Role of midbrain ventro-lateral tegmental area (VTA) adrenergic mechanisms in facilitation of hypothalamically-induced predatory attack behaviour.

Bipolar concentric electrodes were implanted in extreme lateral regions of hypothalamus having coordinates (A12.5 mm, L 3.5-3.7 mm, V 3.0-3.7 mm). These sites were electrically stimulated using biphasic square wave pulses (1 ms, 60 Hz) at a current strength ranging from 300-800 microA to evoke predatory attack on an anaesthetized rat. At lower current strengths of 300 microA only altertness with pupillary dilatation was produced. Gradual increase in the current strength led to recruitment of somatic and affective components and a full blown predatory attack on a rat was produced at a mean current strength of 700 microA. A scoring system allowed the construction of stimulus response curve, which remained fairly constant when repeated over a period of 3-4 weeks. In ventrolateral tegmental area (VTA), bilateral microinjections of norepinephrine (NE 10 micrograms in 0.5 microliter saline, pH 7.4) lowered the mean threshold current strength to 100 microA while predatory attack was produced at 500 microA. Clonidine (5 micrograms in 0.5 microliter propylene glycol, pH 7.4) an alpha-2 agonist similarly lowered the mean threshold to 100 microA and predatory attack threshold to 400 microA. The effects of clonidine appeared within 20 min of microinjection and persisted up to 6 hr. Yohimbine, an alpha-2 antagonist (4 micrograms in 0.5 microliter propylene glycol. pH 7.4) when microinjected into the same locus (VTA), completely blocked predatory attack behaviour for 3 days, the peak period of the blocking effects were between 3-8 hr, after microinjection. Isoproterenol (beta agonist), propranolol (beta blocker), prazosin (alpha-1 antagonist) and phenoxybenzamine (alpha antagonist) failed to produce any effect. Normal saline and propylene glycol in similar volumes served as controls. The excitatory effects of NE and clonidine and inhibitory effects of Yohimbine were significant at P < 0.01 and P < 0.05 respectively with Wilcoxon's signed rank test. The present study indicates the involvement of alpha-2 adrenoceptive mechanisms operating at the midbrain (VTA) level in the elicitation of predatory attack from lateral hypothalamus.

Plasticity of hippocampal and motor cortical pyramidal neurons induced by self-stimulation experience.

The self-stimulation (SS) induced neuronal plasticity was observed in CA3 hippocampal and layer V motor cortical pyramidal neurons. SS experience was allowed daily for a total of 1 hour for 10 days through four bipolar electrodes implanted bilaterally in lateral hypothalamus (LH) and substantia nigra-ventral tegmental area (SN-VTA) in adult male Wistar rats. Examination of pyramidal neurons stained by rapid Golgi technique was made in a total of 1,600 neurons out of 80 rats consisting of 4 groups. The dendritic intersections were quantified upto 200 and 120 microns radial distances in apical and basal dendrites respectively. The CA3 hippocampal and layer V motor cortical pyramidal neurons of SS group revealed significant increase (P < 0.001, two-way ANOVA) in dendritic intersections in both apical and basal dendrites, compared to normal control (NC), sham control (SH) and experimenter-administered (EA) group of animals. These results demonstrate that SS experience promotes increase in dendritic length in hippocampal and motor cortical pyramidal neurons.