The interaction of disrupted type II neuregulin 1 and chronic adolescent stress on adult anxiety- and fear-related behaviors.

The incidence of anxiety, mood, substance abuse disorders and schizophrenia increases during adolescence. Epidemiological evidence confirms that exposure to stress during sensitive periods of development can create vulnerabilities that put genetically predisposed individuals at increased risk for psychiatric disorders. Neuregulin 1 (NRG1) is a frequently identified schizophrenia susceptibility gene that has also been associated with the psychotic features of bipolar disorder. Previously, we established that Type II NRG1 is expressed in the hypothalamic-pituitary-adrenal (HPA) axis neurocircuitry. We also found, using a line of Nrg1 hypomorphic rats (Nrg1(Tn)), that genetic disruption of Type II NRG1 results in altered HPA axis function and environmental reactivity. The present studies used the Nrg1(Tn) rats to test whether Type II NRG1 gene disruption and chronic stress exposure during adolescence interact to alter adult anxiety- and fear-related behaviors. Male and female Nrg1(Tn) and wild-type rats were exposed to chronic variable stress (CVS) during mid-adolescence and then tested for anxiety-like behavior, cued fear conditioning and basal corticosterone secretion in adulthood. The disruption of Type II NRG1 alone significantly impacts rat anxiety-related behavior by reversing normal sex-related differences and impairs the ability to acquire cued fear conditioning. Sex-specific interactions between genotype and adolescent stress also were identified such that CVS-treated wild-type females exhibited a slight reduction in anxiety-like behavior and basal corticosterone, while CVS-treated Nrg1(Tn) females exhibited a significant increase in cued fear extinction. These studies confirm the importance of Type II NRG1 in anxiety and fear behaviors and point to adolescence as a time when stressful experiences can shape adult behavior and HPA axis function.

Effects of anterolateral and posterolateral cuts around the medial hypothalamus on the immunoreactive ACTH and beta-endorphin levels in selected brain regions of the rat.

To evaluate the relative weight of the ACTH-ergic and beta-endorphin-ergic pathway(s) leaving the medial hypothalamus (MH) in anterior or posterior directions immunoreactive ACTH and beta-endorphin (ir-ACTH and ir-betaE) were quantified in selected brain regions of the rat 7-8 days after placing anterolateral (ALC) or posterolateral (PLC) cut around the MH. Retrograde accumulation of both peptides was observed in the MH after ALC, but not after PLC. ALC resulted in dramatic decrease in ir-ACTH/ir-betaE concentrations in all extra-MH brain regions tested (extra-MH hypothalamus, septum, thalamus, hippocampus, amygdala, and medulla oblongata). In contrast, ir-ACTH and ir-betaE levels decreased only in the thalamus and in the medulla oblongata after PLC. The present data indicate (a) ACTH- and betaE-like substances synthesized in the arcuate region of the hypothalamus are axonally transported to extrahypothalamic brain regions by neuronal pathways leaving the MH primarily anterolateral, anterodorsal, or anteromedial direction (even the fibers of certain posteromedial or posterolateral projections leave the MH some anterior directions); (b) the posterior ACTH-/betaE-ergic projections seem to be of minor importance except for the thalamus and the medulla oblongata where it contributes to about one-third of the peptide content. Our biochemical study provide quantitative complementary data to the detailed immunohistochemical picture of the ACTH/betaE-ergic projections in the rat brain described by Khachaturian et al.

Galanin-containing neurons in the paraventricular nucleus: a neurochemical marker for fat ingestion and body weight gain.

The physiological function of the peptide galanin (Gal) remains to be established. It is known to exist in high concentrations within the hypothalamus and to modulate the secretion of specific hormones, as well as to potentiate food consumption. Our study provides evidence for an essential function of neuronal Gal, within a specific hypothalamic area, in stimulating the behavioral process of fat ingestion and body weight gain. Through analyses of peptide levels via RIA and of gene expression via in situ hybridization, a close positive association is established between Gal in the paraventricular nucleus (PVN), particularly its midlateral region, and fat ingestion. No such relationship is detected for Gal in other brain areas or between PVN Gal and ingestion of carbohydrate or protein, supporting the behavioral and anatomical specificity of this relationship. Through PVN injection studies with antisense oligonucleotides to Gal mRNA, a dramatic decline in fat ingestion and body weight suggests that endogenous Gal contributes to the natural appetite for fat. Thus, Gal in the PVN is identified as a neurochemical marker for fat ingestion and, consequently, body weight gain.

Adrenalectomy alters discrete galanin mRNA levels in the hypothalamus and mesencephalon of the rat.

Using solution and in situ hybridization techniques we have studied the effects of adrenalectomy with or without restitution therapy with corticosterone on galanin mRNA levels in discrete regions of the male rat brain. Galanin peptide levels were also measured using a radioimmunoassay. The solution hybridization showed a two-fold increase in galanin mRNA 7 days, but not 14 days, after adrenalectomy in the preoptic area including the hypothalamic paraventricular nucleus (PVN). No changes were observed in the mediobasal hypothalamus including the arcuate nucleus. In situ hybridization showed that the increase in galanin mRNA was localized to the PVN and that the arcuate nucleus was not affected. The changes observed could be fully counteracted by corticosterone treatment. Radioimmunoassay showed decreased galanin levels in the median eminence 14 days, but not 7 days, after adrenalectomy and increased levels in the anterior pituitary and neurointermediate lobe. The results give evidence for a regional regulation of galanin gene expression and galanin peptide synthesis by adrenocortical steroids.

Diurnal rhythm of galanin-like immunoreactivity in the paraventricular and suprachiasmatic nuclei and other hypothalamic areas.

The peptide galanin (GAL), when injected into the rat hypothalamus, is known to stimulate feeding behavior and affect the secretion of various hormones, including insulin and the adrenal steroid, corticosterone. To determine whether endogenous peptide levels shift in relation to natural rhythms of feeding and circulating hormone levels, rats were sacrificed at different times of the light/dark cycle, and their GAL levels were measured, via radioimmunoassay, in medial hypothalamic dissections and micropunched hypothalamic areas. The results suggest the existence of two distinct diurnal rhythms for hypothalamic GAL. One rhythm, detected exclusively in the area of the SCN, is characterized by bimodal peaks of GAL, threefold higher than basal peptide levels, around the onset of the dark and light periods. The second rhythm shows a single peak of GAL towards the middle of the nocturnal feeding cycle, specifically between the third and sixth hour. This latter rhythm is evident in the dorsal region of the medial hypothalamus, localized specifically to the lateral portion of the PVN. Moreover, it is inversely related to circulating insulin but unrelated to the adrenal steroids, suggesting a possible association between this pancreatic hormone and GAL in the PVN.

Estrogen stimulation of galanin gene expression and galanin-like immunoreactivity in the rat and its blockade by the estrogen antagonist keoxifene (LY156758).

Rat galanin (rGAL) gene expression is stimulated potently by 17 beta-estradiol in the anterior pituitary. Neuroendocrine tissue extracts of were purified by chromatography and analyzed for rGAL-like (-LI) immunoreactivity. Greater than 90% of rGAL-LI eluted at the same position as the synthetic rGAL standard in untreated anterior pituitary, median eminence and neurointermediate lobe tissues. Additional immunoreactive forms were detected in the hypothalamus, anterior pituitary and MtT/W15 adenoma tissues, particularly after 17 beta-estradiol treatment. We examined rGAL and its encoding mRNA in the anterior pituitary of immature female rats after the injection of pregnant mare serum gonadotropin (PMSG). One and two days after PMSG injection, serum 17 beta-estradiol increased 3-fold and 4-fold, respectively. This resulted in a surge of endogenous gonadotropin 2 days after PMSG. At this time, rGAL-encoding mRNA was increased 40-fold over controls. Three days after PMSG, there was a 6-fold increase in anterior pituitary and a 41% increase in plasma rGAL-LI concentrations. Plasma 17 beta-estradiol one day after injection of PMSG and the consequent anterior pituitary rGAL-LI concentrations 2 days later were positively correlated. This stimulation of rGAL and its encoding mRNA by PMSG was inhibited by treatment with the estrogen antagonist keoxifene (LY156758).

Effects of Hypnorm (fentanyl) on ACTH/beta-endorphin levels in plasma, pituitary and brain of 10-day old rats.

Administration of Hypnorm, an anaesthetic containing the known mu-opiate receptor agonist fentanyl, elicited dose- and time-related elevation of plasma ACTH, beta-endorphin and corticosterone levels in 10-day old rat pups. Pretreatment with specific antibodies (raised against CRH, AVP and ACTH resp.) revealed that Hypnorm administration activated the ACTH-corticosterone system in the 10-day old rat and its effect is mediated by CRH and/or AVP. Hypnorm anaesthesia was associated with significant decrease in the ACTH and beta-endorphin levels in the pituitary lobes as well as in beta-endorphin content of the hypothalamus and medulla oblongata. Latter results may indicate that the beta-endorphinergic system in the brain of the 10-day old rat is activated by Hypnorm, an effect most probably elicited by the opiate agonist fentanyl.

Effects of mediobasal hypothalamic lesion on immunoreactive ACTH/beta-endorphin levels in cerebrospinal fluid, in discrete brain regions, in plasma, and in pituitary of the rat.

One week after complete destruction of the mediobasal hypothalamus, immunoreactive adrenocorticotropin (ACTH) and beta-endorphin levels were determined in cerebrospinal fluid, trunk blood, as well as in brain and pituitary tissue samples collected from anaesthetized and cisternally cannulated rats. Control rats were sham operated. In lesioned rats we observed: (a) 60% decrease in the immunoreactive beta-endorphin concentrations in the cerebrospinal fluid, (b) decreased immunoreactive ACTH and beta-endorphin levels in the hypothalamus, in the thalamus and in the amygdala, (c) unaffected immunoreactive ACTH/beta-endorphin levels in the septum and in the hippocampus, (d) decreased immunoreactive beta-endorphin levels both in the anterior and neurointermediate pituitary but unchanged immunoreactive ACTH contents in the anterior lobe, and (e) unaffected immunoreactive ACTH and beta-endorphin levels in the plasma under stressful conditions. From these findings the following conclusions can be drawn: (1) more than 50% of the beta-endorphin-like peptide content of the cerebrospinal fluid originates from the periventricular nuclei of the hypothalamus and thalamus in the rat; (2) the loss of the hypothalamic control probably enhances the intracellular proteolytic degradation of beta-endorphin both in the anterior and neurointermediate pituitary lobe; (3) rats with mediobasal hypothalamic lesion cannot react to the stressful stimuli of ether anaesthesia or cisternal cannulation with elevated plasma immunoreactive ACTH and beta-endorphin levels.

Sexually dimorphic expression of the growth hormone-releasing hormone gene is not mediated by circulating gonadal hormones in the adult rat.

The sexual dimorphism characterizing GH secretion in the rat is thought to be related to differences in the hypothalamic synthesis and release of the GH-regulating peptides, GH-releasing hormone (GHRH), and somatostatin. Therefore, the influence of gender and sex steroid hormones on hypothalamic expression of the GHRH gene in adult rats were examined. GHRH messenger RNA (mRNA) levels were measured in individual rat hypothalami by Northern hybridization analysis using a 32P-labeled complementary DNA encoding rat GHRH. Destruction of hypothalamic GHRH neurons by neonatal treatment with monosodium glutamate caused similar 3-fold reductions in the levels of GHRH mRNA in adult male and female animals. In three separate experiments, hypothalamic GHRH mRNA concentrations in male rats were 2- to 3-fold greater than in randomly cycling females (four or five rats per group; P less than 0.01). In spite of the greater abundance of GHRH mRNA abundance in the male rat hypothalamus, circulating gonadal steroids lacked the ability to modulate GHRH gene expression in adult animals, since neither gonadectomy nor pharmacological sex steroid replacement changed GHRH mRNA levels in the hypothalamus of male and female adult rats. Furthermore, GHRH mRNA concentrations in female rats were similar during the proestrus, estrus, and diestrus phase of the estrous cycle. Also, GH inhibited hypothalamic GHRH gene expression in a sex-specific manner. Exposure to high levels of GH secreted by the MtTW15 tumor for 4 weeks reduced GHRH mRNA concentrations 7-fold in male rats (P less than 0.001) but only 2-fold in females (P less than 0.05). These studies demonstrate that GHRH gene expression in the rat hypothalamus is sexually dimorphic. Basal mRNA levels are greater in male rats, and expression in male hypothalami is more sensitive to feedback inhibition by GH than expression in females. There is no evidence for regulation of GHRH mRNA levels by either testosterone or estrogen in adult rats. These gender differences in GHRH gene expression likely contribute to the generation of a sex-specific pattern of GH secretion.

Neonatal treatment with monosodium glutamate: effects of prolonged growth hormone (GH)-releasing hormone deficiency on pulsatile GH secretion and growth in female rats.

Administration of monosodium glutamate (MSG) to neonatal rodents produces permanent lesions of hypothalamic arcuate neurons that secrete GH-releasing hormone (GHRH). The present study was intended to determine the consequences of GHRH deficiency on the pulsatile GH secretory pattern and growth in MSG-treated female rats and to compare these effects with those observed in male littermates. Male and female rats were injected with MSG [4 mg/g body wt (BW), sc] or saline (controls) on days 2, 4, 6, 8, and 10 after birth. Immunoreactive GHRH concentrations were decreased in the hypothalamus (by 60%) and in the median eminence (by 95%) of adult male and female MSG-treated rats. In contrast, somatostatin concentrations were unaffected. BW and linear growth were severely impaired in male MSG-treated rats, but in MSG-lesioned females BW was not different from controls, and the attenuation of longitudinal growth was less severe and the obesity more pronounced than in males. These sex differences occurred despite similar reductions (by 55%) in serum insulin-like growth factor I concentrations in male and female MSG-treated rats. MSG treatment also produced decreases in pituitary wt and GH content (by 60%), independent of sex. Pulsatile GH secretion was studied by serial blood sampling of chronically cannulated, freely moving rats. Plasma GH patterns were analyzed by the PULSAR program. Compared to controls, treatment with MSG led to a marked inhibition (by 90%) of GH secretion in both sexes. Significant reductions in GH pulse amplitude (-95%) and pulse duration (-62%) were observed in males, whereas pulse amplitude (-85%), pulse frequency (-67%), and baseline GH concentrations (-80%) were markedly reduced in females. The GH responses to an iv bolus injection of rat GHRH (1 microgram/rat) was severely blunted in both male and female MSG-treated rats. This study demonstrates that GHRH deficiency in female rats results in a marked inhibition of GH pulses, as in males, but also causes severe and sex-specific reductions in GH basal secretion and pulse frequency. These observations suggest that hypothalamic GHRH secretion in female rats is more continuous than in males and is a determinant of the elevated interpulse secretion of GH. Moreover, body wt and linear growth are less severely affected by arcuate lesions in female animals, compared to males. These sex-related differences in growth rates may result in part from the tendency of female MSG-lesioned rats to become more obese than males, and the development of obesity, in turn, may antagonize the factors that tend to slow linear growth.(ABSTRACT TRUNCATED AT 400 WORDS)

Abnormal cortisol secretion and responses to corticotropin-releasing hormone in women with hypothalamic amenorrhea.

Hypothalamic amenorrhea (HA) is a common disorder associated with hypoestrogenemia and has adverse effects. The mechanism of GnRH deficiency in these women is not yet known. To investigate the role of the hypothalamic-pituitary-adrenal axis in HA, we studied 10 women [mean age, 29 +/- 7 (+/- SD) yr] with 0.5-13 yr of amenorrhea (mean, 4.3 +/- 3.7 yr) related to simple weight loss or psychological stress. We investigated cortisol and ACTH responses to a bolus of ovine CRH, 24-h plasma cortisol levels obtained every 10 min, and urinary free cortisol levels in these patients. Results were compared with those obtained in normal women during all phases of the menstrual cycle. We found that mean basal concentrations of cortisol were significantly higher (P = 0.03) in the HA patients (mean, 210 +/- 130 nmol/L) than in the normal women (100 +/- 30 nmol/L). The delta (peak - basal) cortisol was significantly lower (P = 0.004) in the HA patients than in the normal women (320 +/- 100 vs. 440 +/- 90 nmol/L, respectively). ACTH responses to CRH did not differ between HA patients and normal women. The 24-h mean cortisol was significantly higher (P = 0.006) in the HA patients than in the normal controls (280 +/- 50 and 220 +/- 50 nmol/L, respectively), due to higher cortisol levels at night. The urinary free cortisol level was significantly higher (P = 0.005) in the HA patients (230 +/- 70 nmol/day) than in normal women (150 +/- 40 nmol/day). We conclude that women with HA have a blunted cortisol response to CRH administration. In addition, they have hypercortisolism, as demonstrated by elevated 24-h mean serum cortisol levels and urinary free cortisol values. This hypothalamic-pituitary-adrenal axis activation in patients with stress or weight loss may be a mechanism in the development of amenorrhea and may relate to other potential adverse effects of HA.

Galanin-like immunoreactivity is influenced by estrogen in peripubertal and adult rats.

Galanin gene expression in the anterior pituitary is potently stimulated by estrogen in adult rats. To evaluate the influence of estrogen on galanin during the peripubertal period 30- to 32-day-old female rats were treated with pregnant mare serum gonadotropin (PMSG, 10 IU s.c., 10.00 h). Galanin-like immunoreactivity (galanin-LI) in hypothalamic and pituitary tissues was evaluated 1, 2 or 3 days after PMSG treatment between 17.00 and 19.00 h. The PMSG treatment stimulated 17 beta-estradiol secretion, which induced a midafternoon LH surge 2 days after the PMSG treatment. Concentrations of galanin-LI at the time of this LH surge were elevated 82% in the anterior pituitary and 58% in the hypothalamus (without the median eminence) when compared to saline-treated female rats. On the 3rd day after the PMSG injection, galanin-LI was increased 236% in the anterior pituitary, 88% in the neurointermediate lobe and 39% in the median eminence compared to saline-treated female rats. These changes in galanin-LI were not observed in similarly aged male rats or ovariectomized rats treated with PMSG. In adult male rats, daily injections with 17 beta-estradiol valerate (10 micrograms/daily s.c.) for 1 week increased galanin-LI in the median eminence and neurointermediate lobe to an extent similar to that seen in juvenile female rats following PMSG treatment. In contrast, the high serum levels of 17 beta-estradiol achieved after 17 beta-estradiol valerate treatment increased galanin-LI in the anterior pituitary 65-fold. These studies indicate that galanin-LI is influenced by estrogen in peripubertal and adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Sexual differentiation of growth hormone feedback effects on hypothalamic growth hormone-releasing hormone and somatostatin.

To investigate possible sex differences in the feedback regulation of growth hormone (GH) secretion, concentrations of immunoreactive GH-releasing hormone (GRF) and somatostatin (SS) were measured in the median eminence (ME) and the hypothalamus of male and female rats bearing the MtTW15 tumor, which secretes high amounts of GH and prolactin (PRL). Four weeks after tumor implantation in male rats, the GRF concentration in the whole hypothalamus, including the ME, was decreased by 37% (0.29 +/- 0.02 vs. 0.46 +/- 0.02 ng/mg protein in intact male controls; p less than 0.001) and the concentration of SS was increased by 40% (11.5 +/- 0.7 vs. 8.1 +/- 0.3 ng/mg protein in male controls; p less than 0.01). In female rats, the presence of tumor for 4 weeks caused a smaller (18%) reduction in GRF concentrations (0.27 +/- 0.02 vs. 0.33 +/- 0.03 ng/mg protein in intact female controls; p less than 0.05) and no significant change in SS concentrations (10.2 +/- 0.08 vs. 9.7 +/- 0.8 ng/mg protein in female controls). Tumor-related changes in GRF and SS concentrations were also more pronounced in male rats than in females, when determined separately in the microdissected ME and in the remaining hypothalamus. These differences occurred despite similar increases in serum GH, PRL and insulin-like growth factor I concentrations in male and female tumor-bearing rats. To assess which hormone (GH or PRL) was responsible for these changes, intact male rats were treated for 10 days with 2 daily s.c. injections of rat GH (rGH; 100 and 250 micrograms/day), rat PRL (100 and 250 micrograms/day) or vehicle.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex differences in vasoactive intestinal peptide (VIP) concentrations in the anterior pituitary and hypothalamus of rats.

Recent evidence suggests that vasoactive intestinal peptide (VIP), a putative prolactin (PRL)-releasing factor, is both synthesized and released by anterior pituitary cells, to act as a paracrine or autocrine factor. We have investigated the hypothesis that hypothalamic or pituitary VIP levels differ in male and female rats, since neuroendocrine control of PRL is sexually differentiated. Opposite sex differences were found in the hypothalamus and anterior pituitary. Random-cycle female rats had one-third higher VIP levels in the hypothalamus than males. In contrast, anterior pituitary VIP levels were 3 times as high in male rats as in females. Median eminence VIP levels were similarly low in both sexes. These results support a possible role of VIP in the sexually dimorphic regulatory mechanisms of PRL secretion. Moreover, demonstration that hypothalamic and pituitary VIP levels vary in opposite directions suggests that VIP is differentially regulated at the two sites.

Sexual and developmental differences in peptides regulating growth hormone secretion in the rat.

Sex differences in the hypothalamic control of growth hormone (GH) secretion were investigated by measuring rat GH-releasing factor (rGRF) and somatostatin in male and female rats. Rat GRF-like immunoreactivity (rGRF-IR) was higher in the median eminence and hypothalamic tissue outside of the median eminence of adult (90-day-old) male compared to female rats. A similar pattern of rGRF-IR content was found in the median eminence of 35-day-old rats. This sex difference developed between days 25 and 35 of age, during which time serum concentrations of insulin-like growth factor (IGF-1) and body weight increased in both sexes. To a lesser extent, the content of somatostatin-like immunoreactivity (SLI) was higher in the median eminence of adult female rats compared to male rats. Whole hypothalamic rGRF-IR and SLI contents were influenced only moderately by adult gonadectomy or gonadal steroid treatments. For example, estrogen increased rGRF-IR content in castrated rats, but orchidectomy alone or orchidectomy followed by testosterone did not influence rGRF-IR content. Additionally, whole hypothalamic SLI content was unaffected by orchidectomy or orchidectomy followed by testosterone or estrogen. One month after ovariectomy, rGRF-IR and SLI in whole hypothalamic fragments were similar to their respective contents in gonad-intact males. However, ovariectomy followed by estrogen or testosterone did not restore rGRF-IR content and partially restored SLI content to levels seen in gonad-intact females.

Tissue-specific sex differences in galanin-like immunoreactivity and galanin mRNA during development in the rat.

Galanin-like immunoreactivity (Gal-LI), as determined by radioimmunoassay, was detectable in the brain and gastrointestinal tract by day 15 of gestation. Concentrations of Gal-LI increased after birth in the hypothalamus but decreased in the stomach and duodenum. A sex difference in Gal-LI concentrations appeared during puberty in the median eminence, neurointermediate lobe, and the anterior pituitary (AP), where females had higher Gal-LI concentrations compared to males. This difference was most pronounced in the AP; adult females had up to 4-fold greater Gal-LI concentrations and 5-fold more abundant rGal-specific mRNA compared to males.

Characterization of galanin-like immunoreactivity in the rat brain: effects of neonatal glutamate treatment.

Concentrations of the neuropeptide, galanin, were measured using a newly characterized radioimmunoassay in brain regions of adult male rats treated neonatally with monosodium glutamate. Galanin-like immunoreactivity (galanin-IR) was significantly reduced 57% in the median eminence, 15% in the medial basal hypothalamus, and 27% in the septal region when compared to untreated littermates. Concentrations of galanin-IR were reduced 22% in the preoptic region and unchanged in the parietal cortex. These studies suggest that glutamate-sensitive, galanin-containing neurons in the arcuate nucleus project to regions of the basal forebrain of the rat in addition to the median eminence. The galanin projection from the arcuate nucleus to the median eminence suggests that this peptide plays a role in the regulation of anterior pituitary hormone secretion.

Differential effects of haloperidol, clozapine, and fluperlapine on tuberoinfundibular dopamine neurons and prolactin secretion in the rat.

Two atypical neuroleptic agents, clozapine and fluperlapine, produced rapid elevations in plasma PRL concentrations that were similar in magnitude to those produced by haloperidol. However, the PRL response to clozapine or fluperlapine was of much shorter duration than that elicited by haloperidol. Clozapine, but neither fluperlapine nor haloperidol, produced a rapid increase in the activity of tuberoinfundibular dopamine (TIDA) neurons, as evidenced by an enhanced accumulation of dihydroxyphenylalanine (DOPA) in the median eminence after the inhibition of DOPA decarboxylase. The clozapine-induced increase in DOPA accumulation was evident within 30 minutes after its administration and persisted for at least 4 hours. The clozapine-induced increase in the activity of TIDA neurons may account, in part, for the abbreviated PRL response to this neuroleptic. In addition, ability to produce a short-lived increase in PRL secretion in the rat appears to be common to the atypical neuroleptic drugs.