The "One neuron-one neurotransmitter" concept has been challenged frequently during the last three decades, and the coexistence of neurotransmitters in individual neurons is now regarded as a common phenomenon. The functional significance of neurotransmitter coexistence is, however, less well understood. Several studies have shown that a subpopulation of dopamine (DA) neurons in the ventral tegmental area (VTA) expresses the vesicular glutamate transporter 2 (VGLUT2) and has been suggested to use glutamate as a cotransmitter. The VTA dopamine neurons project to limbic structures including the nucleus accumbens, and are involved in mediating the motivational and locomotor activating effects of psychostimulants. To determine the functional role of glutamate cotransmission by these neurons, we deleted VGLUT2 in DA neurons by using a conditional gene-targeting approach in mice. A DAT-Cre/Vglut2Lox mouse line (Vglut2 f/f;DAT-Cre mice) was produced and analyzed by in vivo amperometry as well as by several behavioral paradigms. Although basal motor function was normal in the Vglut2 f/f;DAT-Cre mice, their risk-taking behavior was altered. Interestingly, in both home-cage and novel environments, the gene targeted mice showed a greatly blunted locomotor response to the psychostimulant amphetamine, which acts via the midbrain DA system. Our results show that VGLUT2 expression in DA neurons is required for normal emotional reactivity as well as for psychostimulant-mediated behavioral activation.amphetamine | midbrain | neurotransmission | reward | striatum

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Vinyl sulfide cyclized analogues of the octapeptide angiotensin II that are structurally related to the cyclic disulfide agonist c[Hcy(3,5)]Ang II have been prepared. The synthesis relies on the reaction of the mercapto group of a cysteine residue in position 3 with the formyl group of allysine incorporated in position 5 of angiotensin II. A mixture of the cis and the trans isomers was formed, and these were separated and isolated by RP-HPLC. Thus, the three-atom CH(2)[bond]S[bond]S element of the AT(1) receptor agonist c[Hcy(3,5)]Ang II has been displaced by a bioisosteric three-atom S[bond]CH[double bond]CH element. A comparative conformational analysis of the 13-membered ring systems of c[Hcy(3,5)]Ang II and the 13-membered cyclic vinyl sulfides with cis and trans configuration, respectively, suggested that all three systems adopted very similar low-energy conformations. This similarity was also reflected in the bioactivity. Both of the compounds that contained the ring systems encompassing the cis or trans vinyl sulfide elements between positions 3 and 5 exhibited K(i) values less than 2 nM and exerted full agonism at the AT(1) receptor. In contrast, vinyl sulfide cyclization involving the amino acid residues 5 and 7 rendered inactive compounds. The cyclic vinyl sulfides that have agonist activity were both shown to possess low-energy conformers compatible with the previously proposed 3D model for the bioactive conformation of Ang II.

Studies were conducted to evaluate the effects of s.c. injected recombinant human growth hormone (GH) on the expression of the gene transcript of N-methyl-D-aspartate receptor subunits type 1 (NR1), type 2A (NR2A), and type 2B (NR2B) in the male rat hippocampus. The GH-induced effects on the expression of hippocampal gene transcripts of GH receptor (GHR) and GH-binding protein were also examined. Male Sprague-Dawley rats, kept in four groups of two different ages, was treated with the hormone or saline during 10 days before decapitation and tissue dissection. Brain tissues collected were analyzed for mRNA content by using the Northern blot technique. The results indicated that in adult young rats (11 weeks of age) the hormone elicited a decrease in the mRNA expression of NR1 but an increase in that of the NR2B subunit. In elderly adult rats (57-67 weeks of age) GH induced an increase in the expression of the hippocampal message for NR1 and NR2A. Meanwhile, the hormone induced a significant up-regulation of the GHR transcript in hippocampus of adult young rats but not in elderly adult rats. It was further found that a significant positive correlation exists between the level of GHR mRNA and the expression of the NR2B subunit transcript in adult young rats. The GH-induced increase in the expression of hippocampal mRNA for the NR2B subunit is compatible with a previously observed memory promoting effect seen for the hormone, because overexpression of this N-methyl-D-aspartate receptor subunit is shown to enhance cognitive capabilities.

Clinical studies have demonstrated that growth hormone (GH) promotes learning and memory processes in GH-deficient (GHD) patients. In animal studies, GH also influences the N-methyl-D-aspartate (NMDA) receptor system in the hippocampus, an essential component of long-term potentiation (LTP), which is highly involved in memory acquisition. This study was designed to examine the beneficial effects of recombinant human GH (rhGH) on cognitive function in male rats with multiple hormone deficiencies resulting from hypophysectomy (Hx). The performance of an rhGH-treated group and an untreated control group was appraised in the Morris water maze (MWM). The rhGH-treated group performed significantly better in the spatial memory task than the control animals on the second and third trial days. Further training eliminated this difference between the groups. Hippocampal mRNA expression of the NMDA subunits NR1, NR2A and NR2B, insulin-like growth factor type 1 receptor (IGF-1R), and postsynaptic density protein-95 (PSD-95) was then measured in the animals by Northern blot analysis. The results suggest that there may be a relationship between the NMDA receptor subunit mRNA expression levels and learning ability, and that learning is improved by rhGH in Hx rats. Furthermore, a link between MWM performance and PSD-95 was also suggested by this study.