Pharmacological properties of cholinoceptive neurones in the medulla and pons of the cat

Discriminative stimulus effects of mecamylamine and nicotine in rhesus monkeys: Central and peripheral mechanisms

Mecamylamine is a non-competitive nicotinic acetylcholine receptor (nAChR) antagonist that has been prescribed for hypertension and as an off-label smoking cessation aid. Here, we examined pharmacological mechanisms underlying the interoceptive effects (i.e., discriminative stimulus effects) of mecamylamine (5.6 mg/kg s.c.) and compared the effects of nAChR antagonists in this discrimination assay to their capacity to block a nicotine discriminative stimulus (1.78 mg/kg s.c.) in rhesus monkeys. Central (pempidine) and peripherally restricted nAChR antagonists (pentolinium and chlorisondamine) dose-dependently substituted for the mecamylamine discriminative stimulus in the following rank order potency (pentolinium > pempidine > chlorisondamine > mecamylamine). In contrast, at equi-effective doses based on substitution for mecamylamine, only mecamylamine antagonized the discriminative stimulus effects of nicotine, i.e., pentolinium, chlorisondamine, and pempidine did not. NMDA receptor antagonists produced dose-dependent substitution for mecamylamine with the following rank order potency (MK-801 > phencyclidine > ketamine). In contrast, behaviorally active doses of smoking cessation aids including nAChR agonists (nicotine, varenicline, and cytisine), the smoking cessation aid and antidepressant bupropion, and the benzodiazepine midazolam did not substitute for the discriminative stimulus effects of mecamylamine. These data suggest that peripheral nAChRs and NMDA receptors may contribute to the interoceptive stimulus effects produced by mecamylamine. Based on the current results, the therapeutic use of mecamylamine (i.e., for smoking or to alleviate green tobacco sickness) should be weighed against the potential for mecamylamine to produce interoceptive effects that overlap with another class of abused drugs (i.e., NMDA receptor agonists).

Evolving concepts of sleep cycle generation: From brain centers to neuronal populations

As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

Cholinergic effects on cat retina In vitro: changes in rod- and cone-driven b-wave and optic nerve response

To identify cholinergically mediated components in the optic nerve response (ONR) we studied effects of cholinergic agonists and antagonists in the arterially perfused cat eye. Acetylcholine, carbachol, scopolamine, quinuclidinylbenzilate and mecamylamine were applied intra-arterially in micromolar concentrations. Recordings of rod- and cone-driven ERG accompanied those of the ONR and revealed: (i) cholinergic agonists enhanced the b-wave, particularly under photopic conditions, whereas scopolamine decreased the b-wave. Mecamylamine induced biphasic effects (decrease followed by increase) in the amplitudes of the rod- and cone-driven b-waves. The effects on the cone-driven ERG were more marked than those on the rod-driven ERG. (ii) The ON-component of the ONR was increased, then decreased by acetylcholine. The cholinergic antagonists exerted complex changes in the ONR-ON component depending on dosage and adaptation. Scopolamine increased, then decreased the rod-driven ON-component, but mainly increased the cone-driven ON-component. Mecamylamine tended to increase the cone-driven, but to decrease the rod-driven ON-component of the ONR. (iii) The configuration of the rod- as well as for the cone-driven ONR, in particular the early plateau and OFF-components, were consistently and reversibly changed by cholinergic agonists, as well as by both muscarinic and nicotinic antagonists. Agonists decreased, and antagonists increased the amplitude of the plateau-component. We conclude that the ERG b-wave was enhanced by acetylcholine, but decreased by cholinergic antagonists. Cholinergic agonists and antagonists affect the same specific components of the ONR in a dose-related and reversible fashion, indicating a major contribution of cholinergic mechanisms to information processing in the cat retina.

Pharmacology of nicotinic receptor-mediated inhibition in rat dorsolateral septal neurones

1. Intracellular electrophysiological techniques were employed to investigate the effects of nicotinic receptor stimulation on rat dorsolateral septal nucleus (DLSN) neurones in a submerged rat brain slice preparation. 2. Acetylcholine (in the presence of the muscarinic antagonist, atropine), nicotine or dimethylphenylpiperazinium (DMPP), applied either by pressure ejection or superfusion, produced predominantly a membrane potential hyperpolarization. 3. Following concentration-response comparisons, DMPP appeared to exhibit fewer desensitizing properties and greater efficacy than nicotine with half-maximal hyperpolarizing responses attainable at 3 and 10 microM, respectively. 4. Pharmacological analyses revealed that the agonist-induced membrane hyperpolarization was sensitive to antagonism by mecamylamine (50-100 microM) and neuronal bungarotoxin (0.2-0.3 microM), but not alpha-bungarotoxin (0.5-1.0 microM), curare (10-50 microM) or dihydro-beta-erythroidine (50-100 microM). 5. Hyperpolarizing responses to DMPP were found to reverse near the equilibrium potential for potassium and were sensitive to changes in extracellular potassium concentration as predicted by the Nernst equation. Under single-electrode voltage clamp, application of DMPP produced an outward current (75-100 pA) which approached reversal at around -88 mV. These findings indicated that the hyperpolarizing response to nicotinic receptor stimulation was mediated by changes in membrane permeability to potassium. 6. DMPP-induced membrane hyperpolarization resulted from a direct action on postsynaptic DLSN neurones since the response persisted under conditions of superfusion with calcium-free/high-magnesium media or tetrodotoxin; both conditions blocked orthodromically induced neurotransmission. The hyperpolarizing response remained unaltered in TTX but was diminished in calcium-free/high-magnesium media. Further studies revealed blockade of the DMPP response following intracellular injection of EGTA. This response was also sensitive to antagonism by various calcium-dependent potassium channel blockers including apamin, barium and tetraethylammonium. 7. Our studies reveal a novel class of CNS nicotinic receptor whose action upon stimulation by an agonist results in a membrane hyperpolarization via a calcium-dependent increase in potassium ion conductance.

A cholinoceptive desynchronized sleep induction zone in the anterodorsal pontine tegmentum: spontaneous and drug-induced neuronal activity

The effect of carbachol microapplication (4 micrograms/250 nl per 90 s) on the discharge of neurons in the anterodorsal pons of four cats was studied using a newly devised microinjector-microelectrode assembly. Neurons were classified according to the magnitude of their discharge rate increases (or decreases) in physiological desynchronized sleep as desynchronized-on (or desynchronized-off) before injecting carbachol. When carbachol produced a desynchronized sleep-like state only half (15 out of 30) of the desynchronized-on cells were activated (desynchronized-on/desynchronized sleep-like state-on) while the other half were not (desynchronized-on/desynchronized sleep-like state-not on). Compared with the non-activated cells, the desynchronized-on/desynchronized sleep-like state-on cells had three features consistent with playing an active role in desynchronized sleep generation: these cells had a higher mean discharge frequency in desynchronized sleep and higher ratio of discharge frequency in desynchronized sleep compared with wakefulness; they did not fire in phase with electromyogram excitation of neck muscles; and they were concentrated in the short latency desynchronized sleep-like state induction zone described in the companion paper. The three-way correlation between the optimal anatomical site for short latency desynchronized sleep-like state induction, the selective neuronal discharge pattern in desynchronized sleep and the cholinergic activation pattern in the desynchronized sleep-like state suggest that we may have identified a neuronal population that is cholinoceptively activated as part of the physiological mechanism of desynchronized sleep generation.

Pharmacology of nicotine

In recent years progress in basic neuropsychopharmacology and clinical addiction research have allowed the conclusion that tobacco smoking essentially represents an addiction to nicotine. Parallel to this work, experimental research in biochemistry, physiology and pharmacology has provided detailed descriptions of the structure and function of the nicotinic receptor, the biologic mediator of the many actions of nicotine. This article reviews current knowledge of nicotinic mechanisms in the peripheral and central nervous systems as well as some implications for the notion of smoking as an addiction to nicotine. In particular this review will focus on the effects of nicotine on brain dopamine and noradrenaline systems since these neuronal systems appear to be crucially involved in the rewarding and stimulant effects of addictive drugs.

The effects of tobacco smoking on smooth pursuit eye movements

It has recently been shown that tobacco smoking in normal human subjects induces a transient primary-position upbeat nystagmus. We studied the effects of tobacco smoking on smooth pursuit eye movements and found defects in both vertical and horizontal tracking during the first 5 minutes after smoking one cigarette. The smooth pursuit defect consisted of a reduction in upward tracking velocity and the superposition of saccadic square-wave jerks on both vertical and horizontal tracking eye movements. The degree of impairment in upward smooth pursuit correlated with the intensity of tobacco-induced nystagmus present when recording in darkness. We suggest that these alterations are due to summation of nystagmus on normal tracking eye movements rather than primary defects in the smooth pursuit system.

Effect of parasympathetic blockade on ventilatory and cardiac depression induced by opioids

We have previously shown that delta-opioid agonists decrease ventilation and heart rate. Because of these results and the known interactions between opioid and acetylcholine metabolism, we hypothesized that opioids induce cardiorespiratory changes via the parasympathetic nervous system. To test this hypothesis, we administered atropine sulfate (systemically) at maximal effect of D-Ala-D-Leu-enkephalin (DADLE; a preferential delta-opioid agonist), injected intracisternally, and examined its effect on cardiorespiratory function. All experiments were performed on chronically instrumented and conscious adult dogs. Mean instantaneous minute ventilation or VT/TTOT decreased and PaCO2 increased after DADLE; atropine had little effect on these changes. Naloxone, even in small doses, reversed opioid effects on VT/TTOT and PaCO2. Atropine, however, reversed the DADLE-induced depression in cardiac rate. In doses that reversed this cardiac depression, atropine had no effect on cardiorespiratory function at rest, i.e., with no prior administration of DADLE. We conclude that DADLE decreases heart rate by increasing parasympathetic activity to the heart and induces hypoventilation by a different mechanism. We speculate that the opioid-induced ventilatory depression is due to either direct opioid action on central respiratory regulation or parasympathetic non-muscarinic or non-cholinergic mediating mechanisms.

Tobacco-induced primary-position upbeat nystagmus

The effects of tobacco smoking on the eye movements of normal human subjects were studied using direct current electrooculography and the magnetic search coil technique. Tobacco induced a transient primary-position upbeat nystagmus in the dark, which was suppressed by visual fixation. It obeyed Alexander's law and was associated with oblique upward fast phases that alternated from side to side. Tobacco-induced nystagmus exhibited a latency of onset at 40 to 90 seconds, a duration of 10 to 20 minutes, and maximum slow-phase velocities at 2 to 3 minutes. We suggest that tobacco induces primary-position upbeat nystagmus through the excitatory effects of nicotine on the central vestibular pathways.

Pontine neuronal response to local cholinergic infusion: relation to REM sleep

In behaving cats, a cholinergic agonist, carbachol, or Ringer's solution were infused into the pontine reticular formation (PRF). Extracellular microelectrode recordings were obtained from single neurons located 1-4 mm from the infusion site. Sleep polygraphic variables were also recorded. Carbachol microinfusions into the PRF area between the locus coeruleus and the genu of the seventh nerve readily triggered some or all of the tonic and phasic components of REM sleep. The resulting REM-like states had a mean duration of 30 min. In 56 reticular units studied during carbachol infusions, compared to Ringer's, we observed the following significant responses: 26.6% (16 cells) increased discharge rate, 55.4% (31 cells) decreased discharge rate, 12.5% (7 cells) showed no change, while 3.5% (2 cells) showed a biphasic response. Both incremental and decremental neuronal responses evoked by carbachol preceded or coincided with the onset of REM-like sleep periods, and persisted throughout these periods. The subpopulation of cholinoceptive PRF neurons accelerated during both spontaneous and carbachol-induced REM sleep is most likely to account for REM sleep triggering by carbachol. The PRF neurons found to be decelerated by carbachol displayed under control conditions, the accelerated discharge pattern during REM sleep which is typical of PRF neurons. Since either discharge acceleration or deceleration may occur during REM-like states the discharge pattern of this subpopulation of PRF neurons may not be critical for REM sleep generation.