The effects of glutamate receptor agonists on neurotensin release using in vivo microdialysis.

In the present study, extracellular concentrations of neurotensin were measured from the striatum, nucleus accumbens and the medial prefrontal cortex in the awake, freely moving rat. Using a highly sensitive solid phase radioimmunoassay, basal concentrations of neurotensin were 2-5 pg/sample. In each region, glutamate receptor agonists, N-methyl-D-aspartate (NMDA) and kainic acid, increased neurotensin release 2-3-fold. Preincubation with the Na(+) channel blocker tetrodotoxin abolished the glutamate receptor agonist-induced increases except in the striatum following kainic acid infusion. These findings indicate that activation of glutamate receptors may indirectly stimulate neurotensin release.

Atypical antipsychotic drugs selectively increase neurotensin efflux in dopamine terminal regions.

Typical antipsychotic drugs, such as haloperidol and chlorpromazine, increase synthesis of the neuropeptide neurotensin (NT) in both the striatum and the nucleus accumbens, whereas atypical antipsychotic drugs, such as clozapine and olanzapine, do so only in the nucleus accumbens. By using in vivo microdialysis, we now report that acute administration of haloperidol, clozapine, or olanzapine failed to alter the release of NT in either the striatum or nucleus accumbens. In contrast, chronic administration of haloperidol for 21 days increased NT release in both the striatum and nucleus accumbens, whereas treatment for 21 days with the atypical antipsychotic drugs, clozapine or olanzapine, increased NT release selectively in the nucleus accumbens. These findings suggest that (i) increased NT mRNA expression and NT tissue concentrations are associated with increases in the extracellular fluid concentrations of the peptide and (ii) atypical antipsychotic drugs may exert their therapeutic effects and produce fewer side effects by virtue of their selectivity in limbic compared with striatal, target neurons.

Neuronal release of somatostatin in the rat striatum: an in vivo microdialysis study.

Extracellular levels of somatostatin in the rat striatum were studied using in vivo microdialysis and radioimmunoassay. In vitro studies were performed using three different dialysis membranes at various flow rates and temperatures to assess the optimal recovery of somatostatin. The best results were obtained when a cellulose fibre membrane was utilized at 37 degrees C with a flow rate of 0.5 microliters/min. For the in vivo studies, transcerebral cellulose probes were implanted in the striatum of chloryl hydrate-anaesthetized rats. Basal levels of somatostatin were detected in the striatum of the freely moving animals and found to be 5-15 fmol. Stimulation with 100 mM KCl increased the recovered somatostatin by 138% (P < 0.05). A second stimulation following a 3-h interval increased the somatostatin levels by approximately 60%. The addition of veratridine (100 microM) in the perfusion medium increased the somatostatin levels recovered from the striatum by 85% (P < 0.01). Following a 3-h interval, a second stimulation by veratridine also increased somatostatin levels (43%). The increases observed after the second depolarizing stimulus (KCl and veratridine) were not found to be significantly different from basal levels. Both EGTA and the sodium channel blocker tetrodotoxin attenuated the effect of KCl and veratridine, respectively. However, neither EGTA nor tetrodotoxin had an effect on the basal levels of somatostatin recovered. These results indicate that (i) the somatostatin measured is neuronally released in the striatum and (ii) microdialysis is a useful tool for examining the regulation of somatostatin release in the brain.

Loss of striatal somatostatin neurons following prenatal methylazoxymethanol.

Prenatal administration of methylazoxymethanol acetate (MAM), which kills neuroblasts undergoing mitosis, was used to lesion striatal somatostatin neurons. Previous [3H]thymidine autoradiographic studies had indicated that striatal somatostatin neurons undergo their final mitotic division at Gestational Days (G) 15 and 16. Therefore, pregnant Sprague-Dawley rats received an intraperitoneal injection of MAM (25 mg/kg) on G15. Neurochemical and histological examination of the mature offspring indicated the loss of half the striatal aspiny interneurons in which somatostatin, neuropeptide Y, and NADPH diaphorase coexist, with relative sparing of the cholinergic interneurons and medium spiny projection cells. This prenatal MAM treatment was without apparent effect on the patch-matrix organization of the striatum.

Effects of short- and long-term haloperidol administration and withdrawal on regional brain cholecystokinin and neurotensin concentrations in the rat.

The effects of oral administration of the neuroleptic, haloperidol, on regional brain concentrations of cholecystokinin (CCK) and neurotensin were examined in the rat. Both short-term (3 weeks) and long-term (8 months) haloperidol administration increased the concentration of CCK in the substantia nigra. While short-term administration significantly increased the concentration of CCK in the ventral tegmental area and decreased the concentration of CCK in the cortex, including the medial prefrontal cortex, these effects were not observed following long-term drug administration. In contrast, long-term, but not short-term, haloperidol administration decreased the concentration of CCK in the olfactory tubercle. Withdrawal from long-term haloperidol did not alter CCK concentrations in any of the brain regions examined. Short-term haloperidol administration significantly increased the concentration of neurotensin in the caudate-putamen. Both short- and long-term administration increased the concentration of neurotensin in the nucleus accumbens, but only the increased following long-term administration reached statistical significance. Withdrawal from long-term haloperidol administration slightly decreased the concentrations of neurotensin in the caudate-putamen and nucleus accumbens. These results indicate that dopamine receptor blockade can affect both CCK- and neurotensin-containing neural systems. Furthermore, these two neuropeptides are affected differently depending upon the duration of haloperidol administration and withdrawal from this drug. The results raise the possibility that chronic administration of haloperidol may be toxic to some neurotensin-containing neurons in the basal ganglia.

Effects of systemic and intracerebroventricular cysteamine on dexamethasone-induced suppression of corticosterone levels in the rat.

To investigate whether somatostatin systems plays a significant role in the regulation of the hypothalamic-pituitary-adrenal axis, the effects of cysteamine, a drug which reduces somatostatin levels, on the dexamethasone-induced suppression of plasma corticosterone levels were examined in the rat. Male Long Evans rats were handled daily for 1 week prior to receiving a standard dexamethasone suppression test. On the 1st day, rats received a 9.00 a.m. saline injection and blood samples were taken from the tail at 1.00 p.m. On the 2nd day, rats received dexamethasone or saline at 9.00 a.m. and a second blood sample was taken at 1.00 p.m. Experimental groups were pretreated with systemic injections of cysteamine, 5 min or 14 h, prior to receiving dexamethasone. Additional groups, previously implanted with guide cannulae, were given an infusion of cysteamine or saline into the lateral ventricle 14 h prior to dexamethasone. Circulating corticosterone levels were determined by radioimmunoassay. Rats were sacrificed immediately following each experiment and the hypothalamus dissected and assayed for levels of somatostatin immunoreactivity. The results of the first experiment showed that dexamethasone (10 micrograms/kg) alone reduced plasma corticosterone levels from control values (174 +/- 36 ng/ml) to undetectable levels (less than 25 ng/ml). Pretreatment with cysteamine 5 min prior to dexamethasone, while having no significant effect on basal corticosterone levels, completely blocked the dexamethasone-induced suppression of corticosterone levels. Similar observations were obtained with rats pretreated with cysteamine 14 h prior to dexamethasone. In contrast, intracerebroventricular cysteamine pretreatment did not block the dexamethasone-induced suppression of corticosterone levels. These results add further evidence in support of an involvement of somatostatin systems in the regulation of the hypothalamic-pituitary-adrenal axis.

Intra-cerebral cysteamine infusions attenuate the motor response to dopaminergic agonists.

Previous studies have suggested that somatostatin neurons in the basal ganglia may be involved in motor activity. In the present experiments, the effects of cysteamine, a drug which reduces somatostatin levels, on the basal and dopamine-mediated motor activities were examined in the rat. Neither intra-striatal nor intra-accumbens infusions of cysteamine had any effect on motor activity prior to the administration of dopamine agonists. However, intra-striatal cysteamine infusions reduced the duration of the stereotypic behavior induced by systemic apomorphine. In addition, intra-accumbens infusions of cysteamine produced a slight reduction in the locomotor response induced by amphetamine. The direct intra-cerebral infusion of cysteamine produced a significant depletion in the levels of somatostatin at the site of injections as measured by radioimmunoassay. These results indicate that somatostatin neurons in the basal ganglia may modulate the motor responses following dopaminergic activation, and further support the presence of a dopamine-somatostatin interaction in this region.

Methylmercury-induced movement and postural disorders in developing rat: loss of somatostatin-immunoreactive interneurons in the striatum.

Tissue concentrations of the neuropeptide somatostatin and the specific activities of glutamic acid decarboxylase (GAD) were measured in several regions of the central nervous system in young rats, following chronic postnatal administration of methylmercuric chloride. By the beginning of the fourth postnatal week, these animals exhibited clinical signs of a mixed spastic/dyskinetic syndrome with visual deficits. At the onset of neurological impairment, a significant decrease in GAD activity was detected in the occipital cortex (48-49%) and striatum (45-50%) when compared to either normal or weight-matched controls. At one subclinical stage of toxicity, decreased GAD activity was detected only in the occipital cortex (29-30%). Tissue levels of somatostatin did not change significantly in the occipital cortex of methylmercury-treated animals at any stage of the experiment. However, somatostatin levels in the striatum were significantly reduced at the onset of neurological impairment (55-57%) and at one subclinical stage of toxicity (49-54%). Immunohistochemistry for somatostatin- and neuropeptide Y-immunoreactive neurons confirmed a marked loss of cells in the dorsolateral region of the striatum with atrophy of the surviving neurons. In the cerebral cortex of methylmercury-treated animals the morphology and distribution of somatostatin-positive neurons appeared normal. In view of the reported co-localization of GAD and somatostatin in some non-pyramidal neurons of the cerebral cortex, these results indicate that methylmercury-induced lesions of the developing cerebral cortex involve a subpopulation of GABAergic neurons which are not co-localized with somatostatin. In the striatum, where GAD and somatostatin are not co-localized within the same neurons, methylmercury-induced lesions involve both GABAergic and somatostatin-positive neurons.

Comparison between short- and long-term haloperidol administration on somatostatin and substance P concentrations in the rat brain.

Neuroleptics influence a variety of putative neurotransmitters in the basal ganglia, including somatostatin and substance P. Most studies have been performed in animals after only 3 or 4 weeks of neuroleptic administration and have seldom examined the effects of withdrawal. To understand better the effects of haloperidol on neuropeptide systems, the effects of short-term (3 weeks) and long-term (8 months) administration, as well as withdrawal from long-term administration of haloperidol, on somatostatin and substance P concentrations were examined in the rat. Short-term haloperidol significantly decreased the concentrations of somatostatin in the caudate-putamen, nucleus accumbens, and ventral tegmental area, and decreased the concentration of substance P in the substantia nigra and the nucleus accumbens. However, long-term administration only decreased the concentration of somatostatin in the nucleus accumbens. In addition, a slight reduction in the concentration of substance P in the medial prefrontal cortex was detected after long-term treatment. After withdrawal from long-term haloperidol administration the concentrations of these peptides did not differ from control values in any of the brain regions examined. These results confirm that dopamine receptor blockade can affect the somatostatin and substance P systems in the basal ganglia and indicate that during long-term administration (8 months) tolerance develops to some of the effects that are observed after shorter (3 weeks) treatment periods.

Effects of MPTP poisoning on central somatostatin and substance P levels in the mouse.

The effects of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) on the levels of the neuropeptides somatostatin and substance P, were examined in various brain regions of C57 mice. Two weeks after injections of MPTP (2 X 30, 2 X 40 and 2 X 50 mg/kg i.p.) a dose-dependent decrease in striatal catecholamine levels was observed. There was also a dose-dependent increase in nigral somatostatin immunoreactivity and no reduction in striatonigral substance P levels. These results are in contrast with the changes observed in peptide levels in post-mortem Parkinson's brains.

The effects of cysteamine on dopamine-mediated behaviors: evidence for dopamine-somatostatin interactions in the striatum.

The effects of prior treatment with cysteamine, a drug which appears to deplete selectively the neuropeptide somatostatin, on apomorphine-induced stereotypy and amphetamine-induced locomotor activity and conditioned place preferences were investigated. Twelve hours following systemic cysteamine injections apomorphine-induced stereotypy was attenuated and striatal somatostatin levels were reduced by half. Systemic cysteamine also decreased the motor stimulant effects of amphetamine, without influencing the rewarding properties as determined by the conditioned place preference procedure. Direct injections of cysteamine into the nucleus accumbens also decreased the locomotor response to amphetamine, and produced a local reduction in somatostatin levels in the accumbens. Cysteamine did not appear to alter monoamine turnover in the striatum after either systemic or intra-accumbens injections. These results suggest that somatostatin in the nucleus accumbens and caudate-putamen modulates the motor, but not the reinforcing properties of dopaminergic drugs, possibly via an action postsynaptic to dopamine-releasing terminals. Furthermore, it is evident from these results that cysteamine is an important tool with which to study the central actions of somatostatin.