Effect of 6-hydroxydopamine on hypothalamic norepinephrine and dopamine content, ultrastructure of the median eminence, and plasma corticosterone

B-cell response to seasonal influenza vaccine in mice is amenable to pharmacological modulation through β-adrenoceptor

AIMS

Given that deprivation of noradrenaline acting on lymphocytes through β-adrenoceptor influences antibody response, the effects of propranolol treatment beginning two days before immunization with quadrivalent inactivated influenza vaccine (QIV) on IgG response and underlying cellular molecular mechanism in mice were investigated.

MAIN METHODS

Twenty-one days post-immunization the total QIV antigen-specific IgG titer and IgG subclass titers in sera were determined using ELISA. Additionally, the total counts of germinal centre (GC) B cells, T follicular helper (Tfh) and T follicular regulatory (Tfr) cells in draining lymph nodes (dLNs) and spleens, in vitro proliferation of interacting B cells and Th cells and IL-21 synthesis in Th cells in response to QIV antigens and/or mitogen were attested using flow cytometry analysis. In QIV antigen-stimulated dLN cell and splenocyte cultures were also measured concentrations of INF-γ and IL-4, cytokines upregulating IgG2a and IgG1 synthesis, respectively.

KEY FINDINGS

Propranolol decreased the total QIV antigen-specific IgG titer. This correlated with lower GC B cell count and the shift in Tfr/Tfh cell and Tfr/GC B cell ratio towards Tfr in propranolol-treated mice compared with controls. Consistently, QIV antigen-stimulated proliferation of B cells and Th cells from propranolol-treated mice in vitro was impaired. This correlated with the lower frequency of QIV antigen-specific IL-21-producing cells among Th cells. Additionally, in propranolol-treated mice, in accordance with the changes in INF-γ/IL-4 ratio in dLN cell/splenocyte cultures, serum IgG2a/IgG1 ratio was shifted towards IgG1 reflecting decreased IgG2a response.

SIGNIFICANCE

The study indicates that chronic propranolol treatment may impair response to QIV.

Physiological role of salsolinol: its hypophysiotrophic function in the regulation of pituitary prolactin secretion

We have recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol) produced by hypothalamic neurons can selectively release prolactin from the anterior lobe (AL) of the pituitary gland. Moreover, high affinity binding sites for SAL have been detected in areas, like median eminence (ME) and the neuro-intermediate lobe (NIL) that are known terminal fields of the tuberoinfundibular DAergic (TIDA) and tuberohypophysial (THDA)/periventricular (PHDA) DAergic systems of the hypothalamus, respectively. However, the in situ biosynthesis and the mechanism of action of SAL are still enigmatic, these observations clearly suggest that sites other than the AL might be targets of SAL action. Based on our recent observations it may be relevant to postulate that an "autosynaptocrine" regulatory mechanism functioning at the level of the DAergic terminals localized in both the ME and NIL, may play a role in the hypophyseotrophic regulation of PRL secretion. Furthermore, SAL may be a key player in these processes. The complete and precise mapping of these intra-terminal mechanisms should help us to understand the tonic DAerg regulation of PRL secretion. Moreover, it may also give insight into the role of pre-synaptic processes that most likely have distinct and significant functional as well as pathological roles in other brain areas using DAergic neurotransmission, like striatonigral and mesolimbic systems.

Pharmacological evidence for inhibition of ACTH secretion by a central adrenergic system in the dog

To obtain more information about the transmitter involved in catecholaminergic inhibition of ACTH secretion, the site of this inhibition, and the receptors involved, the secretion of ACTH was studied in pentobarbital-anesthetized dogs that were surgically stressed and treated with drugs which modify central catecholaminergic transmission. The index of ACTH secretion was adrenal venous output of corticoid hormones. Intravenous L-dopa inhibited ACTH secretion, and this inhibition was not modified by blockade of peripheral decarboxylation of L-dopa with carbidopa. Intravenous or centrally administered clonidine inhibited stress-induced ACTH secretion, whereas centrally administered apomorphine did not. When given into the third ventricle, phenoxybenzamine (but not phentolamine) blocked the inhibitory effect of L-dopa and clonidine. Pimozide had no effect. L-propranolol caused a small but significant decrease in stress-induced ACTH secretion. Intraventricular procaine blocked the stress response. The data support the conclusion that the site of catecholaminergic inhibition of ACTH secretion is central, 'inside the blood-brain barrier', instead of the pituitary or the median eminence. They indicate that dopamine is not the mediator involved, and suggest that it is probably norepinephrine, although epinephrine is not ruled out. The receptor on which the released catecholamines act, presumably on the surface of the cells that secrete the hypothalamic hormone that regulates ACTH secretion, appears to be a type of alpha-adrenergic receptor.

Effect of acute levodopa on brain catecholamines after selective MAO and COMT inhibition in male rats

Interactions between a selective catechol-O-methyltransferase (COMT) inhibitor OR-462 and a monoamine oxidase (MAO)-A inhibitor clorgyline were studied measuring concentrations of L-dopa, dopamine and their metabolites in the rat hypothalamus and striatum after administration of levodopa/carbidopa (15/30 mg/kg i.p.). Part of the experiments were performed in rats pretreated with 6-OH-dopamine (6-OHDA) intracerebroventricularly (i.c.v.) to determine whether changes in dopamine metabolism occurred inside or outside catecholaminergic neurons. OR-462 was an effective COMT inhibitor at the doses 3 and 30 mg/kg i.p. Inhibition of 3-O-methyldopa (3-OMD) formation from L-dopa was reflected in the hypothalamus (45-81% decrease) and striatum (87-88% decrease), since 3-OMD penetrates the blood-brain barrier. Homovanillic acid (HVA) was decreased only in the striatum at 30 mg/kg of OR-462. Clorgyline (8 and 32 mg/kg i.p.) decreased 3,4-dihydroxyphenylacetic acid (DOPAC) formation in the hypothalamus and striatum by 61-91%. When given together, OR-462 and clorgyline elevated hypothalamic dopamine levels 3.2-4.6-fold, but striatal dopamine only 1.3-1.9-fold. The formation of 3-OMD and DOPAC remained suppressed and even brain HVA levels were decreased by 51-97%. 6-OHDA treatment decreased striatal and hypothalamic dopamine by 50% and noradrenaline by 75%. In these animals levodopa/carbidopa increased brain L-dopa 2.4-4-fold, those of 3-OMD 1.2-1.7-fold compared to intact animals, but the synthesis and metabolism of dopamine and the effects of COMT and MAO inhibitors were not significantly changed. Levodopa/carbidopa treatment decreased significantly prolactin and thyrotropin levels in serum but none of the additional treatments changed this action.

Differential alpha-1 and alpha-2 adrenergic effects on hypothalamic corticotropin-releasing factor and plasma adrenocorticotropin

There is presently no consensus as to the nature of the catecholaminergic influence on the regulation of corticotropin-releasing factor. The potential role that the alpha-adrenergic system plays was investigated by measuring hypothalamic corticotropin-releasing factor-like immunoreactivity and plasma adrenocorticotropin, following manipulation of alpha-1 and alpha-2 adrenergic receptor activation. Administration of the alpha-1 agonist methoxamine did not significantly alter either plasma adrenocorticotropin or hypothalamic corticotropin-releasing factor. Administration of the alpha-2 agonist clonidine resulted in a 24-fold increase in plasma adrenocorticotropin and a significant decrease in median eminence corticotropin-releasing factor, consistent with its release. Corticotropin-releasing factor in the remainder of the hypothalamus was not altered. Concurrent administration of clonidine with the selective alpha-2 antagonist yohimbine prevented the clonidine-induced changes in plasma adrenocorticotropin and hypothalamic corticotropin-releasing factor, consistent with the clonidine effect being mediated through alpha-2 receptors. Concurrent administration of clonidine with methoxamine did not prevent these effects, suggesting that the effect of clonidine was not mediated through presynaptic inhibition of noradrenergic adrenergic neurotransmission. Inhibition of protein synthesis by anisomycin induced changes in corticotropin-releasing factor and adrenocorticotropin which were not altered by combined treatment with methoxamine or clonidine. These data suggest differential roles for alpha-1 and alpha-2 systems in the regulation of corticotropin-releasing factor. Results from alpha-2 adrenergic activation were consistent with stimulation of corticotropin-releasing factor release, an effect mediated by a postsynaptic alpha-2 mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Central catecholamine depletion impairs in vivo immunity but not in vitro lymphocyte activation

We have previously shown that depletion of central nervous system (CNS) catecholamines by injecting the neurotoxin 6-hydroxydopamine (6-OHDA) into the cisterna magna of C57B1/6 mice markedly impairs the humoral immune response to sheep red blood cells. This work extends these observations by showing that 6-OHDA treatment also inhibits the humoral antibody response to the T-cell-dependent antigen trinitrophenyl-keyhole limpet hemocyanin, but does not affect the response to the T-independent antigen trinitrophenyl-lipopolysaccharide. This treatment also impairs humoral responsiveness at peripheral lymphoid sites in addition to inhibiting natural killer cell activity. However, 6-OHDA treatment in vivo does not affect in vitro mixed lymphocyte responsiveness, mitogen-induced lymphocyte activation or antigen presentation by macrophages.

Modulation of T-suppressor cell activity by central nervous system catecholamine depletion

This study extends our previous findings, which indicate that depletion of CNS catecholamines has a marked inhibitory effect on humoral immune responsiveness. These data show that depletion of CNS catecholamines by injection of 6-hydroxydopamine (6-OHDA) into the cisterna magna in conjunction with immunization enhances the activity of a population of splenic T-suppressor cells as evidenced by the transfer of these cells to normal recipients. Increased suppressor cell activity does not result solely from 6-OHDA treatment, but rather requires concomitant immunization. Further characterization shows that these suppressor cells are not antigen specific. Hypophysectomy abrogates the effects of 6-OHDA injection suggesting that catecholamine depletion modulates immune function via the release of pituitary hormones. Thus, depletion of CNS catecholamines impairs immune responsiveness by inducing enhanced T-suppressor cell activity, providing additional evidence of the involvement of the CNS in regulation of immune responsiveness.

Effect of selective lesions in the hypothalamic-pituitary region on the development of cerebral vasospasm following an experimental subarachnoid hemorrhage in the rat

Intracisternal injection of blood in the rat induced an angiographically demonstrable biphasic cerebral vasospasm with a maximal acute spasm at 10 min and a maximal late spasm at 2 days after the subarachnoid hemorrhage. Systemic administration of 6-hydroxydopamine, which destroys catecholamine fibers in the circumventricular areas characterized by the absence of a blood-brain barrier, prevented the development of both the acute and the late spasm. Isolation or removal of one of the circumventricular organs, the pituitary, from the brain via a stalk transection or a hypophysectomy did not affect the degree of vasospasm. Lesion of the median eminence, another region without a blood-brain barrier, prevented the development of both types of spasm. The median eminence receives projections from the A1 and A2 nuclei in the medulla oblongata. It is suggested that the projections of these nuclei to the internal layer of the median eminence underlie the development of spasm.

Release and turnover of noradrenaline in isolated median eminence: lack of negative feedback modulation

A low volume (tissue holder, 100 microliter; dead space, 300 microliter) perfusion system has been developed for measuring [3H]noradrenaline release from isolated median eminence, where supramaximal electrical field stimulation can be applied. In tissue preloaded with [3H]noradrenaline, the resting release (0.4-2% of the content) was enhanced by electrical stimulation (2-10-fold increase). That the released radioactivity in response to electrical stimulation is mainly due to release of [3H]noradrenaline was confirmed by high pressure liquid chromatography combined with radiochemical detection. Evidence has been obtained that of the stimulation-evoked release of radioactivity 70-80 percent originates from noradrenergic neurons, however, the release observed at rest was not affected by 6-hydroxydopamine pretreatment. 6-Hydroxydopamine pretreatment selectively reduced the concentration of noradrenaline of the median eminence without affecting its dopamine content. The release evoked by electrical stimulation was [Ca2+]- and tetrodotoxin-sensitive. 4-Aminopyridine enhanced both the resting and stimulation-evoked release. The ratio between the amount of [3H]noradrenaline released by two consecutive stimulation periods at 2 Hz (120 shocks) was constant, 0.94 +/- 0.08. In contrast with other noradrenergic axon terminals, the release of [3H]noradrenaline in the median eminence was not subject to negative feedback modulation, yohimbine and xylazine had no effect. This conclusion was substantiated by in vivo study showing that yohimbine, an alpha2-adrenoceptor antagonist enhanced the turnover rate of noradrenaline in the cortex but not in the median eminence. Since noradrenergic axon terminals in the median eminence do not make synaptic contact and the released noradrenaline does not modulate its own release via alpha2-adrenoceptors, it is an interesting anatomical arrangement: the modulatory alpha2-adrenoceptors are located exclusively on the terminals of the hormone-containing neurons.

Effects of intravenously administered 6-hydroxydopamine on the content of monoamines in the median eminence and neurointermediate lobe of the rat

The effects of intravenous administration of 6-hydroxydopamine (6-OHDA) on the content of catecholamines in the median eminence and neurointermediate lobe was studied in male rats. The results confirmed previous findings of a depletion of catecholamines in both regions as a result of this lesion, although the magnitude of the effect was less than that revealed by fluorescence histochemistry. Administration of desipramine before 6-OHDA injection produced a more specific lesion, depleting dopamine but leaving noradrenaline unaffected.

A comparative study of the behavioral effects of the locus coeruleus and the dorsal noradrenergic bundle lesions in the rat

The effects of bilateral lesions of the dorsal noradrenergic bundle (DB) or of the locus coeruleus (LC) on the rat's behavior in different anxiogenic behavioral situations have been studied. The DB rats defecate less but ambulate more than shams in the open-field (O.F.); these data suggest a decrease in the reactivity of these animals to novelty. Furthermore, the LC rats have a behavior identical to that of shams in the O.F. We note moreover that the DB rats do not habituate to a novel stimulus. During the Henderson test, the behavioral inhibition of all lesioned animals seems to be less important than that of shams. These results are discussed in relation to existing hypotheses of the DB function. The lesions of DB induce a decrease in the noradrenaline (NA) cortical level and in the catecholamines level in hypothalamus. The lesions of LC produce 30% loss of forebrain NA. These results reveal a discrepancy between the effects induced by the lesioning of DB fibers and those produced by lesions of LC, which originate in the DB. The presence of non-noradrenergic elements, fibers of passage, which do not travel with the DB fibers or terminals in the LC region may be elements for interpretation.

Effects of intraventricular injections of 6-hydroxydopamine on anterior pituitary cell proliferation

Effects of intraventricular injections of 6-hydroxydopamine (6-OHDA; 200 micrograms/20 microliter vehicle) on anterior pituitary cell proliferation in rats have been investigated by means of the colchicine metaphase-arrest technique. In those groups of animals receiving 6-OHDA alone or 6-OHDA plus desmethylimipramine (DMI, 25 mg/kg body weight i.p.), where the mean mitotic activity rates (MMARs) were initially low at 48 hours, an increase of MMARs was observed at 96 and 144 hours after the drug injections. At 144 hours after drug administration, the MMARs values in the 6-OHDA-injected group and in the 6-OHDA + DMI-treated group were significantly higher than those in the control groups. This increase of MMARs resulted from the enhancement of acidophilic and chromophobe cell proliferation. The low MMARs at 48 hours after 6-OHDA injection are probably a result of dopamine release from damaged nerve endings; the enhancement of MMARs (particularly evident in the 6-OHDA + DMI-treated group) at 96 and 144 hours after drug administration is presumably related to a deficiency of dopaminergic control of anterior pituitary cell proliferation.

Inhibition in corticotrophin and corticosterone secretion following photic stimulation in rats with 6-hydroxydopamine injection into the medial forebrain bundle

Changes in corticotrophin (ACTH) and corticosterone (CS) serum levels, were examined in rats in which either 6-hydroxydopamine or vehicle were injected into the medial forebrain bundle (MFB) and which were sacrificed by decapitation 14 days later. The injection of the neurotoxin, which depleted very significantly the norepinephrine (NE) content of the mediobasal hypothalamus, elevated significantly the basal levels of serum ACTH and CS, but inhibited the rise in the levels of these hormones following photic stimulation. These results would suggest that the hypothalamic NE system has normally an inhibitory effect on basal ACTH and CS secretion and that the hormonal response to photic stimulation is mediated by that system and possibly by NE MFB fibers.

Adrenocortical responses to ether stress and neural stimuli in rats following the injection of 6-hydroxydopamine into the medial forebrain bundle

Adrenocortical responses, as expressed by changes in plasma corticosterone concentrations, after ether stress, or photic, acoustic, or sciatic nerve stimulation were studied in rats, with 6-hydroxydopamine or vehicle injected into the medial forebrain bundle (MFB). The neurotoxin partially inhibited the response to photic stimulation only, indicating the involvement of MFB catecholaminergic fibers in the transmission of this response which stimulates adrenocortical secretion.

Intraventricular 6-hydroxydopamine increases the adrenal cortex mitotic activity in rats

Effects of intraventricular injections of 6-hydroxydopamine (6-OHDA) on adrenocortical cell proliferation in rats have been investigated by means of the colchicine metaphase-arrest technique. In the group of animals receiving 6-OHDA alone, an increase of mean mitotic activity rate (MMAR) was observed at 96 h and 144 h after injection. This rise of MMAR was completely inhibited by pretreatment of animals with desmethylimipramine (DMI) - a blocker of norepinephrine uptake. It is concluded that enhancement of ACTH secretion is responsible for the above mentioned increase of mitotic activity after 6-OHDA administration. This phenomenon is related to abolition of the inhibitory noradrenergic effect on ACTH secretion by 6-OHDA treatment. Additionally, the results suggest little or no involvement of dopaminergic neurons in early changes of ACTH secretion after intraventricular 6-OHDA.

Catecholamine mechanisms in medio-basal hypothalamus influence prolactin but not growth hormone secretion

Medio-basal hypothalamic (MBH) catecholamine mechanisms in the regulation of prolactin and growth hormone (GH) secretion were investigated in unanesthetized rats with chronic indwelling venous cannulae and bilateral MBH directed intracerebral guide cannulae. MBH injections of the catecholamine-specific neurotoxin 6-hydroxydopamine (6-OHDA; 2 micrograms base in 0.5 microliter 0.9% saline) had no effect upon basal prolactin or GH secretion. Examination of catecholamine fluorescence indicated that MBH 6-OHDA treatment produced widespread disruption of MBH catecholamine afferents but did not destroy tuberoinfundibular dopamine neurons of the arcuate nucleus, nor median eminence catecholamine structures. MBH injections (0.5 microliter, 0.032 M solutions) of dopamine, noradrenaline or adrenaline all produced statistically significant increases in plasma prolactin levels. The potency of these 3 catecholamines in evoking prolactin release differed markedly, adrenaline having the greatest effect. MBH catecholamine injections had no effect upon plasma GH levels compared to saline injected controls. The present data suggest that MBH catecholamine afferents are unimportant in the regulation of basal patterns of GH or prolactin secretion. As MBH catecholamine injections stimulate prolactin release this region may contain a prolactin-facilitatory catecholamine mechanism which is capable of generating prolactin surges in response to certain environmental or endogenous stimuli.

Further studies on the effects of intravenously administered 6-hydroxydopamine on the median eminence of the rat

6-Hydroxydopamine (6-OHDA) given i.v. in a dose of 150 mg/kg to adult male Sprague-Dawley rats produces at 24 h a complete depletion of median eminence (ME) and neurointermediate lobe (NIL) catecholamines (CA), as judged by Falck-Hillarp fluorescence histochemistry, leaving the remainder of the hypothalamus substantially unaltered. Restoration of a normal fluorescence histochemical appearance took place over 35 days, apparently due to regeneration of CA-containing terminals. Administration of desipramine prior to 6-OHDA injection modified its effect; depletion was largely confined to the midline region of the external layer of the ME and to the NIL, suggesting that these regions contain dopaminergic terminals. Microspectrofluorometric methods were used in an attempt to verify this conclusion. It is suggested that this technique provides a model for establishing the role of ME and NIL CA-containing structures in control of pituitary gland function.

Possible involvement of nigrostriatal dopamine system in the inhibition of thyrotropin secretion in the rat

The dopaminergic inhibition of cold-stimulated thyrotropin (TSH) secretion was studied in male rats. Serum TSH levels were decreased by apomorphine (1 mg/kg i.p.) but not by dopamine (DA, 0.2--5 mg/kg s.c.). This effect of apomorphine was abolished by haloperidol (1 mg/kg i.p.), metoclopramide and sulpiride (10 mg/kg i.p.) but not by domperidone (0.1--5 mg/kg i.p.). Domperidone does not cross the blood-brain barrier while the other DA receptor antagonists do so. High doses of domperidone itself inhibited the cold-induced TSH secretion whereas the other DA antagonists did not. DA (1--10 micrograms/rat) into the medial basal hypothalamus (MBH) had no effect but 10--50 micrograms/rat into the 3rd ventricle inhibited the cold-stimulated TSH secretion. 6-Hydroxydopamine infusion after desipramine pretreatment (25 mg/kg i.p.) did not affect TSH secretion when given into the MBH (2 micrograms/rat), the 3rd ventricle (100 micrograms/rat) or unilaterally into the substantia nigra (SN, 6 micrograms/nucleus), but bilateral nigral infusions abolished the TSH cold response. The inhibitory effect of apomorphine (0.1 and 0.5 mg/kg i.p.) was amplified only in the rats whose SN was unilaterally destroyed. These results show that tuberoinfundibular DA neurons do not affect TSH secretion. Instead, the inhibition is mediated through the hypothalamic projections of the nigrostriatal DA system.

Effect of manipulating central catecholamines on puberty and the surge of luteinizing hormone and gonadotropin releasing hormone induced by pregnant mare serum gonadotropin in female rats

We have investigated the effect of manipulating central catecholamines on the timing of puberty (as assessed by vaginal opening) in female rats and the surge of luteinizing hormone (LH) and gonadotropin releasing hormone (GnRH) induced by pregnant mare serum gonadotropin (PMSG) in immature female rats. Manipulation of the catecholamines was carried out with either 6-hydroxydopamine (6-OHDA) administered with or without either desipramine (DMI) or pargyline, or alpha-methyl-p-tyrosine (alpha-MPT). The neonatal administration of 6-OHDA delayed puberty, an effect which was potentiated by pretreatment with DMI and was associated with a reduction in the rate of body growth. Catecholamine fluorescence in animals aged 60--65 days that had been treated with DMI followed by 6-OHDA was diminished only in the caudatus--putamen; treatment with 6-OHDA alone resulted in diminished fluorescence in the hypothalamus and in the intermediate but not the external layer of the median eminence. The neonatal administration of alpha-MPT had no significant effect on either the growth rate or the timing of puberty. Regular oestrous cycle occurred after puberty in animals treated with either 6-OHDA or alpha-MPT. The PMSG-induced LH surge was significantly enhanced by 6-OHDA (administered i.v.) plus DMI, and reduced by 6-OHDA injected in to the lateral ventricle (v). The inhibitory effect of 6-OHDA (v) was reduced by DMI, but in animals given 6-OHDA (i.v.) after pargyline there was a marked reduction in the height of the LH surge. There was a good correlation between the changes in the concentrations of LH in peripheral plasma and the concentrations of GnRH in pituitary stalk plasma in that the PMSG-induced surge of GnRH was significantly increased by 6-OHDA (i.v.) plus DMI and reduced by 6-OHDA (v). In animals treated with 6-OHDA (i.v. plus DMI catecholamine fluorescence was reduced only in the external layer of the median eminence, while after 6-OHDA (v) plus DMI degeneration was seen in the medial forebrain bundle. These results demonstrate a marked difference between the long-term and acute effects of 6-OHDA on the gonadotropin control system. Neonatal treatment with 6-OHDA plus DMI significantly delays puberty and the rate of body growth, but does not affect cyclical gonadotropin release and has no persistent effect on the hypothalamic catecholaminergic systems. The acute administration of 6-OHDA, depending upon the route of administration and whether it is given after DMI, can either potentiate or inhibit the PMSG-induced surge of GnRH and consequently LH by mechanisms which involve destruction, respectively, of either dopaminergic terminals in the median eminence or catecholaminergic fibres in the dorsal hypothalamus.

Noradrenergic afferents to median eminence: inhibitory role in rhythmic growth hormone secretion

The involvement of noradrenergic (NA) afferents to the median eminence (ME) in the regulation of growth hormone (GH) secretion was investigated in chronically cannulated, unanaesthetized male rats by using systemically administered 6-hydroxy-dopamine (6-OHDA). One day after 6-OHDA treatment (50 mg/kg, i.v.) GH peak frequency was substantially increased and examination of catecholamine (CA) fluorescence indicated disruption of CA innervation of the ME, but not other central nervous system (CNS) structures. Prolactin secretion at this time was normal and the administration of butaclamol (1.0 mg/kg, i.v.), a dopamine (DA) antagonist, was effective in increasing secretion, indicating that ME DA mechanisms were functionally intact following 6-OHDA treatment. GH secretory patterns returned to normal by 21 days following 6-OHDA treatment and this corresponded with re-emergence of a normal pattern of CA-fluorescence in the ME. In an additional study, the administration of the alpha-NA agonist clonidine (130 microgram/kg, i.v.) increased GH secretion in previously untreated animals. Administration of the alpha-NA agonist oxymetazoline (45 microgram/kg, i.v.), which does not readily pass the blood-brain barrier (BBB), suppressed GH secretion. These findings indicate that ME NA afferents are inhibitory to GH secretion and are a major determinant of the 3 h pattern of GH release in the rat. This inhibitory input is subsidiary to a NA stimulatory input to an as yet unidentified site inside the BBB.

The effects of intraperitoneal injection of 6-hydroxydopamine on the turnover and the levels of the brain catecholamines and the levels of plasma corticosterone in rats

1. The effects of intraperitoneal injection of 6-hydroxydopamine (6-OHDA) on the levels and the turnover of brain catecholamines and the levels of plasma corticosterone were studied in rats. 2. Two weeks after intraperitoneal injection of 6-OHDA (150 mg/kg) a virtually complete disappearance of cardiac noradrenaline was observed. 3. An increment and an accelerated turnover of noradrenaline in the hypothalamus was observed 2 weeks after peripheral administration of 6-OHDA (150 mg/kg). 4. There was no change in the levels and the turnover of noradrenaline in the cortex of the rats so treated. 5. There was not change in the levels and the turnover of dopamine in either the hypothalamus or the cortex of the 6-OHDA-treated rats. 6. An increment and an accelerated turnover of hypothalamic noradrenaline were not associated with any change in plasma corticosterone.

Light and electron microscopic identification of monoaminergic terminals in the central nervous system

A brief critical survey of methods used for light and electron microscopic examination of amine-containing pathways within the CNS. Light microscopic techniques such as fluorescence histochemistry, immunocytochemistry, autoradiography, silver degeneration techniques, and retrograde tracing technique are suitable for studying the topography of pathways but, due to limits of resolution, they are inadequate for identifying terminals. Electron microscopy which is adequate to visualize terminals does not provide an overall view. This review considers various methods which have been devised to specifically detect aminergic nerve terminals. Electrolytic and chemical induced degenerations are described in noradrenergic, dopaminergic, and serotoninergic terminals. Although the individual degenerative alterations are not specific for aminergic terminals, the degenerative process when considered as a whole can be informative. At present no single technique can provide complete information about the origin, course, connections, and terminals of aminergic systems. Concurrent application of light and electron microscopy, experimental surgery, histochemistry, and microsample biochemistry would provide a complete description.

Selective enhancement of the lordotic component of female sexual behavior in rats following destruction of central catecholamine-containing systems

Ovariectomized female rats received two intraventricular injections of 200 microgram of 6-hydroxydopamine (6-OHDA), a treatment which produced 66% depletion of telencephalic norepinephrine and 53% depletion of telencephalic dopamine. Compared to vehicle-injected controls, 6-OHDA-treated animals showed increased lordosis scores when treated with ovarian hormones. This effect was potentiated by additional treatment with 100 mg/kg alpha-methyl-p-tyrosine (AMT), a catecholamine synthesis inhibitor. Besides showing increased frequency and intensity of lordosis, animals treated with both 6-OHDA and AMT retained the lordotic posture significantly longer after the male dismounted than animals given either treatment alone or vehicle controls. The enhancement of lordosis following CA depletion was not prevented by a series of dexamethasone treatments which caused a marked suppression in adrenal steroid (corticosterone) levels. This suggests that normal adrenal function is not a prerequisite for the observed enhancements. It was concluded that the lordotic response is inhibited by the activity of a catecholamine system. Soliciting behavior (hop-darting) was not enhanced by any treatment, suggesting that catecholamine activity has an inhibitory influence on the stop component of sexual behavior, but not on the whole copulatory pattern.

Plasma corticosterone following alterations of hypothalamic catecholamines in rats

The role of central catecholamines in the regulation of resting ACTH secretion has been investigated by relating plasma corticosterone to changes in hypothalamic catecholamines after treating rats with various amine depleting agents. The hypothesis of a noradrenergic inhibitory control is not supported by the data since a correlation between hypothalamic catecholamine content and plasma corticosterone levels could not be established.