Effects of ageing on the behavioural responses to dopamine agonists: decreased yawning and locomotion, but increased stereotypy

A novel mouse Chr5 locus Diht controls dopamine-induced hypothermia

Mice of the strain C3H.PRI-Flv(r), carrying genetically determined resistance to flaviviruses, have been shown to be more sensitive to the hypothermic effect of dopamine than congenic flavivirus-susceptible C3H/HeJARC mice. In the current study, the greater sensitivity to dopamine-induced hypothermia observed in flavivirus-resistant mice was shown to be dose-dependent, with strain differences being the most prominent at a moderate dose of apomorphine (1 mg/kg). In addition, hypothermic responses to apomorphine were shown to be under developmental regulation; aging increased the potency of apomorphine-induced hypothermia and abrogated strain and sex differences observed in young mice. Linkage analysis of mouse strain-dependent coinheritance between flavivirus resistance and greater sensitivity to the hypothermic effect of dopamine was performed using two genetically unrelated flavivirus-susceptible and two highly congenic flavivirus-resistant mouse strains in parallel with C3H.PRI-Flv(r)-and C3H/HeJARC reference strains. This study has revealed a clear segregation between flavivirus resistance conferred by the Flv locus and sensitivity to dopamine-controlled hypothermia conferred by a novel locus, Diht. Parallel studies in F1 and F2 heterozygote mice showed that the high sensitivity to hypothermic effect of dopamine (Dih-thigh) is inherited as the Chr5-linked dominant trait. The novel locus, Diht, has been mapped proximal to the Flv locus on a distal part of mouse Chr5 between microsatellite markers D5Mit41 and D5Mit158.

Increased sensitization to morphine-induced oral stereotypy in aged rats

Sensitization develops to the stereotypic biting behavior that appears with the repeated administration of high dose morphine to rats. Because there is evidence that this behavior is dopamine-mediated and that there are age-related changes in dopamine systems, we compared the development and expression of morphine-induced biting behavior in aged (24 months) and young rats (5 months). Animals were treated with four sensitizing 10 mg/kg doses of morphine or saline, followed by three weekly challenges with 4 mg/kg doses of morphine or saline. By the fourth sensitizing morphine dose and after the administration of each low dose challenge, the biting time was significantly greater for aged than for young morphine pre-treated rats. After the first weekly low dose challenge, the aged but not young animals expressed more biting than when they did after the last 10 mg/kg dose. These results indicate that sensitization to morphine-induced oral stereotypy is significantly greater in aged as compared to young rats. Age-related enhanced sensitivity to morphine-induced oral stereotypy might be related to age-induced increases in vulnerability to opioid-induced insults to the basal ganglia, and may be a model for certain diseases of this pathway.

In situ hybridization analysis of preprotachykinin-A mRNA levels in young and old rats

Preprotachykinin-A (PPT-A) mRNA levels in discrete rat brain regions were examined. Analysis of silver grains revealed a 19.2% and 31.5% statistically significant decrease in PPT-A mRNA in the dorsal and ventral caudate putamen (d-CPu and v-CPu), respectively, a 30% lower expression of PPT-A mRNA in the bed nucleus of the stria terminalis (BNST), a 33.7% decrease in PPT-A mRNA in the habenula (Hb), and a 30% decrease of PPT-A mRNA levels in the posterodorsal part of the medial amygdala (MePD). Results show that aging of the CNS is associated with widespread changes in tachykinin gene expression, suggesting that alterations in the tachykinergic system may have implications in the physiopathology of the elderly.

Motoric behavior in aged rats treated with GM1

Aging is associated with impaired motor function. Nigrostriatal dopaminergic neurons, in part, regulate motoric behavior, and undergo degenerative changes during aging. GM1 ganglioside partially restores pre-synaptic dopaminergic markers and the number and morphology of dopaminergic neurons in the midbrain and striatum of Sprague--Dawley aged rats. These studies investigated whether GM1 treatment, 30 mg/kg, i.p. daily for 36 days, affects locomotor and stereotypic activity, as well as coordination, balance, and strength in aged rats. Under the treatment conditions used, GM1 did not improve the reduced locomotor and stereotypic behavior of the aged rats. While it partially improved performance on a square bridge test, GM1 had no effect on inclined screen and rod suspension tests. Although GM1 restored the decreased content of dopamine and homovanillic acid in the nigrostriatal neurons of the aged rats, it had no effect on the reduced D1 and D2 dopamine receptor binding and mRNA in the striatum. It appears, that despite the morphological and metabolic restoration of aged nigrostriatal neurons, GM1 has limited ability in improving age-associated motor deficits.

Effects of spinal alpha 1-adrenoceptor antagonism on bladder activity induced by apomorphine in conscious rats with and without bladder outlet obstruction

To test the hypothesis that the spinal control of micturition involves alpha 1-adrenoceptors, the urodynamic effects of intrathecal and intraarterial alpha 1-adrenoceptor blockade on apomorphine-induced bladder activity in rats were studied. Continuous cystometry was performed in conscious female Sprague-Dawley rats with and without bladder outflow obstruction. In normal rats, subcutaneous apomorphine, 30 micrograms/kg, induced bladder activity that was abolished or attenuated by the alpha 1-adrenoceptor antagonists indoramin and doxazosin given intrathecally or intra-arterially. In rats with outlet obstruction, apomorphine 30 micrograms/kg caused no change in cystometric parameters. However, at a dose of 100 micrograms/kg the drug induced bladder activity, which was attenuated by intrathecal indoramin or doxazosin. These results suggest that the bladder activity evoked by apomorphine-stimulation of bulbospinal pathways can be influenced by alpha 1-adrenoceptors at the spinal and peripheral levels, both in normal rats and in rats with bladder hypertrophy secondary to outlet obstruction.

Cognitive neuropharmacology: new perspectives for the pharmacology of cognition

Taking its roots both in neuropharmacology and in cognitive science, cognitive neuropharmacology is an emerging approach in the field of psychopharmacology. It attempts to use theoretical knowledge to understand the biochemical bases of cognition and the mode of action of the commonly used drugs and to find new brain-targeted therapeutics. The aim of the present article is to throw up the main characteristics of this way of research. It is defined in comparison with its neighbouring approaches and by presenting its own rationale. Its particular methods mainly concern the animal modelling of the highest human cognitive functions and the original means of intra-cerebral drug administration. Finally, we present an illustrative example of a study in cognitive neuropharmacology and propose further perspectives.

In situ hybridization analysis of preprotachykinin-A mRNA levels in young and old rats

The present study examined the effects of ageing on preprotachykinin-A (PPT-A) mRNA levels in discrete regions of the rat brain. Semiquantitative analysis of silver grains revealed a 16% statistically significant decrease in PPT-A mRNA in the shell of the nucleus accumbens (AcbSh), a 27.6% statistically significant lower level of PPT-A mRNA in the olfactory tubercle (Tu), a 19.2% and 31. 5% statistically significant decrease in PPT-A mRNA in the dorsal and ventral caudate-putamen (d-CPu) (v-CPu), respectively, a 30% statistically significant lower expression of PPT-A mRNA in the bed nucleus of the stria terminalis (BNST), a 33.7% statistically significant decrease in PPT-A mRNA in the habenula (Hb) and a 30% statistically significant decrease of PPT-A mRNA levels in the postero-dorsal part of the medial amygdala (MePD). No changes in PPT-A mRNA levels were found in the nucleus accumbens, core (AcbC), in the islands of Calleja (Icj), and in the medial preoptic area (mPOA). These results show that ageing of the central nervous system (CNS) is associated with widespread changes in tachykinin gene expression, suggesting that alteration in the tachykinergic system may have implications in the physio-pathology of the elderly.

Effect of apomorphine on the conflict-induced jumping stereotypy in bank voles

In conventional laboratory cages, bank voles (Clethrionomys glareolus) develop a jumping up-and-down stereotypy already before the age of one month. Central DA systems are thought to be involved in the expression of these conflict-induced stereotypies (CIS). Stereotypies can also be elicited pharmacologically, most commonly by amphetamine and apomorphine. Hence, administration of apomorphine to jumping bank voles provides the opportunity to compare pharmacologically-induced stereotypies (PHIS) and CIS in that species. A pilot study showed that apomorphine induced stereotyped licking that is qualitatively different from the CIS elicited by captivity. The present study investigated whether apomorphine has an effect on CIS-levels. The lowest dose (0.625 mg/kg) did not elicit licking but neither influenced jumping levels. Higher doses (0.938 and 1.094 mg/kg) lead to the occurrence of licking but also suppressed CIS-levels. However, the discordance in time profiles of licking and jumping argues against a shift from jumping to licking due to further stimulation of already activated DA systems. Therefore, expression of jumping seems to depend on stimulation of other DA receptor subtypes or jumping may even be DA independent.

Tremulous characteristics of the vacuous jaw movements induced by pilocarpine and ventrolateral striatal dopamine depletions

Vacuous jaw movements induced by the muscarinic agonist pilocarpine and striatal dopamine depletions were examined using a slow motion videotape system. With this procedure, rats were videotaped in a Plexiglas tube so that the profile of the head region could be seen. Vacuous jaw movements were analyzed by examining the tape at 1/6 normal speed. An observer recorded each jaw movement using a computer, and the computer program re-calculated the temporal characteristics of jaw movement responses back to normal speed. The interresponse time was recorded for each jaw movement, and each jaw movement interresponse time was assigned to a 50 ms wide time bin. Thus, the distribution of interresponse times could be used to analyze the temporal characteristics of jaw movement responses. In the first experiment, rats were administered saline vehicle, 1.0 mg/kg and 2.0 mg/kg pilocarpine. The rats were videotaped 10-15 min after injection, and the data were analyzed as described above. Pilocarpine induced very high levels of vacuous jaw movements, and the vast majority of all movements occurred in "bursts" with interresponse times of 1.0 s or less. Analysis of the interresponse time distributions showed that most of the jaw movements were within the 150-350 ms range. The modal jaw movement interresponse time was in the 150-200 ms range, which corresponds to a local frequency of 5-6.66 Hz. In the second experiment, the neurotoxic agent 6-hydroxydopamine was injected directly into the ventrolateral striatum in order to produce a local dopamine depletion. The dopamine-depleted rats were observed for jaw movements 7 days after surgery. The overall level of jaw movement activity resulting from dopamine-depletion was much lower than that produced by pilocarpine. There was a significant inverse correlation between ventrolateral striatal dopamine levels and total number of vacuous jaw movements. Videotape analysis indicated that the temporal characteristics of jaw movements induced by dopamine depletions were similar to those shown with pilocarpine. These experiments indicate that vacuous jaw movements induced by pilocarpine and striatal dopamine depletion occur in a frequency range similar to that shown in parkinsonian tremor.

Glial cell line-derived neurotrophic factor induces the dopaminergic and cholinergic phenotype and increases locomotor activity in aged Fischer 344 rats

Glial cell line-derived neurotrophic factor has been shown to affect dopaminergic and cholinergic neuron markers and functions in young rats. However, it is not known if the response to exogenous glial cell line-derived neurotrophic factor is augmented during normal aging. Thus, the effects of chronic intraventricular infusions of glial cell line-derived neurotrophic factor were determined in young adult (three-months-old) and aged (24-months-old) Fischer 344 (F344) male rats. The effects of glial cell line-derived neurotrophic factor were compared to the effects of the neurotrophin nerve growth factor. Growth factors were administered at a dose of 10 mg/day for 14 days. Locomotor activity and weight changes were also examined in all rats. Aged F344 rats showed significantly reduced (by 75-80%) locomotor activity compared to young rats. In glial cell line-derived neurotrophic factor-treated aged and young rats there was significantly increased (242% and 149%, respectively) locomotor activity measured at seven days. There was also a significant increase in locomotor activity measured 14 days after the start of infusion. Both glial cell line-derived neurotrophic factor and nerve growth factor reduced weight gain by 10% in young and old F344 rats. Two weeks following the start of nerve growth factor or glial cell line-derived neurotrophic factor administration the brains were used for neurochemical analyses. Glial cell line-derived neurotrophic factor significantly increased tyrosine hydroxylase activity in the substantia nigra and striatum of aged rats and in the substantia nigra of young rats. Nerve growth factor treatment did not significantly affect tyrosine hydroxylase activity. However, glial cell line-derived neurotrophic factor and nerve growth factor increased choline acetyltransferase activity in the septum, hippocampus, striatum and cortex of aged rats and in the hippocampus and striatum of young rats to a comparable degree. These findings indicate that specific dopaminergic and cholinergic neuron populations remain responsive to glial cell line-derived neurotrophic factor during the life span of the rat and may be involved in maintaining phenotypic expression within multiple neuronal populations. Additionally, the glial cell line-derived neurotrophic factor-induced up-regulation of brain neurotransmitter systems may be responsible for increased locomotor activity in F344 rats.

Dopaminergic function in the neostriatum and nucleus accumbens of young and aged Fischer 344 rats

Age-dependent alterations in behavioral and neuronal functioning were assessed in young (2-3 month), middle-aged (12 month), and aged (24 month) Fischer 344 rats treated with the indirect dopamine agonist amphetamine (2.25 or 5 mg/kg), the D1 agonist SKF 38393 (7.5, 15, 30 mg/kg), or the D2 agonist quinpirole (0.3, 1.0, 3.0 mg/kg). Drug-induced changes in activity and stereotypy were measured during a 90-min testing session, with Fos immunohistochemistry being used to assess the neuronal response to dopamine agonist treatment. As expected, aged rats given amphetamine (5 mg/kg) had fewer activity counts and higher stereotypy scores than young rats. Middle-aged rats also had fewer activity counts but were similar in stereotypy scores to young rats. Amphetamine also induced different patterns of Fos immunoreactivity in the neostriatum and nucleus accumbens of young and aged rats, as Fos expression in aged rats exhibited a distinctive dorsal to ventral pattern of decline. In general, SKF 38393 had few age-related actions, although aged rats did show a slight relative increase in stereotypy. In contrast, the D2 agonist quinpirole substantially enhanced the motor activity and Fos expression of young rats, while only modestly affecting aged rats. Hence, these results suggest that the D2 receptor is more vulnerable to the effects of aging than the D1 receptor.

DA1 and DA2 receptor regulation in the striatum of young and old rats after peripheral vestibular lesion

Anatomical, lesion and functional studies have indicated that the mesostriatal dopaminergic (DAergic) system may serve as supravestibular center in posture and locomotion control. Nevertheless, no data are available on the involvement of DAergic systems during vestibular compensation. This study was designed for the analysis of DA1 and DA2 receptors in the striatum by means of quantitative receptor autoradiography 28 days after unilateral or bilateral lesion of the labyrinth in 3-month-old rats. Considering the severe decline of DA content and receptors in striatum and the difference in behavioral recovery after vestibular lesions in old age, we also analyzed 24-month-old, lesioned and unlesioned rats. In young rats, hemilabyrinthectomy caused a bilateral increase (20-30%) of DA1 receptors and a two-fold increase of DA2 receptors. In old-rats, we observed a similar modification of DA2 receptors, and a 50% increase in DA1 receptors. Bilabyrinthectomy did not modify DA1 receptor density and decreased DA2 receptor density in young animals, whereas it produced an increase in both DA1 and DA2 in old rats. This study provides evidence for the involvement of the DAergic system during vestibular compensation. Our results also indicate great biochemical plasticity of the remaining DA receptors in the striatum of old rats.

Age-dependent induction and maintenance of sensitization to methamphetamine-induced hyperactivity in mice

Repeated administrations of methamphetamine (2 mg/kg, s.c.), 10 times at 3-day intervals, induced ambulatory sensitization in all groups of mice that were 13-, 15-, 19-, 23- and 36-week-old at the start of methamphetamine administration. The most prominent sensitization was observed in the 19-week-old mice. Among five groups of mice, even though the mice of 36 weeks old showed the highest sensitivity to methamphetamine at the first administration, they exhibited the lowest sensitization during the latter stage of repeated methamphetamine administration. Methamphetamine sensitization once established was well reproduced by the post-sensitization period of 8 weeks. Furthermore, the group of mice given methamphetamine with post-sensitization interval of 8 weeks (19-week-old mice) exhibited further enhancement of the sensitization. In contrast, the groups of mice given methamphetamine with post-sensitization intervals of 12 and 25 weeks (the 23- and 36-week-old, respectively) showed a significant reduced sensitization, and the latter group failed to reach the level of sensitization previously established. These results suggest that the induction of and maintenance of methamphetamine sensitization are dependent on the age of the mice, and that methamphetamine sensitization once established completely persists for up to 8 weeks.

Effects of picotesla flux electromagnetic fields on dopaminergic transmission in Tourette's syndrome

Tourette's syndrome (TS), a chronic familial neuropsychiatric disorder of unknown etiology, is characterized clinically by the presence of motor and vocal tics that wax and wane in severity over the time and by the occurrence of a variety of neurobehavioral disorders. It is believed that the tics of TS result from increased dopamine (DA) activity caused by postsynaptic DA receptor supersensitivity. The synthesis and release of DA is regulated presynaptically by a specific class of DA D2 receptors, termed autoreceptors activation of which causes inhibition of DA synthesis and release. In experimental animals and humans administration of small doses of apomorphine, a DA D2 autoreceptor agonist, produces yawning. Recurrent episodes of yawning followed by increased motor tic activity was observed in two patients with TS during exposure to brief, extracranial applications of picotesla flux electromagnetic fields (EMFs). On the basis of these observations it is suggested that recurrent episodes of yawning in response to application of EMFs was induced by activation of presynaptic DA D2 autoreceptors while further exposure to these EMFs caused excessive stimulation of postsynaptic DA D2 receptors resulting in exacerbation of the tics. Thus, the dual effects of picotesla flux EMFs on the DA D2 autoreceptor and the postsynaptic receptor resemble the biphasic pharmacological and behavioral properties of apomorphine, a DA agonist which activates the autoreceptors in low doses while in higher doses causes stimulation of the postsynaptic receptors producing exacerbation of symptoms of TS. These findings demonstrate that picotesla flux EMFs applied extracerebrally may influence nigrostriatal DA transmission at pre- and postsynaptic DA D2 receptor sites.

Dopamine--glutamate--GABA interactions and ageing: studies in the striatum of the conscious rat

The effects of apomorphine, a D1-D2 dopamine receptor agonist, on the extracellular concentrations of glutamate and GABA were investigated in the striatum of young, middle-aged and aged rats. In vivo intracerebral perfusions were undertaken in the conscious rat using a concentric push-pull cannula system. Amino acid concentrations in samples were determined by HPLC with fluorometric detection. Apomorphine produced a concentration-related rise in striatal glutamate and GABA concentrations in young rats. Maximal increases were obtained at 20 microM apomorphine, and concentrations reached 184 and 191% of the basal value for glutamate and GABA respectively. Apomorphine failed to produce similar increases in glutamate concentration in middle-aged and aged rats. Apomorphine, at 10 microM, also failed to produce an increase in GABA concentration in the aged rats. However, at 20 microM apomorphine produced increases in GABA concentration in middle-aged and aged rats similar to those produced in young rats. These data are indicative of a change in threshold for GABA release induced by dopamine receptor stimulation in the aged rat. These results indicate that an interaction among dopamine, glutamate and GABA exists in the striatum of the rat, and that this type of interaction deteriorates with age.

Prefrontal cortical and hippocampal modulation of haloperidol-induced catalepsy and apomorphine-induced stereotypic behaviors in the rat

Effects of prefrontal cortical or hippocampal excitotoxic lesions on behavioral parameters related to dopaminergic transmission in the basal ganglia were investigated in the rat. We examined haloperidol-induced catalepsy and apomorphine-induced stereotypic behaviors after ibotenic acid lesions of the medial prefrontal cortex (MPFC), dorsal (DH), or ventral hippocampus (VH) in adult rats. Haloperidol-induced (1 mg/kg) catalepsy was decreased in rats with either MPFC or VH but not DH lesions. While both DH and VH lesioned animals demonstrated a reduction in apomorphine-induced (0.75 mg/kg) stereotypic behaviors, the VH lesioned animals also showed an enhancement of locomotor activity. MPFC lesioned rats tended towards potentiation of stereotypic behaviors and reduced locomotion after apomorphine administration. These data indicate that loss of prefrontal cortical or hippocampal modulation leads to an enhancement of DA transmission within the basal ganglia, though the pattern of augmentation depends on the area lesioned.

Repeated scopolamine injections sensitize rats to pilocarpine-induced vacuous jaw movements and enhance striatal muscarinic receptor binding

This experiment was conducted to determine if repeated administration of the muscarinic antagonist scopolamine could increase pilocarpine-induced vacuous jaw movements and also enhance muscarinic receptor binding. Rats received daily injections of either scopolamine (0.5 mg/kg IP) or saline for 14 days. On day 15 rats received no injections of scopolamine, but did receive injections of pilocarpine (1.0, 2.0 or 4.0 mg/kg IP) or saline. After administration of pilocarpine or saline, all rats were observed for vacuous jaw movements and rearing behavior. The day after pilocarpine injections, rats were sacrificed and samples of tissue from the lateral neostriatum were removed to assess muscarinic receptor binding using 3H-QNB as the ligand. Analyses of the vacuous jaw movement data indicated that there was a significant dose-related increase in vacuous jaw movements induced by pilocarpine, and also that there was a significant enhancement of pilocarpine-induced vacuous jaw movements in rats pretreated with repeated scopolamine injections. There was not a significant scopolamine x pilocarpine interaction, suggesting that pretreatment with scopolamine produced an apparent parallel shift in the pilocarpine dose-response curve. Pilocarpine significantly suppressed rearing behavior, and scopolamine pretreatment significantly enhanced the suppression of rearing produced by pilocarpine. Analysis of the receptor binding data indicated that there was a significant increase in the number of muscarinic receptor sites (Bmax) in rats that received repeated scopolamine injections as compared to saline-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ageing on tachykinin function in the basal ganglia

The effects of ageing on tachykinin-induced behaviours and tachykinin receptors were investigated in the rat. Infusion of the NK-3 tachykinin agonist senktide (0.25, 0.5 and 1 nmol) into the substantia nigra induced locomotion in young (4-6 months) animals but this response was attenuated in middle-aged (12 months) and old (27 months) animals. In contrast, senktide-induced wet dog shakes were not significantly affected by age. In the ventral tegmental area, senktide induced locomotion and wet dog shakes with bell-shaped dose-response curves which were unaffected by age. Senktide suppressed grooming but the effect reached significance in the older animals only. Quantitative receptor autoradiography revealed no effect of age on NK-1 tachykinin receptor density in the striatum while NK-3 receptor density declined in the ventrolateral striatum and to a nonsignificant degree in the substantia nigra but not in other striatal subregions or the ventral tegmental area. We conclude that ageing of the nervous system is not associated with widespread changes in tachykinin binding but differences in behavioural response to tachykinin agonists may reflect changes in other transmitter systems which respond to tachykinin input.

Vacuous jaw movements in rats induced by acute reserpine administration: interactions with different doses of apomorphine

Two experiments were conducted to study the vacuous jaw movements induced in rats by acute administration of the monoamine-depleting agent reserpine. In the first experiment, different doses of reserpine (1.25, 2.5, and 5.0 mg/kg) were assessed for their ability to induce vacuous jaw movements. Acute administration of reserpine induced a dose-related increase in vacuous jaw movements, with the two highest doses being significantly different from the vehicle control. In the second experiment, interactions between 5.0 mg/kg reserpine and the dopamine agonist apomorphine were investigated. Coadministration of reserpine with the lowest dose of apomorphine (0.1 mg/kg) significantly increased vacuous jaw movements relative to reserpine alone. The two higher doses of apomorphine (0.5 and 1.0 mg/kg) significantly decreased vacuous jaw movements in reserpine-treated rats. These results demonstrate that vacuous jaw movements are induced by acute reserpine treatment in a dose-related manner. In addition, the interactions with apomorphine suggest that vacuous jaw movements are stimulated by decreases in dopamine release produced by low doses of apomorphine that are thought to have mainly presynaptic actions, but that these movements are decreased by higher doses of apomorphine that are known to act postsynaptically.

Effects of acute haloperidol and reserpine administration on vacuous jaw movements in three different age groups of rats

In these experiments three different age groups of rats were tested for vacuous jaw movements. The age groups included rats aged 3 months, rats aged 6-9 months, and rats aged 12-15 months. In the first experiment, rats were given a single IP injection of either 0.3% tartaric acid vehicle or 0.4 mg/kg haloperidol. In the second experiment, rats were given injections of vehicle or 5.0 mg/kg reserpine. Haloperidol and reserpine significantly increased vacuous jaw movements. There were significant effects of age on vacuous jaw movements, with rats aged 6-9 months and rats aged 12-15 months showing more jaw movements than 3-month-old rats. In both experiments, the drug x age interaction was not significant, indicating that the combined effects of age and neuroleptic treatment were additive and not synergistic. Haloperidol and reserpine also reduced rearing behavior in all age groups. It is concluded that age is an important variable in studies of vacuous jaw movements.

Locomotion of aged rats: relationship to neurochemical but not morphological changes in nigrostriatal dopaminergic neurons

Spontaneous locomotion and motor coordination was evaluated in young (5-6 month old) and aged (24-25 month old) rats. Animals were tested for spontaneous locomotor activity in Digiscan Animal Activity Monitors during the nocturnal cycle. Aged animals exhibited a significant hypoactivity compared to their young counterparts. Evaluation of the time course of activity revealed that the young animals had a cyclical pattern of activity during the 12-hour testing period with clear peaks at 2-4 hours after the initiation of testing and at 8- to 10-hour intervals thereafter. In contrast, the aged animals exhibited a blunted initial activity peak. During the remainder of the test period the aged animals activity was stable with no further peaks in activity. Compared to the young animals the aged animals also (a) remained suspended from a horizontal wire for less time, (b) were unable to descend a wooden pole covered with wire mesh in a coordinated manner, (c) fell more rapidly from a rotating rod and (d) were unable to maintain their balance on a series of wooden beams with either a square or rounded top of varying widths. Histological analysis demonstrated that there was no reduction in the number, area, or length of tyrosine hydroxylase-immunoreactive neurons within the A8, A9, or A10 region of the aged animals. Neurochemical analysis revealed that while DA and HVA levels were not decreased in the aged rats, DOPAC levels, as well as the ratios of DA/DOPAC and DA/HVA, were decreased. These results indicate that neurochemical but not morphological changes within the nigrostriatal dopaminergic system underlie the deficits in motor behavior observed in aged rats.

Post-transcriptional regulation of loss of rat striatal D2 dopamine receptor during aging

The mechanism(s) underlying age-associated diminutions in the rat striatal D2 dopamine receptor (D2-receptor) number was investigated. The levels of D2-receptor mRNA in 4-, 12- and 18-month-old rat striata were found not to change. In contrast, the levels of 110 kDa protein, labeled with a D2-receptor specific antibody, decreased in parallel with [3H]YM-09151-2 binding to striatal membranes. These data suggest a role for post-transcriptional mechanism(s) in age-associated decrease in D2-receptor.

Age-related changes in apomorphine-induced erections

Advancing age produces a noticeable and well-documented decline in erectile function in humans. The effects of aging on the ability of apomorphine to stimulate erection and yawning behavior in rats was studied in our bioassay for potency. At the age of seven months, rats failed to respond to the same dose of apomorphine which, just one month earlier, produced erections. Erectile function was then tested in thirty-two seven-month-old rats naive to apomorphine injections, and these rats also failed to respond. Experimentally naive rats of six months of age were then tested and apomorphine produced reliable erections. It is felt that an alteration in dopamine autoreceptor function may be occurring in the central nervous system of rats at approximately seven months of age rendering them incapable of responding to apomorphine with penile erections.

Selective dopamine D1 and D2 agonists independently affect different components of the free-running circadian rhythm of locomotor activity in rats

The effects of selective dopamine D1 (SKF 38393) and D2 (4-propyl-9-hydroxynaphthoxazine or PHNO) agonists on free-running rhythms of locomotor activity in rats were investigated under either constant light (LL) or dark (DD) condition. Rats under the DD condition exhibited clear circadian patterns of activity, while circadian patterns of activity in animals kept under the LL condition were more ambiguous. Under the DD condition, SKF 38393 significantly increased the length of the period of free-running rhythms of locomotor activity but had no effect on the amplitude and the mesor, while PHNO significantly increased the amplitude and the mesor of free-running rhythms but had no effect on the length of the period.

Day and night locomotor activity effects during administration of (+)-amphetamine

Rats were given continuous subcutaneous amphetamine infusions (0, 2, 6, 10 and 20 mg/kg/day) via osmotic minipumps. The effects of these treatments on the locomotor activity of rats were determined over both light and dark phases of a 12-hr light/dark cycle for 336 consecutive hours. It was observed that tolerance to the locomotor stimulant actions of (+)-amphetamine is both dose- and light/dark cycle-dependent. Locomotor stimulation induced by the two highest doses remained high during both day and night throughout the period of treatment, except for the first few days and nights with the highest dose. Tolerance developed only to the effects of the two lower doses, and only during the day. Effects of the low doses on locomotor activity and on circadian patterns of locomotor activity are roughly similar to those previously observed with continuous administration of a selective dopamine D2 agonist. This behavioral similarity suggests that dopamine released by continuous administration of low doses of (+)-amphetamine may be producing its effects via selective actions on DA D2 receptors in vivo.