Neurophysiological changes in caudate nucleus and substantia nigra following morphine treatment

Average sensory evoked responses were recorded simultaneously from the caudate nucleus and substantia nigra of freely moving rats. Permanent semi-microelectrodes were implanted 6–8 days before experiments were begun. The acoustic average evoked responses to 32 successive click stimuli in caudate nucleus and substantia nigra consisted of large amplitude potentials of long duration made up of 5 components (P₁, N₁, P₂, N₂ and P₃) with long latencies. Three different dosages of morphine were examined (10, 30 and 50 mg/kg. i.p.). Evoked response amplitudes were increased or decreased or, in several cases, not altered by morphine. In the recording from the caudate nucleus, the responses observed were related to the dosage of morphine and the time after injection. In the recordings obtained from the substantia nigra, dose and time effects of morphine on the responses were not evident. Naloxone (1 mg/kg, i.p.) reversed the morphine effects. The low dose of morphine (10 mg/kg i.p.) increased motor activity, while the highest dose (50 mg/kg, i.p.) caused catalepsy. The possible anatomical circuits involved in this mechanism are discussed.     

Behavioral indices of beta receptor subsensitivity after chronic treatment with viloxazine in the mouse

The aim of the experiments was to determine whether chronic pretreatment with viloxazine decreased the sensitivity of mice to the sedative effects of a beta agonist clenbuterol. Mice were subjected to chronic oral treatment with viloxazine (128 mg/kg twice daily) and then given a single administration of 32 mg/kg PO followed by clenbuterol (0.125 mg/kg IP) before being tested in a standard photocell activity meter. Imipramine, administered at the same doses in the same experimental conditions, was used as a comparison compound. The results showed that chronic but not acute viloxazine decreased the hypoactivity induced by clenbuterol, suggesting the induction of beta receptor subsensitivity. With imipramine the results were in the same direction but less clear. The findings are discussed in terms of the eventual specificity of the viloxazine effect to subsensitivity in beta-2 receptors.     

Dopamine receptor changes after long-term haloperidol treatment in rats

Tardive dyskinesia is a neurological syndrome associated with prolonged neuroleptic treatment of schizophrenic patients (Crane 1968; Faurbye 1970; Faurbye et al 1964). It has been suggested that tardive dyskinesia results from chemical denervation of central dopamine neurons and subsequent development of supersensitivity of the postsynaptic receptor (Rubovits & Klawans 1972). Experiments based on animal models of tardive dyskinesia support this hypothesis. Chronic neuroleptic treatment increases postsynaptic dopamine receptor sensitivity when measured either behaviourally or biochemically (Christensen et al 1976; Klawans & Rubovits 1972; Moore & Thornburg 1975; Sayers et al 1975; Tarsy & Baldessarini 1974; Von Voigtlander et al 1975). The molecular basis for this behavioural change has been reported to be an increase in the number of receptor sites with no change in their affinity as measured by ³H-neuroleptic binding to striatal membrane homogenates (Burt et al 1977; Klawans et al 1977; Muller & Seeman 1977). Previous biochemical reports of the development of model tardive dyskinesia after neuroleptic treatment of animals have generally used treatment periods of 3 weeks or less (Burt et al 1977; Klawans et al 1977; Muller & Seeman 1977). Only one report treated animals for longer periods (Clow et al 1978). In the present experiments we studied the kinetics of [³H]spiroperidol binding to rat caudate nucleus homogenates after 3 and 10 weeks of haloperidol treatment.     

Sensitization to daily morphine injections in rats with unilateral lesions of the substantia nigra.

Morphine indirectly enhances dopaminergic activity in the nigrostriatal system, and repeated administration of morphine progressively increases the locomotor activity of rats. We used the rotational behavior model to determine if daily morphine produces an increase in turning and produces cross-sensitization to d-amphetamine and cocaine. Rats with unilateral nigrostriatal lesions received daily injections of saline or morphine (10 mg/kg). Repeated morphine administration produced a progressive increase in turning over 13 days. Next, a morphine dose-response curve (1.0-30 mg/kg) was determined. Both the saline and morphine-treated groups showed dose-dependent increases in turning, but, the peak effect in the morphine group was higher than that in the saline group, indicating sensitization to morphine. The morphine-treated group did not show cross-sensitization to either d-amphetamine (0.1-3 mg/kg) or cocaine (1.0-30 mg/kg); in fact, it showed less cocaine-induced turning than the saline group. Seventy-one days after saline or morphine injections began, the morphine group was still significantly more sensitive to turning induced by 10 mg/kg morphine than the saline group was (200 vs. 750). Therefore, repeated daily injections of morphine produce a progressive sensitization to turning induced by morphine in the absence of cross-sensitization to turning induced by psychomotor stimulants.     

Decreased 3H-l-quinuclidinyl benzilate binding and muscarine receptor subsensitivity after chronic gamma-butyrolactone treatment

Summary A significant decrease in the number of ³H-quinuclidinyl benzilate (³H-QNB) binding sites in striatal membranes was observed in rats exposed to gamma-butyrolactone (GBL) during 3 weeks. This decrease in ³H-QNB binding was not due to a direct effect of GBL on muscarine receptors since no alterations of binding were produced by high concentrations of GBL or of its active metabolite gamma-hydroxybutyric acid (GHB) when added in vitro.Challenge doses of pilocarpine were less effective in producing catalepsy and tremors in rats chronically treated with GBL. It is proposed that the partial inhibition of the nigro-striatal dopaminergic pathway produced by chronic GBL treatment leads to a persistent activation of the intrastriatal cholinergic interneurons, which are tonically inhibited by dopaminergic nigrostriatal neurons. The activation of striatal cholinergic interneurons results in a decreased number of muscarine receptors and in a lower sensitivity to cholinominetic agents.Moreover, tolerance to the anesthetic effect of GBL was found to occur after chronic treatment with this drug. The brain levels of GHB at the time of regaining of the righting reflex were 50% higher in GBL pretreated animals then in controls. Indicating the tolerance to GBL-induced sleep is due to a decreased sensitivity of the effector to GHB rather than to a faster inactivation or elimination of the drug.     

Dopamine receptor reappearance after irreversible receptor blockade: effect of chronic estradiol treatment of ovariectomized rats.

It is well established that estrogen modulates central dopamine functions; however, the mechanism of this interaction is still poorly understood. We have used peripheral N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) administration to induce irreversible blockade of dopamine receptors in ovariectomized female rats, which were pretreated with estradiol (10 micrograms, twice each day for 2 weeks) or its vehicle, in order to investigate the effect of estradiol on dopamine receptor repopulation kinetics. As previously observed, chronic estradiol treatment increased both striatal D1 and D2 dopamine receptor densities and left affinities unchanged. Anterior pituitary D2 dopamine receptor density remained unchanged. One day after EEDQ administration, a similar decrease (80%) of [3H]SCH 23390 and [3H]spiperone binding to striatum of estradiol- and vehicle-treated animals was observed. Anterior pituitary D2 dopamine receptor specific binding was reduced by about 50% the day after EEDQ. Recovery after EEDQ administration showed that both receptor production rate and degradation rate constants of anterior pituitary D2 and striatal D1 receptors were slowed after chronic estradiol treatment, whereas recovery rates for striatal D2 dopamine receptors were unaffected. EEDQ administration in vehicle-treated rats did not significantly affect plasma prolactin levels, whereas the combination of estradiol pretreatment and EEDQ administration led to increased plasma prolactin levels, compared with estradiol-treated animals that did not receive EEDQ. This suggests that only a fraction of anterior pituitary dopamine receptors are required for a maximal inhibition of prolactin secretion. Estradiol affected both striatal D1 and D2 dopamine receptor densities but only D1 dopamine receptor repopulation kinetics, suggesting that it may act by different mechanisms on each dopamine receptor. Alternatively, estradiol may affect dopamine receptor interaction. Thus, the present study raises the possibility that a biochemical D1/D2 receptor interaction affects dopamine receptor biosynthesis, turnover, and/or gene expression and that estradiol may influence this dopamine receptor interaction in the striatum.     

The interaction of the mu-opioid receptor and G protein is altered after chronic morphine treatment in rats

Summary The interaction of µ-opioid receptors and G proteins after chronic morphine treatment was investigated in rats. Male Sprague-Dawley rats (200–260 g) were rendered tolerant to morphine by i.p. injections of increasing doses of morphine twice daily for 4 or 6 days. During this period, there was a time-dependent increase in the AD₅₀ values for morphine to inhibit the tail-flick response. In addition, in vitro µ-opioid receptor binding to midbrain P₂ membranes from these animals revealed that the ability of 10 mol/l Gpp(NH)p (guanyl-5-yl imidodiphosphate) to decrease [³H]DAMGO (TyrD-Ala-Gly-McPhe-Gly-ol) binding affinity, i.e., the ratio K _d(+Gpp(NH)p)/K _d(–Gpp(NH)p), decreased significantly from the control value of 3.68±0.40 to 2.37±0.35 after 6 days of morphine treatment (P<0.05).The ability of DAMGO to stimulate low K _m GTPase activity was also investigated. The EC₅₀ significantly increased from 2.7± 1.1 × 10^–8 mol/l in the control group to 10.8±1.5 × 10^–8 mol/l after 4 days of morphine treatment and was further increased to 13.5±2.1 × 10^–8 mol/l after 6 days of morphine treatment. The maximal stimulation by DAMGO decreased significantly from 18.0±1.7% to 12.8± 1.6% after 6 days of morphine treatment.These results indicate that the interaction between µ-opioid receptors and G proteins had been altered after chronic morphine treatment. Correspondence to: P.-L. Tao at the above address     

Dopamine mediates ipsi- and contraversive circling elicited from the substantia nigra

It was found that ipsiversive and contraversive circling could be induced by imposed stimulation of the lateral and medial portions of the substantia nigra pars compacta (SNC), respectively. Stimulation of more ventral sites in the substantia nigra (SN) did not elicit circling. Pimozide, a dopamine (DA) antagonist, dose dependently blocked both ipsiversive and contraversive circling induced by imposed stimulation of the SN, but did not alter circling elicited from the non-dopaminergic cerebral peduncle. However, amphetamine, a catecholamine agonist, did not facilitate stimulation-induced circling elicited from the SN possibly because it releases DA on both sides of the brain and thus fails to exaggerate the imbalance of activity produced by stimulation. Taken together these results suggest that DA is involved in SN mechanisms mediating both ipsiversive and contraversive stimulation induced circling and provide further evidence that the lateral SN may be functionally antagonistic to the medial SN.     

Dopamine autoreceptors in the ventral tegmental area show subsensitivity following withdrawal from chronic antidepressant drug treatment

The suppression by apomorphine of food intake and eating time was used to assay the sensitivity of dopamine cell body autoreceptors during the course of treatment with DMI, amitriptyline and mianserin. Brief (2–4 days) DMI treatment enhanced the effects of apomorphine, administered systemically or centrally to DA cell body regions. During chronic DMI treatment (3–7 weeks) some evidence of autoreceptor subsensitivity was observed with systemic apomorphine, but not with central apomorphine. Responses to apomorphine applied systemically were reduced during withdrawal from chronic DMI, and responses to apomorphine applied to the ventral tegmental area were reduced during withdrawal from all three antidepressants. As evidence of DA autoreceptor subsensitivity was only observed reliably during withdrawal, this effect is unlikely to be of clinical importance.     

Enhanced self-stimulation responding from the substantia nigra after chronic amphetamine treatment: A role for conditioning factors

Rats treated with 2 mg/kg of d-amphetamine and tested for self-stimulation responding supported from the substantia nigra (pre-trial group), showed a progressive augmentation in rates of self-stimulation responding relative to control animals following repeated drug/test pairings for 10 days. A similar behavioral profile was not observed among animals that received behavioral testing followed by drug administration (post-trial group) during the chronic phase. On test day (Day 11), rats that received repeated drug/test pairings during the chronic phase exhibited a facilitated self-stimulation response to a low test dosage of d-amphetamine (0.5 mg/kg) which otherwise had no behavioral effect, whereas rats exposed to chronic test/drug pairings during the chronic phase did not show enhaced self-stimulation rates to the test dosage of d-amphetamine. Animals chronically treated with pre-trial injections of amphetamine also showed facilitated self-stimulation responding when tested with saline, relative to animals that were chronically treated with post-trial injections of amphetamine and tested with saline. These findings were not parallelled by drug-induced changes in locomotor activity. Response sensitization after chronic amphetamine treatment does not appear to involve the accumulation of the drug in adipose tissue. A role for conditioning factors in the development of the response sensitization is discussed.     

Precipitated withdrawal in the substantia nigra in diazepam-dependent female rats

Female rats were exposed to diazepam (DZ) implants (90 mg/week) or to empty capsules (controls) for 5 weeks. Rats were focally injected (1 microl) into the substantia nigra (SNR) with the central (CBR) and peripheral (PBR) benzodiazepine receptor antagonists, flumazenil [(FLU) 6.25, 12.5, or 25 microg], and PK 11195 [(PK) 3.125, 6.25, 12.5, or 25 microg], respectively. Rats were observed for behavioral and EEG manifestation of withdrawal syndrome. In female rats, both FLU and PK induced a dose-related precipitated abstinence score (PAS), tachypnea, and head bobbing. Twitches and jerks tended to increase with increasing dose of both FLU and PK. Furthermore, FLU evoked dose-related turning and head and body tremors. The FLU- and the PK-induced PAS were accompanied by an increase in total power of the EEG in the SNR. The involvement of the CBR and PBR in physical dependence on DZ in the SNR is suggested. The present data in female rats are discussed with regard to similarities and differences with previous studies in male rats.     

Dopamine receptor sensitivity after chronic dopamine agonists

Several previous reports have demonstrated that chronic administration of both directly and indirectly acting dopamine agonists produces a supersensitive behavioral response to challenge doses of dopamine agonists when compared to the responses induced by acute administration of these drugs. That is, a given dose of a dopamine agonist will produce a greater response after chronic dopamine agonist treatment than is observed upon acute administration of that dose. A similar behavioral phenomenon resulting from chronic administration of dopamine antagonists has been suggested to be due to an increase in the number of dopamine receptors present in relevant brain areas. The same hypothesis has been put forward for the hypersensitivity induced by chronic dopamine agonist administration. The present study was designed to investigate the effect of chronic administration of high doses of both direct and indirect dopamine agonists on the dopamine receptors labeled by ³H-spiroperidol. Groups of animals (CD-1 mice) were sacrificed 1, 3 and 5 days following the last chronic injection. Striatal tissue from these mice was incubated with ³H-spiroperidol and dopamine receptor binding evaluated. Affinity of the receptors for the ligand was unaltered by treatments. The receptors labeled by ³H-spiroperidol showed no significant differences in number following the chronic administration of high doses of apomorphine (30 mg/kg). The B _max was significantly decreased at only one time period following chronic administration of dextroamphetamine (4 mg/kg); however, these was a dramatic 30% reduction in the B _max in striatal tissue from those mice treated with N-n-propylnorapomorphine. These results suggest that the hypersensitive behavioral response in mice following chronic administration of direct and indirect acting dopamine agonists is not due to an increase in the number of dopamine receptors in the striatum which are labeled by ³H-spiroperidol.     

Long-term effect of perphenazine on the substantia nigra in rats

A controlled blind study was made of the long-term effect of perphenazine enanthate (3.4 mg/kg/14 days for 12 months and 40 mg/kg/14 days for 6 months) upon the number of nerve cells (light microscopy) and the uptake of ³H-uridine (autoradiography) in the substantia nigra of rats. No significant differences were found between the numbers of nerve cells in treated and untreated animals, neither in the pars compacta nor in the pars reticularis. Nor were any significant differences found with regard to cell cytology or uptake of ³H-uridine. On the basis of the present data and certain clinical and neuroanatomic studies of patients with persistent signs of encephalopathy following long-term neuroleptic treatment it is suggested that perphenazine and related neuroleptics may exert an irreversible neurotoxic effect on the cells in the nigro-striatal system, but that this effect is not manifest until an elevated sensitivity to neuroleptics is present, e.g. in the form of age phenomena and/or psychiatric disease processes.     

The role of the substantia nigra in motility of the rat: Muscular rigidity,body asymmetry and catalepsy after injection of morphine into the nigra

Unilateral injection of morphine (1.0, 2.5 or 5.0 μg/0.5 μl) into the substantia nigra pars reticulata produced, in a dose-dependent way, tonic activity in the electromyogram (EMG) recorded from the ipsilateral gastrocnemius-soleus muscle of unanaesthetized rats. The effect produced by 5 μg of morphine was antagonized by co-administration of 5 μg of naloxone. Bilateral injection of morphine (5 μg each side) also produced a tonic activity in the EMG and catalepsy, but no asymmetry of posture. Unilateral injection of morphine (5 μg) into the substantia nigra pars compacta produced contralateral turning and/or stereotypy, but no tonic activity in the EMG. Naloxone (5 μg), when injected into the substantia nigra failed to produce any tonic activity in the EMG or any catalepsy. Larger doses of naloxone (10 or 20 μg) produced slight to moderate tonic activity in the EMG when injected into the pars reticulata. Injections of saline (0.5 μl) into either the reticulata or compacta were ineffective. Bilateral lesions of the caudate nucleus with kainic acid (l μg each side) did not prevent the development either of the tonic activity in the EMG or of the catalepsy, which were both produced by unilateral injection of morphine (5μg) into the pars reticulata. These results suggest that opioid mechanisms in the nigra might play a role in the regulation of posture and muscle tone and participate in the expression of motor functions controlled by the striatum (and probably also by the nucleus accumbens).     

Dopamine activates inward rectifier K+ channel in acutely dissociated rat substantia nigra neurones

The effect of dopamine (DA) was investigated on acutely dissociated rat substantia nigra pars compacta (SNc) neurones by using patch clamp recording. The SNc neurones could be classified into two groups. About 75% of large neurones (>30 microm in diameter) were tyrosine hydroxylase (TH) positive while almost all small neurones (<20 microm) were TH negative. In the large neurones, DA hyperpolarized the membrane, resulting in a reduction of the frequency of spontaneous action potentials in current-clamp mode and induced an inward rectifier K+ current in voltage-clamp mode. Quinpirole, a D2 receptor agonist, mimicked the DA action. S(-)-sulpiride, a D2 receptor antagonist, inhibited the DA-induced current (I(DA)) more effectively than SKF83566, a D1 receptor antagonist. Intracellular application of either guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS) or pertussis toxin (IAP) suppressed I(DA). Guanosine 5'-O-(3-thiotriphosphate) (GTP-gammaS) sustained the DA response. Modulators for cAMP such as forskolin and isobutylmethylxathine, H-89, a protein kinase A inhibitor, and chelerythrine, a protein kinase C inhibitor, had no effect on I(DA). The frequency of DA-induced single channel currents in the inside-out patch configuration, for which the unitary conductance was 56.6pS, was greatly reduced by the replacement of GTP with GDP perfused at the cytosolic side. These results suggest that DA acts on a D2-like receptor and activates directly an IAP-sensitive G protein coupled with inward rectifier K+ channels, resulting in a decrease in the spontaneous firing activities of rat SNc dopaminergic neurones.     

Changes in fixed-interval behavior during chronic morphine treatment and morphine abstinence in rats

Rats previously trained to a fixed-interval schedule (FI 2 min) were treated twice daily with saline or morphine hydrochloride (final dose 40 mg/kg i.p.) for 44 days.On day 45 an abstinence state was induced by withdrawing morphine or by giving nalorphine (1 mg/kg i.p.). Operant behavior was recorded on alternate days during the period of chronic treatment and during the withdrawal phase (21 days). It was found that the number of lever presses decreased significantly during the first days of morphine administration but increased later over the control values. The quarter-life was not changed during this period. Morphine withdrawal and nalorphine treatment both caused a further increase in lever presses that lasted about 11 days. Again quarter-life was not changed. These results indicate that the effects of morphine on FI behavior in rats not only undergo tolerance but are actually reversed during the chronic treatment.The data obtained during the withdrawal phase are discussed in relation to the secondary abstinence syndrome described by Martin et al. (1963).