Inhibitory control of nociceptive responses of trigeminal spinal nucleus cells by somatosensory corticofugal projection in rat.

The caudal division of the trigeminal spinal nucleus (Sp5C) is an important brainstem relay station of orofacial pain transmission. The aim of the present study was to examine the effect of cortical electrical stimulation on nociceptive responses in Sp5C neurons. Extracellular recordings were performed in the Sp5C nucleus by tungsten microelectrodes in urethane-anesthetized Sprague-Dawley rats. Nociceptive stimulation was produced by application of capsaicin cream on the whisker pad or by constriction of the infraorbital nerve. Capsaicin application evoked a long-lasting increase in the spontaneous firing rate from 1.4±0.2 to 3.4±0.6 spikes/s. Non-noxious tactile responses from stimuli delivered to the receptive field (RF) center decreased 5 min. after capsaicin application (from 2.3±0.1 to 1.6±0.1 spikes/stimulus) while responses from the whisker located at the RF periphery increased (from 1.3±0.2 to 2.0±0.1 spikes/stimulus under capsaicin). Electrical train stimulation of the primary (S1) or secondary (S2) somatosensory cortical areas reduced the increase in the firing rate evoked by capsaicin. Also, S1, but not S2, cortical stimulation reduced the increase in non-noxious tactile responses from the RF periphery. Inhibitory cortical effects were mediated by the activation of GABAergic and glycinergic neurons because they were blocked by bicuculline or strychnine. The S1 and S2 cortical stimulation also inhibited Sp5C neurons in animals with constriction of the infraorbital nerve. Consequently, the corticofugal projection from S1 and S2 cortical areas modulates nociceptive responses of Sp5C neurons and may control the transmission of nociceptive sensory stimulus.

Midkine regulates amphetamine-induced astrocytosis in striatum but has no effects on amphetamine-induced striatal dopaminergic denervation and addictive effects: functional differences between pleiotrophin and midkine.

Midkine (MK), a neurotrophic factor with important roles in survival and differentiation of dopaminergic neurons, is upregulated in different brain areas after administration of different drugs of abuse suggesting MK could modulate drugs of abuse-induced pharmacological or neuroadaptative effects. To test this hypothesis, we have studied the effects of amphetamine administration in MK genetically deficient (MK-/-) and wild-type (MK+/+) mice. In conditioning studies, we found that amphetamine induces conditioned place preference (CPP) similarly in both MK-/- and MK+/+ mice. In immunohistochemistry studies, we found that amphetamine (10 mg/kg, four times, every 2 h) causes a similar striatal dopaminergic denervation in both MK-/- and MK+/+ mice. However, we detected a significant increase of glial fibrillary acidic protein (GFAP)-positive cells in the striatum of amphetamine-treated MK-/- mice compared to MK+/+ mice, suggesting an enhanced amphetamine-induced astrocytosis in absence of endogenous MK. Interestingly, the levels of expression of the MK receptor, receptor protein tyrosine phosphatase (RPTP) ß/ζ, in the striatum were not found to be changed by the drug administration or the mouse genotype. In a similar manner the phosphorylation levels of RPTP ß/ζ substrates with important roles in survival of dopaminergic neurons, Fyn kinase and TrkA, and of the MAP kinases ERK1/2, were unaffected by the drug or the genotype. The data clearly suggest that endogenous MK limits amphetamine-induced astrocytosis through Fyn-, TrkA- and ERK1/2-independent mechanisms and identify previously unexpected functional differences between MK and pleiotrophin, the only other member of the MK family of growth factors, in the modulation of effects of drugs of abuse.

Trigeminothalamic barrelette neurons: natural structural side asymmetries and sensory input-dependent plasticity in adult rats.

In the rodent trigeminal principal nucleus (Pr5) the barrelette thalamic-projecting neurons relay information from individual whiskers to corresponding contralateral thalamic barreloids. Here we investigated the presence of lateral asymmetries in the dendritic trees of these neurons, and the morphometric changes resulting from input-dependent plasticity in young adult rats. After retrograde labeling with dextran amines from the thalamus, neurons were digitally reconstructed with Neurolucida, and metrically and topologically analyzed with NeuroExplorer. The most unexpected and remarkable result was the observation of side-to-side asymmetries in the barrelette neurons of control rats. These asymmetries more significantly involved the number of low-grade trees and the total dendritic length, which were greater on the left side. Chronic global input loss resulting from infraorbital nerve (IoN) transection, or loss of active touch resulting from whisker clipping in the right neutralized, or even reversed, the observed lateral differences. While results after IoN transection have to be interpreted in the context of partial neuron death in this model, profound bilateral changes were found after haptic loss, which is achieved without inflicting any nerve damage. After whisker trimming, neurons on the left side closely resembled neurons on the right in controls, the natural dendritic length asymmetry being reversed mainly by a shortening of the left trees and a more moderate elongation of the right trees. These results demonstrate that dendritic morphometry is both side- and input-dependent, and that unilateral manipulation of the sensory periphery leads to bilateral morphometric changes in second order neurons of the whisker-barrel system. The presence of anatomical asymmetries in neural structures involved in early stages of somatosensory processing could help explain the expression of sensory input-dependent behavioral asymmetries.