Denervation supersensitivity to serotonin in rat forebrain: single cell studies

Noradrenergic denervation prevents sensitization of rat forebrain neurons to serotonin by tricyclic antidepressant treatment

The effect of prior lesioning of noradrenergic neurons with 6-hydroxydopamine (6-OHDA) on the ability of amitriptyline to enhance the responsiveness of hippocampal pyramidal neurons to serotonin (5-HT) was examined in Sprague-Dawley rats. In control rats, amitriptyline (5 mg/kg/day for 14 days), administered either by daily injection or by an osmotic minipump, enhanced the responsiveness of these neurons to microiontophoretically applied 5-HT, leaving their responsiveness to norepinephrine (NE) and to acetylcholine unaltered. However, in 6-OHDA-pretreated rats, amitriptyline failed to enhance the responsiveness of hippocampal pyramidal neurons to 5-HT. The effect of NE and, to a lesser extent, that of 5-HT were prolonged in 6-OHDA-pretreated rats, presumably because NE and 5-HT released by microiontophoresis are taken up by NE terminals in the intact rat. The present results provide additional evidence that the integrity of the NE system is required for antidepressant treatments to modify 5-HT neurotransmission.

Electrophysiological evidence for locus coeruleus norepinephrine autoreceptor subsensitivity following subchronic administration of D-amphetamine

The effect produced by subchronic administration of D-amphetamine (D-AMP) on the sensitivity of norepinephrine (NE) autoreceptors in the rat locus coeruleus (LC) was studied by means of single unit recording and microiontophoretic techniques. Twice daily i.p. administration of 5 mg/kg D-AMP for one week markedly reduced the ability of i.v. D-AMP and microiontophoretic application of clonidine to suppress the firing of LC NE neurons, suggesting strongly that NE autoreceptors became subsensitive. In addition, the firing pattern of NE neurons became 'disorganized' following subchronic AMP treatment.

Does cholecystokinin potentiate dopamine action in the nucleus accumbens?

Extracellular single unit recording and microiontophoretic techniques were used to determine whether sulfated cholecystokinin octapeptide (CCK-8S) potentiates the inhibitory action of dopamine (DA) on neurons in the nucleus accumbens of rats. The results show that CCK-8S attenuated, rather than enhanced the action of DA. The interaction between CCK-8S and DA does not appear to be specific because similar antagonizing actions between CCK-8S and serotonin were observed. In addition to opposing the inhibitory effects of DA and serotonin, CCK-8S potentiated the activation of glutamate. It is concluded that CCK-8S and DA exerted their actions independently via depolarizing and hyperpolarizing the neuronal membrane of the NAc neurons, respectively.

Circadian variation in sensitivity of suprachiasmatic and lateral geniculate neurones to 5-hydroxytryptamine in the rat

Extracellular single-unit recordings were obtained from neurones in the suprachiasmatic nuclei (s.c.n.) of the rat (a putative circadian pace-maker), the ventral lateral geniculate nucleus (v.l.g.n.) and the hippocampus. These areas receive a 5-hydroxytryptamine (5-HT) innervation from the raphe nuclei. Recording of neuronal activity in the s.c.n., v.l.g.n. and the hippocampus revealed a diurnal variation in the response to the ionophoresis of 5-HT. This variation was manifest as a 2-3-fold increase in post-synaptic sensitivity to 5-HT during the subjective dark (active) phase of the circadian cycle. In contrast there was no apparent circadian variation in the sensitivity of s.c.n., v.l.g.n. or hippocampal neurones to ionophoresed gamma-aminobutyric acid (GABA). Neuronal activity recorded in the s.c.n., v.l.g.n. and hippocampus also exhibited a circadian variation in the recovery from 5-HT-induced suppression of firing. This may reflect reuptake processes as recovery can be prolonged by ionophoresis of uptake blockers (imipramine or fluoxetine). Rats (n = 15) expressing circadian arrhythmicity in their rest-activity behaviour induced by long-term continuous illumination (150-200 lx) showed no apparent circadian variation in 5-HT sensitivity. This loss was accompanied by either the development of a 5-6-fold subsensitivity to ionophoresed 5-HT (eleven out of fifteen rats) or a 2-3-fold supersensitivity to ionophoresed 5-HT (four out of fifteen rats). A similar loss of circadian variation and the development of a subsensitivity to ionophoresed 5-HT was also found in three rats sustaining complete electrolytic lesions of the s.c.n. These changes were not found in rats (n = 4) with partial s.c.n. lesions. These results implicate the s.c.n., or fibres passing through it, in the circadian modulation of 5-HT sensitivity in neurones both intrinsic to the s.c.n. circadian pace-maker itself and in the hippocampus and lateral geniculate nucleus (regions remote from the s.c.n.).

Decreased effectiveness of GABA-mediated inhibition in the inferior colliculus of the genetically epilepsy-prone rat

The inferior colliculus (IC) is a critical site for induction of audiogenic seizures in the genetically epilepsy-prone rat (GEPR). Abnormal response properties observed in inferior colliculus neurons of that strain include a high incidence of onset-offset responses which may be a form of afterdischarge. These response abnormalities may involve altered actions of neurotransmitters in that region of the brain. GABA is implicated as a transmitter in endogenous sound-induced inhibition in the inferior colliculus. Endogenous inhibition and the actions of agents that affect GABA receptors were examined in inferior colliculus neurons in epileptic and normal rats. The iontophoretic dose (current) of GABA required to suppress neuronal firing in the epilepsy-prone rat was significantly greater than that required in neurons of the normal rat. A form of endogenous (binaural) inhibition in inferior colliculus neurons, which is proposed to be GABA-mediated, was also significantly reduced in the epilepsy-prone rat as compared with the normal rat. A benzodiazepine (flurazepam) which enhances the action of GABA in many brain sites including the inferior colliculus was significantly less effective than normal when applied iontophoretically onto the same neurons of the epilepsy-prone rat. The GABAA antagonist, bicuculline, which blocks the effect of GABA on inferior colliculus neurons, frequently induced the onset-offset response in neurons not previously exhibiting this pattern. These findings suggest that the reduction of GABA-mediated inhibition in the genetically epilepsy-prone rat may result in the increased incidence of afterdischarges in neurons of the inferior colliculus and may serve as an important mechanism of epileptogenesis in audiogenic seizures in this genetic form of epilepsy.

Altered responsiveness to substance P and 5-hydroxytryptamine in cat dorsal horn neurons after 5-HT depletion with p-chlorophenylalanine

The responsiveness of functionally identified cat spinal dorsal horn neurons to iontophoretically applied substance P (SP) and 5-hydroxytryptamine (5-HT) has been investigated by means of extracellular recording after 5-HT depletion with p-chlorophenylalanine (p-CPA). In addition, the spinal levels of 5-HT, SP, cholecystokinin octapeptide, neurotensin, and vasoactive intestinal polypeptide have been measured in intact and p-CPA-pretreated cats. In the present study we have demonstrated an altered responsiveness of dorsal horn neurons to locally applied SP and 5-HT. We found in p-CPA-pretreated cats that the proportion of neurons responding with excitation to SP and 5-HT was significantly increased. At the same time, depression induced by 5-HT in the dorsal horn cells was virtually absent in p-CPA-pretreated animals. Our finding that spinal level of 5-HT was significantly decreased in p-CPA-treated animals is consistent with previous studies. No convincing alteration in the spinal levels of 4 analyzed peptides was found in p-CPA-treated animals. The present study has shown that pharmacological depletion of 5-HT has two major effects: (1) it increases significantly the proportion of dorsal horn neurons excited by SP and 5-HT; and (2) it is ineffective in inducing 5-HT supersensitivity. Further work is needed to explain mechanisms involved in these effects.

Serotonin-lesion myoclonic syndromes. I. Neurochemical profile and S-1 receptor binding

This paper and the following one describe the effects of L-5-hydroxytryptophan (5-HTP) (after 3 intracisternal injections of 5,7-dihydroxytryptamine (DHT], fenfluramine (FF), p-chloroamphetamine (PCA) and drug combinations on (i) brain regional amine concentration (HPLC with LEC) and serotonin S-1 receptor binding; and (ii) 'serotonergic' behaviors in the same adult rats. Serotonin (5-HT) neurotoxins produced significantly different regional profiles of 5-HT depletion. Multiple DHT injections caused a 90-100% depletion of 5-HT concurrently in neocortex, hippocampus, striatum, septum/accumbens, pons, cerebellum, and cervical cord. Only PCA significantly depleted midbrain. Drug combinations with DHT resembled DHT alone rather than additive depletions, except for PCA + DHT, which produced a hybrid pattern of depletion. The S-1 binding assay, using cold 5-HT to displace [3H]5-HT, was performed with and without ascorbate, EDTA, CaCl2, and pargyline. Without ascorbate, binding was specific, saturable, region-dependent, and non-linear with high (Kd 1-3 nM) and low affinity (10-20 nM) components but no cooperativity (0.8 less than nH less than 1.0). Bmax and Kd did not differ significantly between vehicle- and drug-treated animals in neocortex, hippocampus, striatum, thalamus, hypothalamus, midbrain, pons, medulla, cervical cord, cerebellum, or septum/accumbens two weeks after lesioning, while the assay did detect a 60% reduction in Bmax induced by ascorbic acid (1 mM). The effects of assay conditions exceeded the changes sometimes reported in S-1 receptor Bmax after 5-HT lesions.

Para-chlorophenylalanine prevents feeding induced by the serotonin agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT)

The effects of para-chlorophenylalanine pre-treatment (PCPA, 150 mg/kg IP daily for 3 days) on feeding and stereotyped behaviour elicited by the serotonin agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in rats were investigated. PCPA depleted brain serotonin and 5-hydroxyindoleacetic acid concentrations by 90% and increased feeding during a 2-h day-time test. 8-OH-DPAT (60-4000 micrograms/kg SC) increased food intake in control animals but decreased it in PCPA-treated animals during the 2-h test. PCPA treatment had no effect on 8-OH-DPAT-induced locomotion or serotonin-related stereotyped behaviour (i.e. forepaw treading, headweaving, wet dog shakes, etc). Since PCPA prevents the operation of pre-synaptic serotonergic mechanisms, the failure of 8-OH-DPAT to increase food intake in PCPA-treated rats suggests that 8-OH-DPAT-induced hyperphagia is autoreceptor mediated.

8-OH-DPAT-induced hyperphagia: its neural basis and possible therapeutic relevance

The pharmacological and neurochemical bases of hyperphagia induced by the serotonin agonist 8-OH-DPAT were examined. In addition, the possible therapeutic potential of 8-OH-DPAT and related drugs in the treatment of anorexic pathology was assessed in an animal model of anorexia (as induced by acute immobilization stress). In normal rats 8-OH-DPAT elicited feeding after peripheral injection and after intracerebral application to the brainstem raphé nuclei. Feeding elicited by peripheral injection of the drug was attenuated by pretreatment with the serotonin synthesis inhibitor para-chlorophenylalanine. Following a hyperphagic dose of 8-OH-DPAT, brain serotonin metabolism was reduced, particularly in midbrain and pons-medulla. Our interpretation of these data is that 8-OH-DPAT elicits feeding via an agonist action on serotonin autoreceptors in the raphé nuclei. These receptors are probably of the 5-HT1A subtype as 8-OH-DPAT has a high affinity for this receptor and other putative 5-HT1A agonist (i.e. buspirone, TVX Q 7821) also elicit feeding. In contrast, putative 5-HT1B agonists (i.e. RU-24969 and quipazine) decrease feeding and cause anorexia. 8-OH-DPAT and other 5-HT1A agonists attenuated the anorexia and body weight loss caused by immobilization stress. Therefore, it seems possible that 5-HT1A agonists may be clinically useful in the treatment of anorexia.

Further evidence for, and nature of, the facilitatory GABAergic influence on central noradrenergic transmission

In order to explore the nature of the facilitatory GABAergic control of cerebral noradrenergic neurons, we have studied the effect of a variety of GABA mimetics (given systemically or injected locally into brain areas containing noradrenergic cell bodies or terminals) on several indices of noradrenaline turnover in the rat brain. Systemic administration of both direct and indirect acting GABA mimetics enhanced; 1) the pargyline induced accumulation of normetanephrine in the hypothalamus; 2) total DOPEG levels in a number of brain regions innervated by noradrenergic neurons; 3) both DOPAC and MOPEG levels in noradrenergic cell body areas (A1, A2 and A6). These effects are probably mediated by GABAA receptors as specific GABAA or mixed GABAA/GABAB agonists but not the GABAB agonist baclofen enhanced noradrenaline turnover. Interruption of noradrenergic impulse flow (by local injection of tetrodotoxin or by hemitransection) blocked the ability of progabide to increase DOPEG concentrations in the hypothalamus and cerebral cortex. Similarly, the co-administration of clonidine with progabide antagonized the progabide-induced increase in hypothalamic total DOPEG levels. Co-administration of yohimbine with progabide provoked an additive effect on hypothalamic DOPEG levels at moderate but not at high doses of yohimbine. Thus, the acceleration of noradrenaline turnover induced by GABA mimetics appears to depend on ongoing activity in noradrenergic neurons and occurs via an increase in neuronal discharges. Local injection of muscimol into the nucleus accumbens or hypothalamus failed to affect DOPEG levels in these structures; similarly, local injection of muscimol into the locus coeruleus failed to modify DOPEG levels in corresponding noradrenergic projection areas. These data indicate that the GABAergic influence is not exerted via GABA receptors located on noradrenergic cell bodies or nerve endings. Furthermore, since systemically administered progabide still increased hypothalamic DOPEG levels after ibotenate-induced destruction of the hypothalamic neuronal cell bodies, a presynaptic modulation of noradrenergic neurons by local GABAergic interneurons is excluded. Chemical destruction of serotoninergic pathways or enhancement of 5-HT transmission by quipazine failed to alter the ability of progabide to increase cerebral DOPEG levels. Moreover, scopolamine or naloxone also failed to affect the progabide-induced increase in cerebral DOPEG levels.(ABSTRACT TRUNCATED AT 400 WORDS)

A strong influence of serotonin axons on beta-adrenergic receptors in rat brain

The role of serotonin axons in modulating the norepinephrine neurotransmission system in rat brain was investigated. Selective lesions of the forebrain serotonergic system were made by injecting 5,7-dihydroxytryptamine into the midbrain raphe nuclei. Four to six weeks after the lesion, the uptake of 3H-labeled serotonin in the frontal cortex and the hippocampus was reduced by more than 90 percent, while neither the uptake of 3H-labeled norepinephrine nor the content of norepinephrine was affected in either tissue. The number of beta-adrenergic receptors, as measured by radioligand binding with 3H-labeled dihydroalprenolol, was increased in the frontal cortex and hippocampus of rats with lesions. Similarly, specific lesions of central serotonin axons produced by systemically administered p-chloramphetamine resulted in an increase in the binding of 3H-labeled dihydroalprenolol to beta-adrenergic receptors and in the production of adenosine 3',5'-monophosphate in response to isoproterenol. These results indicate that serotonin axons may regulate beta-adrenergic receptor number and function in brain.

Serotonin turnover and supersensitivity after neonatal 5,7-dihydroxytryptamine

Adult awake rats which received neonatal pargyline and 5,7-dihydroxytryptamine to severely reduce CNS serotonin terminals and perikarya have a reduced rate of accumulation of brain stem 5-hydroxytryptophan after Ro-44602. The rate of accumulation in the cerebral cortex and spinal cord were near normal when adult, even though serotonin and 5-hydroxyindoleacetic acid were sharply reduced in these regions. The respiratory response to 5-methoxy N,N-dimethyl-tryptamine was much more pronounced in pargyline-5,7-dihydroxytryptamine treated rats than in controls. If supersensitivity in serotonin receptors only develops in areas with decreased transmitter turnover, the site of action of serotonin agonists to depress respiration would seem to reside in the brain stem region. The results also suggest that compensatory changes in turnover do not develop to a similar degree in all CNS areas with altered serotonin content.

Electrophysiological evidence for A10 dopamine autoreceptor subsensitivity following chronic D-amphetamine treatment

Extracellular single unit recording techniques were used to determine whether chronic treatment with D-amphetamine (AMP) causes a subsensitivity of dopamine (DA) autoreceptors on A10 DA neurons in the rat ventral tegmental area. Either once daily or twice daily intraperitoneal (i.p.) administration of 5.0 mg/kg AMP for 1 week significantly reduced the ability of intravenous (i.v.) AMP and apomorphine (APO) to suppress the firing of A10 DA neurons when tested 24-32 h after the final administration of i.p. AMP. For both of these treatment regimens, the dose-response curves for AMP and APO induced suppression were shifted approximately 4-fold to the right of control. Following an 8 day abstinence period, only the rats that received twice daily AMP injections exhibited subsensitivity to i.v. AMP and APO; the degree of subsensitivity was reduced by 50% as compared to that observed 24-32 h post-treatment. These results were not due to acute tolerance phenomena since a single i.p. injection of AMP 24-32 h before testing failed to alter sensitivity to i.v. AMP and APO. Rather, the results indicate that chronic AMP treatment reduces the sensitivity of A10 DA neurons to DA agonists. DA autoreceptor subsensitivity was demonstrated further by the finding that the ability of microiontophoretically applied DA to suppress A10 DA neuronal activity was markedly reduced (5.8-fold shift of the dose-response curve) by chronic AMP treatment (2 X 5 mg/kg/day). In contrast, there was no alteration in the ability of iontophoretic gamma-amino butyric acid (GABA) to suppress A10 DA activity in chronic AMP rats. Chronic AMP-treatment also increased the number of spontaneously active A10 DA neurons as well as their basal firing rate. It is suggested that the ability of chronic AMP treatment to decrease the auto-regulatory ability of A10 DA neurons may be related to the phenomena of behavioral sensitization and AMP psychosis.

Interactions of cholecystokinin octapeptide and dopamine on nucleus accumbens neurons

Local microiontophoretic administration of cholecystokinin octapeptide (CCK) increased the firing rates of neurons in the dorsomedial nucleus accumbens (NAc), but exerted little to no effect on lateral NAc neurons. This regionally defined CCK-effect corresponds to the topographical distribution of CCK-like immunoreactive nerve terminal fiber networks and CCK receptors within the NAc. The excitatory effects of CCK were selectively antagonized by the CCK antagonist proglumide. Dopamine (DA) decreased the firing of NAc cells and reversed CCK-induced excitation. These results suggest that CCK and DA may interact to influence the activity of neurons within the dorsomedial NAc.

A10 dopamine neurons: role of autoreceptors in determining firing rate and sensitivity to dopamine agonists

The present experiments investigated the relationship between the spontaneous basal firing rate of A10 dopamine (DA) neurons and their sensitivity to the rate-suppressant effects of intravenously administered apomorphine (APO) and d-amphetamine (AMP) as well as microiontophoretically ejected DA. The results indicated highly significant inverse relationships between basal neuronal activity and sensitivity to DA and DA agonists, i.e. the faster the spontaneous rate of an A10 DA neuron, the less sensitive that cell was to agonist-induced suppression. This relationship was not found for the rate suppressant effects of iontophoretic gamma-aminobutyric acid. There were no significant differences between the effects of iontophoretic DA on pre-glutamate and glutamate-driven activity of the same A10 DA neurons indicating that faster firing rates, per se, did not determine the sensitivity of these cells to DA agonists. Rather, these results suggest that both spontaneous activity and sensitivity to DA agonists may be determined by the density (or sensitivity) of DA autoreceptors on A10 DA neurons. This hypothesis was supported by the finding that antidromically identified mesocortical DA neurons, which were significantly less responsive to DA, APO and AMP exhibited significantly faster firing rates than other A10 DA neurons. Thus, this subpopulation of A10 DA neurons is primarily made up of cells with low autoreceptor density (or sensitivity).

Brain part monoamines in the neuroendocrine mechanisms activated by immobilization stress in the rat

The long suspected and recently demonstrated abnormality in neuroendocrine control in patients with severe depression indicates that the neurochemical and neuroanatomical analysis of the neuroendocrine response to stress may provide valuable information in understanding the etiology of severe endogenous depression. Rats were immobilized for 1, 2, 3 or 5 hours consecutively or 2 hours per day for 5 days, sacrificed and plasma corticosterone (as an index of the release of adrenocorticotropic hormone from the pituitary) and brain part noradrenaline, dopamine, and serotonin concentrations were determined fluorometrically. Plasma corticosterone and brain part monoamines were also measured in other rats given 2 hour immobilization stress one week after the intraventricular injection of the neurotoxins 6-hydroxydopamine and/or 5,6-dihydroxytryptamine. Plasma corticosterone increased by 30% to 50% after all periods of stress and serotonin was increased in all brain parts after 1, 2 or 3 hours of stress but not after 5 hours or chronic stress. Forebrain dopamine was decreased by 30% after 1 hour stress, slowly increased with increasing duration of stress becoming a marked increase of 85% over controls after prolonged or chronic stress. The destruction of catecholamine nerve terminals with 6-hydroxydopamine prevented the stress induced rise in brain part serotonin but had no effect on the plasma corticosterone response to immobilization stress. Destruction of serotonin nerve terminals with 5,6-dihydroxytryptamine potentiated by 50% the stress induced rise in plasma corticosterone. Plasma corticosterone after 2 hours immobilization stress was the same as controls in rats given both neurotoxins. These data support the hypothesis that ACTH release is stimulated by serotonergic neural activity.

Decreased sensitivity of neurons in the basolateral amygdala to dopamine and noradrenaline iontophoresis after a kindling stimulus

The depressant effects of iontophoretically applied dopamine and noradrenaline on glutamate-induced neuronal firing in the amygdaloid complex of cats were significantly reduced 1 and 2 h after induction of a local epileptiform afterdischarge of the kind used in kindling. Neuronal excitation by glutamate and depression by GABA were not significantly changed. This suggests that kindling is associated with a reduction of the inhibitory effects of endogenous catecholamines.

Morphine-induced activation of A10 dopamine neurons in the rat

The effects of intravenous administration of morphine (MOR) on the spontaneous discharge rate of dopamine (DA) neurons in the ventral tegmental area (VTA or A10) and the substantia nigra pars compacta (SNC or A9) were compared. MOR (0.5-3.5 mg/kg) produced a marked increase in the spontaneous firing of both A10 and A9 DA neurons. Naloxone (NAL) reversed the MOR effects. Acute transection of the medial forebrain bundle (MFB) did not interfere with the observed MOR effects on either A10 or A9 DA neurons. However, following chronic lesions of the MFB (6 days), A9 DA neurons were no longer responsive to MOR whereas A10 DA cells were still activated by MOR. Neither radiofrequency lesions of the dorsal raphe nucleus (DRN) nor administration of the 5-HT2 antagonist ketanserin affected the stimulatory effect of MOR on either A10 or A9 DA cells. Thus, it is confirmed that the effects of MOR on A9 DA cells depend on striatonigral feedback pathways. In contrast, it appears that the MOR-induced activation of A10 DA cells does not depend on afferents from the forebrain or on projections from the DRN, suggesting a more direct action of MOR on A10 DA cells. Microiontophoretic application of MOR or enkephalin analogues significantly increased the spontaneous activity of both A9 and A10 DA cells. However, these effects were not reversed by either iontophoretic or intravenous NAL. On the other hand, both intravenously (0.5-1.5 mg/kg) and iontophoretically administered MOR markedly suppressed the electrical activity of non-DA cells found in the vicinity of A10 DA neurons, and this effect was completely reversed by NAL. It is proposed that the MOR-induced activation of A10 DA cells could be mediated indirectly by non-DA cells.

Monoamine involvement in the overeating caused by muscimol injection in the rat nucleus raphe dorsalis and the effects of d-fenfluramine and d-amphetamine

Injections of 5,7-dihydroxytryptamine in the nucleus raphe dorsalis did not significantly modify the eating caused by muscimol (100 ng) injected in the same area of freely fed rats 11 days later. Eating caused by muscimol, like food intake in starved rats, was significantly reduced by phenoxybenzamine (5, 10 and 20 mg/kg i.p.). Penfluridol (2.5 and 5 mg/kg p.o.), a dopamine receptor blocker, markedly reduced muscimol-induced eating, but had no effect on the food intake of starved rats. d-Fenfluramine (2.5 mg/kg i.p.), a releaser of serotonin from nerve terminals, significantly reduced eating in muscimol-injected and starved animals whereas d-amphetamine 1.25 and 2.5 mg/kg i.p. only inhibited the food intake of starved rats. Eating elicited by muscimol injected in the nucleus raphe dorsalis appears to constitute a catecholamine-mediated model of hyperphagia selectively inhibited by agents which increase serotonin transmission.

Changes in nociception after intrathecal administration of 5,6-dihydroxytryptamine in mice

Intrathecal administration of 5,6-dihydroxytryptamine (5,6-DHT) (5 micrograms) to mice selectively lesioned descending serotonergic pathways, reducing spinal levels of 5-hydroxytryptamine (5-HT) by 80%, without significantly changing the levels of noradrenaline. Increased sensitivity to noxious stimulation, as measured by the tail-flick and hot-plate tests, was observed 2 days after injection of 5,6-DHT. The tail-flick latencies returned to normal on day 6, but were again reduced by administration of the 5-HT receptor blocker metergoline, suggesting that the normalization process involved compensatory mechanisms in the remaining 5-HT system. In the hot-plate test, the latencies both to shaking or kicking of a hindpaw (kick) and to hindpaw lick were recorded, but the time course for the changes of these two responses was found to be different. The latencies to hindpaw lick were normalized within 2 weeks, whereas the hindpaw kick latencies remained reduced throughout the 21 day observation period.

Alpha 1-adrenoceptor denervation supersensitivity in brain: physiological and receptor binding studies

Electrophysiological and radioligand binding methods were used to characterize noradrenergic denervation supersensitivity at alpha 1-adrenoceptors in rat thalamus. Denervation was accomplished either by intraventricular or intracerebral injection of the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA). In the physiological studies, the sensitivity of single lateral geniculate neurons to norepinephrine, carbachol, and serotonin was compared in sham and lesioned animals various times after 6-OHDA. Conducted in parallel were radioligand binding studies in which the density and affinity of thalamic alpha 1-adrenoceptors were measured with the specific antagonist [3H]prazosin. The results indicate that denervation produces a selective increase in the sensitivity of geniculate neurons to alpha 1-adrenergic stimulation and a concomitant increase in alpha 1-adrenoceptor density and agonist affinity.

Selective destruction of the serotonergic fibers of the fornix-fimbria and cingulum bundle increases 5-HT1 but not 5-HT2 receptors in rat midbrain

Selected and localized lesions of serotonergic (5-HT) neurons were made by microinjection of 5,7-dihydroxytryptamine (5,7-DHT), after pretreatment with desipramine, into the cingulum bundle and fornix-fimbria; these are the major serotonergic hippocampal inputs from the median raphe nucleus. Two weeks after the lesion, the binding of [3H]5-HT (5-HT1 receptor) was determined in the hippocampus which receives the afferent terminals and, in addition, in the septum/hypothalamus and midbrain from where the fibers originate. Scatchard analysis showed there was no significant change in binding parameters in the hippocampus; however, a significant increase was observed in the Bmax in the midbrain (38%) with no change in the KD. The caudate which receives 5-HT inputs via other 5-HT tracts was not affected by the lesion. The changes in 5-HT1 receptor number or affinity were not observed 6 days or 5 weeks after the lesion. The binding of the ligands [3H]spiroperidol and [3H]ketanserin to the 5-HT2 receptor population was also determined in the same brain areas; no changes in receptor binding occurred two weeks after the lesion. These experiments demonstrate that a selective lesion of the serotonergic system can increase 5-HT1 receptors in the midbrain, which contains the serotonin cell bodies. In addition, as 5-HT2 binding is not altered, this further supports the hypothesis that 5-HT1 and 5-HT2 receptors are distinct populations of receptors.

Effects of quipazine on pre- and postsynaptic serotonin receptors: single cell studies in the rat CNS

Many behavioural and biochemical studies have pointed to an agonistic activity of quipazine on serotonin (5-HT) receptors. In the present electrophysiological study, the effect of quipazine on pre- and postsynaptic 5-HT receptors in the rat was studied. Quipazine, administered intravenously, depressed the firing rate of 5-HT-containing dorsal raphe neurones (ED50 = 0.82 mg/kg). Microiontophoretic applications of quipazine on 5-HT-containing neurones in the dorsal raphe and on neurones of two forebrain regions receiving a 5-HT input (the ventral lateral geniculate nucleus and the dorsal hippocampus) consistently depressed neuronal firing rate as did 5-HT and D-lysergic acid diethylamide (LSD). Quipazine was more potent on 5-HT neurones than on the ventral lateral geniculate nucleus and hippocampal neurones: the post/presynaptic efficacy ratio for quipazine was similar to that of LSD. Following a selective denervation of 5-HT neurones with intraventricular injection of 5,7-di-hydroxy-tryptamine in desipramine-pretreated rats, the responsiveness of neurones in the ventral lateral geniculate nucleus to quipazine, applied microiontophoretically, was increased as was that to 5-HT and to LSD. These results provide direct evidence for the agonistic activity of quipazine on both pre- and postsynaptic 5-HT receptors.

Serotonin receptor activation in rats previously deprived of REM sleep

The effects of serotonin precursors (L-5-hydroxytryptophan and L-tryptophan, with or without MAO inhibitors) and of agonists (quipazine and 5-methoxy-N,N-dimethyltryptamine-MeO-DMT) were studied in 3 day REM-deprived or control rats, by recording the presence of the serotonin syndrome and the number of head shakes. The REM sleep-deprived rats showed a larger incidence of the serotonin syndrome and a greater number of head shakes in comparison to the control animals, when challenged with the serotonin precursors. Conversely, REM sleep deprivation did not modify the responsiveness of rats to 0.75-6.0 mg/kg of MeO-DMT and to 2.4-6.0 mg/kg of quipazine. However, REM-deprived rats reacted less than controls to 0.3-1.25 mg/kg of quipazine. Increased turnover due to REM sleep deprivation could explain the augmented responsiveness of the rats to the serotonin precursors. Conversely, the decreased responsiveness to quipazine could result from receptor hyposensitivity due to intense receptor activation, caused by the increased turnover, during the 3 day period of REM sleep deprivation.

Altered neuronal responsiveness to biogenic amines in rat cerebral cortex after serotonin denervation or depletion

To further investigate monoaminergic mechanisms in cerebral cortex, responsiveness of cortical neurons to microiontophoretic applications of serotonin (5-HT), dopamine (DA) or noradrenaline (NA) was examined in the frontoparietal region of control, 5,7-dihydroxytryptamine (5,7-DHT)- and p-chlorophenylalanine (PCPA)-treated rats anesthetized with urethane. As a rule, 100 nA applications of either one of these biogenic amines induced marked slowing or total interruption of 'spontaneous' firing overlasting the 30 s period of ejection. Given the large amounts of monoamines ejected, it could be inferred that such microiontophoretic applications produced a maximal activation of receptors. In control rats, the responses to 5-HT, DA and NA were of approximately equal duration (approximately equal to 5 min). Two to 4 weeks after denervation with 5,7-DHT, most neurons (75%) exhibited greatly prolonged responses to 5-HT (approximately equal to 14 min), and marked depressions of firing could be induced by small ejection currents (approximately equal to 2 nA) having little or no effect in the controls. In addition, 85% of the units supersensitive to 5-HT showed considerably shortened responses to DA and NA (approximately equal to 1 min). After 2-14 days of depletion with PCPA, there was no change in the responsiveness to 5-HT in spite of a 91% lowering of cortical 5-HT content equivalent to that measured after denervation. Nevertheless, responsiveness to DA and NA was again diminished in a majority (80%) of the units tested. In control or PCPA-treated rats, acute administration of the 5-HT re-uptake blocker fluoxetine increased the duration of depressions induced by 100 nA applications of 5-HT but did not enhance responsiveness to low ejection currents. This suggested that, after 5-HT denervation, the suppression of re-uptake was mainly responsible for the prolongation of 5-HT responses ('presynaptic' component of supersensitivity), whereas a modification of 5-HT receptors accounted for the greater efficacy of small doses of 5-HT ('postsynaptic' component). Responsiveness to the microiontophoretic application of phenylephrine (PHE), a noradrenergic a-agonist, was comparable with that to NA in PCPA- and 5,7-DHT-treated as well as in control rats. Therefore, the hyposensitivity to DA and NA appeared indicative of a desensitization of catecholamine receptors caused by the absence of 5-HT. Such a desensitization may be viewed as an adaptive change resulting from an increased release of endogenous DA and NA. This interpretation would in turn imply that, normally, 5-HT regulates catecholamine release in the neocortex.

Cerebral circulatory response to hypercapnia: effects of lesions of central dopaminergic and serotoninergic neuron systems

The present study explores the possibility that the central dopaminergic and serotoninergic neuron systems influence CBF under normocapnic and hypercapnic conditions. In the first part of the study the effect of unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway on local cerebral blood flow (1-CBF) was measured autoradiographically with [14C]iodoantipyrine as the diffusible tracer. The lesion caused no major effect on CBF under normocapnic or hypercapnic conditions. However, the circulatory response to hypercapnia was slightly enhanced (about 10%) in the denervated caudate-putamen. It is suggested that under hypercapnic conditions the pronounced increase in blood flow in the caudate-putamen is normally modulated by a slight vasoconstriction caused by dopamine release from the nigrostriatal system. In the second part of the study the effect of intraventricular 5,7-dihydroxytryptamine on cerebral metabolic rate for oxygen (CMRO2) and CBF was evaluated using a 133xenon modification of the Kety-Schmidt inert gas technique. The lesion, which removed about 90% of cortical 5-hydroxytryptamine, had no effect on the circulatory response to hypercapnia, not did it alter CMRO2. Under normocapnic conditions, though, the lesion seemed to induced a minor increase in CMRO2, which indicates that the serotoninergic system exerts a depressant resting tone on metabolic rate in the brain.

Multiple serotonin receptors: regional distribution and effect of Raphe lesions

These studies confirm and extend the recent work suggesting that [3H]lysergic acid diethylamide (LSD) labels two distinct binding sites in rat brain resembling serotonin (5HT) receptors. Although Scatchard analyses of [3H]LSD binding to membranes prepared from cortex/hippocampus were linear, the heterogeneity of the [3H]LSD binding sites was clearly demonstrated in displacement studies. The displacement curves for both 5HT and spiperone were bisigmoidal with the concentration required to saturate the high affinity components nearly 3 orders of magnitude lower than the concentration necessary to saturate the low affinity components. Additivity studies suggested that the sites with high affinity for 5HT and spiperone are different, independent sites. These sites are referred to as 5HT, and 5HT2, respectively. Regional analyses showed, that in the frontal cortex, the density of the 5HT2 site was slightly greater than the 5HT1 site, whereas the 5HT1 site was predominant in all other brain areas, including the spinal cord. The pharmacological properties of the two sites have features in common with 5HT receptors; however, electrolytic lesions of the midbrain raphe nuclei did not change the densities or binding constants of the two apparent 5HT receptor subtypes, even though the number of high affinity 5HT uptake sites was markedly reduced.

Experimentally induced supersensitivity of neocortical neurons to microiontophoretically administered serotonin

Central serotonergic fiber systems of the rat were selectively lesioned by intraventricular injection of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). At various times thereafter, the sensitivity of rostral cortical neurons to microiontophoretically administered serotonin (5-HT) was compared in groups of lesioned and sham-operated animals pretreated with the 5-HT uptake inhibitor CGP 6085. Twenty-four hours after the injection of 5,7-DHT, at which time the cortical 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels were both reduced by 40%, there was no significant difference in the sensitivity of cortical neurons to 5-HT. However, 3 days after such treatment, when the cortical 5-HT and 5-HIAA levels were reduced by 52% and 53% respectively, pronounced supersensitivity to 5-HT was noted. The depressant action of 5-HT on neuronal firing was potentiated with regard to both maximal firing depression and duration of the firing inhibition. A similar potentiation of the 5-HT responses was observed 7 days after lesioning. Supersensitivity thus appears to develop between 1 and 3 days after the injection of 5,7-DHT. Seven days after lesioning, the sensitivity of rostral cortical neurons to gamma-aminobutyric acid was unchanged compared to that observed in sham-operated animals.

Effect of morphine on circling behaviour in rats with 6-hydroxydopamine striatal lesions and electrolesions of the raphe nucleus

Morphine antagonized d-amphetamine circling in rats which had received unilateral 6-OHDA lesions of the striatum but failed to reduce the circling in rats with both a unilateral 6-OHDA striatal lesion and a raphe (5-HT) lesion. Naloxone precipitated withdrawal of morphine tolerant rats greatly enhanced d-amphetamine circling when the rats had a 6-OHDA lesion but not when both 6-OHDA and raphe lesions were present. It is concluded that 5-HT is necessary for the morphine-induced inhibition of the circling. The effect of morphine tolerance and naloxone precipitated withdrawal on brain 5-HT function was investigated using a putative 5-HT rotation model in which both a dopamine and a 5-HT agonist were administered to rats with an asymmetrical medial raphe lesion. The findings suggest that chronic treatment with morphine increases striatal 5-HT function.

Circling produced by serotonin and dopamine agonists in raphe lesioned rats: a serotonin model

The serotonin (5-HT) receptor agonists quipazine and 5-methoxy-N,N-dimethyltryptamine caused slow, contralateral circling, which was not dose-related, when given to rats with an asymmetric electrolytic lesion of the midbrain medial raphe nucleus. No circling was recorded after administration of fenfluramine, a compound that releases 5-HT. The dopamine drugs apomorphine and d-amphetamine caused vigorous circling in raphe lesioned rats which was increased further by quipazine but antagonized by fenfluramine. The effects of the 5-HT drugs on d-amphetamine circling were closely dose-related. The results suggest that modulation of d-amphetamine circling by 5-HT agonists in rats with a specific, asymmetric 5-HT lesion could provide the basis for a 5-HT rotation model.

Monoaminergic denervation of the rat hippocampus: microiontophoretic studies on pre- and postsynaptic supersensitivity to norepinephrine and serotonin

The responsiveness of hippocampal CA3 pyramidal neurons to microiontophoretic applications of serotonin (5-HT), norepinephrine (NE), gamma-aminobutyric acid (GABA) and isoproterenol (ISO) was assessed in rats following 5,7-dihydroxy-tryptamine (5,7-DHT) and 6-hydroxydopamine (6-OHDA) pretreatments and bilateral locus coeruleus lesions. The intraventricular administration of 200 micrograms (free base) of 5,7-DHT and of 6-OHDA produced 89% and 93% decreases of 5-HT and NE respectively. None of these pretreatments modified the inital responsiveness to, or recovery from iontophoretic application of 5-HT. In 6-OHDA pretreated and locus-lesioned rats, the initial effectiveness of NE was not altered but its effect was markedly prolonged. However, there was no such prolongation of the effect of ISO which is not a substrate for the high affinity NE reuptake. The effect of GABA was not affected by these pretreatments. Acute pharmacological blockade of the NE reuptake with desipramine (5 mg/kg, i.p.) similarly induced a prolongation of the effect of iontophoretically applied NE, while fluoxetine (10 mg/kg, i.p.) a 5-HT reuptake blocker, failed to alter the recovery of pyramidal cells from iontophoretic application of 5-HT. It is concluded that 5-HT denervation induces neither pre- nor postsynaptic types of supersensitivity in hippocampal pyramidal cells, contrasting with the previously shown supersensitivity of ventral lateral geniculate and amygdaloid neurons following 5-HT denervation. NE denervation fails to induce a postsynaptic type of supersensitivity but leads to a marked prolongation of the response to NE indicative of a presynaptic mechanism. These results underscore the necessity for regional studies of neurotransmitters and drug action.

Effect of chronic tricylic antidepressant treatment on the serotoninergic autoreceptor: a microiontophoretic study in the rat

Chronic treatment with tricyclic antidepressant (TCA) drugs has been shown to enhance the responsiveness of rat forebrain neurons to serotonin (5-HT). In the present study, imipramine (5 mg and 10 mg/kg), iprindole (2.5 mg/kg), desipramine and femoxetine (5 mg/kg) were administered daily for 14 days. The response of dorsal raphe neurons to intravenous injection of LSD (4 microgram/kg) and to microiontophoretic applications of 5-HT and LSD was assessed 24 h after the last dose. The responsiveness to intravenous LSD and the effectiveness of microiontophoretic applications of 5-HT and LSD were not altered by TCA drug pretreatments. Furthermore, the treatments did not change the mean firing rate of these 5-HT neurons. Those results suggest that chronic treatment with TCA drugs does not alter the sensitivity of the 5-HT autoreceptor. Thus, the effect of the previously reported increase of postsynaptic neuron responsiveness to 5-HT would not be dampened by a decreased activity of the presynaptic neurons.

Neurochemical modulation of sensory-motor reactivity: acoustic and tactile startle reflexes

The present review argues that the startle reflex is particularly well suited as a model system to analyze how drugs alter stimulus reactivity and reflex excitability. It then reviews all the literature to date on how drugs or lesions that are thought to alter neurochemical transmitter systems affect acoustic and/or tactile startle. Hypotheses are presented to account for how serotonin, dopamine, norepinephrine, acetylcholine, and opiates modulate startle. Effects on startle plasticity such as habituation, sensitization, and potentiation resulting from prior associative learning are also included.