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

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.

Effects of systemic administration of 6-hydroxydopamine, 6-hydroxydopa and 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP) on tuberoinfundibular dopaminergic neurons in the rat

Using systemic route of administration, the effects of several neurotoxins on hypothalamic tuberoinfundibular dopaminergic neurons were focused in this study. 6-Hydroxydopamine (6-OHDA, 10 or 100 mg/kg b.wt., i.v. or ip) produced a dose (37 vs. 50%)- and time (41 to 29% from day 4 to day 9)-dependent depletion of hypothalamic median eminence dopamine concentrations, and increases of serum prolactin levels in ovariectomized rats. Other central dopaminergic neurons, however, were not significantly affected. Similar treatments with 6-hydroxydopa (6-OHDOPA) were less effective. On the other hand, treatments of 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP, 10 mg/kg b.wt./day, ip) for 7 or 14 days produced significant decreases of dihydroxyphenylacetic acid (DOPAC) levels in the median eminence and periventricular regions, and increases in serum prolactin levels. Other central dopaminergic neurons were not significantly affected, though. These results suggest that systemic administration of 6-OHDA, 6-OHDOPA, or MPTP, can produce specific destructive effects on the tuberoinfundibular dopaminergic neurons.

Effect of lesioning of medullary catecholamine neurons or the median eminence on the development of cerebral vasospasm in the squirrel monkey

Injections of blood into the interpeduncular fossa and cisterna magna in the squirrel monkey produce an angiographically demonstrable, biphasic cerebral vasospasm with a maximal acute spasm at ten minutes and a maximal late spasm at six days after the subarachnoid haemorrhage (SAH). Selective lesioning of the A2 nucleus in the medulla oblongata or the median eminence in the hypothalamus prior to the SAH prevents the development of both the acute and late cerebral vasospasm. The present data indicate that the A2 nucleus and the median eminence participate in the development of vasospasm in the squirrel monkey.

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.

6-Hydroxydopamine induces degenerative changes in innervation of the rat pituitary gland

Administration of 6-hydroxydopamine to adult male rats by peripheral injections induces degenerative changes in nerve terminals innervating cells of the pituitary intermediate lobe. Additional animals were treated with 5-hydroxydopamine, which produced images of nerve profiles containing vesicles with electron-dense centers, indicative of uptake of the false catecholamine (CA) neurotransmitter. Endocrine cells showed cytologic evidence of activation of synthetic and secretory compartments. The ultrastructural observations suggest that innervation to opiomelanocortin cells is sensitive to a CA-specific neurotoxin and strengthens the hypothesis that CA-containing fibers play a role in intermediate lobe neuroregulation.

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.

Stress, dopaminergic blockade and median eminence-neurointermediate lobe catecholamine depletion: effects on hypothalamic, pituitary and plasma immunoreactive beta-endorphin

We have compared immunoreactive beta-endorphin (ir-beta EP) levels in plasma, hypothalamus, anterior pituitary and neurointermediate lobe of adult female Sprague-Dawley rats, in studies in which levels of catecholamines were manipulated. Whole-brain catecholamines were manipulated by intraperitoneal haloperidol and/or bromocriptine; median eminence and neurointermediate lobe catecholamines were manipulated specifically and differentially by intravenous 6-hydroxydopamine (6-OHDA), with and without pretreatment with intraperitoneal desipramine; changes in amine neurons were assessed by fluorescence histochemistry. Haloperidol and 6-OHDA administration produced a selective reduction of neurointermediate lobe ir-beta EP, to levels equivalent to those seen with prolonged stress; the haloperidol effect was blocked by bromocriptine and the 6-OHDA effect by desipramine. Specific depletion of catecholamine nerve terminals in the median eminence and the neurointermediate lobe was associated with elevated plasma ir-beta EP, with no changes in pituitary or hypothalamic levels. These studies confirm and extend previous reports documenting that ir-beta EP levels in different tissues are modulated by different neural stimuli.

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.

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.