James I. Koenig

Thermoregulatory responses to serotonin (5-HT) receptor stimulation in the rat: Evidence for opposing roles of 5-HT2 and 5-HT1A receptors

The effects of serotonergic agonists and antagonists on the body temperatures of rats were investigated. The administration of the serotonin (5-HT) agonist 6-chloro-2(1-piperazinyl)-pyrazine (MK-212) produced a dose-related increase in body temperature. A maximal increase in body temperature of approx. 1.1 degrees C was observed 30 min after the administration of 3 mg/kg of MK-212. In contrast, administration of the putative 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) resulted in marked, dose-related hypothermic responses. Body temperatures were decreased approx. 3 degrees C 30 min after an injection of 0.3 mg/kg of 8-OH-DPAT. Body temperatures were affected differentially by 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). Large doses (3-10 mg/kg) of 5-MeODMT elicited hyperthermic responses, whereas small doses (0.5-1.0 mg/kg) produced hypothermic responses. Treatment of rats with ketanserin (3 mg/kg) completely prevented the hyperthermic effects of 5-MeODMT, and, in fact, converted a hyperthermic response to 5-MeODMT into a marked hypothermic response. Ketanserin (0.1-1.0 mg/kg) selectively antagonized the hyperthermic response to MK-212 but did not alter the hypothermic effect of 8-OH-DPAT. Mianserin (10 mg/kg) and pirenperone (0.03 mg/kg) also selectively antagonized hyperthermia induced by MK-212. In contrast, pindolol (0.03-0.1 mg/kg) and methiothepin (10 mg/kg) selectively antagonized hypothermia induced by 8-OH-DPAT but did not alter hyperthermia induced by MK-212. Spiperone (0.1-3 mg/kg) and pizotifen (10 mg/kg) attenuated the effects of both 8-OH-DPAT and MK-212. Xylamidine, a peripheral 5-HT antagonist, had no significant effect on hyperthermia induced by MK-212 or hypothermia induced by 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of corticosterone and β-endorphin secretion in the rat by selective 5-HT receptor subtype activation

Changes in plasma concentrations of corticosterone and beta-endorphin (beta-END) were determined in male rats after treatment with the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) or the non-selective 5-HT agonist 6-chloro-2-(1-piperazinyl)pyrazine (MK-212). The administration of either 8-OH-DPAT or MK-212 increased plasma concentrations of both corticosterone and beta-END in a dose-related manner. The corticosterone and beta-END responses to 8-OH-DPAT were antagonized by spiperone and (-)-pindolol, both of which have been shown to have high affinity for the 5-HT1A binding site. In contrast, antagonist which are selective for the 5-HT2 receptor or non-selective 5-HT antagonists were without effect on the hormone responses to 8-OH-DPAT. The MK-212-induced increase in plasma concentrations of corticosterone and beta-END were not affected by treatment with the 5-HT1A antagonists spiperone and (-)-pindolol. However, the corticosterone and beta-END responses to MK-212 were attenuated by the selective 5-HT2 antagonists ketanserin, ritanserin and altanserin, as well as by the non-selective 5-HT antagonist metergoline. It is concluded that stimulation of either 5-HT1A or 5-HT2 receptors results in an activation of the hypothalamic-pituitary-adrenal axis in the rat.

High Concentrations of Neuropeptide Y in Pituitary Portal Blood of Rats

Pituitary portal blood was collected from urethane-anesthetized rats and examined for the presence of neuropeptide Y (NPY) using high-performance liquid chromatography and radioimmunoassay. Other rats were perfused with fixative, and coronal sections through the hypothalamus and median eminence were processed for immunohistochemical localization of NPY. Combined high-performance liquid chromatography and radioimmunoassay analysis of pituitary portal plasma and systemic plasma revealed a single peak of NPY immunoreactivity which corresponded in retention time to synthetic porcine NPY. Increasing amounts of portal or systemic plasma produced displacement curves which were parallel to the NPY standard curve. The concentration of NPY immunoreactivity in portal plasma (52.0 +/- 4.0 ng/ml, mean +/- SEM) was three times greater (p less than 0.005) than in systemic plasma (16 +/- 4.5 ng/ml). NPY-labeled fibers were observed in the external zone of the median eminence in the vicinity of hypothalamo-hypophyseal portal vessels. The observation of significantly higher concentrations of NPY immunoreactivity in the portal plasma supports the hypothesis that NPY may be released from the hypothalamus to affect pituitary function.

Differential Effect of Subchronic Treatment with Various Neuroleptic Agents on Serotonin2 Receptors in Rat Cerebral Cortex

Abstract: In addition to antidepressant drugs, some neuroleptic (NL) drugs reduce serotonin, (5‐HT2) receptor binding sites after chronic administration. The present study was undertaken to characterize further this property of NL drugs. Scatchard analysis of [3H]spiperone binding in rat cerebral cortex revealed that 21‐day treatment with chlorpromazine (CPZ), cis‐flupenthixol, and thioridazine reduced 5‐HT2 radioligand binding density by 60, 27, and 18%, respectively. The more selective dopamine‐D2 antagonists haloperidol and sulpiride were totally ineffective in this regard. No reduction in 5‐HT2 ligand binding sites occurred after 1 day of treatment with CPZ but 3‐days of treatment was effective and this reduction persisted, although diminished, for at least 72 h after the last injection. cis‐Flupenthixol and d‐butaclamol were also effective after 3 days of treatment but trans‐flupenthixol and l‐butaclamol were not, indicating stereo‐specificity of the response mechanism. Female rats showed the same response to CPZ as did male rats. Central 5,7‐dihydroxytryptamine‐induced lesions of 5‐HT neurons demonstrated that intact 5‐HT neurons were not required for the reduction of 5‐HT2 receptor ligand binding by CPZ. Since CPZ has high affinity for many receptors, including α1, histamine1, and muscarinic receptors, the role of these effects in producing 5‐HT2 receptor down‐regulation was considered by studying the effects of prazosin, atropine, and pyrilamine administration on 5‐HT2 radioligand binding. Results indicate that no one of these actions appears to account for the down‐regulation of 5‐HT, receptors by CPZ. Several of these effects, in combination, or some unique mechanism, may be involved.

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.

Increase in plasma leptin and Lep mRNA concentrations by food intake is dependent on insulin

Obese (Lep) gene expression and leptin secretion are regulated by changes in food intake. However, the mechanism by which leptin concentrations are altered by fasting and feeding is unclear. Since these changes occur in parallel with changes in plasma insulin, it is possible that the changes observed are mediated by insulin. To test this hypothesis, we studied the role of insulin in the regulation of Lep gene expression in epididymal fat and leptin secretion during feeding. As shown previously, fasted animals showed significant reductions in Lep mRNA, plasma leptin, and plasma insulin concentrations. Conversely, feeding increased plasma insulin, Lep mRNA, and plasma leptin. In streptozotocin (STZ)-treated animals, plasma insulin concentrations were low. This was associated with low Lep mRNA and plasma leptin concentrations. Changes in food intake, whether fasting or feeding, did not significantly alter plasma insulin levels in STZ-treated animals. Under these circumstances, Lep mRNA and plasma leptin concentrations also remained low. Our results demonstrate that the decrease in Lep mRNA and plasma leptin during fasting and the increase with feeding are dependent on changes in the plasma insulin concentration.

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.

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)

Potential involvement of galanin in the regulation of fluid homeostasis in the rat

A physiological role for galanin, a 29-amino acid neuropeptide, has not been established. However, anatomical studies have demonstrated the presence of galanin in brain regions associated with the control of water balance in the rat, most notably in the paraventricular nucleus (PVN) of the hypothalamus and the neurointermediate lobe of the pituitary gland (NIL). In the PVN, galanin coexists with arginine vasopressin (AVP) in magnocellular neurons. The present study demonstrates that homozygous Brattleboro rats, which lack AVP, produce galanin. Galanin concentrations in the median eminence (ME) of the homozygous Brattleboro rat do not differ from the galanin concentrations in the ME of either heterozygous Brattleboro or Sprague-Dawley rats. However, galanin concentrations in the NIL of the homozygous Brattleboro rat were reduced by 75%. Similarly, dehydration induced by salt-loading reduced galanin concentrations in the NIL and produced transient changes in the ME. These data demonstrate that galanin concentrations are influenced by changes in fluid homeostasis and suggest that galanin may be an important component in the regulation of neurohypophyseal function and AVP secretion.

Neuropeptide Y Actions on Reproductive and Endocrine Functions

In this chapter, we have reviewed the current literature concerning the function of neuropeptide Y (NPY) in endocrine and reproductive systems. The reader is referred to other reviews and recent symposia on NPY for consideration of other areas (1–4). The reader is also referred to other chapters in this volume, particularly those by Hendry and by Stanley.