D1 Dopamine receptor-mediated substance P depletion in the striatonigral neurons of rats subjected to neonatal dopaminergic denervation: implications for self-injurious behavior

The present study examined the influences of dopamine (DA) receptor stimulation on enkephalin (Met5-enkephalin; ME) and tachykinin (substance P; SP) systems of basal ganglia of Sprague-Dawley rats, lesioned as neonates with 6-hydroxydopamine (6-OHDA). It has been proposed that the neonatal 6-OHDA-lesioned rat could serve as a model for the DA deficiency and self-injurious behavior (SIB) observed in the childhood neurological disorder. Lesch-Nyhan syndrome. In agreement with earlier work, the present study found that the neonatal 6-OHDA treatment at 3 days of age, reduced DA and caused an increase in ME and a decrease in SP content in the striatum and substantia nigra, when tested as adults. Administration of the DA precursor, L-dihydroxyphenylalanine (L-DOPA), to lesioned animals, induced SIB; increased DA and DOPAC levels; produced a greater decrease (-64%) in SP levels in the striatum and substantia nigra than was observed with lesion alone (-28%). The L-DOPA-induced decrease in SP levels and the SIB observed in the lesioned animals were blocked by pretreatment with the D1 receptor antagonist, SCH-23390. Moreover, administration of the D1 receptor agonist, SKF-38393, but not the D2 agonist, LY-171555, to lesioned animals mimicked the L-DOPA responses in all respects, except that the agonists did not alter DA or DOPAC levels. None of the DA agonists or antagonists treatments affected lesion-induced increase in ME levels in the striatum. These results indicate for the first time, that SIB precipitated by DA agonists in neonatal dopaminergic denervated animals, is associated with a marked and selective decrease in SP in the striatonigral SP neurons. This process has two components: (a) a retarded development of the SP system due to neonatal dopaminergic denervation: and (b) a depletion of the remaining SP, presumably by enhanced release due to D1 DA receptor-mediated activation of striatonigral SP neurons.     

Dopamine D2 receptors exert tonic regulation over discrete neurotensin systems of the rat brain

Blockade of dopamine D₂ receptors with either the selective antagonist, sulpiride, or the non-selective antagonist, haloperidol, induces 2- to 3-fold increases in the content of neurotensin-like immunoreactivity in the striatum and the nucleus accumbens of the rat brain. Quantitatively similar increases were also observed (a) in the striatum following selective degeneration of more than 85% of the nigrostriatal dopamine pathway with 6-hydroxydopamine and (b) in both the striatum and the nucleus accumbens after non-selective depletion of brain dopamine using reserpine plus -methyl-p-tyrosine. Interestingly, treatment of animals with sulpiride or haloperidol, following the depletion of dopamine by either 6-hydroxydopamine or reserpine plus -methyl-p-tyrosine, did not add to the elevation in neurotensin content of either structure caused by the dopamine depletion alone. These data suggest that an intact dopamine system is required for the neuroleptics to exert effects on individual neurotensin systems. In addition, the same mechanism appears to underlie the responses of the neurotensin pathways to treatments with the neuroleptics or dopamine-depleting drugs. A likely explanation for the effects of neuroleptics and dopamine-depleting drugs is that they eliminate tonic activity on D₂ receptors by basally released dopamine in the striatum and the nucleus accumbens. Supportive evidence for this hypothesis is that concurrent administration of the D₂ receptor agonist, LY 171555, with reserpine, completely blocked the effects of reserpine-induced dopamine depletion on neurotensin systems of the striatum and the nucleus accumbens.     

Discontinued drugs in 2005: endocrine and metabolic

This perspective describes the compounds from the endocrine and metabolic area that were discontinued during the calendar year 2005. This is a continuation of a series of perspectives of each of the editorial areas summarized by Expert Opinion on Investigational Drugs. The candidates for this summary were being developed in the areas of treatment of diabetes and diabetes complications, as well as reproductive and urogenital heath issues.     

Light-increased cGMP and K conductance in the hyperpolarizing receptor potential of Onchidium extra-ocular photoreceptors

The phototransduction mechanism of the extra-ocular photoreceptor cells Ip-2 and Ip-1 in the mollusc Onchidium ganglion was examined. Previous work showed that the depolarizing receptor potential of another extra-ocular photoreceptor cell, A-P-1 is produced by a decrease of the light-sensitive K⁺ conductance activated by a second messenger, cGMP and is inactivated by the hydrolysis of cGMP. Here, a hyperpolarizing receptor potential of Ip-2 or Ip-1 was associated with an increase in membrane conductance. When Ip-2 or Ip-1 was voltage-clamped near the resting membrane potential, light induced an outward photocurrent corresponding to the above hyperpolarization. The spectral sensitivity had a peak at 510 nm. The shift of reversal potentials of the photocurrent depended on the Nernst equation of K⁺-selective conductance. The photocurrent was blocked by 4-AP and l-DIL, which are effective blockers of the A-P-1 light-sensitive K⁺ conductance. These results suggested that the hyperpolarization is mediated by increasing a similar light-sensitive K⁺ conductance to that of A-P-1. The injection of cGMP or Ca²⁺ into a cell produced a K⁺ current that mimicked the photocurrent. 4-AP and l-DIL both abolished the cGMP-activated K⁺ current, while TEA suppressed only the Ca²⁺-activated K⁺ current. These results indicated that cGMP is also a second messenger that regulates the light-sensitive K⁺ conductance. The photocurrent was blocked by LY-83583, a guanylate cyclase (GC) inhibitor, but was unaltered by zaprinast, a phosphodiesterase (PDE) inhibitor. Together, the present results suggest that increasing the internal cGMP in Ip-2 or Ip-1 cells light-activates GC rather than inhibits PDE, thereby leading to an increase of the light-sensitive K⁺ conductance and the hyperpolarization.     

Analysis of the effects of intra-accumbens SKF-38393 and LY-171555 upon the behavioural satiety sequence

The outcome of intra-accumbens infusions of the dopamine D₁ receptor family agonist SKF-38393 and the D₂ receptor family agonist LY-171555 upon measures taken during the behavioural satiety sequence was assessed (0.01 µg, 0.1 µg, 1.0 µg in each case). Each drug was infused either separately, or together as a co-infusion in order to examine the functional relationship between these dopamine receptor subtypes within the nucleus accumbens. Measures of feeding did not change following infusions of SKF-38393 or LY-171555, whether infused separately or together. However, following separate infusion of the lowest dose tested of each drug (0.01 µg), the onset of resting was advanced. Moderate to high doses of SKF-38393 and LY-171555 (0.1 µg, 1.0 µg) infused separately resulted in a marked increase in activity at the expense of resting. Co-infusion of 0.01 µg of each drug also resulted in a dramatic increase in activity. Thus, measures of feeding behaviour were unchanged following excitation of D₁ and D₂ dopamine receptor families within the nucleus accumbens. In marked contrast, locomotor behaviour appeared to be under the potent synergistic control of these receptor families.     

Genotype-dependent modulation of LY 171555-induced defensive behavior in the mouse

The selective D2 dopamine receptor agonist LY 171555 increased defensive behavior dose dependently, decreased social investigation and activity, and increased immobility in mice of the C57BL/6 (C57) strain interacting with non-aggressive mice of the seme strain. In the same test situation, DBA/2 (DBA) mice did not present any increase in defensive behavior following injection of the same doses of LY 171555, while exhibiting a marked decrease in activity and increased immobility. Mice of the BALB/c strain were less responsive than C57 mice as far as defensive behavior was concerned, and showed decreased activity and social investigation and increased immobility. These results point to a major genetic role in the modulation of social behavior by D2 receptors.     

Enhanced response to the inhibitory action of LY171555, a dopamine D2-agonist, on in vivo striatal dopamine release in DOCA/NaCl-hypertensive rats

Our previous studies have demonstrated that LY171555 (quinpirole), a specific dopamine (DA) D₂-receptor agonist, has a pressor effect in the conscious rat which is accompanied by increased sympathetic outflow and arginine vasopressin release. To test the hypothesis that LY171555 inhibits in vivo release of DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), from central dopaminergic neurons of the conscious, freely moving rat by activation of presynaptic DA receptors in the central nervous system and that this mechanism may be altered in the desoxycorticosterone acetate (DOCA)/NaCl model of hypertension, we used the in vivo push-pull perfusion method to study the effect of LY171555 on central DA release in normotensive and DOCA/NaCl-hypertensive rats. Levels of the DOPAC and HVA were measured in striatal perfusates by HPLC before and after administration of LY171555 (1 mg/kg, i.v.) of conscious, unrestrained 4-week DOCA/NaCl hypertensive and uninephrectomized H₂O control rats. There were no significant differences in basal striatal HVA (80 ± 12 vs 89 ± 11 pg/min; DOCA/NaCl vs control) or DOPAC levels (37 ± 5 vs 17 pg/min; DOCA/NaCl vs control) during the entire 240-min collection period. LY171555 significantly reduced HVA and DOPAC levels in perfused striatum in both normotensive control and DOCA/NaCl-hypertensive rats. The LY 17555-induced suppression in HVA levels was significantly greater in DOCA/NaCl rats (Δ = 60.7 ± 3.6%) than in H₂O controls (Δ = 49.0 ± 3.5%, P < 0.05). Pretreatment with metoclopramide (10 mg/kg, i.v.), a specific central and peripheral DA D₂-receptor antagonist, completely blocked the suppressive effects of LY171555 on HVA and DOPAC levels. These observations provide direct evidence for the presence of functionally significant presynaptic inhibitory DA D₂-receptors which modulate dopaminergic neuro-transmission in the striatum of conscious, freely moving rats. This regulatory mechanism appears to be altered in the DOCA/NaCl model of hypertension. These results support the concept that DOCA/NaCl-hypertensive rats have altered central dopaminergic activity.     

Antinociceptive effects of N-acetylaspartylglutamate (NAAG) peptidase inhibitors ZJ-11, ZJ-17 and ZJ-43 in the rat formalin test and in the rat neuropathic pain model

The peptide neurotransmitter N-acetylaspartylglutamate (NAAG) acts as an agonist at group II metabotropic glutamate receptors (mGluRs). NAAG is inactivated by extracellular peptidase activity yielding glutamate and N-acetylaspartate. We recently developed a series of potent NAAG peptidase inhibitors, including ZJ-11, ZJ-17 and ZJ-43. In the present study, we examined the effects of intrathecally administered ZJ-11 and ZJ-17 and intravenously administered ZJ-11 and ZJ-43 in the rat formalin test (an inflammatory pain model) and in the rat partial sciatic nerve ligation model (a neuropathic pain model). Intrathecal injection of ZJ-11 or ZJ-17 or intravenous injection of ZJ-11 or ZJ-43 suppressed both phases of the agitation behaviour induced by paw formalin injection. Intrathecal and intravenous injection of ZJ-11 suppressed the expression of Fos-like immunoreactivity, induced by paw formalin injection, in laminae I-II in segments L4-L5 of the spinal cord, suggesting an action on sensory spinal transmission. Partial sciatic nerve ligation induced significant mechanical allodynia 7 days after the nerve injury. Intrathecal injection of ZJ-11 or ZJ-17 or intravenous administration of ZJ-11 or ZJ-43 attenuated the level of mechanical allodynia induced by this nerve ligation. These effects of intrathecally or intravenously administered ZJ compounds in both the formalin test and the partial sciatic nerve ligation model were completely antagonized by pretreatment with LY-341495, a highly selective group II mGluR antagonist. Thus, elevation of extracellular NAAG, induced by the inhibition of NAAG peptidase, activates group II mGluRs and produces an analgesic effect in neuropathic and inflammatory and pain models. In contrast, peptidase inhibition did not affect the threshold for withdrawal from a noxious mechanical stimulus or from an acute thermal stimulus in the hotplate test.     

Effects of serotonin-3 receptor antagonists on the intracranial self-administration of ethanol within the ventral tegmental area of Wistar rats

Abstract Rationale. Previous work from our laboratory indicated that Wistar rats will self-administer ethanol (EtOH) directly into the posterior ventral tegmental area (VTA) and that 5-HT₃ antagonists will inhibit EtOH-stimulated somatodendritic release of dopamine within the VTA. Objectives. The objective of this study was to use the intracranial self-administration procedure to determine the involvement of 5-HT₃ receptors in mediating the reinforcing effects of EtOH within the VTA, and to increase our understanding of central nervous system mechanisms involved in the rewarding effects of EtOH. Methods. Adult female Wistar rats were stereotaxically implanted with guide cannulae aimed at the posterior VTA. After 1 week, rats were placed into standard two-lever experimental chambers for a total of seven sessions (4-h sessions separated by 48 h) and allowed to self-administer vehicle alone, a 5-HT₃ antagonist alone, 200 mg% EtOH alone, or combinations of 200 mg% EtOH with different concentrations of a 5-HT₃ antagonist (n=6–9 per group). Results. Throughout all seven sessions, Wistar rats self-infused more 200 mg% ETOH (25±5 infusions) than vehicle (5±4 infusions) or 5-HT₃ antagonist (6±4 infusions) (P<0.05), and responded significantly more (P<0.05) on the active than inactive lever (e.g., 50±12 vs 12±8 responses in session 1). Co-administration of 50 μM or 100 μM ICS 205,930 with 200 mg% EtOH completely prevented the acquisition and maintenance of EtOH self-infusion into the posterior VTA. Similarly, co-administration of either 25–100 μM LY-278–584 or 10–100 μM zacopride with 200 mg% EtOH completely blocked EtOH-maintained intracranial self-administration behavior. Conclusions. The results of this study suggest that the reinforcing effects of EtOH within the posterior VTA of Wistar rats require activation of local 5-HT₃ receptors. Electronic Publication