Lane J. Wallace

The role of dopamine and AMPA/kainate receptors in the nucleus accumbens in the hypermotility response to MK801.

The purpose of this study was to evaluate the role of endogenous dopamine in the hypermotility response to MK801. The administration of MK801 (0.1 mg/kg, SC) to rats produced an intense stimulation of coordinated locomotor activity, which was not associated with stereotyped behavior. This stimulatory response was inhibited by pretreatment with either reserpine (5 mg/kg, IP) or alpha-methyl-p-tyrosine (2 doses of 250 mg/kg, IP). Similarly, pretreatment with the D2 antagonist eticlopride (0.03 mg/kg, SC) or the D1 antagonist SCH23390 (0.1 mg/kg, SC) produced a marked inhibition of MK801-stimulated hypermotility, and the combination of eticlopride (0.03 mg/kg, SC) and SCH23390 (0.03 mg/kg, SC) produced a greater inhibition of MK801-stimulated locomotion than either agent alone. The administration of SCH23390 or eticlopride directly into the nucleus accumbens inhibited the locomotor response to MK801, with the combination of both drugs producing a greater inhibition than either agent alone. The intra-accumbens administration of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonists DNQX or GAMS also inhibited the locomotor response produced by MK801. These data suggest that the activation of D1 and D2 dopaminergic receptors and AMPA/kainate excitatory amino acid receptors in the nucleus accumbens is required for the stimulation of locomotor activity produced by MK801.

Effects of the AMPA/kainate receptor antagonist DNQX in the nucleus accumbens on drug-induced conditioned place preference

Activation of AMPA/kainate glutamatergic receptors in the nucleus accumbens may be a component of the mechanism of drug induced reward. To test this hypothesis, 6,7-dinitroquinoxaline-2,3-dione (DNQX), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate glutamatergic receptor anatagonist, was injected into the nucleus accumbens before the administration of amphetamine or morphine during the training phase (acquisition) of a conditioned place preference paradigm. Rats were then tested for place preference in the absence of drugs. In other experiments, DNQX was given before testing for place preference (expression) but not during the training phase. Bilateral injection of DNQX (1 microgram/0.5 microliters/side) inhibited acquisition of place preference to amphetamine (1 mg/kg) but not morphine (10 mg/kg). During acquisition, DNQX marginally attenuated the locomotor stimulation elicited by amphetamine during the first but not subsequent training sessions, while the combination of morphine plus DNQX produced marked akinesia during each training session. When given prior to testing, DNQX inhibited the expression of place preference induced by amphetamine and morphine but did not affect locomotor activity. The results suggest that activation of AMPA/kainate receptors is involved in the primary reward stimulation (acquisition of place preference) of amphetamine but not morphine and in behaviors elicited by memory of primary reward stimulation (expression of place preference) for both drugs. Furthermore, locomotor activity during conditioning is not necessary for acquisition of place preference.

DNQX in the nucleus accumbens inhibits cocaine-induced conditioned place preference

Several lines of evidence suggest that activation of both AMPA/kainate receptors and dopaminergic receptors in the nucleus accumbens may be required for psychostimulant drug induced reward. However, it has been reported that dopaminergic antagonists fail to block acquisition of conditioned place preference to cocaine. The goal of these experiments was to determine whether AMPA receptor antagonist injected into the nucleus accumbens could block conditioned place preference elicited by cocaine under conditions where dopaminergic antagonists do not inhibit acquisition of place preference. DNQX (1 microgram/0.5 microliter), injected into the nucleus accumbens just before systemic injections of cocaine (20 mg/kg i.p.) during the training sessions, attenuated the acquisition of place preference. This suggests that AMPA receptors are involved in acquisition of place preference to cocaine. By contrast, fluphenazine (2.5 micrograms/0.5 microliter), injected into the nucleus accumbens during training, did not alter cocaine-induced place preference. Analysis of locomotor activity showed that the ability of flyphenazine to inhibit cocaine-induced hyperactivity progressively decreased with each training session. These observations suggest that the failure of dopaminergic antagonists to block cocaine-induced place preference may be related to adaptations occurring following repeated exposure to these drugs. Both DNQX and fluphenazine blocked the expression of conditioned place preference in rats that had been previously trained with cocaine alone. This result suggests that both AMPA and dopaminergic receptors are involved in the expression of a conditioned place preference to cocaine.

Effect of Age on Cholinergic Muscarinic Responsiveness and Receptors in the Rat Urinary Bladder

The effects of age on urinary bladder responsiveness to muscarinic agonists and on the Bmax and Kd of the binding of [3H]quinuclidinyl benzilate (QNB) to muscarinic receptors of the bladder were studied. Bladder bodies and bases were isolated from Fischer 344 rats, ages seven, 16 and 27 months. No age-dependent change in maximum KCl-elicited isotonic contractions was observed in either bladder region. The bladder base showed an age-dependent increase in the maximum contractions (Emax) elicited by muscarinic agonists. The Emax values for bladder bases from rats 27 months of age were 44 per cent, 58 per cent and 76 per cent greater than those from rats seven months of age for acetylcholine, bethanechol and oxotremorine, respectively. No such alteration in responsiveness was observed in the bladder body with age. There were no age-related changes in ED50 values for the three agonists in either bladder region. Analysis of [3H]QNB binding in the bladder base demonstrated a modest 18 per cent increase in the Bmax (fmol./mg. tissue) from seven to 16 months and a significant 39 per cent decrease from 16 to 27 months. In the bladder body, the Bmax progressively increased by 25 per cent from seven to 27 months. The Kd values of [3H]QNB did not change with age in either region. The data demonstrate that the age-related increase in the responsiveness of the bladder is regionally specific and cannot be explained by a change in the number or affinity of muscarinic receptors.

Effect of Age on in Vivo Urinary Bladder Function in the Rat

The effects of age on micturition in male Fischer 344 rats, ages five to seven, 16 to 18 and 22 to 24 months, were studied. The 24 hr. water intake, 24 hr. urine output, frequency and volume of each micturition were obtained from rats housed individually in metabolic cages. Intravesical pressure and volume at which the micturition contraction occurred were evaluated using natural-fill cystometry. The 24 hr. water intake and urine output increased significantly with advancing age; 22 to 24 months rats showed a 39% increase in water intake and a 93% increase in urine output compared to five to seven month rats. The increase in urine output observed in the 22 to 24 month old rats was manifested by a 95% increase in volume per micturition and a 52% increase in frequency of micturition compared to five to seven month old rats. The pressure at micturition (PAM) was 100% greater in 22 to 24 and 16 to 18 month old rats compared to five to seven month old rats with no age-related difference in bladder volume at micturition (BVM). These studies demonstrate that in vivo micturition changes with age in the male F344 rat. Although there were no overt urological dysfunctions observed in the aging rats, the alterations in function would indicate that there were changes in either the mechanisms controlling micturition, or changes in the musculature itself. These possibilities will be the subject of further investigations.

Age-related changes in sensitivity of rat urinary bladder to atonomic agents

Experiments were done to determine if age-related changes occur in autonomic regulation of rat urinary bladder. The maximum contractile responses to acetylcholine were 63% and 15% greater in isolated bladders from 29-month and 17-month animals, respectively, as compared to 7-month animals. The amounts of [3H]quinuclidinyl benzilate bound to membrane preparations were 46% and 7% greater. In contrast, no age-related changes were observed in phenylephrine-induced contraction or in isoproterenol-induced relaxation of bladder. Thus, the urinary bladder of aged rats appears to develop increased sensitivity to cholinergic stimuli because of an increase in the number of muscarinic cholinergic receptors.

Dopaminergic agonists administered into the nucleus accumbens : Effects on extracellular glutamate and on locomotor activity

The hypothesis to be tested was that increased dopaminergic transmission induced by amphetamine in the nucleus accumbens results in increased glutamatergic neurotransmission in this brain area and that the increase in level of this neurotransmitter contributes to behavioral effects of psychostimulant drugs. Amphetamine (1 mg/kg, i. p.) increased the amount of extracellular glutamate in the accumbens, as measured by in vivo dialysis, and stimulated locomotor activity. Amphetamine (10 mM) infused into the accumbens by reverse dialysis through the probe produced a similar stimulation of locomotor activity as systemic amphetamine but a greater increase in extracellular glutamate levels. Both of these responses to amphetamine were attenuated by either the selective D1 antagonist SCH23390 or the selective D2 antagonist eticlopride. The combination of a D1 and D2 agonist, SKF38393 (20 mM) and quinpirole (50 mM), administered into the accumbens by reverse dialysis also increased extracellular glutamate and stimulated locomotor activity. Administration of a glutamate uptake inhibitor, threo-beta-hydroxy-aspartate (50 mM), increased extracellular glutamate but did not stimulate locomotor activity. Systemic administration of caffeine (15 mg/kg, i.p.) increased locomotor activity but did not increase extracellular levels of glutamate. These data suggest that activation of dopaminergic receptors in the nucleus accumbens results in stimulation of locomotor activity and in activation of glutamatergic transmission in this brain region. However, an increase in glutamate levels in the nucleus accumbens is neither sufficient nor necessary to produce a stimulation of locomotor activity.

Role of dopaminergic mechanisms in the stimulatory effects of MK-801 injected into the ventral tegmental area and the nucleus accumbens

The bilateral administration of 10 micrograms of (+)MK-801, but not (-)MK-801, into either the VTA or the N.Ac. stimulated locomotor activity. The stimulation induced by (+)MK-801 at both sites was inhibited by reserpine (5 mg/kg, SC) and the D1 antagonist, SCH 23390 (0.1 mg/kg, SC). Eticlopride (0.03 mg/kg, SC), a D2 antagonist, inhibited the stimulation produced by MK-801 in the VTA but not in the N.Ac. Baclofen (32 ng), a GABAB receptor agonist, injected into the VTA inhibited the stimulatory response to MK-801 injected systemically, into the VTA, or into the N.Ac., but did not significantly inhibit spontaneous locomotion or the stimulatory response to apomorphine (5 mg/kg, SC). These observations suggest that the stimulatory effects of MK-801 in the VTA and the N.Ac. are dependent on endogenous dopamine. In addition, the effects produced by MK-801 injected into the VTA closely resemble those produced by the systemic administration of low doses of MK-801, suggesting that this is the primary site of action of MK-801.

AMPA/kainate antagonists in the nucleus accumbens inhibit locomotor stimulatory response to cocaine and dopamine agonists.

The purpose of this study was to determine whether AMPA/kainate excitatory amino acid receptors in the nucleus accumbens (NAc) play a role in the locomotor stimulation produced by cocaine and dopamine receptor agonists. The stimulation of locomotor activity produced by the systemic administration of cocaine was markedly attenuated by either the D1 receptor antagonist SCH23390 or the D2 receptor antagonist eticlopride administered directly into the NAc. This indicates that both dopaminergic receptor subtypes in the NAc are involved in the motor stimulant response to cocaine. The intra-accumbens administration of DNOX or GAMS, which have been shown to inhibit the locomotor stimulation produced by the excitatory amino acid agonist AMPA, antagonized the locomotor stimulant response to cocaine administered either systemically or directly into the NAc. DNOX and GAMS also inhibited the stimulation of locomotor activity produced by the coinjection of the D1 agonist SKF38393 and the D2 agonist quinpirole injected into the NAc of normal animals and of animals pretreated with reserpine. These results suggest that the activation of AMPA/kainate receptors in the NAc plays an important role in the locomotor stimulation produced by cocaine and directly acting dopaminergic receptor agonists. The effects produced by the activation of these receptors is independent of endogenous dopamine stores, suggesting that these receptors are located postsynaptic to the dopaminergic nerve terminals.

Effects of age on urinary bladder function in the male rat.

In a previous study we investigated the effects of age on the micturition characteristics and bladder function of male Fischer rats ages five to seven, 16 to 18 and 22 to 24 months. The 24 hr. water intake and urine output increases significantly with age; 22 to 24 month rats showed a 39% increase in water intake and a 93% increase in urine output compared to five to seven month rats. The intravesical pressure at micturition is 100% greater in 22 to 24 month and 16 to 18 month rats compared to five to seven month old rats with no age-related change in bladder volume at micturition. In the present study, in vitro bladder capacity did not differ between the three age groups although the average plateau pressure significantly decreased with advancing age. Using the isolated whole bladder model, the contractile response to the autonomic agonists bethanechol, phenylephrine, and isoproterenol did not change significantly with age. Similarly, there were no age-related changes in the response of the bladder to non-autonomic drugs (histamine, oxytocin, serotonin, substance P, and PGF2 alpha) except for PGF2 alpha which produced an age-related increase in the maximum bladder contraction. In summary, while in vivo micturition clearly changes with age, the in vitro contractility of the bladders to autonomic agents did not. Therefore, age related differences in micturition would be related primarily to the changes in neuronal innervation and central control of micturition rather than alterations in the contractility of the bladder. In addition, these studies show the importance of correlating in vivo bladder function (micturition frequency and volume, cystometry and urodynamics) with in vitro contractile and functional studies.