Frequency-dependent electrical stimulation of fimbria-fornix preferentially affects the mesolimbic dopamine system or prefrontal cortex

BACKGROUND: The fimbria/fornix fiber system is an essential part of the hippocampal-VTA loop, and therefore activities that are propagated through this fiber system control the activity of the mesolimbic dopamine system. OBJECTIVES/HYPOTHESIS: We hypothesized that stimulation of the fimbria/fornix with an increasing number of electrical pulses would cause increasing activity of the mesolimbic dopamine system, which coincides with concurrent changes in neuronal activities in target regions of the mesolimbic dopaminergic system. METHODS:Right fimbria/fornix fibers were electrically stimulated with different pulse protocols. Stimulus-induced changes in neuronal activities were visualized with BOLD-fMRI, whereas stimulus-induced release of dopamine, as measured for the activity of the mesolimbic dopamine system, was determined in the nucleus accumbens with in vivo fast-scan cyclic voltammetry. RESULTS: Dependent on the protocol, electrical fimbria/fornix stimulation caused BOLD responses in various targets of the mesolimbic dopamine system. Stimulation in the low theta frequency range (5 Hz) triggered significant BOLD responses mainly in the hippocampal formation, infralimbic cortex, and septum. Stimulation in the beta frequency range (20 Hz) caused additional activation in the medial prefrontal cortex (mPFC), nucleus accumbens, striatum, and VTA. Stimulation in the high-gamma frequency range (100 Hz) caused further activation in the hippocampus proper and mPFC. The strong activation in the mPFC during 100 Hz stimulations depended not only on the number of pulses but also on the frequency. Thus, short bursts of 5 or 20 high-frequency pulses caused stronger activation in the mPFC than continuous 5 or 20 Hz pulses. In contrast, high-frequency burst fimbria/fornix stimulation did not further activate the mesolimbic dopamine system when compared to continuous 5 or 20 Hz pulse stimulation. CONCLUSIONS: There exists a frequency-dependent dissociation between BOLD responses and activation of the dopaminergic system. Low frequencies were more efficient to activate the mesolimbic dopamine system, whereas high frequencies were more efficient to trigger BOLD responses in target regions of the mesolimbic dopamine system, particularly the mPFC

Contributions of dopaminergic and non-dopaminergic neurons to VTA-stimulation induced neurovascular responses in brain reward circuits

Mapping the activity of the human mesolimbic dopamine system by BOLD-fMRI is a tempting approach to non-invasively study the action of the brain reward system during different experimental conditions. However, the contribution of dopamine release to the BOLD signal is disputed. To assign the actual contribution of dopaminergic and non-dopaminergic VTA neurons to the formation of BOLD responses in target regions of the mesolimbic system, we used two optogenetic approaches in rats. We either activated VTA dopaminergic neurons selectively, or dopaminergic and mainly glutamatergic projecting neurons together. We further used electrical stimulation to non-selectively activate neurons in the VTA. All three stimulation conditions effectively activated the mesolimbic dopaminergic system and triggered dopamine releases into the NAcc as measured by in vivo fast-scan cyclic voltammetry. Furthermore, both optogenetic stimulation paradigms led to indistinguishable self-stimulation behavior. In contrast to these similarities, however, the BOLD response pattern differed greatly between groups. In general, BOLD responses were weaker and sparser with increasing stimulation specificity for dopaminergic neurons. In addition, repetitive stimulation of the VTA caused a progressive decoupling of dopamine release and BOLD signal strength, and dopamine receptor antagonists were unable to block the BOLD signal elicited by VTA stimulation. To exclude that the sedation during fMRI is the cause of minimal mesolimbic BOLD in response to specific dopaminergic stimulation, we repeated our experiments using CBF SPECT in awake animals. Again, we found activations only for less-specific stimulation. Based on these results we conclude that canonical BOLD responses in the reward system represent mainly the activity of non-dopaminergic neurons. Thus, the minor effects of projecting dopaminergic neurons are concealed by non-dopaminergic activity, a finding which highlights the importance of a careful interpretation of reward-related human fMRI data

Developmental stage-dependent regulation of spine formation by calcium-calmodulin-dependent protein kinase IIα and Rap1

The roles of calcium-calmodulin-dependent protein kinase II-alpha (CaMKIIα) in the expression of long-term synaptic plasticity in the adult brain have been extensively studied. However, how increased CaMKIIα activity controls the maturation of neuronal circuits remains incompletely understood. Herein, we show that pyramidal neurons without CaMKIIα activity upregulate the rate of spine addition, resulting in elevated spine density. Genetic elimination of CaMKIIα activity specifically eliminated the observed maturation-dependent suppression of spine formation. Enhanced spine formation was associated with the stabilization of actin in the spine and could be reversed by increasing the activity of the small GTPase Rap1. CaMKIIα activity was critical in the phosphorylation of synaptic Ras GTPase-activating protein (synGAP), the dispersion of synGAP from postsynaptic sites, and the activation of postsynaptic Rap1. CaMKIIα is already known to be essential in learning and memory, but our findings suggest that CaMKIIα plays an important activity-dependent role in restricting spine density during postnatal development

VEGF Polymorphisms Are Associated With Endocardial Cushion Defects: A Family-Based Case-Control Study

Endocardial cushion defects (ECDs) of the cardiac outflow tract are among the most common congenital heart disease phenotypes. VEGF is essential for endocardial cushion formation and derangements in VEGF synthesis lead to ECD. Three functional single nucleotide polymorphisms (SNPs) in the VEGF gene -2578 C>A, -1154 G>A, and -634 G>C play a role in cardiogenesis. In a Dutch case-control family study of triads, 190 case and 317 control children with both parents, we investigated linkage and association between these VEGF SNPs and ECD. Allele frequencies for the three VEGF SNPs were comparable between ECD children and controls. However, VEGF alleles -2578 C and 1154 G were transmitted more frequently to children with ECD (p = 0.003 and p = 0.002), in particular perimembranous ventricular septal defects (p = 0.012 and p = 0.006). The -2578A/-1154A/-634G haplotype was associated with a reduced risk of ECD (OR 0.7: 95% CI, 0.6-1.0) and was significantly less transmitted to children with ECD U) = 0.002). In a Dutch population, we show that the VEGF 2578 C, -1154 G alleles, and the AAG haplotype are associated with ECD. Possible VEGF gene-enviromnent interactions exposures are discussed. (Pediatr Res 67: 23-28, 2010