Tuberoinfundibular Dopaminergic Neuronal Activity Research Articles

Biochemical indices of tuberoinfundibular dopaminergic neuronal activity during lactation: a lack of response to prolactin.

Serum concentrations of prolactin and biochemical estimates of impulse traffic in dopaminergic neurons (neuronal ‘activity’) were compared in rats on the second day of diestrus and in 12-day postpartum lactating rats that were either suckled or were nonsuckled (pup-deprived). Indices of the activities of tuberoinfundibular, tuberohypophyseal and nigrostriatal dopamine (DA) neurons were obtained by measuring the α-methyltyrosine-induced decline of DA concentrations, and the accumulation of dihydroxyphenylalanine (dopa) after the administration of an inhibitor of aromatic L-amino-acid decarboxylase in the median eminence, posterior pituitary and striatum, respectively. Biochemical indices of tuberoinfundibular DA neuronal activity and the content of DA in the anterior pituitary gland were lower in pup-deprived lactating rats than in diestrous controls, and were further reduced by suckling. These changes appeared to be specific for tuberoinfundibular DA neurons since biochemical estimates of tuberohypophyseal and nigrostriatal DA neuronal activity were the same in all three groups of animals. Following pup removal serum prolactin concentrations in the dam decreased to basal levels within 4 h and remained low for at least 72 h. The rate of dopa accumulation in the median eminence of lactating rats increased 4 h after pup removal, but remained significantly lower than diestrous values for 72 h. These results indicate that an acute suckling stimulus activates a neuronal circuit which causes a prompt decrease in tuberoinfundibular DA neuronal activity. This decrease may play a role in the suckling-induced increase in prolactin secretion. Independent of these acute changes, the ‘basal’ activity of tuberoinfundibular DA neurons is reduced in lactating rats. This decrease appears to result from the lack of responsiveness of these neurons to the stimulating actions of prolactin. This is supported by the fact that the rate of dopa accumulation in the median eminence was increased in diestrous animals 12 h after the intracerebroventricular injection of prolactin or 16 h after the administration of haloperidol (which increases serum prolactin concentrations) but these treatments were without effect in lactating animals. The decreased responsiveness of tuberoinfundibular DA neurons in lactating rats to prolactin may contribute to the ability of the anterior pituitary gland in these animals to maintain a high secretion rate of this hormone.

Lack of effect of a behaviorally active dose of?-melanotropin on biochemical indices of dopaminergic neuronal activity in the rat

An attempt was made to correlate behavioral changes in the male rat following an intracerebroventricular (ICV) injection ofα-melanotropin (α-MSH, 10μg) with biochemical estimates of the activities of nigrostriatal, mesolimbic, tuberoinfundibular and tuberohypophyseal dopaminergic neurons. ICV injection ofα-MSH elicited body snaking, stretching, yawning and penile erections. The concentrations of dopamine (DA) and dihydroxyphenylacetic acid and the rate of DA synthesis (accumulation of DOPA after the inhibition of DOPA decarboxylase) in the striatum, nucleus accumbens, olfactory tubercle, septum, median eminence and posterior pituitary were unaltered at any time (30–180 min) after ICV injection ofα-MSH. Intraperitoneal injection ofα-MSH (100μg/kg) also failed to change the rate of DOPA accumulation in these brain regions. These results imply that the behaviors observed afterα-MSH are not associated with changes in the activity of DA neurons.

Effects of prolactin on central dopaminergic neurons

The activities of various dopaminergic neurons in the brain have been estimated biochemically by quantifying the rate of synthesis or turnover of dopamine in brain regions containing the terminals of these neurons. In addition, the activity of tuberoinfundibular dopaminergic neurons have been estimated from the concentration of dopamine in pituitary stalk blood. Manipulations which increase concentrations of prolactin in cerebrospinal fluid or serum (systemic or intracerebroventricular injections of prolactin; injections of dopaminergic antagonists or estrogens; implantation of pituitaries under the renal capsule; prolactin secreting tumors) increase the activity of tuberoinfundibular dopaminergic neurons. The activities of other dopaminergic neuronal systems are not affected by prolactin.

Effect of estradiol benzoate and clomiphene on tyrosine hydroxylase activity and on luteinizing hormone and prolactin levels in ovariectomized rats

The effects of estradiol benzoate (EB) on tyrosine hydroxylase (TH) activity in the medial basal hypothalamus (MBH) and on plasma levels of luteinizing hormone (LH) and prolactin were studied in long-term ovariectomized rats. Administration of 10 μg EB produced significant elevation of TH activity on Days 1 and 3 following injection. LH levels were significantly lower than controls throughout the three day treatment period, although there was a significant increase from Day 1 to Day 2. TH activity and LH levels were inversely related throughout the experimental period. Clomiphene (15 μg/rat/day), a purported estrogen antagonist, was administered over a period of three days to control and EB-treated rats to determine whether the effect of EB on plasma LH levels was causally related to changes in TH activity. In rats receiving both EB and clomiphene, TH activity was lower and plasma LH was higher than after EB alone. The results support the hypothesis that the feedback effects of estradiol on LH release involve an action on the tuberoinfundibular dopaminergic (TIDA) neurons of the MBH and that clomiphene can oppose the inhibitory effect of estradiol on LH release by directly inhibiting TIDA neuron activity. Furthermore, EB-induced release of prolactin does not appear to involve detectable changes in the activity of TIDA neurons.

Selective activation of tuberoinfundibular dopaminergic neurons in the chronically restrained rat

Selective activation of tuberoinfundibular dopaminergic neurons in the chronically restrained rat

Prolactin in CSF selectively increases dopamine turnover in the median eminence

The α-methyltyrosine-induced decline of dopamine was increased in the median eminence of rats at 16 but not at 2 hours after the start of intraventricular injections of 0.2–2 μg of rat prolactin. Intraventricular injections of prolactin did not alter the α-methyltyrosine-induced decline of dopamine in the striatum or olfactory tubercle. These results suggest that prolactin in the cerebrospinal fluid can selectively increase the activity of tuberoinfundibular dopaminergic neurons.