Target-dependent expression of the netrin-1 receptor, UNC5C, in projection neurons of the ventral tegmental area.

We have shown previously that the netrin-1 receptor, unc-5 homologue C (UNC5C), is expressed by ventral tegmental area (VTA) dopamine (DA) neurons of rodents, but only from adolescence onwards (Manitt et al., 2010; Auger et al., 2013). The goal of this study was to characterize the expression of UNC5C by these neurons. Specifically, we assessed whether UNC5C expression is selective to DA neurons that project to the medial prefrontal cortex (mPFC), which undergo significant maturation during the adolescent period. To this end, we injected fluorescent retrograde tracer beads into the mPFC, nucleus accumbens (NAcc) core, or NAcc lateral shell of adult male wild-type C57Bl/6J mice and processed their brains for tyrosine hydroxylase (TH) and UNC5C immunofluorescence 2-3weeks later. VTA neurons with any combination of these immunolabels were visualized and counted using optical fractionator stereology. Our analysis revealed two main findings: (1) there are no differences in the proportions of UNC5C-positive DA neurons projecting to the mPFC, NAcc core, or NAcc lateral shell, and (2) the proportion of non-DA UNC5C-positive neurons targeting the mPFC is greater than the proportions of non-DA UNC5C-positive neurons targeting the NAcc core or lateral shell. These findings show that, contrary to our hypothesis, DA neurons projecting to the mPFC do not express UNC5C selectively. However, UNC5C expression by non-DA VTA neurons is predominantly found in those projecting to the mPFC and, as such, may play a role in their function.

Maternal high-fat intake alters presynaptic regulation of dopamine in the nucleus accumbens and increases motivation for fat rewards in the offspring.

High caloric intake during early postnatal development can have long term consequences for the offspring. We previously reported that the adult offspring of dams fed a high-fat diet during the last week of gestation and throughout lactation display blunted locomotor response to amphetamine (AMP) and reduced sensitization to the drug compared to offspring of control diet dams. Here, we report that the subsensitivity of high-fat offspring to AMP's locomotor stimulant action reflects, at least in part, altered regulation of nucleus accumbens (NAc) dopamine (DA) transmission. When compared to controls, the DA response of high-fat animals to AMP, as measured with microdialysis, was attenuated in the NAc, but unaffected in the prefrontal cortex (PFC). A relatively higher activity of NAc synaptosomal DA transporter sites without changes in vesicular monoamine transporter (VMAT) uptake capacity was also observed in high-fat offspring. Moreover, ventral tegmental area (VTA) D(2) receptor mRNA levels were decreased in high-fat offspring, suggesting a reduction in DA release-regulating D(2) autoreceptors in terminal regions such as the NAc. The magnitude of locomotor response to D(2/3) receptor activation (with quinpirole) was greater in high-fat than in control animals despite having comparable postsynaptic D(2) mRNA levels in the NAc. Finally, while operant responding for a sugar-enriched food reward did not differ between diet groups, high-fat offspring displayed increased operant responding for a fat-enriched reward compared to controls. These findings add to mounting evidence that early life exposure to elevated dietary maternal fat can lead to long lasting changes in DA-mediated behavioral responses to stimulant drugs and fat-enriched foods.

Enhanced glutamatergic phenotype of mesencephalic dopamine neurons after neonatal 6-hydroxydopamine lesion.

There is increasing evidence that a subset of midbrain dopamine (DA) neurons uses glutamate as a co-transmitter and expresses vesicular glutamate transporter (VGLUT) 2, one of the three vesicular glutamate transporters. In the present study, double in situ hybridization was used to examine tyrosine hydroxylase (TH) and VGLUT2 mRNA expression during the embryonic development of these neurons, and postnatally, in normal rats and rats injected with 6-hydroxydopamine (6-OHDA) at P4 to destroy partially DA neurons. At embryonic days 15 and 16, there was a regional overlap in the labeling of TH and VGLUT2 mRNA in the ventral mesencephalon, which was no longer found at late embryonic stages (E18-E21) and postnatally. In normal pups from P5 to P15, only 1-2% of neurons containing TH mRNA in the ventral tegmental area (VTA) and substantia nigra, pars compacta, also displayed VGLUT2 mRNA. In contrast, after the cerebroventricular administration of 6-OHDA at P4, 26% of surviving DA neurons in the VTA of P15 rats expressed VGLUT2. To search for a colocalization of TH and VGLUT2 protein in axon terminals of these neurons, the nucleus accumbens of normal and 6-OHDA-lesioned P15 rats was examined by electron microscopy after dual immunocytochemical labeling. In normal rats, VGLUT2 protein was found in 28% of TH positive axon terminals in the core of nucleus accumbens. In 6-OHDA-lesioned rats, the total number of TH positive terminals was considerably reduced, and yet the proportion also displaying VGLUT2 immunoreactivity was modestly but significantly increased (37%). These results lead to the suggestion that the glutamatergic phenotype of a VTA DA neurons is highly plastic, repressed toward the end of normal embryonic development, and derepressed postnatally following injury. They also support the hypothesis of co-release of glutamate and DA by mesencephalic neurons in vivo, at least in the developing brain.

Inputs to nucleus pontis caudalis from adjacent trigeminal areas.

Recent studies suggest that the nucleus pontis caudalis (nPontc) plays a role in patterning mastication through interactions with the adjacent lateral tegmentum. In this study, we used in vitro intracellular recording and staining to describe the basic membrane properties and morphology of nPontc neurones and to further explore interactions with adjacent structures, using coronal sections of the brainstem of 78 rats, aged 9-28 days. Neurones were large, with dendrites that spread in all directions, and about 64% fired tonically even in the absence of synaptic inputs. Tonic neurones were predominant in the centre of the nucleus. Electrical stimulation of all regions of the nPontc produced mixed excitatory and inhibitory effects on interneurones of lateral tegmental nuclei. Focal inactivation of the dorsal nPontc with injections of tetrodotoxin also had mixed effects on the spontaneous firing of both interneurones and motoneurones but similar injections in the ventral nPontc produced mostly increases of firing. Sixty-five percent of nPontc neurones received synaptic inputs from the lateral tegmental areas and most of these (68%) were excitatory and mediated by glutamatergic receptors. Inhibitory postsynaptic potentials were mediated by GABA(A) or glycinergic receptors. Although most responses occurred at relatively long latencies (> 2 ms), they could follow relatively high-frequency stimulation (> 50 Hz). Excitatory and inhibitory connections between ipsi- and contralateral nPontc neurones were also documented, which could contribute to bilateral coordination of jaw movements. This study provides evidence that the nPontc exerts both tonic and phasic influences on the premotor components of the masticatory central pattern generator.

Clonidine-induced hyperglycemia: evidence against a growth hormone-mediated effect.

The effects of acute clonidine infusion (0.15 mg over 10 min) on several endocrine and metabolic variables have been evaluated in 12 hypertensive patients and 12 normotensive controls. Plasma glucose increased significantly in comparison with a placebo study in both groups; serum growth hormone showed a significant increase in healthy subjects but did not rise in hypertensive patients; there was no correlation between the increments in plasma glucose and in serum growth hormone in the whole group of subjects; serum-free fatty acids, insulin, prolactin and plasma cortisol did not change in any group. These data, although not excluding a central site of the clonidine hyperglycemic action, contrast the view that it may depend on stimulation of growth hormone release.

Metabolic effects of prazosin.

The effects of oral administration of 2 mg prazosin on several metabolic and endocrine variables were evaluated in 12 patients with hypertension (6 with normal and 6 with abnormal glucose tolerance). Prazosin was followed by a rise in plasma glucose and serum free fatty acids (FFA) in both normal and diabetic subjects; there was a trend upward in serum albumin (IRI), but growth hormone (GH), prolactin (PRL), and gastrin did not change. Although these results are in general agreement with metabolic effects of other alpha adrenergic blockers already reported, the rise in plasma glucose is at variance with studies performed with phentolamine.