Detoxification of a local Microcystis bloom by an algicidal antibiotic from Oscillatoria late-virens.

Mouse toxicity due to microcystin (hepatotoxin) was detected in water bloom of a local lake, comprising mainly Microcystis aeruginosa. Both toxicity and biomass content of the phytoplankton varied seasonally. Incubations with an algicidal metabolite from Oscillatoria late-virens and a herbicide 3-(3,4-dichlorophenyl)-1, 1-dimethyl urea (DCMU) caused depigmentation and detoxification in bloom samples. Algicide-incubation also encouraged growth of protozoa and bacteria, pathogenic form included.

Autonomic actions of cocaine.

The development of our knowledge of the physiological, pharmacological, and biochemical actions of cocaine has in essence occurred in parallel with the development of our knowledge about the function of the autonomic nervous system. Cocaine is a sympathomimetic compound with potent local anesthetic properties. The principal hypothesis accepted to date to explain the sympathomimetic effects of cocaine is that this drug inhibits neuronal monoamine neurotransmitter reuptake by binding to a transporter or uptake site thereby increasing the effective concentration of neurotransmitter at adrenergic receptor sites. Much of the available evidence for this hypothesis has come from studies utilizing in vitro or in situ techniques. There have been relatively fewer studies examining the impact of cocaine on the autonomic nervous system in the intact animal. In addition, few studies have examined the effects of cocaine on central autonomic function. Past studies concerning the mechanism of action of cocaine are reviewed and recent data addressing the cardiovascular, respiratory, and central autonomic effects of cocaine are discussed.

Chronic cocaine reduces alpha 2-adrenoceptor elicited mydriasis and inhibition of locus coeruleus neurons.

The effects of chronic cocaine (50 mg/kg per day for two weeks) administration on two alpha 2-adrenoceptor-mediated responses were studied in rats. Chronic administration of cocaine significantly (compared to sham controls) attenuated the alpha 2-adrenoceptor-mediated inhibition of noradrenergic locus coeruleus (LC) neurons as well as alpha 2-adrenoceptor elicited mydriasis. Noradrenergic LC neurons from the cocaine treated and sham sham groups differed significantly in their responsiveness to the inhibitory effects of clonidine (ED50 values micrograms/kg: sham 7.35 +/- 1.13 and cocaine-treated 17.17 +/- 4.40, P less than 0.05). The ED50 values for the mydriatic response were sham 5.71 +/- 0.49 and cocaine-treated 16.42 +/- 0.69 micrograms/kg, respectively, P less than 0.001. No differences in cardiovascular responses to systemically injected clonidine between the chronic cocaine- and sham-treated groups were observed. Chronic cocaine treatment attenuates the two alpha 2-adrenoceptor-mediated responses most likely via an interaction with central catecholaminergic neurotransmission.

Cocaine-elicited mydriasis in the rat: pharmacological comparison to clonidine, D-amphetamine and desipramine.

Agents known to influence adrenergic function were examined for their mydriatic effects in urethane-anesthetized rats. Both the direct acting adrenergic agonist, clonidine, and the "indirect" acting agonists, cocaine, desipramine and amphetamine elicited mydriatic responses. The polar alpha-2 adrenoceptor agonist, 4-hydroxyclonidine, did not elicit a mydriatic response when administered systemically; however, it did produce a pronounced mydriatic response when administered i.c.v. Inasmuch as the selective lipophilic alpha-2 adrenoceptor antagonist, yohimbine, but not the polar alpha adrenoceptor antagonist, phentolamine, reversed the mydriatic effects of clonidine and cocaine, these data suggest that centrally located alpha-2 adrenoceptors elicit the above mydriatic response. The direct acting alpha-2 adrenoceptor agonists, clonidine (i.v.) and 4-hydroxyclonidine (i.c.v.), were the most efficacious of the agents studied in eliciting the mydriatic response. The indirect acting agents, amphetamine, desipramine and cocaine, were less efficacious. The rank order potency (ED-50) of these drugs was as follows: clonidine greater than desipramine greater than cocaine = amphetamine. The mydriatic effects of cocaine were attenuated by yohimbine and reserpine pretreatments. In addition, the local anesthetic, procaine, and the polar cocaine analog, cocaine methiodide, were significantly less efficacious than cocaine. These results suggest that cocaine elicits mydriasis by indirectly acting at central and postsynaptically located alpha-2 adrenoceptors.

Cocaine and central monoaminergic neurotransmission: a review of electrophysiological studies and comparison to amphetamine and antidepressants.

Psychomotor stimulants (e.g. cocaine and amphetamine) and many antidepressants are believed to elicit their psychotropic actions by interacting primarily with central monoaminergic neurons. The acute central neuronal effects of amphetamine and antidepressants have been extensively investigated in rats utilizing extracellular single unit electrophysiological and microiontophoretic techniques in vivo. In recent years the chronic effects of these compounds on the above neuronal systems have also been reported. Such investigations have proliferated because of the realization that the mechanisms underlying the psychotomimetic effects (e.g. amphetamine and cocaine) and mood elevation (i.e. antidepressants) observed with the administration of these drugs are more accurately reflected in chronic studies. For many years it has been assumed that cocaine and amphetamine produce very similar if not identical psychotropic effects through their actions on central monoaminergic neurotransmission. In terms of effects on single monoaminergic neurons, this assumption had gone by untested until two years ago, when the first report of the electrophysiological effects of cocaine on central monoaminergic (locus ceruleus) neurons appeared in the literature (61). This review discusses recent electrophysiological studies with cocaine at the level of single identified monoaminergic neurons and compares such data with that previously reported for amphetamine and antidepressants. In addition to identifying some of the similarities and differences between these compounds, this review also highlights some of the gaps in our knowledge regarding the effects of these drugs on central monoaminergic neurotransmission.

Reciprocal pre- and postsynaptic actions of cocaine at a central noradrenergic synapse.

Single-unit microelectrode studies were conducted to test the effects of systemic cocaine HCl on spontaneously firing single noradrenergic locus ceruleus (presynaptic) and cerebellar Purkinje (postsynaptic) neurons in rats in vivo. The spontaneous neuronal activity of all locus ceruleus neurons was inhibited by cocaine in a dose-dependent manner (0.5 to 2 mg/kg). These doses of cocaine elicited a predominant activation of postsynaptic Purkinje neurons. No effect of cocaine on neuronal action potential amplitude or slope was observed. Similar doses of the local anesthetic agent, procaine, did not affect action potential amplitudes or slopes of either locus ceruleus or Purkinje neurons. In addition, although cocaine elicited a significantly greater absolute change in the discharge rate of locus ceruleus neurons than of Purkinje neurons, the effects of procaine on those neurons were not significantly different from each other. The inhibition of locus ceruleus neurons by cocaine was significantly attenuated by pretreatment either with the alpha 2-adrenoceptor antagonist, yohimbine, or with reserpine. The activation of Purkinje neurons by cocaine was also significantly attenuated by reserpine pretreatment. Systemic cocaine administration (1 mg/kg, i.v.) did not potentiate the inhibitory effects of either locus ceruleus stimulation or local iontophoretic application of norepinephrine on Purkinje neuron discharge rate. We conclude that cocaine potently inhibits locus ceruleus neurons and this effect probably elicits Purkinje cell activation through disinhibition.

Alpha adrenoceptor modulation of the jaw-opening reflex.

In the present study, the nature, role and location of the alpha-adrenoceptors, modulating the tooth pulp stimulation (TPS)-evoked jaw-opening reflex (JOR), was investigated. Clonidine, a lipophilic alpha 2-adrenoceptor agonist, rapidly, reversibly and significantly elevated the threshold of the digastric electromyogram (dEMG) evoked by tooth pulp stimulation which was antagonized by yohimbine or piperoxane (two alpha 2-adrenoceptor antagonists) but not by prazosin (an alpha 1-adrenoceptor antagonist). 4-Hydroxyclonidine and ST-91 non lipophilic alpha 2 adrenoceptor agonists, were ineffective on the tooth pulp stimulation-evoked threshold of the digastric electromyogram. Clonidine was ineffective upon digastric muscle stimulation (DMS)-evoked threshold of the digastric electromyogram of the curarized digastricus and mylohyoid (motor) nerve stimulation (MNS)-evoked digastric EMG. It is concluded that modulation by clonidine of the tooth pulp stimulation-evoked jaw-opening reflex is devoid of a peripheral efferent component and that such modulation is predominantly expressed through interactions with centrally located alpha 2 adrenoceptors.

Cardiovascular effects of cocaine in anesthetized and conscious rats.

This study examined the cardiovascular and respiratory effects of cocaine and procaine in anesthetized and conscious rats. Intravenous cocaine (0.16-5 mg/Kg) elicited a rapid, dose dependent increase in mean arterial pressure of relatively short duration. In pentobarbital anesthetized (65 mg/Kg, i.p.) animals, the pressor phase was generally followed by a more prolonged depressor phase. These effects on arterial pressure were generally accompanied by a significant tachypnea and at larger doses (2.5 and 5 mg/Kg, i.v.), bradycardia. Procaine (0.31 and 1.25 mg/Kg, i.v.) produced similar cardiovascular and respiratory effects (depressor phase, tachypnea) in pentobarbital anesthetized animals. In conscious-restrained animals, both cocaine and procaine (1.25 mg/kg, i.v.) produced pressor responses. The subsequent depressor response was, however, absent in both cases. The cardiovascular effects of cocaine (0.25-1 mg/Kg, i.v.) in urethane anesthetized (1.25 g/Kg, i.p.) animals were essentially similar to those observed in conscious animals. Procaine (1mg/Kg) did not produce any significant cardiovascular effects in urethane anesthetized animals, but did elicit tachypnea. Reserpine pretreatment (10 mg/Kg, i.p.) did not significantly attenuate the pressor response in urethane anesthetized animals. Phentolamine pretreatment (3 mg/Kg, i.v.) did significantly antagonize the pressor effect in urethane anesthetized animals. These results suggest that: the depressor phase is likely due to a interaction between local anesthetic activity (cocaine and procaine) and barbiturate anesthesia, the cardiovascular effects of cocaine in conscious animals are more similar to those observed in urethane anesthetized rats than in pentobarbital anesthetized rats and the pressor effect in urethane anesthetized rats is apparently due to a reserpine resistant catecholaminergic mechanism.

Cocaine modulation of central monoaminergic neurotransmission.

Extracellular microelectrode studies were conducted to test the effects of cocaine HCl on the activity of spontaneously firing single serotonergic dorsal raphe (DRN), noradrenergic locus coeruleus (LC) and dopaminergic ventral tegmental (VTA) and zona compacta (ZC) neurons, and cerebellar Purkinje neurons (PC) in urethane anesthetized rats in vivo. Cocaine (0.0625-4 mg/kg) predominantly inhibited all of the central monoaminergic neurons and predominantly activated cerebellar Purkinje neurons. Cocaine (1 mg/kg, IV) failed to potentiate the inhibitory effects of LC stimulation on PC neurons. The temporal effects of intravenous cocaine on arterial pressure (i.e., pressor response) were not directly correlated with the effects on neurons. Cocaine did not decrease the amplitude or slope of neuron action potentials, and the effects of cocaine on firing rate were not shared by similar doses of procaine. Reserpine pretreatment (10 mg/kg, IP) attenuated the effects of cocaine (1 mg/kg, IV) on DRN, LC, and PC neurons. Specific adrenoceptor antagonists antagonized the inhibitory effects of cocaine on LC (piperoxane, yohimbine) and VTA (haloperidol) neurons. These results suggest that the central effects of cocaine on presynaptic monoaminergic neurons may in part be mediated by augmented monoamine neurotransmission at autoreceptors and that the effects of cocaine on postsynaptic target cells (PC) may be more complex, requiring the analysis of both pre- and postsynaptic elements.

The nociceptive jaw-opening reflex: evidence for alpha 2 adrenoceptor involvement.

The ability of alpha adrenoceptor agonists to modulate the tooth pulp stimulation evoked (TPS) jaw-opening reflex (JOR) was investigated in rats and rabbits. Low doses of clonidine (6.25-50 micrograms/kg, IV) significantly increased dEMG thresholds. These effects were antagonized by alpha 2 adrenoceptor antagonists (e.g., yohimbine), but not by alpha 1 adrenoceptor antagonists (e.g., prazosin) or mu receptor antagonists (e.g., naloxone). Polar alpha 2 adrenoceptor agonists (e.g., ST-91 and 4-hydroxyclonidine) that cross the blood brain barrier (BBB) poorly and lipophilic alpha 1 adrenoceptor agonists (e.g., ST-587) that cross the BBB easily were without affect on the TPS-JOR. Structures of the peripheral efferent neurocircuitry of the JOR (e.g., the digastric muscle and the neuromuscular junction of the digastric muscle and its motor nerve, the mylohyoid) were shown not to be active sites of clonidine's effect on the TPS-JOR. Treatment with phentolamine (an alpha adrenoceptor antagonist that poorly crosses the BBB) completely poorly crosses the BBB) completely antagonized clonidine's initial transient cardiovascular (pressor) effect without altering its TPS-JOR effects. Pretreatment with reserpine (a catecholamine depleting agent) failed to alter clonidine's affects on the TPS-JOR. Our studies suggest that alpha 2 adrenoceptors potently modulate the TPS-JOR and such modulation may be important in understanding trigeminal neuronal circuitries that partake in pain processing.

Electrophysiological actions of cocaine on noradrenergic neurons in rat locus ceruleus.

Extracellular microelectrode experiments were conducted to study the effects of cocaine HCl on the activity of spontaneously firing single locus ceruleus (LC) noradrenergic neurons in vivo. The responses of single identified noradrenergic LC neurons to the systemic (i.v.) administration of cocaine were observed over a wide range of doses (0.0625-2.0 mg/kg). The spontaneous activity of all LC neurons receiving doses greater than the threshold dose (0.0625 mg/kg) was inhibited in a dose-dependent manner. The local anesthetics, procaine and mepivacaine, did not affect LC neuronal activity, action potential amplitude or slope. The inhibitory effects of cocaine on spontaneous LC neuron activity was reversed by the subsequent i.v. administration of the specific alpha-2 adrenoceptor antagonist, piperoxone, but not the opiate receptor antagonist, naloxone. Pretreatment with another alpha-2 adrenoceptor antagonist, yohimbine (5 mg/kg i.p.), attenuated significantly the inhibition of spontaneous LC activity elicited by i.v. cocaine. Intravenous cocaine produced a brief increase in mean arterial pressure which did not appear to be correlated with the more sustained inhibition of LC neurons. Reserpine pretreatment (10 mg/kg i.p.) attenuated significantly the inhibitory effects of cocaine on LC activity. These results suggest that the inhibition of spontaneous LC neuron activity by i.v. cocaine is most likely mediated by an augmented action of catecholamines at central alpha-2 adrenoceptors and not by the local anesthetic effects of cocaine.

Electrophysiological effects of cocaine on central monoaminergic neurons.

The electrical activity of three different single, identified, spontaneously firing central neurons was monitored by extracellular microelectrodes. Intravenous cocaine administration (0.25-2 mg/kg) elicited an activation of cerebellar Purkinje neurons (PN) and an inhibition of serotonergic dorsal raphe (DRN) and noradrenergic locus coeruleus (LC) neurons. These effects were not temporally correlated with the increase in mean arterial pressure elicited by the intravenous administration of cocaine. In addition, the administration of procaine, a structurally related local anesthetic, did not significantly affect any of the three central neurons studied. Our results suggest that cocaine has potent effects on the activity of DRN, LC and PN neurons, which are not directly related to its cardiovascular or local anesthetic actions.

Evidence for alpha adrenoceptor modulation of the nociceptive jaw-opening reflex in rats and rabbits.

The jaw-opening reflex (JOR) in anesthetized rats and rabbits was the pain paradigm studied. The JOR was elicited by the electrical intrapulpal (left maxillary) stimulation and quantified by the measurement of threshold values for eliciting electromyograms (dEMGs) from the ipsilateral digastric muscle which served as the experimental nociceptive index. In both species, the threshold for the JOR was significantly elevated by the systemic administration of clonidine (12.5-50 micrograms/kg i.v.) and these JOR thresholds were inversely correlated with the frequency of stimulation. The analgesia elicited by clonidine was antagonized by pre- and postdrug treatment with the alpha-2 adrenoceptor antagonist yohimbine (1 mg/kg i.v.) but not the alpha-1 adrenoceptor antagonist prazosin (1 mg/kg i.v.) or the opiate receptor antagonist naloxone (1 mg/kg i.v.). The lipophilic alpha-1 adrenoceptor agonist St587 (100-400 micrograms/kg i.v.) had no significant effect on dEMG. Yohimbine did not antagonize the increase in dEMG elicited by morphine or pentobarbital. There was no direct correlation between the antinociceptive and cardiovascular effects of clonidine. Our results suggest that in the JOR nociceptive paradigm, clonidine elicits potent analgesia by activation of alpha-2 and not alpha-1 adrenoceptors.

Alpha 2 adrenoceptors modulate the nociceptive jaw-opening reflex in rats.

The jaw-opening reflex in lightly anesthetized rats induced by intrapulpal (left maxillary) electrical tooth pulp stimulation and quantified by the electromyograms (threshold values) recorded from the ipsilateral digastric muscle was used as the experimental pain index. The threshold for the jaw-opening reflex was significantly elevated by clonidine (12.5 to 50 micrograms/kg, i.v.) and was inversely correlated with the frequency of stimulation. The analgesia elicited by clonidine was antagonized by pretreatment with the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.v.). All doses of clonidine produced an initial transient pressor response followed by a sustained hypotension and bradycardia. However, there was no direct correlation between the antinociceptive and cardiovascular effects of clonidine. It is proposed that clonidine modulates jaw-opening reflex analgesia by specifically activating alpha 2-adrenoceptors.