Unidirectional axonal transport in in vitro adult rat brain explants

Properties and opioid inhibition of mesolimbic dopamine neurons vary according to target location

The mesolimbic dopamine system, which mediates the rewarding properties of nearly all drugs of abuse, originates in the ventral tegmental area (VTA) and sends major projections to both the nucleus accumbens (NAc) and the basolateral amygdala (BLA). To address whether differences occur between neurons that project to these separate areas, retrograde microspheres were injected to either the BLA or the NAc of DBA/2J mice. Whole-cell recordings were made from labeled VTA dopamine neurons. We found that identified neurons that projected to the BLA and NAc originated within different quadrants of the VTA with neither group exhibiting large-amplitude h-currents. Neurons that projected to the NAc exhibited a greater outward current in response to the kappa-opioid agonist (5alpha,7alpha,8alpha)-(+)-N-methyl-N-[7-(pyrrolidinyl)-1-oxaspiro [4,5]dec-8-yl]-benzeneacetamide (U69593; 200 nM), whereas neurons that projected to the BLA exhibited greater inhibition to the mu/delta opioid agonist [Met5] enkephalin (ME; 3 microM). In addition, we found that the presynaptic inhibition of GABAergic transmission at both GABAA and GABAB receptors was differentially regulated by U69593 between the two groups. When dopamine IPSCs were examined, U69593 caused a greater inhibition in NAc- than BLA-projecting neurons. ME had no effect on either. Finally, the regulation of extracellular dopamine by dopamine uptake transporters was equal across the VTA. These results suggest that opioids differentially inhibit mesolimbic neurons depending on their target projections. Identifying the properties of projecting mesolimbic VTA dopamine neurons is crucial to understanding the action of drugs of abuse.

Extracortical descending projections to the rat inferior colliculus

Feedback controlling is an important element in the sensory processing in the auditory system. It has been long recognized that the inferior colliculus (IC) sends direct ascending projections to the medial geniculate body (MGB), but receives feedback regulation from the auditory cortex. In the present study we probed the shorter extracortical projections to the IC, including the direct descending pathway from the MGB. In the rat, the fluorescence retrograde tracers Fluorogold, True Blue or Rhodamine latex microspheres were injected into the IC, and the auditory thalamus and surrounding regions were examined for fluorescent neurones. We did not find any retrograde labelling in the ventral division of the MGB. However, retrogradely labelled neurones were found in the medial and suprageniculate nuclei of the MGB. We also observed densely packed groups of fluorescent neurones in the peripeduncular nucleus and numerous labelled neurones in the nucleus of the brachium of the IC. The existence of a direct descending pathway to the IC from at least some auditory thalamic nuclei challenges the perception of the colliculo-thalamic relationship as one-way traffic and suggests more direct involvement of the auditory thalamus in the feedback regulation of the incoming acoustic signals.