rats after hypothalamic electrical stimulation or injection of synthetic TSH-releasing factor. Science, 168, 1366-1388. REDDING,T. W., BOWERS, C. Y.AND SCHALLY, A. V. (1966) The effects of morphine and other narcotics on thyroid function in mice. Acta endocrinol., 51, 391-399. SAMEL,M. (1 958) Blocking of the thyrotrophic hormone secretion by morphine and chlorpromazine in rats. Arch. int. Pharmacodyn., 117, 151-157.
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SCHREIBER, V., ZBUSEK, V. AND ZBUZKOVA-KMENTOVA, V. (1968) Effect of codeine on thyroid function in the rat. Physiol. bohemoslov., 17,253-258. SEEVERS, M. H. AND DENEAU, G. A. (1963) Physiological aspects of tolerance and physical dependence. In Physiological Pharmacology, vol. 1, W. s. ROOTAND F. G. HOFMAN (Eds.), Academic Press, New York, pp. 565-640. SHIZYME, K., MATUZAKI, F., IINO,S., MATSUDA, K., NAGATAKI, S. AND OKINAKA, S. (1962) Effect of electrical stimulation of the limbic system on pituitary-thyroidal function. Endocrinology, 71, 456-463. SMITH,C. B. (1972) Neurotransmitters and the narcotic analgesics. In ChemicaI and Biological Aspecis of Drug Dependence, S. J. MULBAND H. BRILL(Eds.), CRC Press, Cleveland, pp. 495-504. SZENTAGOTHAI, J. AND MESS, B. (1958) Central control of thyrotropic activity of the anterior pituitary lobe; functional significance of nucleus medialis habenulze. Wien. Klin. Wschr., 70,259-261. VERTES,M., VERTES, Z. AND KOVACS, S. (1965) Effect of hypothalamic stimulation on pituitary-thyroid function. Acia physiol., 27, 229-235. WOODS,L.A. (1956) The pharmacology of nalorphine. Pharmacol. Rev., 8, 175-198.
DISCUSSION ALIVISATOS: In view of our previous reports that morphine interferes competitvely with certain serotoninergic receptors, I wonder whether serotonin was tried in counteracting this effect of morphine? GEORGE: The mechanism by which morphine inhibits thyroid activity is complicated and does not seem to be serotonin-linked. We know that the amines in the hypothalamus are concerned with secretion of the anterior lobe hormones. We also know that morphine influences the levels or the turnover rates of these amines. In studies I didn’t include here, we morphinized animals over a 6 week period and then challenged them with TRF. TRF, in the doses used, produced an increase in thc release of thyroid hormone of normal animals but was blocked in the morphine-treated animals. On the basis of these preliminary findings we suspect that possibly the blocking effect is somewhere between TRF and the pituitary gland. KASTIN:Just to extend to humans, your last remark, although methadone certainly isn’t morphine, we have tested, in collaboration with Drs. Ruiz and Vargas, TRH administration in 4 patients treated with methadone. Three of them gave perfectly normal responses and one gave no response so we are further looking at that. KRIVOY: Would you like to speculate on the relevance of these data to the addiction cycle? GEORGE: I don’t think the pituitary hormones are directly involved in addiction although they may still play a secondary role in the addiction cycle. We all know that the addiction problem is a serious and complex one. For example, the controversy regarding the role of serotonin in the development of physical dependence to morphine has not been resolved. MESS:It appeared that the anterior hypothalamic lesions were ventral and the posterior ones were localized more dorsally. If so, your data would correlate with thermal regulatory data of my experiments with Dr. Donhoffer. GEORGE:The drug effect was most pronounced when placed in the thermoregulatory area and reduced when the drug was injected into a more dorsal site.
Most of the early publications regarding the interrelationship of morphine and the thyroid dealt chiefly with the effects of administration of thyroid preparations or thyroidectomy on morphine toxicity. Data from studies in mice, rats, guinea pigs, dogs and rabbits have been conflicting, although in man the consensus is that hypothyroidism may lead to increased sensitivity to morphine (Krueger et al., 1941). In view of our present knowledge about thyroid physiology and neuroendocrine control mechanisms it is rather surprising that very few studies have been done to determine the effects of narcotic analgesics, agents which are widely used clinically, on thyroid activity. The first studies in which 1 3 1 1 was used for assessing pituitary-thyroid activity during morphine administration were reported by Ssimel (1958). He injected rats subcutaneously for 5 days with morphine (10 mg/kg) and then measured 13'T uptake by the thyroid and the level of protein-bound 1 3 1 1 in blood. Both were reduced significantlywhen compared with uninjected controls, as were the pituitary and thyroid weights. In another experiment %me1 determined the effect of morphine on the rate of release of thyroid hormone by measuring the radioactivity of the neck region of 13'I-labeled thyroids. Injection of morphine was found to inhibit significantly the normal release rate. Redding et al. (1966) have reported similar findings in mice following the administration of morphine, codeine, dihydromorphinone, levorphan, dextrorphan or meperidine. The injection of 500 pg daily of each drug for 5 days significantlyinhibited thyroidal 1 3 1 1 uptake. On the other hand, when each drug was administered acutely as a single injection of 500 pg per mouse, there was a marked increase in release of 1311 from the thyroid and this effect was abolished by hypophysectomy. The inhibitory effect of chronic codeine administration on thyroid activity has been confirmed by Schreiber et al. (1968). They found that daily feeding of codeine (approximately 5 mg/kg) for a period of 14 days significantly reduced the uptake of 1311 by rat thyroids. However, hypertrophy of the pituitary and thyroid glands produced by methylthiouracil was not altered by chronic codeine administration. Thus, it would seem that References p . 344-345
340
R. GEORGE
codeine can lower the basal rate of thyroid function but does not interfere with hypersecretion of thyrotrophin (TSH) where there is thyroid hormone deficiency. Histological findings also have indicated that chronic morphine administration inhibits pituitary-thyroid function. Hohlweg et al. (1961) observed that injection of morphine twice daily (30 and 60 mg/kg) decreased significantly thyroid weights and pituitary TSH content, and prevented the appearance of thyroidectomy cells in the pituitaries of rats. Although all of the data described above show that narcotic analgesics can modify thyroid gland activity, none show that their action is mediated via the hypothalamicpituitary-thyroid axes. Thus, in collaboration with Doctors Peter Lomax and Norio Kokka, we have attempted to determine the site(s) and the mechanism(s) by which morphine alters thyroid function. In all of our studies we have measured thyroid activity by studying the rate of loss of organically bound 13'1from the thyroid gland of rats. This is a more reliable method for measuring thyroid activity than the 13'1 uptake method and provides a continuous measure of thyroid activity (Brown-Grant et al., 1954a). The procedure consisted of injecting each rat with carrier-free radioiodine as [' 311] Na (5-20 pCi intraperitoneally for 3 days prior to the experiments. The radioactivity of the thyroid gland was measured then by placing the animal's neck over a flat field, 5-cm diameter scintillation detector. Prior conditioning of the animals ensured fairly constant positioning without struggling and the mean of three 10-sec counts was recorded. Decay curves of thyroid radioactivity, after correction for isotopic decay, against time and the release rate was calculated from the graph. In an early study (George and Lomax, 1965) we noted that repeated administration of small doses of morphine sulfate (5-10 mg/kg) inhibited thyroid hormone release within 24 h following the first injection. This inhibition persisted throughout the injection period of 66 h and for 24-30 h following the last injection; the release rate then returned to normal (Table I). The injection of chlorpromazine (5 mg/kg) also promptly inhibited thyroid hormone release whereas reserpine, even in near lethal doses, was ineffective. Since morphine and chlorpromazine are known to increase TABLE I SUMMARY TABLE OF THE EFFECTS OF SYSTEMIC ADMINISTRATION OF MORPHINE ON ~ Y R O I D1311 RELEASE RATES IN INTACT, ADRENALECTOMIZED AND HYPOTHALAMIC LESIONED RATS
Number of
animals -
Intact Adrexa Rostra1 hypothalamic lesions Caudal hypothalamic lesions
10 11 9 9
Mean initial release rate f S.E.M. (%per 2 4 h) _ _ - 16.9 0.7 19.5 1.7 14.8 i 0.6 13.4 5 0.8
Mean release rate after morphine
P
(%)
- .
- -
-
3.0 & 1.5 5 3.0 13.0 &
0.8 0.6 1.1 1.0
-
tO.O1
tO.O1 <0.01
>0.5
-
MORPHINE AND THYROID FUNCTION
34 1
adrenal corticosteroid secretion, and since a variety of stresses and corticosteroid administration (Brown-Grant et al., 1954b, c) inhibit thyroid activity, the effects of morphine and chlorpromazine were investigated in adrenalectomized animals. It was found that adrenalectomy did not interfere with the thyroid-inhibiting effect of both morphine and chlorpromazine; in fact, the effect was slightly enhanced (Table I). In addition, experiments were done to determine whether morphine inhibited thyroid activity by interfering with the action of TSH on the thyroid. It was noted that administration of TSH stimulated release of thyroid hormone in morphinized rats to the same degree as that found in the controls. 'Thus, on the basis of these findings and those reported by others, it appeared that the inhibitory action of morphine on thyroid activity is mediated at the level of the pituitary gland or the czntral nervous system (CNS). In order to further localize this inhibitory site of morphine action, studies were designed to investigate the effects of systemic morphine administration on thyroid release rates in rats with hypothalamic lesions (Lomax and George, 1966). Bilateral electrolytic lesions were made in two groups of rats: one in the region of the anterior hypothalamic/ventromedial nuclei and the other in the medial mammillary nuclei. Repeated injections of morphine (5-10 mg/kg) produced a marked inhibition in thyroid activity in rats with anterior lesions. The inhibition was comparable to that found in intact animals. On the other hand, lesions in the mammillary nuclei completely blocked the thyroid-inhibitory effect of morphine and the release curves were virtually identical with those of untreated controls (Table I). The lesion sites, on histological examination were found to involve, rostrally, the paraventricular and anterior hypothalamic nuclei and, caudally, all or part of the medial mammillary nuclei. Most importantly, the median eminence and the infundibular tract were left intact. From these data it would appear that the thyroid inhibitory effect of morphine is mediated via the caudal hypothalamus in the region ofthe medial mammillary nuclei. It is well established that the anterior region of the median eminence of the hypothalamus is concerned with the regulation of pituitary TSH secretion. Electrolytic lesions in this area reduce the basal level of thyroid secretion and prevent propylthiouracil-induced goiter formation while electrical stimulation of this region increases thyroid activity (Harris and George, 1969), and recently it has been shown by Martin and Reichlin (1970) that such stimulation increases plasma TSH levels as measured by radioimmunoassay in rats. And finally, the presence of thyrotrophinreleasing factor (TRF) in this region has been clearly documented. Also, it is apparent that many regions of the CNS which relay to the hypothalamus participate in the regulation of TSH secretion, and the anterior median eminence region probably represents the final pathway through which reflex inhibition or activation of TSH secretion takes place. Supporting evidence for such a concept comes from the following reports: lesions in the region of the habenular nuclei abolish iodine-deficiency goiter and reduce the inhibitory effect of thyroxine on TSH secretion (Szendgothai and Mess, 1958; Bogdanove, 1962); midbrain transection in the dog inhibits thyroidal 13'1 release (Anderson et al., 1957); electrical stimulation of the hippocampus elicited an increase in release of 13*1-labeledthyroid hormone in References p . 344-345
342
R. GEORGE
dogs (Shizume et al., 1962). Inhibition of thyroid hormone release has been noted also following electrical stimulation of the mesencephalic reticular formation in rats (Kovacs et al., 1965) and caudal hypothalamus in rats (Vertes et al., 1965) and rabbits (Harris and Woods, 1958). Morphine is a compound which has a dual action on the CNS, i.e. it may act as a stimulant or a depressant. The dose of morphine used in these studies (5 mg/kg) was found to cause an elevation in body temperature in the rat in contrast to higher doses which produced a marked hypothermia. These temperature changes are due to a direct action of morphine on the thermoregulatory centers in the anterior hypothalamus (Lotti et a/., 1965a). Also, injection of morphine into rats pretreated with nalorphine, or into rats made tolerant to morphine, frequently produced a rise in their body temperature (Lotti et al., 1965b; 1966). If one assumes that tolerance develops to the depressant and not the stimulant effects of morphine (Seevers and Deneau, 1963) and that nalorphine antagonizes, primarily, the depressant effects of morphine (Woods, 1956), then the increase in core temperature in the rats studied by Lotti et al. (1965a and b) must be due to a stimulant effect of morphine on the thermoregulatory centers in the hypothalamus. Thus, if we can extrapolate from these studies to the effect of morphine on thyroid activity, it would seem possible that the dose of morphine used (5 mg/kg) could have had an excitatory effect on the hypothalamic neurons resulting in a reduction in thyroid activity by activation of inhibitory areas in the posterior hypothalamus. To test this hypothesis, studies were undertaken to determine the effects of microinjection of morphine into various regions of the hypothalamus on thyroid function in the rat (Lomax et al., 1970). Guides for injection cannulae were implanted symmetrically on each side of the midline. The guides were positioned so as to allow injection at various sites, within 1 mm of the mid-sagittal plane, extending from the preoptic to the supramammillary nuclei. The position of the injection sites were verified on completion of the experiments by injecting 1 pl of methyl red at the same point as the drug and sectioning the brains on a freezing microtome. For intracerebral injection, morphine sulfate was dissolved in 0.9 % NaCl. The concentration was adjusted so as to allow a constant injection volume of 1 pl. Doses of 5 or 10 pg of morphine were injected. A total of 36 rats received bilateral injections of morphine sulfate into various regions of the hypothalamus. They were injected with 5 pg in each site at 48 and 56 h, then 10pg at 72 and 80 h after the onset of thyroid counting. Of the animals so treated, 14 exhibited marked inhibition of thyroid l3'I release during the period of injections. The injection sites in these 14 rats were found to lie in two principal areas: between the preoptic nuclei and the chiasma or in the region of the posterior and supramammillary nuclei. In the 6 rats with rostra1 injection sites, morphine cimpletely arrested the release of 13'1, but the release returned to normal after the 72nd h, before the final injection of morphine. In contrast to these animals, the release of 13'1 was abolished throughout the period of morphine administration in the 8 animals injected into the caudal hypothalamic regions. Injection of morphine into other hypothalamic sites failed to alter normal thyroid activity.
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The inhibition of thyroid secretion following morphine injection into the rostral hypothalamus was a rather unexpected finding and indicates that the drug does not have a single site of action. These data pointed out that morphine has a more complex action on pituitary-thyroid function than we had visualized from our earlier experiments. As mentioned previously, morphine has a dual action on the CNS, both stimulation and depression of neurons can occur. Tolerance develops to the depressant but not to the stimulant action of morphine. Therefore, it is interesting to note that, in the case of injections into the rostral hypothalamic sites, there was a reversion to the normal thyroid release rate even during the period of morphine administration. This suggests the development of tolerance to morphine. A similar type of tolerance was seen to the hypothermic effect of morphine by Lotti et al. (1966). In contrast to this, tolerance was not detected when morphine was injected into the caudal hypothalamus. It would seem then that there are two target sites for morphine in the hypothalamus: both activation of a caudal site and depression of a rostral site leading to decreased pituitarythyroid activity. This possibility is further supported by the studies of Lotti et al. (1965a) who found that injection of microquantities of morphine into the rostral hypothalamus produced hypothermia, catatonia, respiratory depression and an increase in pain threshold, effects which are commonly noted after a large systemic dose of morphine. Conversely, injection of morphine into the caudal hypothalamus produced hyperthermia and marked excitation, associated with increased motor activity and agressive behavior.
SUMMARY
It appears that narcotic analgesics have a dual action on the secretion of thyrotrophin (TSH) secretion, however, the predominant effect appears to be one which produces inhibition of pituitary-thyroid function. The inhibitory effect of morphine on TSH secretion is mediated via the hypothalamic-pituitary axis and probably by an excitation of inhibitory neurons that lie in the caudal hypothalamus. Although the mechanism(s) by which narcotic analgesics influence hypothalamicpituitary activity has not been clearly elucidated, two possibilities arise. The first is that analgesics exert their effect on the pituitary through a direct action on the hypothalamic neurosecretory cells that contain releasing factors. The second possibility focusses on the presence of the “neurotransmitters”, acetylcholine, dopamine, noradrenaline and serotonin which are found in the hypothalamus. If these transmitters lie in a presynaptic position to neurosecretory cells, it is conceivable that narcotic analgesics can exert their action on the pituitary by interfering with the synthesis and/or release of the neurotransmitters at the presynaptic site, thereby altering the activity of the neurosecretory cells. Of these two possibilities, the latter seems more likely since there is evidence that narcotic analgesics may alter release and turnover of the neurotransmitters (Smith References p . 344-345
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1972) and that the neurotransmitters have been implicated in the control of pituitary function (George, 1971). ACKNOWLEDGEMENT
The preparation and part of the work reported in this manuscript were supported by research grants from USPHS, MH-17691, and MH-20787. REFERENCES ANDERSON, E., BATES,R. W., HAWTHORNE, E., HAYMAKER, W., KNOWLTON, K., RIOCH,D. McK., SPENCE,W. R. AND WILSON,H. (1957) The effects of midbrain and spinal cord transection on endocrine function with postulation of a midbrain hypothalamico-pituitary activating system. Recent Progr. Hormone Res., 13, 21-66. BOGDANOVE, E. M. (1962) Regulation of TSH secretion. Fed. Proc., 21, 623-627. BROWN-GRANT, K., VONEULER,C., HARRIS,G. W. AND REICHLIN, S . (1954a) The measurement and experimental modification of thyroid activity in the rabbit. J. Physiol. (Lond.), 126, 1-28. BROWN-GRANT, K., HARRIS, G. W. AND REICHLIN, S. (1954b) The effect of emotional and physical stress on thyroid activity in the rabbit. J. Physiol. (Lond.), 126, 2940. BROWN-GRANT, K., HARRIS,G. W. AND REICHLIN, S. (1954~)The influence of the adrenal cortex on thyroid activity in the rabbit. J. Physiol. (Lond.), 126, 41-51. GEORGE, R. (1971) Hypothalamus: anterior pituitary gland. In Narcotic Drugs Biochemical Pharmacology, D. H. CLOUET (Ed.), Plenum, New York, pp. 283-299. GEORGE,R. AND LOMAX,P. (1965) The effects of morphine, chlorpromazine and reserpine on pituitary-thyroid activity in rats. J. Pharmacol., 150, 129-134. HARRIS,G. W. AND GEORGE,R. (1969) Neurohumoral control of the adenohypophysis and the regulation of the secretion of TSH, ACTH and growth hormone. In The Hypothalamus. W. HAYMAKER, E. ANDERSON AND W. J. H. NAUTA (Eds.), Thomas, Springfield, Ill., pp. 326-388. HARRIS,G. W. AND WOODS,J. W. (1958) The effect of electrical stimulation of the hypothalamus or pituitary gland on thyroid activity. J . Physiol. (Lond.), 143, 246-274. HOHLWEG, V. W., KNAPPE,G. AND DORMER, G. (1961) Tierexperimentelle Untersuchungen iiber den Einfluss von Morphin auf die Gonadotrope und thyrotrope Hypophysenfunktion. Endokr. Bd., 40, 152-159. KRUEGER, H., EDDY,N. B. AND SUMWALT, M. (1941) Pharmacology of the opium alkaloids. U.S. Public Health Service, Public Health Report Suppl. 165, parts 1 and 2. KOVACS, S., VERTES, Z., SANDOR,A. AND VERTES, M. (1965) The effect of mesencephalic lesions and stimulation on pituitary-thyroid function. Actu physiol. acad. sci. hung., 26,227-233. LOMAX, P. AND GEORGE, R. (1966) Thyroid activity following administration of morphine in rats with hypothalamic lesions. Bruin Res., 2, 361-367. LOMAX, P., KOKKA, N . AND GEORGE, R. (1970) Thyroid activity following intracerebral injection of morphine in the rat. Neuroendocrinology, 6, 146-152. LOTTI,V. J., LOMAX, P. AND GEORGE, R. (1965a) Temperature responses in the rat following intracerebral microinjection of morphine. J. Phurmacol., 156, 135-139. LOTTI,V. J., LOMAX, P. AND GEORGE, R. (1965b) N-allylnonnorphine antagonism of the hypothermic effect of morphine in the rat following intracerebral and systematic administration. J. Pharmacol., 156,420-425. LOTTI,V. J., LOMAX, P. AND GEORGE, R. (1966) Acute tolerance to morphine following systemic and intracerebral injection in the rat. Int. J. Neuropharmacol., 5, 3542. MARTIN,J. B. AND REICHLIN, S. (1970) Thyrotropin secretion in rats after hypothalamic electrical stimulation or injection of synthetic TSH-releasing factor. Science, 168, 1366-1388. REDDING,T. W., BOWERS, C. Y.AND SCHALLY, A. V. (1966) The effects of morphine and other narcotics on thyroid function in mice. Acta endocrinol., 51, 391-399. SAMEL,M. (1 958) Blocking of the thyrotrophic hormone secretion by morphine and chlorpromazine in rats. Arch. int. Pharmacodyn., 117, 151-157.
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DISCUSSION ALIVISATOS: In view of our previous reports that morphine interferes competitvely with certain serotoninergic receptors, I wonder whether serotonin was tried in counteracting this effect of morphine? GEORGE: The mechanism by which morphine inhibits thyroid activity is complicated and does not seem to be serotonin-linked. We know that the amines in the hypothalamus are concerned with secretion of the anterior lobe hormones. We also know that morphine influences the levels or the turnover rates of these amines. In studies I didn’t include here, we morphinized animals over a 6 week period and then challenged them with TRF. TRF, in the doses used, produced an increase in thc release of thyroid hormone of normal animals but was blocked in the morphine-treated animals. On the basis of these preliminary findings we suspect that possibly the blocking effect is somewhere between TRF and the pituitary gland. KASTIN:Just to extend to humans, your last remark, although methadone certainly isn’t morphine, we have tested, in collaboration with Drs. Ruiz and Vargas, TRH administration in 4 patients treated with methadone. Three of them gave perfectly normal responses and one gave no response so we are further looking at that. KRIVOY: Would you like to speculate on the relevance of these data to the addiction cycle? GEORGE: I don’t think the pituitary hormones are directly involved in addiction although they may still play a secondary role in the addiction cycle. We all know that the addiction problem is a serious and complex one. For example, the controversy regarding the role of serotonin in the development of physical dependence to morphine has not been resolved. MESS:It appeared that the anterior hypothalamic lesions were ventral and the posterior ones were localized more dorsally. If so, your data would correlate with thermal regulatory data of my experiments with Dr. Donhoffer. GEORGE:The drug effect was most pronounced when placed in the thermoregulatory area and reduced when the drug was injected into a more dorsal site.