Kinetics of the release of noradrenaline from micropipettes: interaction between ejecting and retaining currents

Mesolimbic component of the ascending cholinergic pathways: electrophysiological-pharmacological study

The cholinergic input from the pontomesencephalic cholinergic neurons to the diencephalic and basal forebrain structures has been implicated in a number of limbically controlled overt behaviors. The cellular mechanism by which the cholinergic terminals initiate behavioral manifestations is not clear. The objective of this study was to investigate the effects of the ascending cholinergic projection from the laterodorsal tegmental nucleus (LDT) on neuronal firing in the anterior hypothalamic-medial preoptic region (AHMP), known to be involved in agonistic behavior. Experiments were performed on urethan-anesthetized rats. Iontophoretic application of carbachol (CCh) into the vicinity of single cells in the AHMP caused a dose-dependent decrease in the mean firing rate of 83% of units and an increase in 10% of units. The inhibitory effect of CCh, but not the excitatory effect, was reversed by iontophoretic pretreatment with scopolamine. The inhibition of the firing rate was repeatable for the same dose of CCh and dose dependent. Electrical stimulation of neurons in the LDT caused a comparable, current-dependent decrease in the mean firing rate of AHMP neurons that also was reversed by pretreatment of neurons in the AHMP with scopolamine. The antagonizing effects of scopolamine were reversible with time. The same units in the AHMP that inhibited their firing to stimulation of the LDT also responded with a similar inhibition to local iontophoretic CCh. Finally, the fluorescent carbocyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, (DiA), has been used as a retrograde axonal tracer and was injected into the recording sites immediately after the electrophysiological recordings. After 1 wk, DiA dye was found in numerous neurons in the LDT as shown by the fluorescence confocal microscopy. Results of the study suggest that LDT cholinergic neurons project and terminate in the AHMP and that their activation causes a decrease in the mean firing rate of the AHMP neurons. It is postulated that this inhibitory effect is implicated in the initiation of some of the behavioral patterns like defensive or alarm vocalization and behavioral inhibition.

Morphine modulation of GABA- and glutamate-induced changes of ventral pallidal neuronal activity

Microiontophoresis was used to investigate the influence of morphine on the GABA- and glutamate-evoked responses of ventral pallidal neurons recorded extracellularly from chloral hydrate-anesthetized rats. Of the GABA-sensitive neurons (50 of 69 tested) in the ventral pallidum, all displayed a decreased firing rate when GABA was applied, whereas all of the glutamate-sensitive neurons (29 of 40 tested) increased neuronal activity in the presence of glutamate. The majority of ventral pallidal cells tested (65 of 83) were sensitive to iontophoretically applied morphine, and both increases and decreases in neuronal activity were observed. The ability of morphine to alter the ratio between amino acid-evoked activity ("signal") and spontaneous firing ("noise") was used as an indicator of morphine modulation. A morphine subthreshold ejection current, i.e. one that did not change spontaneous firing rate, and a morphine ejection current that produced approximately 50% of the maximum opioid-induced neuronal response were chosen for this evaluation. When morphine was co-iontophoresed with GABA or glutamate, attenuation of the amino acid signal-to-noise ratio was generally seen, though some potentiations were observed. These changes were independent of the direction of morphine-induced changes in spontaneous firing rate. Both sub- and suprathreshold ejection currents were capable of affecting GABA- and glutamate-evoked responses. These data suggest that morphine is a robust ventral pallidal neuromodulator. As ventral pallidal amino acid activity is important in the integration of sensorimotor information, opioid modulation of amino acid transmission in the ventral pallidum may have a profound effect on this integration.

Electrophysiological evidence for excitatory 5-HT2 and depressant 5-HT1A receptors on neurones of the rat midbrain tectum

It has been claimed that the aversive behaviour induced by electrical stimulation of the midbrain tectum (MT) has validity as an animal model of panic attack. A great deal of evidence obtained from behavioural studies suggests that 5-HT2 mechanisms phasically inhibit the substrates of aversion in the MT. In order to test this hypothesis we employed the technique of microiontophoresis of drugs onto neurons of the MT to assess the identity of the receptors mediating the effects of 5-hydroxytryptamine (5-HT). The results obtained show that the majority of 5-HT responsive cells in MT are cells excited by 5-HT (72%). These cells were silent or showed very low spontaneous firing activity, whereas cells depressed by 5-HT showed high spontaneous firing activity at baseline. The 5-HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), buspirone and gepirone caused consistent reduction in the firing rate of cells depressed by 5-HT while they did not change the firing activity of cells excited by 5-HT. The excitatory effects induced by 5-HT on MT neurones were clearly attenuated by concomitant application of ketanserin, a highly specific 5-HT2 antagonist. Excitatory responses to DL-homocysteic acid were not affected by ketanserin. Previous administration of zimelidine, a selective 5-HT uptake inhibitor, caused a significant enhancement of the excitatory effects of 5-HT while similar application of gepirone did not affect the size of the excitatory responses to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic modulation of cholinergic responses in rat medial prefrontal cortex: an electrophysiological study

The neuromodulatory action of dopamine (DA) on acetylcholine (ACh)-evoked responses of prefrontal cortex (PFC) neurones were investigated electrophysiologically in rats anaesthetised with a combination of urethane and ketamine. Iontophoretic application of ACh-excited prefrontal cortex neurones. Concurrent application of DA (5-15 nA) resulted in complex changes in the ACh-evoked responses: (1) DA depressed spontaneous background discharges (designated as noise) proportionally more than the ACh-evoked discharges (designated as input signals), thus yielding an enhanced signal/noise ratio. This increase in signal/noise ratio by dopamine was reversed by iontophoretic application of the Da D2 antagonist sulpiride (20-50 nA). Nevertheless, iontophoretic application of D2 agonist quinpirole (5-35 nA) enhanced the ACh-evoked response, but was accompanied by some increase in spontaneous discharge, thus yielding no change in the signal/noise ratio. (2) DA also increased the signal/noise ratio by inducing a net increase of the ACh-evoked response but simultaneously suppressed the spontaneous activity of PFC neurones. This effect was more prominent following blockade of D1 receptors by SCH23390 (6 mg/kg, i.p.), suggesting that D1 receptors may normally inhibit D2 receptor function in the PFC. In addition, endogenous DA in the PFC did not play a significant part in modifying the ACh-evoked responses since the modulation of ACh-evoked response by DA or its D1 and D2 agonists was similar in both saline control and alpha-methyl-p-tyrosine-pretreated rats. (3) When ejected with larger iontophoretic current (16-35 nA), DA suppressed both the ACh-evoked and spontaneous discharge and this effect was mimicked by D1 agonist SKF38393 (5-15 nA). Taken together, these results suggest that complex dopaminergic modulation of the cholinergic responses of prefrontal cortex neurones are mediated by D1 and D2 receptors. This DA action may have a functional role in the cognitive-integrative processes occurring in the prefrontal cortex.

Iontophoresis of norepinephrine onto neurons of the pigeon's lateral geniculate nucleus: characterization of an inhibitory response

A group of neurons in the pigeon's lateral geniculate equivalent nucleus (LGNe) shows associative enhancement of their response to light during visually conditioned heart rate change. The source of the relevant unconditioned stimulus input to LGNe for this enhancement has been identified as the locus coeruleus (LC). Thus, we have begun to examine neurotransmitters synthesized in LC for possible involvement in associative modification of neuronal discharge in LGNe. As a first step, we have examined the responses of LGNe neurons to iontophoretic application of norepinephrine (NE) and identified the receptor mediating one response class. The majority of neurons in LGNe show inhibition of maintained activity in response to iontophoretic application of NE or its agonists. The potency of the NE agonists is alpha-methyl NE greater than epinephrine greater than NE greater than phenylephrine greater than isoproterenol. This profile is characteristic of an alpha 2-adrenoceptor. The alpha 2-agonist clonidine also induces inhibition of maintained activity. The alpha 2-antagonists WB-4101 and yohimbine block the NE-inhibition while the alpha 1-antagonist prazosin and beta-antagonist sotalol do not. It is thus suggested that the receptor mediating the NE-inhibition of maintained activity has the characteristics of an alpha 2-adrenoceptor.

The effects of strychnine on neurons in cat somatosensory cortex and its interaction with the inhibitory amino acids, glycine, taurine and beta-alanine

In area 3b of primary somatosensory cortex, neurons may be classified as either rapidly adapting or slowly adapting to sustained stimuli and may be differentiated further by the presence or absence of a receptive field and by their threshold of activation. It is also possible to use the rate of adaptation of the background activity to a sustained stimulus to divide the cortex into slowly adapting regions or rapidly adapting regions. By blocking GABA-mediated inhibition with iontophoretically administered bicuculline methiodide, others have observed an increase in receptive field size in rapidly adapting regions but not in slowly adapting regions. The present study was designed to look for a different inhibitory transmitter which might control receptive field size in slowly adapting regions. Iontophoretically delivered strychnine was employed as an antagonist because it interferes with glycine-like inhibitory transmitters such as glycine, taurine and beta-alanine. Pharmacological tests were performed on 157 neurons in two series of experiments. In the first series three effects were documented. (i) In rapidly adapting regions, the size of the receptive field increased in 11 out of 25 cases whereas none of the 20 receptive fields tested in slowly adapting regions enlarged. (ii) In 13 of 24 cases a receptive field was revealed for previously unresponsive neurons in rapidly adapting regions whereas only 5 of 22 unresponsive cells tested in slowly adapting regions developed a receptive field. (iii) In 15 of 25 cells with receptive fields tested in rapidly adapting zones, strychnine reduced the threshold for somatic stimuli but only 8 of 20 cells isolated in slowly adapting zones showed this effect. In a second series of experiments, the effect of beta-alanine, glycine and taurine was examined on neurons of the rapidly adapting regions. beta-Alanine and taurine reduced the excitability of all neurons tested. Glycine inhibited most neurons. However, strychnine only antagonized the inhibitory effects of beta-alanine on responses to peripheral stimuli (9 of 11 cases). When neurons could not be driven by peripheral stimuli, the inhibition of spontaneous or glutamate-induced activity could not be blocked by strychnine (0 of 18 cases). We suggest that glycine-like amino acids contribute to the control of receptive field size and the control of neuronal excitability in rapidly adapting regions but not in slowly adapting regions. Our data suggest that strychnine-sensitive synapses are limited only to a subset of cortical neurons driven by somatic inputs.

Evidence for depressant 5-HT1-like receptors on rat brainstem neurones

1. The technique of microiontophoresis was used to evaluate the contribution of 5-HT1-like, 5-HT2- and 5-HT3-receptors to the depressant effects of 5-hydroxytryptamine (5-HT) on neurones in the midline of the medullary brainstem of the rat in vivo. 2. Depressant responses to 5-HT were resistant to antagonism by the 5-HT2-receptor antagonist ketanserin and the 5-HT3-receptor antagonist MDL 72222 applied either microiontophoretically or administered systemically. 3. Microiontophoretic or systemic administration of the 5-HT antagonist metergoline, which shows nanomolar affinity for the 5-HT1-binding site, also failed to attenuate the depressant responses to 5-HT. 4. Systemic administration of high doses of methysergide (30-40 mg kg-1) attenuated the depressant responses to 5-HT but did not block depressant responses to GABA or excitatory responses to glutamate. 5. The depressant effects of 5-HT were potently mimicked by the 5-HT1-like receptor agonists 5-carboxamidotryptamine and 8-OH-DPAT. 6. These results indicate that neither 5-HT2-receptors nor 5-HT3-receptors are involved in the depressant effects of 5-HT on midline brainstem neurones. The depressant effects of 5-carboxamidotryptamine (5-CT) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and blockade of the response to 5-HT by high doses of methysergide suggests the involvement of 5-HT1-like receptors. The lack of effect of metergoline, however, indicates that this receptor may be different from any of the 5-HT1 binding sites yet described.

Evidence for excitatory 5-HT2-receptors on rat brainstem neurones

1. The technique of microiontophoresis was used to investigate the identity of the receptor mediating the excitatory effects of 5-hydroxytryptamine (5-HT) upon neurones in the midline of the medullary brainstem of the rat in vivo. 2. The 5-HT1-like receptor agonists 5-carboxamidotryptamine (5-CT) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) failed to excite the majority of neurones excited by 5-HT. The mobilities of 5-CT and 8-OH-DPAT when tested in vitro were found not to differ significantly from that of 5-HT, suggesting that the lack of effect of these agonists was not due to a lower rate of release from the microelectrodes. 3. The excitatory responses to 5-HT were attenuated by the 5-HT 2-receptor antagonists ketanserin and methysergide when applied microiontophoretically or administered intravenously (0.3 and 1 mg kg-1 respectively). Excitatory responses to glutamate and noradrenaline were not reduced. 4. The 5-HT3-receptor antagonist MDL 72222 failed to attenuate selectively the excitatory response to 5-HT when applied either by microiontophoresis or administered intravenously (1 mg kg-1). 5. Microiontophoretic application of the alpha 1-adrenoceptor antagonist prazosin did not attenuate excitatory responses to either 5-HT or noradrenaline. Intravenously administered prazosin (0.8 mg kg-1) also failed to attenuate excitatory responses to 5-HT, but did block excitatory responses to noradrenaline. 6. These results suggest that 5-HT2-receptors, but not 5-HT1-like receptors, 5-HT3-receptors or alpha 1-adrenoceptors, are involved in the excitatory response of midline medullary neurones to 5-HT.

beta-Phenylethylamine enhances single cortical neurone responses to noradrenaline in the rat

The firing rates of single neurones in the rat cerebral cortex were recorded using multibarrel glass microelectrodes, and the response to drugs applied by microiontophoresis was investigated. A greater number of cells responded to noradrenaline (NA) (30-66 nA) than to beta-phenylethylamine (PE) (30-100 nA). When responses were obtained to both, 90% of the neurones gave the same response to NA and PE. Applications of PE with small currents (0-12 nA) caused an increase in the response to NA without affecting the baseline firing rate or the response to acetylcholine, glutamate, GABA or 5-hydroxytryptamine. An increase was seen in both excitatory and inhibitory responses to NA. The enhancement lasted up to 39 minutes after the end of the PE application. Applications of NA with small currents (0-3 nA) failed to alter responses to NA. Possible mechanisms of the effect of PE on response to NA are discussed. These results provide further evidence for the hypothesis that trace amines can modulate catecholamine neurotransmission.

Involvement of M1-muscarinic receptors in the excitation of neocortical neurones by acetylcholine

The technique of microelectrophoresis was used to investigate the cholinoceptor pharmacology of spontaneously active single neurones in the parietal cortex of the rat. Acetylcholine, carbachol and the selective M1-muscarinic receptor agonist, McN-A-343, were each potent excitants (rank order of apparent potency: carbachol greater than acetylcholine greater than McN-A-343). When measured in vitro, the apparent mobilities of carbachol and acetylcholine were similar although significantly less than that of McN-A-343, suggesting that the lower potencies of acetylcholine and McN-A-343 probably reflect a genuine biological phenomenon. In addition to excitation, carbachol also evoked biphasic (excitation/depression) and depressant responses. In contrast to the other cholinoceptor agonists, nicotine produced weak and inconsistent excitations. Excitatory responses to acetylcholine and carbachol were significantly attenuated by the selective M1-muscarinic receptor antagonist, pirenzepine, at a time when the excitatory response to McN-A-343 was also significantly reduced. Responses to phenylephrine were not diminished. On several cells an excitatory response to carbachol was converted to a depression by pirenzepine. These results suggest that the excitatory responses of cortical neurones to cholinoceptor agonists are mediated predominantly by M1-muscarinic receptors. The identity of the receptor mediating the depressant response to carbachol remains uncertain, although nicotinic cholinoceptors do not appear to be involved.

Acetylcholine permits long-term enhancement of neuronal responsiveness in cat primary somatosensory cortex

Acetylcholine (ACh) was administered iontophoretically to single neurons in cat somatosensory cortex. Using extracellular recording techniques, neuronal responsiveness was determined at regular intervals from the number of action potentials produced either by iontophoretically applied glutamate or by tactile stimulation of the cutaneous receptive field. The responses were altered in only 21% (13/61) of the neurons following the application of ACh alone. In contrast, 75% (66/88) of the neurons displayed altered responses during administration of ACh simultaneously with either iontophoretically administered glutamate or with tactile stimulation of the receptive field. Forty-seven percent (29/62) of the responses potentiated in the presence of ACh remained enhanced for periods lasting from 8 min to over 1 h. The responsiveness of cortical neurons to afferent inputs changes during the reorganization of somatotopic maps that occurs after deafferentation, and perhaps during some forms of learning. As ACh has been implicated in some of these processes, it may be that the changes in responsiveness observed here following iontophoretically applied ACh are similar to those which facilitate the acquisition of neuronal responses to altered or novel afferent inputs.

Quantification of angiotensin iontophoresis

A method of value for studying the effects of angiotensin II (AII) and angiotensin III (AIII) on the brain is microiontophoresis combined with single unit recording. The purpose of this study was to quantitate the release of angiotensins under various experimental conditions thus providing a firm basis for the iontophoretic application of angiotensins. Quantification of release was accomplished by adding the appropriate [3H]angiotensin to 1 X 10(-3) M solutions of AII and AIII and then measuring the counts released from the tip of the microiontophoretic pipette in vitro into a small volume of Ringer solution. Although both AII and AIII were released by diffusion from micropipettes, this release could all but be eliminated with a retaining current of 20 nA. The release of AII and AIII was linear with respect to the amount of ejecting current applied up to 60 nA, the highest current examined. Angiotensin II at pH 4.5 and AIII at pH 3.5 were released at similar rates of 41.5 and 45.1 fmol/min/nA respectively. Raising the pH of the AII solution to 4.5 reduced the rate of release to 17.9 fmol/min/nA. Transport numbers were determined as follows: AII pH 3.5-0.115; AII pH 4.5-0.035, and AIII pH 4.5-0.145. It can be concluded that angiotensins are readily released by microiontophoresis, the response is linear with respect to application current, and that with the use of the appropriate pH the rate of release of AII and AIII are comparable.

Dark-reared cats: unresponsive cells become visually responsive with microiontophoresis of an excitatory amino acid

The visual system of kittens reared in total darkness is grossly abnormal. Although estimates vary, substantial proportions of cells in the visual cortex of these animals are unresponsive to visual stimulation. Additional cells are weakly responsive or erratic. We have considered the possibility that these neurons receive subthreshold input which might be evident if an excitatory neurochemical agent is applied during extracellular recording with a microelectrode. To test this notion, we have recorded from cells in the striate cortex of dark-reared kittens during microiontophoretic application of an excitatory amino acid, DL-homocysteate (DLH). Using this technique, we find that virtually all cells in the visual cortex of dark-reared kittens are responsive to visual stimulation. Prior to application of DLH, 27% of the cells were unresponsive to visual stimuli. Following iontophoresis of DLH, half of these cells responded with excitatory discharge to visual stimuli and the other half exhibited an inhibitory response in that the elevated maintained activity was suppressed during presentation of a visual stimulus. Additional cells from these animals, which were initially visually responsive, were also studied. For some of these units, responses were weak prior to administration of DLH and we were able to obtain a more clear estimate of selectivity for stimulus orientation during microiontophoresis of the drug. In these cases, and for the few cells which were initially responsive and orientation selective, we observed no major differences in selectivity before and after DLH application.

Excitatory neuronal responses to dopamine in the cerebral cortex: involvement of D2 but not D1 dopamine receptors

The technique of microelectrophoresis was used to evaluate the relative contribution of D1 and D2 dopamine receptors towards the mediation of the excitatory response of single neurones to dopamine in the somatosensory cortex of the rat. The selective D1 dopamine receptor agonist, SKF 38393, failed to excite any of the cells to which it was applied. In contrast, the selective D2 dopamine receptor agonist, LY 171555, excited the majority of cells tested. The apparent potency of LY 171555 was significantly lower than that of dopamine. When the mobilities of SKF 38393 and LY 171555 were assessed by an in vitro method, they were found to be at least as great as those of dopamine and phenylephrine, suggesting that the lack of effect of SKF 38393 and the lower apparent potency of LY 171555 compared to dopamine reflect genuine biological phenomena. The alpha 1-adrenoceptor antagonist, prazosin, discriminated between excitatory responses to the alpha 1-adrenoceptor agonist, phenylephrine, and LY 171555: responses to phenylephrine were more susceptible to antagonism than were those to LY 171555. The dopamine receptor antagonist, haloperidol, produced the reverse discrimination: responses to LY 171555 were more affected than were those to phenylephrine. Neither antagonist reduced the response to the control agonist, acetylcholine. When applied continuously with low ejecting currents, LY 171555 antagonized the excitatory response to dopamine while the response to phenylephrine was relatively preserved. The response to acetylcholine was unaffected. When similarly applied, SKF 38393 had no selective action on the response to dopamine. 6 These results suggest that D2 dopamine receptors are involved in mediating the excitatory neuronal response to dopamine in the cerebral cortex, whereas DI dopamine receptors are unlikely to be involved. LY 171555 appears to act as a partial agonist at D2 dopamine receptors in this test system.

Dual effects of the snake venom polypeptide vipoxin on receptors for acetylcholine and biogenic amines in Aplysia neurons

Vipoxin, a 13,000-dalton polypeptide component of Russell's viper venom, has a dual pattern of effects on the responses of voltage-clamped Aplysia neurons to acetylcholine and biogenic amines. Application of low doses of vipoxin by pressure ejection reversibly antagonized all three types of ionic response to acetylcholine and carbachol. The blockade by vipoxin of acetylcholine responses was not prevented by eserine. The order of susceptibility of acetylcholine responses to blockade by vipoxin was Na+ greater than K+ greater than Cl-. Low doses of vipoxin also produced a reversible potentiation of excitatory responses to dopamine with a slower time course of onset and recovery. Inhibitory responses to dopamine (Cl-, K+) and both excitatory and inhibitory responses to histamine and 5-hydroxytryptamine were little affected by vipoxin. Higher doses of vipoxin directly evoked current responses which were always of the same ionic type as that evoked by acetylcholine or carbachol. Responses to cholinergic agonists and vipoxin were both blocked by cholinergic antagonists but not by antagonists of biogenic amine receptors, which reversibly antagonized the responses to amines on the same cell. These results suggest that vipoxin, which has no demonstrated actions on vertebrate acetylcholine receptors, acts as a partial agonist at all three types of acetylcholine receptor in Aplysia neurons. Our observations thus provide evidence for some degree of phylogenetic difference between vertebrate and molluscan acetylcholine receptors.

Neuronal responses to noradrenaline in the cerebral cortex: evidence against the involvement of alpha 2-adrenoceptors

The technique of microelectrophoresis was used to test the hypothesis that alpha 2-adrenoceptors are involved in mediating the excitatory responses of single neurones to noradrenaline in the somatosensory cerebral cortex of the rat. In the first series of experiments the effects of two alpha 2-adrenoceptor antagonists, yohimbine and idazoxan (RX-781094), were compared on excitatory responses to noradrenaline, phenylephrine and acetylcholine. The response to noradrenaline was not more susceptible to antagonism by these drugs than the response to the alpha 1-adrenoceptor stimulant, phenylephrine. Yohimbine antagonized responses to all three agonists equally, while idazoxan antagonized responses to noradrenaline and phenylephrine equally with relative preservation of responses to acetylcholine. In the second series of experiments the effects of the selective alpha 2-adrenoceptor stimulant, UK-14304, were examined. UK-14304 produced weak and inconsistent excitations on a small number of cells; however, most of the cells did not respond to this drug. When applied continuously using low ejection currents, UK-14304 selectively and reversibly antagonized responses to noradrenaline and phenylephrine without affecting responses to acetylcholine. These results suggest that, in the somatosensory cortex of the rat, neuronal excitation to noradrenaline is unlikely to be mediated either wholly or partly by alpha 2-adrenoceptors. The antagonism of neuronal responses to noradrenaline and phenylephrine by idazoxan probably reflects the alpha 1-adrenoceptor antagonistic properties of the drug which is known to occur at higher concentrations. The low agonistic potency of UK-14304 and the antagonism of responses to noradrenaline and phenylephrine by UK-14304 suggest that this drug, like clonidine, may act as a partial agonist at alpha-adrenoceptors.

A comparison of the effects of morphine, enkephalin, kyotorphin and D-phenylalanine on rat central neurones

1 Morphine, Met-enkephalin, kyotorphin and D-phenylalanine have been applied by microiontophoresis to neurones in the globus pallidus and cerebral cortex of rats anaesthetized with urethane. 2 In the pallidum, most cells were inhibited by all the agonists, with a high correspondence between cells inhibited by Met-enkephalin and D-phenylalanine and by Met-enkephalin and kyotorphin. Whereas responses to Met-enkephalin were readily antagonized by naloxone, responses to kyotorphin and D-phenylalanine were not. 3 In the cerebral cortex a high proportion of cells was excited by all four agonists and antagonism by naloxone was less consistent than in pallidum. 4 It is concluded that the naloxone-reversible analgesic effects of kyotorphin and D-phenylalanine may be mediated indirectly, rather through an activation of opiate receptors.

Effects of ionophoresed noradrenaline on the spontaneous activity of neurones in rat primary somatosensory cortex

Changes in spontaneous activity of rat S1 cortical neurones with identified receptive fields were investigated in reply to ionophoresed noradrenaline (NA). Extracellular levels of NA were maintained constant by continuous electrochemical analysis at the carbon fibre recording tip of the multibarrel micro-electrode. In the absence of NA there were clear differences in spike amplitude, firing rate and pattern of firing of deep (800-1400 micron) and superficial (0-800 micron) cells. Superficial cells responded to low (5 X 10(-8) to 5 X 10(-7) M) NA concentrations with simple inhibition. Recovery occurred within a minute or so of extracellular NA concentrations falling below detectable (10(-8) M) levels. Increases in local concentration merely stopped cells firing. In contrast, cells located in the deep zone could often be excited by very low NA concentrations (less than 10(-8) M), with inhibition occurring at levels 10-100 times greater. Most cells, however, were inhibited, with threshold doses for a 50% change in firing rate much higher than for superficial cells. Some cells in the deep zone showed sustained increases in firing rate following an ionophoretic trial. This could occur for periods of up to 1 h after ceasing a trial. Such effects could be produced by levels as low as 10(-7) M-NA. Interspike interval analysis for deep cells suggested that their spontaneous activity resembled that established for slow-wave sleep. During and after excitation by NA the pattern of firing of small groups of these cells changed to that established for the waking state. The effect could persist for up to 1 h following a short (2-5 min) ionophoretic trial.

Inhibition of the activity of sympathetic preganglionic neurones and neurones activated by visceral afferents, by alpha-methylnoradrenaline and endogenous catecholamines

The responses of electrophysiologically identified sympathetic preganglionic neurones (SPGN) and neurones activated by visceral afferents (VA) to iontophoretic application of: (1) the intraneuronal metabolites of alpha-methyl-DOPA (alpha-MD): alpha-methylnoradrenaline (alpha-MNA) and alpha-methyldopamine (alpha-MDA); (2) noradrenaline (NA) and adrenaline (Ad); and (3) N-methyl-beta-hydroxy-phenylethylamine (NMPEA), were tested in the upper and lower segments of the thoracic spinal cord of the cat. alpha-Methylnoradrenaline, NA and Ad had an inhibitory action on the majority of spontaneously firing neurones. Inhibition of the activity of preganglionic neurones and neurones activated by visceral afferents induced by alpha-MNA was usually equal to the effect of NA, but in some neurones alpha-MNA was more potent. The transport numbers of both amines are similar. The alpha adrenoceptor antagonists, thymoxamine, piperoxan and yohimbine, antagonized the inhibitory effects of alpha-MNA in spontaneously-active preganglionic neurones and neurones activated by visceral afferents. Piperoxan antagonized also the inhibitory effects of NA. The inhibitory effect of alpha-MDA was weaker and that of NMPEA was much weaker than that of alpha-MNA and NA. The inhibitory effects of alpha-MNA and NA in the cerveau isolé preparation resembled those observed in anaesthetized animals. In reserpinized cats, with catecholamine levels in brain stem and spinal cord reduced by 98-99%, the inhibitory effects of alpha-MNA were preserved. It is postulated that alpha-MNA modulates the activity of preganglionic neurones and that this action, leading to a decrease in sympathetic output and mediated by alpha 1 and alpha 2 adrenoceptors, could be the most important factor in the antihypertensive effect of alpha-MD. The inhibition of the activity of neurones activated by visceral afferents by alpha-MNA indicates that alpha-MD may also attenuate the response of the central nervous system to the input from the heart and other organs.

The effect of microelectrophoretically applied clonidine on single cerebral cortical neurones in the rat. Evidence for interaction with alpha 1-adrenoceptors

The technique of microelectrophoresis was used in order to examine the effects of clonidine on single neurones in the somatosensory cortex of the rat, and to compare its actions with those of noradrenaline and phenylephrine. Clonidine evoked only excitatory responses on cortical neurones. The clonidine-sensitive neurones were also excited by noradrenaline and phenylephrine. Clonidine had a consistently lower apparent potency than either noradrenaline or phenylephrine. Responses to clonidine had a slower time-course than responses to the other two adrenoceptor agonists, both the latencies to onset and the recovery times being longer for responses to clonidine than for responses to noradrenaline and phenylephrine. When the mobilities of clonidine and phenylephrine were compared using an in vitro method, no significant difference was found between the mobilities of the two ionic species, suggesting that they have similar transport numbers. Thus the difference between the potencies and time-courses of responses to clonidine and phenylephrine are presumably of biological origin. Responses to clonidine were antagonised by microelectrophoretically applied prazosin; responses to phenylephrine were equally antagonised, while responses to acetylcholine were not affected. Clonidine could reversibly antagonise excitatory responses to both noradrenaline and phenylephrine, without affecting responses to acetylcholine. The results suggest that clonidine may act as a partial agonist at excitatory alpha 1-adrenoceptors on cortical neurones.

Comparison of the effects of ethylenediamine analogues and gamma-aminobutyric acid on cortical and pallidal neurones

1 The actions of ethylenediamine (EDA) and structurally related compounds were investigated by microiontophoresis in Wistar rats. 2 EDA inhibited, via a bicuculline-sensitive mechanism, the spontaneous firing rate of all cortical and pallidal cells tested. 3 The results with the analogues suggest that two amine groups are required for this neuronal depressant action whereas a carboxyl grouping is not. N-substitution reduces the depressant effect. The length of the molecule is also critical, more than 3 methylene components seriously reducing its effectiveness. A rigid analogue of EDA, piperazine, was also active. In addition the apparent transport numbers of EDA and gamma-aminobutyric acid (GABA) were calculated, showing a close similarity between the two. 4 The results are discussed wih respect to the possibility that EDA may represent a new class of GABA-mimetics, or may indicate the existence of a novel diamine receptor mediating bicuculline-sensitive inhibition in the rat CNS.

Comparison of the effects of methoxamine with those of noradrenaline and phenylephrine on single cerebral cortical neurones

1 The technique of microelectrophoresis was used to compare the actions of methoxamine, noradrenaline and phenylephrine on single neurones in the somatosensory cerebral cortex of the rat.2 Methoxamine evoked only excitatory responses on cortical neurones. The methoxamine-sensitive cells were also excited by phenylephrine; cells excited by methoxamine could either be excited or depressed by noradrenaline.3 Methoxamine appeared to be less potent than either noradrenaline or phenylephrine in evoking excitatory responses.4 Responses to methoxamine had a slower time course than responses to either noradrenaline or phenylephrine, both the latencies to onset and the recovery times being longer for responses to methoxamine than for responses to noradrenaline or phenylephrine.5 When the absolute mobilities of methoxamine, noradrenaline and phenylephrine were compared using an in vitro method, no significant differences were found between the mobilities of the three ionic species, suggesting that the three drugs have similar transport numbers. Thus the differences in potency between methoxamine and the other two drugs, and the difference between the time courses of responses to methoxamine and the other two drugs, are presumably of biological origin.6 The alpha-adrenoceptor antagonist, phenoxybenzamine, antagonized equally excitatory responses to methoxamine and noradrenaline, and responses to methoxamine and phenylephrine, without affecting responses to acetylcholine.7 When responses to methoxamine and noradrenaline and responses to methoxamine and acetylcholine were summated on the same cells, the net responses were smaller than those expected on the basis of additive effects; the deviation from additivity was greater in the case of the summation of responses to methoxamine and noradrenaline than in the case of summation of responses to methoxamine and acetylcholine. This observation is consistent with the hypothesis that the interaction between methoxamine and noradrenaline follows the model of competitive dualism, whereas the interaction between methoxamine and acetylcholine follows the model of functional synergism.8 The results suggest that methoxamine may act as a partial agonist at excitatory alpha-adrenoceptors on cerebral cortical neurones.

A procedure for comparing the mobilities of unlabeled drugs used in microelectrophoresis experiments

A novel method for comparing the absolute mobilities of unlabeled compounds released from micropipettes in microelectrophoresis experiments is described. The method is based on the principle that the introduction of a "foreign" ion into an electrolyte reduces the transport number of a "reference" ion present in the electrolyte. Using [14C]-noradrenaline as the "reference" ion, the mobilities of two "foreign" ions, methoxamine and phenylephrine, were compared. No significant difference was found between the mobilities of the two drugs. It was concluded that the two drugs probably have similar transport numbers when released from solutions of equal molarity in microelectrophoresis experiments in vivo, and thus the previously reported difference between the apparent potencies of the two drugs is presumably of biological origin. The method described here may be of use in comparing the mobilities of other compounds, the radiolabeled forms of which are either unavailable or prohibitively expensive.

Histamine actions on activity of cultured hypothalamic neurons: evidence for mediation by H1- and H2-histamine receptors

Tuberal hypothalamic tissue cultures were used to investigate the actions of histaminergic agents on neuronal activity using extracellular glass micropipettes. Histamine and H1- and H2-agonists were applied locally onto single active neurons by iontophoresis, while histaminergic antagonists were perfused through the bathing medium. Peri-event histogram and ratemeter analysis showed histamine to both excite and depress unit activity. Excitations were only antagonized by putative H1- and not H2-histamine antagonists, whereas inhibitions were antagonized by H2- and H2-antagonists. Dimaprit, a specific H2-agonist, elicited inhibitions of activity, while 2-(2-pyridyl)ethylamine, a putative H1-agonist, elicited both excitations and inhibitions. Two pharmacologically distinct populations of histamine receptors may exit in the hypothalamus: excitatory H2-receptors and inhibitory H2-receptors.

The physics of iontophoretic pipettes

A theoretical study has been made of the release of drugs from iontophoretic pipettes. The chief predictions are: (1) in the steady state, release becomes linear with current and independent of pipette geometry when the ejecting voltage exceeds approximately 100 mV; (2) release rises relatively slowly during an ejecting pulse to approach its steady state value. At the end of the pulse, release falls abruptly. This asymmetry of the release curve is greatly accentuated by the prior application of a retaining current. It provides an explanation for the characteristic time course of response commonly seen when drugs are applied iontophoretically to central neurones: (3) the above effects occur on a time scale which depends strongly on the geometry of the pipette. Release from pipettes of large tip diameter and nearly cylindrical bore, such as those generally used in the central nervous system, should be thousands of times slower than from pepettes of small tip diameter and larger taper angle. Experimental observations of the release of an ionized fluorescent compound quantitatively confirm prediction (1) and qualitatively confirm predictions (2) and (3).

The action of microelectrophoretically applied (3,4-dihydroxy-phenylamino)-2-imidazoline (DPI) on single cortical neurones

1. The technique of microelectrophoresis was used in order to compare the actions of the imidazoline derivative, (3,4-dihydroxy-phenylamino)-2-imidazoline (DPI), with those of dopamine and phenylephrine on single neurones in the cerebral cortex of the rat anaesthetized with halothane. 2. DPI and phenylephrine were almost exclusively excitatory, whereas dopamine could evoke both excitatory and depressant responses. 3. In the case of excitatory responses, DPI appeared to be more potent than dopamine, and was approximately equipotent with phenylephrine. 4. The dopamine antagonist, haloperidol, could discriminate between excitatory responses to DPI and dopamine: responses to dopamine were abolished, whereas responses to DPI, and to a control agonist, acetylcholine, were unaffected. 5. The alpha-adrenoceptor antagonist, phenoxybenzamine, antagonized equally excitatory responses to DPI and phenylephrine. Responses to acetylcholine were not affected. 6. It is concluded that DPI does not stimulate dopamine receptors on cortical neurones; the excitatory responses of these cells to DPI may be mediated by alpha-adrenoceptors.

Comparison of the responses of single cortical neurones to tyramine and noradrenaline: effects of desipramine

1 The technique of microelectrophoresis was used in order to compare the actions of tyramine and noradrenaline on single neurones in the cerebral cortex of the rat.2 Tyramine could both excite and depress cortical neurones. Each tyramine-sensitive cell was also sensitive to noradrenaline. There was a high correlation between the directions of responses to tyramine and noradrenaline, most cells excited by tyramine being excited by noradrenaline, and most cells depressed by tyramine being depressed by noradrenaline.3 In the case of both excitatory and depressant responses, tyramine appeared to be less potent than noradrenaline.4 Tyramine evoked ;slower' responses than noradrenaline, both the latencies to onset and the recovery times being longer for responses to tyramine than for responses to noradrenaline.5 When the rates of release of tyramine and noradrenaline from micropipettes were measured in vitro, no significant difference could be observed between the transport numbers of the two drugs. Thus the difference in potency between the two drugs, and the difference in the time courses of responses to the two drugs, are presumably of biological origin.6 Desipramine could discriminate between neuronal responses to tyramine and noradrenaline: responses to tyramine were antagonized, while responses to noradrenaline were either potentiated or unaffected. Responses to DL-homocysteic acid were not affected by desipramine.7 The results are consistent with the hypothesis that tyramine is an indirectly acting sympathomimetic amine in the brain, and desipramine acts by blocking the uptake of both tyramine and noradrenaline into presynaptic noradrenergic nerve terminals.

Responses of motoneurones to electrophoretically applied dopamine

1. The effect of electrophoretically applied dopamine upon motoneurone excitability has been investigated. Field potentials originating from antidromically activated motoneurones were recorded from the ventral horn of the rat lumbar spinal cord. 2. Field potentials showed an increase in amplitude following electrophoretic application of dopamine. Dopamine was shown to be less potent than noradrenaline and 5-hydroxytryptamine in producing these changes. 3. Measurement of the transport number of dopamine suggest that the relatively low potency of dopamine cannot be attributed to differences in ionic mobilities between the amines. 4. Electrophoretic application of alpha-flupenthixol was shown to discriminate between dopamine and 5-hydroxytryptamine responses. Dopamine responses were profoundly reduced. 5. Electrophoretically applied alpha-flupenthixol also discriminated between dopamine and noradrenaline. Noradrenaline responses were consistently potentiated by alpha-flupenthixol. The possibility is discussed that dopamine may not merely be a precursor for noradrenaline in the rat spinal cord.

The pharmacology of adrenergic neuronal responses in the cerebral cortex: evidence for excitatory alpha- and inhibitory beta-receptors

1. The technique of microelectrophoresis was used to compare the actions of a range of adrenoceptor agonists on single cortical neurones in the rat anaesthetized with halothane. 2. Phenylephrine and methoxamine were exclusively excitatory, whereas salbutamol was entirely depressant. Noradrenaline and isoprenaline could evoke both excitatory and depressant responses. Lower doses of isoprenaline usually evoked depressions, whereas higher doses, on the same cell, evoked excitatory responses. 3. The alpha-adrenoceptor blocking agents, phentolamine and phenoxybenzamine, reversibly antagonized excitatory responses to adrenoceptor agonists, without affecting depressant responses to adrenoceptor agonists or excitatory responses to acetylcholine. 4. The beta-adrenoceptor blocking agents, propranolol and sotalol, reversibly antagonized both depressant and excitatory responses to adrenoceptor agonists, without affecting responses to acetylcholine. When the effect of sotalol on excitatory and depressant responses to adrenoceptor agonists was compared on the same cell, the depressant responses could be selectively antagonized, without affecting the excitatory responses. 5. It is concluded that (a) responses of cortical neurones to adrenoceptor agonists are mediated by both alpha- and beta-receptors; (b) these alpha- and beta-receptors give rise to opposite effects: the alpha-receptors being excitatory and the beta-receptors being inhibitory; and (c) responses of many neurones reflect the presence of both types of receptor.

Effect of calcium removal on monoamine-elicited depressions of cultured tuberal neurons

The role of extracellular calcium in monoamine responses of central neurons was ivestigated using explant cultures of tuberal hypothalamus. The spontaneous activity of neurons in cultures was recorded in balanced salt and calcium-deficient salt solutions. This increased firing rate was counteracted by the addition of magnesium. Addition of magnesium also regularized the pattern of firing. Iontophoretic application of putative monoamine neurotransmitters reversibly decreased the rate of firing in both normal and calcium-deficient salt solutions. These results suggests that monoamine inhibitions are not primarily mediated by transmembrane calcium fluxes.

Effect of antipsychotic drugs on the firing of dorsal raphe cells. II. Reversal by picrotoxin

As reported in the preceding study, the ability of certain antipsychotic and adrenolytic agents to inhibit the spontaneous firing of serotonergic 5HT neurons in the dorsal raphe nucleus appeared to be related to adrenergic blocking efficacy. However, the interaction between adrenergic and serotonergic systems was apparently indirect. In this phase of the study we investigated the hypothesis that another transmitter system could mediate this interaction. We examined the effects of two inhibitory amino acid transmitters (GABA and glycine) for possible effects on dorsal raphe cell firing using single cell recording and microiontophoretic techniques. In addition, the ability of the GABA antagonist, picrotoxin and the glycine antagonist, strychnine to reverse the effects of the antipsychotic and alpha-blocking drugs on dorsal raphe firing was tested. Both GABA and glycine were found to inhibit raphe cell firing selectively, allowing for a possible neurotransmitter function for these amino acids within the dorsal raphe nucleus. However, picrotoxin but not strychnine was found to reverse the effects of the antipsychotic and alpha-blocking drugs on raphe firing. Based on these results, we propose that the adrenergic input may influence 5HT neurons indirectly via a GABAergic interneuron or interposed GABA neuron.

The action of microelectrophoretically applied L-3,4-dihydroxyphenylalanine (DOPA) on single cortical neurones

The technique of microelectrophoresis was used in order to compare the actions of L-3,4-dihydroxyphenylalanine (DOPA) and noradrenaline on single neurones in the cerebral cortices of cats and rats. DOPA could both excite and depress cortical neurones. Cells excited by DOPA were also excited by noradrenaline and cells depressed by DOPA were also depressed by noradrenaline. In the case of both excitatory and depressant responses, DOPA appeared to be less potent than noradrenaline. Responses to DOPA and noradrenaline could be antagonized by phentolamine and propranolol. Responses to acetylcholine were not affected. Responses to acetylcholine, but not responses to DOPA, were antagonized by atropine. The results indicate that locally applied DOPA may mimic the actions of noradrenaline on cortical neurones. Possible mechanisms for these effects of DOPA are discussed.

Dopamine evoked inhibition of single cells of the feline putamen and basolateral amygdala

1. In cats under pentobarbitone or halothane anaesthesia, neurones of the putamen and basolateral amygdala were inhibited with a similar time course by iontophoretic applications of dopamine and gamma-aminobutyric acid (GABA), ejected with relatively short (20 sec) low intensity (less than 40 nA) pulses of positive current from five and seven barrelled extracellular micropipettes. The use of a stereotaxically positioned guide tube, sealed to the skull with dental cement, made it possible to obtain stable recording conditions and to correlate the stereotaxic position of the cells with the position of the micro-electrode tracks determined histologically by the post-mortem reconstruction of serial sections. 2. Since in cats anaesthetized with pentobarbitone none of the cells were found to be spontaneously active, the relative potency of dopamine and GABA were compared on glutamate excited cells. Approximately 2-5 times more current was required to release sufficient dopamine to cause just submaximal inhibition, equal in magnitude and duration to that evoked by GABA. 3. In nitrous oxide/halothane anaesthetized cats, approximately one quarter of the cells were spontaneously active. Relative potency studies showed that for dopamine, currents 2-0 and 1-6 times larger than those used for GABA were required to inhibit glutamate excited and spontaneously active cells respectively. 4. When the depth distribution of the cells was compared with the sensitivity of the cells to dopamine and GABA, the most sensitive cells were found to lie within the putamen and the basolateral amygdala. 5. On more than one third of the cells tested, iontophoretic application of the neuroleptic, alpha-flupenthixol of more than 3 or 4 min in duration, greatly reduced or abolished the inhibition of the cells by dopamine without impairing their sensitivity to GABA. 6. In four cats, large I.V. injections of alpha-flupenthixol (10 mg/kg) and the more potent neuroleptic pimozide (1 mg/kg) had no significant effect on the dopamine or GABA sensitivity of seventy cells in the putamen and basolateral amygdala. 7. Our results are in keeping with the view that dopamine has a predominantly inhibitory action in the mammalian forebrain. However the failure of I.V. neuroleptics to modify the sensitivity of the cells to dopamine suggests that the dramatic effects of neuroleptics on animal behaviour may not be explicable simply in terms of a generalized blockade of dopamine receptors at post-synaptic sites.

Effects of iprindole on responses of single cortical and caudate neurones to monoamines and acetylcholine

1 The technique of microelectrophoresis was used to study the effects of iprindole on single neurones in the cerebral cortex and caudate nucleus of the rat. 2 Iprindole, when applied for a brief period, did not affect the firing rate of the vast majority of neurones tested. 3 Both potentiation and antagonism of neuronal responses to noradrenaline, dopamine, and 5-hydroxytryptamine could be observed after a brief application of iprindole. Potentiation and antagonism often occurred after the same application of iprindole, antagonism always preceding potentiation. 4 Responses to acetylcholine were affected by iprindole similarly: both potentiation and antagonism of the responses could be observed. 5 Responses to glutamate were not affected by iprindole. 6 It is concluded that the potentiation of responses to monoamines by iprindole cannot be explained on the basis of uptake blockade; this potentiation may be due to the blockade of masked receptors on the post-synaptic cell. 7. It is suggested that the common pharmacological action of the tricyclic antidepressants may be the ability to block both monoamine and acetylcholine receptors in the brain.

A quantitative microiontophoretic analysis of the responses of central neurones to noradrenaline: interactions with cobalt, manganese, verapamil and dichloroisoprenaline

1. A new experimental procedure has been devised for the study of pharmacological antagonism in the central nervous sytem using automated microiontophoresis to deliver pulses of agonists and computer-generated histograms to quantify the neuronal response. The system allows study of potential antagonists having direct depressant effects and also of neurones with irregular or slow discharge rates. 2. The histogram analysis reveals the necessity for regular, periodic delivery of agonists during the assessment of agonist-antagonist interactions. Without regular repetitive delivery, many agonists, such as noradrenaline, exhibit an apparent but artifactual decrease in inhibitory potency after an interruption of agonist pulses. Examples of this phenomenon are shown, using cerebellar Punkinje cells and cerebral cortical neurones in rats anaesthetized with halothane. 3. Preliminary results with these computer-generated drug response histograms revealed manganese, cobalt, and verapamil to be generally ineffective as antagonists of noradrenaline, despite their direct depressant effects. 4. Conversely, dichloroisoprenaline (DCI), a beta-adrenoceptor antagonist, was effective in blocking noradrenaline-induced depressions of firing in the cerebral cortex at doses which caused over 50% decrease in spontaneous discharge.

Effects of desipramine on neuronal responses to dopamine, noradrenaline, 5-hydroxytryptamine and acetylcholine in the caudate nucleus of the rat

1 The sensitivity of single neurones to microelectrophoretically applied dopamine, noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholing (ACh) was investigated in the caudate nucleus of the rat, anaesthetized with halothane. Both excitatory and depressant responses could be observed to each of the agonists. There was a high correlation between the direction of responses to dopamine and noradrenaline, whereas there was no significant correlation between the direction of responses to dopamine and ACh. 2 The effect of desipramine was studied on both excitatory and depressant responses to dopamine, NA and 5-HT, and on excitatory responses to ACh. Both potentiation and antagonism of neuronal responses to monoamines and ACh could be observed after a brief application of desipramine. 3 Excitatory responses to glutamate were not affected by desipramine. 4 The observation that responses to dopamine and NA can be potentiated by desipramine in the caudate nucleus suggests that uptake blockade is not a prerequisite for potentiation. 5 It is suggested that the potentiation of neuronal responses to dopamine by desipramine may be responsible for the therapeutic efficacy of desipramine in Parkinson's disease.

The effect of tricyclic antidepressants on cholinergic responses of single cortical neurones

1 The technique of microelectrophoresis was used in order to study the effects of tricyclic antidepressants on responses of single cortical neurones to acetylcholine. 2 Both potentiation and antagonism of excitatory responses to acetylcholine could be observed after a brief application of imipramine or desipramine. A higher dose of the antidepressant was required to evoke antagonism than to evoke potentiation. 3 Responses to carbachol were affected by desipramine similarly, suggesting the inhibition of cholinesterase is not responsible for the potentiation of cholinergic responses. 4 A brief application of atropine also had a dual effect on responses to acetylcholine. 5 It is suggested that the potentiation of excitatory cholinergic responses by atropine and the antidepressants may be due to the blockade of masked inhibitory receptors.

Effects of imipramine and desipramine on responses of single cortical neurones to noradrenaline and 5-hydroxytryptamine

1 The technique of microelectrophoresis was used in order to study the effects of imipramine and desipramine on single neurones in the somatosensory cortex of the cat, anaesthetized with halothane.2 Imipramine and desipramine, when applied for a brief period, did not affect the firing rate of the vast majority of the neurones tested.3 Both potentiation and antagonism of excitatory responses to noradrenaline could be observed after a brief application of either of the antidepressants. Four drug-interaction patterns could be distinguished: potentiation of immediate onset; potentiation reaching its maximum after a delay; antagonism followed by potentiation; antagonism followed by recovery.4 When different doses of the same antidepressant were applied, it was found that the drug-interaction patterns were related to the dose of antidepressant applied, a lower dose causing potentiation, and a higher dose antagonism of the response.5 Both potentiation and antagonism of depressant responses to noradrenaline could be observed.6 Both excitatory and depressant responses to 5-hydroxytryptamine were modified by imipramine and desipramine: a smaller dose of the antidepressant potentiated, and a higher dose antagonized the responses.7 Excitatory responses to glutamate were not affected by imipramine and desipramine.