Hypophysectomy-induced striatal hypersensitivity and mesolimbic hyposensitivity to apomorphine

Prenatal ethanol preferentially enhances reactivity of the dopamine D1 but not D2 or D3 receptors in offspring

Reports of prenatal ethanol (ETOH) effects on the dopamine system are inconsistent. In an attempt to clarify this issue, dams were given 35% ethanol-derived calories as the sole nutrient source in a liquid diet from the 10th through the 20th day of gestation (ETOH). Controls were pair-fed (PF) an isocaloric liquid diet or given ad libitum access to laboratory chow (LC). Prenatal exposure to both liquid diets reduced body weight of offspring relative to LC controls, more so for ETOH than for PF exposure. Prenatal ETOH also decreased litter size and viability, relative to both LC and PF control groups. On postnatal days 21-23, male and female offspring were given an injection of saline vehicle or one of eight specific dopamine receptor agonists or antagonists. Immediately after injection subjects were placed in individual observation cages, and over the following 30 min, eight behaviors (square entries, grooming, rearing, circling, sniffing, yawning, head and oral movements) were observed and quantified. No prenatal treatment effects on drug-induced behaviors were observed for dopamine D2 (Apomorphine, DPAT or Quinpirole) or D3 (PD 152255, Nafadotride, Apo or Quin effects on yawning) receptor agonists or antagonists, or for the vehicle control. In contrast, prenatal treatment effects were seen with drugs affecting the dopamine D1 receptor. Both D1 agonists (SKF 38393) and antagonists (SCH 23390 and high doses of spiperone) altered behaviors, especially oral and sniffing behaviors, in a manner which suggested enhanced dopamine D1 drug sensitivity in both ETOH and PF offspring relative to LC controls. These results suggest that at this age, both sexes experience a prenatal undernutrition-linked increase in the behavioral response to dopamine D1 agonists and antagonists, which can be intensified by gestational exposure to alcohol.

Quantitative autoradiographic analysis of D1 and D2 dopamine receptors in rat brain in early and late pregnancy

This study examined the levels of D1 and D2 dopamine receptors in the rat brain during pregnancy, a physiologically unique and important naturally occurring state. We are particularly interested in changes in the dopamine receptor complement of the brain during pregnancy because these receptors might support some components of the immediate postpartum onset of normal maternal behavior. Quantitative in vitro receptor autoradiography was applied particularly focusing on brain areas that control maternal behavior. The D1 dopamine receptor selective antagonist -3H-SCH23390 and the D2 dopamine receptor selective antagonist [3H]spiperone were used as the ligands. We examined the levels of binding to D1 and D2 dopamine receptors in brains in females on day 2 (early pregnancy) and day 21 (late, but prepartum pregnancy) of pregnancy. In addition, brains from females on diestrus-1 and from males provided reference points to the existing literature. Late in pregnancy females had significantly 18-27% lower levels of binding to D1 dopamine receptors in the lateral striatum, the medial striatum, and the nucleus accumbens when compared to all other groups. Late in pregnancy, females had also significantly 11-25% lower levels of binding to D2 dopamine receptors in the lateral striatum, the anterior striatum, the nucleus accumbens and the olfactory tubercle compared to all other experimental groups. We examined all of the brain regions already established to be important for maternal behavior, and found that dopamine receptor binding changed across pregnancy only in one such region, the nucleus accumbens. Thus pregnancy, perhaps the hormones of pregnancy, reduces the levels of D1 and D2 dopamine receptors in the striatum, and the nucleus accumbens, but not in other brain regions.

Apomorphine-induced motor behavior in rats exposed prenatally to alcohol

We psychopharmacologically examined dopamine function in rats exposed to ethanol prenatally. Pregnant rats received liquid diets of 35% or 0% ethanol-derived calories (EDC), or ad lib lab chow (LC). Twenty-eight-day-old offspring received systemic doses of apomorphine chosen to stimulate predominantly presynaptic (0.02 or 0.1 mg/kg) or postsynaptic dopamine receptors (2.0 or 5.0 mg/kg). Behavior was scored automatically for 60 min in an "open field." For males, prenatal ethanol exposure resulted in a dose-response shift to the left for locomotor activity. Females exposed to the liquid diet, with or without ethanol, showed less of an increase in locomotor activity following the 5.0 mg/kg dose of apomorphine than did LC controls. There were no effects of prenatal treatment on repetitious motor behavior in the automated "open field" or on stereotypy scored by direct observation in separate groups of rats. The results are consistent with an hypothesis that prenatal ethanol exposure alters the sensitivity of postsynaptic (perhaps mesolimbic) dopamine systems important to locomotor activity in young male rats.

The ontogeny of SCH 23390-induced catalepsy in male and female rats exposed to ethanol in utero

The maturation of sensitivity to the cataleptic actions of selective D1 dopamine receptor blockade was assessed in rats exposed prenatally to ethanol. Pregnant rats received an average of 12.68 g/kg body weight of ethanol per day through a liquid diet. Control dams were either pair-fed an isocaloric liquid diet without ethanol or given ad lib lab chow. Separate groups of male and female offspring of these dams were administered either 0.33 or 1.00 mg/kg of the selective dopamine D1 receptor antagonist SCH 23390 at postnatal days 13, 17 or 21, and catalepsy was measured 0, 15 and 30 min later. The results are consistent with a hypothesis that prenatal ethanol exposure alters behaviors mediated by nigrostriatal dopamine systems. Overall, prenatal ethanol-exposed animals showed less catalepsy than control rats. Further, there is a gender effect in that dopamine antagonist-induced catalepsy matures earlier in normal male than in normal female controls. Prenatal alcohol exposure appears to delay the emergence of mature behavioral responses to dopamine blockade in male but not female rats.

A permanent dopamine receptor up-regulation in the ovariectomized rat

Although a permanent supersensitivity to dopamine agonists can be induced by lesions of the nigrostriatal dopamine tract, the ovariectomized rat is the first animal model of a permanent hypersensitivity to dopamine agonists in which the neurons survive. This permanent behavioral hypersensitivity to direct acting dopamine agonists is accompanied by an increase in D2 dopamine receptor density in the striatum.

Commentary: psychopharmacological assessment in neurobehavioral teratology

Pharmacological agents can be teratogenic but they can also be valuable tools in behavioral teratology. In this paper we describe some considerations we believe to be important in the use of psychopharmacology to assess the neurological underpinnings of behavioral deficits resulting from teratogenic exposure. Pharmacological challenges of animals exposed in utero to teratogenic agents can increase the sensitivity of behavioral tests and the specificity of the assessment of neural dysfunction. We discuss also some general guidelines for the conduct and interpretation of psychopharmacological experiments with teratogen-exposed animals.

Influence of hypophysectomy on dopamine receptors and dopaminergic behaviors

Hypophysectomy (Hypox) has been proposed to alter the behavioral and biochemical indices of striatal dopamine (DA) function. Since the regulation of striatal DA receptors by hormones may involve the pituitary, it was relevant to reevaluate the effects of Hypox in male and female rats. Behaviorally, dopaminergic agonists exerted enhanced activity in Hypox male and female rats. It has been suggested that these changes are due to altered metabolism since no increase in the DA receptor populations was observed. Dopaminergic antagonists showed equivalent behavioral actions in male and female rats, whether intact or Hypox. Biochemically, neither the density nor the affinity of the striatal DA receptors is altered by Hypox in female rats for 1-2 weeks or male rats for 5-6 weeks. However, in female rats at 5-6 weeks after Hypox there is a significant decrease in receptor number. This decrease in density is not reflected in behavioral changes to either DA receptor agonists or antagonists. Therefore, all dopaminergic behavioral changes do not result from alterations in DA receptors and changes in DA receptors do not necessarily dictate altered behavioral responses to dopaminergic agents.

Effects of estrogen on the basal ganglia

Recent research suggests that estrogen regulates the activity of dopamine-containing fibers originating in the midbrain and terminating in the basal ganglia, and/or dopamine-sensitive cells in the basal ganglia. The mechanism by which estrogen acts is not clear, since cells in neither of these regions concentrate estrogens. Nevertheless, estrogens clearly affect behaviors mediated by the basal ganglia, as illustrated in human patients suffering from extrapyramidal disorders. Both biochemical and behavioral research in animals has confirmed that estrogen modulates basal ganglia function, but there has not been agreement concerning either the locus, the direction, or the mechanism of its action. These topics are the focus of this review. The effects of estrogen on behaviors mediated by DA in the basal ganglia depend on the dose of estrogen administered, the time interval between estrogen treatment and testing, the behavior measured, and the part of the basal ganglia from which the behavior is elicited. A high dose of estrogen results in an initial suppression and later enhancement of DA-related behaviors elicited from the striatum. However, no later enhancement of these behaviors occurs if a low dose of estrogen is given. Even after low doses of estrogen, the latency to behavioral suppression varies depending upon the behavior measured. These varying latencies suggest that more than one mechanism is involved in the effects of estrogen on basal ganglia output. In addition, estrogen may also act on some regions in the mesolimbic DA system. While estrogen may act indirectly via the catechol estrogens and prolactin, it has been demonstrated that estrogen can act directly on the striatum. These findings are related to the effects of estrogen on human extrapyramidal disorders.

Enhancement of hypophysectomy-induced dopamine receptor hypersensitivity in male rats by chronic haloperidol administration

It has been reported that hypophysectomy (HYPOX) would antagonize the development of a neuroleptic-induced dopamine receptor hypersensitivity, and suggested that the neuroleptic-induced dopamine receptor hypersensitivity may be mediated by the neuroleptic-induced hyperprolactinemia. Conversely, we and others have reported on the ability of HYPOX animals to develop a neuroleptic-induced dopamine receptor hypersensitivity. The present study was undertaken to define the possible role(s) of prolactin in the modulation of striatal dopamine receptor sensitivity. The data from these studies indicate: that HYPOX alone will result in the development of a striatal dopamine receptor hypersensitivity; that the HYPOX-induced dopamine receptor hypersensitivity could be increased by the chronic administration and withdrawal of haloperidol; that administration of prolactin to HYPOX rats would partially antagonize the development of the neuroleptic-induced dopamine receptor hypersensitivity; and that the administration of prolactin alone had minimal effects on the apomorphine-induced behavior or neurochemistry of the HYPOX animals. These results suggest that the neuroleptics do not require the presence of a pituitary secretion (specifically, prolactin) to induce a striatal dopamine receptor hypersensitivity; however, they do indicate that a pituitary secretion, perhaps prolactin, may have the ability to modulate striatal dopamine sensitivity.