Karen Bohmaker

Evidence that striatal synthesis-inhibiting autoreceptors are dopamine D3 receptors

The activation constants (KA; dose required to occupy 50% of receptors) for reversal of gamma-butyrolactone (GBL)-induced elevation of striatal L-3,4-dihydroxyphenylalanine (L-DOPA) levels via stimulation of presynaptic dopamine receptors were determined for apomorphine and two dopamine D3 receptor-selective agonists, quinpirole and LY163502 (quinelorane). The KA values correlated significantly with the affinities (Ki) of the agonists for the D3 (r = 0.999, P < 0.05) but not the D2 (r = -0.13) receptor, suggesting that striatal synthesis-inhibiting autoreceptors are of the D3 rather than the D2 subtype.

Receptor reserve at striatal dopamine autoreceptors: Implications for selectivity of dopamine agonists

The dose response curve for apomorphine reversal of gamma-butyrolactone (GBL)-induced L-DOPA accumulation in rat striatum was shifted almost 6-fold to the right after partial irreversible blockade (83%) of dopamine (DA) autoreceptors with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ); however, the maximal response was not reduced. In contrast, the major effect of a similar degree of irreversible blockade (86%) on the dose-response curve for the autoreceptor-selective agent EMD 23,448 was a reduction in maximal response (60% of control), indicating that EMD 23,448 is a partial agonist. A large receptor reserve therefore exists at the DA autoreceptor, which may explain in part why many DA agonists are more potent in models pre- than postsynaptic receptor activation.

Comparative effects of chronic 8-OH-DPAT, gepirone and ipsapirone treatment on the sensitivity of somatodendritic 5-HT1A autoreceptors

Chronic treatment with the full 5-hydroxytryptamine1A (5-HT1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), either by twice daily subcutaneous injection (0.2 or 2.0 mg/kg) or continuous subcutaneous administration via osmotic minipumps (0.4 or 4.0 mg/kg/day), for 14 days, had no effect on the dose-response curves for inhibition of 5-hydroxytryptophan (5-HTP) accumulation in rat cortex or hippocampus by 8-OH-DPAT or the partial 5-HT1A agonist BMY 7378. In contrast, chronic treatment with the nonbenzodiazepine putative anxiolytic gepirone via osmotic minipumps (20 mg/kg/day) resulted in a small but significant rightward shift (1.8-fold) in the dose-response curve to 8-OH-DPAT challenge in the cortex and a slightly larger shift (2.4-fold) in the hippocampus. Similarly, chronic treatment with another putative anxiolytic, ipsapirone, administered via twice daily subcutaneous injections (20 mg/kg), also resulted in a rightward shift (2.6-fold) in the dose-response curve to 8-OH-DPAT in the cortex and a slightly smaller shift (2.3-fold) in the hippocampus. Neither drug, however, decreased the maximal response. The present results are consistent with the suggestion that the clinical anxiolytic effects of gepirone and ipsapirone, and not of 8-OH-DPAT, may be related to their ability to desensitize somatodendritic 5-HT1A autoreceptors; other potential mechanisms are discussed.

Serotonin 5-HT1a receptor-mediated hypothermia in mice: Absence of spare receptors and rapid induction of tolerance

The mixed 5-hydroxytryptamine1A (5-HT1A) receptor agonist/antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol-[4.5]- decane-7,9-dione (BMY 7378) (5 mg/kg) did not significantly depress body temperature, but pretreatment with BMY 7378 blocked hypothermia induced by the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). In contrast, another partial 5-HT1A agonist, pindolol (10 mg/kg), slightly but significantly depressed body temperature by itself but did not attenuate hypothermia elicited by 8-OH-DPAT. Attempts to identify the synaptic locus of the receptor were unsuccessful because depletion of central serotonin (5-HT) by treatment with para-chlorophenylalanine (PCPA; 3 x 150 mg/kg) did not alter the hypothermic response to 8-OH-DPAT. Partial, irreversible 5-HT1A receptor inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (1 mg/kg) reduced the maximal hypothermic effect of 8-OH-DPAT (to 53% of control) without altering its ED50 (0.96 mg/kg). Analysis of the data indicated a linear relationship between 5-HT1A receptor occupancy and hypothermic response, that is, absence of receptor reserve. When groups of mice were treated with each of five different doses of 8-OH-DPAT (0.04, 0.16, 0.63, 2.5, and 10 mg/kg) 48 h apart, there was a significant reduction in hypothermic response after the second injection, but only at the three highest doses. The results demonstrate that 8-OH-DPAT-induced hypothermia in mice is mediated by a 5-HT1A receptor whose synaptic localization is uncertain but that has no receptor reserve. In addition, tolerance is observed after only a single agonist treatment.

Enhancement by the D1 dopamine agonist SKF 38393 of specific components of stereotypy elicited by the D2 agonists LY 171555 and RU 24213

Dose-effect curves were obtained for five specific components of stereotyped behavior (locomotor hyperactivity, rearing, sniffing, grooming and oral activity) elicited by the nonselective dopamine agonists N-propylnorapomorphine (NPA; 0.001-4.5 mg/kg) and apomorphine (APO; 0.3-10.0 mg/kg), the selective D2 agonists LY 171555 (0.1-18.0 mg/kg) and RU 24213 (0.7-20.0 mg/kg) and the selective D1 agonist SKF 38393 (0.6-40.0 mg/kg) in male Sprague-Dawley rats. All the agonists except SKF 38393 elicited dose-dependent sniffing. Both NPA and APO produced robust oral activity at high doses, with concomitant reductions in other behaviors. Neither RU 24213 nor SKF 38393 elicited dose-dependent oral behavior, whereas LY 171555 induced a maximal level of oral activity which was much less intense than that produced by NPA or APO. SKF 38393 alone induced only dose-dependent grooming. Co-treatment of Sprague-Dawley rats with SKF 38393 (5-40 mg/kg) and either LY 171555 (0.1-1.6 mg/kg) or RU 24213 (6.7 mg/kg) produced significant enhancement of sniffing but not of oral activity. Significant enhancement of both behavioral components was observed in male Wistar rats. Differences in specific components of behavioral response on treatment with combinations of D1 and D2 agonists may reflect use of animal strains with different ratios of D1/D2 innervation and/or different behavioral criteria.

Behavioral recovery after irreversible inactivation of D-1 and D-2 dopamine receptors

Irreversible inactivation of both D-1 and D-2 dopamine (DA) receptors by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) resulted in complete loss of stereotypy response to R-(-)-N-propylnorapomorphine (NPA; 0.1-1.0 mg/kg, s.c.) 24 hr later. Stereotyped sniffing recovered much more rapidly than oral behaviors. The D-2 antagonist sulpiride (200 mg/kg) and the putatively nonselective antagonist cis-flupenthixol (2 mg/kg), administered prior to EEDQ, prevented the loss of NPA-induced sniffing but only partially protected against loss of oral behaviors 24 hr later. Complete protection of both behaviors was seen after pretreatment with a combination of sulpiride and the selective D-1 antagonist SCH 23390 (1 mg/kg); pretreatment with the selective D-1 antagonist SCH 23390 alone, however, did not modify the rate of recovery of either behavioral response. The results suggest that either different populations of DA receptors mediate expression of these behaviors or stimulation of a small fraction of the total DA receptor pool may be sufficient to elicit sniffing but not oral responses. Furthermore, maintaining a normal complement of D-2 rather than D-1 receptors appears to be a critical determinant for the elicitation of these behaviors.

The effects of pertussis toxin on dopamine D2 and serotonin 5-HT1A autoreceptor-mediated inhibition of neurotransmitter synthesis: relationship to receptor reserve.

Irreversible inactivation of striatal D2 dopamine (DA) autoreceptors with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) or inactivation of striatal guanine nucleotide binding proteins (G proteins) with pertussis toxin (PT) shifted the dose-response curve for N-n-propylnorapomorphine (NPA)-mediated inhibition of gamma-butyrolactone (GBL)-induced elevation of L-3,4-dihydroxyphenylalanine (L-DOPA) to the right, with a decrease in the maximum response. For the partial agonist (+)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(+)-3-PPP], in contrast, there was little shift in the ED50, after inactivation of either D2 receptors or G proteins. Completely analogous effects were found at the somatodendritic 5-HT1A autoreceptor in the raphe nuclei, mediating inhibition of the synthesis of serotonin (5-HT); the full agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the partial agonist, buspirone were utilized to inhibit the synthesis of 5-HT, as measured by changes in levels of L-5-hydroxytryptophan (5-HTP). Additionally, in both systems, combined treatment with pertussis toxin, followed by EEDQ, reduced the maximum effect, when compared to either agent alone but had little further effect on the ED50. In systems exhibiting a large receptor reserve for agonists, such as those described above, the same pattern of response seen after inactivation of receptors or G proteins may reflect the operation of a common mechanism underlying the phenomenon of receptor reserve.

Chronic Treatment with Antipsychotic Drugs Does not Alter G Protein α or β Subunit Levels in Rat Brain

Groups of rats received once daily subcutaneous treatments for 22 days with haloperidol (0.5 mg/kg), clozapine (20 mg/kg), SCH 23390 (0.2 mg/kg) or vehicle. Quantitative immunoblots for Gi alpha 1, Gi alpha 2, G(o alpha), Gs alpha (45 kD), G beta 35 and G beta 36 were performed on membranes from the following brain regions: striatum, nucleus accumbens, substantia nigra, ventral tegmental area (VTA), prefrontal cortex and hippocampus (CA1). No significant alterations were found in the levels of any of these G protein subunits, in any brain region in treated vs control rats.

Chronic haloperidol does not alter agonist affinity for dopamine receptors in vitro

Agonist competition for [3H]spiperone binding to striatal dopamine D2 receptors was studied in rats rendered supersensitive by chronic treatment with haloperidol. The classical dopamine agonist (-)-N-n-propylnorapomorphine displaced [3H]spiperone biphasically, with IC50 values of 0.5 and 140 nM for the high and low affinity components, respectively. Neither the relative density nor the affinity of either site for (-)-N-propylnorapomorphine was affected by chronic haloperidol treatment. On the other hand, the novel agonist EMD 23 448 displaced [3H]spiperone monophasically. Although this agent only displays potent dopaminergic agonism in supersensitive animals, chronic treatment with haloperidol likewise did not alter the affinity of this drug for [3H]spiperone binding sites. The results suggest that the enhanced in vivo potency of certain agonists in supersensitive animals is probably not mediated by changes in D2 receptor affinity.

Differential effects of chronic clorgyline and amfonelic acid on desensitization of striatal dopamine receptors

Chronic treatment of rats with the MAOI clorgyline significantly reduced the density (Bmax) of cortical beta-adrenergic receptors but did not alter either the Bmax or dissociation constant (Kd) of 3H-spiperone binding to striatal DA receptors. Clorgyline co-treatment also did not significantly affect either behavioral supersensitivity to apomorphine or the increase in 3H-spiperone binding induced by chronic haloperidol. In contrast, repeated treatment with the DA uptake inhibitor amfonelic acid elicited behavioral subsensitivity and reduced striatal 3H-spiperone binding. Furthermore, amfonelic acid co-treatment prevented haloperidol-induced behavioral and receptor binding changes. The possible relevance of these findings in relation to drug choice in clinical trials of receptor sensitivity modification are discussed.