Ghrelin postsynaptically depolarizes dorsal raphe neurons in rats in vitro

Ghrelin promotes growth hormone (GH) secretion and feeding. Recent studies further showed that ghrelin displayed a defending effect against the depressive-like symptoms and affected sleep in animals and humans. Serotonergic system is considered to be implicated in feeding, depression and other mood disorders, and sleep. The dorsal raphe nucleus (DRN) utilizes serotonin (5-HT) as its major neurotransmitter and expresses GH secretagogue receptors (GHS-Rs). Therefore, the present study was carried out to examine the electrophysiological effect of ghrelin on rat DRN neurons in vitro and determine the ionic mechanism involved. Whole-cell recording revealed that ghrelin depolarized DRN neurons dose-dependently in tetrodotoxin-containing artificial cerebrospinal fluid (TTX ACSF). Pretreatment with [d-Lys³]-GHRP-6, a selective antagonist for GHS-Rs, antagonized the ghrelin-induced depolarization. The depolarization was significantly reduced in a low-Na⁺ TTX ACSF and in a high-K⁺ TTX ACSF and was abolished in the combination of both ACSFs, suggesting that the ghrelin-induced depolarization is mediated by a dual ionic mechanism including an increase in nonselective cationic conductance and a decrease in K⁺ conductance. The experiments on the reversal potential also supported an involvement of the dual ionic mechanism in the ghrelin-induced depolarization. On the basis of their electrophysiological and pharmacological properties, approximately 80% of DRN neurons were classified as putative 5-HT-containing neurons and ghrelin depolarized 75% of them. These results suggest that DRN neurons, especially 5-HT-containing neurons, might be involved in the neural mechanisms through which ghrelin participates in the development and/or regulation of feeding behavior, sleep-wake states and depressive-like symptoms.     

Differential distribution of immunoreactive angiotensinogen in the hind-brain and spinal cord of neonatal and adult rats

The present communication deals with the cytochemical localization of angiotensinogen (ATG) immunoactivity in the hind-brain and spinal cord of neonatal (1-day-old) and adult (3-month-old pregnant) female rats. In the neonatal hind-brain, the immunoreactive cells were more numerous than in that of adult rats. In the adult rat hind-brain, the number of ATG-positive cells was quite limited in each nucleus. Further, in some nuclei, only neurons or neuroglia were positive, while in others the immunoactivity was observed in both the components. Spinal cords of neonatal rats showed a few undifferentiated ATG-positive cells in the grey matter. Contrary to this, the spinal cord of adult animals contained numerous immunoreactive glial cells in the grey matter, fasciculus cuneatus and fasciculus gracilis. Immunoactivity in the neurons was localized in the Nissl bodies.     

帕金森病大鼠中缝背核5-羟色胺能神经元电活动的变化

本实验采用玻璃微电极细胞外记录法,观察了帕金森病(Parkinson’s disease,PD)大鼠中缝背核(dorsal raphe nucleus, DRN)5-羟色胺(5-hydroxytryptamine,5-HT)能神经元电活动的变化。在大鼠右侧中脑黑质致密部内微量注射6-羟多巴胺(6- hydroxydopamine,6-OHDA)制作PD模型。结果显示,对照组和PD组大鼠DRN中5-HT能神经元的放电频率分别是(1．76±0．11)spikes／s(n=24)和(2．43±0．17)spikes／(n=21),PD组大鼠的放电频率显著高于对照组(P<0．001)。在对照组大鼠,92％(22／24)的神经元呈规则放电,8％(2／24)为爆发式放电;在PD组大鼠,具有规则、不规则和爆发式放电的神经元比例分别为9％(2／21)、43％(9／21)和48％(10／21),爆发式放电的5-HT能神经元比例明显高于对照组(P<0．001)。在对照组大鼠,DRN内局部注射5-HT1A拮抗剂WAY-100635(3μg/200nL)显著增加5-HT能神经元的放电频率而不影响其放电形式(n=19,P<0．002);而WAY-100635不改变PD组大鼠5-HT能神经元的放电频率和放电形式(n=17,P>0．05)。结果提示,用6-OHDA损毁黑质致密部造成的PD模型大鼠中神经元5-HT1A受体功能失调,并且DRN参与PD的病理生理学机制。     

帕金森病大鼠中缝背核5-羟色胺能神经元放电频率和放电形式的变化

实验采用玻璃微电极细胞外记录法, 观察了帕金森病(Parkinson’s disease, PD)大鼠中缝背核(dorsal raphe nucleus, DRN)5- 羟色胺(5-hydroxytrypamine, 5-HT)能神经元电活动的变化。结果发现, 对照组和 PD 组大鼠 DRN 中 5-HT 能神经元的放电频率分别为(1.61 ±0.56) Hz 和(2.61 ±1.97) Hz, PD 组大鼠的放电频率显著高于对照组(P<0.05)。在对照组大鼠, 79% 的神经元呈现规则放电, 21% 为爆发式放电;在 PD 组大鼠,具有规则、不规则和爆发式放电的神经元比例分别为 36%、16% 和47%, 爆发式放电的 5-HT 能神经元比例明显高于对照组(P<0.05)。结果表明,帕金森病大鼠 DRN 中 5-HT 能神经元的放电频率增高, 且爆发式放电增多。     

The ultrastructure and distribution of 5-HT-containing neurons in the intestine of a teleost fish,Aldrichetta forsteri

Summary The intestine of Aldrichetta forsteri was examined ultrastructurally for evidence of 5-HT-containing intrinsic neurons. A population of nerve cell bodies and processes characterized by the presence of amine-containing vesicles was found after the destruction of adrenergic processes with 6-hydroxydopamine. The distribution of the neurons matched that of the 5-HT-containing neurons previously seen with formaldehyde-induced fluorescence and 5-HT immunohistochemistry in A. forsteri and other teleosts. The axons made close contacts with the circular smooth muscle layer and the muscularis mucosae. The axons also lay exposed to the epithelial cells of the mucosa but did not make contact with neurons or with the longitudinal smooth muscle layer.     

CHEMICAL, ENZYMATIC AND ULTRASTRUCTURAL CHARACTERIZATION OF 5-HYDROXYTRYPTAMINE-CONTAINING NEURONS FROM THE GANGLIA OF APLYSIA CALIFORNICA AND TRITONIA DIOMEDIA

Abstract— Several identified neurons in Aplysia and Tritonia ganglia were shown to contain measurable quantities (4–6 pmol/cell body) of 5-hydroxytryptamine (5-HT). A metabolic correlate for the limited distribution of 5-HT among the neurons of Tritonia is provided by the finding that the enzyme, aromatic acid decarboxylase (AAD), is 500 times more active in nerve cells containing 5-HT than in neurons devoid of the amine. Although all Aplysia neurons have some AAD activity, 5-HT cell bodies in this species are 10-fold more active than cell bodies which do not contain 5-HT. The cytoplasm of 5-HT cell bodies in Aplysia and Tritonia characteristically contains granules that have minimum diameters of approx. 1000 Å and eccentric opaque cores. This type of granule was not found in somata which did not contain measurable 5-HT. These data illustrate the metabolic and morphological specialization in 5-HT-containing neurons of molluscs.     

Age-Related Changes of NADPH-Diaphorase Positivity in the Rat Rostral Migratory Stream

Summary Accumulating evidence confirms that nitric oxide (NO), a versatile diffusible signaling molecule, contributes to controling of adult neurogenesis. We have previously shown the timing of NADPH-diaphorase (NADPH-d) positivity within the rat rostral migratory stream (RMS) during the first postnatal month. The present study was designed to describe further age-related changes of NO presence in this neurogenic region. The presence of NO synthesizing cells in the RMS was shown by NADPH-d histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry. The phenotypic identity of nitrergic cells was examined by double labeling with GFAP and NeuN. Systematic qualitative and quantitative analysis of NADPH-d-positive cells was performed in the neonatal (P14), adult(5 months) and aging (20 months) rat RMS. 1. Nitrergic cells with different distribution pattern and morphological characteristics were present in the RMS at all ages examined. In neonatal animals, small, moderately stained NADPH-d-positive cells were identified in the RMS vertical arm and in the RMS elbow. In adult and aging rats a few labeled cells could be also detected in the RMS horizontal arm. NADPH-d-positive cells in adult and aging rats were characterized by long varicose processes and displayed dark labeling in comparison to the neonatal group. 2. Double immunolabeling has revealed that nNOS-immunoreactivity co-localized with that of NeuN. This indicates that nitrergic cells within the RMS are neurons. 3. Quantitative analysis showed that the number of NADPH-d-positive cells increases with advancing age. The presence of NO producing cells in the RMS of neonatal adult and aging rats indicates, that this proliferating and migratory area is under the influence of NO throughout the entire life of the animals.     

Mitochondria are More Resistant to Hypoxic Depolarization in the Newborn than in the Adult Brain

Hypoxic-ischemic brain injury subsequent to asphyxia represents a major cause of morbidity and death in the newborn. The newborn brain has been considered more resistant to hypoxia than the adult brain because of lower energy demand. The mechanisms underlying hypoxic brain injury, in particular the age-related vulnerability, are still only partially understood. The mitochondrial function is pivotal for the function and survival of neurons. Acutely isolated CA1 neurons from neonatal (3-6 days) and adult rats (5-6 weeks) were loaded with Rh 123, and the effect of hypoxia on the inner mitochondrial membrane potential (Delta psi(m)) was compared. During prolonged hypoxia (15 min), Delta psi(m) was lost in a majority of the neonatal neurons (83%) and in all the adult neurons. During hypoxia (5 min) followed by reoxygenation the mitochondria in 23% of the neonatal neurons were completely depolarized, whereas 85% of the adult neurons demonstrated a complete loss of Delta psi(m). In conclusion hippocampal CA1 mitochondria in the newborn rat are more resistant to hypoxic depolarization than in the adult rat.     

Sodium Salicylate Suppresses GABAergic Inhibitory Activity in Neurons of Rodent Dorsal Raphe Nucleus

Sodium salicylate (NaSal), a tinnitus inducing agent, can activate serotonergic (5-HTergic) neurons in the dorsal raphe nucleus (DRN) and can increase serotonin (5-HT) level in the inferior colliculus and the auditory cortex in rodents. To explore the underlying neural mechanisms, we first examined effects of NaSal on neuronal intrinsic properties and the inhibitory synaptic transmissions in DRN slices of rats by using whole-cell patch-clamp technique. We found that NaSal hyperpolarized the resting membrane potential, decreased the input resistance, and suppressed spontaneous and current-evoked firing in GABAergic neurons, but not in 5-HTergic neurons. In addition, NaSal reduced GABAergic spontaneous and miniature inhibitory postsynaptic currents in 5-HTergic neurons. We next examined whether the observed depression of GABAergic activity would cause an increase in the excitability of 5-HTergic neurons using optogenetic technique in DRN slices of the transgenic mouse with channelrhodopsin-2 expressed in GABAergic neurons. When the GABAergic inhibition was enhanced by optical stimulation to GABAergic neurons in mouse DRN, NaSal significantly depolarized the resting membrane potential, increased the input resistance and increased current-evoked firing of 5-HTergic neurons. However, NaSal would fail to increase the excitability of 5-HTergic neurons when the GABAergic synaptic transmission was blocked by picrotoxin, a GABA receptor antagonist. Our results indicate that NaSal suppresses the GABAergic activities to raise the excitability of local 5-HTergic neural circuits in the DRN, which may contribute to the elevated 5-HT level by NaSal in the brain.     

Distribution of biogenic amines in the slug,Limax maximus

Summary The distribution of monoamines inLimax maximus was studied by the histochemical fluorescent method of Falck and Hillarp. The number of 5-HT-containing and catecholamine-containing perikarya in the central nervous system is small compared with the non-fluorescent perikarya. However, all the ganglia except the proto-cerebral ganglia have some amine-containing neurons. There are relatively larger numbers of fluorescent cells in the cerebral, visceral, pedal and right parietal ganglia than in the other ganglia. A single, giant 5-HT-containing neuron was observed in each meta-cerebral ganglion.Monoamine neurons are localised in a number of peripheral tissues (heart, integument, tentacles, penis retractor muscle, sole of foot, kidney, alimentary canal, reproductive organs and tentacular, pharyngeal and cephalic retractor muscles). Neurons containing catecholamine are mostly associated with sensory structures such as the statocysts, the retina of the eye and the integument of the tentacles, whereas 5-HT-containing nerve fibres are mainly observed in muscle tissues.We wish to thank the Wellcome Trust for financial support.     

Role of dorsal raphe nucleus serotonin 5-HT1A receptor in the regulation of REM sleep

Cholinergic neurons in the laterodorsal (LDT) and the pedunculopontine (PPT) tegmental nuclei act to promote REM sleep (REMS). The predominantly glutamatergic neurons of the REMS-induction region of the medial pontine reticular formation are in turn activated by cholinergic cells, which results in the occurrence of tonic and phasic components of REMS. All these neurons are inhibited by serotonergic (5-HT), noradrenergic, and presumably histaminergic (H2 receptor) and dopaminergic (D2 and D3 receptor) cells. 5-Hydroxytryptamine-containing neurons in the dorsal raphe nucleus (DRN) virtually cease firing when an animal starts REMS, consequently decreasing the release of 5-HT during this state. The activation of GABA(A) receptors is apparently responsible for this phenomenon. Systemic administration of the selective 5-HT1A receptor agonist 8-OHDPAT induces dose-dependent effects; i.e. low doses increase slow wave sleep and reduce waking, whereas large doses increase waking and reduce slow wave sleep and REM sleep. Direct injection of 8-OHDPAT or flesinoxan, another 5-HT1A agonist into the DRN, or microdialysis perfusion of 8-OHDPAT into the DRN significantly increases REMS. On the other hand, infusion of 8-OHDPAT into the LDT selectively inhibits REMS, as does direct administration into the DRN of the 5-HT1A receptor antagonists pindolol or WAY 100635. Thus, presently available evidence indicates that selective activation of the somatodendritic 5-HT1A receptor in the DRN induces an increase of REMS. On the other hand, activation of the postsynaptic 5-HT1A receptor at the level of the PPT/LDT nuclei decreases REMS occurrence.     

Acanthoscurria gomesiana spider-derived Mygalin in the prelimbic prefrontal cortex modulates neuropathic pain and depression comorbid

Depression has a high rate of comorbidity with neuropathic pain. This study aims to investigate the effect of Mygalin, an acylpolyamine synthesized from a natural molecule in the hemolymph of the Acanthoscurria gomesiana spider, injected into the prelimbic (PrL) region of the medial prefrontal cortex on chronic neuropathic pain and depression comorbidity in rats. To investigate that comorbidity, neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve in male Wistar rats. The biotinylated biodextran amine (BDA) bidirectional neural tract tracer was microinjected into the PrL cortex to study brain connections. Rodents were further subjected to von Frey (mechanical allodynia), acetone (cold allodynia), and forced swim (depressive-like behavior) tests. BDA neural tract tracer-labeled perikarya were found in the dorsal columns of the periaqueductal gray matter (dPAG) and the dorsal raphe nucleus (DRN). Neuronal activity of DRN neurons decreased in CCI rats. However, PrL cortex treatment with Mygalin increased the number of spikes on DRN neurons. Mygalin treatment in the PrL cortex decreased both mechanical and cold allodynia and immobility behavior in CCI rats. PrL cortex treatment with N-methyl-D -aspartate (NMDA) receptor receptors attenuated the analgesic and antidepressive effects caused by Mygalin. The PrL cortex is connected with the dPAG and DRN, and Mygalin administration into the PrL increased the activity of DRN neurons. Mygalin in the PrL cortex produced antinociceptive and antidepressive-like effects, and the NMDA agonist reversed these effects.     

A comparison of lipid metabolism in hepatocytes isolated from fed and starved neonatal and adult rats.

1. The utilization of [1-14C]palmitate by hepatocytes prepared from fed and starved neonatal and adult rats has been examined by measuring isotopic incorporation into various products. 2. In cells from fed adult rats the principal products were esters (triglycerides and phospholipids) but ketone bodies were the main metabolic end products in cells from starved adult and fed and starved neonatal rats. Production of triglycerides exceeded that of phospholipids in fed adult cells whereas phospholipid formation always predominated in neonatal cells. 3. The high rate of fatty acid oxidation and hence NADH formation by neonatal cells is reflected by a lower acetoacetate--3-hydroxybutyrate ratio at the earlier stages of incubation of neonatal cells. 4. The addition of glycerol modified quantitatively the products of palmitate metabolism by adult hepatocytes but no such effects were observed with neonatal cells. 5. Compared with adult cells, neonatal hepatocytes showed very low rates of lipogenesis that were only enhanced a little by addition of lactate/pyruvate and did not show any effects of glucose concentration upon incorporation of tritium from 3H2O into lipids.     

Tryptamine Concentrations in Areas of 5-Hydroxytryptamine Terminal Innervation After Electrolytic Lesions of Midbrain Raphe Nuclei

The possible existence of tryptamine-containing neurons originating in the midbrain raphe is suggested by several reports of tryptamine-mediated responses to electrical stimulation of the raphe nuclei. To assess this hypothesis, we have investigated the effects of electrolytic lesions of the median and dorsal raphe nuclei on striatal, hypothalamic, and hippocampal concentrations of tryptamine, 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid. In addition, the rat striatal tryptophan concentrations were also determined. No changes in the concentrations of tryptamine were observed at 1 or 2 weeks after lesioning the dorsal and median raphe nuclei, at which time the other 5-hydroxyindoles were markedly reduced; furthermore, no reductions were observed in tryptamine concentrations in the striatum, hypothalamus, or hippocampus of rats pretreated with a monoamine oxidase inhibitor. The only change observed in these rats was a limited increase in striatal tryptamine and tryptophan observed at 1 day after lesioning. The results indicate that tryptamine concentration is independent of the integrity of 5-HT-containing neurons of the midbrain raphe nuclei. Furthermore, if tryptamine-containing neurons that have terminal projections to the striatum, hypothalamus, and hippocampus exist, their cell bodies are located in regions outside the dorsal and median raphe nuclei. Another possibility could be that tryptamine is located in glial cells.     

Chronic Functional Consequences of Adult Infraorbital Nerve Transection for Rat Trigeminal Subnucleus Interpolaris

In adult rats, transection of the infraorbital nerve and subsequent regeneration have been shown to result in altered somatotopic organization and changes in response properties of primary afferents within the trigeminal ganglion. The present study examined how these changes affect the postsynaptic targets of these neurons within subnucleus interpolaris of the trigeminal brainstem. Extra-cellular recordings were made from 330 cells in normal rats and 424 cells in rats surviving 57-290 days after transection of the infraorbital nerve in adulthood. Adult infraorbital nerve transection resulted in significant functional reorganization within subnucleus interpolaris. Relative to normal rats, the major changes can be summarized as follows: (1) a decrease in the dorsoventral extent of infraorbital representation; (2) a disruption of inter- and intradivisional somatotopic organization; (3) an increase in the proportion of cells with no discernible receptive field; (4) an increase in receptive field size for cells with infraorbital receptive field components; (5) the appearance of a significant proportion of cells with discontinuous receptive fields; (6) an increase in the proportion of cells exhibiting interdivisional convergence; (7) significant changes in the types of receptor surfaces activating local-circuit neurons with infraorbital receptive field components; (8) the appearance of a significant proportion of cells exhibiting convergence of different receptor surfaces; (9) significant changes in the dynamic response characteristics of cells with infraorbital receptive field components; and (10) an increase in the proportion of spontaneously active infraorbital-responsive cells. The changes observed were quite similar to those reported in adult subnucleus interpolaris following neonatal infraorbital nerve transection. The majority of changes observed in both studies can be most parsimoniously explained by alterations of primary afferents. However, central mechanisms may be more likely substrates for others. Regardless of the mechanism, the mature rodent trigeminal system appears capable of considerable functional reorganization following peripheral nerve damage.     

Chronic functional consequences of adult infraorbital nerve transection for rat trigeminal subnucleus interpolaris

In adult rats, transection of the infraorbital nerve and subsequent regeneration have been shown to result in altered somatotopic organization and changes in response properties of primary afferents within the trigeminal ganglion. The present study examined how these changes affect the postsynaptic targets of these neurons within subnucleus interpolaris of the trigeminal brainstem. Extracellular recordings were made from 330 cells in normal rats and 424 cells in rats surviving 57-290 days after transection of the infraorbital nerve in adulthood. Adult infraorbital nerve transection resulted in significant functional reorganization within subnucleus interpolaris. Relative to normal rats, the major changes can be summarized as follows: (1) a decrease in the dorsoventral extent of infraorbital representation; (2) a disruption of inter- and intradivisional somatotopic organization; (3) an increase in the proportion of cells with no discernible receptive field; (4) an increase in receptive field size for cells with infraorbital receptive field components; (5) the appearance of a significant proportion of cells with discontinuous receptive fields; (6) an increase in the proportion of cells exhibiting interdivisional convergence; (7) significant changes in the types of receptor surfaces activating local-circuit neurons with infraorbital receptive field components; (8) the appearance of a significant proportion of cells exhibiting convergence of different receptor surfaces; (9) significant changes in the dynamic response characteristics of cells with infraorbital receptive field components; and (10) an increase in the proportion of spontaneously active infraorbital-responsive cells. The changes observed were quite similar to those reported in adult subnucleus interpolaris following neonatal infraorbital nerve transection. The majority of changes observed in both studies can be most parsimoniously explained by alterations of primary afferents. However, central mechanisms may be more likely substrates for others. Regardless of the mechanism, the mature rodent trigeminal system appears capable of considerable functional reorganization following peripheral nerve damage.     

5,7-双羟色胺损毁大鼠中缝背核后底丘脑核的神经活动增强

采用玻璃微电极在体细胞外记录法,观察了5,7-双羟色胺(5,7-dihydroxytryptamine,5,7-DHT)损毁大鼠中缝背核(dorsalraphenucleus,DRN)后,底丘脑核(subthalamicnucleus,STN)神经元电活动的变化。结果发现,对照组和DRN损毁组大鼠STN神经元的放电频率分别是(6.93±6.55)Hz和(11.27±9.31)Hz,DRN损毁组大鼠的放电频率显著高于对照组(P<0.01)。在对照组大鼠,13%的神经元呈现规则放电,46%为不规则放电,41%为爆发式放电;而在DRN损毁组大鼠,具有规则、不规则和爆发式放电的神经元比例分别为9%、14%和77%,爆发式放电的STN神经元比例明显高于对照组(P<0.01)。结果显示,DRN损毁后大鼠STN神经元的放电频率增高,爆发式放电增多,提示在正常大鼠DRN抑制STN神经元的活动。     

Differential Effect of NMDA on Extracellular Serotonin in Rat Midbrain Raphe and Forebrain Sites

Abstract: The contribution of NMDA receptors to regulation of serotonin (5-HT) release was assessed by in vivo microdialysis in freely behaving rats. During infusion of NMDA (30, 100, and 300 µ M ) into the dorsal raphe nucleus (DRN), 5-HT was increased by ∼25, 100, and 280%, respectively. Competitive and noncompetitive NMDA-receptor antagonists blocked this effect on DRN 5-HT. Infusion of NMDA (300 µ M ) into the DRN also produced an 80% increase in extracellular 5-HT in the nucleus accumbens. During infusion of NMDA (100 and 300 µ M ) into the median raphe nucleus (MRN), 5-HT was increased by ∼15 and 80%, respectively. NMDA-receptor antagonists blocked this effect on MRN 5-HT. Infusion of NMDA into the MRN also produced a significant increase in hippocampal 5-HT. In contrast, infusion of NMDA into the nucleus accumbens, frontal cortex, or hippocampus produced small decreases in 5-HT in these forebrain sites. Taken together, these results suggest that NMDA receptors in the midbrain raphe, but not the forebrain, can have an excitatory influence on 5-HT neurons and, thus, produce increased 5-HT release in the forebrain. Furthermore, in comparison with the MRN, DRN 5-HT neurons were more sensitive to the excitatory effect of NMDA.     

Sex steroid effects on the development and functioning of the growth hormone axis

Summary 1. The secretory pattern of growth hormone (GH) is sexually dimorphic in the adult rat. However, this difference between the sexes does not become apparent until after the onset of puberty, suggesting that pubertal sex steroids play an important role in the manifestation of this phenomenon.2. We have addressed the question as to whether there exists a sexual dimorphism in the hypothalamic neuropeptides that regulate GH release from the anterior pituitary,i.e., somatostatin (SS) and growth hormone-releasing hormone (GHRH). In addition, we have investigated whether the developmental changes in the GH secretory pattern are correlated with changes in these neuropeptides. The effect of testosterone treatment on SS and GHRH neurons during both the neonatal period and adulthood have also been studied.3. We have found that the synthetic capacity, as reflected in relative messenger RNA (mRNA) levels, of both SS and GHRH neurons changes throughout development in both male and female rats. These mRNA levels are sexually dimorphic at certain times during maturation and can be modulated by changes in testosterone levels, suggesting that sex steroid modulation of these two neuropeptide systems could at least partially account for the sexual dimorphism seen in the adult GH secretory pattern.4. The neonatal steroid environment has also been suggested to be involved in the generation of the final adult GH secretory pattern, although the mechanisms underlying this effect are even less well understood. In support of the hypothesis that the neonatal steroid environment plays an important role in organizing the GH axis, we have found that the number of GHRH neurons in the adult brain, as well as their sensitivity to adult steroids, is modulated by neotatal testosterone treatment. The number of SS neurons in the periventricular and paraventricular nuclei were not modulated by neonatal steroids; however, the synthetic capacity of these neurons does appear to be influenced by the neonatal steroid environment.5. These studies suggest that both the neonatal and adult sex steroid environments influence the adult GH secretory pattern by modulating GHRH and SS neurons.