Stuart Fielding

Effects of 5-HT1A receptor agonists and NMDA receptor antagonists in the social interaction test and the elevated plus maze

The effects of several 5-HT1A agonists and excitatory amino acid antagonists were compared to the standard benzodiazepines, diazepam and chlordiazepoxide (CDP) in two assays predictive of anxiolytic activity, the social interaction and elevated plus maze procedures. Indicative of anxiolytic effects the 5-HT1A agonists, buspirone, gepirone and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) all significantly increased social interaction time and open arm exploration time in the social interaction and elevated plus maze procedures, respectively. Likewise, anxiolytic activity in these assays were also produced by the competitive N-methyl-D-aspartate (NMDA) antagonists, 2-amino-5-phosphonovaleric acid (AP-5), 2-amino-7-phosphonoheptanoic acid (AP-7), 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and the non-competitive NMDA antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) while NMDA produced anxiogenic effects. Furthermore, the anxiolytic effects of these agents were of equal magnitude to the benzodiazepines. These two classes of compounds were differentiated in the yohimbine-induced seizure assay, with the NMDA antagonists dose dependently antagonizing seizures similar to the benzodiazepines while the 5-HT1A agonists were inactive. These results suggest that the 5-HT1A agonists and the NMDA antagonists may be potential non-classical anxiolytic agents with different mechanisms of action.

Antinociceptive activity of clonidine and its potentiation of morphine analgesia.

Abstract The activity of clonidine and its interaction with morphine was assesed in the mouse tail flick assay. In this assay, clonidine was found to be 10 times more potent than morphine. Clonidine potentiated morphine antinociceptive activity approximately five-fold and morphine potentiated clonidine activity four-fold. Clonidines agonistic activity was not reversed by naloxone hydrochloride 910 mg/kg) while the potentiating effect of clonindine by morphine was. Tolerance to the antinociceptive effect of morphine was observed in morphine pellet-implanted mice byt no cross tolerance was observed for clonidine. These data indicate that clonidine-induced analgesia is not a result of an interaction at morphine receptors; but rather, common pathway(s) are present which appear to complement the agonistic interaction of each.

Evidence that GABA mechanisms mediate the anxiolytic action of benzodiazepines: a study with valproic acid.

Abstract Valproic acid and diazepam were tested for “anti-anxiety” activity in two animal tests known to predict the anxiolytic action of drugs. In one test, male hooded rats were trained to discriminate the “anxiomimetic” action of pentylenetetrazol from saline by responding on one of two levers for food reinforcement after pentylenetetrazol (1450 μmol/kg) injection and the other lever after saline injection. Once the pentylenetetrazol-saline discrimination was acquired, pretreatment with either diazepam (4.4–35.0 μmol/kg) or valproic acid (278–2220 μmol/kg) produced a dose-dependent antagonism of the “anxiomimetic” stimulus in this test. In the other test, male Wistar rats were trained to respond for milk reinforcement and to suppress those responses when the reinforcement was accompanied by the simultaneous delivery of foot shock. Treatment of these rats with diazepam (7–28 μmol/kg) or valproic acid (1110–2220 μmol/kg) antagonized the suppression of responding induced by the foot shock in a dose-dependent manner. In both tests, diazepam and valproic acid showed anxiolytic activity at doses which did not result in an overall suppression of responding. The demonstration of anxiolytic activity by the GABAmimetic drug, valproic acid, suggests that GABA mechanisms may mediate the anxiolytic action of benzodiazepines.

GABAmimetic agents display anxiolytic-like effects in the social interaction and elevated plus maze procedures

Benzodiazepine (BZD) anxiolytics, through their activation of the BZD-GABA receptor complex, display robust anxiolytic-like effects following systemic administration in both conditioned and non-conditioned behavioral procedures. The present results show that the GABAA agonists muscimol (0.5–1.0 mg/kg), THIP (2.5–10.0 mg/kg), and isoguvacine (25.0 mg/kg) as well as the GABA transaminase (GABA-T) inhibitor AOAA (aminooxyacetic acid; 5.0–20.0 mg/kg) following intraperitoneal administration exert anxiolytic-like activity of similar magnitude to that of diazepam in two nonconditioned procedures, namely the social interaction and the elevated plus maze tests. We have also extended our original findings that the anti-epileptic drug sodium valproate exerts an anxiolytic-like effect in the Geller conflict paradigm, to show this agents robust activity in the social interaction and elevated plus maze tests following systemic administration (100–400 mg/kg). These results show that GABAergic agents that facilitate GABA transmission are effective following systemic administration in non-conditioned anxiety procedures and may indicate potential therapeutic efficacy in certain anxiety states.

Behavioral Actions of Neuroleptics

The term “neuroleptic” was proposed by Jean Delay and Pierre Deniker in 1955 to the French Academy of Medicine. Delay and Deniker defined a neuroleptic as an agent which induced a syndrome of behavioral and psychic symptoms that is observed in patients after administration of chlorpromazine-like drugs. By this definition, the neuroleptic drugs are those chemical substances which produce a characteristic neuroleptic syndrome in man as, at that time, the typical effects of these drugs in animals were not known. It may be pointed out that the neuroleptic syndrome is a rather acute effect of these drugs. The long-term effect that is desired of neuroleptics is to provide a cure for schizophrenia.

Effect of valproic acid on anxiety related behaviors in the rat

Abstract Valproic acid was tested for anxiolytic activity in two behavioral tests most often used to predict anxiolytic activity in man. In one test, male hooded rats were trained to discriminate the anxiomimetic action of pentylenetetrazol by responding on one of two levers for food reinforcement after pentylenetetrazol injection and on the other lever after saline injection. Pretreatment of these rats with valproic acid (320 mg/kg) antagonized the discriminative stimulus produced by pentylenetetrazol. In the other test, male Wistar rats were trained to respond for milk reinforcement in a conflict procedure where some of the reinforced responses resulted in the simultaneous delivery of footshock. Treatment of these rats with valproic acid (320 mg/kg) antagonized the suppression of responding induced by the footshock. In both tests, valproic acid showed anxiolytic activity comparable to diazepam. These data suggest that valproic acid may be useful in treating clinical anxiety.

Mu-opioid component of the ethylketocyclazocine (EKC) discriminative stimulus in the rat

The opioid receptor selectivity of the EKC discriminative stimulus was characterized in Fischer rats trained to discriminate 0.3 mg/kg EKC (SC) from saline in a twochoice discrete-trial avoidance paradigm. The putative kappa-opioid receptor agonists EKC and U50,488H completely generalized with the EKC aue at doses of 0.3 and 10 mg/kg, respectively. The putative mu-opioid receptor agonists morphine (M) and fentanyl also dose-dependently generalized with the EKC stimulus. The generalization of M with EKC was not symmetrical, EKC and U50,488H produced little or no M-appropriate responding in rats trained to discriminate 3.0 mg/kg M (SC) from saline. This generalization pattern may reflect a lack of opioid receptor selectivity of the EKC stimulus. However, distinct muopioid and kappa-opioid components of the EKC cue could be identified using graded doses of naloxone in EKC-trained rats. The discriminative effects of morphine and fentanyl were blocked completely by doses of 0.1–1.0 mg/kg naloxone, whereas doses of naloxone 3–10 times greater were necessary to block the discriminative effects of EKC and U50,488H. These results suggest that EKC produces a complex discriminative stimulus with mu-opiod and kappa-opioid components that can be separated using antagonists such as naloxone.

The antinociceptive effects of GABA agonists and antagonists in mice

Abstract Changes in GABA-associated systems in brain and spinal cord have been implicated in morphine (MO) activity. The antinociceptive activity of the GABA agonists, muscimol (MUS) and aminooxyacetic acid (AOAA) and the GABA antagonists picrotoxin (PIC) and bicuculline (BIC) were investigated in the phenylquinone-induced abdominal constriction (PQ) and tail flick (TF) procedures. In PQ after SC administration, PIC and MUS were slightly less active than MO ( ED 50 =0.29 mg / kg ) while PIC and AOAA were 3 and 15 times less potent respectively. The activity of AOAA ( ED 50 =4.2 mg / kg ) was increased 5-fold after pretreatment with naloxone. Using subconvulsant doses, maximal increases in TF latencies ranged from 0.39 sec for MUS to 1.31 sec for PIC. Significant changes in TF latency between sham and spinally-transected mice was found only for picrotoxin (2.5 mg/kg). Although GABA systems are involved in the perception of various nociceptive stimuli no distinction in activity could be made between agonists and antagonists.

The effects of GABA agonists and antagonists on apomorphine-induced climbing behavior

Abstract Apomorphine administered at a dose of 1.5 mg/kg SC induces an intense climbing behavior in mice lasting for approximately 30 min. It is known that neuroleptics attenuate apomorphine climbing behavior (AC) but the influence of GABA agonists and antagonists on this behavior is unclear. The GABA agonist, muscimol, blocked AC with an ED50=0.9 mg/kg IP. Pretreatment (5 hr) with aminooxyacetic acid (50 mg/kg IP), a GABA-T inhibitor, produced a 65% inhibition of AC. The GABA antagonists, picrotoxin and bicuculline, had little effect on AC. Baclofen had no effect on AC from 0.5 to 4 mg/kg IP. Interaction studies, combining muscimol and haloperidol were conducted to evaluate the interaction between GABA-ergic and dopaminergic systems on AC. In the first studies mice were treated with haloperidol (0.04-0.15 mg/kg SC) then challenged with inactive (0.2 mg/kg IP) and active (0.4 mg/kg IP) doses of muscimol. In the second series of studies mice were treated with muscimol (0.25-1 mg/kg IP) then challenged with inactive (0.04 mg/kg SC) and active (0.08 mg/kg SC) doses of haloperidol. In all of these experiments, there was an enhanced suppression of AC. Our studies suggest that in addition to dopamine antagonists, GABA agonists are also capable of blocking AC. Our results further suggest that the effect of GABA agonists in inhibiting AC may be mediated through dopaminergic neurons.

Enhancement of salt intake by choline chloride

The ingestion of salt (NaCl) in most societies far exceeds levels that are considered necessary and safe. To reduce salt intake, a rat model of salt taste acceptance was created to identify novel, more palatable salt substitutes/enhancers. To induce salt craving, male Sprague-Dawley rats were placed on a low-salt diet and injected weekly (SC) with either 100 mg/kg of deoxycorticosterone acetate (DOCA) or its vehicle, peanut oil. Following 16 h of fluid deprivation, intake was evaluated in a one-bottle acceptance procedure. The DOCA-injected animals consumed large volumes of salt-containing solutions (up to 120 ml in 2 h) and showed sensitivity to and specificity for the salt taste (generally lacking acceptance of dextrose, KCl, ammonium chloride, and water solutions). Using this model, choline chloride was identified as having salt taste-enhancing properties. Choline chloride (0.1-0.7%) significantly enhanced the intake of a dilute salt solution (0.1%). The relevance of the rat model was demonstrated in human taste trials; choline chloride enhanced the palatability of and preference for foods containing this compound. The results of the present experiments support the use of this animal model for evaluating compounds for salt-like taste qualities and suggest that choline chloride will be an effective salt taste enhancer in man.