Barbiturates block divalent cation action potentials in leech nociceptive cells

The involvement of adenosine neuromodulation in pentobarbital-induced field excitatory postsynaptic potentials depression in rat hippocampal slices

We investigated the contribution of adenosine neuromodulation to mechanisms of pentobarbital-induced depression of excitatory synaptic transmission in vitro. Transverse hippocampal slices were prepared from brains removed from isoflurane-anesthetized male Wistar rats. Field excitatory postsynaptic potentials (fEPSPs), elicited by orthodromic electrical stimulation of Schaffer collateral at 0.05 Hz, were recorded from the CA1 region in oxygenated artificial cerebrospinal fluid. Amplitude of fEPSP was analyzed for assessing drug effects. Pentobarbital (100 microM) transiently depressed fEPSPs (P<0.01); i.e., fEPSP was initially depressed to approximately 60% of control and then recovered to approximately 80% of control. The fEPSP depression was partially suppressed by pretreatment with 50 microM aminophylline, a nonselective adenosine receptor antagonist, and 0.2 microM 3, 7-Dimethyl-1-propagylxanthine, an adenosine A(1) receptor antagonist (P<0.01 each). However, the fEPSP depression was not affected by pretreatment with 10 microM 8-cyclopentyl-1, 3-dipropylxanthine, an A(2) receptor antagonist, or 10 microM bicuculline, a gamma-aminobutyric acid (GABA) A receptor antagonist. The results indicate that adenosine neuromodulation through A(1) receptors and other undefined mechanisms, which are independent from GABAergic mechanisms, are involved in pentobarbital-induced depression of excitatory synaptic transmission.

The anesthetic agents pentobarbital and chloralose block phase shifts of a neuronal in vitro circadian pacemaker

The anesthetic pentobarbital (6 mM) is capable of blocking light or high K(+)-induced phase shifts of the circadian pacemaker in the isolated eye of Bulla. Pentobarbital alone was effective in generating phase shifts consistent with phase response curves obtained to either extracellular low Ca2+ or hyperpolarizing pulses. Patch clamp recordings from the circadian pacemaker cells indicate that pentobarbital reduces the Ca(2+)-dependent K+ current. Together, these data suggest that pentobarbital acts on the pacemaker by reducing an inward Ca2+ current. Chloralose (3 mM) was effective in blocking light, but not high K(+)-induced phase shifts, and did not generate phase shifts when applied alone, suggesting that chloralose may act as a weak Ca2+ channel inhibitor.

Different actions of a short-acting barbiturate on sodium and potassium conductances in invertebrate and vertebrate neurons

In this study, the effects of methohexital are compared on the voltage-gated sodium (Na+) and potassium ion (K+) conductances of Retzius cells in the leech Macrobdella and of dorsal root cells of the chick in culture. Under current-clamp conditions methohexital prolonged the Na+-dependent action potential of neurons in the leech. This prolongation occurred in the absence of changes in resting membrane potential or the maximum rate of depolarization of the spike. The prolonged action potentials were identical to those recorded in the same neurons in the absence of outward currents [i.e. in Ca2+-free Ringer's solution containing Mn2+, tetraethylammonium chloride (TEA) and 4-aminopyridine (4-AP)]. They consisted of an initial spike, followed by a plateau lasting several hundreds of milliseconds. Both components of the action potential were Na+-dependent and resistant to tetrodotoxin (TTX), while the plateau was selectively blocked by saxitoxin (STX), suggesting that it originated from the flow of Na+ through a conductance different from that underlying the spike potential (Johansen and Kleinhaus, 1987). Similarly, the plateau of the action potential prolonged by methohexital, described in this study was abolished by 50 microM saxitoxin. These results suggest that the action of the drug resulted from a block of repolarizing K+-conductances. This was confirmed by voltage-clamp experiments which showed that methohexital (100-1000 microM) reduced both IK and IA in the Retzius cell, essential mimicking the combined effects of TEA and 4-AP (Johansen and Kleinhaus, 1986b). In contrast, in dorsal root cells, methohexital decreased the amplitude of Na+ and K+ currents equally. This modulation of ionic conductances by methohexital may be important for the sedative and anesthetic actions of the drug.

The effects of procaine, strychnine and penicillin on nociceptive neurons in leech segmental ganglia

Procaine, strychnine and penicillin selectively depolarized the membrane potential and prolonged the action potential recorded in the lateral but not the medial nociceptive (N) cell in the hirudinid leech Macrobdella decora. In contrast, procaine did not differentiate between medial and lateral N cells in two other hirudinid leeches Hirudo medicinalis and Haemopis marmorata. In these species, the drug equally decreased the amplitude of action potentials in both types of N cells without effecting their resting membrane properties. In the nociceptive neurons of the glossiphoniid leech Haementeria ghilianii which possesses only one type of N cell, procaine produced a depolarization and prolonged the action potential. This finding indicates that the single pair of N cells in Haementeria is of the lateral type. The results suggest that the lateral type N cell in Macrobdella and Haementeria share a unique Na+-dependent conductance which is selectively opened by the local anesthetics procaine and strychnine as well as by penicillin. This conductance is either not present or insensitive to the drugs in the homologous N cells in the two other leech species examined.