Effects of saline substitution on responding and plasma corticosterone in rats trained to self-administer different doses of cocaine.

Research from our laboratory has explored the role of the hypothalamo-pituitary-adrenal (HPA) axis in cocaine reinforcement. These experiments were designed to determine the involvement of the HPA axis in extinction. Male Wistar rats were trained to self-administer cocaine [0.125, 0.25, or 0.5 mg/kg/infusion (inf)] and food pellets (45 mg) under a multiple, alternating schedule of reinforcement. When self-administration was stable, saline was substituted for cocaine. Blood samples were taken at the end of the sessions following cocaine self-administration, the first exposure to saline substitution (first); and once the criteria for extinction were met (final). Plasma corticosterone was measured using radioimmunoassays. Although there was a significant increase in the number of infusions obtained during the first saline substitution test by rats trained with 0.5 mg/kg/inf of cocaine, there was a decrease in infusions received when 0.125 mg/kg/inf of cocaine was tested. Following repeated exposure to the extinction conditions, responding by rats trained to self-administer all three doses of cocaine was decreased to similar levels. In addition, there were significant differences in plasma corticosterone in rats trained with different doses of cocaine. Lever-pressing behavior and plasma corticosterone varied during extinction in relation to the training dose of cocaine and according to whether the rats had been exposed to single or repeated extinction testing. These data are discussed in terms of the potential difficulties involved in interpreting the effects of compounds intended to reduce drug reinforcement.

N-methyl D-aspartate receptor-mediated bidirectional control of extracellular signal-regulated kinase activity in cortical neuronal cultures.

N-Methyl D-aspartate (NMDA) receptor activation of extracellular-signal regulated kinase (ERK) was examined in primary cortical cultures. Tetrodotoxin, NMDA receptor antagonists, or reduced extracellular calcium (0.1 mm) greatly decreased basal levels of phospho-ERK2, indicating that activity-dependent activation of NMDA receptors maintained a high level of basal ERK2 activation. This activity-dependent activation of phospho-ERK2 was blocked by pertussis toxin and inhibition of calcium/calmodulin-dependent kinase II and phosphatidylinositol 3-kinase but not by inhibition of protein kinase C or cAMP-dependent protein kinase. Addition of a calcium ionophore or 100 microm NMDA decreased phospho-ERK2 in the presence of 1 mm extracellular calcium but enhanced phospho-ERK2 in 0.1 mm extracellular calcium. The reduction in basal phospho-ERK2 by 100 microm NMDA was also reflected as a decrease in phospho-cAMP response element-binding protein. Inhibition of tyrosine phosphatases and serine/threonine phosphatases protein phosphatase 1 (PP1), PP2A, and PP2B did not prevent the inhibitory effect of NMDA. In the presence of tetrodotoxin, NMDA produced a bell-shaped dose-response curve with stimulation of phospho-ERK2 at 10, 25, and 50 microm NMDA and reduced stimulation at 100 microm NMDA. NMDA (50 microm) stimulation of phospho-ERK2 was completely blocked by pertussis toxin and inhibitors of phosphatidylinositol 3-kinase and was partially blocked by a calcium/calmodulin-dependent kinase II inhibitor. These results suggests that NMDA receptors can bidirectionally control ERK signaling.

Fatigue properties of hydroxyapatite-coated dental implants after exposure to a periodontal pathogen.

We studied the fatigue properties of rods (4 mm diameter) of hydroxyapatite-coated, titanium alloy implant material after it was exposed to a periodontal pathogen, Actinobacillus actinomycetemcomitans (Aa). We varied the crystallinity of the hydroxyapatite (HA) coating in these rods to the levels of, 60.5%, 52.8%, and 47.8%. Each rod was first inoculated with Aa in the log phase of its growth cycle. After 48 h, we counted the adhered cells. We measured the dissolution of HA coating due to bacterial exposure alone by determining the calcium and phosphate concentrations in the bacterial growth media. Once the adherent bacteria were removed from these rods, we subjected them to 5 million cycles of fatigue testing after immersion in Lactated Ringer's solution. We then determined the calcium and phosphate concentrations in the fatigue media. We found additional coating loss after fatiguing of the samples. This coating loss was a cumulative effect of bacterial exposure and fatigue loading of the hydroxyapatite-coated dental implant alloy. The lower crystallinity sample showed a higher loss of coating within the range of crystallinity studied here. The HA coating in implants during clinical use may undergo such changes, because they are exposed to the same bacteria.

Inhibition of cyclic AMP phosphodiesterase (PDE4) reverses memory deficits associated with NMDA receptor antagonism.

Rolipram, a selective inhibitor of type 4 cyclic AMP phosphodiesterase (PDE4), completely reversed the amnesic effects of MK-801 on working and reference memory (F[4,64] = 11.10; p <.0001 and F[4,64] = 2.53; p <.05, respectively) at doses of 0.01-0.1 mg/kg in the radial-arm maze task. Similar antagonism by rolipram of the effects of MK-801 was observed on inhibitory avoidance behavior (F[3,35] = 190.8; p <.0001). In vitro evidence suggests that an increase in cAMP concentrations may mediate the observed behavioral effects of rolipram. In the absence of PDE4 inhibition, NMDA did not increase cAMP concentrations in primary cultures of rat cerebral cortical neurons. However, when PDE4 was inhibited with rolipram, NMDA markedly elevated cAMP. These observations suggest that PDE4 is an integral component of the NMDA receptor-mediated signal transduction pathway involved in memory processes. Inhibitors of PDE4 may act on this pathway to produce their effects on memory and may represent a new class of cognitive enhancers.