Faculty Bibliography

We compared a conventional method of measurement of sciatic motor and sensory nerve conduction velocity, with a novel procedure that measures conduction in an 8-mm segment of the rat sural nerve. Conventional procedures gave reductions in velocity of 20% and 14% for motor and sensory fibers, respectively, whereas sural sensory fibers showed a 40% reduction (P <0.05). Changes were attenuated by treatment with either an aldose reductase inhibitor or a gamma-linolenic acid-alpha-lipoic acid conjugate, such that values from conventional procedures were not significantly different from controls and the sural sensory deficit halved. Putative motor fibers of the sural nerve showed no conduction velocity deficit in diabetic rats. Measurement of chronaxie and rheobase in sural sensory fibers revealed mild reductions in excitability in diabetics, with prevention of the chronaxie change by the treatments. Thus, measurement of sensory conduction in distal nerve segments show more profound defects in diabetic rats and may give a truer picture of preventive drug efficacy.

Aberrant neurofilament phosphorylation occurs in many neurodegenerative diseases, and in this study, two animal models of type 1 diabetes--the spontaneously diabetic BB rat and the streptozocin-induced diabetic rat--have been used to determine whether such a phenomenon is involved in the etiology of the symmetrical sensory polyneuropathy commonly associated with diabetes. There was a two- to threefold (P < 0.05) elevation of neurofilament phosphorylation in lumbar dorsal root ganglia (DRG) of diabetic rats that was localized to perikarya of medium to large neurons using immunocytochemistry. Additionally, diabetes enhanced neurofilament M phosphorylation by 2.5-fold (P < 0.001) in sural nerve of BB rats. Neurofilaments are substrates of the mitogen-activated protein kinase (MAPK) family, which includes c-jun NH2-terminal kinase (JNK) or stress-activated protein kinase (SAPK1) and extracellular signal-regulated kinases (ERKs) 1 and 2. Diabetes induced a significant three- to fourfold (P < 0.05) increase in phosphorylation of a 54-kDa isoform of JNK in DRG and sural nerve, and this correlated with elevated c-Jun and neurofilament phosphorylation. In diabetes, ERK phosphorylation was also increased in the DRG, but not in sural nerve. Immunocytochemistry showed that JNK was present in sensory neuron perikarya and axons. Motoneuron perikarya and peroneal nerve of diabetic rats showed no evidence of increased neurofilament phosphorylation and failed to exhibit phosphorylation of JNK. It is hypothesized that in sensory neurons of diabetic rats, aberrant phosphorylation of neurofilament may contribute to the distal sensory axonopathy observed in diabetes.

1. Fast cyclic voltammetry was used to investigate the effect of 7-OH-DPAT (7-hydroxy-N,N-di-n-propyl-2-aminotetralin), a putative D3 receptor agonist, on electrically stimulated endogenous dopamine release in slices of rat nucleus accumbens. 2. 7-OH-DPAT inhibited single pulse stimulated dopamine release in a concentration-dependent manner with a maximum inhibition of 95.5%. Analysis of concentration-response curves to 7-OH-DPAT showed that they were biphasic, with the high affinity component contributing 18.0% to the total inhibition and the low affinity component 77.5%. 7-OH-DPAT exhibited a 560 fold selectivity between the high and low affinity components (0.015 nM compared to 8.4 nM). 3. Concentration-response curves to the non-selective D2/D3 agonist, apomorphine, were monophasic. The maximum inhibition was 93.1% and the EC50 value 82 nM. 4. The selective D2 antagonist, haloperidol (30 nM), antagonized the low affinity component of the concentration-response cuve to 7-OH-DPAT whilst the high affinity component was essentially unaffected. The pKB values calculated for the high and low affinity components were 7.89 and 9.45 respectively. 5. In conclusion, these results demonstrate that 7-OH-DPAT inhibits stimulated dopamine release by acting at two different sites. Furthermore, the results are consistent with the hypothesis that the high and low affinity components of the concentration-response curve to 7-OH-DPAT may reflect activation of functional D3 and D2 release-regulating autoreceptors respectively. However, the possibility that the biphasic nature of the curve may reflect different subtypes of the D2 receptor cannot be excluded.

Behavioural sensitization to the locomotor stimulant effect of (+)-amphetamine or quinpirole was induced in rats by intermittent drug administration. Once established, endogenous dopamine release (DA) was measured in slices containing nucleus accumbens using fast cyclic voltammetry. DA release induced by single pulse electrical stimulation did not differ between vehicle-, (+)-amphetamine-or quinpirole-treated groups. Multiple pulse stimulation resulted in enhanced DA release in quinpirole-sensitized rats and an attenuation of DA release in (+)-amphetamine-sensitized rats. In the presence of sulpiride, DA release was increased, at low stimulation frequencies, in vehicle- and quinpirole-treated animals, but not in amphetamine-treated animals. The sensitivity of axon terminal D2 DA receptors was assessed in vitro by measuring the concentration of quinpirole required to inhibit single pulse release of DA by 50% (EC(50)). Quinpirole EC(50) was significantly increased in the quinpirole-treated animals and significantly attenuated in the (+)-amphetamine-treated animals. The results suggest that the increase in DA release following quinpirole may arise from the desensitization of release-regulating D2 autoreceptors in the nucleus accumbens. The sensitization of axon terminal D2 autoreceptors and the decrease in DA release following behavioural sensitization with (+)-amphetamine is difficult to reconcile with a unitary explanation of behavioural sensitization to both quinpirole and (+)-amphetamine. It is suggested that the effects following (+)-amphetamine may be secondary to decreased axon traffic caused by DA displacement in the ventral tegmental area, and that the drugs examined in this study may induce behavioural sensitization by different mechanisms at different sites.

Fast cyclic voltammetry using carbon fibre microelectrodes in rat brain slices, was used to investigate regional differences in electrically-evoked dopamine (DA) efflux at 10 different sites in the anterior caudate putamen (aCPu) and 10 sites in the posterior caudate putamen (pCPu). For each site DA overflow was evoked by both single pulse (1P) stimulation and by trains of 25 pulses applied at a frequency of 50 Hz (25P/50 Hz). Peak DA efflux evoked by 1P was about 58% greater in the aCPu (0.19 mumol/l DA) than in the pCPu (0.12 mumol/l DA), but showed no mediolateral variation in either region. Peak DA efflux evoked by 25P/50 Hz relative to 1P efflux also varied between the two regions; the aCPu contained predominantly low ratio (25P/50 Hz: 1P) sites ranging from 1.47 to 3.71, whereas in the pCPu these ratios were higher, ranging from 2.73 to 9.40, and were particularly high in the dorsomedial region of the pCPu. Efflux detected in low ratio sites of the aCPu showed little dependence on the frequency (10 to 500 Hz), or the number of pulses (5 to 20) in a train. By contrast DA efflux evoked in high ratio sites of the pCPu responded in a pulse and frequency dependent manner, the maximum ratio (approximately 8 times 1P) being at 20P/20 Hz. Interestingly the frequency response relationship obtained in the pCPu resembled the profile observed in the nucleus accumbens (NAc). Voltammetric evidence and experiments with selective reuptake blockers indicated that only DA was measured in our studies and 5-HT did not significantly contribute to the frequency dependent pattern of efflux detected in high ratio sites of the pCPu, where striatal 5-HT concentrations are highest. Experiments with the selective D2 receptor antagonists metoclopramide or (-)sulpiride revealed that under our experimental conditions, DA efflux in the aCPu was not modulated by DA autoreceptor activation. By contrast, autoreceptor modulation did occur in high ratio sites of the pCPu at stimulations lasting longer than approximately 1000 ms. These observations support the concept that the caudate putamen is heterogeneously organised with respect to the frequency characteristics of evoked DA release. The factors controlling frequency dependent release under these conditions may be a function of A10 innervation, since high ratio release sites occur in areas where the density of such innervation is greatest, for example, the dorsomedial pCPu. This is supported by the observation that high ratio release sites are also found in the NAc, which receives dopaminergic fibres predominantly from an A10 region.(ABSTRACT TRUNCATED AT 400 WORDS)

Chronic exposure to mild unpredictable stress caused a decrease in rats' consumption of a palatable weak sucrose solution, which was reversed by chronic (5 weeks) administration of imipramine (5 mg/kg/day). Dopamine (DA) release in the nucleus accumbens (NAc) and caudate putamen (CPu) was measured in vivo using fast cyclic voltammetry, following electrical stimulation of the medial forebrain bundle. Experiments were performed under chloral hydrate anaesthesia 48 h after the termination of stress and the final imipramine injection. DA release was increased in the NAc of both stressed and imipramine-treated animals; imipramine did not normalize the increased DA release in stressed animals. In a further experiment, brain slices from stressed animals tended to be subsensitive to the inhibition of DA release in the NAc by quinpirole. No changes were observed in the CPu in any experiment. We discuss the relationship of these effects to stress-induced anhedonia.