Expression of delta opioid receptor mRNA and protein in the rat cerebral cortex and cerebellum is decreased by growth hormone

Addiction and the cerebellum with a focus on actions of opioid receptors

Increasing evidence suggests that the cerebellum could play a role in the higher cognitive processes involved in addiction as the cerebellum contains anatomical and functional pathways to circuitry controlling motivation and saliency. In addition, the cerebellum exhibits a widespread presence of receptors, including opioid receptors which are known to play a prominent role in synaptic and circuit mechanisms of plasticity associated with drug use and development of addiction to opioids and other drugs of abuse. Further, the presence of perineural nets (PNNs) in the cerebellum which contain proteins known to alter synaptic plasticity could contribute to addiction. The role the cerebellum plays in processes of addiction is likely complex, and could depend on the particular drug of abuse, the pattern of use, and the stage of the user within the addiction cycle. In this review, we discuss functional and structural modifications shown to be produced in the cerebellum by opioids that exhibit dependency-inducing properties which provide support for the conclusion that the cerebellum plays a role in addiction.

Growth Hormone (GH) and Gonadotropin-Releasing Hormone (GnRH) in the Central Nervous System: A Potential Neurological Combinatory Therapy?

This brief review of the neurological effects of growth hormone (GH) and gonadotropin-releasing hormone (GnRH) in the brain, particularly in the cerebral cortex, hypothalamus, hippocampus, cerebellum, spinal cord, neural retina, and brain tumors, summarizes recent information about their therapeutic potential as treatments for different neuropathologies and neurodegenerative processes. The effect of GH and GnRH (by independent administration) has been associated with beneficial impacts in patients with brain trauma and spinal cord injuries. Both GH and GnRH have demonstrated potent neurotrophic, neuroprotective, and neuroregenerative action. Positive behavioral and cognitive effects are also associated with GH and GnRH administration. Increasing evidence suggests the possibility of a multifactorial therapy that includes both GH and GnRH.

Mode of GH administration and gene expression in the female rat brain

The endogenous secretion of growth hormone (GH) is sexually dimorphic in rats with females having a more even and males a more pulsatile secretion and low trough levels. The mode of GH administration, mimicking the sexually dimorphic secretion, has different systemic effects. In the brains of male rats, we have previously found that the mode of GH administration differently affects neuron-haemoglobin beta (Hbb) expression whereas effects on other transcripts were moderate. The different modes of GH administration could have different effects on brain transcripts in female rats. Hypophysectomised female rats were given GH either as injections twice daily or as continuous infusion and GH-responsive transcripts were assessed by quantitative reverse transcription polymerase chain reaction in the hippocampus and parietal cortex (cortex). The different modes of GH-administration markedly increased Hbb and 5'-aminolevulinate synthase 2 (Alas2) in both brain regions. As other effects were relatively moderate, a mixed model analysis (MMA) was used to investigate general effects of the treatments. In the hippocampus, MMA showed that GH-infusion suppressed glia- and neuron-related transcript expression levels, whereas GH-injections increased expression levels. In the cortex, GH-infusion instead increased neuron-related transcripts, whereas GH-injections had no significant effect. Interestingly, this contrasts to previous results obtained from male rat cortex where GH-infusion generally decreased expression levels. In conclusion, the results indicate that there is a small but significant difference in response to mode of GH administration in the hippocampus as compared to the cortex. For both modes of GH administration, there was a robust effect on Hbb and Alas2.

Different modes of GH administration influence gene expression in the male rat brain

The endogenous secretion pattern in males of GH is episodic in rats and in humans, whereas GH administration is usually even. Different types of GH administration have different effects on body mass, longitudinal bone growth, and liver metabolism in rodents, whereas possible effects on brain plasticity have not been investigated. In this study, GH was administered as a continuous infusion or as two daily injections in hypophysectomized male rats. Thirteen transcripts previously known to respond to GH in the hippocampus and parietal cortex (cortex) were assessed by RT-PCR. To investigate the effects of type of GH administration on several transcripts with different variations, and categories of transcripts (neuron-, glia-, and GH-related), a mixed model analysis was applied. Accordingly, GH injections increased overall transcript abundance more than GH infusions (21% in the hippocampus, P<0.001 and 10% in the cortex, P=0.09). Specifically, GH infusions and injections robustly increased neuronal hemoglobin beta (Hbb) expression significantly (1.8- to 3.6-fold), and GH injections were more effective than GH infusions in increasing Hbb in the cortex (41%, P=0.02), whereas a 23% difference in the hippocampus was not significant. Also cortical connexin 43 was higher in the group with GH injections than in those with GH infusions (26%, P<0.007). Also, there were differences between GH injections and infusions in GH-related transcripts of the cortex (23%, P=0.04) and glia-related transcripts of the hippocampus (15%, P=0.02). Thus, with the exception of Hbb there is a moderate difference in responsiveness to different modes of GH administration.

Local overexpression of GH and GH/IGF1 effects in the adult mouse hippocampus

GH therapy improves hippocampal functions mainly via circulating IGF1. However, the roles of local GH and IGF1 expression are not well understood. We investigated whether transgenic (TG) overexpression in the adult brain of bovine GH (bGH) under the control of the glial fibrillary acidic protein (GFAP) promoter affected cellular proliferation and the expression of transcripts known to be induced by systemic GH in the hippocampus. Cellular proliferation was examined by 5-bromo-2'-deoxyuridine immunohistochemistry. Quantitative PCR and western blots were performed. Although robustly expressed, bGH-Tg did not increase either cell proliferation or survival. However, bGH-Tg modestly increased Igf1 and Gfap mRNAs, whereas other GH-associated transcripts were unaffected, i.e. the GH receptor (Ghr), IGF1 receptor (Igf1r), 2',3'-cyclic nucleotide 3'-phosphodiesterase (Cnp), ionotropic glutamate receptor 2a (Nr2a (Grin2a)), opioid receptor delta (Dor), synapse-associated protein 90/postsynaptic density-95-associated protein (Sapap2 (Dlgap2)), haemoglobin beta (Hbb) and glutamine synthetase (Gs (Glul)). However, IGF1R was correlated with the expression of Dor, Nr2a, Sapap2, Gs and Gfap. In summary, although local bGH expression was robust, it activated local IGF1 very modestly, which is probably the reason for the low response of previous GH-associated response parameters. This would, in turn, indicate that hippocampal GH is less important than endocrine GH. However, as most transcripts were correlated with the expression of IGF1R, there is still a possibility for endogenous circulating or local GH to act via IGF1R signalling. Possible reasons for the relative bio-inactivity of bGH include the bell-shaped dose-response curve and cell-specific expression of bGH.

Application of in vitro [³⁵S]GTPγ-S autoradiography in studies of growth hormone effects on opioid receptors in the male rat brain

Chronic treatment with opiates may inhibit cell growth and trigger apoptosis. On the contrary, growth hormone (GH) has been demonstrated to stimulate neurogenesis and counteract apoptosis. We recently demonstrated that recombinant human GH (rhGH) may reverse opiate-induced apoptosis in cells derived from prenatal mouse hippocampus. Thus, GH might be able to prevent the impaired cognitive capabilities that may occur in both humans and other mammals in connection to chronic opiate treatment. In order to explore the mechanism by which GH exerts its beneficial effects we here examined the impact of GH treatment on the levels of delta and mu opioid peptide (DOP and MOP, respectively) receptors in the male rat brain. The rats were treated with rhGH (Genotropin®) at two different doses (0.07 and 0.7 IU/kg), twice daily, during 7 days. Following decapitation, the levels of DOP and MOP receptor functionality were determined using [³⁵S]GTPγS autoradiography. The results demonstrate that rhGH affects the levels of the MOP receptor functionality in certain areas of the brain. These alterations were seen in e.g. amygdala and thalamus, i.e. regions that recently have been implicated in learning and memory. The activity level of DOP receptors was not affected. Thus, the data support that the beneficial effect of GH on counteracting apoptosis might involve a direct or indirect effect on the MOP but not the DOP receptor.

Peripheral administration of bovine GH regulates the expression of cerebrocortical beta-globin, GABAB receptor 1, and the Lissencephaly-1 protein (LIS-1) in adult hypophysectomized rats

Growth hormone (GH) therapy substantially improves several cognitive functions in hypopituitary experimental animals and in humans. Although a number of biochemical correlates to these effects have been characterized, there are no comprehensive analysis available examining effects of GH on the brain. Hypophysectomized female rats were given replacement therapy with cortisol and thyroxine (=hx). Subcutaneous infusions of bovine GH (bGH, henceforth designated GH) were supplied in osmotic minipumps for 6 days (=hx+GH). To evaluate whether GH normalized specific transcript expression levels in cerebral cortex, pituitary-intact rats were used as normal controls. DNA microarrays (Affymetrix) of cerebrocortical samples showed that 24 transcripts were changed by more than 1.5-fold by GH treatment in addition to being normalized by GH treatment. The expression of three selected highly regulated transcripts was confirmed by quantitative real-time polymerase chain reaction analysis. These were the GABAB receptor 1, Lissencephaly-1 protein (LIS-1), and hemoglobin b or beta-globin. A similar regulation was found for hemoglobin b also in the hippocampus. Both the GABAB receptor 1 and hemoglobin b may have importance for the previously described neuroprotective and perhaps cognitive potential of GH treatment. Altogether, these results show that short term GH treatment affects a number of transcripts in cerebral cortex with various biological functions. These transcripts represent potential novel mechanisms by which GH can interact with the brain.

Expression, cellular distribution, and heterogeneity of growth hormone in the chicken cerebellum during development

Although growth hormone (GH) is mainly synthesized and secreted by pituitary somatotrophs, it is now well established that the GH gene can be expressed in many extrapituitary tissues, including the central nervous system (CNS). Here we studied the expression of GH in the chicken cerebellum. Cerebellar GH expression was analyzed by in situ hybridization and cDNA sequencing, as well as by immunohistochemistry and confocal microscopy. GH heterogeneity was studied by Western blotting. We demonstrated that the GH gene was expressed in the chicken cerebellum and that its nucleotide sequence is closely homologous to pituitary GH cDNA. Within the cerebellum, GH mRNA is mainly expressed in Purkinje cells and in cells of the granular layer. GH-immunoreactivity (IR) is also widespread in the cerebellum and is similarly most abundant in the Purkinje and granular cells as identified by specific neuronal markers and histochemical techniques. The GH concentration in the cerebellum is age-related and higher in adult birds than in embryos and juveniles. Cerebellar GH-IR, as determined by Western blot under reducing conditions, is associated with several size variants (of 15, 23, 26, 29, 35, 45, 50, 55, 80 kDa), of which the 15 kDa isoform predominates (>30% among all developmental stages). GH receptor (GHR) mRNA and protein are also present in the cerebellum and are similarly mainly present in Purkinje and granular cells. Together, these data suggest that GH and GHR are locally expressed within the cerebellum and that this hormone may act as a local autocrine/paracrine factor during development of this neural tissue.

Cocaine withdrawal-induced trafficking of delta-opioid receptors in rat nucleus accumbens

Interactions between the opioidergic and dopaminergic systems in the nucleus accumbens (NAcb) play a critical role in mediating cocaine withdrawal-induced effects on cell signaling and behavior. In support of this, increased activation of striatal dopamine-D1 receptors (D1R) results in desensitization of delta-opioid receptor (DOR) signaling through adenylyl cyclase during early cocaine withdrawal. A potential cellular substrate underlying receptor desensitization is receptor internalization. The present study examined the effect of cocaine withdrawal on subcellular localization of DOR in dendrites of the NAcb core (NAcbC) and shell (NAcbS) using immunoelectron microscopy. Female and male rats received binge-pattern cocaine or saline for 14 days and subsequently underwent 48 h withdrawal. Animals were transcardially perfused and tissue sections were processed for immunogold-silver localization of DOR. Semi-quantitative analysis revealed that cocaine withdrawal caused an increase in the percentage of DOR localized intracellularly in the NAcbS of male and female rats and the NAcbC of male rats compared to saline controls. In contrast, in the NAcbC of female rats, there was an increase in DOR associated with the plasma membrane following cocaine withdrawal. To determine whether modulation of D1R could directly impact DOR containing neurons, the hypothesis that DOR and D1R co-exist in common neurons of the NAcb was examined in naïve rats. Semi-quantitative analysis revealed a subset of profiles containing both DOR and D1R immunoreactivities. The present findings demonstrate a redistribution of DOR in the NAcb following cocaine withdrawal and provide anatomical evidence supporting D1R regulation of DOR function in a subset of NAcb neurons.

Distinct neuronal growth hormone receptor ligand specificity in the rat brain

A cerebral growth hormone axis is activated during recovery from brain injury and centrally administered growth hormone can rescue injured neurons. It remains unclear, however, whether this treatment effect occurs directly via neuronal growth hormone receptors. Immunohistochemistry confirmed growth hormone receptor protein on neuronal cell bodies in the rat cortex. Surprisingly, we found that central treatment with bovine growth hormone, which is equipotent to rat growth hormone in the rat periphery, failed to rescue cortical neurons following hypoxic ischemic injury. We further investigated the actions of rat and bovine growth hormone on primary neuron-enriched cultures of fetal rat cortex. In agreement with the in vivo treatment studies, rat but not bovine growth hormone rescued neurons from nutrient deprivation-induced cell death (p<0.05). This neuroprotective effect was inhibited by the selective growth hormone receptor antagonist G120D (p<0.001). Furthermore, rat but not bovine growth hormone had trophic effects on uninjured cultures (p<0.001). Immunocytochemistry showed growth hormone receptor on neurons within the neuron-enriched cultures. We show for the first time that the protective and trophic effects of rat growth hormone are mediated via growth hormone receptors on neurons and that the rodent neuronal growth hormone receptor exhibits unique ligand specificity.

Comparison of opioid receptor distributions in the rat central nervous system

The opioid receptors, mu, delta and kappa, conduct the major pharmacological effects of opioid drugs, and exhibit intriguing functional relationships and interactions in the CNS. Previously established hypotheses regarding the mechanisms underlying these phenomena specify theoretical patterns of relative cellular localisation for the different receptor types. In this study, we have used double-label immunohistochemistry to compare the cellular distributions of delta and kappa receptors with those of mu receptors in the rat CNS. Regions of established significance in opioid addiction were examined. Extensive mu/delta co-localisation was observed in neuron-like cells in several regions. mu and kappa receptors were also often co-localised in neuron-like cell bodies in several regions. However, intense kappa immunoreactivity (ir) also appeared in a separate, morphologically distinct population of cells that did not express mu receptors. These small, ovoid cells were often closely apposed against the larger, mu-ir cell bodies. Such cellular appositions were seen in several regions, but were particularly common in the medial thalamus, the periaqueductal grey and brainstem regions. These findings support proposals that functional similarities, synergy and cooperativity between mu and delta receptors arise from widespread co-expression by cells and intracellular molecular interactions. Although co-expression of mu and kappa receptors was also detected, the appearance of a separate population of kappa-expressing cells supports proposals that the contrasting and functionally antagonistic properties of mu and kappa receptors are due to expression in physiologically distinct cell types. Greater understanding of opioid receptor interaction mechanisms may provide possibilities for therapeutic intervention in opioid addiction and other conditions.

The Role of Growth Hormone in Neural Development

Growth hormone (GH) is integrally involved in the development of the central nervous system (CNS), as well as during its recovery from injury, two processes that share many similarities and may influence CNS functionality. This review discusses some of the most recent findings in the field and, in particular, the ontogeny, distribution, regulation and putative functions of GH and its receptor within the CNS, particularly during development. The relative roles of peripheral GH, acting in part through insulin-like growth factor-I, and of the autocrine/paracrine GH system within the brain are considered. The potential role of GH as a therapeutic agent to influence brain development and function is discussed.

Comparison of immunoblotted delta opioid receptor proteins expressed in the adult rat brain and their regulation by growth hormone

It has previously been suggested that exogenous growth hormone (GH) affect quality of life and higher brain functions through the endogenous opioid system. Recently, we showed that GH down-regulate 72 and 48 kDa delta opioid receptor (DOR) proteins in the adult rat cerebral cortex and cerebellum. In the present study, we found that an antiserum raised against the N-terminus of the DOR also recognizes a 36 kDa protein, not recognized by a C-terminus-directed antiserum. We aimed to investigate the identity of the 72, 48 and 36 kDa proteins and to further study the effects of GH on their expression in different brain regions. The expression was studied in hypophysectomized (Hx) and untreated normal female rats. One subgroup of Hx rats received GH as a daily subcutaneous injection for 19 days. Our data show that treatment with GH in Hx rats normalized the expression of the 72 kDa protein in the cerebral cortex, whereas no significant effect were observed for the 48 or 36 kDa proteins. However, GH significantly reduced the ratio between the 72 and 36 kDa proteins in different brain regions of Hx rats. Our data suggest that GH reduces the levels of a 72 kDa DOR that likely represents a dimeric form of a 36 kDa DOR post-translationally truncated at the C-terminus, and that altered receptor dimerization may be involved in GH induced effects in the central nervous system.