Collicular picrotoxin alleviates akinesia but not sensory neglect in rats with bilateral 6-hydroxydopamine lesions of ventral midbrain

Defense reaction mediated by NMDA mechanisms in the inferior colliculus is modulated by GABAergic nigro-collicular pathways

Electrical stimulation of the inferior colliculus (IC) causes a behavioral activation together with autonomic responses similar to fear reactions to threatening situations. GABAergic mechanisms exert a tonic inhibitory control on the neural substrates of aversion in the IC insofar as local injections of GABA agonists or antagonists inhibit or mimic these defensive behaviors, respectively. Recently, we have shown that systemic injections of the GABA-A receptor agonist muscimol unexpectedly enhanced the freezing and escape responses provoked by gradual increases in the intensity of the electrical stimulation of the IC. Taking into account that the neural circuits mediated by excitatory amino acids (EAA) in the IC may be responsible for the integration of fear states, in the present study we examined whether the defensive behavior induced by local injections of NMDA into the IC is influenced by prior treatment with systemic muscimol and also whether this GABAergic control could be exerted by GABAergic fibers that project to the inferior colliculus from the substantia nigra pars reticulata (SNpr). Rats were implanted with two guide-cannulae aimed at the IC and SNpr through which drug microinfusions with glass micropipette could be made with reduced brain damage. One week after surgery, the animals received either NMDA (7 nmol/0.2 microl) or saline into the IC and were placed into the middle of an enclosure where behavioral responses such as freezing, crossings, jumping, rearing, and turnings could be measured as an indirect index of unconditioned fear. These animals were pretreated either with saline or muscimol (0.5 mg/kg, IP) or with brain injections of saline or muscimol (1 nmol/0.2 ìl into SNpr). NMDA applied into the IC produced a behavioral activation with significant increases in all behavioral measures. IP injections of muscimol or into the SNpr enhanced the defense reaction caused by microinjections of NMDA into the IC. These findings give support to the idea that unconditioned defensive responses generated in the IC may be mediated by NMDA mechanisms. Additionally, a reduction of the inhibitory control exerted by nigrocollicular GABAergic neurons seems to be responsible for the unexpected pro-aversive action of systemic injections of muscimol on the neural substrates of aversion mediated by NMDA in the IC.

GABAergic nigro-collicular pathways modulate the defensive behaviour elicited by midbrain tectum stimulation

Midbrain tectum (MT) structures such as the dorsal periaqueductal gray matter and deep layers of superior colliculus are well-known for the organization and generation of defensive behaviour. Electrical stimulation or microinjection of GABA antagonists into these structures produce aversive behaviour. In order to determine whether the nigrocollicular GABAergic fibers exert some control over this behaviour, rats bearing neurochemical lesions with kainic acid in the substantia nigra, pars reticulata (SNpr) and compacta (SNpc), were submitted to MT microinjections of bicuculline or electrical stimulation at aversive thresholds. The same procedure was carried out after enhancement or inhibition of GABAergic transmission in SNpr through microinjections of muscimol or bicuculline, respectively. Animals with SNpr neurochemical lesion exhibited a significant decrease in the aversive thresholds and an increase in the responsiveness to bicuculline microinjections. An opposite effect was observed following microinjections of bicuculline into the SNpr. The enhancement of the GABAergic transmission into the SNpr following microinjection of muscimol mimicked the effects produced by the lesion with kainic acid. These results suggest an inhibitory control of GABAergic fibers from the substantia nigra, pars reticulata, on aversive behaviour induced by midbrain stimulation.

Effect of increasing doses of intracaudate haloperidol upon motor expressions that require an intact substantia nigra pars reticulata and/or superior colliculus in cats

In the present study it was investigated whether intracaudate (CN) injections of increasing doses of haloperidol progressively affect movements that require the integrity of the substantia nigra pars reticulata (SNR) and/or the deeper layers of the superior colliculus (dl-SC) in cats. Therefore, cats were trained to walk from one side of a narrow bar to the other side both under full light and under stroboscopic illumination (2 flashes/s). Previously it has been shown that this set-up is useful for assessing changes in the execution of dl-SC and SNR-related non-externally guided targeting movements. Moreover, cats were trained to step out of a startbox on a rotating cylinder which, according to previous studies, also implies the execution of a dl-SC- and SNR-related non-externally guided targeting movement. CN application of 25 micrograms/5 microliters haloperidol, but not CN injection of 12.5 micrograms/5 microliters haloperidol or apomorphine (0.6 micrograms/5 microliters), reduced the occurrence of these dl-SC- and SNR-related targeting movements. Dl-SC injection of picrotoxin (100 ng/0.5 microliter) did not restore the haloperidol (25 micrograms/5 microliter) induced decrement in the occurrence of these targeting movements. In addition, CN application of 25 micrograms/5 microliter haloperidol, but not 12.5 micrograms/5 microliter haloperidol, reduced the feline ability to execute SNR-specific limb movements while crossing a revolving cylinder. The present results show that only a severe but not a moderate inhibition of dopamine receptors within the CN affects motor expressions characteristic of the SNR and/or dl-SC.

Responses resembling defensive behaviour produced by microinjection of glutamate into superior colliculus of rats

Electrical stimulation of the superior colliculus in rats elicits not only orienting movements, as it does in other mammals, but also behaviours resembling such natural defensive responses as prolonged freezing, cringing, shying, and fast running and jumping. To investigate the location of the cells mediating these behaviours, the superior colliculus was systematically mapped with microinjections of sodium L-glutamate (50 mM, 200 nl), and the resultant behavioural changes as assessed in an open field were analysed for defence-like responses. The main regions that gave defensive behaviour were (i) rostromedial superior colliculus (all layers), and (ii) both medial and lateral parts of the caudal deep layers. Cells in these areas project into the ipsilateral descending pathway. However, the cells of origin of this pathway are also found in collicular regions, such as rostral intermediate gray and parts of far caudal colliculus, that did not give defensive movements in response to glutamate stimulation. It is unclear whether this is because only parts of the ipsilateral pathway mediate defensive behaviours, or because glutamate is a relatively inefficient stimulating agent for these systems. An unexpected feature of the results was that at a number of collicular sites the nature of the defensive response changed with successive (up to three) injections of glutamate, often appearing to become more intense. Whether the mechanism underlying this potentiation is related to the conditioning of natural defensive behaviour is unknown.

The striato-nigro-collicular pathway and explosive running behaviour: functional interaction between neostriatal dopamine and collicular GABA

The nature of the functional interaction between neostriatal dopamine activity and collicular GABA activity was studied. To this end we analyzed the ability of apomorphine injections into the neostriatum (50-500 ng/0.5 microliters per side) to reinitiate explosive running behaviour in rats pretreated with a subthreshold dose of picrotoxin into the colliculus superior (30-80 ng/0.5 microliters per side). Apomorphine was found to reinitiate the former behaviour dose dependently and its effect could be antagonized by a very low dose of neostriatally applied haloperidol (250 ng/0.5 microliters per side). It was possible to delineate the most critical region within the structures studied. Evidence is presented in favour of the previously reported hypothesis that increasing dopaminergic neostriatal activity within the terminal region of the nitrostriatal pathway results in a decreasing GABAergic activity within the terminal region of the GABAergic striato-nigro-collicular pathway.

Rotational behavior following cholinergic stimulation of the superior colliculus in rats

Rate which received microinjections of carbachol into the superior colliculus exhibited pronounced dose-dependent rotational behavior contralateral to the site of injection (Experiment 1). Wet dog shakes were also observed in some animals. Similar injections in the midbrain reticular formation produced immobility with slight contralateral flexion of the neck. Convulsions were observed in some rats after injections into either anatomical location. In Experiment 2, circling induced by carbachol in the superior colliculus was blocked by prior injection of either the muscarinic receptor antagonist scopolamine or the nicotinic receptor antagonist mecamylamine, suggesting that both nicotinic and muscarinic receptors are involved in the effect. In Experiment 3 contralateral rotational behavior was induced by intracollicular microinjections of the combination of acetylcholine chloride and physostigmine. The results suggest that collicular mediation of contralateral rotational behavior, and perhaps orientation, might involve cholinergic receptors.

Unilateral injection of GABA agonists in the superior colliculus: asymmetry to tactile stimulation

A unilateral microinjection of each one of three different GABA agonists (Muscimol: 1.4 nmoles; Baclofen: 0.8 nmole; THIP: 10.7 nmoles) into the superior colliculus was found to result in a reversible asymmetry in the rat's responsiveness to tactile stimulation. The rat was hyporeactive to stimulations applied contralaterally and hyperreactive to stimulations applied ipsilaterally to the infusion site. Furthermore, the rat showed ipsiversive turning in response to tactile stimulation applied either ipsi- or contralaterally to the infusion site. The results are discussed in relation with motor and sensory asymmetry produced by unilateral manipulations affecting the striato-nigro tectal system.

Picrotoxin microinjections into the brain: a model of abrupt withdrawal 'jumping' behaviour in rats not exposed to any opiate?

Picrotoxin microinjections (0.5 microliter per side) given into the locus coeruleus, superior colliculus and central grey of freely moving Wistar rats produced an accelerating galloping locomotion, most often forwards but also upwards. Systemic administration of apomorphine (0.7-2.0 mg/kg s.c.) given 20 min earlier increased the number of rats that displayed the so-called 'running and jumping' behaviour after picrotoxin (0.1-0.2 microgram). The dose-effect curve, however, varied according to the brain structure selected. Systemic administration of morphine (1.0-10.0 mg/kg i.p.) was found to reduce the occurrence of running and jumping behaviour when it was elicited via the locus coeruleus, but not the superior colliculus and central grey. It is suggested that the running and jumping behaviour which is elicited via the locus coeruleus might be a suitable model of a partial morphine abstinence syndrome, i.e. withdrawal jumping, in experimental animals not exposed to any opiate. The superior colliculus and central grey appear to be involved in mediating an opiate-unrelated phenomenon of 'withdrawal' jumping in rats not exposed to any opiate.